Trends in Metro Atlanta’s Stormwater Management Regulations

How Developers Can Get Ahead of Varying Requirements

A growing movement towards sustainable stormwater management is affecting the way many metro Atlanta municipalities interpret and enforce the current Georgia Stormwater Management Manual (GSMM) update, or the “Blue Book”. For developers, the trend adds another layer of complexity to land development because interpretations among local authorities having jurisdiction (AHJs) vary. Both factors are impacting the way developers should approach land development site selection and design.

With growing pressure to reduce runoff and improve water quality from the Environmental Protection Agency (EPA) and Atlanta’s surrounding counties, many AHJs require developers to perform site infiltration testing, regardless of whether projects are ground-up or redevelopments. Depending on the AHJ’s requirements, testing results can vastly impact site layout and influence the type of stormwater management practices design teams can use.

Approaching Site Selection and Design

I recommend addressing compliance as early as possible in the property evaluation process, given the extent stormwater requirements can impact a project and the widely varying requirements between jurisdictions.

Start by engaging a site civil engineer who understands the AHJ’s unique requirements and a geotechnical consultant who can perform the required testing during due diligence. Conducting tests at this point enables developers and their design teams to learn critical information affecting site design well before the design process starts.

With AHJ requirements varying between standard percolation tests to extensive testing across the entire site, your site civil engineer should direct the location, number, spacing, depth, and type of infiltration tests needed to comply.

Benefits of Early Infiltration Testing

Testing a site’s infiltration rates early in the development process allows development teams to:

• Have information that will influence site design and layout. Depending on the AHJ, design teams may be limited to locating stormwater management facilities in the areas with the highest infiltration rates. Moving forward with design without this critical information puts development teams at risk of incurring costly redesigns later.

• Better understand stormwater management costs. In some cases, typical hydrodynamic separators are no longer permitted, and more costly filtration or infiltration stormwater management devices are needed. In other cases, development teams have been directed to incorporate swales or bioretention basins, which restrict space, reduce yield, and increase project budgets.

• Maintain project momentum. Waiting until design plans are submitted for review, only to be told that more comprehensive infiltration testing is needed, could significantly slow down permitting and overall project schedule.

Take Control

While the Georgia Blue Book guidelines are intentionally ambiguous to allow AHJs the flexibility to customize them, developers can still take control of a project’s design, budget, and schedule. Here’s how developers can make more informed decisions:

• Know the AHJ’s requirements. Engage both geotechnical and site civil engineering consultants to confirm the approved methodology for infiltration testing in your municipality. I also recommend meeting with the AHJ’s staff to get their buy-in before performing the testing.
• Understand what site features impact infiltration. Learn how certain elements can impact infiltration to help with site selection. Update your selection criteria to include factors like clay soil types, high groundwater tables, and the presence of rock, which can all result in low infiltration rates. Being aware of these factors positions development teams to evaluate sites quickly, and potentially arrive at a go/no-go decision before testing.
• Work with your consultant team to minimize the effects of stringent requirements. In some situations, Bohler’s engineers have identified opportunities to obtain exemptions from the infiltration testing requirements. Some redevelopments, utility improvements, Americans with Disabilities Act (ADA) improvements, permeable pavers, and other project types can be mostly or completely exempt from stormwater management requirements.

Addressing Poor Infiltration

If early testing yields a low infiltration rate, there are still ways to move forward. An increasing number of AHJs in the metro Atlanta area no longer allow the hydrodynamic separator for water quality treatment, the typical device used for sites with infiltration rates below 0.5 inches per hour. Thus, design teams are likely to use a filtration device in conjunction with a water detention facility to hold water on site. Common filtration devices include:

• Sand filters. These can be either above ground or underground with a sand media and underdrains to treat stormwater before it leaves the site. These can be costly, especially if underground, and require significant long-term maintenance. They can also be used in conjunction with a stormwater detention facility.
• Swales and bioretention. This design option can reduce rainwater runoff and provide some water quality treatment. Swales and bioretention are typically used in conjunction with other stormwater control measures to meet full runoff volume reduction, water quality treatment, and/or detention requirements.
• Proprietary filter devices. These are typically underground systems that use filter cartridges to treat runoff. They have specific design criteria and can require significant upfront and long-term maintenance costs. They are used in conjunction with a separate detention facility.

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