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TABLE OF CONTENTS CHAPTER I

Page

I INTRODUCTION

1.1 Thepurpose. . . . . . . . . . . . . . . . . . . . . . . 1.1 1.2 Application of Sedimentation Ponds . . . . . . . . . . . 1.1 1.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 1.4 Design Manual U s e . . . . . . . . . . . . . . . . . . . . 1.3

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I. INTRODUCTION

1.1 The Purpose

The need f o r control of sediment eroded from areas disturbed by coal

mining operations has been w e l l documented. Presently, several erosion and

sedimentation control measures are avai lable t o the operator.

various methods, sedimentation ponds have been the mos t widely used and are

required by federal regulations. Sedimentation ponds are typica l ly the last

treatment measure applied before runoff leaves the permit area. Therefore, it

is paramount t h a t sedimentation ponds be designed, constructed, and maintained

t o provide sediment removal to meet regulatory e f f luent l imi ta t ions and main-

t a i n the hydrologic balance.

Of these

Previously, federal and state regulations have required design of sedi-

( 1 ) to provide a spec i f ic s torage mentation ponds f o r two general criteria:

capacity based on the amount of disturbed area and ( 2 ) provide a required

storage capacity t o r e t a in the runoff from a design precipitation event f o r a

spec i f ied period of time.

ponds designed t o meet the above criteria do not necessarily meet applicable

e f f luen t l imitat ions. This inconsistency is addressed by the regulat ions

Recent s tudies have shown that the sedimentation

', curren t ly published by the Office of Surface Mining (OSM), whereby seaimen-

t a t i o n ponds are required to meet ef f luent l imitat ions and the se lec t ion of

sedimentation pond design criteria such as storage volume, pond geometry, and

detention time is l e f t to the design engineer. Thus, t he design of sedimen-

t a t i o n ponds should be based on the pond's a b i l i t y to achieve specific

effluent limitations.

\

1.2 Application of Seamentation Ponds

As stated previously, sedimentation ponds are the last treatment measure

applied before the runoff leaves the permit area. However, it should be

understood t h a t sedimentation ponds are not t he only means of sediment and

erosion control , but simply an i n t e g r a l part of an overa l l plan.

a complete sediment and erosion cont ro l plan before, during, and a f t e r mining

operations based on sound engineering knowledge is necessary to minimize

p o t e n t i a l environmental damage from surface mining activities. Further, it is

e s s e n t i a l that the designer r ea l i ze t h a t t he drainage basin i n t h e permit area

is only one part of a larger, mre complex drainage system. The drainage

The need f o r

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.. .. . . 1. . . . . . . .-. .. .

1.2

network i n the pepnit area i n t e r a c t s w i t h other parts of the l a rge r drainage

system i n a complex fashion.

established a state of balance or quasi-equilibrium. The mining opera t ion , or

any other large-scale disturbance, w i l l a f f e c t this balance or equilibrium and

can r e s u l t i n dynamic responses through the system. The &signer must

recognize this s i t u a t i o n i n order to restore the disturbed topography and

drainage t o a condi t ion where it w i l l again properly function as part of the

larger system.

Over time this complicated system has

Sedimentation ponds as referred to in this manual are used f o r the re-

moval of sediment due to erosion from dis turbed areas during the a c t i v e mining

phase and during the reclamation phase u n t i l adequate revegetation has been

e s t ab l i shed . Sedimentation ponds are used i n a l l OSM regions, w i t h a l l types

of d n i n g methods, on n a t u r a l drainageways and in conjunction with d ivers ions .

The major c o n t r o l l i n g f a c t o r in the app l i ca t ion of sedimentation ponds is

topography of the s p e c i f i c site.

normally -associated with eas t e rn mines in the Appalachian Hountain range,

l imited mining is conducted on steep sloped t e r r a i n i n the Rocky Mountain sta-

tes. There are also r o l l i n g and f l a t t e r r a i n areas i n southeastern parts of

Although mining in steep sloped t e r r a i n is

. . t he United States. Therefore, techniques f o r app l i ca t ion and design of sedi- \

mentation ponds cannot be spec i f i ed by region, but are very dependent on the

topography of the si te being analyzed.

1.3 SCOP

The procedures presented h this manual are based on a comprehensive

literature review and assessment of the best technology cu r ren t ly available.

Se lec t ion criteria f o r i nc lus ion in t h e design manual for t h e range of design

methodologies a v a i l a b l e included consideration of the phys ica l environment of

surface mine opera t ions , cu r ren t design procedures employed, the problems with

e x i s t i n g sedimentation ponds, and the l e v e l of effort requi red t o provide

compliance with e f f l u e n t l imi t a t ions . Modeling methods f o r design of sedimen-

t a t i o n ponds are considered state-of-the-art procedures. However, based on

the capabilities and p resen t procedures used by most opera tors , modeling is not included i n the manual.

i nc lud ing some methods i n comwn use, are presented i n t h i s manual.

I n con t r a s t , many of t h e s impl i f i ed procedures,

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1.3

This manual addresses all aspects of the pond that affect the removal of

suspended solids including, but not limited t o 8 type of mining, topography,

location soil types8 pond geometry, inlet and outlet control, and maintanence.

No attempt is made to present information on structural design.

To help meet the needs of designers and operators, contacts were made

with appropriate agencies in states where significant active mining operations

occur. E’urther, contacts were made with operators to develop a background of

their capabilities, problems in sedimentation pond performance, innovative

techniques, and present design procedures. This information provided insight

for development of a useable design manual.

1.4 Design Manual Use

The methodologies and considerations in design of sedimentation ponds

have been presented to provide the designer or operator with an understanding

of the processes involved to remove suspended solids and what effect6 these

processes have. In Chapter 11, preliminary considerations of watershed

characteristics and sources of sediment are discussed. In Chapter 111, com-

putational methods for water routing and removal efficiency are presented

\ along with a discussion on the Characteristics of sediment removal to meet . . effluent limitations. This chapter contains the data requirements and the

methodologies that are used to design a sedimentation pond.

discussion in this chapter is that pertaining to sediment data, specifically

the particle size distribution.

tions is greatly dependent on the particle size distribution. great care should be taken to develop an accurate representative size distri-

An important

The design of ponds to meet effluent limita-

Therefore,

bution. Chapter N presents modifications that

performance of the sedimentation pond. Chapter

sediment removal. Maintenance of sedimentation

enough. Lack of pond maintenance is one of the

the per-

can be made to improve the

V deals with maintenance and

ponds cannot be emphasized

major problems in

fonnance of existing sedimentation ponds and the development of a maintenance

program is a significant part of pond design.

sections are interrelated in the design process.

Chapter V I presents how these

To bring the information and methodology together, the final chapter pre-

sents the procedural steps for design along with a comprehensive design

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1.4

example. Users of the design manual are encouraged to carefully review the

example presented in Chapter VI to better understand the design methodology.

With a l i t t l e practice, the complete design process w i l l become familiar and

8 traight f orward.

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TABLE OF CONTENTS CHAPTER I f

Page

I1 . PRELIMINARY CONSIDERATIONS FOR APPLICATION AND USE OF SEDIMENTATION PONDS FOR SURFACE MINING

2.1 Off ice of Surface Mining Regulations and Environmental Pro tec t ion Agency Water Quality Standards . . . . . . . . . . . . . . . . .

2.2 Watershed Charac te r i s t i c s . . . . . . . . . . . . . . . . 2.2.1 Climatology .................... 2.2.2 Geology . . . . . . . . . . . . . . . . . . . . . 2.2.3 soils . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Vegetation . . . . . . . . . . . . . . . . . . . . 2.2.5 Topography . . . . . . . . . . . . . . . . . . . . 2.2.6 Iiydr010gy

2.3 Location of Major Sources of Sediment . . . . . . . . . . 2.3.1 Haul and Access mads . . . . . . . . . . . . . . 2.3.2 Areas of Active Mining . . . . . . . . . . . . . . . 2.3.3 Areas Being Cleared for f i n i n g Activit ies . . . . 2.3.4 Areas in Process of Reclamation . . . . . . . . .

2.4 Typ es and Applications of Sedimentation Ponds . . . . . . 2.4.1 Excavated Sedimentation Ponds . . . . . . . . . . 2.4.2 Embankment and Combination npbankment/

Excavated Sedimentation Ponds . . . . . . . . . . 2.4.3 Sedimentation Pond Spillway Type . . . . . . . . . 2.4.4 Multiple Pond Systems . . . . . . . . . . . . . . 2.4.5 Physical/Chemical Treatment Ponds . . . . . . . . 2.4.6 Dry Basin versus Permanent Pool . . . . . . . . .

2.5 Summary of Preliminary Considerations . . . . . . . . . .

2.1 2.2

2.2 2.3 2.3 2.4 2.4 2.6

2.6

2.7 2.7 2.8 2.8

2.9

2.9

2.10 2.11 2.12 2.12 2.13

2.14

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LIST OF FIGURES C E U P E R I1

Page

Figure 2.1. Example of r i l l and gully erosion . . . . . . . . 2.5

. ..

. . \

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11. PRELIMINARY CONSIDERATI013S FOR APPLICATION A?$D USE OF SEDIMENTATION PONDS FOR SURFACE MINING

Off ice of Surf ace Mining Regulations and Environmental Pro tec t ion Agency Water Qua l i t y Standards

Design criteria f o r s e m n t a t i o n ponds are es t ab l i shed through f e d e r a l

2.1

and state regulations. OSM sedimentation pond design criteria are intended t o

prevent, to t h e ex ten t possible, a d d i t i o n a l con t r ibu t ions of suspended solids

t o stream f l o w or runoff outside t h e permit area and t o achieve appl icable

federal and state w a t e r q u a l i t y standards.

dards include e f f l u e n t l imi t a t ion guide l ines f o r c o a l mining po in t sources

e s t a b l i s h e d by t h e Environmental Pro tec t ion Agency (EPA). Therefore, the

requirements of the EPA are d i r e c t l y t i e d t o the performance standards for the

design criteria of sedimentation ponds.

These d n h m water q u a l i t y stan-

OSM cons iders sedimentation ponds i n conjunction with a l t e r n a t i v e c o n t r o l

measures as t h e best practical technology (BPT) c u r r e n t l y ava i l ab le , as

e s t a b l i s h e d by t h e EPA, for the con t ro l of sediment.

s p e c i f i c a l l y defined by OSM t o include any barrier, -8 or excavated

depression which slows water runoff allowing sediment to settle.

A sedimentation pond is

',

0% sedimentation pond design criteria must enable compliance with the 5 . . \ e f f l u e n t l imi t a t ions , water q u a l i t y standards, and t h e s a f e t y requirements of

state and f e d e r a l governments. The determination of sedimentation pond design

cri teria is l e f t up to the operators and t h e r e g i s t e r e d p ro fes s iona l engineers

designing the ponds and reviewed by the regula tory a u t h o r i t i e s .

Currently, t h e requirements of the EPA e f f l u e n t Umi ta t ions are the

result of t h e Federal Water Pol lu t ion Control A c t Amendments of 1972. I n 8um-

mary, p o i n t source water discharges are required to meet e f f l u e n t l i m i t a t i o n s

r e q u i r i n g t h e app l i ca t ion BPT cur ren t ly available by' Ju ly l, 1977, - and best -.'

available technology (BAT) economically achievable by Ju ly I, 1983. The C l e a n

Water A c t of 1977 rev ised t h e control program for nontoxic po l lu t an t s .

detailed discussion of appl icable e f f l u e n t l imi t a t ions , see Section 3.3.

Suggested design criteria f o r sedimentation ponds are based on t h e c u r r e n t

e f f l u e n t lFmits found in many states.

vary; however, t h e s ta te agency requirements must be taken i n t o account by the

design engineer.

For

E x a c t requirements f o r each agency

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2.2

2.2 Watershed Characteristics

The location, design, construction, and methods of treatment used i n

sedimentation ponds a re dependent on the character is t ics of the watershed

where mining a c t i v i t y takes place.

logy, geology, s o i l s , . vegetation, topography, and hydrology of the watershed

w i l l help f a c i l i t a t e the design of e f f i c i e n t sedimentation ponds t h a t m e e t

water quality standards and regulations. These charac te r i s t ics do vary on a

watershed-by-watershed basis; proper consideration of this must be made i n

order to design sedimentation ponds properly.

Preliminary consideration fo r the climato-

2.2.1 Clfmatology

Climatological elements in sedimentation pond design include temperature

and precipi ta t ion.

be t r e a t e d as w e l l as how ef fec t ive ly it is treated i n a pond.

ambient temperature changes the viscosi ty of water which changes the s e t t l i n g

ve loc i ty of sediment particles. Consequently, the detention t i m e required t o

settle out a waterborne particle of a given size changes. Ambient temperature

a l s o influences the magnitude and seasonal dis t r ibut ion of runoff.

I n general, temperature can a f f ec t the amount of runoff to

Variation in

. . \ Prec ip i ta t ion i n the form of snow causes l i t t l e or no erosion. However, snow

melting i n the Spring on p a r t i a l l y frozen ground is a source fo r higher rates

of runoff and erosion. I f low temperatures exist fo r a s ign i f icant period of

time, ice formations in sedimentation ponds may be a problem. I f not taken

i n t o consideration i n pond design, ice formations may cause f a i l u r e of embank-

ments o r damage energy d iss ipa tors such as baff les or riprap. L o w tem- peratures f o r a s igni f icant period of t i m e may disrupt, damage, or halt

chemical treatment processes that may be used as part of water treatment i n

sedimentation ponds.

Prec ip i ta t ion is the mos t important element of climate in determining the

rate of erosion. The magnitude of annual precipi ta t ion, i n addition t o the

frequency, in tens i ty , duration, and seasonal dis t r ibut ion of prec ip i ta t ion

events a f f e c t the magnitude of runoff and erosion. Precipi ta t ion charac-

teristics a l so e f f e c t the type of o u t l e t and operation of a sedimentation

pond.

from an event.

have s u f f i c i e n t l y settled t o meet w a t e r qual i ty standards.

For example, gated o u t l e t s a r e normally used t o store a l l of the runoff

The gates are opened t o discharge the runoff after sediments

This would be

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2.3

d i f f i c u l t to design f o r a pond on a watershed i n t h e eas t e rn region of the

United States where annual p r e c i p i t a t i o n is much greater and precipitation

even t s that occur are c h a r a c t e r i s t i c a l l y of a longer duration, l o w e r inten-

s i t y , and amre frequent. Gated o u t l e t s are much more suitable f o r ponds i n

the western reg ion where annual p r e c i p i t a t i o n and water y i e l d are much lower

and p r e c i p i t a t i o n events are of high i n t e n s i t y but for a r e l a t i v e l y s h o r t

dura t ion .

Seasonal d i s t r i b u t i o n of p r e c i p i t a t i o n r e s u l t s i n a seasonal d i s t r i b u t i o n

The eros ion p o t e n t i a l is greater during the high of annual e ros ion p o t e n t i a l .

p r e c i p i t a t i o n period such as Spring than it is during Winter. This is impor-

t a n t in es t ima t ing t h e y i e l d and concentration of seUiment i n t h e runoff. For

f u r t h e r cons idera t ion of seasonal e ros ion p o t e n t i a l refer to %=face Water

Hydrology and Sedimentology Manual" (OSM, 1982). -- - --

2.2.2 Geology

The geology of a watershed interacts somewhat with a l l hydrologic charac-

teristics of t h e watershed. The geology has a s i g n i f i c a n t e f f e c t on the

topography. Topography is a major cons idera t ion in the loca t ion and shape of

. . a sedimentation pond. \

~n important cons idera t ion of the geology is t o eva lua te the d i f f e r e n t

types of overburden material t h a t w i l l be exposed during t h e mining process.

Overburden e ros ion characteristics should be considered when e s t ima t ing sedi-

ment y i e l d s and runoff concentrations.

2.2.3 Soils

Watershed Soil c h a r a c t e r i s t i c s depend t o a l a rge ex ten t on t h e pa ren t

geologic materials and the predominant weathering processes.

c h a r a c t e r i s t i c s of the watershed determine runoff and eros ion from the

watershed. The magnitude of runoff and eros ion is a function of s o i l

i n f i l t r a t i o n , s o i l permeabili ty, moisture content, s t r u c t u r e , texture, and

con ten t of organic matter.

These soil

The sediment s i z e d i s t r i b u t i o n of t h e runoff is the main design criteria

i n s i z i n g a sedimentation pond. As the s i z e of particle to be removed becomes

smaller the s i z e of a sediment pond requi red becomes la rger . Often t i m e s , the

s ize of pond is impractical and chemical treatment may be necessary to remove

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2.4

the f i n e particles and a l l ev ia t e the high cos t of constructing a very large pond.

2.2.4 Vegetation

Vegetation plays an important ro l e i n the water balance and s o i l stabi-

l i t y i n a watershed. Through interception, t ranspirat ion, and evaporative

processes, vegetation can subs tan t ia l ly reduce water y ie ld from a watershed.

Vegetation plays an important part i n protecting the soil from erosion.

Vegetation can reduce the amount of erosion through: interception, reduced

r a i n f a l l i n t ens i t i e s by providing an energy-absorbing cover, reduced landsl ide

hazards through binding and lowering soil water content, and reduced overland

and channel erosion by providing added roughness which retards water velocity.

Removal of vegetation exposes underlying soils to great ly increased erosive

forces. Thus, one of the =re cri t ical periods during the mining process is

when reclaimed area s o i l s have not Beveloped s igni f icant vegetation.

, \ .

2.2.5 Topography

Topography, including land and channel slopes, aspect, and surface and

. . channel geometry, is an expression of the morphological, geologic8 and other erosive forces that have acted on the watershed. t u r n 8 these fac to r s are

modified by the topography they have created.

governs water discharge rate and sediment transport rate. Conversely, the

discharge rate and the sediment t ransport can alter the channel form to pro-

duce a new slope.

of w a t e r flow, thus a l t e r i n g sediment transport . Primary topographic con-

s idera t ions i n control l ing erosion are slope and length of slope.

i n slope increases the t ransport capacity of surface runoff, thus increasing

erosion. An increase i n length of slope allows f o r concentration of sheet

flow. This concentration of sheet flow forms rills and gul l ies by erosion

(see Figure 2.1).

\

For example, channel slope

Channel and surface geometry a f f e c t the depth and ve loc i ty

An increase

The watershed shape or geometry can a l so be considered part of watershed

topography. t h e watershed where flow concentrates and the time required.for f l o w

concentration.

The geometry of the watershed determines the amount of area of

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2.5

WIND EROSION

RAINDROP EROSION

SHEET EROSION

\ '

R l l l AND GULLY ER,OSION

5 . . \

STREAM AND CHANNEL EROSION

FIGURE 2.1 EXAMPLE OF RILL AND GULLY EROSION (SCS, 1980)

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2 . 6

Many t i m e s topography c o n t r o l s the loca t ion of a sedimentation pond or

Regulations state that sedimentation the type of pond that is constructed.

ponds should be loca ted as close to the source of sediment as possible.

Normally the only f a c t o r that limits loca t ing a pond is s t e e p t e r r a i n ; ponds

are usua l ly cons t ruc ted i n va l l eys or hollows a short d is tance downstream t o

fac i l i t a te cons t ruc t ion .

2.2.6 Hydrology

Runoff that reaches a sedimentation pond is the r e s u l t of complex

i n t e r a c t i o n of a l l t h e prev ious ly discussed watershed c h a r a c t e r i s t i c s ; clima-

tology, geology, s o i l s , vege ta t ion , and topography i n t e r a c t t o make up the

hydrologic process that inc ludes runoff from a p r e c i p i t a t i o n event.

includes the i n t e n s i t y , frequency, duration, d i s t r i b u t i o n of p r e c i p i t a t i o n

events , and how that p r e c i p i t a t i o n uccurs, e i t h e r in the form of snow or r a i n

depending on temperature.

C l i m a t e

Vegetative cover intercepts a f r a c t i o n of this pre-

, c i p i t a t i o n by evapot ranspi ra t ion . The ex is tence and degree of vege ta t ive

cover inc reases w i t h favorable clFmate and so i l conditions.

f r a c t i o n of p r e c i p i t a t i o n , s o i l conditions determine what f r a c t i o n w i l l

. . i n f i l t r a t e and what f r a c t i o n w i l l become su r face runoff. The remaining f r ac -

t i o n that becomes surface runoff w i l l concentrate i n a watershed according t o

t h e geologic and topographic f e a t u r e s of the watershed. I n this manner the

i n t e r a c t i o n of watershed c h a r a c t e r i s t i c s governs t h e magnitude and d i a t r ibu -

O f the remaining

\

t i o n of runoff f r o m p r e c i p i t a t i o n and t h e sediment y ie ld . Consequent

q u a l i t y of runoff f r o m the disturbed area is a f fec t ed by the dynamic

equilibrium of the watershed.

2.3 Location of Major Sources of Sediment

There are four categories for sources of sediment from su r face m

Y, the

ning

a c t i v i t i e s in a watershed. These are the d n e d por t ion of the watershed,

t h e mined or mining po r t ion of the watershed, spoil banks or areas where spoil

is s tockpi led , and haul or access roads necessary f o r mining activit ies. Of

these four categories, the unmined por t ion of the watershed genera l ly y i e l d s

t h e least amount of sediment and is not considered a major source of sediment

y i e ld . The remaining three ca t egor i e s are a direct consequence of mining

opera t ions . Of these three, haul or access roads and s p o i l banks commonly

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2.7

generate the greatest sediment y ie ld per un i t area. For example, a study done

on a monitored watershed I n Appalachia (EPA, 1976) gave the following com-

parison f o r rates of erosion:

Area Y i e l d

(tons/square mile) ~~ ~~ -~

Unmlned watershed

Mined watershed

Spoi l bank

Eaul road

28

1,930

27 , 000

57,600

However, due t o the much larger percentage of

the mined or mining portion of the watershed,

' most sediment.

2.3.1 Haul and Access mads \ Both haul roads and.access roads to mine

of sediment. In addition, roadways generally

'. . .

the t o t a l watershed covered by

these areas generally y i e ld the

sites cons t i t u t e

remain as a main

a major source

source

throughout the l i f e - o f the mine.

due to an increased r a t e of runoff resu l t ing from a r e l a t lve ly Impermeable

surface, and steep cu t or f i l l slopes associated with roadways.

cept sheet runoff and act as a t ransport mechanism v ia roadside ditches.

Roadways occasionally require c lear ing and steepening of s ide slopes which

increase slope erosion.

y i e ld from roadways include:

Roadways y ie ld la rger amounts of sediment

Roads inter-

Other fac tors which contr ibute to a greater sediment

- Poor locat ion of mads.

- Improper construction/maintenance of roads and s ide slopes.

- Improper planning/construction of road drainage systems.

2.3.2 Areas of Active f in ing

The nature and extent of sediment y ie ld from areas of ac t ive mining are-

dependent on the mining method or procedure u t i l i zed . Four coamon methods of

surface mining are: area mining, contour mining, boxcut mining, and mountain

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. .

