Sedimentation/Filtration systems are the primary stormwater treatment device used in Austin. Runoff is first diverted into a sedimentation basin, where particulate pollutants are removed via gravity settling, followed by filtration through an 18” layer of sand. These systems can achieve removal rates of 40-90% for suspended solids, heavy metals, and organics. Properly operating systems will typically capture 90% or more of all runoff from the contributing drainage area, and release it at a slow rate that enhances baseflow and reduces stream erosion.
Sedimentation/filtration systems are not allowed in Barton Springs Zone (BSZ)watersheds as a stand-alone water quality control, as they are not capable of achieving a non-degradation level of treatment.
Two design variations are allowed in Austin. In “full” sedimentation/filtration systems the entire water quality volume is held in the sedimentation basin, which then slowly discharges runoff to the filtration basin via a perforated riser pipe. The alternative “partial” sedimentation/filtration design foregoes the perforated riser pipe, and distributes the water quality volume between the filtration basin and a sediment chamber. The latter is then separated from the filtration bed by a gabion wall or other porous structure. The “full” design is required when the City of Austin is responsible for maintenance.
Design guidelines for full and partial sedimentation/filtration ponds are provided in Section 1.6.5.of the Environmental Criteria Manual (see 1.6.5.A for “full” systems and 1.6.5.B for “partial” systems).