January-February 2013

Integrating Stormwater Runoff Quantity and Quality Requirements

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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%.

The county has several unique coastal characteristics: 50% of the county consists of open areas and salt marshes, there is little upstream freshwater input, shellfish harvesting and fishing are major economic and recreational activities, and population growth has been rapid in recent decades.

The impetus for the county’s stormwater regulations came from shellfish closures in the mid-1990s (Hilton Head Island, shown at the lower right in Figure 1). These closures led to heightened public awareness and political will on the part of the county council. The first round of the regulations was adopted in 1998. The innovative approach used at the time was based on an antidegradation target for phosphorus of 10% “equivalent impervious cover” (EIC). EIC is a metric that measures how effectively impervious surface runoff is reduced relative to predevelopment pervious surface runoff. In other words, degradation could be halted if land-development projects limited their EIC to 10%. EIC limits can be reached either by reducing impervious cover or treating impervious cover with a stormwater best management practice (BMP) that earns credits based on its size assigned in the BMP manual. In subsequent years, the EIC concept was extended to antidegradation goals for bacteria and nitrogen. The county also established a stormwater utility in 2001 based on a recommendation from citizen action in response to the 1990s shellfish bed harvesting closings. This utility is a dedicated funding source for stormwater-related initiatives.

After another shellfish closure in the May River in 2009 (station 19-19 in Figure 1), the county investigated possible causes, and the volume of stormwater came under increased scrutiny. Increased stormwater volume from development projects was implicated in salinity changes, increased discharges into wetlands with observed increases in fecal coliform bacteria concentration at wetland outlets, and impacts to fisheries. With direction from the county council, the county stormwater utility developed a volume-based criterion based on the 95th percentile storm event (derived from the federal facilities standard in the 2007 Energy Independence and Security Act). This storm in Beaufort County is 1.95 inches of rainfall in 24 hours.

In 2009 the county amended its stormwater ordinance to include volume controls and in 2010 updated its BMP manual. The revised manual details volume reduction and EIC credits for six stormwater practices that infiltrate, evapotranspire, and/or reuse runoff:

• 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

The updated manual outlines EIC credits for various combinations of practices, soils, and ponding depth/storage. The manual also contains a compliance worksheet to calculate EIC resulting from using a combination of practices.

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.

 

With the adoption of new development controls, questions were raised about developments that had been approved before runoff volume controls, but that had not been built. An analysis was conducted to determine the extent of this unbuilt universe. The analysis indicated that there were more than 22,000 residential lots of record that could be built without volume controls. More than 15,000 of these vacant lots were in previously approved subdivisions and planned urban developments (PUDs). This was significant, because the built universe of improved single-family residential structures was 39,000 units. It was determined that the additional stormwater runoff from this permitted future building could make the volume problems worse and could lead to further water use impairments.

Ordinance changes were developed to require on-lot volume reduction BMPs for individual new homes and modifications of existing homes that are more than 50% of assessed value. This was required only if the volume of the lot was not being treated by a development plan or other method. This allowed the voluntary option for subdivisions and PUDs to opt for a neighborhood retrofit in lieu of only on-lot controls. These controls were adopted in June 2011 after some workshops with a new stakeholder in the stormwater control arena, the local homebuilders. As part on this process, the county developed a web-based program to assist in compliance with these on-lot controls. This Web-based program was developed to allow homebuilders and homeowners to develop solutions without resorting to technical support. More than 50 homes received permits utilizing this web-based approach. Additionally, a number of developments have analyzed their existing controls and have documented their compliance with the 2009 volume controls on a development-wide basis. The county has developed a spreadsheet to enter lot features such as disconnected impervious area and cisterns; it can calculate rain garden size required to meet volume control requirements after the other site features are taken into account. This allows for evaluation of low-impact-development (LID) benefits on an on-lot basis.

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.

The one thing driving both the quantity and quality designs has been impervious surface. Development in Beaufort County has increased impervious surfaces. Researchers have linked increased percentage of impervious surface in watersheds with deceases in stream health and increased water use impairments. Increased impervious surface causes three impacts:

• 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)

When the county adopted controls for the third impact in 2009, many questions were raised concerning the benefits of volume controls on peak shaving and their impact on existing water-quality controls.

The county discovered that the EIC concept historically used for water-quality compliance could be adapted for the new volume control criterion. It was determined that the LID features designed to meet the 95th percentile storm (1.95 inches) control requirement would also reduce long-term runoff to a level that would be expected from a site with 10% equivalent impervious cover, which was similar to the goals set for many of the targeted pollutants. This led to a realization that runoff volume controls were a different way to handle stormwater runoff and not an additional set of controls. The county’s BMP manual was reformatted to recognize the water-quality benefits of volume reduction before going on to address the water-quality controls themselves. This BMP manual reformatting was finalized in 2012 after additional input from local stakeholders and users.

Impacts of runoff volume control were carried over into peak shaving requirements. Analysis indicated that by applying volume controls and maintaining the same peak shaving volume requirements in the county’s current 25-year-event shaving requirement, we could control to the 100-year-event requirement. Although there was some benefit to going to the higher-level controls, it was decided to maintain the current peak shaving volume requirement. This was because the local engineering community had already started to explicitly incorporate the runoff volume control features into its peak shaving calculations. Therefore, by utilizing volume controls, most water-quality and some of the peak shaving requirements are also addressed.

Summary
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.

 

Author Bios:

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.