STORMWATER MANAGEMENT

1. STORMWATER MANAGEMENT By 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. Goals: Control Runoff Quantity Control Runoff Quality Control Groundwater Recharge Implementation: Non-Structural Strategies Structural Strategies * BMP can be found at www.njstormwater.org Best Management Practices*

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. 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 Channel flow time, min Drop in channel elevation,m or ft 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 timeΔt 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