Using Bioretention Retrofits to Achieve the Goals of Virginia's New Stormwater Management Regulations

Virginia's new stormwater regulations involve the use of the Runoff Reduction Method (RRM), which requires the product of the peak flow rate and runoff volume (QRV) from the one-year storm event in the post-development condition to be reduced to eighty percent of the pre-development QRV to protect against channel erosion. This study models different bioretention cell sizes 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 determined between the size of the cell required to meet the RRM and the sub-watershed's Natural Resources Conservation Service (NRCS) curve number. However, the required size for some of the cells is much larger than those typically seen. With the RRM applied for all of the sub-watersheds, the resulting hydrograph at the watershed outlet has a lower peak than the pre-development condition.