ABSTRACT INTRODUCTION

Transcription

1 AN EVALUATION OF DETENTION POND DESIGN USING CONTINUOUS SIMULATION Michael F. Schmidt, P.E., Senior Water Resources Engineer, Camp Dresser & McKee Inc., Jacksonville, Florida and Thomas F. Quasebarth, Senior Environmental Scientist, Camp Dresser & McKee, Annandale, Virginia, and Theodore J. Johnson, P.E., Water Resources Engineer, Camp Dresser & McKee, Denver, Colorado ABSTRACT Detention ponds are a frequently used Best Management Practice (BMP) for flood control and the control of nonpoint source pollutant loads from urban land uses. Various design criteria have been developed for site-specific applications. In many cases, long-term monitoring data is not available to evaluate the hydrologic and water quality performance of these devices over extended periods. Currently, 'Many BMP assessments simply assign a constant pollutant removal efficiency to estimate long-term performance based upon average pollutant removal efficiencies reported under the U. S. Environmental Protection Agency's (EPA's) Nationwide Urban Program (). The reported that while detention basins overall were very effective, monitored pollutant removal efficiencies varied from excellent to very poor. Monitoring these ponds typically requires inflow and outflow data from the same storm events to characterize pollutant removal rates. In practice, some monitored storms will exhibit inflow runoff volumes which are higher than outflow runoff volumes because excess storage capacity is available at the start of runoff due to drawdown of the permanent pool during dry weather interevent periods as a result of evaporation or infiltration. A long-term continuous analysis can be used to better characterize detention times, and thus, the pollutant removal efficiencies that can be achieved over time. This paper compares the features and results of three public domain models for detention pond evaluations. INTRODUCTION As part of the National Urban Program (), the U. S. Environmental Protection Agency (EPA) compiled a variety of storm-related water quality data to characterize non-point source pollutant loads and efficiencies of best management practices (BMPs). Detention ponds are a frequently used Best Management Practice (BMP) for flood control and the control of nonpoint source pollutant loads from urban land uses. Various design criteria have been developed for site-specific applications. In many cases, long-term monitoring data is not available to evaluate the hydrologic and water quality performance of these devices over extended periods. Currently, many BMP assessments simply assign a constant pollutant removal efficiency to estimate long-term performance based upon average pollutant removal efficiencies reported under the U. S. Environmental Protection Agency's (EPA's) Nationwide Urban Program (). The reported that while detention basins overall were very effective, monitored pollutant removal efficiencies varied from excellent to very poor. Monitoring these ponds typically requires inflow and outflow data from the same storm events to characterize pollutant removal rates. In practice, some monitored storms will exhibit inflow runoff volumes which are higher than outflow runoff volumes because excess storage capacity is available at the start of runoff due to drawdown of the permanent pool during dry weather interevent periods as a result of evaporation or infiltration. A long-term continuous analysis can be used to better characterize detention times, and thus, the pollutant removal efficiencies that can be achieved over time. The database needs to be updated by long term detention pond removal efficiencies, interstorm effects (back-to-back storms), and long term fate of pollutants captured. Most programs sampled a few storms for a given pond. It would be most desirable to monitor a full hydrologic year. In order to further the technology, some programs are monitoring more storm to storm effects on pond removal efficiencies, such as the Rouge River National Wet Weather Demonstration Program in Detroit, Michigan. The purposes of this paper are to: - evaluate a monitored detention pond for potential design improvements - compare "off-the-shelf' continuous simulation models for analysis of detention pond performance - identify continuous simulation model operational requirements - discuss detention pond design requirements Continuous simulation models such as Storm Water Management Model (), Storage Treatment Overflow Model (STORM), and Program for Predicting Polluting Particle Passage through Pits, Puddles, and Ponds (P8) are used to provide estimates of flows and concentrations over extended simulation

