Beyond the Build

Construction Trends

Keeping Stormwater Below Deck

Underground DetentionWe are in the early stages of a parking structure design for a new project which also includes a storm water management system below the deck. While a number of storm water management systems exist in the market, our design is part of the parking deck and fairly simple compared to most. It features precast double “T” sections, supported by stone mat with geogrid and fabric. Storm water enters the detention system and slowly infiltrates through the stone bed back into the groundwater.

While our system is creating the void using the double “T’s, we are also evaluating cast-in-place vaults as an alternative. During large rain events, a weir and outflow allows the excess storm water beyond the capacity of the system and jurisdictional requirements to daylight off the site.

We are working with a “bottomless” system, but there are also closed or concrete bottom systems. Site conditions and other variables will play into which system is used. As EPA standards have become more strict, and thus state and local standards, we find that most jurisdictions are generally requiring the capture of the first 1” of rainfall on the site to be kept there. Alternative systems are designed with infiltration in mind to put as much of the storm water back into the aquifers as possible.

These systems offer total void storage for full accessibility and are fairly easily to maintain, maximizing the storm water storage while minimizing the overall project footprint. It is all about efficient land usage.

Merrill Stewart Jr.

Merrill Stewart is Founder and CEO of The Stewart/Perry Company, a commercial building contractor based in Birmingham.