Along the border between Montgomery and Delaware Counties in the southeast corner of Pennsylvania lies a 41-acre urban watershed. The watershed consists of more than 16 acres of impervious surface, including Villanova University's parking lots, dormitories, office buildings, railroads, highways, and housing areas. An existing storm water detention basin on the university's property was targeted as an ideal site for a 319 retrofit project. This basin had the potential to treat the runoff that forms the headwaters of a watershed listed as medium priority on the state's degraded watershed list and to treat flows that affect a high-priority stream segment on the state's section 303(d) list.
An existing storm water detention basin was targeted for a 319 retrofit project.
The purpose of the 319 project was to make a storm water wetland out of the existing detention basin, creating a water quality treatment facility. Water quality considerations were not part of the original design. The existing storm water detention basin was originally designed to reduce the increased peak flows coming from the university campus. Runoff entered the basin through sheet flow from a large parking lot and through two major pipes. The site had an existing 12-inch underdrain that quickly carried the water through the basin, directly connecting the parking lots to the headwaters of a small first-order stream. The site was designed to remain dry except during storm events, but there was always some flow through the underdrain, supporting the concept that the site was ideal for creating a storm water wetland.
One goal of the project was to prove that retrofitting could be accomplished easily on an existing structure without violating the original design concept. The retrofit of the basin therefore concentrated on retaining small storms while not violating the original storm water peak flow controls required by law.
The basin was redesigned by removing the underground pipes, moving earth to create a meandering flow path, adding a sediment forebay, and modifying the structure outlet. Wetland plantings were conducted; plants were selected for diversity and based on their ability to thrive at different inundation levels.
Low flows would now travel through the sediment forebay to give particles a chance to settle out. Flows would continue through a meandering wetland channel, maximizing contact with the plants, and finally through a deeper pool and the outlet structure. The flow path for larger storms would provide for the flow to go over a berm, preventing resuspension of the sediments collected in the structure, thus using the original design for peak flow management while avoiding damage to the low-flow components.
A meandering channel was designed to reduce flow velocity and allow particles to settle out.
Because it is located on the university's property, this storm water wetland is not only aiding in the reduction of pollutants for this headwater but also serving as a permanent research and demonstration site. To date, hundreds of visitors have toured the site, and the site is being incorporated into a demonstration "theme park" of multiple BMPs (including signage) on Villanova's property.
The wetland project was completed at the end of 2000, and the current plan is to wait a year for the wetlands to mature before starting to collect water quality samples. Hydrologic and hydraulic monitoring is already under way, and flowmeters and a rain gauge also have been installed to collect data. It is projected that total suspended solids will be reduced by 70 percent, total phosphorus by 40 percent, total nitrogen by 20 percent, and lead by 75 percent.