Design of Micropool for Stormwater Detention Basin

A micropool is a small wet pool placed in front of the outlet structure installed at a stormwater extended detention basin. An extended detention basin is sized with a user-specified drain time in order to meet the stormwater quality criteria. The outlet structure is often composed of a vertical perforated plate and a top horizontal grate to release the loaded basin over a long period of time. When the perforated plate becomes clogged, the siphon system housed in the micropool becomes surcharged and can continually drain the stored water volume to meet the design criteria on the basin’s drain time. This study presents a new design method to size a micropool. It is recommended that a micropool be sized by the debris particle’s float velocity, water quality capture volume, and basin’s drain time. The depth of a micropool shall take into consideration evaporation loss and sediment dead storage volume. The debris samples collected in the field represent typical urban floating particle sizes observed in stormwater basins. It is recommended that a micropool be sized to capture up to 80%–90% of floating particles accumulated in a detention basin. This new method provides a reasonable surface area and pool depth to achieve the goals set forth for micropool designs.