ETHEKWINI DESIGN MANUAL - · PDF fileengineering unit coastal stormwater & catchment management department design manual: guidelines and policy for the design of stormwater drainage

ENGINEERING UNITCOASTAL STORMWATER & CATCHMENT MANAGEMENT DEPARTMENT

DESIGN MANUAL: GUIDELINES AND POLICY FOR THE DESIGN OF STORMWATER

DRAINAGE AND STORMWATER MANAGEMENT SYSTEMS(Vade Mecum)

Issued by :

R. KASSERCHUN PrENG.DEPUTY HEAD : CSCMMUNICIPAL CENTRE 166 OLD FORT ROADDURBAN4001

Revision Date: May 2008 1

ETHEKWINI MUNICIPALITY

DESIGN MANUAL: GUIDELINES AND POLICY FOR THE DESIGN OFSTORMWATER DRAINAGE AND STORMWATER MANAGEMENTSYSTEMS

INTRODUCTION

These guidelines and policies are applicable to the design of minor stormwater reticulationand collection systems and for the management and control of stormwater runoff from sitesby means of soakpits and attenuation tanks and runoff discharge controls.

These guidelines include recommendations for the sizing and design of stormwater soakpits,attenuation structures/ponds, outlet discharge controls/overflows, kerb inlets, manholes, roadedge channels, watercourses, underground pipelines and small channels. These must bedesigned to effectively collect, control and convey run-off from storms to larger or majordrainage systems.

The design of all major stormwater systems (whether culverts, pipes, canals, including roadcrossings, bridges etc.), for catchments greater than 1 km where hydraulic analysis of thewaterway is necessary, should be carried out by checking by a registered professionalengineer proficient in this field for approval by Coastal, Stormwater & CatchmentManagement Unit.

Limited information related to the drainage and storage facilities to be used in a majordrainage system for the control of floods has been included in these guidelines.

These guidelines should be read in conjunction with the Stormwater management section ofthe “Guidelines for the provision of Engineering Services and Amenities in ResidentialTownship Development” (the Red book) issued by the National Housing Board.

Rail and highway crossings and jacked pipes normally require special measures and shouldbe referred to the Deputy Head: Coastal Stormwater and Catchment Management.


CONTENTS

1 STORMWATER MANAGEMENT POLICY page 5

1.1 Provision of soakpits1.2 Provision of attenuation and other storage structures1.3 Brief outline of minimum information required in a stormwater management plan

2 THE RATIONAL METHOD page 10

2.1 Restriction on usage2.2 Area of catchment 'A'2.3 Time of concentration 'Tc'2.4 Run-off coefficient 'C'2.5 Rainfall intensity 'I'

3 DESIGN OF A MINOR SYSTEM page 14

3.1 Critical points3.2 Rainfall intensity at critical points3.3 Minimum diameter3.4 Minimum velocity and gradient3.5 Materials3.6 Anchor blocks3.7 Curved alignment3.8 Servitudes3.9 Layout of stormwater sewers in servitudes3.10 Layout of stormwater sewers in road reserves3.11 Manholes3.12 Manhole covers3.13 Benching in manholes3.14 Channels3.15 Minimum cover3.16 Bedding3.17 Invert levels at manholes3.18 Inlets

4 SELECTION OF CLASS OF PIPE page 20

5 PARTIAL FLOW IN PIPES page 20

6 SUBSOIL DRAINS page 20

7 STORAGE PONDS page 20

8 DESIGN OF SHORT LENGTH CULVERTS UNDER EMBANKMENTS page 21

9 CALCULATING TIME OF CONCENTRATION page 21

10 EXAMPLES OF CALCULATING THE RUNOFF COEFFICIENT C page 22

11 METHOD OF STORMWATER RETICULATION DESIGN page 24

12 GENERAL METHODOLOGY page 24

13 WORKED EXAMPLE page 27


14 DESIGN OF STORMWATER KERB INLETS page 33

15 CALCULATIONS OF INVERT LEVELS AT MANHOLES page 34

LIST OF APPENDICES page 35

PART R Stormwater Disposal SABS 0400 - 1990 page 36Hydraulic elements of circular sections page 38Bend losses page 39Details of anchor blocks page 40Discharge through box culverts : inlet control page 41Discharge through pipe culverts : inlet control page 42Kerb Inlet chart 1: cross fall 2% page 43Kerb Inlet chart 2: cross fall 2,5% page 44Kerb Inlet chart 3 & 4 : cross fall 4% and 6% page 45Pipe bedding details page 46Protection of pipes at reduced depths of cover page 47Recommended layout of services - 9.5 metre reserve page48Recommended layout of services - 12 metre reserve page 49Recommended layout of services - 16 metre reserve page 50Recommended layout of services - 19 metre reserve page 51


1 STORMWATER MANAGEMENT POLICY

Municipal stormwater availability

In many instances erven do not have access or connections to municipal stormwater. Insuch cases the onus is placed on the developer to manage the excess stormwater resultingfrom any hardening of the site area. This management can take the form of soakpits if thesoil and geological profiles allow or attenuation facilities such as tanks, ponds or areasdesigned to retain water. Where possible the emphasis should be placed on improving thepotential for groundwater infiltration.

Municipal infrastructure generally provided stormwater systems designed on the basis thatnot more than 40% of the area of residential properties would be hardened. As such, anydevelopment in such areas in excess of a 40% limitation naturally implies that the developermust be held responsible to manage the excess runoff from such a site for the proportion ofhardening in excess of 40%.

Where sites have historically disposed of stormwater into soakpits on site, this method ofdisposal must be maintained. Notwithstanding, soakpits cannot function effectively forever(life spans are limited to 5 to 15 years depending on the design and maintenance). Theobligation is thus placed on the owner to maintain and upgrade the existing soakpit capacitywhere any further development or hardening of the site occurs. These responsibilities are necessary and due. Changing conditions and trends have lead toincreased development densification. Increasingly, paving is replacing higher maintenancelawns and garden areas and a process of maximizing land usage to supplement incomes.These changing trends in residential and semi-urban areas have seen rezoning and newsectional type developments resulting in higher rates and volumes of runoff.

Therefore, In accordance with the National Building Regulations “PART R StormwaterDisposal SABS 0400 – 1990” (See Appendix), the eThekwini Municipal policy requires thatprivately owned sites may be required to manage and make provision for their ownstormwater runoff.

The general level of management required is that of controlling all runoff emanating fromsuch a site in excess of that which would have occurred if the site was in its natural ororiginal virgin state. Such stormwater management may well be the provision of soak pits,structures such as attenuation ponds or tanks (with controlled outlets where necessary, all toensure that the rate of runoff is reduced to predevelopment states and that runoff is notconcentrated onto adjacent neighbouring sites or other infrastructure, be it road drainage orvalley lines, streams etc.

1.1 Provision of soakpits

In general, where a site was originally developed with attenuation structures/ponds orsoakpits controlling runoff, these systems should remain. These sites are alsorequired to maintain and upgrade their systems to accommodate both existing andany new additional hardening on the site.

Any application for approval of development on any open site without access tomunicipal stormwater will be required to provide a stormwater management planbased on a rational design method demonstrating that the soakpit/attenuation andother proposed controls controlling the additional stormwater runoff generated due toall the development of the site will not adversely affect downstream and neighbouringareas. All designs are subject to the approval of the Coastal, Stormwater &Catchment Management Department of the eThekwini Engineering Unit.


Additionally, existing single erven or properties that have a municipal stormwaterconnection, but where the site has been developed to the point where more than 40%of the site area has been hardened may be required to manage the excess runoffgenerated on the site/erf itself.

Hardening of the site may be by either roofed areas or by otherwise generating morestormwater runoff by providing paved driveways or other semi or impermeablestructures cumulatively covering in excess of 40 % of the area of the site or erf. If thegeological and stability profile of the site is suitable soakpits sized on the basis of theexcess hardened coverage of 40 % are to be provided or other engineeredattenuation measures based on an engineers design and the Rational method ofdetermining pre to post development runoff.

Many existing/old soakpits are very rudimentary e.g. sheet iron covered rubble filledholes. These older soakpits had a effective lifespan rarely in excess of 10 years.They cannot be cleaned or maintained and the initial storage volume provided wasgenerally minimal. Properly designed structures provide access to allow for essentialperiodic removal of accumulated silt, organic material and other windblown materialsfrom the soakpit to allow it to continue to function effectively. Regular maintenanceextends the soakpits life span and maintains storage volume.

