14.9 BMP T820 - Linear Sand Filter
Linear sand filters are typically long, shallow, two-celled, rectangular vaults. The first cell is designed for settling coarse particles, and the second cell contains the sand bed. Stormwater enters the second cell via a weir section that also functions as a flow spreader.
Figure 4 - 35: Linear Sand Filter illustrates a linear sand filter.
14.9.2 Design Criteria for Linear Sand Filters
Comply with Section 14.4 Design Criteria for All Sand Filtration BMPs and Section 14.5 Construction Criteria for All Sand Filtration BMPs in addition to the criteria below.
Divide the two cells by a divider wall that is level and extends a minimum of 12 inches above the sand bed.
Stormwater may enter the sediment cell by sheet flow or a piped inlet.
The width of the sand cell must be 1-foot minimum to 15 feet maximum.
The sand filter bed must be a minimum of 12 inches deep and have an 8-inch layer of drain rock with perforated drainpipe beneath the sand layer.
The drainpipe must be 6-inch diameter minimum and be wrapped in geotextile and sloped a minimum of 0.5 percent to promote positive drainage. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C.
Maximum sand bed ponding depth: 12 inches.
Must be vented as described above for sand filter vaults.
Linear sand filters must conform to the materials and structural suitability criteria specified for wetvaults in BMP T1120 - Wetvaults.
Sediment cell width shall be selected based on sand filter width as follows:
Linear sand filters shall only be used if the contributing area is 2 acres or less.
Figure 4 - 35: Linear Sand Filter