By: Wesley Holmes

SS Credit 6.1: Stormwater Design—Quantity Control

1 Point

Intent

To limit disruption of natural hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff and eliminating contaminants.

Requirements

Case 1-Option 1. For sites with existing imperviousness 50% or less implement a stormwater management plan that prevents the post development peak discharge rate and quantity from exceeding the predevelopment peak discharge rate and quantity for the 1- and 2-year 24-hour design storms.

When land is altered for development, the way stormwater moves through the landscape is fundamentally altered. In an undeveloped landscape, trees, shrubs and other vegetation reduce stormwater runoff volumes through various processes. When rain falls to the ground the leaves of branches and shrubs catch or intercept the rain. This process, termed interception, lowers the total amount hitting the ground and reduces impact on topsoil. Transpiration is a more complex process where water captured in root systems is processed through the plant and released through leaves. In addition, the topography of an area usually will have natural depressions which collect water allowing evaporation. Ultimately these processes work to reduce the amount of water flowing through collecting streams and storm drains. According to a 2008 report issued by the National Research Council on Urban Stormwater Management in the United States in addition to entrainment of chemical and microbial contaminants as stormwater runs over roads, rooftops, and compacted land, stormwater discharge poses a physical hazard to aquatic habitats and stream function, owing to the increase in water velocity and volume that inevitably result on a watershed scale as many individually managed sources are combined.

When land is typically prepared for development it is graded, compacted and in many places paved. Grading removes the native soils and natural depression areas that once worked to retain rainfall and stormwater runoff on site. Compaction reduces the infiltration capacity of the underlying soils and increases the amount of rainfall that is converted to stormwater runoff. The addition of roads, parking lots, rooftops and other impervious surfaces work to further increase stormwater runoff volumes and flow. In the end, much of the rainfall that was once retained in the landscape is now converted to a flow of stormwater runoff. This increase in flow rates can have significant erosion impacts on receiving streams and riparian vegetation.

LEED Site Selection Credit 6.1 requires that the peak discharge rate and quantity of stormwater after development does not exceed the predevelopment peak discharge rate and quantity for the 1- and 2-year 24-hr design storm (Really heavy and lasting rain). Essentially this requires that the Academic Center development to maintain the rate and amount of runoff from the site. The runoff control is being achieved through a retention and treatment pond which will be covered in next weeks post on stormwater quality control.

External Links of Interest

EPA Urban Stormwater BMP Performance Tool

EPA National Menu of Stormwater Best Management Practices

Center for Watershed Protection

Tags: LEED, Runoff, Stormwater, Sustainable Development

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