In 2007, the Philadelphia Water Department conducted an inventory of stormwater basins in the Wissahickon Watershed and found, evaluated, and ranked 153 basins. The survey determined that most of the basins were designed to capture and hold large amounts of water during major rain events and then slowly release the water into the nearest stream. However, major rain events, over 2” in 24 hours, represent less than 20% of annual rainfall as compared to minor rain events, which are typically under 2”. Most stormwater basins in the region were not designed to capture and hold smaller amounts of rain water. Instead, water from minor rain events flows directly through such basins, leading to erosion, pollution, and sedimentation downstream.
In recent years, there has been a renewed interest in how existing stormwater basins detain stormwater on-site. Basins can be re-designed (or “retrofitted”) to retain/infiltrate runoff from smaller storms by replacing the basin’s mown grass with deep-rooted native plants, redesigning the angle of the basin’s sides, and making the outflow pipe smaller. Such retrofits slow the flow of water through the basin, allowing it to soak into the soil. These practices are typically referred to as naturalizing stormwater basins.
Whitpain Township, a municipality in Montgomery County roughly 20 minutes from Philadelphia, offers an excellent example of a successful stormwater basin retrofit. In 2009, Whitpain began working with the Wissahickon Valley Watershed Association (WVWA) on a re-design of a stormwater basin it owns in the Village Circle subdivision, originally constructed in the 1970s. The Township hopes the re-designed basin will serve as a model for other municipalities in the region as their storm sewer systems age, climate change brings major storms more frequently, and they must comply with stricter stormwater control requirements.
Whitpain first worked with WVWA and the Pennsylvania Environmental Council to create a retrofit design using funding from the Schuylkill River Restoration Fund. The Township issued a request for bids for the retrofit construction, but rejected every proposal due to their high costs. More information was needed on current design conditions and future maintenance costs. Whitpain subsequently withdrew its request for bids.
While Whitpain re-evaluated its plan for the Village Circle basin, it naturalized two other basins and applied the lessons learned from those projects to their new proposal. In 2015, Whitpain released a new request for bids with more specific measurements of the basin, a less diverse planting regimen, and less stringent operations and maintenance requirements. The bidding process was competitive, but Whitpain eventually chose Land Concepts LLC to lead the redesign.
Land Concepts first conducted an updated survey of the Village Circle basin to understand how much regrading would be required. As part of this survey, Whitpain Township staff helped to dig 6- to 8-feet down in several spots around the basin to see what was underneath the topsoil (it turned out to be dry sandstone, meaning they could re-design for infiltration). The basin design includes a meandering flow path through the basin, infiltration beds (which detain stormwater long enough to allow it to infiltrate into the soil), and a modified outlet structure. Land Concepts also added a forebay—which also retains stormwater and allows it to infiltrate before it reaches the outflow point—and hardy native vegetation to their retrofit plan. Finally, Land Concepts hired a construction company, JMC Contractors from Glen Mills, PA, and broke ground.
Other than a few minor hiccups—2016’s dry summer that prevented the earth mix from taking hold and erosion on the forebay and outlet discharge area—the construction went smoothly. According to David Cavanaugh, a landscape architect at Land Concepts, the retrofitted basin now provides excellent habitat for birds and insects and has greatly reduced downstream erosion on an un-named tributary of Willow Run Creek.
“We went to the basin during a heavy rainstorm and saw that it was about one third full, meaning the forebay was doing its job. We also saw bubbles rising to the surface in the middle of the basin, so infiltration was occurring,” said Mr. Cavanaugh. Pending scientific data to prove its efficacy and the two years David expects it will take to fully develop, the Village Circle basin re-design looks to have been successful.
Starting in 2013, the William Penn Foundation began funding projects under its Delaware River Watershed Initiative (DRWI) aimed at revitalizing local streams and rivers. As part of this process, the Foundation is funding monitoring and data collection that documents how successful practices, like naturalized stormwater basins, are at slowing stormwater and improving water quality. Documenting the performance of these projects will support broader efforts to retrofit the many traditional basins constructed in the Wissahickon Watershed—and in Suburban Philadelphia more broadly. These retrofits can be a relatively affordable and simple way to improve water quality, now a specific requirement set forth in municipal stormwater permits.
Leading the DRWI data-collection effort in the Wissahickon Watershed are Dr. Laura Toran, P.G., of Temple University’s Department of Earth and Environmental Science, and Lindsay Blanton of WVWA. When WVWA heard that Whitpain was re-designing the Village Circle basin, the organization thought it was an excellent opportunity to learn more about the impact of such retrofits on stormwater management in the watershed. Dr. Toran was brought on board to help conduct the monitoring.
The team is currently using two data loggers: one that measures water levels and temperature and another that measures turbidity, or how much sediment a body of water is carrying. Dr. Toran placed these two data loggers in the Village Circle basin before its retrofit and gathered about a year’s worth of data to use as a baseline. In January, several months after the re-design was completed, WVWA, with Dr. Toran’s help, placed another couple of loggers in the same area to gather data on how water is moving through the basin after its retrofit. Her hope is that the data they are currently collecting will show a reduction in the overall amount of water being released from the basin during rain events, meaning infiltration is occurring.
Dr. Toran is hesitant to say anything definite about the Village Circle basin’s success before the data is collected, but both her and Ms. Blanton believe that stormwater basin retrofits are “the key” to improving water quality in suburban areas. “So many stormwater basins exist nationwide that retrofitting or naturalizing even just a fraction could have major benefits on water quality regionally and nationally,” said Ms. Blanton.
A word of warning from Dr. Toran, though: “it is often hard to say whether something like a stormwater basin retrofit is really helping to improve local water quality, because there are so many stressors on urban and suburban streams. A stormwater basin can be retrofitted and have no effect on areas downstream, or it can have an effect that doesn’t show because of other issues like climate change or rapid urbanization.” This is why, Dr. Toran emphasized, it is so important to fund environmental data monitoring programs. The more robust our understanding of our relationship with the environment is, the easier it will be to encourage municipal officials and funding sources to undertake and support projects like stormwater retrofits.
Other municipalities in the region should take note of Whitpain’s efforts to retrofit the Village Circle basin. Hopefully the data Dr. Toran and Ms. Blanton are collecting on the Village Circle basin will confirm the positive effects that David Cavanaugh has already seen on the local ecosystem. Municipalities across suburban Philadelphia should stay tuned as the Village Circle basin continues to develop, and continue to learn from Whitpain’s successes and challenges in the endeavor.
Feel free to reach out to the Pennsylvania Environmental Council with any questions regarding this article. You can also contact David Cavanaugh, Lindsay Blanton, Laura Toran, or Jim Blanch (Whitpain Township Engineer) directly for more detailed information on the Village Circle Basin.
Article by Zhenya Nalywayko