Narberth Park consists of basketball and tennis courts, a playground, asphalt parking, and a compacted turf athletic field. The Park was developed on the historic floodplain of Indian Creek, a tributary of Cobbs Creek, and is located in the upper portions of the highly developed Darby Creek Watershed. Both the Darby and Cobbs Creeks suffer from serious degradation, affecting water quality and water levels. This has been caused mainly by nonexistent or inadequate stormwater management practices. The portion of Indian Creek flowing through the park has been buried in a culvert and the floodplain has been filled. The playfield and park area appear to be major low points of drainage within the watershed and are situated along and overtop the original natural stream channel. Existing stormwater management on the site is minimal and includes several surface drainage inlets, which convey runoff as rapidly as possible to Indian Creek.
The Park currently experiences semi-annual flooding during large, high-intensity storms. Flooding reportedly inundates the playfield and its surroundings, as well as the lower levels of surrounding structures, some of which are private residences. Given the extreme downstream sensitivities and the site’s upper watershed location, it is very important that additional stormwater management techniques be employed to retain runoff and use rate control measures instead of diverting large volumes of stormwater offsite.
To remedy the current conditions, a conceptual retrofit design by Cahill Associates, Inc. suggests adding: a Large Subsurface Storage/Infiltration Bed, Porous Pavement and a Subsurface Infiltration Bed, and Neighborhood Frontyard/Backyard Stormwater BMP Program. The Borough hopes to implement some of these suggested upgrades in coordination with Montgomery County’s Green Fields/ Green Towns Open Space Program in 2009.
Large Subsurface Storage/Infiltration Bed
It is recommended that a large subsurface infiltration bed be installed under the existing turf playfield (approximately 3.4 acres) in order to alleviate surface flooding and to provide on-site storage. The storage/infiltration system should be designed to maximize the storage capacity during extreme flood events to manage some portion of the estimated flood volume generated by the site and the Upper Darby and Cobbs Creek Watersheds. This type of infiltration system consists of a subsurface storage/infiltration bed (depth varies based on storage requirements) wrapped with geotextile fabric and topped with soil and vegetation or with a porous or standard asphalt surface. The infiltration bed should be designed with a positive overflow that allows the system to fill up before overflowing into the existing stormwater conveyance system to ensure maximum storage capacity within the bed. Due to the flood mitigation requirements, this system must be designed after careful hydrologic and hydraulic analysis of the upper watershed. For a typical design of a subsurface infiltration bed by Cahill Associates, Inc. click here.
Porous Pavement and Subsurface Infiltration Bed
It is recommended that the tennis and basketball courts be re-paved with porous pavement underlain by subsurface storage/infiltration beds. These areas are important as they could provide additional stormwater storage areas. For a porous pavement diagram by Cahill Associates, Inc. click here.
Neighborhood Frontyard/Backyard Stormwater BMP Program
The purpose of this program is to encourage private homeowners to voluntarily disconnect downspouts and other connections to the Borough’s stormwater collection system and to install small-scale BMPs (e.g., rain gardens, vegetated swales, rain barrels, green roofs) to manage stormwater on their properties. Larger properties are valuable land for larger-scale BMPs like vegetated swales, subsurface infiltration beds or constructed wetlands. All rooftop and other impervious area runoff can be re-directed into a variety of potential BMPs designed for smaller storms. The cumulative impact could be significant: reduced downstream flooding problems, enhanced groundwater recharge, moderated stream base flow, and significantly improved water quality.
Watershed: Darby-Cobbs Creek
Narberth Park is located at the intersection of North Wynnewood Avenue and Haverford Avenue in Narberth Borough and is approximately 5.2 acres surrounded by mixed-use urban development.
$831,710 – $2,681,710 – Estimated cost contingent on type of storage bed placed under fields. This range excludes an estimate for the Frontyard/Backyard BMP Program.
Technical assistance for the design concept (Cahill Associates, Inc.) was provided through the Pennsylvania Environmental Council’s (PEC’s) Stormwater Retrofit Technical Assistance Program, which is funded through a Pennsylvania DEP Coastal Zone Management Program Grant and through the William Penn Foundation.
Pennsylvania Environmental Council SE Office
123 Chestnut Street, Suite 401
Philadelphia PA 19106