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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: 1) Large
subsurface storage/infiltration bed; 2) Porous pavement and
subsurface infiltration bed; and 3) 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. 1)
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.
2)
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.
3)
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.
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