When drawings are submitted for approval of building additions and alterations andthe property had existing soakpits, then the developer must be able to demonstratethat the existing soakpits are functioning. These existing soakpits will have over timebeen filled with detritus and in many cases have been displaced by excavations forpools or other structures etc. The original soakpits cannot be accepted unless theywere constructed properly and can be accessed for cleaning or inspection. Currentpolicy dictates that approval for any new addition on sites with soakpits must makeprovision for installing new soakpits unless a professional engineer is able to inspectthe old/existing soakpits and certify the capacity and condition. As such, new properly designed soakpits for stormwater shall be built, sized on thebasis of 1 (one) cubic metre of clear volume to drain each and every 40 squaremetres of all roofed, paved or otherwise hardened areas on that site.

There are some alternatives to using soakpits and these include the provision ofrainwater tanks which can be used for watering gardens and/or engineered ponds.These may be an attractive alternative to soakpits providing a stored source ofirrigation water. The acceptable storage ratio in terms of using rainwater tanks as analternative to soakpits is 60% of the volume of the tank. In other words, whencalculating the volume of storage required (on the 1 m3 to 40 m2 area) then 60% of therainwater tank volume may be claimed on the assumption that the tank is 40% full atany given time.

Notes:

Hardened areas shall include roofed areas, all paving, surfaced driveways, poolsurrounds, etc.

Grass blocks or other permeable paving are considered as hardened areas but thearea can be reduced by up to 50% in such cases for determining the storage required(i.e. as being actually hardened).

A typical detail of the soakpit design showing a plan and sectional view is to beprovided as part of any drawing submission for approval where a soakpit is to beinstalled.


Drawings must table the extent of the hardened areas to demonstrate that therequired soakpit volume/s provided is adequate (i.e. 1 cubic metre of volume for every40 square metres of area hardened).

Soakpits should be constructed 3 – 4 metres (minimum) from any structure orboundary to prevent damage to buildings, foundations and to reduce a risk of pipingunder boundary walls etc. Any reduction in this distance must be supported by ageotechnical report.

All soakpits provided shall be drawn to scale in position on the plan view, labeled as"new" together with the volume and overall dimensions.

The pipe drainage must collect and drain the rain water from collection points, (eg.down pipes) to the soakpits and sized to ensure that the soakpits will fill duringsignificant storm events.

1.2 Provision of attenuation and other storage structures

Where economics, site geology, impermeable rock layers or clay soil types, slopestability or other engineering considerations argue against the suitability of using asoakpit then other attenuation controls or structures are mandated. On individualerven which are suitable for soakpits, it may not generally require a professional civilengineer’s design for soakpits for stormwater management.

Generally, for multi-unit residential or commercial developments or where coverageexceeds 40% of an individual residential site, notwithstanding the suitability of the sitegeology for soakpits, a stormwater management plan is required. This is based on theincreased risk of failure and higher percentage runoffs. Soakpits tend to fill in higherorder storm events before the peak stormwater runoff from the site is reached. Inother words the higher order peak runoff from the site is not reduced and the risk ofdownstream flood damage is not diminished.

A competent professional civil engineer will be required to produce a Storm-waterManagement plan modeling the run-off for the Pre and Post development scenariosfor at least the 1:10 and 1:50 year Recurrence Interval storms.

• The Storm-water Management Plan provided must provide logical andcoherent detailed information allowing for verification of the engineers design.The SMP must provide as a minimum, the information as listed in paragraph1.3 below.

The pre-development runoff coefficient (C) should ideally be derived on thecomponent method where the slope component (Cs), soil permeability typecomponent (Cp) and vegetation cover component (Cv), are assessed and summed toderive the runoff coefficient used for the site/drainage area as a whole.

The post development runoff coefficient (C) for the overall site can generally simplybe derived assessed on a proportional basis of the pre-development runoff coefficientC and the post dev areas that have actually been hardened with C = 0.95. Someexamples are shown later in these notes.

Generally, a Time of concentration Tc of not less than 15 minutes for residential sitesand Tc of not less than 10 minutes for industrial sites must be used ( i.e. where thecalculated Tc is less).

The Rainfall statistical data to be used is available on the municipal website:


www.durban.gov.za/eThekwini/Services/Engineering/CSCM

This data is listed based on a latitude and longitude grid for a variety of recurrenceintervals from 2 to 1000 years for storm durations from 5 minutes to several days. Thedata is given as point data (i.e. In “mm” of rain falling in the storm time period shown)and must be converted to the rainfall intensity (mm/hr) corresponding to the Rationalformula.

Storage or attenuation/infiltration measures must provide for the difference betweenthe Pre development and Post development 50 year storm runoffsgenerated/calculated.

The attenuation storage required can be assessed on a simple plot of the peak runoffvalues Q calculated for the pre and post scenarios versus the Time/s of concentration(Tc) for each scenario. i.e. plotting superimposed pre and post hydrographs assimplified triangles.

For small catchments runoff is assumed to be zero at both Time = zero and at 2 x Tcminutes. The volume of attenuation storage is represented by the area of the postdev hydrograph which lies outside/above the pre dev hydrograph plotted. In effect anyrunoff greater than that of the pre dev plot (50 year RI storm) must beattenuated/retained.

The rate of outflow for any recurrence interval storm must be restricted to the pre-development runoff. For example: runoff/outflow for the Post-developed site in a 10year RI storm is not to exceed the 10 year pre-development runoff peak. The staticwater head generated in the attenuation tank when the 10 year post developmentcalculated storage volume is reached should not result in a greater discharge from theattenuation tank than the 10 year Pre-development runoff calculated. Similarly, the 50year storm storage/outflow relationships must be attained.

The stormwater Management Plan must demonstrate with sufficient detail that theproposed measures/structures/ponds can be provided and are practical and workable.Where sites are steep the extended cut and fill banks which may result in providing apond or structure should be shown. Cut and fill banks obviously cannot extend intobuilding platforms or neighbouring properties and the drawings should demonstratethat this has been catered for in the design.

The drainage system provided must be capable of delivering the volumes associatedwith the attenuation structure. There is no point in providing for an attenuation pond orother structure when the site layout and stormwater system is incapable of channelingor conveying the increased and excess runoff generated. The increased runoff maynot bypass the attenuation provided and flow offsite/downstream without beingreduced to predevelopment rates of discharge.

Depending on the individual site characteristics and the location of the attenuationfeature, a degree of over-design may be inherently required to satisfy the requirementof limiting runoff to pre-development conditions in the larger recurrence interval stormevents. Competent design can resolve such problems and minimise costs.

The use of level parking or flat areas as shallow attenuation tanks, creating wet or drylined or unlined ponds are all alternatives to costly conventional retention structures.

The objective is to maximize groundwater infiltration and/or to reduce peak runofffrom artificially hardened development further eroding streams and furtherexacerbating the potential for flooding lower down in any catchment.


All Final stage/Construction drawings or building plans must include complete detailsof all storm-water structures, all outlet controls, reticulation layout and pipe sizes andall erosion protection measures required for construction. The architects submissionmust incorporate the approved engineered stormwater management layout.

1.3 List of minimum information required in a stormwater management plan (forboth the pre and post development scenarios)

• Street address and description of erf• Latitude and longitude or X, Y co-ordinates of site (Especially if no street address

exists)• Area of site• Length of estimated drainage flow path• Fall or slope of site• Runoff Coefficients used (Cv - vegetal cover, Cs - slope, Cp - permeability/soil

type)• Time of Concentration • Existing and future areas that have or will be developed (hardened)• Actual Runoff coefficients used eg:

o Roofs, premix driveways (C=0.95), o paving (C=0.95), o grass blocks (C=0.5)

• Rainfall intensities I10, I50

• Runoff calculations Q10, Q50

• Attenuation volumes

All of the above should be tabulated for both the Pre and Post development for atleast the 10 and 50 year recurrence interval storms.


2 THE RATIONAL METHOD

2.1 Restriction on Usage

The Rational Method is still probably the quickest and most commonly used methodof estimating the peak runoff value of stormwater run-off generated from urban andrural areas in spite of its limitations in application and accuracy. Municipalguidelines/policy based on the National Building Regulations presupposes at least aRational method of determination.

