Weekly “What is it?”: Stormwater Detention Vaults

Landscape designers describe an underground stormwater project at New Bethel Church in Biloxi, MS. Photo credit: Carrie Stevenson, UF IFAS Extension

In August 2005, large swaths of Biloxi, Mississippi were underwater. In particular, Main Street and historic New Bethel Missionary Baptist Church—0.7 miles from the coastline—were inundated with 12 feet of floodwater. While the levee failures and desperation in New Orleans grabbed most news headlines after Hurricane Katrina made landfall, storm surges in coastal Mississippi reached an unbelievable 21 feet.

The landscaped strip at the edge of this parking lot is part of a highly engineered stormwater vault system. Photo credit: Carrie Stevenson, UF IFAS Extension

Years later, New Bethel and surrounding homes and businesses have dried out but not completely recovered. For years, when it rained, the church’s parking lot filled with standing water, forcing parishioners in their Sunday best to wade ankle-deep in water back to their vehicles.

As part of a broader effort to revitalize and increase the overall community resilience of East Biloxi, a creative arm of Mississippi State called the Gulf Coast Community Design Studio stepped in to help. They had a creative solution that not only addressed the flooding problem, but also improved the aesthetics of the open lot adjacent to the church.

These black plastic cells can be connected adjacent to and above or below similar cells to create underwater stormwater storage capacity. Photo credit: Carrie Stevenson, UF IFAS Extension

While I have written about bioretention here before, the New Bethel project is a bit different. Instead of digging a deeper open-air swale and planting it with vegetation, this effort incorporates an underwater stormwater vault. The designers placed hard plastic open-structure “stratavault” cells below ground, wrapped them in geotextiles, and covered them in landscape soil. The vaults formed an open cavity for water storage, in which the parking lot runoff could be directed. Hidden below ground, the vault was planted over with grasses and other low-growing vegetation.

Illustration of how the stratavaults work underground. Courtesy Citygreen.com

While other options, including pervious pavement, were considered for the project, funding was limited, so they looked for less expensive options. The designers had access to a stockpile of free stratavault cells, so they decided to use them as a demonstration project. So far, it has worked beautifully and kept the parking lot dry.

Many sites that use underground stormwater vaults do so to save space.  Open real estate is expensive, especially on beaches or urban sites. So, many builders use this technique under the foundation of a building or directly below sidewalks and parking lots. Because structures are built over these underground vaults, care must be taken to provide access for future maintenance. The design of underground vaults varies from gridded cells like the East Biloxi project (often used due to their load-bearing capacity) to open concrete or metal tanks.

For more information on the Gulf Coast Community Design Studio and resilience projects in East Biloxi, check out their websites below.

Mississippi State University Gulf Coast Community Design Studio: https://www.gulfcoaststudio.msstate.edu/current-work

Resilient East Biloxi


Posted: April 25, 2024

Category: Natural Resources, Water
Tags: Native Plants, Water Quality, Weekly What Is It

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