Underground Detention Practice

All of the integrated management practices (IMP's) at the back of the site drain to the gravel bed detention area through the underdrain network. The gravel bed detention area is an underground basin filled with crushed angular gravel. The basin ranges from 3.5’ to 5’ deep and is approximately 7,900 square feet in area.

A perforated, 10” pipe allows water to move from the gravel detention bed into a manhole where a 1.625” orifice restricts water from leaving the site. This mechanism is designed to detain the 1-year storm over 30 hours, reducing stress on the receiving stream. An emergency overflow swale is located near the manhole and allows water to flow into the adjacent wetlands if necessary. However, because the 10” pipe is at a 0% slope and water fills approximately 10,500 cubic feet of void space before overflowing, the use of the emergency overflow has never been necessary - even in storms nearing the 100-year event mark.

More Information on WSSI’s Underground Detention

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