Pocket wetlands are constructed shallow marsh systems designed and placed to control stormwater volume and facilitate pollutant removal. As engineered constructed facilities, pocket wetlands have less biodiversity than natural wetlands but still require a base flow to support the aquatic vegetation present. Pollutant removal in these systems occurs through the settling of larger solids and course organic material and also by uptake in the aquatic vegetation. Pocket wetlands are designed with three distinct zones: a forebay immediately after the inlet to receive stormwater, the wetland area, and a micropool immediately prior to the outfall. The forebay and micropool allow for sediment control. Pocket wetlands have more limited application than other stormwater BMPs that may have superior aesthetic or functional capabilities.
Several design variations exist for stormwater wetland systems, with the differences occurring in the amount of storage volume provided, pond depth, and dry storage. While other design variations exist, including shallow wetlands, extended detention shallow wetlands, and pond/wetland systems, only pocket wetlands have realistic application in urban environments. The other design variants require a large drainage area to sustain wetland water levels. Pocket wetlands may be supported by drainage areas of 5-10 acres. Pretreatment of stormwater prior to its introduction into the wetland may be used to prevent sediment and grit loading. Pretreatment options include filter strips, swales, catch basins, and oil and grit separators.
Stormwater Management Objectives
Pocket wetlands are designed to treat stormwater runoff from a 5-10 acre drainage area. Hydraulic detention of stormwater is achieved through an increase in the water depth of the wetland. Detention times are 24 hours or less and the water depth increase is no more than three (3) feet. The wetland should also be designed to contain the runoff volume of 90 percent of the annual storm events.
Increasing the duration of discharge and controlling stormwater volume enables pocket wetlands to significantly reduce peak discharge. Detaining and extending runoff times reduces the total energy of the runoff event.
Improving water quality is a primary focus of pocket wetland design. Settling and vegetative uptake are the primary mechanisms of pollutant removal with removal rates anticipated to be 80 percent for TSS, 40 percent for phosphorous, 30 percent for nitrogen, and 50 percent for metals.
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