The formula used in this method is –

Q = ft x C x I x A/360 cumecs

Where

Q = the maximum/peak rate of run-off in cumecs (m3/s)ft = an adjustment factor for the recurrence interval storm considered.C= run-off coefficient (see applicable tables for determination)I = the rainfall intensity (mm/hr)A = area of catchment in hectares (1 ha = 10 000m2)

2.2 Area of catchment (A)

The area of catchment is the total area above a point of interest that will contribute tothe run-off at that point, either from naturally occurring stream flow or from overlandflow.

Such an area can be measured on a topographical map by drawing a line from theoutfall point and following the crests of ridges, spurs or high areas which can beidentified from contours and encloses those valleys or low areas which will draintowards the outfall point. In manual mapping of any catchment, conceptually, runoffflows as a vector line equally bisecting the angle formed when drawn through thecontour lines. This concept determines the extent of natural catchments.

The area of the catchment once defined on a topographic map is determined bydividing the area into measurable geometric shapes (triangulation etc.), or by usingthe measuring tools on the GIS.

2.3 Time of concentration (Tc)

The time of concentration can be regarded as the time it takes for the excess rainfallresulting in run-off from the furthest significant part of a natural catchment to reachthe point being considered. The shape of the catchment has significant implicationson assessing the length of the flow path. For more accurate estimates of Tc it issometimes necessary to create several smaller catchments especially where theshape of the catchment includes long narrow upper reaches which will notsignificantly contribute to the overall volumes or peak flow generated.

It is difficult to accurately assess or determine the time of concentration with greatconfidence since an iterative approach would be required to assess the depth ofsheet or concentrated flows with innumerable other considerations. However anumber of tables and equations are included for estimation purposes. To obtain ameasure of consistency, a general approach is recommended as follows:

(a) Sheet flow naturally concentrates becoming stream flow with greater flow depthand generally higher flow velocities. The catchment’s size and shape is usually


significant in terms of determining the overland flow path to assess the time ofconcentration. The flow path length must be assessed appropriately. The runoffhydrograph for a long thin catchment differs significantly from a square catchmentwith the same area and characteristics. The rates of runoff will differ and bespread over longer periods of time. The total volume of runoff will tend to differalso depending on the time surface water has to infiltrate into the ground. Theinitial rates of infiltration depends on soil, moisture content and innumerable otherconsiderations.

(b) In assessing any flow-path when confronted with irregular shapes use yourengineering judgement to apportion a flow length that is appropriate, i.e. thatconforms to the runoff for the bulk of the area of the site to the point beingassessed. Similarly when assessing the slope of a catchment use a slopeconforming to the bulk or major portion of the site and use this in the Kerbyformula (see later). (NB. the slope component of the runoff coefficient is not whatis being considered here).

Guidelines suggest the use of the height difference on the 1085 method or equal areamethods. The 1085 height is the elevation difference measured from downstream atthe 10 and 85% points along the full length of the flow-path.

Whatever method is used it must be representative of the bulk of the catchment areaconsidered.

Since the methods used are subjective and inherently provide an estimate only it isnot necessary to exhaustively analyze most catchments in a municipal environment.In large complex catchments the best method though laborious, is to break thecatchment down into smaller areas and individually assess and cumulatively add eachcomponent areas runoff to derive a better estimate of the peak runoff of the whole. Inthe municipal environment with generally small catchments or developments beingassessed, it is not necessary to exhaustively analyze but a sensitivity check should bedone.

Generally sheetflow only occurs in the first or initial say 200m portion of the upper endof the catchment. Thereafter, a further time of flow in conduits/streams/gutters etcbecomes applicable and then Manning type equations or charts providing typicalvelocities for various terrain types or lined conduits can be used to estimate thebalance of the travel time to the point under consideration.

There may be several different catchments contributing flow to the point underconsideration. The longest travel time calculated of all the different contributing runoffroutes to that point is assumed to be the time of concentration.

Similarly the area A used in the formula Q=CIA to determine the peak flow at thatpoint is the sum of the areas of all the catchments contributing runoff to that point.

Since the bulk of development in the city/metro region takes place where catchmentsare relatively small (< 1 km square) we advocate using a minimum time ofconcentration of 15 minutes for all undeveloped/rural/residential type sites. In otherwords if the calculated time of concentration for a residential site is less than 15minutes use 15 minutes, and where a site is predominantly hardened i.e. fullydeveloped commercial/industrial sites) then use a minimum of 10 mins if thecalculated time of concentration is less.

2.4 Run-off coefficient (C)


The run-off coefficient is a factor ranging between 0 and 1 which compensates forvariations in rainfall over the catchment, infiltration and overland flow velocity during astorm, the shape of the catchment, ground slope, etc. Because of variousindeterminate factors including ground moisture content, vegetation, permeability ofsoils, varying slopes, rainfall intensity etcetera, the coefficient 'C' is difficult to assessand a widely diverging range of estimated runoff coefficients can result. To minimisewidely disparate results and allow for uniformity and consistency in approach, thetable method used by the Dept of Water Affairs & Forestry (DWAF) is to be used andan excell version is available (shown below).

C can be derived from applicable tables for determination. Where several catchmentsor sub-catchments contribute runoff to the point under consideration then unless C isuniform for all (i.e. the same slopes and vegetation etc exist) then a modified Capplies which must be calculated as follows:

Coverall = (Sum of Ci x Ai)/(Sum of Ai) for all of the differing sub-catchments “i”.

For undeveloped sites the value for ‘C’ must be derived from the sum of thecontributions of the ground slope Cs, the vegetative cover Cv and the permeability orsoil type Cp. In urban/industrial areas a combination of the percentage areacontribution of the hardened areas and the balance of the site area assessed in termsof Cs, Cy and Cp above is logical/appropriate:

Ci for the catchment “i” = Csi + Cvi + Cpi

Ci may be considered to remain constant during any particular storm for smallercatchments (<5 km2). However we do advocate using a modification factor ft toreduce the runoff for lower order storms.

RI Storm year ReductionFactor ft

2 year 0.55 year 0.5510 year 0.620 year 0.6750 year 0.83100 year 1

But in a Pre and Post development assessment of runoff to estimate the attenuationstorage required to reduce “post” runoff, the difference in C between pre and postdevelopment values of C determines the volume to be attenuated. The allowabledischarge should be determined by applying the above reduction factor in order toobtain a lower value for the predevelopment peak flow since the rational peak flow isgenerally considered to be conservative (high) for design of systems.

Values of 'C' for different types of catchment conditions and surfaces are shownbelow. A spreadsheet calculator for estimating the ‘C’ value is included in the laterexamples in these guidelines.


CALCULATION OF RUNOFF COEFFICIENT DWA METHODPRE/RURAL Runoff Coefficient POST/URBAN Runoff Coefficient RURAL URBAN % Steepness/Slope Cs % > 900mm Lawn sandy<2% 0 0.08< 3% 20 0.05 Lawn sandy>7% 0 0.183-10 % 50 0.11 Lawn heavy<2% 0 0.1510 - 30 % 15 0.20 Lawn heavy>7% 0 0.30> 30 % 15 0.30 Residential single 0 0.40 Cs 100 0.14 Flats/dense townships 0 0.60Permeability Cp % Industry , light 0 0.65Very perm (Dunes) 0 0.05 Industry , heavy 0 0.70Perm (light soil) 10 0.10 Business local 0 0.60Semi (most soils) 80 0.20 Business CBD 0 0.85Imperm (rock, paving) 10 0.30 Streets/roofs 100 0.95 Cp 100 0.20 100 0.95Vegetal growth Cv % Dense bush, forest 10 0.05 AREA WEIGHTING FACTORSCult land, sparse bush 5 0.15 % DWAGrassland 75 0.25 RURAL 0 0.57Bare Surface 10 0.30 URBAN 100 0.95 Cv 100 0.23 LAKES 0 0.00

Ct = Cs + Cp + Cv = 0.57Cpost/design 100 0.95

In determining the value of 'C', the effect of both the present and future land use onrun-off must be considered if upgrading or new stormwater infrastructure is beingconsidered.

The Mean Annual Precipitation (MAP) for Durban/eThekwini region is predominantly>= 900 mm MAP and even if the MAP is lower, we advocate using 900 MAP as aminimum.

2.5 Rainfall Intensity 'I'

The rainfall intensity is the average rainfall in mm/hr for a design storm of a givenfrequency having a duration equal to the Time of Concentration Tc. Rainfall figurescan be obtained from the eThekwini website once you have identified the appropriateLatitude and Longitude for the site (available on the GIS). The figures given for thetimes shown are point rainfalls (i.e. in mm) and must be converted to an intensity (i.e.mm/hr)

www.durban.gov.za/eThekwini/Services/Engineering/CSCM.

