Proceedings, 2001National Conference on Beach Preservation Technology, pp. 274-283
William C. Seabergh
Abstract: Much focus is placed on beach erosion on the open coast. However, coastal processes
often occur on sandy shorelines interior to inlets that can lead to severe erosion. These shorelines lie
adjacent to coastal inlets and extend around the inlet from the ocean to bay. In particular, an
examination of coastal inlets with jetties or terminal groins that are connected to a sandy shoreline
develop inner-bank erosion in the absence of preventive measures. Many mature projects show
eroded regions that required extensive revetment. Typically, if the erosion is permitted to proceed
unabated, a crenulate-shaped shoreline region will develop from the terminus of the jetty, extending
both bayward and laterally into the adjacent beach. This expansion of erosion leads to loss of
property and difficulties in reclamation as a shallow water environment develops. The eroded
sediment moves into the channel creating shoaling problems. Many times the eroded region may
flank the jetty structure, leaving it isolated from the shore. Isolation of the jetty may lead to potential
problems of tidal current scour near the structure, opening the already eroded embayment to
increased wave activity and additional erosion, and permitting increased wave attack on the jetty
itself. As part of the Coastal Inlets Research Program (U.S. Army Corps of Engineers), this type of
erosion was studied in a movable bed physical model of an inlet. After the governing processes were
understood, several preventive techniques were investigated. A case study is included.
Examination of maps and photographs of many coastal inlets reveals a characteristic curved
eroded shoreline on the inner-bank, or channel side of a jetty where it terminates in a sandy shore.
At many inlets, that region of lateral expansion shoreward of the jetty terminus has been revetted or
protected in some way with rock, bulkheads, or sheet-pile structures. The result of this erosion
process is shown at St. George Inlet, Florida, which is located on the Gulf Coast in the eastern part of
the Florida panhandle. Figure 1 shows the inlet in its natural state, and Figure 2 shows the inlet a
few years after parallel jetties were constructed. Erosive forces have created the curved shorelines at
the base of the jetties. Figure 3 shows the entrance channel at Panama City Harbor, Florida, created
by cutting through an elongated spit in 1933. The construction included jetties extending to the -3.7
m, mllw (mean lower low water) contour. The inner 90 m of the jetties flared outward and were
anchored in sand. Between 1935 and 1956, landward extensions of the jetties were made to prevent
continued erosion of the inner-bank shoreline. Flanking of the jetties was a major concern.
1) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls
Ferry Road, Vicksburg, MS 39180-6199 USA. William.C.Seabergh@erdc.usace.army.mil


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