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Project Design: Jason
Wheatley, Natural Resources Conservation Service (NRCS)
Primary Contractor: Stanley
Construction
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OLAND
(January - February 2001)
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Project
Background
The Oland site is
on a 37-acre parcel of land off of Old Highway 64 West near the
community of Brasstown, NC. The project is located on the east and west
banks of Brasstown Creek, about one mile upstream from the Brasstown
community and the confluence of Brasstown Creek and the Hiwassee River.
The Oland project was constructed in winter 2001 and is unique in that
it is the only monitored project not designed by a private consulting
firm. Natural Resources Conservation Service personnel designed the
structures and provided primary construction oversight for the project.
The site is
directly downstream from a very old privately maintained bridge across
Brasstown Creek. The constriction of the creek at the stone bridge
piers was the primary cause of bank erosion and channel widening
problems downstream. During a bankfull event, the channel width was
reduced from 72 feet to 36 feet at this bridge. The natural channel
downstream could not support the increased velocity and force of the
flow resulting from this constriction. The left bank had been severely
eroded for a distance of about 300 feet below the bridge. At that point,
the channel width at bankfull was over 100 feet. The creek had lost its
sediment carrying capacity and had begun to aggrade dramatically. A
shallow transverse bar crossed the entire channel at a point where a
pool should naturally occur. As the banks eroded, several large trees
fell into the creek, adding to the instability of this section of
stream. Bankfull elevation was very near low bank elevation. Flows
from storm events in excess of the 1.5 year storm (typical return
interval for bankfull event) would spill over the low bank onto the
adjacent flood plain.
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Before
Bankfull
width is 104 feet across.
Large mid-bar is sending
velocities into both banks. |
After
Crossvane
installed to protect both banks.
Deposition of sand and
wood in less than one year.
Goal is to eventually have a
bankfull width of 77 feet. |
Since the historic
bridge was a fixture not likely to be removed at any time in the
foreseeable future, it was necessary to restore dimension and profile of
the stream downstream of the bridge. The goal was to dissipate the
energy downstream of the bridge constriction in a way that would not
continue the cycle of bank erosion and channel widening that had
occurred in the past.
A rock cross vane
was installed at a location downstream of the bridge and upstream of the
eroded bank area.
This structure would effectively turn flows away from the banks and
dissipate energy in a mid-channel, plunge pool during bankfull events.
The top of this structure tied into the banks at bankfull elevation and
at the appropriate bankfull width (75 feet) for the drainage area. Tree
revetments were secured along the eroded bank section in sufficient
quantity to reduce flow velocities, to restore optimal bankfull channel
width, and to trap sediment so that a stable soil medium could be
established to support permanent vegetation. Root wads installed into
the eroded bank for stabilization helped anchor the revetment. A
50-foot riparian buffer was established as part of the restoration.
2008 Status
The entire 600 feet of restored stream at Oland was
monitored in 2006 and 2007, and it is very stable. The cross vane that
was constructed successfully provides grade control, maintains velocity,
and protects stream banks. The restored stream reach is currently
classified as an E4 channel. The streambed slope is 0.004 feet/foot,
with a sinuosity of 1.34. The median particle size is currently very
coarse gravel.
The restored reach contains three riffles, one deep scour pool, and a
few shallower pools. Riffles are associated with the straight portions
of the channel, while the pools are present downstream of the cross vane
and in the meander bend. The 2007 channel pattern and profile is very
similar to that observed following construction in 2001. Stream bank
erosion was not observed within the channel.
In 2007, the macroinvertebrate assessment downstream of the
Oland project yielded a bioclassification of Good, compared to Good in
2006 and Excellent in 2005. Between 2005 and 2007, EPT taxa richness
and abundance decreased as with other Brasstown Creek sites.
Disturbance of existing vegetation during Oland construction
was minimal, and the vegetation has recovered well, stabilizing the
banks and providing structure on the terrace. Oland is distinctive
among Hiwassee projects for its diversity and concentration of exotic
invasives. Beaver activity continues to remove planted trees.
3-Year Study
Recommendations
In
addition to continuing to monitor this reach in subsequent years,
recommended actions include:
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Remove
invasives;
Oriental bittersweet (Celastrus orbiculatus) in particular
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Place
beaver fence
around young planted trees
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If
the property is
sold, conduct an early walkthrough with the new owner to identify
and preserve planted vegetation
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