Conventional Approach (i.e., pipes and ponds)
New Approach i.e., bioretention)
Collect runoff to one point; centralize it.
Increase storage and drainage. A few large detention basins.
Locate BMPs where runoff is produced; keep it dispersed.
Increase infiltration and evapotranspiration. Many small retention basins.
Complex, large scale.
Simple, small scale. Role of Vegetation
Significant, several functions.
Multidimensional with added benefits of aesthetics and water quality improvement.
One example of a bioretention BMP is the rain garden, which is a modified infiltration system (Ferguson, 1994). This BMP was developed in the late 1980s by Larry Coffman in Prince George's County, MD (Bitter and Bowers, 1994; Engineering Technologies Associates and Biohabitats, 1993), and it is similar to other biofiltration systems. A rain garden is an engineered BMP designed to treat stormwater from a small drainage basin such as a parking lot or rooftop (Figure 3.17). It consists of an area with reconstructed soil stratigraphy and planted vegetation that is oriented in such a way as to receive runoff from the drainage basin. The soil of the rain garden is designed to encourage infiltration. The first layer (30 cm) is typically composed of a mixture of 50% sand, 30% top soil, and 20% mulch. This is the active zone in which most pollutant absorbtion takes place in terms of nutrients and metals. Sand or gravel are sometimes used below this layer, and the latest designs employ an under drain, as in a septic tank drain field, leading to a stormwater catchment system. The rain garden is intended to model a terrestrial system rather than a wetland in order to encourage infiltration. This objective requires design so that ponding occurs but is minimized. This is a critical element that can have long-term hydrologic implications. If ponding is too long, wetland conditions are favored which reduce infiltration capacity. The rain garden is thus designed to absorb the first flush of storm runoff and then to overflow with excess runoff leading to other
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