218
R.H. Charlier and Chr. P. De Meyer
Exposed marshes have been eroding in North Carolina at over 1 m/year (3.28
ft/year). Planting success is unlikely if the fetch is greater than 4,8 km. Along
with fetch, the shoreline configuration (cove, straight or headland) and sediment
grain size are the best indicators of successful plantings.
A fully established marsh is reasonably stable and can provide the divided
shoreline protection but the outer edge of the root mat begins to erode, gradually
resulting in the destruction of the marsh and the loss of any erosion protection for
upland property.
A 3 to 7 year functional lifetime is relatively short compared to the 20 to 30 year
expected lifetime of other erosion control options. Several types of structures have
been used it conjunction with marsh grass plantings. Temporary structures can
reduce the time it takes the planting to become fully established. Permanent
structures have been used to maintain marshes in higher wave climates.
Hay bales as temporary protection bales were washed out by storms 5 weeks after
placement. Two tiers of tires permitted planting survival and growth was
considered successful after two growing seasons. Half the length of the
breakwater was removed at that time. Approximately one quarter of the
developing marsh width was rapidly eroded along the seaward edge. The
remaining marsh appeared to stabilize during the second year after removal.
A fixed breakwater of wooden posts and metal planks supporting tires began to
deteriorate soon after construction and the planting failed, but a floating tire
breakwater provided sufficient protection for the site to be fully vegetated though
partially sank due to sediment accumulation in the tires. Growth was slower were
less protected.
Breakwaters provide temporary protection for marsh establishment, but are not
suitable for permanent installation. Tires, an inexpensive raw material, are
difficult to connect and anchor as functional wave barriers. A tire breakwater
requires high maintenance and for marsh protection is not low cost.
Grasses cannot stabilize areas outside a breakwater and unprotected shorelines
will continue to erode. When a breakwater protecting a marsh was removed the
shoreline response was rapid : marsh seaward of the bulkhead washed out in less
than six months. Two years after removal of breakwater, only a few remnants of
peat remained and the beach had almost disappeared. The combination of a low
inexpensive breakwater and marsh grasses clearly can control erosion for an
extended period of time, but even a well established marsh cannot survive the
wave climate after breakwater removal. Erosion control with vegetation alone
will not be effective unless a small but permanent wave barrier is maintained.
R.H. Charlier and Chr. P. De Meyer
Exposed marshes have been eroding in North Carolina at over 1 m/year (3.28
ft/year). Planting success is unlikely if the fetch is greater than 4,8 km. Along
with fetch, the shoreline configuration (cove, straight or headland) and sediment
grain size are the best indicators of successful plantings.
A fully established marsh is reasonably stable and can provide the divided
shoreline protection but the outer edge of the root mat begins to erode, gradually
resulting in the destruction of the marsh and the loss of any erosion protection for
upland property.
A 3 to 7 year functional lifetime is relatively short compared to the 20 to 30 year
expected lifetime of other erosion control options. Several types of structures have
been used it conjunction with marsh grass plantings. Temporary structures can
reduce the time it takes the planting to become fully established. Permanent
structures have been used to maintain marshes in higher wave climates.
Hay bales as temporary protection bales were washed out by storms 5 weeks after
placement. Two tiers of tires permitted planting survival and growth was
considered successful after two growing seasons. Half the length of the
breakwater was removed at that time. Approximately one quarter of the
developing marsh width was rapidly eroded along the seaward edge. The
remaining marsh appeared to stabilize during the second year after removal.
A fixed breakwater of wooden posts and metal planks supporting tires began to
deteriorate soon after construction and the planting failed, but a floating tire
breakwater provided sufficient protection for the site to be fully vegetated though
partially sank due to sediment accumulation in the tires. Growth was slower were
less protected.
Breakwaters provide temporary protection for marsh establishment, but are not
suitable for permanent installation. Tires, an inexpensive raw material, are
difficult to connect and anchor as functional wave barriers. A tire breakwater
requires high maintenance and for marsh protection is not low cost.
Grasses cannot stabilize areas outside a breakwater and unprotected shorelines
will continue to erode. When a breakwater protecting a marsh was removed the
shoreline response was rapid : marsh seaward of the bulkhead washed out in less
than six months. Two years after removal of breakwater, only a few remnants of
peat remained and the beach had almost disappeared. The combination of a low
inexpensive breakwater and marsh grasses clearly can control erosion for an
extended period of time, but even a well established marsh cannot survive the
wave climate after breakwater removal. Erosion control with vegetation alone
will not be effective unless a small but permanent wave barrier is maintained.