The next time you park your car in Houston, you might be just a few feet above a system of chambers holding hundreds of thousands of cubic feet of stormwater.

Underground stormwater detention systems are becoming an increasingly attractive option in densely developed urban areas where large above-ground detention basins aren’t feasible.

We don’t always think about stormwater detention systems when we visit apartment complexes, big-box shopping centers, and other large developments, but they’re there. Developers are required to include detention systems on a site to ensure stormwater doesn’t run off the newly developed site and flood nearby homes and businesses — or run into a nearby bayou and flood people downstream.

Detention systems traditionally include an excavated area — something that resembles an empty pond or lake — and a channel system that directs stormwater into it. The systems are designed to allow the collected water to drain out at a controlled rate over time.

While traditional detention systems are effective, we’re seeing increased interest in underground options. The alternative systems work just like above-ground basins — they collect, hold, and gradually release stormwater — but they’re built below ground. While the design of underground systems varies, they generally begin with a foundation of crushed rock. A system of boxes or chambers that hold stormwater is placed on top and then backfilled with more rock before concrete is poured on top.

Underground systems are certainly not the most affordable option. Creating an underground system costs roughly $10 to $12 per cubic foot while the EPA estimates that a traditional detention pond costs less than $1.00 per cubic foot to construct. An underground system spanning 5-10 acres could easily cost millions of dollars. That said, an underground system is a good option for developments in densely packed areas like Houston, where buying enough land to create a large detention pond isn’t always affordable — or even possible.

At WGA, we regularly help developers determine how much land they’ll need for a detention system on their site. If a client can’t fit everything on the land available to them, we sometimes suggest a combination stormwater system with traditional and underground systems working together.

We also remind clients that it is possible to realize a return on investment for underground detention. Generally, detention takes about 15% of the developable land. With an underground system, the developer can utilize the space for an additional building. Profits on the additional square footage often pay for the cost of the underground system within a short period of time.

Beyond helping developers determine what kind of detention system is best for their needs, experienced engineers understand how to successfully plan and construct it. If an underground system is beneath a parking lot, for instance, that lot must be able to support normal traffic loads. Engineers also need to be wary of trees and landscaping on the property, so a growing tree’s roots don’t become intertwined with the underground system.

Ultimately, the key to a successful stormwater detention system is always going to be the knowledge and experience of the engineering team on the project.

For more information about WGA Consulting Engineers and stormwater detention systems we’ve worked on, contact us at 713-789-1900, email info@wga-llp.com, or visit our projects page.