Rainfall intensity figures are also included in the accompanying excell file.

In the design of a drainage system using the rational method, the duration of thedesign storm is assumed to be equal to the time of concentration 'Tc' estimated at theappropriate nodal point in the drainage system being designed.

'Tc' must be computed independently at each inlet or low spot. Thus, as the designprogresses downstream, the drainage area increases in size, 'Tc' will increase and asa result, the rainfall intensity 'I' will correspondingly decrease.

Remember that the minimum value of Tc to be used is at least 15 mins (in rural orurban residential areas) and 10 mins (in commercial/Industrial areas since thehardened areas are proportionally much greater). In other words, where the actual


assessed/calculated Tc is less than 15 or 10 minutes then use 15 or 10 minutes asappropriate as a minimum.

An excell file is also available giving rainfall intensities for storms of up to 8 hrs wherethe latitude and longitude are known.

3 DESIGN OF A MINOR SYSTEM

3.1 Critical Points

The effectiveness of stormwater design depends largely on the identification in acatchment area of those areas or points where flooding cannot be tolerated more thanonce in 10 years due to the likelihood of heavy economic losses or socialinconvenience. Such points are termed critical points, and while they can sometimesbe pinpointed on a topocadastral map, they should always be identified during a fieldinspection.

Critical points may occur:(a) at low points in a road (where ponding will occur) and ponding water may overflow

the verge on the fill side, thus eroding fill embankments and flooding low-lyingproperty;

(b) at the intersection of a steep road with a flat road where water flowing down thesteep road could flood the intersection or overshoot the opposite verge;

(c) at the site of an important drainage structure eg. The confluence of a road with amajor stream. Where potential flooding of a development may cause higheconomic losses.

Any design or planning for the overall system must entail an assessment of “what if”scenarios. For example: ensure that you allow for an overland flood route below lowpoints in a system considering that the pipe system could fail. This may be allowingfor an access way or a footpath below such points.

3.2 Rainfall Intensity at Critical Points

At critical points, both sufficient inlet capacity and pipe capacity must be provided tocope with the 10 year storm event. At all other points in a catchment, except forcertain cases discussed below, design generally is only for the 3 year storm.

In special cases e.g. in areas where uncontained stormwater resulting from the use ofa 3 year design storm would cause severe wash-a-ways in soft ground, theStormwater/Catchment Manager may require that the design be based on a stormfrequency of 5 years for non-critical points, and of 20 years for critical points.

In flat areas of the City with considerable commercial and industrial development, run-offs should be based on a 10 year storm period. "Major" disposal systems (i.e. thosetaking flows of about 10 cumecs or more) should be designed for 20 year storms andin some special cases e.g. systems crossing Transnet/S.A.T.S. reserves, designintensities may be required to be based on 50 or even 100 year storms.

3.3 Minimum Diameter

Note that downstream pipes should never be smaller in diameter than the upstreampipe notwithstanding that hydraulic considerations (such as steeper hydraulicgradelines or slopes) may support/allow this. Downstream pipes will obviously tend tobe blocked by any debris/objects transported down larger upstream pipes. Theminimum diameter of pipe shall be as follows:300 mm in a servitude; and


375 mm in a road reserve.

3.4 Minimum Velocity and Gradient

The desirable minimum full flow velocity shall be 1,5 m/sec and the absolute minimumfull flow velocity should be 0,9 m/sec which is acceptable only in unusualcircumstances.

Desirable and absolute minimum gradients are shown in the following table:

Diameter Desirable Gradient 1/300 80375 110450 140525 170600 200675 240750 280825 320900 3501050 4401200 520

3.5 Materials

In general, stormwater pipes shall have rubber ring joints, be spigot and socket spunconcrete pipes complying with S.A.B.S. 677 but fibre reinforced cement pipes arepermissible provided they comply with S.A.B.S. 819. Ogee type pipes are NOTacceptable.

Other acceptable/suitable pipe types are : “Weholite” and “Ribloc” type pipes for usewhere steep grades or to maximise the use of labour but their use is NOTrecommended in road reserves and road crossings .

3.6 Anchor Blocks

20 mPa concrete anchor blocks to details shown in Appendix 5 should be provided asfollows :

Grade (%) Spacing for 2,44 m pipe lengthsOver 50 every joint30 to 50 inc. alternate joints20 every 4th joint10 every 8th joint

Spacing for intermediate grades can be interpolated.

3.7 Curved Alignment

In normal circumstances straight alignment between manholes should be used, butcurved horizontal alignment is acceptable subject to the following limitations:

(a) the minimum radius of curvature for an effective pipe length of 2,44 m is asfollows :

Pipe diameter (mm) Radius of curvature (m)300 70375 70


450 93525 93600 112675 to 900 inc. 1401 050 1861 200 1861 350 2781 500 278

(b) curved alignment is only permissible with pipes having approved flexible joints.

3.8 Servitudes

The width of sewer and drainage servitudes is dependant upon the diameters of pipesto be laid within the servitude area and should not normally be less than 2m.However this width may be reduced at the discretion of the Deputy Head.

3.9 Position of Stormwater Sewers in Servitudes

Stormwater sewers in servitudes should be positioned as follows:in 3 m servitudes - 1,0 m from a property boundary;in 2 m servitudes - in the centre of the servitude.

3.10 Position of Stormwater Sewers in Road Reserves

Recommended layout of services in road reserves are shown in the Appendices forvarious road widths. In existing roads already containing services, a stormwatersewer should be laid in the verge at least 1 m clear in a horizontal direction from thewater main.

3.11 Manholes

Manholes should be placed at every change in horizontal and/or vertical direction orat a maximum spacing of:

100 m for pipes up to and including 900 mm diameter;150 m for pipes over 900 mm up to and including 1 200 mm diameter;200 m for pipes over 1 200 mm in diameter.

Details of standard precast concrete ring manholes and brick manholes are shown ondrawing nos. 38570/1/2/3 and 38850/1/4.

Manholes may be constructed using 1 000 mm internal diameter class A precastconcrete ring units in accordance with SABS. 1294 to a maximum depth of 5 m forpipes up to 375 mm diameter where a junction occurs and up to 600 mm with nojunction.

In all other cases including changes of direction and for manholes deeper than 5 m, aspecialised design is usually necessary. Provision is to be made for a landing in allmanholes deeper than 5 m in compliance with the requirements of the relevantregulations including the Occupational Health and Safety requirements. Step irons areconsidered unnecessary and should not be provided.

3.12 Manhole Covers

Where manholes occur in roadways, standard D.C. heavy duty cast iron covers andframes in accordance with SABS. 558 Type 2B as shown on drawing number


DMW1281 should be used. Heavy duty precast concrete covers should be providedin footways and verges and wherever vehicular traffic may be expected other than inroadways. In all other cases light duty precast concrete covers are acceptable.

A new composite polymer specification manhole cover and frame has been fieldtested and approved for use. Contact the CS&CM catchment or stormwater managersfor details.

Details of heavy duty and light duty precast concrete covers are shown on drawingnos. 38853 and 38852 respectively.

Where manhole covers are to be sloped to suit road gradients, they should be laid onshaped brickwork or in-situ concrete.

3.13 Benching in Manholes

All manholes should be benched with a smooth concrete channel formed to the soffitof the pipe and every attempt should be made to streamline the "inlet to outlet" flow ofwater.

3.14 Channels

The minimum roadway cross fall on any black top surface should be 2,5% and theminimum longitudinal gradient should be 0,5% for concrete channels and 1% forasphalt channels.

3.15 Minimum Cover

The minimum allowable depth of cover to the outside of the barrel of the pipe forstormwater sewers is as follows:

(a) in servitudes 0,8 m(b) in footways and verges 1,0 m below final kerb level(c) in roadways 1,2 m below final constructed road level

If the required depth of cover cannot be achieved by importing additional material anda lesser depth of cover is unavoidable, the pipe should be protected from damage, atthe discretion of the Deputy Head, by means of:

the placement of case-in-situ or precast concrete slab(s) over the pipe, isolated fromthe pipe crown by a soil cushion of 100 mm minimum thickness. The protecting slab(s) should be wide enough and so designed to prevent excessive superimposed loadsbeing transferred directly to the pipe (see Appendix);

the use of structurally stronger pipes able to withstand superimposed loads at thedepth concerned;

Where the depth of cover in roadways or footways and verges is less than 600 mm orwhere the depth of cover in servitudes is less than 300 mm, protection of the pipefrom damage must be provided.

