EM 1110-2-1100 (Part V)
31 Jul 2003
(2) Normal storm wave conditions are expected once every two years or maybe every year. More
intense, less frequent storms will reach the foredune, cliffs, structure or vegetation line. Beach stabilization
structures can provide upland protection beyond the baseline for these rarer storm events. At a minimum,
these structures should be designed to provide the minimum, dry beach width for shore protection.
(3) Figure V-3-11c,d,e depict the three most common beach erosion mitigation structures, namely
headland breakwaters, nearshore breakwaters, and a groin field. And, each schematic displays the minimum,
dry beach width, Ymin that is required for design. In each case, it is located in the gap area with greatest wave
energy. The EST methodology discussed in V-3-1-c can be applied to determine the probability distribution
of dry beach widths including the minimum for normal storm conditions. Functional design of these
structures based on empirical knowledge is presented in the next sections. Two key factors are the minimum
dry beach width (or volume) and the natural, sediment transport processes at the site. Explicit
acknowledgment of Ymin as design criteria is often missing in coastal engineering design.
c. Headland breakwaters.
(1) Background and definitions. Natural sandy beaches between rocky headlands have been called a
variety of names in the literature, related to the curved shape of the bay found at many coasts around the
world. Silvester and Hsu (1993) summarize the literature. See also Part III-2-3-i. for a list of references.
Because of their geometry, they have been called spiral beaches, crenulate-shaped bays, log-spiral and
parabolic-shaped shorelines, headland bay beaches and pocket beaches. Half-Moon Bay in California is a
good example as first discussed by Krumbein (1944) and shown as Figure 4.3 in Silvester and Hsu (1993).
Many researchers have studied the dynamic processes of this geomorphic feature, but Silvester (1960) was
the first to examine their static equilibrium and propose the creation of artificial headland breakwaters as a
shore protection structure. Figure V-3-12 presents a sketch of an artificial headland system and beach plan-
form (from EM 1110-2-1617, "Coastal Groins and Nearshore Breakwaters."). Normal wave conditions with
a predominant swell direction produce a maximum indentation between two fixed points (breakwater
structures) and a fully equilibrated, planform shape. Thus man can mimic nature by building the headland
breakwaters and letting nature sculpture the beach with a limiting indentation and shoreline that is stable.
(2) Physical processes. Waves from one persistent, dominant direction, β, diffract around the upcoast
headland and refract into the bay. Waves will break at angles to the shoreline causing sediment transport and
shoreline shape adjustment (nonequilibrium shape) until a full equilibrium shape is reached. At this stage,
waves break simultaneously around the entire periphery, no longshore currents and no littoral drifts occur
within the embayment. The tangent section, adjacent to the downdrift headland is exactly parallel to the
normal wave crest direction from offshore. Such a bay is said to be in static equilibrium (i.e., it is stable until
there is a shift in the dominant wave direction). Minimal amounts of additional sediments enter or leave past
the headland boundaries. Bidirectional, dominant, wave impact (swells and storms) and sediment bypassing
the headlands are two reasons for littoral drift to continue around the bay. These bays are said to be in
dynamic equilibrium and can be predicted within certain tolerances. Only the static equilibrium shapes can
be related to wave input. The ability to calculate the static equilibrium shape and maximum indentation are
needed for the functional design of headland breakwaters.
(3) Functional design. Early investigators employed a log-spiral curve to fit the planform shape (Yasso
1964; Silvester 1970). In practice it is difficult to apply because the center of the log-spiral does not match
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