EM 1110-2-1100 (Part V)
31 Jul 2003
Figure V-3-20. Definition schematic for nearshore breakwaters
wave induced, mean water level change (setup) in the exposed, gap areas is larger than in the sheltered areas.
Longshore variability in the wave setup produces gradients in the mean water surface. Water flows from the
elevated levels in the gap area towards the lower, sheltered area to accelerate the longshore current flowing
towards the sheltered area behind the structure from the left side. These gradients also change the direction
of the current which is driven away from the breakwater in the region immediately downdrift of the
breakwater (right side). These two current systems (littoral current and setup current) merge behind the
structure to give rise to complex circulation patterns. The acceleration of the littoral current updrift causes
initial erosion of the beach on the updrift side. The same occurs in the area immediately downdrift. These
currents carry the eroded material towards the sheltered area, where it deposits. These mechanisms cause the
patterns of deposition behind and erosion on either side that is observed in nature (see Figure V-3-20). The
above physical description had been confirmed by a two-dimensional, numerical (horizontal plane) joint
processes (waves, currents, sediment transport) morphological modeling system (Zyserman et al. 1998).
(b) Storm processes and response. Protection afforded by the breakwater will limit erosion of the salient
during significant storms. The exposed gap area will be eroded with sediment dragged offshore during
storms. Breakwater height, length, wave transmission characteristics and distance from shore contribute to
its effectiveness to provide a minimum dry beach width, as discussed further in the following paragraphs.
(3) Functional design. Prototype experience for the functional design of nearshore breakwaters in
the United States is generally limited to sediment-starved shores with fetch-limited wave climates on
the Great Lakes, Chesapeake Bay, and Gulf of Mexico shores (Pope and Dean 1986). Table V-3-4 is a
summary of U.S. projects up to 1993 (Chasten et al. 1993). Nearshore breakwaters for shore protection have
also been used extensively for shore protection in Japan and Israel (Toyoshima 1982; Goldsmith 1990
(unpublished))1 and in Denmark, Singapore and Spain (Rosati 1990). Detailed summaries of the literature,
previous projects, and design guidance are provided in a number of references (Lesnick 1979; Dally and Pope
1
Goldsmith, V. 1990. "Engineering Performance of Detached Breakwaters Along the Coast of Israel," draft report, Coastal
Engineering Research Center, Ft. Belvoir, VA.
V-3-48
Shore Protection Projects
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