Slide 24 of 52
Notes:
The pocket sand filter is a simplified and low cost design that may be used on smaller sites. Runoff is usually diverted within a manhole. A bypass pipe sends excess runoff along the storm drain system, and a flow diversion pipe routes the water quality volume into the system. Pretreatment is provided by a concrete flow spreader, a grass filter strip and a plunge pool. The filter bed is also a relatively simple affair. A shallow basin is excavated, and contains the sand filter layer. Most of the water quality volume is temporarily stored above the filter bed. The surface of the filter bed contains a soil layer and grass cover crop. In the event of clogging, the pocket sand filter has a pea gravel �window� to direct runoff into the sand, as well as a cleanout and observation well. In most cases, the filtered runoff is allowed to exfiltrate into the underlying soils, although underdrains may be needed if the soils are not suitably permeable.