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Practical Application of Modeling Results to Land Development Policies in Contra Costa County

This presentation discusses the practical application of modeling results for land development policies in Contra Costa County, California. Topics covered include stormwater NPDES requirements, low impact development, model approach and features, using results for LID design, and enhancements in progress.

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Practical Application of Modeling Results to Land Development Policies in Contra Costa County

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  1. Practical Application of Modeling Results to Land Development Policies in Contra Costa County California Water Environmental Modeling Forum Watershed and Urban Hydrology Modeling Workshop 22 June 2007 Dan Cloak Environmental Consulting

  2. Topics • Stormwater NPDES requirements • Low Impact Development • Model approach and features • Using results for LID design • Enhancements in progress

  3. NPDES requirementsfor new developments • Minimize imperviousness • Control pollutant sources • Treat stormwater prior to discharge from the site • Match peaks and durations to pre-project conditions (HMP) • Maintain treatment and flow-control facilities in perpetuity

  4. Low Impact Development • Stormwater treatment and flow control • Minimize imperviousness • Disperse runoff • Use Integrated Management Practices (IMPs)

  5. Swale

  6. Planter Box

  7. Dry Well

  8. Showing Treatment Compliance • NPDES Permit sizing criteria for treatment control: • “collect and convey” drainage design • conventional, “end of pipe” treatment • use of “C” factors to determine design inflow or volume

  9. Sizing criterion for treatment 0.2 inches/hour BMP Area/Impervious Area = 0.2/5 = 0.04 Planting medium i = 5 inches/hour

  10. Application of sizing factor

  11. LID for flow control • Can LID facilities mitigate increased peaks and volumes of flows from impervious areas? • How would we demonstrate that? • What are the design criteria?

  12. Who needs a model? • Needed: A conservative “best guess” for appropriate sizing factors • Proposed solution: Implied equivalence to pre-project condition by • Weighted “C” (rational method) • Curve number (NRCS) • ΔS (initial storage in NRCS) • Any other values for input parameters to a hydrologic model • However: Water Board staff specified continuous simulation

  13. HSPF analysis of unit-acre runoff • 33 years hourly rainfall • Pre-project condition • 100% impervious condition • Hydrologic soil groups A, B, C, D • Swales, Bioretention Areas,In-ground and Flow-through Planters • Underdrain with flow-restrictor in C&D soils • Dry wells, infiltration trenches and basins

  14. Modeling Approaches • “Bathtub” approach • Pervious land surface over gravel • Two-layer approach • Stage-storage discharge relationships represented in FTABLEs. • Soil moisture content recalculated at each time step • Matric head within soil pores and hydraulic conductivity recalculated for each time step

  15. Results: Control of Peak Flows

  16. Results: Flow Duration Control

  17. Sizing Factors for Flow Control

  18. Adjustment to annual rainfall

  19. Using LID with Sizing Factors • Step-by-step instructions • Intuitive interface • Can be used by developer’s designer (engineer, architect, or tech) • Demonstrates compliance with both treatment and flow-control requirements

  20. Example Design Using the Sizing Calculator

  21. DMA PAVE-3 4,826 SF DMA PAVE-2 2,737 SF DMA LS-3 1,207 SF IMP PL-2530 SF DMA LS-2 1,112 SF IMP PL-3515 SF DMA ROOF-1 4,681 SF DMA PAVE-1 7,651 SF DMA LS-1 6,205 SF IMP PL-1 750 SF

  22. DMA PAVE-3 4,826 SF DMA PAVE-2 2,737 SF DMA LS-3 1,207 SF IMP PL-2530 SF DMA LS-2 1,112 SF IMP PL-3515 SF DMA ROOF-1 4,681 SF DMA PAVE-1 7,651 SF DMA LS-1 6,205 SF IMP PL-1 750 SF

  23. How “real” are sizing factors? • Applied a model intended for watershed scale to site scale • Sizing factors are minimums, facilities are actually built larger • Used textbook or countywide values for input parameters • Didn’t account for losses or inefficiencies in drainage systems

  24. Next: Improved IMPs

  25. Abating the drop-off problem 2" 6" Overflow 10" 6" Soil Mix 18" Gravel 18" Under drain

  26. Floodable pavement 2" Overflow 10" 6" Soil Mix 18" Gravel 18" Under drain

  27. Cistern and bioretention Overflow 2" 2" 4" Soil Mix 18" Gravel 18" Under drain

  28. Enhanced swale section 6" 1 4 18" min. Soil mix Gravel 24" min. Under drain

  29. Enhanced underground storage 2" Overflow 10" Soil Mix 18" 18"

  30. Conclusions • Design criteria for stormwater facilities are always “best professional judgment.” • Modeling provides some insights, but perhaps just as many illusions. • More useful insights come from observations and tinkering. • That requires building and operating many facilities over a long period.

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