Parametric study to evaluate the adequacy of current design practices and policies for stormwater and sediment ponds

Ponds are used extensively throughout much of the United States to regulate increases in runoff and sediment resulting from urban development. Local regulations in South Carolina must comply with SCDHEC standards, which require stormwater detention and sediment retention ponds be designed based on runoff from 2- and 10-year 24-hour rainfall events. There is growing evidence these ponds are not performing acceptably, as indicated by greater incidence of downstream flooding and channel scouring, siltation, and widening due to bank undercutting. It is time to re-examine and perhaps revise controlling regulations and accepted methods for engineering analysis and design.; This research critically assessed existing management practices and policies for stormwater and sediment ponds. A parametric study was designed to answer questions pertaining to pond performance during storms other than the regulatory 2- and 10-year 24-hour design storms, during variable frequency, depth, and duration storms, and longterm. A Windows-based stormwater drainage system simulation program Drain:Hydro 2.0 with added features for sediment retention ponds, daily weather simulator, channel evolution and sediment transport, and coupling with ArcGISRTM was developed as the experimental tool for the parametric study. Each sub-model was tested and validated using field monitoring and literature data, and by comparing simulation results with output from accepted professional programs.; Outcomes from the parametric study show the downstream impacts of pond discharges, their long-term performance, and, most important, the limitations of current pond design standards. The study demonstrates that stormwater and sediment ponds designed in accordance with current policies and practices are not sufficient to control downstream flooding and channel aggradation-degradation. The study shows that 2- and 10-year 24-hour controls are not sufficient to protect downstream channels. For better control of downstream flooding and channel aggradation-degradation, 24-hour extended detention of the 1-year 24-hour storm should be implemented. The present study also suggests that stormwater detention and sediment retention ponds should be sized based on their performance during runoff from storms having a return period equal to or less than 1-year.; The performance of stormwater detention and sediment retention ponds differs among events due to variable water and sediment loadings. As expected, it was observed that sediment trapping efficiency decreases during the life-cycle of a pond without periodic cleanout. During construction phase, sediment ponds should be cleaned out, minimally, on a one year interval.; Sediment retention ponds provide better performance than stormwater detention ponds in terms of sediment trapping efficiency and peak effluent settleable solids concentration. They also provide better channel protection. Sediment retention ponds can be designed to maintain post development (control) hydrograph shape, volume, timing and more importantly, sediment transport capacity at pre-development levels. The permanent pool volume should equal the difference between post-development and pre-development runoff hydrograph volumes for a 1-year 24-year storm event. This volume should be drained over 24 hours following cessation of rainfall. To meet current SCDHEC regulations for water quality design, this volume should not be less than one-half inch of runoff from the entire watershed.