2 . 8

top removal.

tour mining due to containment of runoff on disturbed areas.

have a long, narrow geometry with more spoil and bench area po ten t i a l i t y ,

draining off the disturbed area d i r ec t ly i n t o an off-s i te drainage system.

addition, t he receiving waters are generally c loser t o the mining operations

for contour mining than for area mining.

In general, area mining is a lesser source of sediment than con-

Contour mines

In

Boxcut mining is a form of contour mining used i n steeper terrain t h a t

reduces spo i l dumped downhill by moving it l a t e r a l l y along the boxcut and

placing it i n areas where coal has been removed.

t h e o r ig ina l grade of the h i l l .

and contour mining as a source of sediment.

This f a c i l i t a t e s maintaining

Boxcut mining f a l l s somewhere between area

Mountain top removal is another form of contour mining used when the eco-

nomics of overburden removal and coal seam thickness f a c i l i t a t e removing the

e n t i r e h i l l t op .

sediment than contour mining. This is due to the regional nature of , and the

Mountain top removal is a l s o considered a lesser source of

\ reduction in , r e l i e f the t o mountain top removal. This generalization is t r u e

only i f the surface drainage is control led internal ly .

The mining methods could be ranked from the la rges t __ - _- con@$bufor -- cf_$edi- . . ment t o the least as contour mining, boxcut mining, mountain top removal, and \

area mining. This is an wet generalization since the sediment y i e ld from a

mined area is si te spec i f ic and is dependent on the amount of on-site erosion

cont ro l measures t h a t are taken.

2.3.3

The areas being cleared and grubbed as pretreatment for surface mining

Areas Being Cleared f o r Mining Act iv i t ies

a c t i v i t i e s a r e one of the major sources of sediment yield.

grubbing expose s o i l on steep slopes and can c rea te a s o i l surface that im-

pedes i n f i l t r a t i o n and/or concentrates runoff. Other fac tors t h a t increase

sediment y i e id are:

overclearing or clear ing too far ahead of the p i t exposing the area f o r a

longer period, and improper placement of the salvaged topsoil .

Clearing and

failure to i n s t a l l perimeter control measures,

2.3.4 Areas i n Process of Reclamation

This last major source occurs from the start of grading operations and

lasts u n t i l s t ab i l i za t ion of the s o i l occurs by vegetative and/or s t r u c t u r a l

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. ..I . . . - . -

2.9

.,easures.

y i e l d include cons t ruc t ing excessively steep, long slopes or s t r u c t u r a l l y

uns t ab le drainage channels. Also, improper t i l l a g e practices, p l a n t material

s e l e c t i o n , seedbed preparation, and maintenance can increase sediment y i e l d

Other f a c t o r s i n the reclamation process that can increase sediment

from reclamation areas.

reclamation than a t any o the r t i m e during mining operations; t he re fo re , recla-

mation practices such as t i l l i n g , mulching, and revegetation are very impor-

t a n t i n the con t ro l of erosion.

The p o t e n t i a l f o r e ros ion is greater just after

2.4 5p es and Applications of Sedimentation Ponds

Sedimentation ponds can be an e f f e c t i v e way t o c o n t r o l sediment from

On-site e ros ion p ro tec t ion measures can be l eav ing t h e mine permit area.

taken t o reduce the sediment yield from the mine site, but such measures

r a r e l y c o n t r o l the sediment t o t h e ex ten t that e f f l u e n t requirements can be

- m e t .

dn operator uses before the runoff leaves the mine permit area and e n t e r s

n a t u r a l drainageways downstream. Improper con t ro l of t h e sediment leav ing the

Sedimentation ponds are t y p i c a l l y the last sediment c o n t r o l measure that

t . mine penuit area may cause severe o f f - s i t e damages. \

Sedimentation ponds are generally constructed with embankments, by excaz

va t ion , or a combination of both. The sedimentation ponds presented i n the

following sec t ions ca tegor ize the types of ponds that are cu r ren t ly used by

the mining industry.

exist and may be used i n t h e sediment c o n t r o l plan.

which must be considered at each site to detenaine which basic type of eedi-

mentation pond, or va r i a t ion , w i l l provide the maximum c o n t r o l of sediment. The following sec t ions give a desc r ip t ion of the var ious types of sedimen-

t a t i o n ponds and where their app l i ca t ion is most practical.

However, s eve ra l combinations of t h e var ious types&&' -- There are many f a c t o r s

2.4.1 Excavated Sedimentation Pond

Excavated sedimentation ponds are cons t ruc ted by excavating a p i t or

"hole" i n t h e ground with the use of a bulldozer or backhoe. Generally, t hese

types of sedimentation ponds are l imi ted to conta in sur face runoff from

d i s tu rbed areas a t sur face mines located i n r o l l i n g t o f l a t t e r r a i n and from

s m a l l drainage areas. Sedimentation ponds which are constructed s t r i c t l y by

excavation are not used i n steep sloped t e r r a i n due t o t h e large amount of

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. . . , , . .. . . _.. ...- - . . . . . ...... . - - , . . . ._

2.10

excavation t h a t would be required to ,achieve the applicable storage volume

requirements.

drainageway.

These types of ponds are generally located off a natural

The excavated sedimentation pond has been used in conjunction w i t h the

mine p i t which serves as a preliminaG s e t t l i n g basin.

disturbed areas within the mine s i t e is directed i n t o the mine p i t where

s e t t l i n g of the larger s ize particles' occurs.

is pumped i n t o the excavated sedimentation pond where f i n a l a e t t l i n g occurs.

Using pumps to control the inflow in to the excavated sedhenta t ion pond allows

cont ro l of the detention time within $he sedimentation pond. Additionally,

t h e storage volume w i t h i n the p i t can: be u t i l i zed , thereby reducing the sedi-

mentation pond storage requirements as long as the storage volume within the

The runoff from

mom the pi t , the mine atainage

I

I

mine p i t does not i n t e r f e re with mining operations.

There are nmny disadvantages wit+ the excavated sedimentation pond.

is- l imited to applications i n r e l a t ive ly f l a t terrain and control l ing surface

runoff from s m a l l drainage areas. Ins t a l l a t ion of &watering devices i n these

types of ponds is generally very expensive and therefore, they are rarely

ins ta l led .

thus reducing the avai lable storage vblume when a storm event occurs.

r e s u l t of t h i s w i l l be decreased detention times and therefore, decreased

e f f luen t qual i ty . In addition, it i s l d i f f i c u l t t o provide separate pr inc ipa l

and emergency spillways. For these reasons, applications of the excavated

It

1

This leads t o the pond s tor ing water f o r a long period of time

The . . \

sedimentation pond are very W t e d . ~

j

2.4.2 Embankment and Combination PPbankment/Excavated

An embankment sedimentation pond'can be used in any type of te r ra in .

Sedimentation Ponds

Generally, these types of ponds are located on a draincgeway. An embankment

is constructed across the atainageway to form the sedimentat ion pond.

t h e drainageway bed is excavated upstream of the embankment, a combination

When

embankment/excavated sedimentation pond is formed. Excavation upstream of the

embankment provides addi t ional storage volume capacity t o an embankment sedi-

mentation pond.

I

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2111

A variety of ou t l e t s may be used,with the embankment and combination

embankment/excavated sedimentation pond. The most common method is t o use a

pipe o u t l e t for the pr inc ipa l spillway and a channel c u t i n t o the top of t h e

embankment as the emergency spillway. Although, several other types of

o u t l e t s are used and are presented in Section 3.9.

As previously mentioned, embankment sedimentation ponds are generally

The topography of ten dictates t h a t t h i s type of constructed on drainageways.

pond be constructed.

There is a p o s s i b i l i t y o f embankment failure due to poor construction or the

use of poor construction materials.

decrease the sedfioent removal e f f ic iency of the pond by adding sediment t o the

Rowever, this type of pond has some disadvantages.

Bank sloughing may occur t h a t can

pond.

increases the maintenance requirements.

t a t i o n pond is generally control led by topography.

Bank sloughing reduces the storage volume capacity.and therefore ,

The Shape of t h e embankment sedfmen-

The combination embankment/excavated sedimentation pond has the advantage

of providing additional storage volume' without increasing the height or s i z e

of t h e embankment. However, exposure ,of t he side slopes due t o upstream exca-

vation may require t h a t the slopes be btabi l ized. I I

I

2.4.3 Sedimentation Pond Spillway Type

Sedimentation ponds have of ten b& c l a s s i f i e d according to the type of I

pr inc ipa l and emergency spillway used.:

binat ion of pr incipal and emergency spillways be provided to safe ly pass the

runoff from a 25-year, 24-hour p rec ip i t a t ion event or larger event spec i f ied

by t h e regulatory authority. I n addition, the elevation of t he crest of the

emergency spillway s h a l l be a t least me foot above the crest of t h e p r inc ipa l

spillway.

emergency spillways.

OSM regulations require t h a t a com-

Therefore, separate o u t l e t s must be provided fo r the p r inc ipa l and

Pr inc ipa l spillways are usual ly constructed by using some type of pipe

ou t l e t . Pnergency spillways are generally constructed by excavating an exit

channel through the embankment or natural ground.

be used for the emergency spillway but due to the costs involved, t h e exca-

vated e x i t channel is the most feas ib le . A complete discussion of spil lways

i s presented i n Section 3.9.

Other types of o u t l e t s may

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2.12

2.4.4 Multiple Pond'Systems

A multiple pond system is considered to be the use of two or more sedi-

mentation ponds i n a series (one downstream of the other) . The concept of

mult iple ponds is a l so accomplished through compartmentalization of a s ingle

pond.

Solids with higher s e t t l i n g ve loc i t ies w i l l settle i n the f i r s t pond or com-

partment and the f ine r sediments w i l l be s e t t l e d in the f i n a l pond or compart-

ment.

maintenance ( i .eer sediment removal) is l i m i t e d t o the first s e t t l i n g pond or compartment. Also, f i e l d applications have shown t h a t multiple sediment ponds

i n a series are more e f f i c i e n t i n removing f i n e r particles than a s ingle pond

of equal surface area. One disadvantage i n t he use of multiple ponds is that more area is disturbed due t o the construction of additional ponds.

de ta i led discussion on the application and design of multiple pond systems is given in Section 4.3.

The concept of multiple ponds is the occurrence of staged se t t l i ng .

One par t icu lar advantage t o this type of system is t h a t most of the

A

' .

2.4.5 Physical/Chemical Treatment Ponds

Physical/chemical treatment i den t i f i e s the.process of adding chemicals to 5 . . enhance the physical s e t t l i n g charac te r i s t ics of the sediment particles t o be

removed by gravity se t t l ing .

coagulants o r f locculant aids. Chemicals are added to the inf luent of the

sedimentation pond, whereproper mixing must occur, and then the sediment is

f locculated and settled i n the pond. Coal f ines may not be f locculated when

chemicals are added due t o the electrical nature of the coal f ines . Other

treatment measures may be required to remove the coal f ines .

\

The chemicals added are generally re fer red t o as

Physical/chemical treatment is a standard pract ice i n the treatment of

water fo r d o m e s . t i c use. More recently it has been applied i n the mining

industry fo r the removal of sediment t o m e e t e f f luent l imitations. To m e e t

t h e ex i s t ing and proposed ef f luent l imitat ions w i l l s o m e t i m e s require removal

of very f i n e clay and silt sedislents. The volume and surface area of a sedi-

mentation pond required for removal of very f ine sediments are unreasonably

large.

moval of f i n e sediments.

Therefore, physical/chemical treatment measures are required fo r re-

Physical/chemical treatment measures have been used i n the field i n

conjunction w i t h multiple ponds i n a series. In this type of appl icat ionr the

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2.13

la rge sediment is settled i n the first pond, reducing the sediment load t o the

second pond where it is only used fo r s e t t l i n g of the f i n e r sediments. The

chemical dose required fo r s e t t l i n g is generally d i r ec t ly related t o the

concentration of solids. Therefore, by reducing the solids concentration i n

t h e f i r s t pond, the amount of chemical required is reduced below t h e amount

t h a t would be required using only one sedimentation pond.

on the ac tua l physical/chemical treatment process and types of coagulants is

given in Section 4.3.

Detailed discussion

Some disadvantages do exist i n the appl icat ion of physical/chemical

treatment.

material s e t t l e d in the sedimentation pond; therefore , consideration m u s t be

given to sediment storage volume and/or frequency of maintenance.

consideration is f i n a l disposal of the s e t t l e d sediment. It should be

rea l ized that the sediment contains the chemicals used f o r coagulation. The

The use of chemicals i n s e t t l i n g solids adds to the volume of

Another

type t i o n

', . . \

such

and/or d i f f i cu l ty of f i n a l disposal w i l l depend on the type of coagula-

chemicals and flocculant aids used.

2.4.6

The dry basin is characterized as a basin which has a dewatering device

Dry Basin versus Permanent Pool

t h a t the Sedimentation pond does not s t o r e the w a t e r runoff from any one

p rec ip i t a t ion event indefini te ly . Examples of dewatering devices include

tr ickle tubes and perforated riser pipe o u t l e t s t h a t &water t o the sediment

s torage l eve l i n the pond.

D r y basins MY be located e i the r on or off drainageways. For dry basin ponds, t he dewatering device is &signed t o dewater the sediment pond a t a

rate which achieves and maintains the required detention time t o achieve

appl icable e f f luent l imitations.

pond is either dewatering o r dry.

Between prec ip i ta t ion events this type of

The permanent pool sedimentation pond is designed to provide a permanent

s torage volume of water a f t e r the pr incipal spillway has stopped discharging.

The maximum permanent pool elevation is the elevation of the p r i n c i p a l

spillway crest.

and perennial streams when approval of the regulating authori ty is given.

Permanent pool sedimentation ponds can a l so be located off drainageways.

These types of ponds are of ten located on small drainageways

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2.14

The main difference between dry basins and permanent pool basins is t h a t

t h e permanent pool provides a constant storage volume fo r water while t he dry

bas in does not.

des a water supply f o r dust control on haul and access roads during dry

periods and the permanent pool helps minfmize resuspension of sediment t h a t

has already settled.

needs t o be designed f o r the addi t ional permanent storage volume of water.

The advantages of the permanent pool basin are t h a t it provi-

The disadvantage of the permanent pool basin is t h a t it

The advantage of the d r y basin pond is t h a t it does not have to be

designed f o r an addi t ional storage volume and therefore, the e n t i r e storage

volume of the basin can be u t i l i z e d during large runoff events.

tage of dry bas ins is t h a t resuspension of the s e t t l e d sediment may occur i f

cont ro l measures a t the i n l e t a r e not taken.

The disadvan-

2.5 Summary of Preliminary Considerations

Preliminary consideration f o r the application and use of a sedimentation -_

I pond i n a surface raining operation is based on the per t inent regulations and

standards t h a t ou t l ine the l eve l of performance required.

knowledge of the watershed charac te r i s t ics , sediment t ransport mechanisms, and

major sediment source locations, the type of sedimentation pond and its appfl-

W i t h a w r u n g

\

ca t ion can be chosen and designed to f a c i l i t a t e meeting regulations and s tan-

dards f o r pond performance.

The criteria for design of sedimentation ponds is established federa l ly

through regulations by the Office of Surface Mining, Environmental Protection

Agency, and the Mine Safety and Health Administration (MSHA).

ter ia a r e intended to prevent, to the extent possible, addi t ional contribu-

t i ons of suspended so l id s outside of the permit area and achieve water qual i ty

standards. Sedimentation ponds are a l so required to meet inspection and large

dam criteria of the ?&HA (30 CFR 77.216) and requirements of state and loca l

agencies with jur i sd ic t ion over the design, construction, or discharge of

sedimentation ponds.

OSM design cri-

The climate, geology, soils, vegetation, topography, and r e su l t an t hydro-

logy of a watershed a f f e c t the magnitude and r a t e of erosion and sediment

t ranspor t t h a t must be treated by a sedimentation pond.

t o r s include t h e seasonal var ia t ion and range of temperature and the magni-

tude, in tens i ty , frequency, duration, and seasonal d i s t r ibu t ion of

Climatological fac-

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2.15

precipitation.

face adjustments affect watershed climate and hydrology.

rocks, large-scale erosive forces, and bed structure dictate the general

topography. all of the other watershed characteristics.

Geological processes creating long- and short-term land sur-

Erosion-resistant

Thus, the geology of a watershed is important because it affects

The magnitude of runoff and erosion is a function of soil permeability,

moisture content, structure, texture, and content of organic matter. The

amount of vegetative cover, which is a function of climate and soil charac- teristics, affects the stability of the soil and water yield from a watershed.

Primary factors of topography are the length and steepness of slopes and the

geometry of the watershed. The complex interaction of all these watershed

characteristics determines the magnitude and distribution of runoff from a

precipitation event, which is the element of the hydrologic process of

concern.

There are four categories of sediment sources from surface mining activi-

ties in a watershed. These are the unmined portion of the watershed, the

mined or mining portion of the watershed, spoil banks or areas where spoil is Stockpiled, and haul or access roads necessary for mining activities.

these four categories, the unmined portion of the watershed generally yields

the least amount of sediment and is not considered a major source of sediment yield.

operations.

of

\

The remaining three categories are a direct consequence of mining

The type of pond can be classified into one of three general categories:

excavated ponds; embankement ponds1 and combination excavated/embankment

ponds.

topography of the proposed site.

The major factor in the selection of the type of pond is the

When conditions for treatment of runoff from the disturbed urea warrant,

chemical treatment systems or multiple pond configurations are used.

Normally, chemical treatment systems are utilized when high concentrations of

colloidal size particles are present in runoff from the disturbed area and

gravity-settling methods are not sufficient.

when the cost of constructing a single pond is high because of topographic

constraints, or the gravity-settling method of treatment in a single pond is

not sufficient to meet effluent limitations.

Multiple ponds are normally used

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2.16

With sufficient knowledge of the preliminary considerations discussed

previously, the type and application of a sedimentation pond which will be

effective in sediment control can be selected.

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... TABLE OF CONTENTS

CHAPTER 111

Page

I11 . SEDIMENT REMOVAL THROUGH SEDIMENTATION PONDS

3.1 3.1 Site Selection . . . . . . . . . . . . . . . . . . . . . 3.1.1 General Considerations . . . . . . . . . . . . . 3.1.2 Topography Considerations . . . . . . . . . . . . 3.1

3.2

3.1.2.1 Steep Sloped Terrain . . . . . . . . . . 3.1.2.2 Mild Sloped Ter ra in . . . . . . . . . . 3.2

3.3

3.1.3 Sedimentation Ponds m c a t e d on

3.1.4 Sedimentation Ponds b a t e d of f

3.1.5 Source of Sediment . . . . . . . . . . . . . . . 3.1.6 Access ib i l i t y . . . . . . . . . . . . . . . . . . 3.1.7 Mining Considerations . . . . . . . . . . . . . . 3.1.8 F i e l d Inves t iga t ion . . . . . . . . . . . . . . .

Main Drainageways . . . . . . . . . . . . . . . . . Main Drainageways . . . . . . . . . . . . . . . . .

3.4

3.4 3.5 3.5 3.6 3.6

3.2 Data Requirements . . . . . . . . . . . . . . . . . . . 3.8

4 3.2.1 HydrOlOgY b

3.2.2 Sediment Data . . . . . . . . . . . . . . . . . . 3.8 3.8

3.2.2.1 I n f l u e n t Sediment Size Di s t r ibu t ion . . 3.2.2.3 Sediment Yield . . . . . . . . . . . . . 3.9

3.12

3.2.3 Inflow Suspended Solids Concentration . . . . . . 3.12

3.3 E f f l u e n t Limi ta t ions . . . . . . . . . . . . . . . . . . 3.13

3.3.1 Suspended Sol ids Limitation . . . . . . . . . . . 3.3.2 Settleable Solids Limitation . . . . . . . . . . 3.13

3.13

3.4 Trapping Eff ic iency . . . . . . . . . . . . . . . . . . 3.5 S e t t l e a b l e So l ids Concentration . . . . . . . . . . . . 3.6 Storage Volume Requirement . . . . . . . 3.7 Available Storage Volume . . . . . . . . . . . . . . . . 3.8 Sedimentation Pond Configuration . . . . . . . . . . . .

3.16 3.20 3.29 3.30 3.36

3.8.1 Ideal S e t t l i n g . . . . . . . . . . . . . . . . . 3.8.2 Nonideal S e t t l i n g . . . . . . . . . . . . . . . . 3.36

3.39

3.8.2.1 Flow Currents . . . . . . . . . . . . . 3.8.2.2 Delta Formation . . . . . . . . . . . . 3.39

3.40

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TABLE OF CONTENTS (continued) CHAPTER 111

\

Page

3.8.2.3 Short C i rcu i t i ng and Turbulence . . . . 3.40 3.8-2.4 Scour and Resuspension . . . . . . . . . 3.40

3.8.3 Control of Nonideal S e t t l i n g . . . . . . . . . . 3.40

3.8.3.1 Short-Circuit ing Factor . . . . . . . . 3.43 3.8.3.2 3.8.3.3 3.8.3.4

Length-to-Width Ratio . . . . . . Permissible I n l e t Velocity . . . . Permissible Flow-Through Velocity . .

3.43 3.44 3.44

3.9 Sedimentation Pond Outlet Control Measures . . . . . . . 3.46

3.9.1 Pr inc ipa l Spillways . . . . . . . . . . . . . . . 3.46

3.9.1.1 Op en Channel Spillways . . . . . . . . . 3.46 3.9.1.2 Drop I n l e t Spillways . . . . . . . . . . 3.47 3.9.1.3 Pipe Culvert Spillways . . . . . . . . . 3.47 3.9.1.4 Eff ic iency of Pr inc ipa l Spillways . . . 3.50

3.9.2 Dewatering Devices . . . . . . . . . . . . . . . . 3.50 3.9.3 P r i n c i p a l Spillway Modifications . . . . . . . . 3.54

3.9.3.1 Weir Troughs . . . . . . . . . . . . . . 3.56 3.9.3.2 Floa t ing Discharge . . . . . . . . . . . 3.56 3.9.3.3 F i l t e r i n p . . . . . . . . . . . . . . . 3.58 3.9.3.4 Gated Spillways . . . . . . . . . . . . 3.58 3.9.3.5 Anti-Vortex Devices . . . . . . . . . . 3.60

3.9.4 Emergency Spillways . . . . . . . . . . . . . . . 3.60 3.9.5 Erosion Control B e l o w Spillways . . . . . . . . . 3.65

3.9.5.1 General . . . . . . . . . . . . . . . . 3.65 3.9.5.2 Riprap . . . . . . . . . . . . . . . . . 3.66

3.10Sunanary . . . . . . . . . . . . . . . . . . . . . . . . ‘ 3 . 6 7

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LIST OF TABLES CHAPTER I11

Page

- Table 3.1 Suggested P a r t i c l e S ize Distribution '

for Soil Textural Class . . . . . . . . . . . . . . . . 3.11

Table 3.2 Development of Settleable So l ids S i z e Distribution . . . . . . . . . . . . . . . . . . 3.24

Table 3 . 3 S i ze Distribution for Part i c l e s Smaller than 0.011 orrm . . . . . . . . . . . . . . . . . 3.29

Table 3.4 Stage-Storage Relationship Development . . . . . . . . . 3.36

Table 3.5 Permissible Velocities for Vegetated Spillways . . . . 3.63

Table 3.6 Permissible Spillway Velocities after Aging . . . . . 3.64

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.....................

