New stormwater regulations adopted in Maryland in 2010 introduced the concept of “Environmental Site Design (ESD).” ESD tasks the engineer and the owner to look to the site first for cues in stormwater management. It requires engineers to map the site for sensitive features worth protecting. Once your site has been mapped, the designer is supposed to “fingerprint” site improvements that avoid impacting these features. Then, once the site has been laid out, the designer turns to more “structural” means of treating stormwater, both for quality and quantity.
Despite best efforts to implement ESD to the “Maximum Extent Practicable (MEP),” it is not always feasible or practical to the developer. The development of a new mini-warehouse storage facility fell into this category. To meet the requirements of both water quality and quantity management using ESD, a large area of the site was dedicated to bio-retention ponds.
The developer of the site had long term experience maintaining bio-retention ponds on a variety of properties. In his experience, the maintenance of these ponds is very time consuming and very expensive. The owner asked the engineers at Frederick, Seibert and Associates to take a second look at the proposed BMP. Upon further analysis, a determination was made to install a StormFilter, a stormwater treatment system comprised of a concrete structure that houses rechargeable, media-filled cartridges that can treat stormwater to the Maryland Department of the Environment standards.
Normally, these concrete vaults with filter cartridges are reserved for urban areas where water is stored in pipes upstream of the structure and slowly drawn through the filter media at a designed rate. In this case, the structure was installed directly in the berm of the basin. The only difference is that the storage is taking place in a pond with a grass bottom.
The runoff will enter the basin via direct sheet flow along the entire basin perimeter adjacent to the pavement. A stone trench at the edge of the pavement is used to settle fines and reduce flow velocity before flowing into the pond. There is no traditional underground storm drain other than the outlet pipe from the basin. As water ponds up in the basin, runoff will drain into the StormFilter structure located in the berm via a small inlet orifice and trash-rack on the outside wall of the structure. Some evaporation of runoff in the basin will occur as it fills and slowly drains, which adds the benefit of at least some minimal volume/runoff reduction. Volume in excess of the ESDv will exit the basin through a new outlet control structure that provides overflow relief.
With an impervious lining, there is no opportunity for infiltration and recharge of groundwater, so the bio-retention pond is just a filter, discharging clean water at a certain rate. At the end of the day, this solution offered all of the same stormwater management benefits for a similar cost and much less maintenance, which is exactly what the developer wanted.