ERDC/CHL CETN-IV-22
December 1999
forming a spit that encroached the navigation channel (Douglass 1987, Figure 3). It is likely that
the proposed deflector dike would have retained sediment in the deposition basin.
Raise and sand-tighten jetties
Permeable jetties and jetties with elevations that are low relative to the adjacent beach can
contribute to erosion of the adjacent beach and shoaling of the inlet channel (Figure 1). Jetty
elevation below that of the berm elevation of the adjacent beach allows wave runup to transport
sand over the top of a jetty and into the inlet, exacerbated during times of storms and high water
level. This sediment-transport pathway is an unnecessary loss of material from the adjacent
shoreline, and increases shoaling of the entrance channel. This loss is especially detrimental for
down-drift beaches, which tend to lose sand because of a near persistent local transport reversal.
As an example of this phenomenon, consider Ocean City Inlet, Maryland (Figures 4a, 4b, and
4c).
Figure 4a shows an aerial photograph of Ocean City Inlet taken in 1952, prior to elevating and
sand-tightening of the south jetty. Note the shoal on the northwest corner of the south beach.
Superimposed on this photograph are the sediment-transport pathways believed to create the
shoal, based on studies by Dean and Perlin (1977).
Figure 4a. Ocean City Inlet, Maryland with sediment-transport
pathways over and through jetty (based on Dean and Perlin
(1977); photograph dated 8 June 1952)
Figure 4b shows the elevation of the berm adjacent to the south jetty, indicating an increase in
this elevation with distance from the south jetty. Because the jetty was lower than the adjacent
beach berm, a downhill gradient existed from the beach towards the jetty. The downhill gradient
in beach elevation allowed the flow of water and sand over and through the south jetty.
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