BMP Monitoring Sites - Urban Drainage and Flood Control District

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UDFCD BMP MONITORING SITES Sand Filters SAND FILTERS A sand filter extended detention basin is a stormwater filter that consists of a runoff storage zone underlain by a sand bed with an underdrain system. During a storm, accumulated runoff ponds in the surcharge zone and gradually infiltrates into the underlying sand bed, filling the void spaces of the sand. The underdrain gradually dewaters the sand bed and discharges the runoff to a nearby channel, swale or storm sewer. A sand filter is generally suited to onsite configurations where there is no base flow and is put in operation when the upstream catchment no longer has construction or grading/landscaping activities. Primary advantages of sand filters include effective water quality enhancement through settling and filtering. The primary disadvantage is a potential for clogging if a moderate to high level of silts and clays are allowed to flow into the facility. For this reason, the sand filter should not be put into operation while construction or major landscaping activities are taking place in the tributary catchment. Also, this BMP should not be located close to building foundations or other areas where expansive soils are a concern, although an underdrain and impermeable line can ameliorate some of this concern. Since an underdrain system is incorporated into this BMP, sand filters are suitable for about any site; presence of sandy subsoil is not a requirement. This BMP has a flat surface area, so it may be more challenging to incorporate it into steeply sloping terrain. Sand Filters Monitored by District
UDFCD BMP MONITORING SITES Open Bed Sand Filter OPEN BED SAND FILTER Description Location Typical Details Installation Photo Gallery Monitoring Data Other Examples Description The open bed sand filter that is monitored for flow volume and water quality by the District is located in Lakewood and was constructed in March/April of 2007. The vault is made of reinforced concrete with two cells separated by a concrete wall. The sand filter cell is 100’ long by 5’ wide by 4’-4.5’ deep. The outlet cell is 4’ long by 5’ wide by 5’ deep. The sand filter cell contains a 15”-18” layer of sand, a nonwoven geotextile fabric, and a 5”-8” layer of aggregate. At the bottom of the sand filter cell there is a 4” perforated underdrain pipe that collects the filtered water and carries it to the outlet cell. An orifice plate with a 5/8” hole controls the outflow under regular flow conditions and a sharp-crested weir at the top of the wall separating the cells controls flow under high flow conditions. Pressure transducers located in the middle and at the end of the sand filter cell measure water depths. The sampling equipment for the inlet is stored in a wooden box near the upstream end of the sand filter and the equipment for the outlet is stored in a shed adjacent to the outlet of the sand filter. A rain gage on top of the shed measures rainfall and signals the ISCO samplers inside the box and the shed to begin sampling after 0.1 inches of rain falls. The samplers then draw a sample of water from the inlet and outlet after a designated amount of flow has passed, and then every 15 minutes until 12 hours after the storm has ended. The total cost to construct the open bed sand filter was $74,000 in 2007. The costs included reinforced concrete, reinforced concrete pipe, removal and replacement of asphalt, earthwork, curb and gutter, geotextile, C-33 sand, #67 aggregate, grating, orifice and weir plates, and perforated pipe.
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BMP Monitoring Sites - Urban Drainage and Flood Control District

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