EM 1110-2-1100 (Part V)
31 Jul 2003
miles) that include guidelines for distance B (Figure V-3-14) related to storm surge and wave conditions.
Much more research is needed to relate Ymin to design storm conditions for the functional design of headland
breakwater systems. The next section discusses analytical methods to estimate Ymin during storm conditions.
d. Nearshore breakwaters.
(1) Background and definitions. Nearshore breakwaters are detached, generally shore-parallel
structures that reduce the amount of wave energy reaching a protected area. They are similar to natural bars,
reefs or nearshore islands that dissipate wave energy. The reduction in wave energy slows the littoral drift,
produces sediment deposition and a shoreline bulge or salient feature in the sheltered area behind the
breakwater. Some longshore sediment transport may continue along the coast behind the nearshore
breakwater.
(a) Figure V-3-17 displays a salient behind a single breakwater and a multiple breakwater system with
both salient and a tombolo when the shoreline is attached to the breakwater. The tombolo may
occur naturally or be forced during construction to produce a headland breakwater as discussed in
the previous section. The tombolo blocks normal, longshore sediment transport behind the structure. Daily
tidal variations may expose a tombolo at low tide while only a salient feature is visible at high tide as
occurs at the Winthrop Beach, Massachusetts, nearshore breakwaters constructed in 1935 (Dally and
Pope 1986). Figure V-3-18 displays the single 610 m long, rubble-mound breakwater at Santa Monica,
California, and salient feature (circa 1967). Periodic dredging is needed to prevent tombolo formation. The
multiple, nearshore breakwater system at Presque Isle, Pennsylvania, is shown in Figure V-3-19 (Fall 1992).
Fifty-five breakwaters were built in 1989-1992 to protect 8.3 km (13.8 miles) of Lake Erie shoreline (Mohr
1994).
(b) In general, the primary objectives of a nearshore breakwater system are to:
Increase the fill life (longevity) of a beach-fill project.
Provide protection to upland areas from storm damage.
Provide a wide beach for recreation.
Create or stabilize wetland areas.
(c) In addition, adverse effects on downdrift beaches should be minimized by consideration of the impact
on longshore sediment transport.
(d) Numerous variations of breakwater types exist. Here, the focus is on detached, offshore breakwaters
not connected to shore by any type of sand-holding structure. They may be low-crested to permit increased
wave transmission and lower construction costs. They also may be reef-type breakwaters constructed of
homogeneous stone size as opposed to the traditional, multilayer, cross-section design. Headland breakwaters
(natural or constructed tombolos) are discussed (Part V-3-3). Another type of shore-parallel, offshore
structure is called the submerged sill or perched beach and is discussed in (Part V-3-3). Additional
Shore Protection Projects
V-3-43
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