Integrating Stormwater Runoff Quantity and Quality Requirements in a Coastal County

摘要

Beaufort County, in coastal South Carolina, is proud of its water resources and efforts to protectrnthem through water quality control. The county adopted water quality control requirements in itsrnBest Management Practices (BMP) Manual in 1998, and realized a 30 percent increase inrnpopulation without any additional water quality impairments through 2009. Further controlrnrequirements were considered in 2009 when the state restricted shellfish harvesting in a sectionrnof the May River. Specifically, the BMP Manual and development regulations have beenrnmodified to add stormwater runoff quantity control requirements. The approach taken byrnBeaufort County provides a relatively simple and scientifically based approach for evaluatingrnrunoff volume for new development. The threshold of the runoff control is an “equivalent”rnimperviousness of 10 percent, which will not necessarily reduce runoff to pre-developmentrnlevels, but will limit runoff to a level of uncontrolled imperviousness that has been associatedrnwith limited receiving water impacts. The county has also modified the developmentrnrequirements to control the runoff volume from undeveloped lots that are located inrndevelopments that had a BMP plan approved prior to runoff volume control requirements. Forrnthese “Step 2” controls, the county has developed a spreadsheet to evaluate runoff volumerncontrol under the post-development lot condition, and also allows for an engineering analysis tornassess whether a development will meet the equivalent impervious goal without additional on-lotrncontrols. The county has also explored the benefits of the runoff volume controls in meetingrnexisting peak flow attenuation for extreme storm events.