Relief for an Overburdened Watershed
Radnor Township manages storm water by offering above-ground community recreation area and below-ground relief of surface floodingFounded in 1682 and located 13 miles west of Philadelphia, the Radnor Township municipality is the home of more than 30,000 residents. Radnor Township lies along the Main Line, a group of wealthy Philadelphia suburbs. For several decades, however, the township has required effective storm water management because the area is frequently plagued by huge storm events resulting in significant flooding, sometimes 2 to 3 ft deep. As a result, the township’s Board of Commissioners passed a comprehensive storm water management ordinance in May 2005.
The township has recently installed two storm water management systems to help with runoff issues. Both systems have been installed under existing parking lots. In 2007, in a joint effort with the Radnor Township School District, the township began work on installing another storm water management system at the middle school in Wayne, Pa.
According to Craig J. Thorne, building support personnel for the Radnor Township School District, the school district wanted to develop a new middle school facility on the existing site that was to be certified LEED construction with a small footprint.
By building a new middle school on the existing site, the old middle school building could be torn down and used as the storm water management site.
“The concept was to stay in the center of town and utilize the existing track of land in the best possible manner so as to promote an above-ground community recreation area and below ground to relieve surface flooding at the site and downstream flooding as well,” Thorne said.
Both the school district and the township would benefit from this storm water management project. The district would be able to use the space as a playing field and the township takes the storm event surge off of the residents downstream in this overburdened watershed.
Challenges and Procedures
Challenges. Site restrictions played into every aspect of construction. “Keeping the middle school in the heart of town was the ultimate goal for the community,” Thorne said.
According to him, this was the only large area that was available in this specific watershed that was suitable for this type of project because the same square footage also had to fit a geothermal well field as well as playing fields. Groundwater was also a challenge. The area has natural springs, many of which are under the old school building.
“We knew the groundwater table was high and that we were going to encounter natural springs during the drilling,” Thorne said.
More than 100 wells, 150 ft deep were drilled on site. The geothermal well fields were installed in the summer of 2007 and will heat and cool the new middle school.
Procedures. According to Thorne, the elevation of the underground piping was set at a height that would allow the system above to work properly. Approximately 5 ft of elevation was required for the underground basin. Because the site was cleared of topsoil from the well boring, there was no need for a strip-and-store process of this soil. The existing elevation was roughly 1 ft lower than finish grade and only 3 ft of earth needed to be removed at the ends of the playing area; however, 5 ft had to be removed in the center for the proper backfill and the crown of the football field. The area to receive the basin is roughly 342 ft by 168 ft.
Geo-tech fabric was used to keep debris out of the crushed stone that will surround the basin. Storm water will then be diverted from an existing underground brick arch culvert and surface inlets to the new storage basin. Adequate topsoil will then be reinstalled to support the new natural grass playing fields. Horst Excavating, located in Lancaster, Pa., was responsible for the project.
Storm Water Tank Modules
When the school district and the township first started looking for a way to collect storm water runoff, they hired a consulting engineering firm who got in touch with Brentwood. According to Rob Adams, product manager, storm water, for Brentwood Industries, they initially offered StormPac, a first generation media product.
“As the project moved along, there were some minor design changes and other unforeseen changes that delayed the storm water portion of the project. [During] that time Brentwood fully released the StormTank, which the engineer and school liked very much,” Adams said. “The design was based around the StormTank and the project moved along quickly from that point.”
In order to meet the site challenges, Brentwood had to help create a footprint that would fit under and around an athletic field and at the same time be a shallow enough system so it would not disturb the deep wells in the same area. This was done using the 18-in. StormTank modules and laying out a footprint that matched the property being used for the athletic field and surrounding area, according to Adams.
Radnor Township expects the storm water management project to be finalized by the fall of 2008. When completed, the area will have a storm water management system that maximizes the best use of space, collects surface runoff and relieves downstream flooding.
“The community wanted a certified green building with a small footprint and made it possible by deciding to stack, if you will, these three projects: geothermal well field, underground storm water management and playing fields on top of one another,” Thorne said. “It just goes to show you how things can be accomplished with a little forethought and a lot of determination from the stakeholders.”