1. STORMWATER MANAGEMENTBy
John Gribbin, P.E.
Revised by Prof. Washington for CET413

2. 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

3. 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

4. Areas Impacted
Streams
Lakes
Wetlands
Private Property
Public Property

5. Best Management Practices*
Goals:
Control Runoff Quantity
Control Runoff Quality
Control Groundwater Recharge
Implementation:
Non-Structural Strategies
Structural Strategies
* BMP can be found at

6. 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.

7. Structural Strategies
Detention Basin
Dry Wells
Manufactured Treatment Devices
Infiltration Basin
Pervious Paving System
Bioretention Basin
Constructed Stormwater Wetlands
Vegetative Filter

8. Modified Rational Method
Runoff Calculations
Rational Method
Modified Rational Method
NRCS Method

9. Rational Method Qp = Aci Where Qp = peak runoff, cfs
A = drainage area, acres
c = runoff coefficient
i = rainfall intensity, in/h

11. 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

12. Drainage Area

15. Time of Concentration

16. C- composite runoff coefficient

17. Typical Runoff Coefficients

18. 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

19. Tc – Overland Flow

20. Channel Flow Drop in channel elevation,m or ft Channel flow time, min
Length of Channel, m or ft.

21. i-d-f chart (rainfall intensity)

22. 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)

23. 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

24. CN-SCS runoff curve number
Typical Runoff Coefficients
Land use description
Meadows
Forests
Grass - Lawns
Commercial-Business
Residential
Pavement- Roofs

25. SCS RAINFALL –RUNOFF CURVES

26. Unit Peak Discharge, csm/in
Csm/in =cu. Ft. per sec. per square mile of watershed per inch of runoff

27. 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

28. Triangular Hydrograph for Design
Height = Qmax
2 Tc Tc
Base = 3 Tc

29. Storage and Flow Factors