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Project Design: Buck
Engineering
Primary Contractor: Byers
Construction
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BELL
(March - April 2002)
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Project
Background
The site is
along 3,900 feet of Brasstown Creek and includes the lower portions of
Gumlog and Winchester creeks on the Opal Henson and the late Allen Bell
farms. Gumlog Creek, in particular is a major tributary of Brasstown
Creek; most of its drainage area lies in Georgia and is unrestored.
Because of channel incision in Brasstown Creek along the Bell reach, the
floodplain was only accessible during extreme flood events. Due to
agricultural practices, little to no established riparian buffer existed
in some areas along this reach of the creek. This resulted in stream
bank erosion, which in turn led to increased sedimentation within the
channel, and in some areas, to the channel becoming overly wide. The
increase in bankfull width had reduced sediment transport capacity,
resulting in bar formations in mid-channel. The mid-channel bars
further accelerated bank erosion, altered riffle-pool sequences, and
decreased the quality of aquatic habitat. Total estimated soil loss
from this reach of Brasstown Creek was 1,300 tons per year.
Previously, the
lower reach of Gumlog Creek was altered by beaver activity. Water
flowing along the sides of the dams had caused bank erosion and the
right bank was severely eroded for about 75 feet along the outside of a
bend. The lower section of Winchester Creek exhibited a belt width that
was too tight and the stream was about to cut through.
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Before
Vertical
bank in outside bend.
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After
J-Hook
installed to divert erosive energies away from bank. Bankfull
bench installed to allow proper rooting depth. |
The Bell
project was completed in spring 2002. The goals were to improve water
quality and aquatic habitat by reducing sedimentation, to improve stream
stability, riparian and floodplain functionality, and the natural
aesthetics of the stream corridor. The design for the Bell site called
for the installation of rock vanes and deflectors to move the thalweg
away from the banks, reduce near-bank stress and bank erosion, and
prevent bar formation in the channel. Bankfull benches were excavated
in areas with excessively high banks on which no established buffer
existed. In these areas mature trees were not disturbed. Root wads
were installed throughout the reach. Bank erosion in the lower reaches
of the two tributary streams was mitigated by sloping and matting the
banks and by installing root wads and rock vanes. A riparian buffer was
reestablished throughout the reach of the project with plantings of
native trees and shrubs.
During HRWC’s
2005 annual project inventory, a 200-foot section of bank was found to
be eroding as a result of return flow following three large storm events
(flood events) in fall of 2004. HRWC organized a volunteer workday to
reshape the bank, apply erosion control matting, and install live
stakes. Students from Young Harris College provided most of the labor
for the repair.
2008 Status
A 2,180-foot reach of the Bell project was monitored in
2005, 2006 and 2007, and this portion of the stream remains very
stable. In a very tight meander bend, approximately 300 feet of the
restored channel has historically been subject to severe bank erosion,
but this area did not see increased erosion between 2005 and 2007.
Thirteen structures provide bank protection and grade control within the
monitored reach. All structures associated with the 6-year old project
appear to have very good to excellent function, except for one j-hook
vane in the aforementioned meander bend, which may require minor
repair. The restored stream reach is currently classified as a C4
channel. The streambed slope is 0.003 feet/foot, with a sinuosity of
1.44. Between 2005 and 2007, the streambed material became slightly
coarser.
The restored reach contains several riffles and pools.
Riffles are typically associated with the structures, while pools are
present in the expected locations at meander bends. Since 2005, riffles
have generally had aggradation in the bed, while pools have generally
become deeper. Significant stream bank erosion is not observed within
the channel.
Vegetation in the buffer is generally dense and reproductive
trees line both banks. However, much of the understory vegetation is
invasive, dominated by exotic Chinese privet (Ligustrum sinense),
a mixture of exotic and native dewberry and blackberry (Rubus spp.),
and native river cane (Arundinaria gigantea).
An increase in the Total Suspended Solids concentration
between upstream and downstream monitoring stations was reported at the
Bell project reach. Five of the seven samples had higher TSS downstream
with an average of 241% increase. These observed increases underscore
the importance of continuing to pursue restoration work in the Georgia
portion of the watershed.
In 2007, the benthic macroinvertebrate assessment at Bell
yielded a bioclassification of Good/Fair, the same as in 2005. Between
2005 and 2007, EPT taxa richness increased slightly, while EPT abundance
decreased slightly.
3-Year Study
Recommendations
In
addition to continuing to monitor this reach in subsequent years,
recommended actions include:
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Remove
dense privet
and replace with native shrubs, trees, and river cane
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Evaluate
options for stabilizing both sides of the stream bank at the meander
bend near station 11+00
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Closely
monitor the
currently stable areas with high shear stress; specifically in three
locations between stations 16+85 and 18+59, 15+22 and 16+09, and
9+50 and 14+30
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