Example 1 developed a model for initial runoff on a 29-acre site. Additional models simulated post-development runoff, both with and without surface collection routing (Example 2), for 2-, 10-, and 100-year storms. Example 3 prompts designing a detention pond downstream of urban development to prevent flooding and ensure water quality in the receiving stream. The pond must match undeveloped site peak discharges for 2-, 10-, and 100-year storms and offer extended detention for a specific Water Quality Capture Volume (WQCV).
Table 3-1 outlines discharges managed by the pond, based on Example 1’s pre-development peaks and Example 2’s post-development peaks (Table 2-7). SWMM input files, Example1-Pre.inp and Example2-Post.inp, contain 2-, 10-, and 100-year rainfall hyetographs.
Apart from discharge control, the design requires managing a Water Quality Capture Volume (WQCV), defined as a volume detained long enough to achieve pollutant removal. Required volume and drawdown time vary by stormwater policies. Here, the WQCV must be released over 40 hours, removing significant particulate pollutants from urban stormwater.
For safety, the final design limits storage depth to 6 feet. The detention pond segregates minor (WQCV and 2-year storm) and major storms (10- and 100-year) runoff into trapezoidal prism-shaped sections. Figure 3-1 shows the pond’s location within the study area.