LIST OF FIGURES CHAPTER I11

Page

Figure 3.1 Imhoff cone test apparatus . . . . . . . . . . . . . . . 3.15

Figure 3.2 Requi red trapping eff ic iency t o meet various e f f luen t l imi ta t ions . . . . . . . . . . . . . 3.18

Figure 3.3 Definit ion of trapping eff ic iency f o r various particle s i zes . . . . . . . . . . . . . . . . 3.19

Figure 3.4 Inf luent sediment s ize d is t r ibu t ion . . . . . . . . . 3.21

Figure 3.5 Set t leab le s o l i d s s i z e d is t r ibu t ion . . . . . . . . . 3.24

Figure 3.6 Definit ion of percentage in t e rva l s . . . . . . . . . . 3.27

Figure 3.7 Water rout ing curver S/V versus Tb . . . . . . . . . . 3.31

Figure 3.8 Water routing curve. QI/Qo versus Tb . . . . . . . . . 3.32

Figure 3.9 Definit ion sketch to determine incremental storage volume . . . . . . . . . . . . . . 3.33

Figure 3.10 P a r t i c l e s i z e versus detention . . \ time f o r various depths . . . . . . . . . . . . . . . 3.38

Figure 3.11 Schematic of de l t a formation . . . . . . . . . . . . . 3.41

Figure 3.12 Sedimentation ponds with dead storage spaces . . . 3.42

Figure 3.13 Typical drop i n l e t spillway . . . . . . . . . . . . . 3.48

Figure 3.14 Typical pipe cu lve r t spillway . . . . . . . . . . . . 3.49

Figure 3.15 Subsurface drain . . . . . . . . . . . . . . . . . . . 3.52

Figure 3.16 Single perforat ion of riser barrel . . . . . . . . . . 3.53

Figure 3.17 Siphon dewatering methods . . . . . . . . . . . . . . 3.55

Figure 3.18 Weir trough . . . . . . . . . . . . . . . . . . . . . 3.57

Figure 3.19 Floating w e i r . . . . . . . . . . . . . . . . . . . . 3.59

Figure 3-20 Anti-vortex device . . . . . . . . . . . . . . . . . . 3.61

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111. SEDIMENT REMOVAL THROUGH SEDIMENTATION PONDS

The design of sedimentation ponds is based upon sa t i s fy ing e f f luent l i m i -

t a t i ons . These l imitat ions are established fo r specif ied stream water qua l i ty

c r i t e r i a and design precipi ta t ion runoff event. Sedimentation ponds are

usually designed in the preliminary stages of the kine plan and therefore ,

t he re is l i t t l e or no available information on base flow conditions. Because

of t h i s , ponds are designed for the design prec ip i ta t ion runoff event and the

pond ef f luent fo r base flow conditions is tested a f t e r the pond becomes opera-

t iona l . I f base flow ef f luent l imitat ions cannot be satisfied, modifications

can be made to the pond. Generally, the design runoff event w i l l cont ro l the

sedimentation pond design unless sediment inflow fo r base flow conditions is

composed of high concentrations of f ine silts and clays.

The following sections describe the information that is required f o r

designing a sedimentation pond to m e e t e f f luent limitations.

is based upon ideal s e t t l i n g conditions with conservative fac tors incorporated

The pond design

i n t o the design t o account for nonideal s e t t l i n g conditions.

The design begins by select ing a particle s i z e which m u s t be removed i n

Determining the pond t h e pond such t h a t e f f luent l imitat ions are sa t i s f i ed . 5 . . configuration requires an in te rac t ive process which begins by assuming a

depth. The required storage volume and the avai lable storage volume are

determined and compared t o each other. When the required storage volume is

l a rger than the avai lable storage volume, a new depth is assumed and the

design process is repeated. Once the avai lable storage volume is adequate,

t h e pond configuration is checked based upon nonideal s e t t l i n g conditions.

The design procedure and example presented in Chapter vf show how the infor-

mation presented i n the following sections is used in sedimentation pond

design.

\

3.1 Site Selection

3.1.1 General Considerations

Selecting a sedimentation pon location requires consideration of several

fac tors . In a l l cases, sedimentation ponds must be constructed in locat ions

where it w i l l be possible t o direct or d iver t a l l surface runoff from

disturbed areas i n t o sedimentation ponds throughout the l i f e of mining opera-

t ions . Other fac tors which are of primary importance and should be considered

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3.2

in se lec t ing a sedimentation pond location include the mpography of t he mine

site, locat ing major sources of sediment, access ib i l i ty of the sedimentation

pond, a v a i l a b i l i t y of construction materials, and the direct ion of mining. I n

many instances these fac tors will l i m i t the number of viable locations t h a t

are avai lable f o r sedimentation pond construction. In. par t icu lar , availabi-

l i t y of su i t ab le sites for a sedimentation pond location w i l l be control led,

t o a large extent , by the topography of the mine site. In addition, ponds

must be constructed pr ior to any disturbance of the mine area. Through care-

f u l planning practices and f i e l d investigation, the sedimentation pond loca-

t i o n s which w i l l meet t h i s objective can be ident i f ied.

3.1.2 Topography Considerations

3.1.2.1 Steep Sloped Terrain

Throughout the United States surface mining operaions are of ten located

i n s teep sloped te r ra in . This is t rue for the Appalachian mining region, the

Rocky Mountain, and parts of northern California and Washington.

regions which are characterized by steep sloped t e r r a in , the topography beco-

mes the most important control l ing factor i n the site se le t ion f o r a sedimen-

t a t i o n pond location.

pond locat ion is t o determine where an adequate storage volume can be

provided.

I n the

!

The main problem i n finding a su i tab le sedimentation

Where surface mining operations are located in the upper part of a

watershed, the topography is characterized by steep slopes and v-shaped

drainageways.

close t o the mining operation as possible; therefare , the only site that is often avai lable for a sedimentation pond location is the v-shaped drainageway

d i r e c t l y downstream of the surface mine operation.

incorporates the use of an embankment w i l l have t o be used.

of the sedimentation pond can be increased by excavating upstream of the

embankment. Often t i m e s the storage capacity provided by the embankment

including any upstream excavation does not provide the storage volume required

to achieve e f f luent l imitations. To Overcome the problem of sedimentation

pond locat ion i n steep sloping t e r r a in , there are two a l te rna t ives avai lable

t o the operator.

It is usually desirable to locate the sedimentation pond as

A sedimentation pond which

The storge volume

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. - . . . . . - . . . .

3 . 3

. -

The f i r s t a l t e r n a t i v e i s to cons t ruc t a sedimentation pond a t a loca t ion

f a r t h e r downstream within t h e watershed where the slope of t h e drainageway

becomes milder and t h e shape of t h e drainageway becomes u-shaped.

vantage of this a l t e r n a t i v e is t h a t runoff from a much l a r g e r area w i l l need

/

The bisad-

to be contained and treated, thereby r equ i r ing t h e cons t ruc t ion of a much

larger s t ruc tu re .

The second a l t e r n a t i v e is to cons t ruc t a series of sedimentation ponds

located i n the steep, narrow drainageways where t h e runoff from t h e disturbed

mining areas passes through each sedimentation pond (mul t ip le sedimentation

ponds).

It should be noted here t h a t t h e r e are o the r a l t e r n a t i v e s t o seaimen-

t a t i o n ponds f o r sediment and eros ion con t ro l .

area, s e v e r a l devices and techniques have been used.

Sediment Control Measures f o r Small Areas in Surface C o a l Mining" (OSH, 1982)

'for f u r t h e r consideration of a l t e r n a t i v e s to sedimentation ponds.

Depending on t h e size of the

Refer t o "Design of

3.1.2.2 Mild Sloped Terrain

There is much more f l e x i b i l i t y in s e l e c t i n g a sedimentation pond loca t ion 5 i n mild sloped t e r r a i n . The phys ica l c o n s t r a i n t s imposed by the topography

are less than for s teep ' s loped t e r r a i n and the re fo re , more a t t e n t i o n may be

directed toward t h e o the r primary f a c t o r s considered in t h e s e l e c t i o n of a

sedimentation pond site.

\

Sedlmentation ponds may be located on or of f drainageways. Small draina-

geways are o f t en s e l e c t e d for a sedimentation pond location where an embank-

ment Is used with or without excavation t o provide t h e s to rage volume

required. Due to t h e milder drainageway p r o f i l e and milder slopes of t h e

v a l l e y , t h e sedimentation pond located in t h e mild sloped t e r r a i n w i l l nor-

mally have a greater length and width f o r any he ight of dam spec i f i ed , thereby

provid ing more s torage capacity.

O f f drainage loca t ions are genera l ly p re fe r r ed when t h e r e is a suitable

l o c a t i o n available f o r sedimentation pond cons t ruc t ion .

areas are good loca t ions f o r sedimentation ponds. An embankment can be

cons t ruc t ed across t h e downstream end of t h e depression area and t h e s to rage

volume may be increased by excavation.

Natural depression

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3.4

3.1.3

Sedimentation ponds located on main drainageways are usua l ly found i n

Sedimentation Ponds irocated on Hain Drainageways

su r face mining opera t ions located i n steep t e r r a i n . Due to topographic

c o n s t r a i n t s , this may be the arost cos t -e f fec t ive way t o control sediment.

main drainageway may be either an ephemeral or perennial stream. The sedimen-

t a t i o n pond located on a perennia l stream will have a permanent pool, whereas

sedimentation ponds loca ted on main drainageways which are ephemeral may be

e i t h e r a dry bas in or permanent pool.

The

The disadvantage of l oca t ing a sedimentation pond on 01 main drainageway

is that t h e su r face runoff from both d is turbed an6 undisturbed areas w i l l have

t o be detained long enough t o achieve e f f l u e n t l imi t a t ions . This requires

much l a r g e r s to rage volume be provided and the re fo re , the cons t ruc t ion of a

much l a r g e r sedimentation pond structure. I n addi t ion , chances of a sedimen-

t a t i o n pond being washed away during a major flood event are increased due t o

c o n t r o l of runoff from a l a rge r drainage area. Sedimentation ponds located on

drainageways which are perennial streams must be desgined t o m e e t base f l o w

w a t e r q u a l i t y l imi t a t ions . When t h e sedimentation pond is removed, reclama-

t i o n of t h e drainage channel w i l l be required. The channel w i l l have t o be . . \ restored t o i ts o r i g i n a l shape, slope, and channel pro tec t ion .

It is p r e f e r r a b l e t o select a sedimentation pond loca t ion which w i l l no t

be located h a main drainageway. However, t h e topographic c o n s t r a i n t s of t h e

mine site area may be such that the main drainageway is t h e only possible site

for a sedimentation pond location.

3.1.4 Sedimentation Ponds irocated of f Main Drainageways

Off nutin drainageways sedimentation ponds are genera l ly used in r o l l i n g

and mild t e q r a i n where t h e topography does not restrict t h e loca t ion t o t h e

e x t e n t a t do s i n steep sloped t e r r a i n . The types of sedimentation ponds '-7-

used'

embankment and excavation. The sedimentation ponds may be cons t ruc ted as

e i t h e r a permanent pool or dry basin.

- *'U main drainageway loca t ions are embankment or some combination of

The of f main drainageway loca t ion has seve ra l advantages over t h e on main

drainageway loca t ion .

t i o n can genera l ly be constructed closer t o t h e sediment source and the re fo re ,

designed f o r a smaller i n f l u e n t volume. This location avoids unnecessary

A sedimentation pond i n an of€ main drainageway lOCa-

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.. . - .. . _ _ - - . .

3.5

treatment of runoff from undisturbed areas. E s s e n t i a l l y , the base flow is

zero, therefore the pond is generally designed based on precipi ta t ion events.

Gated o u t l e t s can be used on sedimentation ponds located i n an off main

drainageway s i te to control the detention t i m e between runoff events. ~ l s o ,

the chances of an apbanhent sedimentation pond f a i l i n g during a major storm

event are reduced since only the runoff from a much smaller area w i l l have to

be controlled.

whether an off main drainageway location w i l l be feasible fo r the operator t o

construct.

However, again topography w i l l play an important r o l e as t o

3.1.5 Source of Sediment

During the development of a mine plan, the locations that w i l l be major

sources of sediment i n

Major sediment sources

spoil p i l e s , and areas

major sediment sources

throughout t he l i f e of

stages.

the surface mining operation should be ident i f ied.

include haul and access roads, areas being cleared,

being reclaimed. As mining progresses the locations of

change. Thus, the location of sediment sources

the mine should be considered during the planning

Sedimentation ponds should be located as close to m j o r seaFment sources

as possible.

t ages from both a sediment control and construction viewpoint.

t h e sediment as close t o the source as possible may require the construction

Locating sedimentation ponds i n t h i s manner has several advan-

Controlling

of several smaller sedimentation ponds as opposed to one or two la rger ponds.

The smaller seaFmentation ponds may be constructed d i r ec t ly downstream of the

major sediment sources thus requiring sediment control of only the disturbed

areas. The net e f f e c t of this is the inf luent volume is reduced by avoiding

co l lec t ion of runoff from undisturbed areas, thereby reducing the required

storage volume to achieve eff luent l imitations.

3.1.6 Accessibil i ty

Improper, o r lack of , maintenance for sedimentation ponds is one of the

major reasons fo r poor sediment removal eff ic iencies . Often the l a c k of main-

tenance is due t o inaccess ib i l i ty to the location of the sedimentation pond.

Sedimentation ponds are often constructed i n locations that are remote from

t he surface mining operation and therefore, access roads t o the sedimentation

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3.6

pond are given l i t t l e regular a t t e n t i o n and maintenance.

a sedimentation pond should be a primary consideration i n t h e planning and

design of sedimentation ponds.

The access road should be designed considering t h e type of equipment

The a c c e s s i b i l i t y t o

which w i l l use the road.

such t h a t t h e road drainage w i l l be directed i n t o t h e sedimentation pond.

Other sediment c o n t r o l measures or t h e construction of another sedimentation

pond w i l l be requi red downstream of t h e access road t o prevent o f f - s i t e damage

i f road drainage i n t o the sedimentation pond is not possible.

Where poss ib le , t h e access road should be designed

3.1.7 Mining Considerations

Throughout t h e l i f e of t h e sur face mining opera t ions , t h e loca t ions of

t h e major sources of sediment w i l l cons tan t ly change due to the progression of

mining. Sedimentation pond loca t ions should be selected cons ider ing t h e

d i r e c t i o n of mining so it w i l l be poss ib le t o direct or d i v e r t a l l s u r f c e

&off from d i s tu rbed areas i n t o t h e pond throughout t h e l i f e of t h e mining

opera t ions .

a minimum t o avoid f i l l i n g of t h e sedimentation pond prematurely.

I n a l l cases, t i m e of exposure of cleared land should be kept t o \

. . \

3.1.8 Field Inves t iga t ion

F i e l d i n v e s t i g a t i o n is e s s e n t i a l in the development of an e f f e c t i v e sedi-

ment c o n t r o l plan.

t h a t must be considered in s e l e c t i n g a sedimentation pond loca t ion , several

pre l iminary sedimentation poncl l oca t ions can be selected using the most r ecen t

topographic maps of t h e mine area.

ducted t o v e r i f y information from topographic maps, survey t h e phys ica l

f e a t u r e s of each s i te , and i d e n t i f y any problems which may be encountered a t

each si te.

Af te r the operator has an understanding of the factors

A f i e l d inves t iga t ion should then be con-

There are several su r face f e a t u r e s t h a t should be noted a t each p o t e n t i a l

sedimentation pond loca t ion . These f ea tu res inc lude s o i l type, vege ta t ive

cover, p r e f i l e and s ide slopes of t h e drainageway, channel shape, channel pro-

t e c t i o n , and t h e c a p a b i l i t y of each s i t e to provide the design s to rage volume.

The previous ly mentioned f ea tu res are a l l interrelated in inf luenc ing t h e

e r o s i o n a l p o t e n t i a l of t h e s i te and t h e s u i t a b i l i t y of t h e s i te as a sedimen-

t a t i o n pond location. An overview of each si te should be conducted not ing any

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3.7

problems r e l a t ing t o unstable s o i l s or erosion.

denced by small landslides, bank sloughing, and gully or till erosion.

These features w i l l be evi-

The s o i l type and vegetative cover a t the s i t e should be investigated and

noted.

s o i l . I f a large area w i l l be cleared fo r mining operations and the construc-

t i o n of the sedimentation pond, protective measures may be required to protec t

t h e b a r r e n so i l .

desirable t o d is turb s o i l s which would become unstable. This w i l l happen f o r

s o i l s which have l i t t l e cohesion and the problem is increased i f the sedimen-

t a t i o n pond is constructed in steep sloped te r ra in .

disturbed, a continous sloughing of the banks w i l l occur which w i l l reduce the

sediment removal eff ic iency of the pond as w e l l as threaten the s t a b i l i t y of

t he embankment. However, sedimentation ponds located in s o i l s which have a

high clay content may pose a problem in achieving ef f luent l imitations.

t o turbulence within the pond and wave action on the banks, high con-

centrat ions of col lo ida l particles could r e s u l t and w i l l require the addition

of coagulants or f locculants under base flow conditions.

Vegetation reduces the erosion poten t ia l and tends t o s t a b i l i z e the

Where s o m e type of excavation might be required, it is not

d

Once unstable s o i l s are

Due

i

I n s teep sloped areas where sedimentation ponds are o f t e n constructed on

drainageways, the drainageway channel shape and protection should be invest i -

gated.

w i l l have to be r e s to r id after the removal of t he embankment.

have developed an armoring Layer of a ce r t a in s i z e particle o r it may be pro-

tected only by vegetation.

downstream of the sedimentation pond and the magnitude of scouring W i l l be

greater fo r the vegetation-lined drainageway than fo r the drainageway which

has already developed an armoring layer.

\

The drainageway channel protection should be noted since the channel

The channel may

It can be expected t h a t scouring w i l l occur

Once i n the f ie ld , the designer should ver i fy the information on the

topographic maps and survey the physical features a t each site.

f i e l d investigation, a review and comparison of the advantages and disadvan-

tages of each s i te can be evaluated to select the best sedimentation pond

location.

After the

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3.8

3.2 Data Requirements

Information t h a t is required t o design a Sedimentation pond can be broken

i n t o three categroies: hydrology, sediment data, and inflow suspended so l id s

concentration. The following sect ions describe what spec i f ic information is

required fo r sedimentation pond design.

3 2 1 Hydrology

Hydrologic information required t o design a sedimentation pond includes

the peak inflow rate and the runoff volume fo r the design storm event. In

addition, where ponds receive the inflow from the mining p i t , pumping, or are

located on a perennial stream, the inflow rate f o r base flow conditions must

be determined.

For the design storm event, an inflow hydrograph must be developed from

which the peak inflow r a t e and runoff volume can be determined. There are

severa l references available which describe inflow hydrograph development

(OSM, 1982; Bureau of Reclamation, 1977; Barfield, 1981; Soi l Conservation

Service, 1975 1. I .

For sedimentation ponds which receive inflow by pumping, the designer

! w i l l have to determine the inflow r a t e f o r base flow conditions. For ponds \

which a re located on perennial streams, the designer can use h i s t o r i c a l data

i f available.

Therefore, the inflow rate for base flow conditions or from pumping w i l l

generally have to be measured a f t e r the pond has been constructed.

However, t h i s type of information may be very limited.

3.2.2 Sediment Data

The sediment data required f o r pond design are the p a r t i c l e s i z e distri-

bution and total sediment yield during a runoff event. The design of a pond

occurs during the planning stages before ac tua l mining starts. Therefore,

Information on the particle s i z e d is t r ibu t ion of the sediment runoff from the

disturbed area is not generally avai lable f o r the spec i f ic s i t e .

following sections discuss the methods of obtaining sediment s i z e d is t r ibu t ion

and sediment yield.

The

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3.9

3.2.2.1 Influent Sediment Size Distribution

The most important sediment data required to design a sedimentation pond

to meet effluent limitations is the particle size distribution of the sediment

influent. The particle size distribution should represent the worst condition

during the life of the mine. From past and present experience, the worst con-

dition occurs during the reclamation phase. Two conditions during the recla-

mation phase must be considered.

The first condition to be considered is before the topsoil or "A" horizon

has been replaced.

size distributi

replaced. L l G - Pondition, the particle size distribution of the eroded soil will be represented by - the txpsd-1.

particle size distribution with the highest percentage of particle sizes in

the silt range (0.001 to 0.074 m) will be selected for the design influent particle size distribution.

distribution is to obtain size distribution Information from previous anh nearby mining operations. When mining operations within the same area or

areas with the same soil texture exist, determination of particle size distri-

butions of sediment runoff from existing analysis can be used.

ticle size distribution from a nearby site is used, several considerations and

comparisons must be made so the information does represent the site under

consideration.

The soil which is eroded, and hence the influent particle 1 be represented by the graded overburden. The second -

condition to be - - 1 7 onsider d is after the topsoil or "A" horizon has been 8'

Whichever condition results in a c- -

\ The best way to estimate the particle size

'

t \

Before a par-

1. Soil characteristics at both sites should be very similar including soil types below the surface which are disturbed during mining.

2. Slopes, drainage, and sediment transport characteristics of both sites should be evaluated and compared.

3. The type of mining and amount of area disturbed at both sites should be evaluated.

4. Data from as many samples and sites should be collected and eva- luated to provide a good estimate.

5. The magnitude of the runoff event during which the sample was collected should be considered.

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When these data do not ex i s t , but nearby sites do exist, sampling and labora-

to ry analysis should be conducted whenever possible.

Another method f o r developing particle s ize d is t r ibu t ion information is based on the site spec i f ic soil textural class and physical properties.

Generally, s o i l physical propert ies occurring at a spec i f ic si te can be iden-

t i f i e d using information given in s tandard Soil Conservation Service (SCS)

s o i l surveys. These invest igat ions c o n s i s t of c lass i fying physical, chemical,

and biological cha rac t e r i s t i c s of s o i l s extending to depths of up t o s i x feet.

The U.S. Department of Agriculture (USDA) publishes reports and maps of their

soil surveys, usually on a county basis.

A procedure f o r determining particle s i ze d is t r ibu t ion based on s o i l

textural class is presented f o r use w i t h t h i s manual.

simply a name given each s o i l which designates the ranges of sand, silt, and

c lay s i zes it contains.

descriptions, other soil survey data i n t he vicini ty , s o i l data from the mine

p lan , f i e l d estimation by a soil s c i e n t i s t , or laboratory analysis. After

determining the textural c lass i f ica t ion , the corresponding particle s i z e

groups are then determined from Table 3.1.

