This treatment is commonly used beneath parking lots. Like other infiltration/detention treatments, it has a tremendous capacity to reduce peak flow. Since it does not require an associated retention pond, more land is available for parking. It can be used for detention and infiltration, depending on subbase and groundwater characteristics.

It is comprised of two units in series: a water quality unit (WQU) and a larger detention/infiltration unit (DIU). Both are made of high-density polyethylene pipe. The WQU is a series of weirs constructed from 60-inch diameter pipe. The DIU consists of three, 40-foot sections of 48-inch diameter perforated pipe, connected by headers. The top and sides of the excavation basin are wrapped in geotextile. Stormwater flows of 1 cubic foot per second (cfs) go fi rst through the WQU and then into the DIU. Flows exceeding 1 cfs bypass the WQU through a pipe leading into the DIU. This prevents re-suspension of solids. From the DIU, stormwater infiltrates into the sandy subbase.

The WQU has two manholes for access and cleanout. Its maintenance includes removal of accumulated solids and floatables. DIU maintenance is minimal as pretreatment occurs in the WQU. Proper maintenance of the WQU prevents costly maintenance of the larger DIU.