Abstract
Virginia’s new stormwater regulations involve the use of the Runoff Reduction Method (RRM), a methodology to estimate a volume reduction in predicted runoff. Regulations require that for downstream erosion control, the product of the peak flow rate and runoff volume (Q*RV) from one-year storm events in the post-development condition be reduced to less than pre-development Q*RV. This study models different bioretention sizing scenarios in a developed watershed in Blacksburg, Virginia to determine the performance at both the sub-watershed and watershed levels. In addition, models of “optimal” bioretention cells, sized to meet the RRM for each sub-watershed, are evaluated. A direct relationship is observed between the size of the cell required to meet the RRM and the sub-watershed’s developed Natural Resources Conservation Service (NRCS) curve number, and a sizing analysis is provided. Modeling shows that the required size of “optimal” cells for many sub-watersheds exceeds conventional bioretention designs. Upon applying the RRM for all sub-watersheds, the resulting hydrograph at the watershed outlet more closely resembles the pre-development hydrograph than existing development.
Highlights
Low Impact Development (LID) is a design methodology that seeks to restore a developed site’s hydrologic response to a storm to its pre-development condition (Prince George’s County, 1999)
A direct relationship is observed between the size of the cell required to meet the Runoff Reduction Method (RRM) and the sub-watershed’s developed Natural Resources Conservation Service (NRCS) curve number, and a sizing analysis is provided
Bioretention, a common LID practice, accepts runoff, allows the water to pond on top of it, and lets water percolate through its engineered soil media to either the underlying soil or an underdrain
Summary
Low Impact Development (LID) is a design methodology that seeks to restore a developed site’s hydrologic response to a storm to its pre-development condition (Prince George’s County, 1999). Bioretention, a common LID practice, accepts runoff, allows the water to pond on top of it, and lets water percolate through its engineered soil media to either the underlying soil or an underdrain. Retrofitting urbanized areas with LID and Best Management Practice (BMP) technologies is an effective way of reducing runoff in a watershed (Damodaram et al, 2010). Many BMPs exist that could be used in urban stormwater infrastructure retrofits, bioretention is a practice that has increasingly become attractive to designers. This study strictly focuses on the volume reduction benefits of bioretention; other reasons that make it a widely used practice are its high removal efficiency of nutrients and pollutants and creation of canopy and wildlife habitat for small species in urban settings
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