A textural class is

This class can be obtained from SCS s o i l series

I

r Where the mining area has several s o i l textural c l a s s i f i ca t ions within

For \

the drainage bowdary, a composite s i z e dis t r ibut ion can be developed.

each pa r t i cu la r soil t ex tu ra l c lass i f ica t ion , the sediment s i z e d is t r ibu t ion

given in Table 3.1 W i l l be multiplied times the f rac t ion of the disturbed area

that each s o i l textural class covers. The values fo r each soil textural class

are then aUded together t o form a representative composite s i z e d is t r ibu t ion . An example of developing a representative composite s i z e d is t r ibu t ion is given

i n the design example i n Chapter VI.

The sediment s i z e d is t r ibu t ion based on tex tura l class is not recommended

f o r use i f amre detailed soil data are available a t the mine site. Also, it

is impor tan t t h a t the s o i l data describing the material b e l o w the surface

(exposed during mining) be considered during development of t he particle s i z e

d is t r ibu t ion .

than the information on which it is based.

changes and modifications t o the pond after construction, the particle s i z e

d i s t r ibu t ion u t i l i z e d should be a conservative estimate.

The designer should r ea l i ze that the design can be no better

To help eliminate s ign i f i can t

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3.11

n a Y)

F A

V Y

n n

0 a - - a L

E a I-

s1

m n

m -

0 c

N

U C

In a

0 N

0 w

0 N

m c

m

u C

v) a

it 0 a

0 *)

0 hl

0 hl

m N

m

E

-I

6 C

v) a

m (Y

m N

0 c

0 n

0 -

I

m c

m

0 W

0 N

I

m

m

0 c

5:

0 c

m N

m N

E 0

3 * a 0 % 0 c .I v)

-

0 -

0 c

0 c

0 w

0 R

I

m

0 c

0 (Y

m W

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. . .. . . . .

3.12

The s i z e d is t r ibu t ion for base flow conditions can be s igni f icant ly d i f -

f e r e n t from the s i ze d is t r ibu t ion based upon the surface and overburden so i l s .

Generally, the s i z e d is t r ibu t ion fo r base flow conditions w i l l be composed of

smaller particle s izes . Sampling of the base flow s i z e d is t r ibu t ion is recom-

mended to accurately design for base flow ef f luent Limitations.

For sedimentation ponds which receive inflow by pumping, the sediment

s i z e d is t r ibu t ion is very d i f f i c u l t to predict.

s i z e d is t r ibu t ion can be developed from the overburden so i l . Once the pond is

operational, the ef f luent w i l l have to be t es ted and pond modifications may be

required.

An i n i t i a l estimate of the

3.2.2.2 Sediment Yield

Sediment y ie ld of the mining area is required to determine the sediment

s torage volume of the pond and ca lcu la te the average e f f luent concentration

f o r the design storm event.

upon the annual sediment yield and the frequency of sediment removal.

lef t to the designer to decide how often the sediment w i l l be removed from the

pond.

Loss Equation (USLE). There are several references which are avai lable which

describe the use of the'USLE (OSM, 1982; Barfield, 1981).

The required sediment storage volume is dependent

It is i

The annual sediment y ie ld can be determined using the Universal Soil t \

Sediment y ie ld fo r the design stonn event must be determined 80 the

average e f f luent concentration can be calculated. The Modified Universal Soil

Loss Equation (MUSLE) can be used to ca lcu la te the sediment y ie ld from the

design storm event. of MUSLE.

The previously mentioned references also describe the use

3.2.3 Inflow Suspended .Solids Concentration

The inflow suspended so l ids concentration is required f o r both base flow

conditions and the design runoff event. For base flow conditions, the

in f luen t suspended solids concentration w i l l have to be measured since it is very d i f f i c u l t to predict.

For the design runoff event, the average inf luent suspended so l ids con-

c e n t r a t i o n can be computed knowing the storm runoff volume and sediment yield.

The average inf luent suspended so l id s concentration is computed as:

6

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3.13

where, CI = average i n f l u e n t suspended solids concentration (mg/l),

Y = storm sediment y i e l d (lbs),

y = u n i t weight of water (62.4 1b / f t3 ) , and

It should he recognized t h a t t h i s concentration is t h e average suspended

So l ids concent ra t ion during t h e storm and higher suspended s o l i d s con-

c e n t r a t i o n s would be expected when t h e peak inflow rate occurs.

/ 3.3 E f f l u e n t Limitations ,-> I .

Design procedures f o r sedFmentation ponds developed in this manual are- L r based on meeting So l ids e f f l u e n t l imi t a t ions .

t h a t t h e r e are o the r e f f l u e n t q u a l i t y limitations on i ron , manganese, and pH.

It assumed that t h e manual w i l l be used for design of sedimentation ponds

i n t h e planning s t ages of mining and that t h e mining operation is con t ro l l ed

by New Source Performance Standards (NSPS).

l i m i t a t i o n s are based on flow condition and t h e state of mining operation.

The opera tor should be aware

'1 The NSPS solids e f f l u e n t q u a l i t y

! \ 3.3.1 Suspended Solids Limitation

Solids e f f l u e n t q u a l i t y l i m i t a t i o n during base f l o w for a c t i v e su r face

mining, underground mining, and coal prepara t ion areas is 35 mg/l total

suspended solids (TSS) for the average of d a i l y values for 30 consecutive days

and a maximum of 70 mg/l TSS f o r any one day. Por post-mining cona i t ions , ' t he

discharge f r o m underground mine drainage is a l s o subject t o these suspended

solids l i m i t a t i o n s . 3.3.2 S e t t l e a b l e Sol ids Lfmitation

During any discharge or w e r f l o w r e s u l t i n g from a p r e c i p i t a t i o n event

less than or equal to t h e 10-year, 24-hour p r e c i p i t a t i o n event, t h e discharge

i s subject to s o l i d s e f f l u e n t q u a l i t y l i m i t a t i o n s of 0.5 m l / l settleable

solids (SS).

event greater than t h e 10-year, 24-hour p r e c i p i t a t i o n event the discharge is

During any discharge or werflow r e s u l t i n g from a p r e c i p i t a t i o n

I

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3.14

not s u b j e c t t o a s o l i d s e f f l u e n t q u a l i t y l imi t a t ion . These a l t e r n a t e limita-

t i o n s during p r e c i p i t a t i o n events only apply i f :

1. The treatment f a c i l i t y is designed, constructed, operated, and main- t a i n e d to contain a t a minimum t h e volume of water which would d ra in i n t o t h e treatment f a c i l i t y from a c t i v e ' n l n i n g areas and reclamation areas during t h e 10-year, 24-hour p r e c i p i t a t i o n event (or snowmelt of equivalent v0lume)t

2. The treatment f a c i l l t y is designed, constructed, operated, and main- t a i n e d t o cons i s t en t ly achieve t h e e f f l u e n t l i m i t a t i o n s set by the regula tory agencies f o r a l l e f f l u e n t q u a l i t y l imi t a t ions ;

The volume of settleable solids i n the e f f l u e n t from a sedimentation pond

is determined by a simple procedure known as t h e Imhoff cone test (see Figure

3.1).

mixed sample.

cone are gent ly s t i r r e d with a rod to f r e e any particles which may be c l ing ing

t o t h e s i d e s of t h e cone, and s e t t l i n g is allowed to occur f o r an a d d i t i o n a l

15 minutes. The volume of settleable s o l i d s i n t h e cone is then recorded as

The Imhoff cones are f i l l e d to t h e one - l i t e r mark with a thoroughly

S e t t l i n g is allowed to occur for 45 minutes, t h e s i d e s of the

' milliliters per l i t e r (from %tandard Methods f o r Examination of Water and

Wastewater,' 15th e d i t i o n ) .

perature 25OC, 77.F) or t h e r e s u l t s are ad jus ted to room temperature. It

should be poin ted ou t that s o m e d i f f i c u l t y exists in reading the Imhoff cones.

When dea l ing with f i n e particles such as silt, it requires practice in

def in ing t h e volume of settleable solids. It is recommended t h a t t hese

readings be taken in t h e presence of persons who have experience in performing

the Imhoff cone test.

The test is normally performed a t room texn-

: \

P a r t i c l e s i z e s smaller than one micron (0.001 mu) are assumed non-

Therefore, particle sizes settleable under gravitational fo rces alone.

smaller than one micron are not considered settleable s o l i d s i n this manual. - A well-designed sedimentation pond w i l l remove p r a c t i c a l l y a l l of the

sand-sized particles. Therefore, t h e settled volume in t h e b o t t o m of t h e

Imhoff cone w i l l be composed pr imar i ly of silt.

The sma l l e s t particle which w i l l set t le through the e n t i r e he ight of t h e

Imhoff cone during t h e test can be computed. Based upon Stoke's Law8 test

condi t ions , and assuming a s p e c i f i c g rav i ty of t h e particle to be 2.65, this particle size is computed as 0.011 rmn (GI. Stokes's f a w is based upon i d e a l

s e t t l i n g and t h e r e are aeveral re fe rences ava i l ab le which d iscuss Stoke's L a w

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... _ r . . . - . .. .

3.15

FIGURE 3.1 IMHOFF CONE TEST APPARATUS (SAWYER, 1978)

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3.16

(Barfield, 1981; Shames, 1962). A l l particles larger than 0.011 nm~ would

sett le during the test.

mm would be expected to settle depending upon the concentration of each par-

t icle s i z e within the sample.

select a particle s i z e of a par t icu lar s i z e d is t r ibu t ion t h a t must he removed

so that the se t t l eab le so l id s concentration meets e f f luent l imi ta t ions when

t h e sample is placed i n the Imhoff cone.

only a percentage of the pa r t i c l e s smaller than 0.011

The objective of *is design manual is t o

3.4 Trapping Efficiency '

To m e e t e f f luent l imitat ions, sedimentation pond design must be based on

sediment s ize d is t r ibu t ion and TSS concentration of t he base flow or design

storm runoff entering the pond.

common method f o r developing the pond design c r i t e r i a to meet a specif ied

e f f luent l imitat ion is by determining the percent of sediment removal

required.

(E) and is equal to the weight of sediment flowing i n t o the pond minus the

weight of sediment leaving the pond divided by the weight of sediment flowing

i n t o the pond and then multiplied by 100 to obtain eff ic iency In percent.

Thus, the trapping eff ic iency is given by:

Based on present state of the art , the most

The percent of sediment removal is ca l led the trapping eff ic iency

L

: \

wo x 100 wI - E = wI '

where, wI - weight of sediment flowing i n t o the pond,

Wo = weight of sediment flowing out of the pond,

During base flow, the sedimentation pond w i l l be i n a steady-state

condition where the water inflow volume equals the water outflow volume. The

w a t e r volume can be changed to a weight of w a t e r .

sediment by t h e weight of water w i l l y ie ld a concentration of TSS.

t h e trapping eff ic iency becomes

Dividing the weight of

Therefore,

x 100 cI - co E = ( 3 . 3 )

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.. . ... . . .. .. . .

3.17

where, CI = average sediment concentration

Co = average sediment concentration

i n t o the pond,

o u t of t h e pond.

For base f l o w , e f f l u e n t l imi t a t ions are stated as a concentration of TSS.

Therefore, a r e l a t i o n s h i p between the i n f l u e n t TSS Concentration and t h e

t r app ing e f f i c i e n c y can be developed i f t h e e f f l u e n t TSS concentration is

known. Once t h e i n f l u e n t TSS concentration has been measured, t h e required

t r app ing e f f i c i e n c y can be determined i f the e f f l u e n t TSS concentration is

known. Figure 3.2 presen t s t h i s r e l a t i o n s h i p f o r a range of e f f l u e n t con-

c e n t r a t i o n l imi t a t ions . Knowing t h e i n f l u e n t TSS concentration, the required

t r a p p i n g e f f i c i e n c y to limft the e f f l u e n t concentration to a s tandard can be

determined from Figure 3.2.

During t h e design p r e c i p i t a t i o n runoff event, t h e development of pond

The condition during a storm runoff is design criteria is more d i f f i c u l t .

.dynamic i n that the inflow to t h e pond is represented by a runoff hydrograph;

t h e outflow is based on t h e water surface e l eva t ion in t h e pond and t h e

discharge capac i ty of t h e outflow device. I n addi t ion , e f f l u e n t l i m i t a t i o n s

for the design p r e c i p i t a t i o n runoff event are stated as a concent ra t ion of

b settleable sediment. !

To design f o r t h e design runoff event requires that a practical approach

be taken. The method used to route the inflow hydrograph through the sedimen-

t a t i o n pond is based upon the inflow volume being equal to the outflow volume.

(Water rou t ing is discussed in Section 3.6.) Therefore, the t rapping e f f i -

c iency for the design runoff went can also be computed using Equation 3.3.

However, for t h e design runoff event, t h e suspended solids concent ra t ion and

t h e t r app ing e f f i c i e n c y are both unknown. - By d e f i n i t i o n , t h e t rapping e f f i c i ency is t h e weight of sediment removed

i n the pond.

c e n t r a t i o n and s i z e d i s t r i b u t i o n . I n addi t ion , it is assumed t h a t the

i n f l u e n t sediment is evenly d i s t r i b u t e d in t h e water inflow. Therefore, when

a

pe rcen t of sediment removal or t rapping e f f i c i e n c y is equal to t h e percent of

I t h e s i z e d i s t r i b u t i o n t h a t is l a r g e r than di . Figure 3.3 p re sen t s t h e def i -

The i n f l u e n t sediment is represented by the sediment con-

sedimentation pond is designed t o remove a c e r t a i n particle size ( a i l , t h e

n i t i o n of t h e t rapping e f f i c i ency f o r various particle s i zes . This estimate

of t r app ing e f f i c i e n c y is conservative s ince it assumes none of the particles

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100

90

80

n

a? Y * 0

w' 70 3 :

w

- LL

>LL

a z 60 a a

0: t-

50

40

. I 10

- . ... . . . . . . . ....

3.113

I 100 lob0

INFLUENT SUSPENDED SOLIDS CONCENTRATION (mQ./L.)

FIGURE 3.2 REQUIRED TRAPPING EFFICIENCY TO MEET VARIOUS EFFLUENT LIMITATIONS

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. . . . . . .

. . . - . .. . . .. , . .. , . . .~-,. ._. . _ _ ...._ , . . _.

3.20

smaller than the selected particle s i ze ( d i ) w i l l set t le i n the pond.

Actually, a percentage of the particles smaller than d i w i l l settle.

Therefore, f o r each particle s ize , a trapping eff ic iency can be determined

from the inf luent s i z e d is t r ibu t ion , and the suspended so l id s concentration

can be calculated by rearranging Equation 3.3.

E co = ( 1 - - 100) cI (3.4)

To determine whether the e f f luent requirements a re s a t i s f i e d , a rela-

t ionship between the suspended so l ids concentration and the settleable solids

concentration is required. This re la t ionship is presented in the following

section.

3.5 Set t leab le Solids Concentration

Effluent l imi ta t ions during runoff events and post-mining reclamation are

stated in terms of a volume of s e t t l eab le so l ids per one l i ter of sample. To - relate the settleable solids J i d t a t i o n -+ -the- *sign - . - . of - sedimentation ponds, c-- .__c_

a relat ionship between settleable solids and total suspended solids m u s t be

considered.

settle in the bottom of an Imhoff cone i n one hour of quiescent se t t l i ng .

-- Set t leab le solids are defined as the volume of particles t h a t . _

Knowing the inf luent sediment s i z e dis t r ibut ion, a particle s i z e t o be

settled in t h e pond is selected and the se t t l eab le so l id s concentration is determined. If the settleable so l id s concentration is l a rger than e f f luen t

l imitat ions, a smaller particle s i z e is selected and a new settleable so l id s

concentration is computed. Likewise, if the settleable so l id s concentration

is Qnaller than the e f f luent l imitat ions, a larger particle s i z e is se lec ted

and the ettleable so l id s concentration is computed. Therefore, an I

interat i i process is required to determine the particle s i z e that t h e s ea - / e entat ion pond must remove so the pond ef f luent satisfies the settleable

s o l i d s l imitat ion.

The first s t ep in computing the se t t l eab le sol ids concentration is t o

ad jus t the inf luent sediment s i z e d is t r ibu t ion by subtract ing out the non-

s e t t l e a b l e s i z e s ( < 0.001 man). Given the s i z e d is t r ibu t ion in Figure 3.4, it

can be seen that ten percent of the sediment is smaller #an 0.001 CPILI.

Therefore, the 90 p e r c e n t of the s i z e d is t r ibu t ion which is settleable m u s t be

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. . . _ _ - - , . - . _ _ .

3.21

6 0 0 6

0 0 0 0 0 QD (0 0 cu

tl3NId l N 3 3 U 3 d

8 0

0 9

E E Y

W

w J 2 I- a a a

w .tn

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3.22

red is t r ibu ted 60 that it makes up 100 percent of the s i z e dis t r ibut ion.

3.2 shows how t o develop a s i z e d is t r ibu t ion i n which a l l particle s i zes are

se t t l eab le .

Table

The settleable s i z e d is t r ibu t ion is presented i n Figure 3.5.

A re la t ionship between the ef f luent suspended solids concentration, t he

s e t t l e a b l e particle s i z e dis t r ibut ion, and the settleable so l ids concentration

is required.

suspended so l id s concentration to se t t l eab le so l id s based on d iscre te particle

s e t t l i n g and the geometry of the Imhoff cone. The volume of settleable so l id s

is given by

Barf ie ld (1981) developed an equation f o r the conversion of

A

d i 6 [ ( l - X0’ + c (TI Axil C* ss = - W

i = 1 0

where, SS = settleable so l id s concentration (mg/l),

C* - average e f f luent suspended so l id s concentration fo r the settleable s i zes (mg/l),

W = dry bulk density of the settled so l ids (mg/ml),

(3.5)

Xo = f rac t ion of particles in the e f f luent d i s t r ibu t ion smaller than = 0.011 nun,

d, = smallest particle which w i l l settle through the e n t i r e height of an Imhoff cone (0.011

d i = mean particle s i ze of the in t e rva l (m)8 and

A X i = f r ac t ion of e f f luent sediment s i z e d is t r ibu t ion which has a mean particle s ize of d i .

The average e f f luent suspended so l id s concentration fo r the settleable s i zes

is given as

where, E, y v# Y are 88 defined previously, and

k = f rac t ion of the pa r t i c l e s in the inf luent s i z e d is t r ibu t ion which are se t t leab le .

I n the previous example, k would equal 0.90 since 90 percent of the inf luent

s i z e d i s t r ibu t ion is se t t leab le .

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3.23

Table 3.2. Development of Settleable Solids Size Distribution.

Settleable Solids Influent Size Distribution

Particle Size Distribution column 3 x (100/90) (mm) ( 0 finer) column 2 - 10 ( 0 finer) (1 ) (2) ( 3 ) ( 4 )

0.001

0 0042

0.01

10

16

26

0

6

16

0.0

6.7

17.8

. . 0.04 50 40 44.4

0.10

0.20

0.66

72

90

100

62

80

90

68.9

88.9

100.0

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3 - 2 4

PARTICLE SIZE (mm)

FIGURE 3.5 SETTLEABLE SOLIDS SIZE DISTRIBUTION

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3.25

The dry bulk dens i ty of the settled solids (W) should be r ep resen ta t ive

of settled silt s ince t h i s is the s i z e range tha t w i l l settle during t h e

Imhoff cone test. A r ep resen ta t ive value of W for settled silt is 7 0

l b / f t 3 or approximately 1120 mg/l.

The f r a c t i o n of the particles in t h e e f f l u e n t size d i s t r i b u t i o n which are

smal le r t h a n a, (0.011 mm) is denoted as X,,. When a particle s i z e t o be

removed i n a pond is equal to or smaller than 0.011 nun, w i l l always be

1.0 and a l l of the particle s i z e s i n t h e e f f l u e n t are equal to or smal le r than

0.Ollmm. A l l of the particle s i z e s which have a diameter of 0.011 x n or

l a r g e r w i l l settle i n an Imhoff cone test.

determines what percent of the particle sizes smaller than 0.011 w i l l settle

during t h e test.

The second term i n Equation 3.5

When a particle s i z e to be removed i n t h e pond is l a r g e r than 0.011 nun,

x, than 0.011 mm. For this condition, the e f f l u e n t w i l l conta in particle sizes

greater than 0.011 mu. A l l particle s i z e s greater than 0.011 rmm w i l l settle

in the Imhoff cone during the test. The first term in Equation 3.5 describes

the percent of the e f f l u e n t s i z e d i s t r i b u t i o n which is l a r g e r than 0.011 rmn

and the re fo re , w i l l sett le during the Imhoff cone test. For t h i s condi t ion ,

is equal t o t h e percent of the e f f l u e n t size d i s t r i b u t i o n which is smaller

2b can be completed as

% of settleable s i z e d i s t r i b u t i o n smal le r than 0.011 nrm x = o % of settleable size d i s t r i b u t i o n smaller than s i z e to

be removed i n sedimentation pond

The design of a sedimentation pond to meet e f f l u e n t l i m i t a t i o n s requires

t h a t a particle s i z e to be removed be se lec ted .

select a particle size of 0.011 mn. This makes x, i n Equation 3.5 equal t o

1.0.

smaller than 0.011 ran. To eva lua te the second term in Equation 3.5, the par-

t ic le sizes smaller than 0.011 w must be r e d i s t r i b u t e d i n t o a s i z e d i s t r i b u -

t i o n i n which the particle s i z e s smaller than 0.011 rn comprise the e n t i r e

s i z e d i s t r i b u t i o n .

3.5, it c a n be seen that 19.5 percent of the settleable size d i s t r i b u t i o n is

smaller t h a n 0.011 nm. This percentage of the settleable s i z e d i s t r i b u t i o n is

t hen r e d i s t r i b u t e d t o be 100 percent.

A good s t a r t i n g p i n t is t o

Therefore, the e f f l u e n t size d i s t r i b u t i o n is made up of particles

Using the settleable size d i s t r i b u t i o n presented i n Figure

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_ _ _ . . . - . . . . .

3.26

This procedure starts by breaking up the settleable s i z e d i s t r i b u t i o n

smaller than 0.011 mm i n t o s e v e r a l percentage i n t e r v a l s . This is shown i n

Figure 3.6. The s i z e range f o r each increment is then t abu la t ed and the mean

size ( d i ) is determined. This procedure is shown i n Table 3.3. I n this

example, percentage increments of 0.04 were chosen. There is no set value for

the percent increments.

r e s u l t . The particle s i z e range f o r each increment is then t abu la t ed (column

1, Table 3.3). The particle s i z e ( d i ) i n the middle of each increment is then

t a b u l a t e d in column 2 of Table 3.3 as mean s i z e .

r e d i s t r i b u t e t h e s i z e d i s t r i b u t i o n smaller than 0.011 m. This is

accomplished by div id ing each percent increment (column 3) by the sum of

column 3. For this example, the first four e n t r i e s in column 4 are found by

d iv id ing 0.04 by 0.195. Column 4 is the A X i va lue used in Equation 3.5

corresponding t o the di value (column 2) . Knowing this information, the

'settleable s o l i d s concentration in the e f f l u e n t can be determined from

Equation 3.5.

