ERDC/CHL CHETN-II-44
September 2001
Locations receiving finer sediments will erode faster, predominantly through cross-shore
transport. This process will continue until the finer material is transported away or is covered by
coarser material. Development of EHSs by this process can be reduced by specifying an
acceptable range in grain size or by overfilling. One strategy is to cover the finer sediment with
a coarser cap (Kieslich and Brunt 1989).
Residual Structure-Induced Slope: This process has been identified for groins and may
also be applicable to detached breakwaters. A groin will tend to hold the upper beach profile,
but it cannot prevent removal of sediment seaward of its tip as erosional processes continue. If
the groin is removed and oversteepening of the more-offshore profile is not recognized, a greater
amount of fill will move offshore to re-establish the profile. Recognizing the profile mismatch
and supplying sufficient additional fill volume can mitigate this type of local EHS.
Wave Transformation over Borrow Pit: If borrow material is mined in relatively shallow
water (determined by the ratio of depth at the borrow pit to the length of the predominant waves),
the pit will act as a lens and redirect waves by refraction and, possibly, by reflection, diffraction,
and dissipation. A persistent nonuniform wave climate will be imposed along the beach, and the
resultant longshore transport will preferentially redistribute sediment to create hot spots and cold
spots. Because waves diverge from a borrow pit, the shoreline in the direct lee of the pit is
expected to accumulate sediment. This EHS type is a special case of type 11, "Wave
transformation over offshore bathymetry."
Combe and Soileau (1987) documented shoreline response to borrow pits located too close to
shore at Grand Isle, LA. The resultant cuspate shoreline was subsequently predicted by
refraction analysis (Gravens and Rosati 1994). Such situations can be avoided by conducting a
refraction analysis as part of the design process to determine allowable location, dimensions, and
depths of borrow pits (e.g., Kraus et al. 1988; McKenna, Brown, and Kraus 1995).
Gaps in Bars (Also Called Breaks in Bars): Breaking waves typically form one or more
longshore bars along the coast. The bar system is not always uniform alongshore or two-
dimensional, with gaps in the bars formed as the nearshore bottom becomes more three-
dimensional. Waves can enter the gaps and approach closer to shore before breaking. The direct
incidence of higher waves through the gap and their spreading by diffraction produces a three-
dimensional circulation pattern and erosion directly leeward of the gap (Figure 1). Gaps might
be produced by rip currents by random nonuniformity in waves alongshore, or by other causes.
If the gaps appear randomly, then erosion is expected to be temporary at a given location. If the
gaps are produced by rip currents, it is possible that their location might be semipermanent,
because rip currents often tend to appear in the same location, or oscillate cyclically along the
shore (Davis and Fox 1972). This EHS type is expected to remain as long as the gap in the bar
remains.
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