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Extended Detention Basins and Their Structures: Extended detention basins function similarly to wet detention basins, except they perform their treatment without much (if any) open water (dead storage). They are designed to force water to pool long enough for the majority of pollutants to settle out by the normal settling processes. This forced detention is accomplished through the construction of specially designed, multi -tiered outlet structures. The following describes the design considerations and design parameters for extended detention basins. Extended Detention Basin Design Considerations: There are four key considerations for extended detention basin design: 1. The primary removal process for most water quality treatment basins is particulate settling. The longer water is held (or detained) in a basin, the more likely pollutant particles are to settle out, hence, the term extended detention. The EPA"s National Urban Runoff Program (NURP) documents indicate that there may be diminishing returns on particulate removal for detention times beyond 24 hours. 2. From numerous case studies, the vast majority of the mass -volume of pollutants carried by runoff occurs during; storms of 1-year or 2-year magnitude or less. For much of Minnesota, a 2.5-inch rainfall in 24 hours is considered to be a 1-year storm event. 3. Further, as published in the EPA's NURP documents, the average time between storm peaks in Minnesota is about 87 hours, or a little more than 3 days. If flows are to be detained in a basin, the basin typically must be dewatered in no more than 2 days to enable the basin to be ready for the next storm. 4. The NURP documents reveal that the surficial size of a basin (the wetted area for the water quality treatment portion of a basin) is important to the basin's treatment efficiency. In general, the bigger the surface area the better. But there are diminishing returns on basins with wetted surface areas that are larger than Y2% to 1 % of the total untreated tributary area of the basin. Extended Detention Basin Design Parameters: Based on the key considerations described above, the following are the key design parameters for extended detention basins and their structures: r- h7lsis 23 MN fi' 2362I0fi2 Suter Vitw Bond Assessinrnl Wnrkf'iies Reporl Silver View Pond Assrssmcnl-final.doe% 23