However, smaller s i z e d increments w i l l y i e l d a better

The f i n a l s t e p is to

I f the settleable s o l i d s e f f l u e n t limitations are not s a t i s f i e d , a par-

t i c le s i z e smaller than 0.011 nm is chosen to be removed. The va lue of &, i n Equation 3.5 w i l l still be equal to 1.0. However, the particle size range

i n column 1, Table 3.3 w i l l change. The particle s i z e range w i l l now have the

upper l i m i t of the selected particle size instead of 0.011 mu. Therefore, the

t r app ing e f f i c i ency , e f f l e u n t concent ra t ion , particle size range, increment

size, and A X i

cen t ra t ion can be computed.

w i l l have new values and the new settleable solids con-

When the computed settleable s o l i d s concentration is less than the

e f f l u e n t l i m i t a t i o n s , larger s i z e d particles w i l l be allowed in the e f f l u e n t .

Therefore, a particle s i z e l a r g e r than 0.011 nun is selected to be removed i n

the pond. I n Equation 3.5, the second term w i l l r e sa in the same as that which

w a s computed f o r a particle s i z e of 0.011 m bu t w i l l be reduced by a factor

of %. poin t . The va lue of w i l l no longer be equal to 1.0. For this condition,

rr, can be computed as defined previously. W i t h the new t rapping e f f i c i ency , e f f l u e n t concentration, and value of G, the settleable solids concent ra t ion

can be computed using Equation 3.5.

i nc rease r ap id ly as the particle s i z e t o be removed i n the pond is increased

This is one of the main reasons f o r s e l e c t i n g 0.011 nun as a s t a r t i n g

The settleable solids concentration w i l l

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. ... ~ .,.. .

3.28

Table 3.3. Size Distribution for Particles Smaller than 0.011 nun.

( 1 ) Particle Size Range

( 3 ) Percent in Size

Range of ( 2 ) Settleable Size

Mean Size Distribution

I

0.001 - 0.0023 0.0015

, 0.0023 0.0046 0.0035

0.0046 - 0.0064 0 . 0054

0.0064 - 0.0088 0 0075

0.0088 - 0.011 0.0100

c

0.04

0.04

0.04

0.04

0.035 - 0.195

0.205

0.205

0 205

0 - 205

0.180

1.0

-

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3.29

since a l l particles larger than 0.01 1 mn w i l l s e t t l e i n an m o f f cone.

Therefore, when a new pa r t i c l e s i z e is selected, a particle s i z e i n t he range

of 0.015 mm t o 0.02 xmn should be t r i e d so the designer can understand how f a s t

t h e se t t l eab le so l ids concentration increases. . When the designer has calculated the particle s i z e which must be removed

in the sedimentation pond to meet ef f luent Limitations, criteria €or the sedi-

mentation pond design can be determined. The determination of the design par-

t i c l e s i z e to meet e f f luent .limitations may seem confusing. Following through

the example given i n Chapter V I w i l l help the operator understand how t o

design a sedimentation pond to s a t i s f y se t t l eab le so l ids e f f luent lLmitations.

3.6 Storage Volume Requirement

I n the design process, there is an i t e r a t i o n procedure t h a t is required

between t he information presented in Sections 3.6, 3.7 and 3.8. Knowing the

particle s i z e t o be removed (Section 3.5)# a depth is assumed and the

corresponding required detention time is determined (Section 3.8). The

avai lable storage volume for the selected depth is determined (Section 3.7). The required storage volume is then determined (Section 3.6) and compared t o

\ t h e avai lable storage volume. I f the avai lable storage volume is less than :

t he required storage voiume, the depth is Increased and the In te ra t ion is

repeated. When the avai lable storage volume is greater than the required

storage volume, the depth, detention time, storage volume, and outflow rate

are established. The pond surface area, length, and Width are then checked to

ensure t h a t the selected particle size is s e t t l e d in the pond.

Flow routing through a sedimentation pond is determined by the rate of

inflow, storage capacity of the pond, and outflow capacity €or given reservoi r

levels . Numerous methods of reservoir routing have been developed which

include the b d l f i e d Puls Method, Rippl Mass Curve, and several others.

Descriptions of these methods can be found in hydrology texts and manuals.

A simplified method is used in this manual. The simplified rout ing

method is used to determine the required storage volume and s i z e the p r inc ipa l

spillway t o produce the required detention time so t h a t e f f luent requirements

are m e t . The simplified routing procedure requires that the peak inflow rate

and runoff volume are known.

determined from the inflow hydrograph. This method implies two assumptions,

The peak Inflow r a t e and runoff volume can be

\

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3.30

t he shape of the inflow and outflow hydrographs are t r iangular and the i n i t i a l

water surface elevation is a t the elevation of the pr incipal spillway c re s t .

Therefore, the areas under the inflow and outflow hydrographs are equal.

Water routing through sedimentation ponds can be solved using Figures 3.7

and 3.8. Figure 3.7 is a graph showing the relat ionship between the time base

of the inflow hydrograph (a) and the r a t i o of the required storage volume (S)

t o the runoff volume (V) f o r a range of detention t i m e s . Figure 3.8 presents

t h e

the

the

relat ionship between Tb and the r a t i o of t he peak outflow rate (Qo) t o

peak inflow rate (QI)

inflow hydrograph is determined as:

f o r a range of detention times. The time base of

V 1800 Qx Tb =. (3.7)

where, = time base of. inflow hydrograph (hours),

V

Q~ = peak inflow rate ( c f s ) .

= water runoff volume ( f t 3 ),

I

The time base can be

t hydrograph. Knowing

detention time f o r a

t he required storage

Figures 3.7 and 3.8.

b

computed based on the information from the inflow

the t i m e base of the inflow hydrograph and the required

selected particle size to be s e t t l e d (Section 3.8.11,

volume and peak outflow rate can be determined using

3.7 Available Storage Volume

The sedimentation pond storage volume should provide an adequate sediment

s torage volume and an adequate detention storage volume so ef f luent limita-

t i o n s a re sa t i s f i ed . A t each sedimentation pond site, a relat ionship between

the depth and the storage volume is required s ince the trapping eff ic iency

depends on depth and storage volume of the pond.

The method u t i l i z e d to develop the depth and storage volume relat ionship

requires a topographic map of the location of the pool area and embankment of

t he sedimentation pond.

pool elevations can be determined using a planimeter and the scaled topography

map. For example, i n Figure 3.9 the incremental storage volume between a pool

a t elevation E2 and a pool a t elevation E3 is d e t e d n e d by measuring (with a

An incremental value of storage volume between two

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3 . 3 3

FIGURE 3.9 DEFINITION SKETCH TO DETERMINE INCREMENTAL STORAGE VOLUME

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3.34

planimeter) the pool surface area a t elevations E2 and E3-

storage volume is then calculated as the increase i n elevation (E3 - E2) t i m e s

t he average surface area of the pool [(AE Thus, a t ab le r e l a t ing

storage t o stage can be developed (see Table 3.4). A graph of t he s tage ver-

The incremental

+ AE~)/~]. 2

sus storage volume is then plot ted.

It is l e f t ' to the designer to decide how often sediment w i l l be removed

from the pond.

removal can be computed using procedures described in Section 3.2.2.2.

sediment y i e ld is converted to a storage volume by dividing the y ie ld by the

The sediment y ie ld during the time period between sediment

The

u n i t weight of the deposited sediment. Lara and Pemberton (1963) developed an

equation to ca lcu la te the unl t weight of the settled sediment based upon sedi-

ment s i z e d is t r ibu t ion and type of reservoir operation. This equation appears

i n several references (Barfield, 1981; Bureau of Reclamation, 1977) and the

designer may consult these to compute the u n i t weight of the settled sediment.

A un i t weight of 70 lbs/ft3 is suggested t o simplify the design.

value of u n i t weight, the required sediment storage volume can be computed.

Using this

The corresponding depth of the sediment i n the pond can be found from the

stage-storage curve.

: The characteristics of sediment deposition are such that the large sized \

particles wil l settle near the i n l e t of the pond resu l t ing i n the formation of

a del ta . Delta formation is described in Section 3.8.2.2. Because the la rger

s ized particles settle near the i n l e t , the sediment storage volume should be

provided near t he i n l e t of the pond.

provided a t this location, accumulated sediment a t the inlet w i l l require f re-

quent remova 1 e

If t h e sediment storage volume is not

The detention storage volume is the storage volume required to produce

the required detention t i m e . This is the volume that is used in the w a t e r

routing procedure presented i n Section 3.6.

determined from the stage-storage curve and is measured as the avai lable

s torage volume above the elevation of the pr inc ipa l spillway crest. The ele-

vation of the pr inc ipa l spillway crest is usually chosen as the maximum depth

of the sediment storage volume unless a permanent pool is provided.

permanent pool is provided, the permanent pool elevation w i l l be a t the eleva-

t i o n of the pr inc ipa l spillway crest.

The detention storage volume is

When a

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. ..

3.35

Table 3.4. Stage-Storage Relatlonshlp Developent.

Stage (feet)

Storage (feet31

0

E2

E3

E4

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3.36

3.8 Sedimentation Pond Configuration

The design of the sedimentation pond conf igura t ion is based upon ideal

s e t t l i n g condi t ions . I n actual f i e l d s i t u a t i o n s , ideal s e t t l i n g conditions

are o f t e n d i f f i c u l t to reproduce.

f a c t o r s h t o the design which account f o r nonideal s e t t l i n g conditions.

following s e c t i o n s d iscuss the design of the pond conf igura t ion based upon

ideal s e t t l i n g , f a c t o r s which produce nonideal s e t t l i n g , and what f a c t o r s are

used to compensate for nonideal s e t t l i n g conditions.

This n e c e s s i t a t e s the need to incorpora te

The

3.8.1 Ideal S e t t l i n g

Based upon ideal s e t t l i n g conditions, t h e r e is a direct r e l a t i o n s h i p

between t h e de ten t ion s to rage depth of the pond and t h e detention t i m e . This r e l a t i o n s h i p

I where, Vs =

D =

TD =

can be expressed as

D 3600 TD

=

particle s e t t l i n g ve loc i ty ( f p s ) , deten t ion storage depth ( f t ) , and

de ten t ion time (hours).

The particle s e t t l i n g ve loc i ty is defined by Stoke's L a w and is dependent

upon temperature of the water, particle size, and specific g rav i ty of t h e par-

ticle. To determine t h e design particle size as presented in Section 3-58 the

temperature of the water was assumed 7 7 0 F, s i n c e this is part of the Imhoff

cone test and sets t h e criteria which must be s a t i s f i e d . I n t h e f ield, t h e

temperature of the water runoff w i l l be closer t o 500 F. For the same par-

t i c l e size, s e t t l i n g w i l l t ake longer in the water which is 50° F t h a n i n t h e

water which is 77O F.

upon the water being 50. F. A s s d n g a water temperature of SO0 F and the

s p e c i f i c g r a v i t y of t h e particle to be 2.65, Stoke's Law may be wri t t en as

'

Therefore, design of the sedimentation pond is based

Vs = 2.254 d2 (3.9)

w h e r e , Vs 5 particle s e t t l i n g ve loc i ty (fps) and

d = particle diameter (m).

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3.37

The r e s u l t of combining Equations 3.0 and 3.9 is

D 3600 TD 2.254 d2 = (3.10)

Figure 3.10 presents the relat ionship between the particle diameter and deten-

t i o n time f o r various depths using Equation 3.10. To sett le any size par-

t icle, the required detention time f o r various depths can be found from Figure

3.10 or computed by Equation 3.10.

There is a l so a direct re la t ionship between the flow length of the pond

and the detention time. This re la t ionship is represented as

L u '€I 3600 TD

where, V, = horizontal flow veloci ty through the pond ( fps ) ,

L = f l o w length of the pond (f t) , and

T~ = detention time (hours).

The horizontal flow veloci ty through the pond can be computed as

QO VH = - wD1

where, Qo = peak outflow rate ( c f s ) ,

W = average width of t he pond (f t) , and

D1 = total depth of the pond (ft).

Combining Equations 3.11 and .3.12 results i n

3600 T Q Dl O L =

wDl

(3.11)

(3.12) ,

(3.13)

where, TI), G8 W are as defined i n Equation 3.11 and

D1 = sediment storage depth plus detention storage depth, and

TD, = detention time f o r depth D1 from Equation 3.10.

Equation 3.13 gives the required flow length of the pond t o settle the design

p a r t i c l e s ize . This equation is used as a check a f t e r the pond storage volume

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3.39

and outflow rate have been established. The total depth is used i n Equation

3.13 since the p a r t i c l e w i l l be required to sett le this depth j u s t after the

pond construction is completed.

achieved, .measures described in Section 4.2 can be taken to increase the flow

If the required flow length cannot be

length of the pond.

3.8.2 Nonideal Se t t l i ng

As presented so f a r , t he design of sedimentation ponds have been based on

ideal s e t t l i n g conditions.

Impossible t o provide i d e a l s e t t l i n g conditions.

s e t t l i n g are caused by severa l factors .

conditions; they are a l l i n t e r r e l a t ed and cause deviations from ideal s e t t l i n g

reducing the eff ic iency of the sedimentation pond.

va r i a t ion form ideal s e t t l i n g are:

However, i n the f i e l d it is d i f f i c u l t and of ten

Variations from ideal

These factors are not independent

The conditions causing

- - Reservoir deposition

Flow c u r r e n t s within the pond

- Short c i r cu i t i ng and turbulence

- Scour and resuspension

3.8.2.1 Flow Currents

V a r i o u s types of flow currents can exist within sedimentation ponds. The

most common being those caused by wind blowing Over the surface of the pond.

Convection currents can a l s o exist due t o s ign i f icant differences of

temperature within the pond.

Often during storm runoff events, the inflow to the pond is t yp ica l ly

more dense due t o high suspended so l id s concentrations. This r e s u l t s i n a

densi ty current t h a t flows along the bottom of the pond. This local ized

increase i n flow can cause scour and resuspension of settled solids and signi-

f i c a n t l y reduce the t r a p eff ic iency i f the o u t l e t t o the pond is located near

the bottom.

A l l types of currents t ransport suspended material throughout the pond

both v e r t i c a l l y and horizontal ly and d i s t o r t the flow pa t te rn from that

assumed under ideal se t t l i ng .

the pond.

The r e s u l t is a reduction in the performance of

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3.40

3.8.2.2 Delta Formation

As the sediment-laden inflow passes from the i n l e t channel t o the sedi-

mentation pond, the forward velcotiy of the flow is reduced due t o increase i n

f l o w width and flow depth. This re su l t s in the larger sized particles being

deposited almost immediately as flow enters the sedimentation pond.

Deposition of the larger sized particles near the inlet of the pond w i l l

r e s u l t i n t he formation of a delta. Figure 3.11 shows a del ta formation near

t h e i n l e t of the pond. The &lta will continue t o grow larger and w i l l gra-

dually migrate downstream within the pond.

mation is reduced detention time.

given enough t h e t o settle to the bottom of the pond.

The consequence of a delta for-

Therefore, small p a r t i c l e s i zes are not

3.8.2.3 Short Circuit ing and Turbulence

Short c i r cu i t i ng is the flow of water through a Sedimentation pond

d i r e c t l y from the i n l e t to the o u t l e t resu l t ing in dead storage areas and

reduced detention t i m e s .

shapes which have shor t c i rcui t ing.

caused by flow currents (as previously (Uscussed), high i n l e t w l O C i t i e 6 , high

o u t l e t flow rates, sedimentation pond geometry, and improper location of

i n l e t s and outlets. When short c i r cu i t i ng 0ccum8 the ef fec t ive width of the

f l o w area through the pond is reduced and t h e l l o w veloci ty through the pond

is greater. This e f f e c t reduces the s e t t l i n g charac te r i s t ics and increases

po ten t i a l f o r scour and resuspension of s e t t l e d sediments.

Figure 3.12 presents some typical sedimentation pond

Short c i r cu i t i ng and turbulence are !

3.8.2.4 Scour and Resuspension

Scour and resuspension is caused.by density c u r r e n t s and high flow rates

through the sedimentation pond. The scour veloci ty is defined as t h a t velo-

c i t y of flow required t o i n i t i a t e motion of a discrete par t ic le .

of the design p a r t i c l e s i z e u i l l r e s u l t i n e f f luent l imitat ions not being

s a t i s f i e d .

Resuspension

r

3.8.3 Control of Nonideal Se t t l ing

The s ign i f i can t fac tors which a f f e c t ideal s e t t l i n g are shor t c i r cu i t i ng ,

turbulence, and scouring of the s e t t l i n g sediment.

c r i t e r i a areset t o minimize these e f fec ts . These criteria are length-to-width

In the following sect ions

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3.4:.

POUD .

FIGURE 3. . I 1 SCHEMATIC OF DELTA FORMATION (AFTER GRAF, 1974)

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RISER (OUTLET) Dt FLOW PATHS

NORMAL POOL

. DEAD STORAGE

. .

;

. DEAD STORAG

FLOW PATHS . .

ISER (OUTLET) DEAD STORAGE

DEAD STORAGE

NORMAL POOL FLOW PATHS

INFLOW

FIGURE 3.12 SEDIMENTATION PONDS WITH DEAD STORAGE SPACES (AFTER HAAN & BARFIELD, 1978 )

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3.43

r a t i o , sho r t - c i r cu i t i ng f a c t o r , permissible inlet ve loc i ty , and permissible

flow-through ve loc i ty . The sedimentation pond must meet these criteria t o

ensure that the desired sediment removal is a t t a ined .

3.8.3.1 Short-Circuit ing Fac tor

Research by Camp (1946) on various types of s e t t l i n g basins has r e s u l t e d

in the development of a shor t - c i r cu i t i ng compensation f a c t o r based on the

shape of the bas in geometry. It has been recommended tha t the su r face area of

a s e t t l i n g bas in be increased t o account for nonideal s e t t l i n g condi t ions

according t o

(3.14)

where, A = su r face area of the pond a t the e l eva t ion of t h e p r i n c i p a l sp i l lway crest ( f t 2 ) ,

FSC = shor t - c i r cu i t i ng f a c t o r ,

QO = outflow rate ' ( c f s ) , and

vs = s e t t l i n g ve loc i ty of the design particle size (fps). : \

The value of FSC is genera l ly 1.2. Equation 3.14 w i l l y i e l d the required

surface area that is needed in the pond a t the e l eva t ion of the p r i n c i p a l

sp i l lway crest.

face area does no t meet the requirement of Equation 3.14.

There are three measures that can be taken if the pond sur-

The pond side

slopes can he excavated, the e l eva t ion of the p r i n c i p a l sp i l lway crest can be

raised, or app l i ca t ion of multiple ponds.

3.8.3.2 Lenqth-to-Width Ratio

The ratio between the f l o w length and the e f f e c t i v e width of the sedimen-

t a t i o n pond is used as a design a i d to minimize short c i r c u i t i n g .

a length-to-width ratio allows f o r u t i l i z a t i o n of the f u l l su r f ace area of the

sedimentation pond and helps maintain a cons tan t ho r i zon ta l v e l o c i t y through

the sedimentation pond.

t h e water m u s t flow from the inlet to the o u t l e t of the pond. The width used

i n the computation is the e f f e c t i v e width of the sedimentation pond. This is

determined by div id ing the sur face area by the length from the i n l e t t o the

Specifying

The length that is used is the shortest d i s t ance that

i

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3.44

ou t l e t . The surface area of the pond is measured a t the elevation of the

pr inc ipa l spillway crest.

It is general practice to specify a minimum length-to-width r a t i o of

2: l . Larger length-to-width r a t io s w i l l promote improved performance and

values of up t o 5: l have been recommended. The length-to-width r a t i o is

determined after the pond storage volume and outflow rate have been

established. I f t h i s r a t i o cannot be sa t i s f i ed , the flow length can be

increased. Section 4.2 describes the measures that can be taken to increae

t h e flow length.

Both the short-circuit ing factor and the length-to-width r a t i o compensate

f o r nonideal s e t t l i n g conditions. The short-circui t ing factor determines only

t h e required surface area, whereas the l e n g t h - W i d t h r a t i o defines the shape

of the surface area.

3.8.3.3 Permissible I n l e t Velocity

For a sedimentation pond to be ef fec t ive in sediment removal, the velo- . -- I \ c i t y of the flow i n t o the pond mus t be small enough t o prevent short cir-

cuiting. The criteria is used t o l i m i t the Roude number in the i n l e t channel

t t o 1.0. The Froude number is defined as

V

\

(3.15) Fr = - 4z

where, Fr = Froude number,

v P veloci ty in the inlet channel (fps) 8

g = gravi ta t iona l acceleration (32.2 ft/sec2) 0 and

D = depth of flow in the inlet channel ( f t ) . I f the Froude number in the i n l e t channel is greater than 1.0, inlet cont ro l

measures w i l l be required. These measures are discussed in Section 4.2.

3.8- 3.4 Permissible Plow-Through Velocity

The horizontal flow velocity through the pond must be less than the scour

veloci ty of the design particle s i ze to avoid resuspension of the s e t t l e d

sediment. The scour velocity fo r a specific particle s ize is determined by

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3.45

(3.16)

where, Vsc - scour velocity,

B = Shields' crt icial shear stress parametr (0.047 for uniform sand),

g = gravitational acceleration ( 32.2 f t/sec 2) ,

6, = specific gravity of particle (usually 2.6 t o 2-81,

d = diameter of spherical particle ( f t ) 8

F = Darcy-Weisbach friction factor (usually 0.02 t o 0.03).

Assuming that Shields' cri t ical shear stress parameter is equal t o 0.05, the

. specific gravity of the particle is 2.65, and the Darcy-Weisbach f r i c t i o n fac-

tor is 0.025, Equation 3.16 can be reduced to

1/2 V '= 1.67 d SC

I where, Vsc - scour velocity (fps) and

d = particle diameter (mm). t \ The horizontal velocity through the pond is

QO va = - WD

where, vx = horizontal flow velocity (fps),

QO = outflow rate (cfs),

w - average width of sediment pond ( f t ) , and

D = detention storage depth ( f t ) .

(3.17)

(3.18)

I f the horizontal velocity through the pond is greater than the scour velocity

for the particle that must be settled, the depth can be increased to reduce

the horizontal velocity which will also increase the width and decrease the

outflow rate.

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3.46

3.9 Sedimentation Pond Outlet Control Measures

Sedimentation ponds must provide a pr inc ipa l spillway and an emergency

spillway. Pr incipal spillways are designed to provide su f f i c i en t detention

time during the design prec ip i ta t ion event t o meet the ef f luent l imi ta t ions

and dewater the pond. Rnergency spillways are designed to work in conjunction

with the pr inc ipa l spillway and pond storage to safe ly discharge the peak

runoff r e su l t i ng from the design s t o m . The design procedure presented i n

Chapter V I develops the design discharge f o r s iz ing the pr inc ipa l spillway and

the elevat ion above the b o t t o m of the pond.

no t covered in this section. Several references provide design procedures f o r

s i z i n g standpipe and culvert-type spillways (U.S. Department of

Transportation, 1965; Barf ie ld 19811 Bureau of Reclamation, 1974; Soi l

Conservation Semice, 1969). The following discussion presents important

design considerations f o r pr inc ipa l and emergency spillways, various types and

Actual design of the o u t l e t is

configurations, and effectiveness.

