The design of the Infiltration Trench needed to meet multiple purposes. In addition to the stormwater management and monitoring capabilities of the site, it was also necessary to provide an attractive and functional picnic and gathering area for Villanova students and faculty.
The Infiltration Trench is located between the St. Augustine Center and a bi-level parking garage. The drainage area is approximately 20,400 ft2 and 100% impervious. Before construction began, a test pit was dug to determine the depth to bedrock and soil properties. Percolation tests were performed to determine whether the soil was suitable for infiltration. The recommended infiltration test from the Maryland BMP manual was performed, along with a percolation test using a constant-head infiltrometer. The investigation showed that there was sufficient depth to bedrock and that the soil was suitable for infiltration.
The Trench is approximately 10'x13' and 6' deep and is filled with large washed aggregate. This provides approximately 300 cubic feet of storage, which is equivalent to less than 1/4" over the entire drainage area to the Infiltration Trench. Inflow is brought into the Infiltration Trench via a system of 4" PVC pipes that connect drains from the parking deck. The inflow is first piped into the monitoring/pretreatment bench located against the parking garage. This bench houses monitoring equipment and provides pretreatment of the runoff. Stilling pools allow sediment and grit to settle out of stormwater runoff, while a mesh screen with ¼” openings removes leaves and debris prior to the runoff entering the Infiltration Trench.
The Trench is overlain with Eco-Pavers donated by EP Henry. The Eco-Pavers create a strong, durable and most importantly, porous surface for the picnic area. A 6" overflow pipe connects the top of the bed (just beneath the pavers) with a nearby existing storm drain. When the capacity of the overflow pipe is exceeded, the Eco-Pavers act as an additional overflow. Excess runoff flows up from between the pavers and flow over 2' of grass into the storm drain.
Instrumentation for water quality and quantity monitoring of the site was incorporated into the design from the onset of the project. Rainfall is measured with a tipping bucket rain gage located on the roof of the parking garage. Inflow is measured with a pressure transducer and V-notch weir located inside the monitoring bench. The weir was constructed using a 1/4" thick aluminum plate per ASTM standards (D 5242-92). Outflow from the overflow pipe is also monitored using a pressure transducer and a 6" diameter Palmer Bowlus flume. The flume dimensions were specially retrofitted to fit the small confined space in the existing storm drain. Two monitoring wells were included for redundancy, with each containing a set of two lysimeters installed beneath it, one at 2' and one at 4'. One of the monitoring wells has a pressure transducer installed to measure the depth of water inside the storage bed. The site is also designed to take inflow water quality samples with an automated sampler.
The site continues to be included as part of various Stormwater tours at Villanova. Like all the other SCMs on Villanova's campus, the Infiltration Trench has educational signage explaining the design and function of the BMP.