2 periods. This paper presents the use of these three continuous simulation models for evaluating the effectiveness of detention pond performance over long-term periods, including a discussion of model strengths and weaknesses for detention basin design. Modeling wet weather responses for water quantity and water quality can be very complex and timeconsuming. Figure I shows the hydrologic cycle and some of the major phenomena that must be considered and/or modeled. This paper will focus on the nonpoint source pollutant load buildup, washoff by rainfall, delivery as inflow to a detention pond, removal processes in the pond, and outflow from the pond. Figure 2 shows how the improved information/approach for BMP efficiency evaluation can b utilized to refine a watershed management program. Better information on BMP performance will allow improved decisions on capital improvements. For this paper, a detention pond monitored during the Lansing, Michigan study(2) was used to compare the three models and their ability to represent storm specific runoff and water quality data for 28 specific storms. The models can then be applied to a continuous rainfall record for the area in order to evaluate long-term continuous pond efficiencies and pollutant loads. Of the three models tested, and P8 come the closest in representing actual physical, in-pond phenomena from- April 1980 to November 1981 (the monitoring period). The Lansing was performed by Environmental Control Technology Corporation (Ecq) for the Ingham County, Michigan Drain Commission and the Tri- County Regional Planning Commission. Figure 3 shows the general location map for the study area which is part of the Bogus Swamp Drain within the Grand River Watershed, and Figure 4 shows the site location for monitoring Station 6 (pond inflow) and Station 5 (pond outflow). The pond is considered an in-line retention pond but functions as a detention pond. Its contributing area is approximately 70 acres with 37 acres of golf course and parkland contributing in a swale to the southern end of the pond, and 33 acres of mixed commercial/residential land use contributing to the Station 6 inflow location. Figure 5 presents plan and section views of the pond. Note that the inflow and outflow are close together, indicating the potential for short circuiting. The normal water level is at ft-ngvd providing a permanent pool of approximately 13 days. The inflow pipe to Station 6 is a 36" storm-sewer and the outflow structure is a multistage inlet weir/grate at elevation ft-ngvd that is outlet-controlled by an 18 CMP. A valved drawdown orifice has been constructed for maintenance. The reported 34 rainfall events of which 29 were monitored at the inlet and 28 at the outlet for various parameters, including which is the target pollutant for this study. Figure 6 and 7 show results for plotted regression equations relating runoff/outflow volumes (ft3) and concentrations to rainfall in inches. Notice that observed loads are higher at the inlet than estimated and that the estimated concentrations in the outflow are lower than observed. In order to evaluate the pond in a continuous simulation, the, STORM, P8 were applied and results were compared to the monitored results. The following paragraphs present background information on the three public domain models. - Figure 8 shows a simplified version of the processes modeled by and STORM. Version 4.21 RUNOFF and STORAGE/ TREATMENT(') blocks were used for the processes as shown in Figure 9. RUNOFF hydrology included directly connected impervious area (DCIA), pollutant buildup and washoff, pervious infiltration, and routing to the pond. Figure 10 shows simplified mathematical descriptions of buildup and washoff relationships used in for this application. The STORAGE/TREATMENT block was used as a plug flow detention pond for pond routing of flows and using the discrete particle settling option. The Lansing recorded in six distinct particle size ranges by fraction. These were used and assigned specific gravity values based upon typical particle sizes. STORM was written by CDM for the U. S. Army Corps of Engineers (USACOE)(4). It is used for combined sewer overflow (CSO) evaluation. It provides continuous simulation to predict quantity and quality of stormwater runoff and wastewater. It also calculates required storage capacities and treatment rates required to achieve runoff control. quantity is predicted using the runoff coefficient method (rational), and runoff quality is predicted using daily accumulation rates. STORM does not model detention pond treatment processes.

3 The P8/Urban Catchment Model(') is a continuous simulation model for predicting the generation and transport of stormwater runoff pollutants from a watershed and treatment in downstream BMPs. P8 simulations are driven by continuous hourly rainfall and daily air temperature time series. P8 was originally developed by William Walker for the Naragansett Bay Project (NBP). Figure II is a schematic of the mass balance relationships considered by P8. For detention pond simulations, P8 computes removal efficiencies based on particle size, settling velocity, and actual storm characteristics. This allows simulation of storm-tostorm variability. P8 includes design features that allow the user to refine the design of a detention pond to meet a target removal efficiency. Water quality simulations under P8 are based on five generic "particle" size classes. P8 applies a particle composition for each particle size class to simulate up to ten pollutants. P8 computes runoff from pervious areas using the SCS curve number technique. For impervious areas, the model assumes that any rainfall exceeding the defined depression storage in the basin is runoff. P8 computes "particle" concentrations for both pervious and impervious areas, then sums the results. Particle concentrations from pervious areas are simulated using an empirical relationship similar to the sediment rating model in. Impervious areas are simulated using particle buildup and washoff and/or fixed runoff concentrations. P8 uses buildup/washoff algorithms derived from. RESULTS - RUNOFF was calibrated to the monitored data by adjusting buildup and washoff factors and hydrologic parameters such as initial abstractions and DCIA. STORAGE/TREATMENT was calibrated by varying stage-discharge characteristics for the pond along with specific gravities for the various particle distributions. Tables 1 and 2 present summary statistics for vs results for the monitored storms at the inlet and outlet for runoff volume (ft3) and (mg/i-). It can be seen that well represented the observed data. Continuous Simulation Results Station 6: Basin Inlet Table I Nurp Rainfall (inches) ft3 3,000 2,100 52,000 51,000 20,574 18,(00 EMC Continuous Simulation Results Station 5: Basin Outlet Table 2 SWIM (ft3) ,000 5,700 68,000 69,000 26,414 27,000 EMC