\

! 3.9.1 Pr inc ipa l Spillways

The pr inc ipa l spillway i s sized to provide a discharge rate as determined

: through design of t h e sedimentation pond.

uat ion of local drainage .conBitions, water r igh ts , economics, land-use

cons t ra in ts , and requirements of local, state and federa l regulations. Design

of the p r inc ipa l spillway should not be independent of the design of the ear th

embankment and emergency spillway.

A c t u a l design should include eval- \

The types of pr inc ipa l spillways commonly used can be c l a s s i f i e d into

three categories: open channel, drop inlet and pipe culverts. The type of

spillway used is based on local site-specific conditions.

3.9.1.1 Op en Channel Spillways

open channel spillways should only be used when a l l other a l t e rna t ives

have been shown to be infeasible .

dewatering the pond.

drainage basins. Design of open channel spillways to meet e f f luent standards

during base f l o w and design storm conditions is very d i f f i c u l t . When an open

channel is used €or the pr inc ipa l spillway it often is designed f o r the

emergency capacity, or the emergency spillway is a l so an open channel.

This type of spillway provides no means of

Typically open channel spillways are located on small

Open

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3.47

channel spillways do not provide as much control of discharge or f l e x i b i l i t y

t o modification as drop i n l e t s or pipe culverts. Further discussion of open

channel spillways is covered in Section 3.9.4 on emergency spillways.

3.9.1.2 Drop I n l e t Spillways

Drop i n l e t spillways are one of the most common types of pr inc ipa l

spillways used for sedimentation ponds. A drop inlet spillway is quite

f l ex ib l e i n design, offers good control of dbcharge, and is w e l l adapted t o

sedimentation ponds.

i n diameter.

cleaning. When the design discharge f o r meeting eff luent requirements r e su l t s

I n a spillway s i z e smaller than 12 inches in diameter, a 12-inch pipe is used

with an o r i f i c e of the required s i z e opening affixed to the inflow end of the

drop inlet.

A recommended minimum s i z e fo r drop inlets is 12 inches

This m i m i m u size provides access ib i l i ty for maintenance and

Configuration of a typical drop inlet is shown In Figure 3.13. A drop

i n l e t has two main features , the barrel and riser. The riser and barrel can

be of concrete, reinforced concrete polyvinyl chloride (PVC) 8 corrugated or smooth m e t a l pipe.

\ The select ion of the type of material used should consiUer

! s i te conditions and economics. \

I n designing drop inlets an impor tan t consideration is anchoring of the

riser and barrel on the bottom of the pond, and seepage along the barrel.

Fai lure of the riser to s t ay anchored is a k o n problem.

rit3erS should consider the s i z e of the riser, local soil type, type of pond,

and weather condltions. I f the pond is a permanent pool it is susceptible to

freezing, and the forces created by the forming Ice should be considered.

Anchoring of

Seepage along the barrel is o f t e n the cause of dam embankment failure.

The problem generally occurs due t o the lack of compaction around the barrel . during construction of the embankment.

3.9.1.3 Pipe Culvert Spillways

Another type of pr inc ipa l spillway commonly used is the pipe cu lver t ,

also re fer red t o as a “ t r i c k l e tube.” It’ consis ts of a pipe l a i d in the earth

i n such a manner that the entrance elevation of the pipe ( a t the upstream end)

establishes the normal pool elevation in the pond. Figure 3.14 shows a typ-

i c a l pipe culver t arrangement. Pipe cu lver t spillways require the same con-

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3.48

/

FOQT IN G AHTI -SEEP 2d

COLLARS RIP RAP PROTECT8QN

FIGURE 3.13 TYPICAL DROP INLET SPILLWAY

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3.49.

PROTECTION COLLARS

FIGURE 3.14 TYPICAL PIPE CULVERT SPILLWAY

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3.50

s i d e r a t i o n as drop i n l e t spil lways do for seepage, minimum s i z e and type of

material.

3.9.1.4 Eff ic iency of Pr inc ipa l Spillways

I n t h i s s e c t i o n the e f f i c i ency of p r inc ipa l spil lways 3s q s c u s s e d . l n

general . I n Section 3.9.3 various modifications to p r i n c i p a l spil lways and

t h e i r e f f e c t on discharge q u a l i t y are presented. The s ize of the p r i n c i p a l

spil lway is designed to convey the discharge required to achieve removal of

sediment. The design discharge is determined during design of the sediment

pond.

c i p a l spil lways on discharge q u a l i t y is based on loca t ion , i n r e l a t i o n t o the

geometry of t h e pond, and flow c h a r a c t e r i s t i c s a t the i n l e t end of t h e

Once t h e design discharge is properly determined, the e f f e c t of prin-

Spi 11-y

The primary concerns i n loca t ion of t h e p r i n c i p a l spil lway are an ef fec-

?ive sur face area and s h o r t c i r c u i t i n g . As discussed in Sections 3.8.2 and

!S. 3, t h e length-to-width ra t io should be a minimum of 2: I , and a ratio of

5: l is recommended. As the d i s t ance between t h e spil lway and t h e pond i n l e t ! \decreases, t h e e f f e c t i v e sur face area decreases. As the e f f e c t i v e sur face

area decreases, the occurrence of short c i r c u i t i n g and turbulence is more

l i k e l y , and the overall e f f i c i ency of t h e pond is reduced.

pond e f f i c i e n c y is variable and based on site-specific conditions.

a n es t imate of the reduced e f f i c i ency can be based on the reduction i n effec-

t i ve -face area of t h e pond.

The level a t which the inlet of t+e p r i n c i p a l spil lway e.xists wi th in the

Because the sediment settles to the

The reduction i n

Bowever,

pond a f f e c t s the e f f i c i ency of the pond.

bottom of the pond, it is clear t h a t there w i l l be less sediment a t the sur- face than near t h e bottom of the pond. Thus discharging from near t h e sur-

face of the pond can improve t h e e f f ic iency . This characteristic has been

shown through use of f l o a t i n g weir devices and is fXschssed f u r t h e r in Section

3.9.3.2.

Due to the tu rbu len t nature a t the p r i n c i p a l spil lway inlet, scour and ' resuspension of s e t t l e d sediment is l ike ly .

s i o n around a spil lway is related tn the elevation of the s e t t l e d sediment.

As the l e v e l of settled sediment approaches the e leva t ion of the i n l e t of the

sp i l lway, scour and resuspension increase. Scour and resuspension are o f t en

The amount of scour and resuspen-

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3.51

associated w i t h dewatering.

avoid some resuspension and scour a t a l l t i m e s .

t e r i n g methods is presented in the following section.

When dewatering is required it is impossible to

Comparison of various dewa-

3.9.2 Dewatering Devices

Dewatering is usually required to drain the sediment pond between runoff

storms so adequate storage volrnae within the pond is maintained.

devices a re not necessary when draining below the pr incipal spillway is not

required. Several methods of dewatering are used, including perforated

risers, subsurface drainage, a s ingle perforation with associated use of a

skimmer baf f le or a type of gate valve, siphon arrangement attached t o the

riser and/or pumping.

Dewatering

The use of perforated s tandpipe or riser for dewatering i s required by

some states. However, sediment is carried out of the pond through the per- dorat ions because of resuspension of settled so l id s due to turbulence near the

perforat ions or because sediment is allowed to accumulate too high along the

riser barrel. \ U s e of a perforated riser is not recommended.

In the subsurface drain arrangement, a (four-inch) perforated plastic

t pipe network is l a i d in a trench in the bottom of the pond and covered with a

f a b r i c f i l t e r and sand as shown in Figure 3.15. The pipe is connected to the

riser and the pond is dewatered through the sand f i l t e r /per fora ted pipe

arrangement by gravity.

\

There are two advantages of a subsurface drain arrangement: ( 1 ) complete

dewatering of the s e t t l e d sediment I s possible to a id in removal and disposal,

and (2 ) no turbulence or resuspension of settled sediment is associated w i t h

t h i s method.

f i l t e r fabric due to the nature of the settled sediment, the permeability of

t h e settled sediment could r e su l t in exceedingly long dewatering t i m e , and the

added expense of i n s t a l l i ng t h i s type of pipe arrangement.

However, major disadvantages are clogging of the sand f i l t e r and

A s ingle perforation at the sediment cleanout leve l w i t h a skimmer-baffle

is shown in Figure 3.16.

and is capable of completely draining the pool to the sediment clean-out

level . With a skimmer, the perforation is non-clogging, f a i r l y easy t o

construct , and an e f f i c i e n t skimmer of surface debris. S o m e type of valve can

a l s o be used to gate the perforation which allows control over the desired

The s ingle perforation method is easy to construct

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3.52

PERFORATED PIPE IN TRENCH EDGE OF POOL

A L

NT

P L A N NOTE : S = 15' TO 25' -

* EMBANKMENT

1 I ,

1

PERFORATED PIPE IN TRENCH

0.5% MINIMUM GRADE J-

S E C T I O N A - A

FIGURE 3.15 SUBSURFACE DRAIN (EPA, 1977)

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3.53

L

i RISER

4" MAX. DIA. HOLE I, v

MAXIMUM SEDIMENT .

1 STORAGE 1"'" .L v LL

I ' I I SEDIMENT CLEANOUT LEVEL

(60% OF MAXIMUM SEDIMENT I , STORAGE LEVEL)

SINGLE PERFORATION CROSS SECTION

MAXIMUM SEDIMENT STORAGE LEVEL

CUT IN HALF LENGTHWISE

S I N G L E PERFORATION WITH SKIMMER E L EVATI 0 N

FIGURE 3.16 SINGLE PERFORATION OF RISER BARREL ( E P A , 1977)

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3.54

detention t i m e before dewatering the pond. With the perforation, gated

dewatering can be done a f t e r the runoff event is Over and the required removal

of sediment has Occurred, thus reducing the amount of sediment discharged

during dewatering.

With the siphon methods of dewatering, a ( f o u r h c h ) pipe siphon can be

subs t i tu ted fo r the s ing le perforation as described previously (Figure 3.17).

The length of siphon depends on the dewatering t i m e desired. In each case,

t h e i n l e t t o the siphon is placed a t the elevation of the sediment clean-out

l e v e l t o f a c i l i t a t e drainage without removing sediment. The siphon is a l s o an

e f f i c i e n t skimmer of surface debris, w i l l always drain the pond to the sedi-

ment clean-out level , and has a higher discharge capacity than the s ing le per-

fo ra t ion method with the same s i z e of opening.

For excavated ponds without a permanent pool, risers may not be prac-

tical.

pond.

Therefore, a self-priming or portable pump can be used to dewater t h e

The e f f e c t of dewatering devices on the discharge qua l i ty depends grea t ly

on the leve l of sediment in the pond. When sediment is allowed to accumulate

up t o the dewatering o u t l e t , the amount of scour and resuspension of settled

! sediment increases, decreasing the discharge quali ty. Therefore proper main- \

tenance and s e d h e n t removal can decrease the e f f ec t s of dewatering. It is

recommended t h a t the sediment be cleaned out when it reaches 60 percent of t he

design sediment storage.

devices, the a b i l i t y t o maintain the discharge qua l i ty can be r e l a t ed to the

level of cont ro l a t the dewatering device.

forations provide less control than a s ingle perforation with a b a f f l e skimmer or a siphon type arrangement. A gate on the dewatering opening provides t h e

most cont ro l by enabling the operator to vary the detention t i m e and physi-

c a l l y ver i fy t h a t the sediment has settled before dewatering the pond.

For properly designed and constructed dewatering

Perforated risers and s ing le per-

3.9.3 Principal Spillway Modifications

The purpose of modifications to atop i n l e t or pipe culver t spil lways is

t o reduce shor t c i rcu i t ing , eliminate turbulence, and thus increase t rapping

e f f i c i ency of the pond. After proper s iz ing, the effect iveness of a p r inc ipa l

spil lway is related to location within the pond, discharge po in t from within

t h e pond, and turbulent flow conditions a t the out le t . As discussed i n

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- . . . . . . . . . . . . - - . - . . . . . .

I/i1 HOLE AT SEDIMENT _ _ CLEANOUT LEVEL AND I " FROM 'END OF PIPE

3.55

- - . . .

RISER

FLOW 4

4" PIPE

SEDIMENT CLEANOUT LEVEL

!/: HOLE .AT SEDIMENT CLEANOUT LEVEL AND I" FROM END OF PIPE

I- I 2"

I I

. . A. SHORT SIPHON CROSS - SECTION

RISER -

FLOW .+--

u 41' PIPE

B. LONG SIPHON CROSS-SECTION

FIGURE 3.17 SIPHON DEWATERING METHODS (€PA, 19801 '

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. . . . . . . . . .. . . . . .. . - . __ . . .

3.56

Section 3.8.2.3, shor t c i r cu i t i ng and turbulence can reduce the trapping

ef f ic iency of the sedimentation pond. Studies of ex is t ing ponds have shown

poor pond performance as a r e su l t of short c i r cu i t i ng and turbulence a t the

o u t l e t of the pond (reference EPA, 1980; EPA, 1979; EPA, 1976). Several stu-

dies have made recommendations as t o what spillway modifications can help

improve the sediment removal eff ic iency of a par t icu lar pond.

discussion presents some of the more commonly used modifications and t h e i r

effects on discharge q u a l i t y .

The following

3.9.3.1 Weir Troughs

Drop inlets and cu lver t spillways are s ingle point ou t l e t s t h a t usually

c r e a t e shor t c i r cu i t i ng and resuspension.

t h e point discharge is a w e i r trough connected t o the out le t .

One modification t h a t elfminates

A w e i r trough

.discharges along the length of the w e i r and creates less turbulence than a

s ing le discharge point.

flow-through area and ef fec t ive surface area are a l so increased. Thus, by

By discharging from more than a s ing le point t he

I reducing turbulence and increasing the e f f ec t ive surface area, a weir trough

o u t l e t can provide improved discharge qua l i ty over that of a s ingle poin t

: discharge ou t l e t . Figure 3.18 shows a typical w e i r trough arrangement. I n . \

appl icat ion of a weir trough, s t r u c t u r a l i n t eg r i ty and maintenance are

required f o r e f f ec t ive operation and performance. Weir troughs are suscep-

t ib le to the same s t ruc tu ra l problems as baffles (see Section 4.2.2.2).

3.9.3.2 Float ing Discharge

Typical pr inc ipa l spillways are generally fixed and discharge from the

same point elevation. As the water surface elevation in the pond rises above

t h e spillway elevation, the concentration of sediment increases due t o the

s e t t l i n g of particles from the surface down. om this it is easy t o see that

the ndnFmum concentration within the pond w i l l be near or a t the water surface

elevation. Therefore, the discharge qua l i ty can be improved by discharging

from the surface of the pond.

f i e l d tested by OSM.

The r e s u l t s of the tes t showed settleable so l id s concentration in the

discharge to be consis tent ly less f o r the p ivota l e l b o w type o u t l e t as com-

pared to a typical perforated riser ou t l e t .

A variable elevation discharge o r i f i c e has been

Three tests were conducted using d i f fe ren t o u t l e t s izes .

This device w a s adapted from

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I

3 .57

P L A N

rTROUGH NORM

EMERGENCY SPILLWAY

PROTECTION RAP

E L E V A T I O N

FIGURE 3.18 WEIR TROUGH

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3.58

f loa t ing w e i r spillways or ig ina l ly suggested fo r management of f i s h ponds.

Figure 3.19 shows the device tes ted by OSM. The f loa t ing weir (or incl ined

am) allows removal of surface water regardless of surface elevation. The

f loa t ing w e i r consis ts of a riser pipe connected to the d ra in via a p ivota l

90° e l b o w designed and constructed to enable quarter rotat ion about the axis

of the drain.

weights and f lo t a t ion jugs t o maintain the o r i f i c e t o two to four inches below

t h e water surface (OSM, dra f t report) . Discharge is controlled by varying the

o r i f i c e size.

dewatering of the pond.

Buoyancy and submerged depth of the o r i f i c e are adjusted by

Another advantage t o this device i s t h a t it can a l so provide

3.9.3.3 F i l t e r i n g

As i n municipal water treatment, f i l t e r i n g of the discharge can grea t ly

improve the qua l i ty by removing f ine r sediments t h a t do not sett le out in the

pond.

riiser pipe f i l t e r s include cloth or f iberg lass wraps and gravel cones placed

: around the f i l t e r s . The f i l t e r wraps have been found to be f a i r l y e f f ec t ive

Riser pipe f i l t e r s have been used t o improve trapping efficiency.

i n trapping f i n e particles. €lowever, the f i l t e r wraps become clogged very

! rapidly (Oscaryan, 1975). This clogging may cause the water leve l in the pond \

to rise above the riser crest, thus negating the f i l t e r e f fec t .

Another inexpensive f i l t e r i n g mechanism is the use of s t r a w bales around

an out le t . Straw bales,

l i k e f i l t e r wraps, are effect ive in trapping f ine particlest however, they

require frequent maintenance.

This method is nrost applicable to pipe culverts.

3.9.3.4 Gated Spillways

A gated spillway gives the operator complete control of the discharge

from the pond.

p l e t e lys to re the runoff from a r a i n f a l l event.

f o r s e t t l i n g of the sediment, the gate valve is opened and the pond allowed to

dra in .

With the gate closed the pond is allowed to f i l l and com-

After an adequate time period

Gated spillways a re applicable only t o ponds t h a t do not have a constant

base flow o r ponds on ephemeral drainages.

s t o r e the e n t i r e runoff volume from the design ra i .n fa l l event. Often times

ponds with gated spillways are designed to s to re t w i c e the design runoff

The pond should be designed to

,

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3.59

I

, . - \

OR1 F ICE

/-

PIVOTAL ELBOW

(FIGURE 3.19 FLOATING WEIR

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3.60

volume due t o the nature of one or more r a i n f a l l events to occur within a

shor t t i m e period.

Sl ide gates or bu t t e r f ly valves a r e usually used for control at the

downstream end of the o u t l e t conduit.

end (within the pond) of the spillway, however, access must be provided when

t h e pond is full. Again, proper maintenance is necessary to keep the valves

Gates can also be used a t the upstream

i n good working condition.

Gated spillways have an ind i r ec t e f f e c t on discharge water qual i ty .

s t a t e d previously, a gated spillway enables the operator to increase the

detention time within the pond. Thus, t he longer the operator is able to

store the runoff i n the pond, the amre s e t t l i n g can take place, and thus

improve the discharge vater quality.

As

3.9.3.5 Anti-Vortex Devices

An anti-vortex device is used t o reduce turbulence a t the o u t l e t and to

I reduce the range of headwater depth where slug-flow act ion preva i l s and t o

a l l o w f u l l pipe flow to occur a t a lower headwater depth. Slug-flow ac t ion

f r e s u l t s from the induction of air into the conduit by entrance drawdown and b

vor t i ces immediately upsteam of the i n l e t .

discharge eff ic iency values may be reduced *by up t o 50 percent (scs8 1975).

If no anti-vortex device is used,

Anti-vortex devices include grills, racks, vertical plates, of f ixed

s o l i d hoods placed t o break up the vor t ices or t o prevent their formation

where they could feed air i n t o the conduit (Figure 3.201. In order t o be

ef fec t ive , M e hood or grill must be placed immediately above the entrance and

the area between the ins ide of the anti-vortex device and the outside of the

riser must be equal t o or greater than the mea ins ide the riser.

Another anti-vortex device is a th in , vertical plate normal to the cen-

t e r l i n e of t h e dam and firmly attached to the top of the riser.

p l a t e must equal the diameter of the riser plus 12 inches and height must equal t h e diameter of the barrel.

Length of the

3.9.4 Emergency Spillways

Emergency spillways are used to convey large flood events sa fe ly out of

the pond without overtopping or breaching the dam. For dams less than 20 f e e t

i n height or 20 acre-feet i n ac t ive storage, OSM requirements cal l for design-

Page 92: Design Manual for Sedimentation Control Through Sedimentation ...

3.G!.

! -

! \

L L - W - a a

m . t

z - I K

f, u w

F I I

a I U

2 .o

I- 0 w v)

-

w 0 - > W 0 X w I-

0 > I

z

a

F: a

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3.62

ing the combination of the pr incipal and the emergency sp i l lway t o safe ly con-

vey the runoff resu l t ing from the 25-year, 24-hour precipi ta t ion event. For

l a rger dams, the spillways must safely dicharge the runoff resul t ing from the

100-year, 24-hour precipi ta t ion event or a larger event as required by the

regulatory agency.

account the design Uscharge of the pr inc ipa l spillways. In general,

emergency spillways consis t of a crest section, a conveyance section, and a

discharge section. mere are two types of emergency spillways, overflow

spillway and channel spillway.

The design of the emergency spillway should take i n t o

Selection of the type of emergency spillway is dependent on the s o i l s and

Vegetated emergency spillways have higher protection climate of the site.

from damaging erosion than earth spillways.

where a vigorous grass growth can be sustained by normal maintenance without

I r r iga t ion .

They are applicable t o sites

Earth spillways are used in those areas where vegetative growth cannot be

I maintained. They are similar t o vegetated spillways but are designed fo r

later permissible ve loc i t ies and less frequent use. Normally, they w i l l

require more maintenance a f t e r a flow event. ! h Rock emergency spillways are applicable on undisturbed land where parent

bedrock material is present.

locities must be ascertained for the spec i f ic site based on a knowledge of

hardness, condition, durabi l i ty , weathering charac te r i s t ics , and s t ruc ture of

t h e rock formation.

Allowable frequency of use and permissible ve-

Excavated open channel spillways are t o have cut-and-fill slopes in earth

and rock which are stable against s l iding. If the dam is to be permanent, cu t

slope s t a b i l i t y is t o be evaluated for the long-term natural moisture con-

dit ions. Side slopes s h a l l be s t ab le f o r the material i n which the spillway

is constructed and s h a l l not be steeper than 3 horizontal t o 1 v e r t i c a l i n

ea r th and 1 horizontal t o 1 vertical in rock.

The exit channel should be s t r a igh t whenever possible. Slope of the

constructed exit channel should f a l l within the range established by discharge

requirements and permissible ve loc i t ies based on spillway material (Tables

3.5 and 3.6).

veloc i t ies .

Riprap may be used t o stabilize the spillway f o r higher design

Spillway discharge should be a t a point hwnstream from any part

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_..... .. ... .

3.63

Table 3.5. Permissilbe Velocities for Vegetated Spillways (So i l Conservation Service, 1981).

