Slide 31 of 52
Notes:
Sedimentation basins are the preferred method of pretreatment for stormwater filters because the basins are constructed in conjunction with the filter bed and maintenance requirements are relegated to one location. In addition, the performance and sizing criteria for sedimentation basins are reasonably well established. Sedimentation basins can also be constructed underground in high density areas where space is limited.
The water quality volume is used as the basis for sizing the pretreatment chamber for all types of stormwater filters, except the �pocket system.� According to an extensive literature review conducted by the City of Austin, TX, removal of discrete particles by gravity settling is primarily a function of surface loading (the rate of outflow divided by the basin surface area) and is independent of basin depth. However, a minimum basin depth of 3 feet is recommended to minimize particle resuspension and turbulence effects. Therefore, surface area is the primary design parameter for sedimentation affecting removal efficiency (E). E is also a function of particle size distribution. Silt sized particles are used as the target particle size for sedimentation basin design (i.e.,?20 microns).
For sites with imperviousness of greater than 75%, which have a higher percentage of coarse grained sediments, the target capture particle is approximately 40 microns, decreasing the required filter area.
The equation above is used to size pretreatment settling basin surface area. It was derived by the Washington State Department of Ecology from the Camp-Hazen Equation.
As = Sedimentation basin surface area (ft2)
E = Trap efficiency; which is the target removal efficiency of suspended solids (set equal to 90%)
w = Particle settling velocity; for target particle size (silt) use settling velocity = 0.0004 ft/sec (0.0033 ft/sec for I > 75%, where I is percentage impervious area)
Qo = rate of outflow from the basin; which is equal to the water quality volume (WQv) divided by the detention time (td); use 24 hours.
The equations simplify to:
As = 0.066 ( (WQV) ft2 for I < 75%
As = 0.0081 (WQV) ft2 for I >= 75%
The length to width ratio of the sedimentation basin should be 2:1 or greater. Inlet and outlet structures should be located at extreme ends of the basin. Baffles may be used to mitigate short-circuiting and/or dead storage problems. The basin bottom shall have a minimum depth of 3 feet to minimize resuspension and turbulence. The basin bottom shall be nearly level to facilitate sedimentation.
