EM 1110-2-1100 (Part V)
31 Jul 2003
prototype data. Seiji, Uda, and Tanaka (1987) gave conditions on the Lg/Y ratio for no, possible, and certain
erosion, Yg opposite the gap. The magnitude of Yg was not determined, but gap erosion occurred for Lg/Y
greater than 0.8. Hallermeier (1981) gave an equation for the water depth to locate nearshore breakwaters
when tombolo formation was to be avoided. The relation requires knowledge of wave height and period
statistics at the 12 hr per year exceedance level. Rosati (1990) also evaluated Hallermeier's relation with the
limited, available field data. The correlation was said to be good for seven of the nine data points tested.
These relationships to quantify Ys, Yg and ds can be found in Chasten et al. (1993).
Walker, Clark, and Pope (1981) discuss a procedure to apply diffraction analysis to determine the
approximate shoreline configuration behind a breakwater. Their studies indicate that if the isolines
of the Κt=0.3 diffraction coefficients are constructed from each end of the breakwater for a range
of incident wave directions (monochromatic waves) and they intersect seaward of the postproject
shoreline, a tombolo will not form. This corresponds to Lg/Y, where Y is after placement of the
beach fill, as part of the project shown in Figure V-3-21. Waves coming around each end of the
breakwater meet each other before the undiffracted, incident wave (outside the breakwater's shadow)
reach the shoreline. The postproject shoreline is estimated as a smoothed crest pattern for all
diffracted crests and a balance in the sediment volume.
The Japanese Ministry of Construction presented a step-by-step interactive procedure for nearshore
breakwater design (Japanese Ministry of Construction 1986). Rosati (1990) and Rosati and Truitt
(1990) found that 60 percent of the designs produced tombolos and therefore the JMC method was
more suitable for headland breakwater design. All are for nonpermeable, high-crested, nearshore
breakwaters.
(c) Other design factors. The crest elevation and crest width, permeability, slope of front face, and type
of construction are additional design factors that influence functional performance. No general guidelines
presently exist.
Generally, low crests allow more energy to penetrate into the lee of the breakwater to prevent
tombolo formation or remove a tombolo by storm waves. Wide crests on low breakwaters can
promote breaking to diminish wave energy penetration and encourage tombolo formation. Permeable
structures can allow significant amounts of energy to propagate through them to prevent tombolo
formation. Types of construction including nontraditional, patented devices are discussed in
Part III-3-5.
Waves in the lee of the breakwater are determined by three processes: diffraction around the ends,
wave transmission by overtopping and wave transmission through the structure. For diffraction
around single and multiple breakwaters with gaps, see Part II-7-2 for irregular waves and the Shore
Protection Manual (1984) for many cases with monochromatic waves. Wave transmission due to
overtopping and through the structure by permeability is discussed in Part VI-5-2. Wave reflection
is covered in Part VI-5-2. The Automated Coastal Engineering Systems (ACES) (Leenknecht,
Szuawalski, and Sherlock 1992) provides an application to determine wave transmission coefficients
and transmitted wave heights for permeable breakwaters with crest elevation at or above the still-
water level.
Breakwater impact on littoral currents and creation of wave setup gradients to produce setup currents
was previously discussed. If the crest elevation is low enough to permit wave overtopping, the mass
carried over the structure causes a net seaward return flow of water through the gaps. Seelig and
Walton (1980) present a method for estimating the strength of the seaward flowing currents. The
effect of the combined littoral and setup currents on the longshore sediment transport to produce
salient features with adjacent erosional areas was also previously discussed (physical processes).
V-3-56
Shore Protection Projects
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