Permissible Velocity in fps

Erosion Ftesistant Soils2 Easily Erodible Soils2

Slope of Exit Channel i n percent

Slope of E x i t Channel i n percent

Vegetation 0 to 5 5 t o 10 o t o s 5 t o 1 0

Bermudagrass \ Bahiagrass

a

Buffalograss 5 Kentucky bluegrass

Smooth bromegrass 7 . Tal l fescue Reed Canarygrass

Sod-f orming grass legume mixtures

5

7

6

4'

Lespedeza sericea Weeping lovegrass Y e l l o w bluestem 3 . 5 N/A3 Native grass mixtures Annuals

6

5

4

2.5

5

4

3

N/A

SCS-TP-61

As defined i n TR-52

Use on slopes steeper than 5 percent is not recommended.

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...

3.64

Table 3.6. Permissible Spillway Velocities a f t e r Aging l .

Original Material Excavated Feet per second

Fine sand, non-colloidal

Sandy loam, non-colloidal

S i l t loam, non-colloidal

Alluvial silts, non-colloidal

Ordinary firm loam

Volcanic ash

3.00

3.50

3.50

3.50

_ . . , Fine gravel 5.00

' St i f f clay, very colloidal

Graded, loam to cobbles 8 non-colloidal

! Alluvial s i l t68 C O l l O i d a l \

Graded, S i l t to cobbles, C O U O i & l

Cobbles and shingles

Coarse gravel, non-colloidal

Shales and hardpans

5.00

5.00

5.00

5.50

5.50

6.00

6.00

Recommended i n 1926 by Special Committee on Irrigation Research, American Society of Civil mglneers.

Values shown apply to water transporting colloidal silts.

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3.65

of the ear th embankment. If this is not prac t ica l , a wing d ike should be

constructed t o prevent flows from encroaching on the downstream toe of the

dam.

Elevation of the crest of the emergency spillway is dependent upon the

type of sp i l lway t o be used. In a l l cases, the design depth of water over the

spillway must be a minimum of one foot below the elevation of the settled

height of the &am.

emergency spillway for events less than the lO-year, 24-hour prec ip i ta t ion

event as long as ef f luent l imitations are achieved. Therefore, t h e elevation

difference between the principal spillway and the emergency spillway is site

dependent.

design storm.

The current OSM regulations allow discharge through the

Lucal state requirements may specify f u l l containment of the

3.9.5 Erosion Control Below Spillways

3.9.5.1 General

During operation, the ou t l e t discharge from the pr incipal spillway of a

sedimentation pond is a highly concentrated, fast-moving jet (with its asso- c i a t ed turbulence) t h a t has considerable poten t ia l f o r causing damage

downstream.

erosion downstream of the s t ructure; undenuine the ou t l e t ; form a wide, deep

acour hole i n the o u t l e t area; and possibly endanger the aafety of the dam

embankment. Protection is necessary to prevent the jet and i ts associated

turbulence from causing erosion u n t i l the jet flow has dissipated to a d l d e r ,

non-scouring flow. The m6t conanon method of protect ing the channel from ero-

sive forces caused by high ve loc i t ies and turbulent flow is to l i n e the chan-

nel w i t h riprap. I n this manner, the channel is protected from erosion u n t i l

the outflow jet has dissipated to a milder flow condition of decreased ve-

l o c i t y and turbulence.

! \

If protect ive measures are not taken, pond discharge can cause

Where the pond w i l l discharge onto an area which had not previously been

exposed to flow, there is the likelihood of severe erosion from the flow over

loose so i l s . On the other hand, if the pond discharges i n t o a well-armored

na tura l channel, the downstream erosion a f f ec t s w i l l be minimal.

the o u t l e t and the channel at the o u t l e t should be straight so Uischarge does

no t impinge on any of the channel banks a shor t distance Bownstream. By pro-

per ly choosing an ou t l e t location and geometry, the amount of downstream ero-

Alignment of

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3.66

sion is minimized.

below principal spillways are discussed in the following sections.

The application and design of r iprap f o r erosion control

3.9.5.2 Riprap

Riprap consis ts of a layer of d i scre te fragments of durable rock

possessing su f f i c i en t s i z e t o withstand the dynamic, erosive forces generated

by the flow of water. Riprap should be hard, dense, and durable t o withstand

long exposure to weathering. In surface mining operations, r ip rap is t he mos t

common and economical means of preventing erosion of channel bed and banks

upstream and par t icu lar ly downstream of sedimentation ponds where there is a

high erosive poten t ia l due to contraction of flow, flow alignment, changes i n

slope, and etc. When the material is of su f f i c i en t s i ze , shape, gradation,

and hardness, r iprap is excellent erosion protection.

The important fac tors to be considered i n designing rock r iprap protec-

t i o n are: rock durabi l i ty , density, s ize , weight, shape, and angularity;

direction and maganitude of the velocity of flow near the rockt bed or bank

slope; and angle of repose of the rock. In addition, the desired l eve l of

.p ro tec t ion may not be provided by the r iprap i f design criteria concerning

! rock gradation, placement, r ip rap thickness, and f i l t e r design are not \

considered . There a re many means and methods by which r iprap protect ion can be

constructed and placed.

methods of placement:

Following is a categorization of r iprap materials and

- Dumped r iprap

- Hand-placed r iprap

- Wire-enclosed r iprap (gabion)

- Grouted r iprap

When available i n su f f i c i en t s ize , dumped rock riprap is usually the most

economical material f o r bank protection. Dumped rock r iprap has many advan-

tages over other types of protection, including i ts f l e x i b i l i t y and the ease

of local damage repair .

manner but is not complicated.

embankment, rocks can be dumped d i rec t ly from trucks from the top of the

embankment.

dropping down the slope in a chute or pushed downhill with a bulldozer.

Construction must be accomplished in a prescribed

I f r iprap is placed during construction of the

To prevent segregation of s izes , rock should never be placed by

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3.67

Dumped r iprap can be placed with a minimum of expensive hand work.

appearance of dumped r iprap is natural , and after a time, vegetation w i l l grow

between the rocks: Final ly , i n temporary channels when usefulness of the pro-

t ec t ion is finished, the rock is salvageable.

The

m p e d r iprap is extensively used on surface mine sites due to the

ava i l ab i l i t y of rock and the ease of placement.

f o r the proper s t a b i l i t y and erosion control.

r iprap are avai lable (Simons, Li ti Associates, fnc., 1982; ~ a r f i e l d , 19818

Bureau of Reclamation, 1977).

Sizing of r iprap is Important

Several references f o r s iz ing

3.10 Summary

Sedimentation pond design is based upon meeting e f f luent l imitat ions f o r

t h e design storm runoff event.

pond is determined such t h a t e f f luent l imitat ions are sa t i s f i ed .

configuration i s then determined t o provide the required s e t t l i n g conditions.

.'This requires an i n t e ra t ive process.

lished, the principal spillway is sized to produce the required detention time

and the emergency spillway is then shed so that the combination of pr inc ipa l

and emergency spillways are adequate. The f i n a l step in the design process is

A particle s i z e t h a t must be removed in the

The pond

\ Once the pond configuration is estab-

! \

to check the e f f luent f o r base flow conditions a f t e r the pond is operational.

The design example i n Chapter VI presents h o w the previous sect ions are

in t e r r e l a t ed in the design process.

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4.7

81 ~ORIGINAL STREAM BANK

A

L A

-1

s..J

PLAN VIEW ,

SECTiON B-B

"""~-~~~ FIGURE 4. '3 LOOSE ROCK

(II ~ ~(,\~~~T ~~~VI~~ CHECK DAM

'Q7~'

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4.20

POINTS A SHOULD BE HIGHER THAN POINT B

PROPER PLACEMENT OF STRAW IN DRAINAGEWAY (OSM, 1981

BALE BARRIER

\. 10

FIGURE 4.10

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. . . . . . . . . .

TABLE OF CONTENTS CHAPTER V

Page

V. MAINTENANCE FOR WATER QUALITY CONTROL

5.1 Pond Maintenance . . . . . . . . . . . . . . . . . . . . 5.1

5.1.1 Accessibility . . . . . . . . . . . . . . . . . . 5.2 5.1.2 Monitoring/Maintaining

Sediment Storage Volume . . . . . . . . . . . . . 5.2 5.1.3 Bank Stability and Maintenance . . . . . . . . . . 5.3

5.1.3.1 Vegetation Stabilization . . . . . . . . 5.3 5.1.3.2 Riprap Stabilization . . . . . . . . . . 5.4 5.1.3.3 Rill and Gully Control . . . . . . . . . 5.4

5.1.4 Maintenance of Inlet and Outlet Structures . . . . 5.4

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V. MAINTENANCE FOR WATER QUALITY CONTROL

5. I Pond Maintenance

The useful life of a pond is partially a function of the maintenance of

the pond and the embankment.

serve the structural integrity of the dam to ensure that essential design

features of the outlets are maintained, and to ensure adequate storage and

capacity for the pond.

major problems.

program than it is to make repairs after an extended period of negligence.

Tie primary purposes of maintenance are to pre-

Hinor problems should be repaired before they become

It is usually less costly to implement aregular maintenance

Written instructions for maintenance and operation of the structure (and

any r e w r e d monitoring equipment) should be prepared as part of the design. These instructions should establish the frequency, and describe the nature of,

inspections.

inlet and outlet structures.

gates, specific instructions should be given regarding the operation of the

gates

Instructions should also be provided for routine maintenance of

If a spillway is controlled by manually operated

’ ‘1 A record of all inspections and any maintenance performed. on the pond should be kept. The date, last major rainfall, sediment storage level, and

any problems should be noted. If maintenance has been performed, the date and

type of repair should be noted. These two records w i l l aid the operator in

determining if chronic problem areas exist in the pond design.

t \

After an area has been stabilized and successfully revegetated, sedimen-

tation ponds may be removed.

Sedimentation pond is usually addressed in the d n h g and reclamation plan

submittal.

this chapter.

The decision for removal or retention of a

The options available to the operator will be discussed later in

Repairs of embankments and emergency spillways are extremely important

for the proper functioning of the sedimentation pond system.

regions, embankments are usually stabilized by mulching and then by

establishing a good vegetation cover.

establishment of a vegetative cover, riprap or mulching may be used. The use

of either a vegetative cover or riprap is not specifically required by OSM.

However, both of these measures will aid in stabilizing the embankment or

spillway and will usually reduce the required maintenance. The design

In humid

Where conditions do not allow the

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- ._ . .- . . _-. . . .. ... ._

5.2

engineer should check w i t h the regulatory authority t o determine regionally

accepted methods t o s t a b i l i z e the embankment and spillway.

Studies invest igat ing the performance of exis t ing sediment ponds f o r the

control of erosion from surface mining operations .have shown- t h a t once the

pond has been constructed and operated maintenance of sediment level , i n l e t s

and ou t l e t s is one of the main causes of poor pond performance (EPA 1980, EPA

1979).

5 -1 .1 Accessibi l i ty

Location of t he pond is of prime importance. The pond should be

accessible f o r construction, k n i t o r i n g , and maintenance.

maintenance should be considered during the planning of the pond.

engineer should consider the type of equipment used f o r construction and main-

tenance and the r o o m required fo r this equipment to function e f f i c i en t ly . I n

a ' w e l l designed pond, the heavier sediment w i l l *posit near the i n l e t of the

Accessibi l i ty for

The design

- . pond. Therefore, access t o the i n l e t end of the pond is essent ia l . A w e l l i ' constructed and regular ly maintained road is very helpful for providing proper

maintenance, including sediment removal, r iprap repair, or embankment repair.

Adequate access ib i l i t y is v i t a l if chemical f locculation is being used. : \

5.1.2 Monitoring/Maintaining Sediment storage Volume

Most sedimentation ponds are designed with su f f i c i en t annual sediment

storage volume for a number of years. ow ever, designing for excess s torage

volume aOes not guarantee that this storage volume w i l l not be exceeded by a

l a rge storm event.

I n order to ensure adequate storage volume, the avai lable sediment

storage volume i n a pond must be monitored.

is helpful f o r monitoring.

clean-out l eve l is t o i n s t a l l a s t a f f gage i n the pond and to determine the

sediment accumulation l eve l t h a t requires clean out. Most design manuals

(Virginia Soil and Water Conservation COmmission, 1980) recommend clean out

when the accumulated sediment reaches 60 percent of the design sediment

s torage volume. An acceptable schedule should be established by the design

engineer, operator, and regulatory agency. It is the respons ib i l i ty of the

operator t o ensure t h a t the schedule is followed.

Pre-defining the clean-out leve l

One of the simplest means of pre-defining the

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5.3

Clean out of sediment is usually handled by a small dragline, clamshell

bucket, or a backhoe f o r w e t ponds and by a front-end loader fo r dry ponds.

If a front-end loader is used, the ponds should be su f f i c i en t ly dr ied i n order

t o support t he weight of equipment.

by dragl ines operating from the banks, cleaning is =re d i f f i c u l t .

cases dredging may be necessary.

professionals experienced i n this procedure.

For large ponds which cannot be cleaned

I n such

Dredging w i l l of ten require the service of

Sediment removed from a pond is usually incorporated i n t o the s p o i l

material. I f the removed sediment I s found to contain acid- or toxic-forming

materials, the sediment w i l l have to be disposed of in a more control led

manner.

U s e of t h i s material as a topsoi l subs t i t u t e may be very useful f o r

underground coal mining a c t i v i t i e s where the amount of avai lable topso i l is

limited. I f chemical f locculat ion is used to improve the eff ic iency of the

pond, use of the accumulated sediment w i l l probably not be su i tab le as a top-

soil subs t i t u t e because of the possible toxic e f f e c t s of the chemicals.

Sediment removed from a pond may be used as a subs t i t u t e for topsoi l .

1 Chemical and physical analyses are needed before any material can be used as a

s u b s t i t u t e f o r topsoi l .

5.1- 3

Depending on the pond surface area, location, and local climate, main-

Bank S t a b i l i t y and Maintenance

tenance of s ide slopes is important. Wave action, excavation, and removal of

vegetation w i l l promote the erosion of side elopesand subsequent increase i n

solids concentration of the pond.

are dbcussed.

Several methods and maintenance procedures

5.1 . 3.1 Vegetation S tab i l iza t ion

Maintenance of vegetative measures should occur on a regular basis, con-

s i s t e n t with favorable p lan t growth, soil , and climatic conditions. This

involves regular seasonal work for f e r t i l i z i n g , liming ( i f appl icable) ,

pruning, f i re controls , reseeding, and weed and pest control. open channel

spil lways are subject to rapid infes ta t ion of w e e d s and woody plants .

, should be eradicated or cu t back since they of ten reduce drainageway effi-

These

ciency. Well-maintained vegetation w i l l provide a comfortable margin of ero-

s i o n cont ro l .

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5.4

5.1.3.2 Riprap Stab i l iza t ion

Large storms may displace the r iprap and allow erosion of the underlying

material. Displacement and damage to the r iprap w i l l usually occur where flow

v e l o c i t i e s are highest. Typically, discharge s t ruc tures and spillway areas

experience the mos t damage. I f displacement of the r iprap has occurred, t h e

r ip rap f i l ter blanket should be checked fo r damages. Repairs should be made

as soon as prac t ica l . These areas should be checked fo r erosion or s i l t i n g of

t h e channel i n order to assess the impact of the carrying capacity of the

channel.

Riprap is commonly used to control the upstream embankment of the dam

from damages due to wave action. I f r ip rap is damaged, the s i z e of the r ip rap

may have to be increased.

damages wherever the r iprap is displaced.

The r iprap f i l ter blanket should be checked f o r

5.1.3.3 Rill and Gully Control

Concentrated water flow w i l l cause rills and gu l l i e s on the embankment

slope. Vegetation and r iprap will often s top their development; however, a

c e r t a i n amount of erosion is expected on any earth embankment.

densi ty a t which rills and gu l l i e s become uncontrollable is d i f f i c u l t to

define and is dependent on the s o i l s , climate, and land use of the local area.

OSM requires rills or gu l l i e s deeper than nine inches in reclaimed areas to be

f i l l ed , graded, or otherwise s t ab i l i zed (i-e.., s t r a w mulch) (30 CFR 816.106).

U s e of this r u l e as a guideline for embankment s t ab i l i za t ion is suggested and

w i l l preclude the formation of large gul l ies .

more s t r ingen t maintenance program.

: The s i z e and \

Sta t e agencies may require a

5.1.4 Maintenance of I n l e t and Oultet Structures

Maintenance of i n l e t and o u t l e t s t ruc tures is M extremely important

requirement i n achieving e f f ec t ive sediment control. A l l water-handling

s t ruc tu res should be inspected after every major storm. Erosion damages

requi re prompt repair to prevent fur ther damage and to help prevent similar

damage i n t h e future.

Sediment buildup i n the i n l e t section and behind check dams and f i l ter

barriers should be checked. Sediment and other debris removed from these

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5.5

areas should be disposed of in a manner t h a t w i l l prevent sediment from being

carried back in to the waterways a t the mine. Possible use of t h i s material as

a subs t i t u t e s o i l medium should be considered.

barriers should be replaced before they become clogged or overtopped.

Straw bales and sandbag

When vegetation is used to s t a b i l i z e the area or as a vegetative f i l t e r ,

t o p dressing with f e r t i l i z e r is usually required. F e r t i l i z e r w i l l help keep a

dense s tand and provide growth of desirable plants. Areas where f a i l u r e s have

been experienced i n the establishment of vegetative protection m u s t be

promptly treated. Timely maintenance w i l l reduce cos ts in t he long run. I f

t h e area continues t o exhibit vegetation f a i l u r e due to e i t h e r high or pro-

longed w a t e r flow, mote extensive s t ab i l i z ing measures such as r iprap may be needed.

Pipe culver t spillways should be examined fo r s t ructural s t a b i l i t y both

a t the i n l e t and a t the discharge point. Trash racks should be cleaned of

debris. I f gates or valves are wed, they should be t e s t ed to see t h a t they

- \ work freely. I

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TABLE OF CONTENTS CHAPTER VI

Page

. . Vf. DESIGN PROCEDURES AND MAMPLE

6.1 Design Procedure . . . . . . . . . . . . . . . . . . . . 6.1 6 . 2 DesignBcample . . . . . . . . . . . . . . . . . . . . . 6.4

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. . . . . . . . . -

LIST OF TABLES CHAPTER VI

Page

Table 6.1 Development of Influent Size Distribution . . . . . . . 6.6

Table 6.2 Summary of Sedimentation Pond Design . . . . . . . . . . 6.20

LIST OF FIGURES CXAPTER V I

Page

F i g u r e 6.1 Sedimentation pond site . . . . . . . . . . . . . . . . 6.5

F i g u r e 6.2 Sediment s i ze distribution . . . . . . . . . . . . . . 6.7

Figure 6.3 Stage-storage area and stage-storage curves of Pond Si te No. 1 . . . . . . . . . . . . . . . 6.11

Figure 6.4 Sedimentation Pond S i t e No. 2 . . . . . . . . . . . . . 6.15 : \ Figure 6.5 Stage-storage area and stage-storage

curves of Pond Si te No. 2 . . . . . . . . . . . . . . . 6.17

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V I . DESIGN PROCEDURES AND MAMPLE

6.1 Design Procedure

Step 1. Site selection (Section 3.1)

The sedimentation pond loc t ion is selected considering the f a c t o r s presented i n Section 3.1.

The peak inflow r a t e and runoff volume f o r the design storm event are determined.

Step 3. I n f l u e n t sediment size d i s t r i b u t i o n (Section 3.2.2.1)

The s i ze d i s t r i b u t i o n of the inflowing sediment is required. Where e x i s t i n g information from t h e mine site or nearby mine sites is a v a i l a b l e , it should be used. When there is no e x i s t i n g data, a s i z e d i s t r i b u t i o n can be developed using information from so i l

. surveys.

! Step 4. Sediment y i e l d (Section 3.2.2.2)

Determine the annual sediment y i e l d and the storm sediment y ie ld . \

Step 5. Inflow suspended s o l i d s concentration (Section 3.2.3)

Using t h e storm sediment y i e l d and the storm runoff volume, deter- mine the average in f luen t suspended solids concentration.

Step 6. Settleable solids concentration (Section 3.5)

Develop t h e settleable sediment s i z e d i s t t i b u t i o n (particles > 0.001 mm) f r o m the in f luen t sediment size d i s t r i b u t i o n . Select a particle size t o be removed i n t h e pond. Determine t h e t rapping e f f i c i e n c y from the settleable sediment size d i s t r ibu t ion . Determine the average e f f l u e n t suspended s o l i d s concentration using t h e t rapping e f f i c i ency , sediment y i e ld , and runoff volume (Equation 3.6). Calcula te the settleable solids concentration (SS) from Equation 3.5. I f SS > 0.5 m l / l , select a smaller s i z e particle and repeat procedure. I f SS E 0.5 d/l, go t o Step 7 and design pond to remove selected particle s i ze . If SS < 0.5 m l / l , select a l a r g e r s i z e particle and repeat procedure.

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6 . 2

Step 7 . Available storage volume (Section 3.7)

Develop stage-storage curve for sedimentation pond location. Determine the required sediment storage volume and the corresponding depth. Determine the available detention storage depth by

I) = embankment height - (embankment settlement + required

freeboard + emergency spillway depth..+ sediment

storage depth)

Determine the available detention storage volume above the sediment storage depth from the stage-storage curve.

Step 8 . Required storage volume (Section 3.6) . A s s u m e a detention storage depth and determine the required deten- t i o n t i m e fo r the design particle s i z e from Figure 3.10. t he t i m e base of the inflow hydrograph. storage volume from Figure 3.7. from Figure 3.8. avai lab le storage volume. If t he avai lable storage volume is less than the required storage volume, e i t h e r

Calculate Determine the required

Determine the required outflow rate Compare the required storage volume t o the

(a) Increase the embankment height and determine the new avai lable storage volume. Repeat Step 8.

(b) Excavate ehe pond side slopes and a develop new stage- storage curve. R e p e a t Step 8.

(c ) Construct a pond downstream and re turn t o Step 1.

I f the available storage volume is larger than the required storage volume, check the required surface area (Section 3.8.3.1). If t he measured surface area is less than the required surface area, (1) excavate pond side slopes or ( 2 ) r a i s e pr incipal spillway crest. I f the measured surface area is greater than the required surface area, check length-width r a t i o (Section 3.8.2.2) and ca lcu la te required length t o settle design particle size (Section 3.8.1). I f the length is not large enough, increase the flow length (Section 4.2). If the length criteria is met , check scouring (Section 3.8.3.4). I f t h e scouring velocity is smaller than the horizontal velocity, increase the depth and return to Step 7 . is greater than the horizontal veloci ty , go t o Step 9.

I f the scouring veloci ty

Step 9. Principal spillways (Section 3.9.1)

Select pr incipal spillway type and design for the peak outflow rate and the corresponding head.

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6.3

Step 10. Emergency spillway (Section 3.9.4)

Select emergency spillway type and design the spillway system to pass the peak discharge from the 25-year, 24-hour runoff event.

Step 11. Erosion control b e l o w the spillways (Section 3.9.5)

Size the riprap b e l o w the principal spillway.

Step 12. Check base f l o w conditions after pond is operational.

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6 . 4

6.2 Design Example

The design example is to i l lus t ra te the procedures of s i z ing a detention

pond t o meet the e f f luen t Standard. Design of pr inc ipa l spillway (Step 9 ) ,

emergency spillway (Step 101, erosion control below the spillway (Step 111,

and base f l o w condition (Step 12) are not Included because methodologies can

be found in various texts and references.

