Project Announcement: Andorra Stormwater Management and Sedimentation Reduction Project

Current Projects // February 22, 2018

By Peg Shaw, Director of Land Management

Phase I of FOW’s Green Stormwater Infrastructure (GSI) project in Andorra is set to begin construction in March and be completed by early June. This unique project, funded by the Pennsylvania Department of Environmental Protection, takes a watershed-approach to mitigating damage from uncontrolled surface stormwater flows through a series of features designed to intercept, treat, convey and infiltrate stormwater originating from Northwestern Avenue and adjacent minimally vegetated parkland. The result will greatly decrease sedimentation carried from this micro-watershed into the Wissahickon Creek, while greatly increasing the quality of the day-use area of the Wissahickon Environmental Center (WEC) and adjacent trails.

Throughout 2017 and into the winter of 2018, the environmental engineering firm A. D. Marble, based in King of Prussia, has guided the project from concept to final design. Their construction partners, Lancaster-based Flyway Excavating, is the contractor on site turning the two-dimensional drawings into 3D.

When completed, park visitors can expect to see an enlarged and re-vegetated basin at the upper parking lot with a series of vegetated rock cascades leading from the basin down the hill through the forested canopy. At the WEC day use area, visitors will enjoy a vegetated rain garden and swale leading to the wetland downhill and along the drive from the WEC. This lower reach of the watershed is Phase II of the project and will be addressed at a later date.

Safety During Construction

FOW consulted with the Parks and Recreation staff at the WEC, District 4, and Natural Lands Restoration as we developed plans with the design and construction team to ensure public safety and minimize impacts on park users during construction. With that in mind, we ask visitors to the Andorra Natural Area during the construction period to please heed the work zone and safety signage at all times — even if the crew is not working.

Areas of Impact During Construction

The work zone will be closed to the public from early March to early June.

Upper Parking Lot
During construction, the Upper Parking lot above the Wissahickon Environmental Center on Northwestern Avenue (Roxborough Side) will be closed to vehicles since it will be the main staging area for the project and will be used for material stockpiles and equipment.

Bike Bypass
This trail will be closed weekdays during construction, and may be also closed on various weekends depending on the construction schedule.

Areas NOT Impacted During Construction
The following park amenities at or near the work zone will not be impacted during construction. Nevertheless, please remain aware that construction is underway nearby and any related safety signage should still be followed.

Red Forest Loop and Connector Trail
This old rocky road bed that runs behind the WEC and along the edge of the day-use area will not be impacted during construction, except for a few specific days based on the construction schedule. From the WEC, this trail will provide the most direct route to Andorra Meadow.

Wissahickon Environment Center and Lower Parking Lot
WEC, the “Tree House,” and public restrooms will be open during normal hours throughout the construction period. The access road from Forbidden Drive and associated parking lot will also be open during this time.


STATUS: In Progress Spring 2018
SCHEDULE: Phase I Construction March – May 2018
FUNDING: PA Department of Environmental Protection, Growing Greener Grant Program
COST: $379,500
PROJECT TEAM: A.D. Marble Engineering – Design and Permitting, Flyways Excavating – Construction
PARTNERS: Philadelphia Parks and Recreation, Philadelphia Streets Department, Springfield Township


The Andorra Natural Area was once part of the largest commercial nursery on the east coast. After decades of intensive use, the land’s natural hydrologic functions are now severely compromised. Soil compaction, overbrowsing by deer and the removal of leaf litter by invasive earthworms combine to restrict stormwater infiltration, tree regeneration and a healthy vegetated understory. These conditions also promote invasive plant establishment, further degrading the habitat.  Additionally, uncontrolled stormwater from Northwestern Avenue pours into the area with high-volume and fast-moving flows. With limited capacity to slow and infiltrate stormwater, erosive gullies develop, leading to increased sedimentation of the Wissahickon Creek.


Taking a watershed approach, the project incorporates a series of green stormwater infrastructure features designed to capture, treat, infiltrate and slow stormwater. The project begins at the upper parking lot and descends through the vegetated hillside and below the Wissahickon Environmental Center to Forbidden Drive. There are two phases of implementation. Phase I, scheduled for 2018, addresses the site area from the upper parking lot to the WEC day use area. Phase II will address the site below the WEC along the main access road. Design and permitting for the second phase is within the scope of Phase I and will be used for fundraising.


  • Northwestern Ave – Re-pitching roadway to redirect stormwater into Green Stormwater Infrastructure
  • Upper Parking Lot – Enlarged Vegetated Infiltration Basin with Sediment Treatment Forebay
  • Forested Hillside – Regenerative step pools to control stormwater flow, remove sediment and enhance infiltration
  • WEC Day Use Area – Rain Garden and Vegetated Swales providing additional stormwater control and treatment
  • Provide Environmental Education Opportunities throughout