How to Improve the Quality of Post-Development Stormwater Runoff

In 1990, the EPA amended the National Pollutant Discharge Elimination System (NPDES) program to include stormwater runoff. This was a monumental step in advancing water quality nationwide. This revision required permitted local governments to establish regulations for new development stormwater runoff to be as close as possible to its pre-development composition. However, a single uniform approach was not developed to help local governments implement this new NPDES standard.  

The absence of a formalized structure meant municipal separate storm sewer systems (MS4s) were on their own to develop an effective and efficient regulatory approach. Without a clear path forward, multiple methods of varying effects were created. Several state and local governments gravitated toward the "first flush" approach, a process offering simple calculation and ease of use. Others implemented the "percent reduction" method, which focused on capturing the sediment in stormwater runoff to subsequently collect most of the pollutants. The "volume control with infiltration or treatment" standard utilized by MS4s, which centered on capturing a large amount of runoff.  

While these regulatory approaches were valid options to meet NPDES requirements, Greenville County identified a way to streamline this approach by calculating the quality of pre-development and post-development stormwater runoff. These calculations would allow Greenville County's design community to make evidence-based decisions when designing best management practices to bring post-development stormwater runoff as close as possible to its pre-construction condition.  

The only hitch? Greenville County needed a tool that could easily and accurately support this calculation. Fortunately, the county discovered the Integrated, Design, Evaluation and Assessment of Loadings (IDEAL) model.   

IDEAL Catches Greenville County's Attention 

The South Carolina Department of Health and Environmental Control's (SCDHEC) Office of Ocean and Coastal Resource Management (OCRM) originally developed and implemented the IDEAL model for the eight coastal counties of South Carolina. Tasked with managing coastal resources, including stormwater, OCRM noticed the need for a tool that could safeguard water quality associated with large site development. Since this kind of model did not exist, the agency acquired a National Oceanic and Atmospheric Administration (NOAA) grant to fund its creation in 2002.  

The development team consisted of Bill Barfield, PE, Ph.D., of Oklahoma State University; John Hayes, PE, Ph.D., of Clemson University; and Woolpert staff members. Together, this group created IDEAL. The model began as a simple spreadsheet that could help engineers predict the trapping efficiencies of dry ponds, wet ponds and vegetated filter strips for nutrients, bacteria and total suspended solids (TSS) for one watershed at a time.  

OCRM included IDEAL in its “Post Construction BMP Design Aid Manual,” which was explicitly created to help municipalities on the South Carolina coast meet water quality regulations. The coastal design community had access to IDEAL for free whenever OCRM required a quantitative and qualitative water quality report. Once the model was made available, it did not take long for Greenville County to see it as the tool needed to successfully meet water quality standards for their stormwater program.  

Greenville County Reimagines IDEAL  

In 2003, Greenville County started working with the IDEAL team to develop a customized version of the tool that they could include in the county's new design manual as a component of its NPDES Phase I program.  

Understanding the county's regulatory approach comes down to knowing the composition of stormwater runoff. Stormwater runoff is rarely, if ever, acutely toxic to aquatic organisms. Instead, the impact of stormwater runoff is similar to death by a thousand papercuts, meaning the pollutant load of runoff over time is more of an environmental threat than the concentrations associated with a single storm. Therefore, the basis for this approach is that post-development annual pollutant loads cannot exceed pre-construction levels for the pollutants of concern. How does this work?  

Say a local government has a waterbody that is impaired by E. coli bacteria. Designers would need to determine the annual load for bacteria from the site in its pre-developed condition and then design a treatment train for the post-developed condition that would ensure the annual bacteria load did not increase. IDEAL is able to provide that calculation by utilizing historical rainfall data for the region and running a probabilistic annual simulation for the site. Such an approach reduces many of the issues associated with a single storm (e.g., 1- to 2-inch, 24-hour storm) calculation since no single storm can represent the pollutant buildup during interarrival times, antecedent moisture conditions and growing or dormant seasons, all of which play an important role in pollutant runoff. 

"This regulatory approach is easy for council members to understand and approve because you're saying Mother Nature got it right," said James Riddle, PE, a Woolpert project manager who has served Greenville County since 2008. "You're getting the stormwater runoff closer to its original composition before the land was developed, which is what the EPA wants."  

Equipped with an innovative calculation and a sophisticated model to support it, Greenville County was able to institute a process to effectively meet the NPDES requirement for stormwater runoff. As water quality standards have evolved, Greenville County has helped IDEAL evolve as well.  

Greenville County Complies With New Standards 

Today, the Phase I MS4 Permit in Greenville County requires officials to support a post-development permitting program, meaning the county's standards supersede statewide post-construction water quality requirements. County officials determine the standard by assessing the location of a project and the nature of the development or redevelopment initiative. Greenville County has refined IDEAL so that it helps designers meet these new water quality requirements.  

Some of those enhancements include: 

• The ability to model multiple BMPs and watersheds. 
• The expansion of available BMPs for designers to assess. 
• The inclusion of pollutant (TN, TP, TSS, bacteria) Event Mean Concentration (EMC) data for different land uses. 
• A GIS interface.  

These enhancements enable the model to meet evolving water quality regulations, leading the county to require engineers to use the tool to ensure essential standards are met.  

"Within the Reedy River, which is impaired for nutrients, the engineers must demonstrate using IDEAL that there is no increase in annual loading for total phosphorus," said Judy Wortkoetter, PE, county engineer for Greenville County.  

Requiring the use of IDEAL also enables designers to fulfill other county requirements, including the following: 

• Any development or redevelopment project outside the Reedy River watershed that is greater than or equal to 1-25 acres and not discharging to an impaired waterbody must trap 85% of the annual TSS load. If the project is discharging to an impaired waterbody, designers must trap 85% of the annual TSS load and comply with anti-degradation rules for the pollutant of concern.  
• For any development or redevelopment project in Greenville County disturbing 10,000 square feet or 0.99 acres, designers must ensure the annual TSS load is less than or equal to 600 pounds per acre. 

IDEAL helps engineers calculate and quantify pollutants discharged to help meet regulations. However, the tool also helps local governments like Greenville County comply with local and state requirements.   

"The benefit we have seen is having tangible results in meeting water quality requirements for impaired waters, sites over 25 acres and nutrient management in the Reedy River," said Wortkoetter, adding that the county is able to demonstrate to SCDHEC that it has established a process to meet the anti-degradation rule. "We also have clear results in meeting the state regulation on trapping total suspended solids instead of first flush."  

Increasing adoptions of IDEAL 

Because Greenville County has implemented a successful tool to assist in meeting water quality regulations, other entities throughout the state have followed suit. The University of South Carolina and Clemson University are using IDEAL in their engineering schools for senior design projects. Government entities such as Richland County, Anderson County, Mount Pleasant and Horry County are also using IDEAL for new development permitting. Additionally, the city of Columbia, Anderson County, Hilton Head Island, Richland County, Fort Jackson and the South Carolina Department of Transportation have used IDEAL to model water quality retrofit projects and establish design standards.  

The widespread use of IDEAL in South Carolina has also led to its use in other states. For example, it's been used to demonstrate water quality benefits on projects in West Virginia, North Carolina, Georgia, Florida, Ohio, Oklahoma, Kentucky, Virginia and Texas.  

Still, the county is not solely an example of what IDEAL can do today. Greenville County's stormwater regulatory approach has been adjusted throughout the decades to account for new priorities, and IDEAL has continued to evolve with additional regulations. As Greenville County uses the model's latest enhancements to meet its evolving water quality goals, the county will also serve as a primary example of what IDEAL can do in the years to come.