STORMWATER MANAGEMENT By John Gribbin, P.E. Revised by Prof. Washington for CET413.

STORMWATER MANAGEMENTSTORMWATER MANAGEMENT

ByByJohn Gribbin, P.E.John Gribbin, P.E.

Revised by Prof. Washington for CET413Revised by Prof. Washington for CET413

Topics

• Effects of Land Development• Stormwater Management Regulations

• Best Management Practices• Runoff by Rational Method

• Runoff by NRCS Method• Runoff Hydrographs• Reservoir Routing

• Detention Basin Design

Effects of Land Development

• Increase of Runoff Rate• Increase in Runoff Volume

• Decrease in Recharge• Increase in Pollutants• Increase in Erosion

• Potential for Flooding Downstream

Areas Impacted

• Streams• Lakes

• Wetlands• Private Property• Public Property

Best Management Practices*

Goals:• Control Runoff

Quantity• Control Runoff Quality• Control Groundwater

Recharge

Implementation:• Non-Structural

Strategies• Structural Strategies

* BMP can be found at

www.njstormwater.org

Non-Structural Strategies

• Protect areas that provide water quality benefits.• Minimize impervious surfaces.• Maximize the protection of vegetation.• Minimize the decrease in “time of concentration.”• Minimize clearing and grading.• Minimize soil compaction.• Provide low maintenance landscaping (minimize the use

of lawns).• Provide vegetated open-channel conveyance systems.

Structural Strategies

• Detention Basin• Dry Wells• Manufactured Treatment Devices• Infiltration Basin• Pervious Paving System• Bioretention Basin• Constructed Stormwater Wetlands• Vegetative Filter

Runoff Calculations

• Rational Method

• Modified Rational Method

• NRCS Method

Rational Method

Qp = Aci

Where Qp = peak runoff, cfs

A = drainage area, acres

c = runoff coefficient

i = rainfall intensity, in/h

Rational Method

Procedure:• Delineate the drainage area• Measure the size of the drainage area• Compute composite c• Delineate hydraulic path• Compute time of concentration, tc, min.• Select rainfall frequency in years• Determine i using I-D-F curve• Compute peak runoff using Qp = Aci

Drainage Area

Time of Concentration

C- composite runoff coefficient

1 1 2 2 2 2

1 2 3

1( .....)

,

basin area (A A A ...)

T

T

C C xA C xA C xAA

where

C composite runoff coefficient

A total

Typical Runoff Coefficients

COMPUTE THE Tc (time of concentration)

Tc= Overland Flow + Channel Flow

where,

overland flow (fig. 9-3)

channel flow (fig. 9-4)

or Time= Distance/velocity

Tc – Overland Flow

Channel Flow

Dro

p in

cha

nnel

ele

vatio

n,m

or

ft

Leng

th o

f C

hann

el,

m o

r ft

.

Cha

nnel

flo

w t

ime,

min

i-d-f chart (rainfall intensity)

NRCS Method (was known as SCS method)

qp = AmquQ

where qp = Peak runoff, cfs

Am = Drainage area, s.m.

qu = Unit peak discharge, csm/in

Q = Runoff, in (R in textbook)

NRCS Method

Procedure:• Delineate the watershed• Measure the watershed area• Compute Composite CN

• Compute time of concentration tc

• Select rainfall frequency• Determine 24-hour precipitation P• Determine rainfall distribution

• Determine Ia

• Determine Q

• Determine qu

• Compute peak runoff using qp = AmquQ

CN-SCS runoff curve number

Land use description

Forests

Meadows

Grass - Lawns

Commercial-Business

Residential

Pavement- Roofs

Typical Runoff Coefficients

SCS RAINFALL –RUNOFF CURVES

Unit Peak Discharge, csm/in

Csm/in =cu. Ft. per sec. per square mile of watershed per inch of runoff

Basin Routing

• Routing is a mathematical procedure for computing an outflow hydrograph when the inflow hydrograph is known.

• Routing relies on the so-called continuity equation which is a statement of conservation of mass of water entering and leaving the basin.

• Continuity equation:

_ _

I – O = ΔS/Δt

_

where I = mean flow into basin during time Δt

_

O = mean outflow from basin during time Δt

ΔS = change in basin storage during time Δt

Δt = incremental time period

Triangular Hydrograph for Design

Base = 3 Tc2 TcTc

Hei

ght

= Q

max

Storage and Flow FactorsBasin Volume

Total Rain VolumePeak Outflow

Peak inflow Existing Peak Discharge

Proposed Peak Discharge

1

Storage Factor

FlowFactor

FlowFactor

Storage Factor FlowFactor