About Subsurface Gravel Wetland
General Information Performance Data Water Quality Treatment Process Design


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Design

This subsurface gravel wetland was designed by UNHSC. Its rectangular footprint occupies 5,450 square feet and can accommodate runoff from up to one acre of impervious surface. It includes a pretreatment sedimentation forebay that preserves the filter media, followed by two flow-through treatment basins. (Other pretreatment approaches may be used.)

Each treatment basin is lined and topped with two feet of gravel and eight inches of wetland soil. The lining is for research; in practice, a lining is only needed at sites with soils belonging to groups “A” (sand, loamy sand, or sandy loam with high infiltration rates) and “B” (silt loam or loam with moderate infiltration rates). Gravel wetlands depend on horizontal filtration; however, trenches to promote infiltration (downward flow) can be incorporated at the end of the system.

The wetland is designed to retain and filter the water quality volume (WQ_v)—10 percent in the forebay and 45 percent in each treatment basin. It can detain a channel protection volume (CP_v) of 4,600 cubic feet, and release it over 24 to 48 hours. The conveyance protection volume (Q10) is bypassed. For small, frequent storms, each treatment basin filters 100 percent of the influent it receives. For larger storms that do not exceed design volume, stormwater bypasses the first treatment basin and is processed by the second. When storms exceed design volume, the first inch of rain (first flush) is treated, while the excess is routed to conveyance structures or receiving waters.

Since standing water of significant depth is not expected, except during heavy rains, the side slopes of the system are graded at 3:1 or flatter to facilitate maintenance. With the exception of the forebay, the wetland hosts a healthy, diverse mix of native wetland grasses, reeds, herbaceous plants, and shrubs.