Why All the Fuss?
In 2016, the Georgia Stormwater Management Manual (also referred to as the Blue Book) was revised. The main thrust of the updated guidelines is to design stormwater management systems to retain the first one inch of rainfall on the site to the extent that is practicable. The result is a relevant tool to help manage stormwater runoff on projects located throughout the state.
After January 2021, local municipalities across metro Atlanta began adopting these updated standards and incorporating them into the permitting process. As new requirements are adapted and applied, we are working with the local jurisdictions to understand and design to these new requirements, which we feel are important to share with our clients and design partners.
Let’s Start with the Basics
Water quality, as a practice, is one of the main focuses of stormwater design. As engineers, we design systems that limit the impact of stormwater runoff created by increased impervious areas that often come with new development.
The goal of water quality requirements is to limit or reduce – and ultimately eliminate – additional contaminates in the overall stormwater system in all of Georgia. Prior to this revision the primary methodology has been volume storage, which is essentially holding site runoff via detention to allow effluents and sediments to settle out. However, runoff reduction and infiltration evolved from this approach to the new standards of recharging the groundwater system. The result is that the rain or stormwater is slowed enough to penetrate below surface waters.
So, What’s the Approach?
At this point, it’s essential to understand that while stormwater runoff reduction and infiltration are often used interchangeably, they’re not the same thing. Runoff reduction is the methodology, and infiltration is the actual act of infiltrating. Not all best management practices (BMPs) accomplish the infiltration goal. Instead of detaining a volume of water, the goal is to retain a volume of water allowing it to infiltrate into the groundwater system.
The changing requirements from water quality volume to runoff reduction and infiltration are impacting the development process. In addition, the “tried and true” solutions that were implemented on previous projects are not always being approved. The result is extensive project redesigns and extended time in the permitting process, delaying anticipated project completion dates and increasing construction costs.
How is the determination made?
The Blue Book doesn’t mandate stormwater reduction and infiltration for all sites, extending the authority to local jurisdictions. Thus, the most prudent way to determine whether or not runoff reduction or infiltration is applicable is to perform a feasibility study of the site. After the feasibility study, an owner can determine if the standard water quality volume approach or the runoff reduction approach is preferred by the local jurisdiction.
Why spend money on a feasibility study?
Because each jurisdiction treats projects differently requiring unique solutions, we like to look at project sites very early in the process to note several things. The first thing is to determine if the soils are suitable for infiltration. Georgia has an abundance of red clay and granite, so infiltration is not always an option under those conditions. Groundwater is another condition that makes it infeasible to provide infiltration. If you have a high groundwater table, you can’t put more water into the soil than it can hold in a saturated state.
Ultimately, it is up to each jurisdiction to determine whether runoff reduction is required or if a feasibility study will be accepted. In some cases, consideration may be granted while not eliminating the runoff reduction requirement, allowing for a combination approach of runoff reduction and standard water quality BMPs to be utilized.
We advise our clients to have an infiltration test performed on the potential development site early in the process. A geotechnical firm can perform this service. While it differs from soil boring tests, both tests can be performed simultaneously. Once this information is in hand, it can be a crucial piece of data to support a feasibility study. It’s a small up-front cost that could serve our clients well in the long run.
With the evolving complexity of stormwater management systems and jurisdictional requirements, we find that thorough site research and analysis and conversations with jurisdictional authorities, design and permitting time during the development process is reduced.
Do you or your site need stormwater runoff reduction solutions? Reach out to our team of experts.