ERDC/CHL CHETN-II-44
September 2001
Figure 1. Gaps in bars (marked with arrows) allow higher waves to approach closer to shore,
which proceed by diffraction through the gaps
Mechanically Placed Fill: It is possible that some portions of a beach fill may be placed
mechanically and other portions hydraulically. Also, design experience with the longevity of
hydraulic fills may be transferred inappropriately to projects involving mechanical fill. Because
some portion of the sediment-water slurry pumped during a hydraulic fill will move offshore in
the course of achieving the design cross section, a hydraulically placed project profile will
receive some portion of overfill. In contrast, mechanically placed material will have little or no
overfill. All other factors being equal, because of the disparity in amount of material placed, a
mechanically placed fill will undergo greater adjustment across shore in achieving equilibrium.
Dean, Liotta, and Simn (1999) state that the void ratio for mechanically placed material tends to
be greater than that of hydraulically placed fill, indicating that mechanically placed material
would produce less dry beach width after equilibration and consolidation for the same initial bulk
volume of sediment.
Profile Lowering in Front of Seawalls: At a beach that has experienced long-term erosion
in front of a seawall, the existing beach profile may be lower than at the surrounding beach.
Extra fill should be placed in front of such as seawall to assure achievement of the design width
after sediment moves seaward on the profile to return it to equilibrium condition.
produce a locally straight shoreline. However, on many beaches, natural projections into the
water (headlands) or constructed projections of short length (such as buildings and scenic
viewing areas) exist. Fill placed in front of such hard protrusions or headlands will erode and
have less width than the adjacent beach on the sides that is set back from the headland or
encroachment.
Figure 2 shows a section of Monmouth Beach, NJ, that projects seaward of a seawall that follows
the coastal trend. Groins hold the local beach in this area. Smith and Kraus (1999)
recommended that no modifications of the groins be made to increase their sand-retention
efficiency. Through shoreline change numerical modeling they found that a beach fill to be
placed to the south (updrift) would act as a feeder beach. The alternatives were quantified by the
number of temporary landward violations of the shoreline past a trigger distance that would
normally dictate placement of fill. The violations were temporary, until feeder material arrived.
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