3.16Bedding

Bedding shall generally be in accordance with the requirements of StandardEngineering Specification Part "DB" : Earthworks for Pipe Trenches.


The various bedding classes and their relevant load factors for rigid pipes are shownin the appendix.

In saturated ground, steps should be taken to provide adequate drainage in trencheswith a minimum layer of 150 mm of 19 mm stone placed under the pipe. Should siteconditions warrant, filter fabric may be placed between in-situ material at the trenchbottom and the stone mat. A cut-off drain placed at the seepage side of the trenchbottom and connected to a stormwater manhole may be necessary. Furtherinformation on bedding of pipes is contained in the "Concrete Pipe Handbook"published by the Concrete Society of Southern Africa, a copy of which is available inthe Technical Library of the Engineering Unit.

3.17 Invert Levels at Manholes

General

The standard practice of laying pipes "crown to crown" with benching formed to half-pipe level normally creates no problems but in some cases insufficient attention isgiven to the adjustment of invert levels at manholes to compensate for loss of energybetween inlet and outlet pipes to prevent surcharge occurring in the manhole. In otherwords the water level rises in the manhole to compensate for the loss of energy and inextreme cases, the water level rise may be more than the manhole height and watersurges out of the manhole dislodging the manhole lid.

Standard Conditions

At manholes with no drop inlets or no substantial side inflows or where the change inhorizontal direction is less than 4 ̊ the difference in invert levels is determined by thedifference in pipe diameters only provided pipes are laid "crown to crown" andbenching complies with the requirements of paragraph 3.13 above.

Special Conditions

In the following circumstances conditions within a manhole warrant detailedexamination:

(a) where the velocity head from the inlet pipe is destroyed e.g. at a drop manhole;(b) where a relatively large inflow enters a manhole from an inlet or from one or

more subsidiary lines.

When considering (a) and (b) above, the following criteria should be taken intoaccount in calculating the required invert level of the outlet pipe:(i) full pipe flow at entry to outlet pipe;(ii) the water level in the manhole is not to be above crown level of the pipe

carrying the major incoming flow and the crown of other incoming pipes shouldnot be lower than this level.

The velocity head required at the entry to the outlet pipe above the crown (the top orsoffit) of the pipe =V²/2g

where V = required velocity at entry to outlet pipe in m/s, and g = 9,81 m/s², and inorder to satisfy conditions (i) and (ii), the difference in invert level, between the inletand outlet pipes = V²/2g + difference in diameter of pipes.

Note: the downstream pipe should not be a smaller diameter than the upstream pipenotwithstanding that the available grade may allow for this. This is to preventpotential blockages where an obstruction enters the system upstream).


Bend Losses

Bend losses should be taken into account where there is a change in horizontaldirection greater than 4 ̊ and although opinions vary on the extent of such losses inmanholes, a loss of 50% of the velocity head of the inlet pipe is consideredreasonable.

For a 90 ̊ bend

Velocity head at inlet pipe = Vi²/2gBend loss = 0.5 x Vi²/2gVelocity head at outlet pipe = Vo²/2gAvailable velocity head downstream of the bend = Vi²/2g - 0.5 Vi²/2g= 0.5 x Vi²/2g

The difference in invert level between inlet and outlet pipes equals the difference inpipe diameters plus the difference between required and available velocity head.

=difference in pipe diameters + Vo²/2g - 0.5 x Vi²/2g

For a 45 ̊ bend:

From the graph in Appendix 12, the available velocity head downstream of the bend

=Vi²/2g - 0.75 x 0.5 Vi²/2g = 5/8 x Vi²/2g

and the required difference in invert level

= difference in pipe diameters + Vo²/2g -5/8 x Vi²/2g

Where a substantial inflow from a subsidiary line occurs at a bend, the conditionsdetailed in clause 3.17.3 apply.

3.18 Inlets

Where the grade of a road is flatter than 0,5%, graded channels are used and singleinlets are provided at ± 30 m intervals. In this Service Unit, for roads having gradessteeper than 1 in 200, the usual practice is to provide inlets at ± 40 m with depressedchannels extending 1 m on the upstream approach.

A more economical design could be obtained if more detailed investigations were tobe conducted into inlet opening length, upstream channel length, road grade and thewidth of stream flow in the road and to encourage such analysis, Charts have beenincluded in the appendix to these guidelines.

These charts are based on the following:

(a) maximum allowable stream width to be 2,5 m and at critical points, 3,2 m;(b) interception at inlets to be 80% of flow and at critical points, 100% of flow(c) depressed channels to be provided regardless of the type of kerb used; and(d) inlets to have openings in units of 1,2 m varying in length up to 5,6 m

(N.B. A 2 bay inlet consists of 2 No. 1.2 metre wide units but after deducting thethickness of brickwork i.e. 0,2 m, the waterway area is reduced to 2 m).

4 SELECTION OF CLASS OF PIPE


The class of pipe to be used can be obtained from the "Concrete Pipe and PortalCulvert Handbook" issued by the Concrete Manufacturers Association.

5 PARTIAL FLOW IN PIPES

In the design of a new reticulation system, it is assumed that pipes are flowing full andpartial flow is not normally considered. However, it is sometimes necessary to knowthe velocity and discharge of pipes flowing partially full and in such cases, the graphin Appendix 19 may be used or the Manning Equation which is included in theaccompanying spreadsheet.

6 SUBSOIL DRAINS

A subsoil drain should consist of either of the following:

(a) for small volumes of seepage water, 19 mm or 25 mm grade single size stone asper SABS 1083 wrapped in drainage grade filter fabric with a 200 mm overlap atthe top of the drain to form a nominal 200 mm by 200 mm square section with a100 mm layer of coarse clean sand placed on either side of the 200 mm x 200 mmsection;

(b) for regular and high seepage flows, subsoil pipes wrapped in drainage grade filterfabric with a minimum overlap of 100 mm situated at the top of the pipe andcovered with a clean coarse clean sand compacted to 95% Mod AASHTO.

It is often convenient to incorporate a subsoil drainage system in the same trench asthe stormwater sewer, running alongside or in the road. This technique allows foreasy interception of transverse subsoil drains from under the road. If a subsoil drainis piped using perforated pitch fibre, slotted concrete or no-fines concrete pipes, it canbe connected to a stormwater manhole or inlet but it is advisable to use fibre cementor spun concrete pipes for the connection through the brickwork.

Where subsoil drainage is required to cut off seepage, e.g. under a road, it may beconnected into a conveniently situated stormwater manhole or catchpit by means of ano-fines concrete block built into the side wall of the brick chamber instead of bricks.

7 LARGE STORAGE PONDS

A major drainage system may consist of natural and artificial watercourses, largeconduits, stormwater storage facilities, servitudes and floodplains. Should such asystem be intended to cope with storms of 100-year frequency, the severity of thestorm and consequent disruption of certain activities may allow playing fields,carparks, open spaces and similar areas to be used for on-site storage of stormwater.

Such storage ponds can be used to regulate the rate of run-off and also for thecontrol of pollution. They are classified as either attenuation (retarding) ponds, whenonly a slowing down in the rate of run-off is required, or retention ponds, when, in theabsence of a positive outlet, the run-off is retained for future use.

A major drainage system incorporating storage ponds will only operate infrequently,the more so in Durban because of the topography and consequently the design ofsuch systems is rarely necessary. However, if required, more information on thedesign of storage ponds can be found in Part D, Urban Stormwater Management, ofthe Guidelines for the Provision of Engineering Services for Residential Townships, acopy of which is available in the Technical Library of the Engineering Unit.


8 DESIGN OF SHORT LENGTH CULVERTS UNDER EMBANKMENTS

The characteristics of flow in a culvert are complicated since the flow is controlled bymany variables, including inlet geometry, slope, size, roughness, approach, tailwaterconditions etc., and to obtain an accurate determination of flow would requirelaboratory or field investigations.

An estimate of the capacity of a culvert CANNOT be made using the Manning (orsimilar) formula for steady state pipe flow as the result would, in general, give aculvert size which is far too small.

However, an approximate solution to the problem can be obtained using the followingcharts and formula. However, in all cases, the design of culverts under embankmentsshould either be checked by, or referred to, the Coastal, Stormwater & CatchmentManagement Department of the Engineering Unit.

