interchange and a larger off-site creation project. The wetland creation
site, also
located in the Town of Wells, was a 1.6 ha abandoned gravel mined area abutting
the Turnpike (Figure 1). The site lies adjacent to Branch Brook, which supports
a
native brook trout (Salvelinus fontinalis) population and serves as the principal
water
supply for the surrounding three communities. The Kennebunkport and Wells
Water
District withdraws surface water through infiltration galleries along the
banks of
Branch Brook at the treatment plant located 2.5 km downstream of the mitigation
site.
BRANCH BROOK SITE DESIGN
The design objectives for the Branch Brook mitigation project include: 1)
reclaim
gravel-mined lands and construct more productive wetland habitat; 2) allow
for
containment in the event of an accidental spill along the Turnpike; 3) alleviate
existing erosion problems by providing floodwater storage and reducing peak
discharge rates, and 4) enhance effectiveness of existing grassed swales
in treating
highway runoff by incorporating the water quality benefits of wet detention
basins
and created wetlands. The opportunity to meet these objectives was provided
by
diverting into the mitigation site stormwater runoff from an existing grassed
swale
draining approximately 2.3 land-km with total watershed of 5.2 ha. In order
to
achieve the project objectives, the mitigation site design includes a small
sediment
basin along the roadway embankment and two wet detention basins supporting
a
diverse wetland fringe, each equipped with a specialized outlet control structure.
The design also provides direct access from the Turnpike in the event of
an
emergency.
The design of the outlet control structures within each basin incorporates
several
considerations. With the pool elevation controlled by the pipe exiting the
catch
basin, the pipe entering the catch basin remains submerged at all times (Figure
2).
This allows the wetland basins to: 1) retain floatable contaminants; 2) discharge
cooler waters at depth to reduce thermal impacts to the sensitive cold water
fishery
(Schueler 1987), and 3) discourage beaver activity by confining sounds of
running
water within the berm. Each inlet pipe is also fitted with an epoxy-coated
wedge
gate valve to contain accidental spills within the wetland basins. The outlet
pipe is
sized to reduce peak discharge rates of the 10, 25 and 100-year storms by
approximately 90% (HNTB, unpublished data).


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