Integrating Stormwater Runoff Quantity and Quality Requirements in a Coastal County

Beaufort County, in coastal South Carolina, is proud of its water resources and efforts to protect them through water quality control. The county adopted water quality control requirements in its Best Management Practices (BMP) Manual in 1998, and realized a 30 percent increase in population without any additional water quality impairments through 2009. Further control requirements were considered in 2009 when the state restricted shellfish harvesting in a section of the May River. Specifically, the BMP Manual and development regulations have been modified to add stormwater runoff quantity control requirements. The approach taken by Beaufort County provides a relatively simple and scientifically based approach for evaluating runoff volume for new development. The threshold of the runoff control is an “equivalent” imperviousness of 10 percent, which will not necessarily reduce runoff to pre-development levels, but will limit runoff to a level of uncontrolled imperviousness that has been associated with limited receiving water impacts. The county has also modified the development requirements to control the runoff volume from undeveloped lots that are located in developments that had a BMP plan approved prior to runoff volume control requirements. For these “Step 2” controls, the county has developed a spreadsheet to evaluate runoff volume control under the post-development lot condition, and also allows for an engineering analysis to assess whether a development will meet the equivalent impervious goal without additional on-lot controls. The county has also explored the benefits of the runoff volume controls in meeting existing peak flow attenuation for extreme storm events.