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Somerset
County Wetland Mitigation Bank
Project
Background In
1998, Somerset County began planning an 8.1-acre wetland mitigation
bank. The project had three
objectives: 1.
Serve as compensation for wetland impacts associated with past
Somerset County infrastructure projects; 2.
Serve as a credit or bank for future infrastructure projects
planned by Somerset County; and 3.
Be an amenity to the County Park System. Mark
Gallagher, Vice President of Princeton
Hydro, LLC, developed the conceptual design for the project.
The formal engineering design was prepared under the direction of
Geoffrey Goll, PE, also of Princeton Hydro. Carl Andreassen, PE,
the Principal Hydraulic Engineer for Somerset County, served as the
County’s project manager. The
goal of the mitigation project was to create a floodplain wetland system
composed mostly of forested wetland that would provide several important
wetland functions, including flood storage, sediment removal and
wildlife. The project site
is located in an agricultural area of Branchburg Township, along the
South Branch of the Raritan River.
The selected site was chosen due to its proximity to the South
Branch and the presence of wetlands similar to those planned for the
project downstream. The
naturally occurring wetland was used as a model for the mitigation bank.
Somerset
County is also working to develop this site as a County park with public
access. Project
Design
The
primary objective of the wetland mitigation was to construct 8.1 acres
of emergent and floodplain forested wetlands from an existing upland
agricultural field, thereby creating wetland habitat and increasing the
flood storage capacity of the floodplain. Extensive earthwork was
required in order to achieve the desired goals.
The surface elevation was lowered and an existing backflow
channel expanded to establish wetland hydrology.
By lowering the surface elevation, the retention of storm water
and stream backflow during flood events was facilitated.
The retention of surface water in the mitigation site will
increase the duration of inundation/saturation typical of a floodplain
wetland. The wetland was
designed to flood at a frequency of four times a year.
The desired flood frequency and depth of flooding was determined
through the use of hydrologic
modeling. The model was
also used to evaluate tractive force and flow velocity at different
flood stages, depths and cross sections to guide the mitigation design. In
the early developmental stages of the project, the wetland vegetation
will be an a patchwork of areas dominated by shrubs or trees, thus
increasing the wetlands structural complexity and habitat value.
A system in which there are patches of trees and shrubs in a
matrix of herbaceous plants will serve as recruitment for
bird-disseminated seeds, leading to a plant community that will, over
time, appear more similar to a natural wetland than a mitigation site.
The
proposed mitigation design will incorporate three distinct vegetation
planting types. Floodplain
forested wetlands will provide the framework for the development of a
riparian-forested wetland. Scrub-shrub
wetlands will increase the habitat complexity of the site by providing
good cover and/or food for wildlife.
Emergent wetland plantings will help establish a diverse wetland
habitat. The species
selected for the project were limited to indigenous species that are
common components of the local riparian or floodplain flora.
The
project is located within a County park.
Therefore, a trail system was included, as part of the project
design, and a small pedestrian bridge will provide access to the created
wetlands and the South Branch for the public.
The County also entered into the Partners for Wildlife Program
sponsored by the USFWS to control the invasive species reed canary grass
(Phalaris arundinaceae) and to re-vegetate the treated areas with
woody plants. Trout
Unlimited has also worked on stabilizing the bank of the segment of the
South Branch that is on the property.
Although,
only a few months old the completed project is not only a naturally
looking feature of that South Branch Raritan River floodplain but
already supports a diverse wildlife community.
One
of the important elements of this mitigation project was that it was
designed to fit naturally into the site’s landscape and complement
other existing natural communities.
The County’s willingness to participate in the USFWS’s
Partners for Wildlife Program served to further enhance the wildlife
value of the this portion of the South Branch.
The efforts of the USFWS in concert with the County’s
mitigation project has created the foundation for the development of a
diverse floodplain community. For
more information on this project, contact: Carl Andreassen, Somerset County Engineering Department andreassen@co.somerset.nj.us
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