Project Specifications:

Location:  Durham, NC

Application: Detention and Treatment – Sand Filter

Product used: 8’-0” DoubleTrap – Sand Filter; 6’-0” DoubleTrap

Number of pieces: 28 |132

Number of systems: 2

Total water stored: Total Detention System Volume = 31,284 CF   Total Sand Filter Volume = 6,533 CF

Market: Residential

Project Details: The 3.97-acre site, located in the downtown area of the City of Durham, North Carolina was primarily open space with a few impervious parking areas and was approximately 17% impervious in the pre-development condition. Proposed development on this site consisted of the construction of apartment units, along with surface and underground parking and the supporting utility/stormwater management improvements. The site required a stormwater management system to meet the City of Durham’s water quantity and water quality regulations.  In the post-development condition, the site is approximately 85% impervious with most of the site being treated by the stormwater management system before discharging into the existing storm drainage network.

Challenge:  The City of Durham required a stormwater management system on the site to meet stormwater management performance standards for development. The regulations have peak runoff control and water quality requirements.  Jeremy Finch, P.E., Project Manager for McAdam Company explains that “peak runoff control requirements stipulate that for development that increases the peak runoff rate from the 1-year, 2-year, and 10-year storm events from pre-development conditions, stormwater management facilities should be designed and provided such that there is no net increase in peak runoff rate.” In regards to water quality requirements, the stormwater management system design had to provide 85% TSS removal and a minimum 30% on-site reduction in nitrogen and phosphorus levels (i.e. 30% of the required difference between post-development loading rates and the allowable site export).

Solution: To meet the city requirements, an underground detention system and underground sand filter were installed in the site to treat and manage stormwater runoff from the proposed development. The water quantity treatment was provided with the traditional detention StormTrap system and the water quality treatment was provided by modifying the traditional StormTrap system design to incorporate sand media with underdrains to allow the system to function as a sand filter. “StormTrap detention system was designed with a sump to allow the floor of the detention vault to actually serve as the sediment chamber for the sand filter” explains Finch. The engineer states that StormTrap was chosen for the project because it offered the best stormwater solution “StormTrap gave us a cost-effective solution, the precast concrete system allowed for more durable long lasting design, and it was quicker to install than some of the comparable systems”.