9 CALCULATING TIME OF CONCENTRATION

Note: Where the calculated value of Tc is less, then use the following minimum valuesfor Tc

Thick vegetation (bush, wooded areas) 15 minsCultivated areas and parks 15 mins Residential areas 15 minsFully developed, commercial, industrial 10 mins

Time of Overland Flow

The KERBY formula can be used where flow is overland/sheetflow, ie streamflow isnot yet developed:

Tc (in minutes) = 36x (r x L/1000/S0.5)0,467

Time of overland flow Units Type of surface r factor Tc - Mins Smooth paving 0.02 L - Distance (m) not greater than 200mclean soil 0.10 s - Slope (m/m) sparse grass 0.30 r - roughness value for type of surface mod grass 0.40 EXAMPLE thick bush/grass 0.80Tc overland MINS 19.96

The above table is not to be used for an overland distance (L) where L is a lot greaterthan 200 m. Here streamflow would be well established. Use stream flow equationand/or the manning equation and/or table values below in combination with the Kerbyequation where L exceeds 200m.

Bransby Williams Streamflow equation -Natural Watercourses onlyTc (in minutes) = 60(.87L2/1000000000/S)0.385

This table gives a guide to the time taken with slope and distance shown

The units are the same as tabulated above and yield the following typical results% slope Slope length (m) m/m 100 200 400 500 800 10001 0.01 4.0 6.8 11.6 13.7 19.7 23.42 0.02 3.0 5.2 8.9 10.5 15.1 17.915 0.15 1.4 2.4 4.1 4.8 7.0 8.3


30 0.3 1.1 1.8 3.1 3.7 5.3 6.350 0.5 0.9 1.5 2.6 3.0 4.4 5.2

The following table can be used to estimate the lower limit of the flow time becausethe table gives relatively high velocity values for large catchments indicative ofestablished runoff/flows.

% slopesheet flowpaved (m/s)

grassedwaterway (m/s)

near bareground (m/s) lawn (m/s) cultivated (m/s)

forest/meadow(m/s)

1 0.6 0.45 0.3 0.2 0.15 0.12 0.8 0.65 0.4 0.3 0.2 0.15 1.4 1 0.7 0.5 0.3 0.210 1.9 1.45 0.9 0.7 0.45 0.2515 2.4 1.8 1.1 0.8 0.55 0.320 2.7 2.05 1.3 0.9 0.65 0.3525 3 2.3 1.4 1 0.7 0.35

As examples:

the depth of sheet flow to achieve the paved velocities shown above would be approx20 mm.

the depth of flow in a natural stream sloping at 1 % would be some 200 mm deep toreach a velocity of 0.45m/s.

the depth of flow would be approx 50 mm in the case of the grassed waterway toreach the velocities shown

10 EXAMPLES OF CALCULATING THE RUNOFF COEFFICIENT C

In a rural (undeveloped) area the catchment being assessed has slopes where 20%of the area < 3%, 50% is between 3-10%, 15 % of the site area has slopes beween10-30% and the remaining 15% of the area is very steep with a slope of> 30%.Entering these values will generate a runoff slope component (Cs) of 0.14.

on the same site/catchment the soil profiles/permeability characteristics over thesurface show 0% of the area is very permeable, 10% of the area is permeable (lightsoil), 80% of the site is semi-permeable (most soil types), and the remaining 10% ofthe area is visible sheet rock. Entering these values will generate a runoff permeabilitycomponent (Cp) of 0.20.

on the same site/catchment the vegetation profiles across the surface shows 10% ofthe area is dense bush, 5% of the area is cultivated land or sparse bush, 75% of thesite is grassland, and the remaining 10% of the area is bare rock. Entering thesevalues will generate a runoff vegetal cover component (Cv) of 0.23.

The cumulative runoff component for the entire site (C)=(Cs)+(Cp)+(Cv) =0.14+0.2+0.23=0.57

(See the following table…)


DWA METHODPRE/RURAL Runoff Coefficient Catchment MAP MAP URBAN %Catchment Slope CS % > 900mm Lawn sandy<2% 0 0.08< 3% 20 0.05 Lawn sandy>7% 0 0.183-10 % 50 0.11 Lawn heavy<2% 0 0.1510 - 30 % 15 0.20 Lawn heavy>7% 33 0.30> 30 % 15 0.30 Residential single 0 0.50

100 0.14 Flats/dense townships 0 0.60Soil Permeability Cp % Industry , light 0 0.65Very perm (Dunes) 0 0.05 Industry , heavy 0 0.70Perm (light soil) 10 0.10 Business local 0 0.60Semi (most soils) 80 0.20 Business CBD 0 0.85Imperm (rock, paving) 10 0.30 Streets/roofs 67 0.95

100 0.20 100 0.74Vegetal growth Cv %Dense bush, forest 10 0.05 AREA WEIGHTING FACTORSCult land, sparse bush 5 0.15 % DWAGrassland 75 0.25 RURAL 40.00 0.57Bare Surface 10 0.30 URBAN 60.00 0.74

100 0.23 LAKES 0 0.00Rural Catchment coeff Ct = 0.57 Cdesign 100 0.67

POST/URBAN Runoff Coefficient

At a later stage the same site is developed as follows:

The site is partially developed with 40% of the area hardened (being roof, accessroads and paving), 20 % of the site is landscaped into flat lawns, and the balanceremains in a pre-development condition.

“In the above excell table above, the POST/Urban portion of the table, 40% hardeningof the overall site equates to 67% of the developed/Post portion, and the 20%landscaped portion equates to the balance or 33% of the developed/Post portion.”

The post development runoff coefficient is a combination of 40% of the site with arunoff C = 0.95, 20% of the site with C=0.3 say, and 40% of the site with say theoriginal undeveloped C=0.57 as previously calculated/shown. The combined C cansimply be estimated by the calculation C= (40% x 0.95 + 20% x 0.3 +40% x 0.57) /100 = 0.67.

11 METHOD OF STORMWATER RETICULATION DESIGN

1 Identify the catchment area on a topographical plan and locate the naturaloutfall.

2 Determine the points in the catchment at which flooding is intolerable andmark these on the plan as critical points.

3 Build up a provisional network of drainage on the plan, starting at the top ofthe catchment and working downwards until the outlet point is reached.

4 At this stage the field inspection should be done. After the network has beenchecked and adjusted, if necessary, to suit local conditions, the stormwaterlines are surveyed in order to establish levels, lengths, grades, etc.

5 Mark on the plan the position of those inlets which are obviously necessary(e.g. at local low points, intersections, critical points, etc.) And judge whereintermediate inlets are required. Rough calculations of local flows and


reference to the Kerb Inlet Charts : Appendices 13, 14 and 15 should be madeto assess spacings.

6 Establish the critical flow paths; i.e the flow paths from each critical point to theoutlet point of the catchment. These define the spinal system of criticalpipelines which is required to carry the 10 year storm flows from each criticalpoint to the outlet point.

7 Code the reticulation system so that each branch of the network and eachlength of pipe between manholes or inlets of any branch is indentifiable : forexample “A-B” would represent the pipe length between manholes or inlets “A-B’ on Figure 1 of this Appendix.

8 Calculate the area of catchment above the inlet.

9 Determine the time of concentration Tc in minutes.

N.B. The longest time of flow for all of the catchments (upstream catchments) thatcontribute runoff to the point considered must be used when calculating the designrunoff at that point. If two or more catchments contribute runoff to the point inconsideration, then the flow time for each catchment should be assessed. Theflowtime will generally consist of an overland flow time, any streamflow (in largercatchments) and or any conduit flowtime (in pipes or channels etc). The longest timeestimated for all of the catchments draining to the point will be the Tc used todetermine the rainfall intensity to be used in the formula Q=CIA/360. The area used inthe calculation will be the cumulative areas of all of the upstream catchments whichdrain to the point in consideration.

12 GENERAL METHODOLOGY

Label each catchment eg Area 1, Area 2 etc.

Determine the overland flow length and average slope for each catchment.

Determine the time of overland flow (use Kerby but L not > 200m) + time of streamflow (for distance > 200 m if applicable) + time of flow in conduit (if applicable thenuse chart or use Manning equation).

For subsequent downstream catchments the time of overland flow (length not >200m) + time of stream flow (if applicable) + time of flow in conduit must bedetermined when considering the design flow.

