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STORMWATER MANAGEMENT

STORMWATER MANAGEMENT. By John Gribbin, P.E. Revised 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

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STORMWATER MANAGEMENT

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  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

  10. 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

  11. Drainage Area

  12. Time of Concentration

  13. C- composite runoff coefficient

  14. Typical Runoff Coefficients

  15. 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

  16. Tc – Overland Flow

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

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

  19. 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)

  20. 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

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

  22. SCS RAINFALL –RUNOFF CURVES

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

  24. 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

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

  26. Storage and Flow Factors

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