HydroQual, Inc. Application of the SWMM Storage-Treatment Block for Analysis/Design of Extended-Detention Ponds Thomas L. Newman II, P.E. Tarig A. Omer Eugene D. Driscoll HydroQual, Inc.

Overview EDPs -popular BMPs for storm water control Current design approach based on: –rules of thumb –guidance documents target large storms drain time: hrs –potential for poor pollutant-removal efficiency Alternate design approach, based on: –long-term simulations –simple screening model –SWMM S-T detention time concept, targets smaller storms goal: optimize pollutant removal efficiency

HydroQual, Inc. Extended-Detention Ponds (EDPs) Basin to detain storm water, then drain dry Popular BMP –Cost-effective –Peak Shaving and Pollutant Removal –Retrofit from DPs, WPs –Brief Detention Time No release of warm or anoxic water fewer safety problems

HydroQual, Inc. Traditional Design of EDPs Rules of Thumb /Guidance Documents –Volume (Capacity) runoff from a large, infrequent design storm –examples: 1-year or 1.0-inch storm –general implication: bigger is better –Drain-out Time typically hours to drain from full to empty –Expected Performance: 90% SS; 50% TN, TP however, Galli 1992 reported most EDPs are not performing that well!

HydroQual, Inc. Analysis of EDP Performance 2 Projects –EDPs designed by others –Traditional design methods Check Pollutant-Removal Efficiency –Evaluate a long-term (2-year) period –SWMM S-T Block depth/volume/surface area/outflow rate completely mixed vs. plug flow pollutant-removal/decay (k) rates

HydroQual, Inc. Model Results for Original EDP Design Poor Removal Efficiencies –TSS: 17%(expected %) –TN: 7%(expected %) –TP: 9%(expected %) Why?? –reasons for low efficiency not clear... –SWMM output not easy to interpret –need a common-sense check of black box

HydroQual, Inc. Modeling Enhancements Post-processing program –to reduce massive SWMM output files –also to calculate volume utilization –calculate number of times the EDP filled to capacity Simple spreadsheet screening model –simplifying assumption: entire storm at once –for any storm, estimate fill level, detention time, pollutant removal E –easy iterations for orifice size, k rate, EDP volume

HydroQual, Inc.

Simple Spreadsheet Model Now able to check SWMM results Easier to develop insight into effects of: – storm size – outlet orifice size – k rate Use in conjunction with SWMM for sensitivity runs of EDP design elements

HydroQual, Inc. Sensitivity Analyses Effect of Orifice on Detention Orifice Size –5 inches (12.7 cm) original diameter –2 inches (5.0 cm) minimum acceptable Smaller sizes –better use of pond capacity for detention –no danger of turning EDP into WP

HydroQual, Inc. Sensitivity Analyses Effect of Orifice on Efficiency Orifice Size –Smaller sizes increase pond detention time –Higher detention time produces better pollutant removals –Example: reducing from 5 inch to 2 inch, TSS from 17% to 70%

HydroQual, Inc. Sensitivity Analyses Effect of EDP Sizing EDP Capacity –1-yr, 24-hr storm =2.71 inch (6.9 cm) (original volume) –3-month, 24-hr storm =1.25 inch (3.1 cm) (45% of original) Same performance with half-sized pond if orifice is 0.5-in smaller

HydroQual, Inc. Detention Time for EDP Design Traditional Guidance –Drain time for large, infrequent storms –not indicative of typical detention time An important step forward would be to develop a useful definition of detention time that is applicable for EDP design Alternative Approach –based on effective detention time over long term –still developing

HydroQual, Inc. Proposed Definition of Effective Detention Time Based on long-term runs: M out =M in e -k te –M = pollutant mass –k = decay rate –te = effective detention time or, in terms of removal efficiency, E: t e = -ln(1-E)/k –(E=removal efficiency)

HydroQual, Inc. Design Application of Effective Detention Time t e = -ln(1-E)/k (E=removal efficiency) Application to design: –use target E, pollutant decay rate k to define t e –design EDP so that attain t e over the long term: Preliminary Design: use screening model to check that t e is met for a frequent storm (i.e. average or 0.5-inch storm) Final Design: long-term simulations with SWMM S-T

HydroQual, Inc. Recommendations Use caution with traditional approach Evaluate performance over long term SWMM-ST is suitable, but Use the simple spreadsheet model to check SWMM and to aid design Developing an alternate approach based on effective detention time (promising, not complete)

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HydroQual, Inc. Summary of Results