NB. If Tc < 15 mins (rural/parks/residential) use 15 mins

OR

If Tc < 10 mins (commercial/industrial/largely hardened sites) use 10 mins

Calculate the relevant runoff co-efficient C for each catchment and for the design ofpipes downstream, the area and longest time of concentration for all the contributingcatchments must be used.

Where more than one catchment contributes flow and the runoff Coefficient differs foreach contributing Catchment then a mean or representative C value must bedetermined on a proportional area basis i.e. C = ((C1 x A1) + (Ci x Ai) etc.) / (A1 +Aietc)


Identify the Latitude and Longitude of the site. (This can be located using the GIS)and obtain the relevant intensity of rainfall for the appropriate storm return period fromthe municipal website or the accompanying rainfall intensity excell spreadsheet.

www.durban.gov.za/eThekwini/Services/Engineering/CSCM

Then calculate the PEAK runoff from Q = CIA/360 cumecs

Using the field information, pipe inverts between manholes/inlets are used to calculatepipe gradients and a nomogram or manning formula can be used to establish thefollowing information:

theoretical pipe diameter;selected pipe diameter;capacity of selected pipe diameter; andvelocity of flow

All the above data should be tabulated for each step in the calculation but do notproceed downstream of a junction until all the adjoining branches have been dealtwith.

When the first leg below a junction is considered in the calculation, the areas of eachcatchment area for each branch leading to the junction are added together to give thetotal area for this leg. This procedure of summing the areas ensures that critical linesare carrying the 10 year runoffs from the full contributing catchment areas to thecritical point considered.

Using the flow value, you can derive the most suitable inlet from the Kerb Inlet Chartsgiven in the Appendix bearing in mind the following:

(a) unless the position of an inlet is fixed by physical conditions, the limiting factorto be used is the maximum allowable stream width of 2,5 m (3,2 m at criticalpoints) which will often lead to greater spacing between inlets and, as a resulta more economical design but care should be taken that the interveningspacing is carefully considered in areas where resulting flood damage couldoccur.

(c) the bypass flow should normally be about 20% of the calculated runoff unless totalcollection is desired (e.g. at a critical or local low point) and the appropriate inletfor approximately 80% interception is selected from the relevant Kerb Inlet Chart.

The information should be tabulated on your sketch plan of the site for ease of laterreference and for review.

At low points select an inlet for a flooded width of 3,2 m (see examples on inletcharts).

When necessary calculate energy losses at manholes using the information given inParagraph 3.17 above and tabulate the information on your design sketch plan layout.

It is important to note that the basic method of runoff calculations, pipe and inletdesign shown in this text, may be adopted for use in designing extensions to existingdrainage systems or upsizing of pipes in existing networks.

However, it is imperative that the design of extensions or relays should not overloadthe capacity of existing downstream pipes. If this situation occurs, please consult the


Stormwater/Catchment Manager for assistance. Attenuation storage or upgradingdownstream may be required.


13 WORKED EXAMPLE


In the above layout, the low point is a critical point “inlet C”, the critical path is C-D-E-F.

Consider point of entry at A

Area drained/Catchment Area is Area 1= 2 ha (= 20 000 m2)Flow Length =150 mHeight of Fall = 4 m Average Grade = 4/150 = 0.03 or 3 %Tc (Kerby formula in minutes) = 36x (r x L/1000/S0.5)0,467

Tc (Using S=0.03, r = 0.4; L = 150) yields Tc = 22.5 minutes

For rural/residential areas Tc should be >= 15 mins therefore Tc = 22.5 mins isin order.

Estimate Runoff Co-eff for Area 1 ie. C1 = Cs + Cp + Cr = 0.05 + 0.2 + 0.25 =0.5

Design is normally for a 3 year storm.

NOTE: In this example rainfall for a 5 year storm return period has been usedbut the same principles otherwise apply for a 3 year storm with theproviso that storm rainfall intensity would have to be interpolatedbetween the 2 and 5 year data since 3 year data has not been included.

The site is at Latitude 29 deg 44 min longitude 30 deg 50 min

To find the rainfall intensity we must interpolate for the Tc calculated of 22.5mins between the values shown in the table for the above Latitude andlongitude between 15 min and 30 mins. The formula used is of the form ITc =(Tc - 15)/(30 - 15)*(I30 - I15) + I15

Tc = 22.5 minutes gives I = 98.5 say 99 mm/hr

The recurrence interval adjustment factor “ft” for 5 year storm is 0.55 from thetable in paragraph 2.4 above.

Q = ft x CIA/360 = 0.55 x 0,5 x 99 x 2/360 = 0.151 cumecs

From information obtained in the field, the grade between A & B is 1 in 42. The slope S is therefore = 0.024

From Manning equation Q = V x A = 1/n x (R)2/3 x S0.5 x A = 1/n x(D/4)2/3 xS0.5 x Pi x D2/4

The manning n value for a concrete pipe is between 0.011 new to 0.013 for anolder pipe

The theoretical pipe diameter = 300 mm using Manning or find the diameterusing the pipe flow charts.

The capacity of the selected pipe (new and flowing full) = 0.180 cumecs

The headwall depth required for overland flow to enter the pipe can be foundin the appendix chart or estimated in the following spreadsheet calculator:


SHORT sloped PIPE CULVERTS <= 250 mm diaHEAD H DIA D (m) H/D slope

1.25 0.09 13.88888889 0.001q=D2(gD)0.50.48(S/0.4)0.05(H/D)1.9 m3/s H/D>0.8 0<h/d<0.8q=D2(gD)0.50.44(S/0.4)0.05(H/D)1.5 m3/s 0.8>H/D>1.2 0.8<H/D<=1.2q=0.6pi (D2/4)(2gh)0.5 m3/s 0.018902997 h/d>>1.2

In the above calculator, a head of 0.7 m only is required to induce a flow of0.157 m3/s in a 300 dia pipe. The actual available head would normally begreater in practice since the minimum cover for a pipe is more.

The velocity of flow in selected pipe at full pipe flow conditions =2.5 m/s

Before considering conditions at inlet B, calculate the time of flow in pipe A-B

For A-B Flow Length = 80 m

Flow Velocity in 300 mm dia Pipe at Grade of 1 in 42 = 2,5 m/s

Flow time = Length of pipe / velocity = 80/2.5 = 0.5 mins

The time of concentration Tc at B = 22.5 mins + 0.5 mins = 23 mins

Consider Inlet at B

Area drained A = 0,5 haFlow Length = 120 mHeight of Fall = 5 mAverage Grade = 5/120 = 0.041 = 4.1%Tc (Using Kerby Formula S=0.041, R= 0.4; L = 120) yields Tc = 18.3 minutes

For rural/residential areas Tc should be >= 15 mins therefore Tc = 18.3 mins isOK

Estimate Runoff Co-eff = C2 = Cs + Cp + Cr = 0.11 + 0.2 + 0.25 = 0.56

Assume 5 Year Storm Return Period and I = 110 mm/hr by interpolatingbetween the tabulated values for I at 15mins and 30 mins because Tc = 18.3mins.

Q = ft x CIA/360 = 0.55 x 0.5 x 110 x 0.5 /360 =0.035 m3/s

With a grade of 1 in 20 between B1 and B and using manning formula, theselected pipe diameter = 300mm

capacity of selected pipe = 0,392 cumecs

the velocity of flow in the selected pipe is negligible.

Consider MH.B


Area contributing to MH B = Area 1 + Area 2 = 2.5 ha

Calculate the longest travel time for contributing areas to MH B gives Tc = 23mins

The combined runoff coeff C for Area 1 and 2 = (C1 x Area1 + C2 x Area2)/(Area1 +Area2) since different runoff coefficients were determined for Area 1and Area 2 (i.e we are calculating a mean representative value) = 0.51

Rainfall intensity I for a 23 min 5 year storm interpolated between the statisticalintensities given for 15 mins and at 30 mins results in I = 93.1 mm/hr

Q = ft xCxIxA/360= 0.55x0.51x93.1x2.5/360 = 0.181 cumecs (= 0 181 m3/s or181 litres/sec)

The surveyed grade is 1 in 80 between B and C

selected pipe diameter = 375 mm

capacity of selected pipe from manning or table = 0,230 m3/sthe velocity of flow in the selected pipe = 2,1 m/sLength of Pipe B-C = 60 m

Flow Time in Pipe B-C = 0.5 mins

Tc at C = 23.5 mins

Consider Inlet C

NB: C is a lowpoint i.e design is for a 10 yr storm as a critical point

Area drained = 1.7 haFlow Length = 180 mHeight of Fall = 14 mAverage Grade = 14/120 = 0.12 or 12 %

Tc (Using Kerby Formula S=0.12, R= 0.4; L = 180) yields Tc = 17.3 minutesFor rural/residential areas Tc should be >= 15 mins therefore Tc = 17.3 mins isOK

Estimate Runoff Co-eff = C2 = Cs + Cp + Cr = 0.2 + 0.2 + 0.25 = 0.65

As the runoff entering inlet C from Area 3 will be required in the sizing of theoutlet, Q should now be calculated for this area.

