Permanent Storm Water Quality Pond Design

摘要

Stormwater runoff quality can be improved permanently by incorporating a wet pool design into a detention facility or by installing a wet pond only design. Practice has shown that 80% of the total suspended solids (TSS) can be removed, for a typical rainfall year, by providing three (3) times the volume of the mean annual stormwater runoff in a properly designed facility. Removal of sediment, which is a pollutant causing high turbidity, also has the effect of removing nutrients, such as phosphorus, which attaches to sediment particles. Otak, Incorporated designed two (2) water quality wet ponds during 2007 for the redevelopment of the 15.8-acre Barbee Mill sawmill into 115 residential lots directly adjacent to Lake Washington in Renton, Washington which is a Seattle, King County, Washington suburb. Stormwater flow control (detention) was not required by the City due to the proximity of the sizable adjacent lake, but Basic Water Quality improvements were required. The North water quality wet pond provides 51,635 cubic feet of water quality treatment volume which is 4 times the expected mean annual storm runoff from the 13.58 acre basin area. Since impervious roof drainage is considered non-pollution generating approximately 2.02 acres of the basin was bypassed around the water quality pond directly to Lake Washington. The South water quality pond provides 11,431 cubic feet of water quality treatment volume which is more than 7 times the expected mean annual storm runoff from the 2.18 acre basin area. Approximately 0.86 acres of the basin was bypassed around the water quality pond directly to Lake Washington since it source was impervious roof drainage. Successful permanent wet pond designs incorporate a normal stilling pool to reduce stormwater runoff energy upon entry into the pool, an inverted outlet pipe to reduce outlet energy, recirculation of standing pool water if the depth is greater than 6 feet to prevent stagnation, at least a 3:1 length to width ratio to promote settling, a tear drop rounded shape to reduce dead zones, maintaining base flow to promote circulation during dry periods, shady shoreline vegetation to reduce thermal heating, an impermeable pond bottom to maintain a permanent pool and are aligned with the summer prevailing winds to encourage circulation. Typical concentrations of the TSS in the area range between 30 and 100 mg per Liter. Successful removal of more than 80% of the TSS is likely since most of the suggested permanent wet pond designs features were incorporated in these ponds.