Step 1. Considerations f o r sedimentation pond select ion have been presented i n Section 3.1. I n this design example, a si te is preselected and presented i n Figure 6.1.

Step 2. Hydrology

Design Event

10-year, 24-hour

25-year, 24-hour

' Step 3. Sediment s i z e d i s t r ibu t ion

Q I (cfs)

77

91.5

V (acre-feet)

2.31

3.35

For t he purpose of i l l u s t r a t i n g the meghodology, develop in f luen t s i z e d i s t r ibu t ion from Soil 'Surveys (Table 6.1).

55 percent of area is sand 4 0 percent of area is sandy clay

5 percent of area is s i l t y c lay

Figure 6.2 presents the inf luent s ize dis t r ibut ion. t i o n is extended with a s t r a i g h t l i n e to a particle size of 0.001 mm.

The dis t r ibu-

Step 4. Sediment y i e ld (use USLE for annual sediment y i e ld and MUSLE f o r storm sediment y i e ld )

Annual sediment y ie ld = 115 tons Storm sediment y ie ld = SO tons (looyear, 24-hour stom)

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... .

6 . 5

FIGURE 6. I SEDIMENTATION ' POND SITE

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6.6

!

A % I 0 , L

w m 0 . 0 - . - -

A

c;-

A

c 1

cc.

I--

8

1 m N

1 m N

? (Y

m 0

0

N c:

m m

c . c

. ( Y

hl 0 x N

8 0' V

x 0 c

m

I

I

I

0 . 2

m N

2 (Y

Q) *\

* 9 c . c

K e4 . -

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...... .

6.7

n

E E Y

w

W -I 52 c

.6 0

6 J cd w K - J c3 -

1J3NId lN33C13d

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6.8

Step 5. Average inflow suspended so l ids concentration

15,926 mg/l 50 tons x 2000 lb/ton x l o 6 = 2 3 2.31 ac-ft x 43,560 f t /ac x 62.4 Ib / f t cI

Step 6. Settleable so l id s concentration

From Figure 6-28. 21 percent of the inf luent s i ze d is t r ibu t ion is non-settleable (< 0.001 mm). The redis t r ibuted se t t l eab le s i z e d is t r ibu t ion is a l so presented i n Figure 6.2.

Star t with d = 0.011 m.

From Figure 6.2, trapping efficiency E = 0.82 and f rac t ion of settleable solids K = 0.798 using Equation 3.6.

(1.0 - 0.82) x 0.79 x 50 tons x 2000 lb/ton

2.31 ac-ft x 43,560 f t /ac x 62.4 l b / f t 2 3 c* = 0

x l o 6 = 2265 mg/l

The s e t t l eab le so l ids concentration is determined as follows:

t b

P a r t i c l e Influent Effluent (di/O- 01 1 l6 Size Range Mean Size 4 4. x Axi

( a i l

0.001 -0.002 0.0014 0.05 0.28 1.19 x 10'6 ,

0.002 -0.0038 0.0028 0.05 0.28 7.62 x 10-5

0.0038-0.0072 0.0052 0.05 0.28 3.12 10-3

0.0072-0.011 0 . 0089 0.16 4.49sx 10-2 - 0.03 - 1.0 0.048 0.18

From Equation 3-58 settleble solids concentration can be calculated as

2265 m /1 ss = l120 mi,d [ (1.0 - l = O ) + 0.0481 0.10 ml/l < 0.5 m l / l

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6.9

Try with a larger size of settleable s o l i d s , d = 0.020 nnn.

% of settleable s i z e d i s t r i b u t i o n smaller than 0.011 m o % of settleable size d i s t r i b u t i o n smaller than 0.020 rmn x =

0.18 0.22 =- = 0.818

From Figure 6.2, E = 0.78 and K = 0.79

(1.0 - 0.78) x 0.79 x 50 tons x 2000 -/ton

2.31 ac- f t x 43,560 f t /ac x 62.4 l b / f t 2 3 c =

x lo6 = 2768 mg/l

2768 mg/l 1120 m g / m l [(loo - 0.818) +‘(0.818 x 0.048)l ss =

= 0.55 ml/l > 0.5 m l f l

Try with a smal le r s i z e of settleable solids, d = 0.018 mn.

% of settleable s i z e d i s t r i b u t i o n smaller than 0.011 nun % of s e t t l e a b l e size d i s t r i b u t i o n smal le r than 0.018 nnu

E

‘0

0.18 0.215 = - = 0.837

From Figixre 6.2, E - 0.785 and K - 0.79 (1.0 - 0.785) x 0.79 x 50 t ons x 2000 &/ton

2.31 ac- f t x 43,560 f t /ac x 62.4 lb/ f t 2 3 C =

x lo6 = 2705 mg/l

ss = 2705 1 120 mg/l mg/ml [(loo - 0.837) + (0.837 x 0.048)J

= 0.49 ml/l E 0.5 ml/l

I n order t o m e e t the 0.5 m l / l standard, the pond is designed to remove a l l particles equal to and larger than 0.018 ma.

Step 7. Available s to rage volume

Based on the selected s i te (Figure 6.l), the stage-storage curve is obtained using the rnethodology described in Section 3.7 and is pre- s e n t e d i n Figure 6.3. f o r three t i m e s the annual sediment y i e l d

The sediment s torage volume should provide

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6.10

I

3 yr x 115 ton/yr x 2000 %/ton p: o.23 ac-ft P

3 2 70 l b / f t x 43,560 f t /ac vs

From Figure 6.3, depth of sediment = 4.2 f t

Total maximum embankment height (assumed) = 16 f t Allow 5 percent settlement (0.5 x 16) = 0.80 f t

Required freeboard = 1.0 f t Allow 2.5 f t for emergency spillway 2.5 f t

Haximum depth of sediment storage = 4.2 f t

Available detention storage depth (Di) 16 - ( 0 . 8 0 + 1.0 + 2.5 + 4.2) = 7.5 f t

Stage a t maximum detention storage depth 7.5 + 4.2 = 11.7 f t

The avai lable storage volume is described by taking the difference between the storage volume at the stage of the maximurn detention s torage depth and the storage volume a t the elevation of t he prin- cipal spillway.

Pram Figure 6.3, avai lable detention storage volume 1.71 - 0.23 = 1.48 ac-ft

t \

Step 8. Required storage volume

The t o t a l depth (+I of the pond is used i n the following com- putat ions and is equal t o the sum of the sediment storage depth, detention storage depth, and the permanent pool depth ( i f used).

T h e base of inflow hydrograph

2 2V 2 x 2.31 ac-ft x 43,560 f t /ac

*I o.73 hours

T b a - = 3 77 f t /sec x 3600 secfhx

For d = 0.018 mm and D = 11.70 f t , T

TD = 4.50 hours (From Figure 3.10)

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6.11

15

10 '

0

SURFACE AREA ( F T ~ )

1.0 2.0

STORAGE VOLUME (ACRE - FEET)

FIGURE 6.3 STAGE - SURFACE AREA AND STAGE - STORAGE CURVES OF POND SITE N0. I

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6.12

i

For Tb = 0.73 hours and TD = 4.50 hours,

- r 0.950 ( P r o m Figure 3.7) V

QO

QI - = 0.05 (From Figure 3.8)

S = 0.950 x 2.31 ac-ft = 2.19 ac-ft > 1.48 ac-ft

Since the pond location w i t h an assumed 16 ft embankment height can- not provide the required storage volume, t he designer has three a l te rna t ives .

(a) Increase the detention storage depth and r e t u r n t o Step 7 and determine the new available storage.

(b) Excavate the pond side slopes no steeper than 2 horizontal t o 1 vertical and re turn to Step 7 and develop new stage- storage curve.

( c ) Construct another pond downstream.

For purposes of i l l u s t r a t i n g multiple pond design, alternative ( c ) is chosen.

t Step 8c. Determine the sediment removal i n the upstream pond \

S 1-48 U-ft o.64 - I c V 2.31 ac-ft

S V

TD = 0.4 hours

For - = 0.64 and Tb = 0.73 hours,

(Figure 3.7)

Qo

QI - L: 0.360 (Figure 3-81

L

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6.13

. . 1

For TD = 0.4 hours and DT = 11.70 f t ,

(Equation 3-10) DT 2.254 d2 = 3600 TD

11.7 f t 2.254 x 0.4 hrs x 3600 aecfhr

d=j7- 2.254 X 3600 TD =

e 0.060

Therefore, only particles larger than 0.060 m can be removed i n t h e first pond based upon idea l s e t t l i n g .

Check surface area

A (measured a t depth - 4.2 f t ) = 3490 f t 2

Qo = 0.360 x 77 CfS = 27.7 CfS

From Equation 3.14, required surface area

A 1.2 X 27'7 ft3/sec = 4096 ft2 > 3490 f t 2 rep 2.254 x (0.06012

Due t o nonideal s e t t l i n g conditions, the smallest particle which w i l l be removed using the ava i lab le surface area is /--

I 0.065 ZUU 2.254 x A 2.254 X 3490 8 . .

Check length-to-width ratio

L (measured a t depth = 4.2 f t ) * 130 f t

130 = 4.9 > 2.0 L W 26.8 f t - =

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.. .

6.14

Check Scouring

V t i = - = *O 27*7 ft3/sec * 0.14 fps (Equation 3.18) WD 26.8 f t x 7.5 f t

V = 1.67 dli2 = 1.67 x (0.065)'/2'= 0.43 fps (Equation 3.17) sc

The smallest particle which w i l l be removed i n the f i r s t pond is 0.065 mn which corresponds t o a removal e f f ic iency of 69.5 percent (Figure 6.2).

Design of Downstream Pond (repeat Steps 1 through 8 )

Step 1.

step 2.

S i t e selection (Figure 6.4). Se lec t a second pond j u s t downstream of t h e f i r s t pond.

Eydrology (10-year, 24-hour design event)

10-year, 24-hour design event

F i r s t pond (%I

Q ( c f s )

27 i7

V (acre-feet)

2.31

Additional contr ibut ing area (Qr) 22.0 0.39

Design parameter (QI Second pond) 49.7 2.70

25=year, 24-hour design event

First pond (h)

V (acre-f eet 1

3.35

Additional contr ibut ing area (Qs) 26.1 0.56

Design parameter 117.6 3.91

Notes: 1. The design inflow peak flow for second pond should be deter- mined by reservoir rout ing and added to t he hydrograph of the addi t iona l contr ibut ing area. For i l l u s t r a t i o n of eedimen- t a t i o n pond design, the peak outflow from the f i r s t pond is added d i r ec t ly t o the peak f l o w of t h e addi t iona l contr ibut ing area.

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I

. * _ . . . . . ..-.

G .15

FIGURE 6.4 SEDIMENTATION POND SITE N0.2

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6.16

2. I f t h e r e is any permanent pool s torage i n the first pond, the stored volume should be subt rac ted from the t o t a l volume for designing t h e second pond.

Step 3. Sediment size d i s t r i b u t i o n

U s e the same sediment size &.istribution.as the f i r s t pond.

Step 40 Sediment y i e l d (use USLE f o r annual sediment y i e l d and MUSLE for storm sediment y i e l d )

From add i t iona l con t r ibu t ing drainage area (10 acres)

Annual sediment y i e l d = 19 t ons S t o m sediment y i e l d - 8.3 t o n s

Step 5. Inflow suspended s o l i d s concent ra t ion

Since the design is to remove a l l particles l a r g e r t h a n 0.018 m, c a l c u l a t i o n of t h e inflow suspended solids concent ra t ion is not required.

.A

I Step 6. Settleable solids concent ra t ion

Must remove a l l particles Larger than 0.018 mn. t \

Step 7. Available storage volume

The stage-storage curve is presented i n Figure 6.5. Since t h e first pond has a t rapping e f f i c i e n c y of 69.5 percent , the sediment storage volume required & t h e second pond is equal t o 30.5 percent of the upstream sediment y i e l d plus the sediment y i e l d from the a d d i t i o n a l con t r ibu t ing drainage area.

(115 ton/yr x 0.305) + 19 ton/yr] x 3 yr x 2000 lb/ton v = I 8 70 lb/ft3 X 43,560 f t 2 / a C

= 0.11 ac - f t

From Piguke 6.5, depth of sediment = 1.3 f t I

Using the same embankment se t t lement , required freeboard, and emergency spil lway depth as the first pond, t h e a v a i l a b l e de ten t ion s to rage depth is

D1 = 16 (0.80 + 1.0 + 2.5 + 1.3) = 10.4 f t

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.. . . . - - . , . - .. ..

6.17

\ \ I I

10

0 I .o 2.0 . 3.0 4.0 5.0

STORAGE VOLUME (ACRE - FEET)

FIGURE 6.5 STAGE - SURFACE AREA AND STAGE -STORAGE CURVES OF S ITE NO. 2

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. _ _ - _ . . . . .

6.18

Stage a t maximum detention stage depth

10.4 + 1.3 11.7

From Figure 6.5. available storage volume is

3.75 - 0.11 = 3.64 ac-ft

Step 8. Required storage volume

Time base of inflow hydrograph

2 2V 2 x 2.70 ac-ft x 43.560 f t /ac

'1 49.7 f t /sec x 3600 sec/hr l.31 hours

T b = - = 3

For d = 0.018 x n and DT = 11.7 f t8

For Tb = 1.31 hours and TD = 4.5 hours8

S - = 0.930. S = 0.930 x 2.70 ac-ft = 2.51 ac-ft < 3.64 ac-ft V

QO

QI - = 0.0708 Qo e 0.070 x 49.7 P 3-48 C f S

Since the required storage volume is less than the available storage volume, decrease the depth and repeat Step 8 .

Assume ~1 - 8.5 ft

From Figure 6 . S 8 Stage a t maximum detention depth

For

For

8.5 + 1-3 a= 9.8 f t

Available storage = 2.80 - 0.11 = 2.69 ac-ft

d = 0.018 and DT P 9.8 ft

TD = 3.9 hours

Tb = 1.31 hours and TD = 3.9 hours

s - = 0.910. S = 0.910 x 2.70 = 2.46 ac-it < 2.69 ac-ft V

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6.19

= 0.09 x 49.7 = 4.47 c f s QO

*I Qo - = 0.09,

Check surface area

A (measured at depth = 1.3 f t ) = 5300 f t 2

- = 6120 f t 2 > 5300 f t 2 4.47

2.254 x (0.018) 2 A P 1.2 X rep

The pond must be excavated t o meet the surface area requirement or raise the p r i n c i p a l spillway crest t o elevation 2.0 f t . - This w i l l provide more sediment storage than is required. w i l l exist i f a dewatering device is not provided.

A permanent pool

A (measured a t depth - 2.0 f t ) = 6500 f t 2

Check ava i lab le storage volume

Available storage = 3.20 - 0.25 = 2.95 ac-ft > 2.46 ac- f t

Check length-to-width ratio

L (measured a t depth = 2.0) - 250 f t

L 250 W 26

. - - = 9.6 > 2.0

Check scourinq

Qo 4.47 ft3/eec e: o.02 fps v,=-- WD1 26 f t x 8.5 ft

= 1.67 d1l2 = 1.67 x (0.018)’/2 = 0.22 fps vS

vH < ’s

A summary of t h e &sign is presented in Table 6.2. I

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6.20

Table 6.2. Summary of Sedimentation Pond Design.

Description Pond N o . 1 PondNo. 2

1 .

2 .

3.

4 .

5.

6 .

'7.

8.

1 9.

10 .

1 1 . ! \

12.

13.

14

15.

16.

17.

18.

19.

=Sign inflow (PI), CfS

Design runoff volume (v), ac-ft

Annual sediment yield, tons

Storm sediment yield, tons (10-year, 24-hour storm)

Inflow suspended solids concentration, mg/l

Minimum s ize of particle se t t l ed , m

Detention t i m e , hours

Pr incipal s p i l l a y elevation, f t

Sediment storage required, ac-f t

Sediment storage provided, ac-ft

-off detention depth provided, f t

Runoff detention volume provided, ac-f t

Surface area required, f t

Surface area provided, f t

Length-to-width ratio

Scour veloci ty (Vsc), f p s

2

2

Flow-through Velocity (v,) 8 f p S

10-year, 24-hour design outflow, cfs

25-year8 24-hour design outflow, c f s

77

2.31

115

50

15,926

0 065

0.4

4.2

0.23

0.23

7 . 5

1-48

3490

3490

4 .9

0.43

0.14

27.7

63.8

49.7

2.7

54

--

--- 0.018

4.5

2.0

0.11

0.25

8.5

2.95

6120

6500

9.6

0.22

0.02

4.47

113.13

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VII. REFERENCES

American Arblic Health Association, American Water Works Association, Water Po l lu t ion Control Federation, 1981, "Standard Methods f o r the Examination of Water and Wastewater," 15th Edition, American public Health Association, Washington, D.C.

Anderson, C. E. and J. M. miggs8 1979, "Planning Erosion Control f o r Coal Mining and Restoration," Iowa State University, Dept. Agr icu l tura l Engineering, Jour. Soil Water Conservation 34: 234-236.

Bar f i e ld , B. J., R. C. Warner and C. T. Haan, 1981, Applied Hydrology and Sedimentology f o r Disturbed Areas, Oklahoma Technical Press; Sti l lwater, Oklahoma.

Bondurant, J. A*, C. E. Brockway and M. J. Brown, 19758 "Some Aspects of Sediment Control, University of Kentucky; ~ e x i n g t o n , =I pp- 117-121.

Bureau of Reclamation, 1974, Design of Small mutts, A Water Resources Technical Publ ica t ion .

camp, T. R., 1946, "Sedimentation and the Design of S e t t l i n g Tanks," Transactions of t h e X C E r V o l . 111, pp. 895-958.

) hen, m a r n g - ~ i n g , 1975, "Design of Sediment Retention asi ins," National Symposium on Urban Hydrology and Sediment Control, University of Kentucky, k f i n g t o n , pp* 285-2930

t \

Curtis8 D. C. and R. H. McCuen, 19778 "Design Eff ic iency of Stormwater Detention Basins," Jour. Water Resources Planning and Management Division, ASCEt VOl. 103, ppo 125-1390

Environmental P ro tec t ion Agency, 1981, "Development of Methods to Improve Performance of Surface Mine Sediment Basins, Phase If and 111," EPA Contract NO. 68-03-2677.

Environmental Protection Agency, 1980, "Development of Methods to Improve Performance of Surface Mine Sediment Basins, Phase I-," EPA 660/7-80-072.

Environmental Pro tec t ion Agency, 1980, "Demonstration of Debris Basin Effec t iveness in Sediment ~ o n t r o l , " EPA 600/7-80-154.

Environmental P ro tec t ion Agency, 1979, "Evaluation of the Perfonnance Capab i l i t y of Surface Mine Sedimentation Basins," EPA 440/1-79-200.

Environmental Protection Agency, 1979, "Removal of Trace Elements from A c i d Mine Drainage," E ~ A 660/7-79-101.

Environmental Protection Agency, 1978, "Erosion and Sediment Control HandhOkt" E P A 440/3-78-003.

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- . _ _ .. .- _ . .. . . . .

7.2

Environmental Pro tec t ion Agency, 1976, "Erosion and Sediment Control: Surface Mining in t h e Eastern United States, Vol. 1 Planning and Vol. 2 Design," EPA 625/3-76-006.

Environmental Pro tec t ion Agency, 1976, "Effectiveness of Surface Mine Sedimentation Basins," EPA 660/2-76-117.

Environmental Pro tec t ion Agency, 1976, "Debris Basins for Control of Surface Mine Sedimentation," EPA 600/2-76-108.

Environmental Pro tec t ion Agency, 1976, "Methods f o r Control of Fine Grained Sediments Resulting from Construction Activity, ' EPA 440/9-76-026.

Environmental P ro tec t ion Agency, 1972, "Guidelines f o r Erosion and Sediment Control Planning and Implementation," EPA R2-72-72-015.

Gilbrea th , J. L., 19798 "The S t a t e of the A r t of Erosion and Sediment Control €or Surface Mined Weas,' U.S. Army Mi l i t a ry Personnel Center, Vi rg in ia , NTIA R e p o r t *A077 709.

Guy8 H. Fe8 1979, "Performance and Environmental E f fec t s of Sedimentation .Ponds Required for Surface Coal Mining Operations,' IES 25th Annual Technical Meeting, Seattle, Washington.

'; Haan, C. T. and B. J. Bar f i e ld , 1978, Hydrology and Sedimentology of Surface Mined Lands, I n s t i t u t e f o r Mining and Minerals Research, Kentucky Center f o r Energy Research Iabora tory , University of Kentucky, Lexington, Kentucky.

: \ Hazen, A * , 1904, 'On Sedimentation" Transactions ASCE Vol. 53:45-71.

H i l l , Re D - , 1975, "Sediment Control and Surface Mining," Polish-United States Symposium Environmental P ro tec t ion in open P i t Coal M.ning, Univers i ty of Denver Research I n s t i t u t e , Denver, Colorado (Septr 89-95).

Lara, J. M. and E. L. Pemberton, 1963, " In i t i a l Unit Weight of Deposited Sediments," USDA Miscellaneous Publication.

Mallory, C. W. and M.A. Nawrocki, 19748 "Containment Area F a c i l i t y Concepts f o r Dredged Ua te r i a l Separation, Drying and &hauling," U.S. Army Engineering Waterways Experiment S t a t i o n , DACW 39-73-(2-0136.

McCarthy, R. E., 1973, 'Preventing the Sedimentation of Streams i n a P a c i f i c N o r t h w e s t Coal Surface Mine," Research and Applied Technology Symposium on Mined Land Reclamation, P i t t sburgh , Pennsylvania (133-143).

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.. . . ..-. ._- - -. -. _._., .. . , , . - .. . . .. .. ..

7.3

i

Office of Surface Mining, 1981, %esign Manual for Water Diversions i n surface Mine Operations," prepared by Shcm68 W. & Associates, Inc., OSM Contract No. J5 10 1050.

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Oscaryan, P. c.8 1975, "Design of Sediment Basins fo r Construction Sites," National Symposium on Urban Hydrology and Sediment Control, University of Kentucky, Lexington, Kentucky.

Reed, L. A * , 1978, "Effectiveness of Sediment Control Techniques U s e d During Highway Construction in Central Pennsylvania," U. S. Geological Survey, Water Supply Paper 2054.

Reed, L- A * , 19758 "Controlling Sediment from Construction rea as," Third Symposium on Surface Mining and Reclamation, fiouisville, Kentucky (Oct.; 48-51 1 .

.Robinson, C. W. and C. E. Brockway, 1980, "Sediment Removal With Vegetal Filters," Proceedings of the 1980 Specialty Conference, I r r iga t ion and Drainage: Todays Challenges, Boise, Idaho (July) .

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_i . - . , . . . . - _. . .. . , . .

7.4

0 . S . Soil Conservation Service, 1961, "Engineering Geology," National Engineering Bandbook, Section 8.

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