Bio retention areas are one of the best management practices (BMP’s), or methods, by which storm water can be filtered as required by many municipalities in metro Atlanta and other areas. A bio retention area is a filter consisting of various layers of filter media. As layered from bottom to top, there is a gravel area with under-drains, a depth of engineered soil designed to drain water, and a 3-inch layer of mulch on top planted with specific landscaping. The landscaping used for bio retention areas is not only tolerant to inundation of water, but also absorbs pollutants. The mulch filters larger particles of pollution in incoming stormwater, holds moisture for the vegetation, and prevents erosion of the soil below. Engineered soil filters the fine particles of pollution, and the gravel under-bed collects the water filtered from above and pipes it into the storm sewer system downstream. All elements act together to filter the initial runoff which contains the greatest amount of pollutants.
One of Partain Engineering’s recently completed projects is a fellowship hall addition to Johns Creek Presbyterian Church in Johns Creek, GA. In order to meet the city’s water quality requirements, we designed a bio retention area to provide water quality for the proposed impervious area. A bio retention area was selected due to the city’s restrictions and site constraints and was added adjacent to the site’s existing stormwater detention facility.
While a bio retention area is more expensive than some BMP’s that might have been allowed in other municipalities, it can be an attractive alternative to treat stormwater while, at the same time, meeting city requirements. This measure can also, in some cases, help reduce or eliminate the need for stormwater detention.
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