4 P8 - Hydrology was calibrated by performing minor adjustment to the estimated SCS curve numbers and depression storage parameters. Computed loads from pervious and impervious watershed area were calibrated by adjustments to a constant "Pollutant Load Factor". The default particle setting velocities for P8 are based upon settling column tests conducted under the (Driscoll, 1983). The treatment efficiency of the Lansing detention pond was calibrated by adjusting the "Particle Removal Scale Factor" which modifies settling, velocities and decay rates to account for device- specific characteristics. Tables 3 and 4 present the P8 result. P8 Continuous Simulation Results Station 6: Basin Inlet Table 3 PS P8 PS PS , ,000 77,500 20,674 20,057 (ft3) EM C P8 Continuous Simulation Results Station 5: Basin Outlet Table 4 P8 PS P8 P ,000 3,900 68,000 86,200 26,414 26,370 (ft3)* EMC on drawdown duration (ranges from 2 to 6 days) STORM - reported no untreated overflows to receiving water indicating retention basin has sufficient storage capacity. The average annual loading was 1,500 lbs/yr for a treatment rate of 0.03 in/hr and equivalent storage capacity of 7.1 ac./ft. STORM runoff quantity calibration involved adjusting coefficients for pervious and impervious areas and depression storage. Four monitored storm events were used: June 1, 1980; June 5, 1980; July 16, 1980; and October 14, 1980; and the runoff volumes fluctuated by -10 percent to +40 percent. The STORM runoff quality calibration included adjusting accumulation rate and washoff decay coefficient. STORM loads fluctuated by -50 percent to +-21 percent. Figures 12 and 13 show STORM results for the four specific storms. CONCLUSIONS AND RECOMMENDATIONS Detention Ponds - The monitored detention pond exhibited removal

5 efficiencies ranging from 80 to 90+ percent. Overall, it was fairly efficient except for an occasional storm which would short-circuit the pond storage. Potential improvements to the pond include: constructing a sheet pile weir to force a "u" direction for flow (Figure 14); and constructing a sediment forebay to assist in maintenance dredging of captured solids. Work by Wanielista() and Hartigan() has shown optimal pond efficiencies for given length to width ratios, overflow rates, pond geometry s (limit short circuiting), maximum and minimum depths, permanent pool sizes, and tributary area to pond area ratios. The following recommendations for detention pond criteria have been proposed in the various reports: two-week hydraulic residence time (actual) for permanent pool, length-to-width ratios of between 4 and 7 to overflow rates for small storms <0.014 ft/hr, maximize distance between inlets and outlets; and a maximum depth of 8 to 12 feet. Models - the following recommendations are made for the three models to improve their ability for use in continuous simulation of detention ponds: STORM - detention routing capability P8 - h ydrology and routing (backwater); land use/pollutant loading relationships; particle resuspension; BMP design criteria (length/width). - number of pollutants simulated; multiple land use/emcs; plug flow routing capacity; combine STORAGE/TREATMENT with TRANSPORT; direct reporting of detention time and in-pond velocities. REFERENCES 1. Results of the Nationwide Urban Program 1983, U. S. Environmental Protection Agency 2. Final Report, USEPA Nationwide Urban Program Lansing, Michigan Evaluation of Urban Stormwater and Management Practices for Controlling Urban Stormwater, 1982 Environmental Control Technology Corporation. 3. EPA Storm Water Management Model Version 4.2 Users Manual, 1992, University of Florida Department of Environmental Engineering Science. 4. STORM Users Manual 1977 USACOE Hydrologic Engineering Center. 5. P8 Urban Catchment Model Program Documentation Version.1.1, 1990 William W. Walker. 6. Final Report on Efficiency Optimization of Wet Detention Ponds for Urban Stormwater Management, 1990, University of Central Florida Department of Civil and Environmental Engineering. 7. Final Report - An Assessment of Stormwater-Management Progra ms, 1985, Camp Dresser & McKee Inc.