Strategically located in one of the fastest-growing areas in the United States, Watkins Centre is a 600- acre mixed-use development in Chesterfield County, a southwestern suburb of the city of Richmond, Virginia. Positioned to become Richmond’s premier business and lifestyle address, according to the regional center’s website, it will be primarily dedicated to commercial uses, including office, retail and light industrial, along with high-density residential.
Westchester Commons is the first phase of retail development at the Watkins Centre site, and encompasses over 100 acres. The site is the first large scale development on the Route 288 corridor and is located on the border of the watershed limits of the Swift Creek Reservoir which provides drinking water for a portion of Chesterfield County.
The site would include a high area of impervious acreage on the site – more than 80 acres – and the Virginia stormwater regulations required a 65% removal of total phosphorous (TP) from runoff. Initially, the consulting civil engineer, Timmons Group in Richmond, opted for a single large surface basin to not only meet the standards, but also detain the required two- and 10-year storm events.
Chesterfield County requires that a surface basin must contain 10 times the water quality volume (defined as ½” over the impervious acreage) to achieve a 65% TP reduction. The resulting footprint of the surface basin was in excess of five acres and completely eliminated an intermittent stream channel. Following review by the Army Corps of Engineers, the plans were rejected because of the eliminated channel. Design engineers went back to the drawing board and decided to separate the pond into two basins that bracketed the stream.
While this option was acceptable to the Corps and to the regulatory agencies, the new layout created further issues. First, the volume of one of the ponds was greater than 50 acre--ft.. This placed the emergency spillway into the purview of dam safety oversight, meaning additional permitting delays and costs and long-term safety monitoring. Then because of the need for a great deal of parking, the ponds would have to be surrounded by 50 foot tall retaining walls. The cost of these walls was excessive and the developer requested a further redesign.
Timmons Group had worked with Contech on previous projects utilizing the Stormwater Management StormFilter water quality system, so they approached Contech with the idea of specifying the system on this project.
The StormFilter system is one of only six products approved as a manufactured BMP system by the Virginia Department of Conservation (VDCR), and included in the Virginia Stormwater Management Handbook (VSMH). Moreover, it is one of only two manufactured systems that is granted a TP removal performance level of 50%. Chesterfield County approved the use of the system.
The next thing to do was to determine the configuration and location of the structure. The sheer size of the drainage area feeding the system required that a brand new configuration of the vault housing the filter cartridges be developed. The maximum number of cartridges that Contech had ever placed in a single structure up to this point was 600. This structure would require 928 filter cartridges.
Typically, large counts of cartridges will either be housed in concrete box culverts or in a large castin- place box. The number of cartridges that were required made either of these options too costly. Therefore Contech proposed the use of their CON/SPAN system, a patented, modular, precast system typically used for the underground detention of stormwater runoff. Its innovative, economical design stands apart from other products through the strength of its distinctive arch design, fast set-in-place construction, and extensive technical support.
The layout of the CON/SPAN StormFilter consisted of six distinct cells with approximately 155 cartridges in each cell. Four cells treated the eastern portion of the site (approximately two-thirds of the entire site), while the other two cells treated the remaining drainage area to the west. The entire footprint of the structure is 66--ft. by 72--ft. with 8--ft. of internal height and approximately 10--ft. of cover above.