ERDC/CHL CHETN-II-44
September 2001
(estimated at 115,000 m3/year) updrift of Chincoteague Inlet. A large cape-like feature (Fishing
Point) has developed and the updrift sand impoundment has a pronounced influence on the
shoreline by sand-starving the barrier islands to the south. The blockage of longshore sediment
by the inlet has starved the downdrift beaches to the extent that a 5-km offset is observed on the
island to the south. Shoreline recession rates of more than 10 m/year are observed for tens of
kilometers to the south. The net result is a highly concave shoreline extending 35 km south of the
inlet. Such a regional trend of high erosion rate and shoreline adjustment must be taken into
account in managing that coast.
Relict Inlet Offsets: A relict tidal inlet can have a marked impact on shoreline rates-of-
change and may manifest itself as an EHS for a period of decades until the shoreline re-
equilibrates in conformance with the offshore contours. In microtidal conditions, unstabilized
inlets exhibit similar behavior. An idealized example is given in Figure 5. Sand is transported
alongshore to the inlet and constricts the inlet throat. The resulting decrease in flow area
increases the current velocity and scour potential in the channel. Because sand is added to
mainly one side of the inlet, the downdrift side erodes preferentially, causing the inlet to migrate
with a characteristic offset in planform. Once the inlet closes, however, the updrift offset
functions as a headland (type 7). Wave energy is focused by refraction, resulting in significantly
higher rates of erosion until the updrift offset aligns with the rest of the shoreline. An example
of this type of EHS is illustrated by an analysis of long-term shoreline change data for Jones
Beach Island New York. Long-term shoreline change data for Jones Beach Island suggest that
the eastern half of the island is an EHS, with shoreline recession rates in excess of 4 m/year
observed.
BAY
TIME 1
DOWNDRIFT BEACH
LONGSHORE SEDIMENT TRANSPORT
TIME 2
ACCRETION
EROSION
TIME 3
INLET MIGRATION
OFFSET
Figure 5. Development of a relict inlet and shoreline offset
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