Maximize your space with underground detention storage

On a tight site, a surface detention pond can consume the acreage that makes the deal work. Underground detention puts the same storage volume beneath the parking lot instead.

What the system is

A network of large-diameter perforated HDPE pipes, or precast concrete or plastic chambers, is bedded in open-graded stone and wrapped in geotextile beneath a paved area. Runoff is routed into the system through inlets and a manifold, stored in the pipe or chamber volume plus the void space in the surrounding stone, and released at a controlled rate through an outlet structure.

Why it gets used

• Recovers the surface area a pond would occupy — often several parking rows or an entire building pad
• Removes the fencing, safety, and aesthetic issues of an open basin
• Works on infill and redevelopment sites where there is simply nowhere to put a pond
• Can be designed to infiltrate where soils allow, reducing volume as well as peak flow

What it requires

Cover depth and load rating have to suit the traffic above. The invert must sit above the seasonal water table and have enough fall to reach the outfall. Structural backfill and bedding specifications are not optional — these systems depend on the stone envelope for their strength.

Design in the maintenance

Every underground system needs an accessible isolator row or sediment sump upstream, manholes at the manifold, and enough clearance for a jet-vac truck to reach the length of the pipe. Sediment that reaches the storage volume is very difficult to remove, so pretreatment is where the design succeeds or fails.

The cost per stored gallon is higher than a pond. On a constrained site, the land it releases is worth considerably more.