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03/07/95 09:43 12503 224 7820 WUUll14AKU- LLYUlr - - Stormwater Ponds and Marshes (Wetlands) Guidance is available for three types of ponds, three variations of marshes (also called wgtliwds) and one variation that is a combination pond/marsh system: • Wet ponds — constructed ponds with a permanent pool of water throughout most of the year (or at least throughout the wet season) for treating incoming stormwater runoff through gravitational settling and other means. Wet ponds typically also provide additional temporary storage for the control of the water quantity design storm runoff. Water levels and flood control are managed by the use of risers, orifices, and other outlet control structures. • Dry extended detention (ED) ponds — constructed ponds whose outlets have been designed to retain the runoff volume from a water quality design storm for some minimum time (usually 24 hours) to allow for the settling of particles and associated pollutants. Dry ED ponds do not maintain a permanent pool between storm events. These ponds can also be used to provide flood control by including additional flood detention storage. • Wet extended detention (ED) ponds — constructed ponds that incorporate both a permanent pool and extended detention to provide storm water treatment. These ponds can also be utilized for flood control by including additional temporary storage for peak flows. Also referred to as "combination ponds ". • Shallow marsh — a stormwater marsh that is mostly between zero and eighteen inches deep and, therefore, can support the growth of emergent wetland plants. A deeper forebay is located at the major inlet to trap sediment, and a deep micropool (smaller permanent pool) is located near the outlet to facilitate sedimentation and prevent clogging. • Extended detention marsh — a shallow stormwater marsh with an additional forebay area upstream of the marsh for the temporary detention of runoff. As much as 50 percent of the total treatment volume can be provided as extended detention storage, which reduces the space requirements compared to a facility that is solely a shallow marsh. The extra runoff is stored to allow pollutants to settle out. • Pocket marsh - a constructed marsh that is usually a tenth of an acre in size, and typically serves residential or commercial sites of ten acres or less. Baseflow is not typically required. Effectiveness has not been proven in the field, but is expected to be comparable to wet swales (Schueler, 1995). • Pond/marsh system — a constructed system with two separate cells, a shallow marsh and a deep pond that leads into it. The pond traps sediments, reduces water velocity, removes pollutants (as does the shallow marsh), and reduces the space required for the system. S1945042na\task02001onsite controls\tcchmcm.ck 11