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Peak Flow and Volume: A Case Study of Watershed-Scale Stormwater Control

Peak Flow and Volume: A Case Study of Watershed-Scale Stormwater Control. Clay Emerson PhD PE Princeton Hydro, LLC. NJAFM 8 th Annual Conference “Breaking the Cycle” Somerset, NJ October 2, 2012. OUTLINE. Objective Methods Results Conclusions. OBJECTIVE.

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Peak Flow and Volume: A Case Study of Watershed-Scale Stormwater Control

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  1. Peak Flow and Volume: • A Case Study of Watershed-Scale Stormwater Control Clay Emerson PhD PE Princeton Hydro, LLC NJAFM 8th Annual Conference “Breaking the Cycle” Somerset, NJ October 2, 2012

  2. OUTLINE • Objective • Methods • Results • Conclusions

  3. OBJECTIVE • Post development peak flow rate not to exceed predevelopment level • Initiated the Release Rate method of SWM in PA • No runoff volume requirements • Designed for 2-100yr storms

  4. OBJECTIVE Over 100 detention basins ~17% Impervious Case Study: Valley Creek Chester County PA

  5. METHODS Precipitation Data • Network of five to seven recording gages • Five minute interval • GageInterp precipitation grids in HEC-DSS format

  6. METHODS

  7. METHODS Detention Basin Information Survey of Detention Basins • 111 Basins Surveyed • Storage vs. Outflow Curves • Photographic Record • GPS Location Record

  8. SURVEY Maintenance

  9. SURVEY

  10. SURVEY

  11. METHODS

  12. METHODS

  13. METHODS Only 4% of the total area drains to a detention basin. However, this accounts for 25% of the directly connected impervious area.

  14. RESULTS • 0.5” Rainfall • Peak Without – 45cfs • Peak With – 46cfs • 2.0” Rainfall • Peak Without – 117cfs • Peak With – 114cfs

  15. RESULTS Rest of watershed Detention basin areas with (red) and without (blue)

  16. RESULTS • Just detention basin drainage areas. (2”)

  17. RESULTS • Hypothetical Volume Control • Volume control of 0.5”, 1.0”, 2.0” of runoff from the basins’ drainage areas.

  18. RESULTS PA IDF Curve Largest storm was only ~2.0” on average.

  19. 2 year storm RESULTS 50 Years Daily Rainfall Record Chadds Ford Rain Gage • 2+ year design • Accounts for only 3% of annual precipitation

  20. RESULTS • 1- through 100-year design storms • uniform distribution

  21. CONCLUSIONS Detention basins in Valley Creek watershed: • have a negligible impact on stormwater flow • are designed for only 3% of the annual rainfall • are often poorly maintained

  22. CONCLUSIONS Focus on hydrology: • Act 167 “release rate” districts? • Recreate predevelopment run-off volume • If run-off volumes are maintained watershed wide peak flow rates will follow • Streambank erosion, baseflow, flow duration relationship

  23. RESULTS “Runoff quantity impacts”

  24. QUESTIONS Clay Emerson PhD PE cemerson@princetonhydro.com Princeton Hydro, LLC Emerson, C.H., Welty, C., Traver, R.G., “A Watershed-Scale Evaluation of a System of Stormwater Detention Basins” ASCE Journal of Hydrologic Engineering, Vol. 10, No. 3, pgs. 237-242 May/June 2005.

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