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Why Do It
Gravel wetlands retain, filter, and slowly release stormwater runoff. Research at the University of New Hampshire Stormwater Center shows they are among the most effective systems at protecting water quality. They are well suited for low permeability soils or high groundwater, and have a natural wetland appearance when completed.
In South Portland, Gravel Wetlands are a recommended approach to meet Basic or Post-Construction Stormwater Management Plan permit requirements.
Gravel wetlands are designed to temporarily retain stormwater flow, allowing water to move horizontally through a vegetated filter system. Gravel Wetlands maintain a saturated gravel bed and provide treatment through plant and soil processes. Plants on the surface of the filter reduce stormwater volume by evapotranspiration. An underdrain releases water slowly from the bottom of the filter. University of New Hampshire Stormwater Center shows that gravel wetlands do a god job of treating nitrogen pollution. Nitrogen reduction is important if sensitive coastal bays or estuaries are downstream. Soil filter systems reduce overall stormwater volumes, delay peak flows, and are effective at removing a wide range of pollutants.
Gravel wetlands are well suited to poorly draining soils. In most cases, there is no need to separate the system from seasonal high groundwater. They also have minimal hydraulic head requirements, suitable for relatively flat areas. They can be adapted to other soil and groundwater conditions by using an impermeable liner to maintain saturated conditions in the gravel bed.
Design and Installation
Gravel Wetlands should be designed by an engineer. They can be constructed using common materials. Wetland soil media composition is critical to the function of the systems and careful attention should be made regarding composition and installation.
General inspection of the wetland and any structural components must occur at least annually. The perimeter is mowed at least annually. The pre-treatment forebay will need occasional removal of sediment (every 5 years, or when 50% of capacity is lost, which ever occurs first). Inspections should ensure that no sediment is reaching the gravel. Wetland plantings typically become well established, but occasional replanting to maintain minimum 50% coverage may be needed. If plants become overcrowded, they may need thinning.