Integrating Stormwater Runoff Quantity and Quality Requirements
By Daniel Ahern, Richard Wagner, and Robert Klink
Beaufort County, SC, is located between Charleston, SC, and Savannah, GA. Because of the prime coastal location, the county has long been an attractive location for resorts and other types of development. The county’s stormwater program has been challenged by its citizens and leaders to be a progressive coastal program that has recently incorporated volume control into its stormwater management criteria. This progressive attitude has kept 85% of our most-sensitive-water-use (shellfish harvesting) waters open since water-quality controls were first adopted in 1998 (Figure 1). This was maintained while the county increased in population by over 30%.
- Rooftop practices (green roofs, evaporative cooling on flat roofs)
- Pervious pavement
- Runoff capture and use for irrigation
- Disconnection of impervious areas
- Rain gardens and other devices
- Swales for runoff from highways and roadways
|Figure 1. Shellfish harvest classification
|Figure 2. Runoff annual goals
Figure 2 shows how the county’s 10% EIC antidegradation target relates to allowed rainfall percentage to runoff by soil type on an annual average. Practices selected from the manual will allow the runoff from the impervious area to be reduced to meet the allowed runoff percentages.
Integrating Stormwater Runoff Quantity and Quality
The Center for Watershed Protection pondered “the computation conundrum,” how to account for small storm runoff reductions in larger storm hydrology. The conundrum highlighted the traditional stormwater approaches in which quantity and quality decisions were kept in separate silos and impacts in one area did not get recognized in the other area. The computational methods were further partitioned by state and regulatory programs. Quantity controls were designed on an event basis, and water-quality controls were developed on an annual average basis.
- Increase in rate of runoff (peak controls)
- Increased loads of pollutants to receiving waters (water-quality controls)
- Increase in total volume of runoff (runoff volume controls)
Beaufort County found in addressing a runoff volume requirement that volume quantity and quality requirements can be integrated by using EIC. Quantity requirements, computed on an event basis, can be related to runoff volume controls sizes and evaluation of the EIC percentage. This percentage can then be related to water-quality requirements that have historically been developed on an annual average basis. Using EIC allows for a systematic linkage of all three impacts of impervious surface increase and links well to research that has related biological and hydraulic health of streams to the impervious surface in the watershed.
Daniel Ahern, P.E., BCEE, is the stormwater manager for Beaufort County, SC.
Richard Wagner, P.E., D.WRE, is with CDM Smith.
Robert Klink, P.E., is the Beaufort County engineer.