Stormwater management is one of the most important considerations on any site design. On larger sites, it’s common to designate a portion of the land for an above ground stormwater management detention pond. But what about tighter and/or smaller sites? As developers look for the best location for their proposed project from the limited selection of available sites, sometimes they are forced to consider a smaller site that has been sitting available for years. This can result in sites potentially smaller than an acre with no room for a traditional above ground detention pond (imagine an apartment building in the middle of a bustling downtown). Our goal is to provide our clients with the most efficient site design possible. On smaller, more constrained parcels of land, underground detention is one of the best ways to maximize return on investment for a new development by allowing for more develop-able space above ground. Below are just a few methods we have used.
- Underground Chambers: We use chamber systems often. One common type of chamber uses hard, plastic arched pieces that interlock. They’re surrounded by a stone base and backfill, so that the stone provides additional detention volume and also allows the stormwater to potentially infiltrate into native soil. Some systems also provide water quality treatment with isolated rows of chambers to catch debris, sediment, and trash typically generated by small rain events. Ports are installed along these isolated rows and the entire system to facilitate routine inspection and maintenance, which is highly recommended. Another advantage of the chambers is that they are lightweight, making them easy to install.
- Corrugated metal pipe systems: One factor that may dictate using a more closed system like oversized corrugated metal pipe is the percentage of expansive clay on site. Expansive clay swells with the intrusion of water and shrinks as the water exits the soil, potentially causing settlement issues. Containing storm water to a more closed system may be very important while controlling the quantity of water leaving a new development. Also, depending on the availability of stone in the area, this option can be less expensive that an underground chamber system.
- Concrete vault: Our project at One Sixty Ross in Auburn, AL, uses a concrete vault under the parking deck for underground detention. We chose this method because it was located below a very heavy concrete structure and made more sense structurally.
There are several factors that help determine which method will work best on a site. One is the depth from the finished ground surface to the existing stormwater discharge invert elevation. Most sites have very limited depth, though we occasionally encounter sites with plenty of depth that give us a lot of detention options. Another factor that leads to underground detention options is the proposed use located on top of the proposed detention system, such as amenities, parking lot or parking deck, etc. Also, as mentioned above, the in-situ soil type may dictate the type of underground detention system.
So if you’re working with one of these smaller parcels for your next project, reach out to us at Foresite Group so we can discuss and vet all options with you. A great site designer can provide creative solutions to stormwater management – we actually enjoy this nerdy stuff!
About Russ Lassiter
Russ Lassiter, PE, graduated from Auburn University with a Bachelor's Degree in Civil Engineering in 2005. Russ currently serves as the Land Development Division Director in the Auburn, AL office. He has a special interest in large mass grading sites, and has recently been able to use this knowledge for faith-based and industrial sites in Auburn.