Slide 35 of 52
Notes:
The overflow elements of the filter chamber consist of a flow distribution vault, a sand or organic media filter bed, underdrain piping, a basin liner (whenever necessary to prevent groundwater contamination), and a high flow overflow structure. In some applications the filter bed will have a cover of either vegetation, gravel or a synthetic geotextile-matrix matting.
The flow distribution vault should be designed to spread the flow uniformly across the surface of the filter bed. V-notch weirs, a level broad crested weir or multiple orifice openings are alternatives. Flow should be dispersed to prevent scouring. The height of the inlet structure should be equal to the filter bed elevation. Rip rap or other suitable erosion protection should be installed immediately below the inlet structure discharge location where necessary.
The filter bed profile generally consists of a cover vegetation planted in a 3" topsoil layer above an 18" layer of 0.02"-0.04" diameter, clean concrete sand or organic media, and over a 6" to 11" gravel underdrain system with a 4� to 6" perforated pipe collection system. An alternative sand bed profile consists of a top layer of 12" to 18" of 0.02"-0.04" diameter sand.
The sand bed depth recommended above should be considered the final consolidated depth. Depending on moisture content and compactive effort, a 5% increase in depth should be considered. The sand and gravel should be separated by a layer of permeable geotextile fabric, meeting the recommended specifications. Some authorities recommend against using geotextile fabrics to separate layers. Four inches of pea gravel may be substituted for the filter cloth. Separating the sand and gravel layers allows for an integrated sand/gravel boundary with a higher matrix potential which allows easier water flow from the sand to the gravel. Lateral pipes are placed in trenches and covered with gravel and geotextile fabric. The laterals are underlain by a layer of drainage matting which provides for adequate vertical and horizontal flow.
The underdrain system consists of perforated collector and lateral pipe system. Perforations should be 3/8" diameter and should be spaced approximately 6" on-center. The lateral pipes should be spaced at a maximum distance of 10' on-center. Pipes should be adequate to accommodate the weight of the sand and gravel above. Pipes should be 6" PVC, Schedule 40 or greater. The entire underdrain system should have positive drainage. (A design slope of at least .5% is recommended). A vertical standpipe should be provided for inspection and cleanout.
The outlet structure may simply be a direct connection of the underdrain piping system to a downstream storm drainage system, channel or waterway. In many cases the outlet structure will be a separate chamber, into which the underdrain system flows. This chamber will then discharge directly to the receiving waters or drainage system. The outlet chamber may also act to collect overflow drainage associated with larger storm events, as applied in the underground sand filter and the perimeter sand filter.
