Project Update – Forbidden Drive Streambank Stabilization

Current Projects // December 19, 2018

by Peg Shaw, FOW Director of Land Management

Overview
For the majority of its length, Forbidden Drive is perched high on the banks of Wissahickon Creek, occupying a transition zone between the Creek and the forested slopes above. With steep slopes above and below, the Drive was designed with a series of swales, inlets, and culverts to handle stormwater. Swales running parallel to and along the inside/upslope of the Drive carry stormwater off the trail, into inlets connected to culverts under the Drive where the water flows out and onto the lower slopes toward the creek below. This drainage system works very well for the Drive, so long as it is regularly maintained. This includes adding corrective grading when necessary, keeping the swales and inlets clear of debris, and inspecting the culverts for repairs and/or replacement. Without maintenance, problems arise‚Äďand grow.

The Problem

Severe erosion along one of the Forbidden Drive collapse sites.

After years of limited or no maintenance of the drainage system, hundreds of stormwater gullies have developed along the Drive and in 2014, three have actually led to major collapses of the hillside/streambank, taking a portion of the Drive with them. These are located downstream from Valley Green Inn, at the Mt. Airy Avenue bridge, and downstream from the Kitchens Lane bridge. Friends of the Wissahickon immediately responded to maintain safety by installing temporary construction fencing. Next, they began fundraising to repair these sites, were awarded several grants from Pennsylvania Department of Community and Economic Development and the Pennsylvania Department of Environmental Protection, and added informational signage to the fencing. While the design process got underway, engineers conducted a thorough inspection and analysis of the drainage system and conditions along the Drive in the fall of 2016 and into the winter of 2017. The findings: there were 417 significant stormwater gullies along the Drive, indicating a severe failure of the drainage system and an urgent need for correction.

Finding a Solution
Several factors dovetailed to create a significant challenge to repairing the three collapsed streambanks along Forbidden Drive. These include steep narrow slopes, stormwater flows in the creek, damaged habitat on the banks, site locations along the most heavily used trail in the park, and trout stocking restrictions impacting the construction schedule, and the need for a design that will ultimately blend into the aesthetic beauty of the park. While the initial projected construction schedule was for fall 2017, design complications arose as we approached the midpoint of the process, necessitating a rework of the design and postponing the construction schedule to fall 2018. In the meantime, FOW and our volunteers and summer Student Conservation Association crews have conducted routine drainage maintenance along the Drive to help stem further development of stormwater gullies and are working with our municipal partners to create a long-term solution.

Construction and Completion
Across the region, significant and frequent rain events have been relentless this year. For many years, climate change scientist have been predicting these changes in weather patterns for the mid-Atlantic region. These weather events have caused delays for hundreds of construction schedules, including the Wissahickon streambank collapse sites along Forbidden Drive. By mid-September, the contractor RiverLogics, informed us the construction schedule would have to move to early summer, 2019. While this is very disappointing, FOW will continue to maintain site security fencing, conduct maintenance activities along the Drive, and partner with City agencies to address the long-term needs of Forbidden Drive, the 2018 PA Trail of the Year.