Inner-bank erosion, which occurs at the shoreward termination of a jetty structure on the channel
side of an inlet in a sandy beach, occurs on all United States coasts. An unabated eroded area takes
the shape of a crenulate or log-spiral beach, similar to that which occurs on the open coast downdrift
of headland features. Investigations in a movable-bed physical model basin have shown that the
erosion is a consequence primarily of wave action, which initially cuts a trough at the jetty structure-
sandy shoreline intersection then gradually widens, allowing diffracted wave energy to widen the
trough, both behind the structure and bayward. Waves refracting up the side of the channel slope
then continue to elongate the eroded region. An equilibrium shape is reached similar to the open
coast crenulate bay. If a flood-flow current is added to the wave environment, the wave angle
becomes steeper and sand at the landward end of the embayment is pushed further inland along the
channel's edges. The waves plus current results in a larger equilibrium embayment than for waves-
only. For a tide-plus-waves condition, the erosion area is greater than the waves-only condition, but
less than the waves plus steady flood current condition.
Laboratory experiments were performed to examine solutions for reducing the erosion other than
the traditional revetment of the shoreline with armor stone. A hard-point stone mound placed some
distance bayward of the jetty termination created a shoreline response similar to that of headland
beaches, creating smaller embayments rather than one large deep-cutting embayment. A diamond-
shaped rock mound on the jetty termination location functioned best, especially because of its
projection into the beach-side of the jetty. The diamond-shaped mound prevented flanking of the
terminal tip of the jetty. Its projection into the channel reduced wave height at the structure-sand
interface. The experiments were conceptual in nature and conducted for limited conditions. Future
work should examine many different wave conditions. The site-specific design of a terminal rock
mound for Half Moon Bay, Grays Harbor, Washington, was evaluated in the inlet laboratory facility.
The final design was constructed in 2000.
The work was conducted under the Inlet Laboratory Investigations work unit, Coastal Inlets
Research Program. Permission to publish was granted by the Office, Chief of Engineers. Thanks to
Ms. Julie Rosati, Mr. Dennis Markle, and Dr. Nicholas Kraus for review comments.
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