Underground Detention Systems Help Reduce the Stormwater Runoff Peak Flows

Nov. 30, 2022

The 200 Old Tappan site is a 19-acre mixed-use development in Bergen County, New Jersey. The proposed project involves the construction of a multi-family apartment and a mixed-use building, 99 townhomes, and a 21,000 sq. ft retail area, as well as associated parking, residential roadways, and associated infrastructure. The site alterations required stormwater management systems to comply with the local water quality and quantity regulations. 

As a redevelopment project, surface stormwater facilities were not feasible to meet the project requirements. PS&S partnered with StormTrap to design three underground detention systems to reduce the stormwater runoff peak flows to pre-development conditions as required by the NJDEP Stormwater Management Rules.  

The challenges for this project included limited areas for placing the stormwater management facilities due to layout requirements, site grading limitations, and the need to convey runoff from the three basins to an existing downstream storm manhole within the municipal storm system via gravity.  

“The invert out of the existing manhole was relatively high, so we had to set up the basins elevations carefully,” explained Gary Yin, P.E., Sr. Engineer at PS&S. The basins were built with 5′-3”, 6’-0’’ and 7’-0’’ DoubleTrap modular units with low cover. “Systems #2 and #3 were also required to be watertight since we couldn’t lower the systems elevations.”  

Two detention systems (#2 and #3) were installed under a lawn area between townhouses. The other unit (#1) was placed in front of a multi-family building and is located under the parking area. All systems have an outlet control structure to manage runoff release and partially provide water quality treatment with the required extended detention time.  

The installation was successfully completed in December 2021. The three basins together can store approximately 139,487 cf of water and “significantly increased the available area for redevelopment while accommodating the site constraints,” stated Yin. 




Jan. 22, 2020