The 4.1 square-mile study area lies in the center of the Wissahickon Creek watershed, consisting of portions of four municipalities (Ambler Borough, Lower Gwynedd Township, Upper Dublin Township, and Whitpain Township). Three sub-watersheds are included in the study area (Rose Valley Creek, Tannery Run, and Honey Run & Stuart Farm Creek). The study area faces problems with water quality and flooding, particularly in the West Ambler section of Whitpain Township. The purpose of the Flooding and Stormwater Management Plan for Ambler Area Watersheds was to mitigate water quality and flooding concerns through identifying and prioritizing stormwater improvements. The study was undertaken by a multi-disciplinary research team from Temple University’s Center for Sustainable Communities (CSC).
An Advisory Committee was formed consisting of representatives from government agencies, municipal authorities, environmental associations, and local residents. Primary data were created through field and municipal surveys, and secondary data were collected from various sources. A stakeholder meeting was held where residents shared their experiences with recent flooding events. Several GIS analyses were conducted regarding watershed characteristics and runoff, including flash flood potential, land use, and precipitation. Analyses were also conducted in order to further understand stormwater and flooding issues in the area. Both Hydrologic and Hydraulic models were used to generate new flood maps and evaluate improvements from stormwater infrastructure facilities. New 100-Yr. and 500-Yr. preliminary floodplains were developed as a result.
Five general types of recommendations were proposed:
- Extended Detention Basins (including retrofitting) – The Project Team looked for opportunities to increase volume and naturalize basins to hold water longer. Thirty-six (36) sites were recommended for either expansion or new construction of basins resulting in an additional potential storage volume of 90.3 acre-ft.
- Infiltration Sites – These recommended sites were based on inspections of areas that can provide storage for runoff from large rooftops, parking lots, and/or athletic fields. Seventeen (17) sites were recommended for infiltration resulting in an additional potential storage volume of 5.5 acre-ft.
- Riparian Buffer Restoration – The total additional potential storage volume resulting from riparian buffer restoration would be 2.84 acre-ft.
- Site-Specific Recommendations – There were six site-specific recommendations proposed, including (a) daylighting and channelization of the Rose Valley Creek, (b) maintaining operating level at Loch Alsh Reservoir four feet lower than in past and reconstruction of Loch Linden Dam (St. Mary’s Lake) & replacement of the lake with a constructed wetland, (c) creation of additional wetland area, excavation, and/or construction of a dam at Ambler Park, (d) continuation of EPA and ACE remediation and mitigation of the BoRit asbestos site, and (e) Church Street and Main Street: channel widening and bridge replacement.
- Low-Impact Green Stormwater Infrastructure Projects – These include measures such as rain gardens, rain barrels, green roofs, and permeable pavement. While the implementation of these projects will not impact flood control to a great extent, they have proven to significantly improve water quality on a local scale.
There are several strategies the municipalities can undertake for implementation, including the adoption and enforcement of new Flood Insurance Rate Maps (FIRMs), implementation of the stormwater improvements and flood control projects, adoption of municipal stormwater ordinances, institution of a flood warning system, and participation in FEMA’s Community Rating System (CRS).