A = 1.7 haTc = 17.3 minsC = 0.6 as calculated aboveI for 10yr storm = 142.4 mm/hr (use 10 yr intensity being a critical point)Q = ft 10year xCxIxA/360= 0.6 x 0.65 x 142.4 x 1.7/360 = 0.262 cumecs (= 0.262m3/s or 262 litres/sec)

For total flow at C

Area Ac = Area1 + Area2 + Area3 = 4.2 ha


Tc = Tc(of Area 1) + Tc(from point A to B) +Tc(from point B to C)=22.5+0.5+0.5=23.5mins (being longest Tc for all contributing areas)

Mean or representative runoff coeff C at point C must be determined. It is aproportional average of all 3 of the contributing catchments, as follows:

= (C1xA1 + C2xA2 + C3xA3)/(A1+A2+A3) =(0.5*2+0.56*0.5+0.65*1.7)/(2+0.5+1.7)

= 0.57

Intensity I10year = 120.8 mm/hr being the inter[polated value for 23.5mins for a 10yr storm

Q = ft 10year x CABCx I xAABC/360= 0.6*0.57*120.8*4.2/360 = 0.482 cumecs (=0.482 m3/s or 482 litres/sec)

With a grade of 1 in 75 between C and D

Selected pipe diameter from manning equation or pipe flowchart = 535 mm diapipe (Capacity full flow = 0.592 l/s)capacity of selected pipe = 0.71 cumecsvelocity of flow in selected pipe = 2.5 m/s

Length of Pipe C-D = 50 mFlow Time in Pipe = Length/Vel = 50/2.5 = 0.33 minsTc at D = Tc at C + 0.33 mins = 23.5 + 0.33 = 23.8 mins

Consider MH.D

Area = A1 + A2 + A3 as for inlet C = 4.2 ha

Runoff coeff = 0.57 as at pt C

Intensity I10year = 119.8 mm/hr from interpolation for Tc = 23.8 mins for 10 yearstorm (C was a low pt or critical pt)

Q = ft10year xCABCD x I x AABCD/360= 0.6*0.57*119.8*4.2/360 = 0.478 cumecs (= 0.478 m3/s or 478 litres/sec)

With a grade of 1 in 80 between D and E

selected pipe diameter = 600 mmCapacity of selected pipe = 0.54 cumecsvelocity of flow = 1.91 m/sLength of Pipe D-E = 100 mFlow Time in Pipe = 100/1.9 = 0.9 mins

Tc at E = 23.8 + 0.9 = 24.7 mins

Consider Inlet E

Flow Length = 70 mHeight of Fall = 3 mAverage Grade =3/70 = 0.04 = 4 %


Tc (Using Kerby Formula S=0.04, R= 0.4; L = 70) yields Tc = 14.4 minutes say15 mins minimum

Estimate Runoff Co-eff for Area 4 = Cs + Cp + Cr = 0.11 + 0.2 + 0.25 = 0.56

For Total Flow at E

Area = A1 + A2 + A3 + A4 = 4.5 haTc = 24.7 mins

C = (C1xA1+C2xA2+C3xA3+C4xA4)/(A1+A2+A3+A4)=(0.5*2+0.56*0.5+0.65*1.7+0.56*0.3)/(2+0.5+1.7+0.3) = 0.57

Intensity I10year = 116.6 mm/hr by interpolation for Tc = 24.7 mins

Q = ft10year xCABCD x I x AABCD/360= 0.6*0.57*116.6*4.5/360 = 0.498 say 0.5 cumecs (= 0.5 m3/s or 500litres/sec)

With a grade of 1 in 10 between E and Fselected pipe diameter = 600 mm (pipe size must conform to upstream pipesize even though a 375 mm pipe has the reqd capacity at a grade of 1/10)capacity of selected pipe = 2.3 cumecsvelocity of flow = 8.1 m/s


14 DESIGN OF STORMWATER KERB INLETS

Once the pipe reticulation designed, consideration should be given to the sizing ofinlets. Examples are shown on the actual charts.

Briefly considering the above example comments are as follows:

Consider inlet B1

Q has been calculated aboveThe gradient of the Road must be knownNote that normally 80% of gutter flow is to be interceptedThe Maximum Stream Width = 2.5 mThe road Crossfall should be determined Using kerb inlet design charts, the number of bays and upstream channellength can be determined

Check that the Stream Width is not exceeded and determine the actualintercepted flow (the balance should be added to the required flow at the nextdownstream inlet Data should be tabulated or put on your design sketch layoutplan

Consider Inlet C

Q will be as calculated plus the bypass flow from inlet B1

As this inlet is at a low point, the approach grade on either side of the inlet willbe reasonably flat and a grade of 1% can be assumed in both cases.

Interception must be 100% and the maximum stream width should not exceed3,2 m.

If flow is large, auxiliary inlets should be provided to intercept part of theupstream flow.

Using the chart for low point conditions Note that at low points there is nonecessity to depress the upstream channel.

Consider Inlet E

Area contributing flow is area 4 only and the Tc for area 4 will be used in thecalculation only.(N.B. a storm return period of 10 years is used only in the design of the pipenetwork) due to the low point or critical point at C

There is no bypass flow from inlet C

Conditions at Point of Entry in Area 1

The available head H must be determined on site = 0,94 m and site conditionsare assumed to be such that this depth of headwater is acceptable


15 CALCULATIONS OF INVERT LEVELS AT MANHOLES

SEE PARAGRAPH 3.17 ON PAGE 17

In the previous example:

Manhole B

if there was a substantial flow from inlet B1 then at MH B it would be assumed thatthe velocity head of the inlet pipe from A was destroyed and consequently bendlosses would in such a case be irrelevant and the reduced invert level would becalculated as follows:

The required difference in invert level required is

= (dout - din) + vout2/2g

NOTES: Vout is the velocity of flow in the outlet pipe using the calculated design runoffflow divided by the full area of the chosen pipeThe velocity head would be measured above the crown of the pipe – i.e.assuming full pipe flow

If there were no inflows but only the energy losses due to the 900 bend of 0.5 x v2/2g.Then the required difference in invert level required would be:

= (dout - din) + vout2/2g –(vin

2/2g - 0.5vin2/2g)

Inlet C11

At this inlet the change in grade of the pipe is small and can be disregarded but asthere is a substantial inflow, a energy or velocity head adjustment is required.

As there is a difference in pipe diameter, the reduced invert level of the outlet pipefrom the exit level of the inlet pipe, is the sum of the velocity head required for theoutlet pipe and the difference in diameter.

i.e The required difference in invert level required is= (dout - din) + vout

2/2g

Manhole D

At manhole D where there is a 900 bend and no difference in pipe diameter and noinflow, the difference in invert level is simply = 0.5vin

2/2g

Inlet E

At this point the effect of the small inflow can be disregarded and the requireddifference in invert level required is

= (dout - din) + vout2/2g - (vin

2/2g - 0.75 x 0.5vin2/2g)

Data should be tabulated in on a sketch of the layout or in a suitable table.


APPENDICES

PART R Stormwater Disposal SABS 0400 - 1990 page 36Hydraulic elements of circular sections page 38Bend losses page 39Details of anchor blocks page 40Discharge through box culverts : inlet control page 41Discharge through pipe culverts : inlet control page 42Kerb Inlet chart 1: cross fall 2% page 43Kerb Inlet chart 2: cross fall 2,5% page 44Kerb Inlet chart 3 & 4 : cross fall 4% and 6% page 45Pipe bedding details page 46Protection of pipes at reduced depths of cover page 47Recommended layout of services - 9.5 metre reserve page48Recommended layout of services - 12 metre reserve page 49Recommended layout of services - 16 metre reserve page 50Recommended layout of services - 19 metre reserve page 51

















ETHEKWINI DESIGN MANUAL - · PDF fileengineering unit coastal stormwater & catchment management department design manual: guidelines and policy for the design of stormwater drainage

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