ERDC/CHL CHETN-II-45
March 2002
Wave Transmission at Detached
Breakwaters for Shoreline
Response Modeling
by Ty Wamsley, Hans Hanson, and Nicholas C. Kraus
evaluates selected available formulas for predicting wave transmission at reef breakwaters and
more conventional multilayer structures, leading to recommendations for the most appropriate
formulas for shoreline-response modeling. The GENESIS shoreline response numerical model is
modified to allow for automated time-dependent calculation of the wave transmission
coefficient, and a case study is presented to illustrate the new predictive capability.
BACKGROUND: Detached breakwaters, reef breakwaters, and spurs attached to jetties are
shore-parallel structures constructed to serve as a shore-protection measure or to intercept
sediment moving alongshore before arrival at inlet entrance channels and harbors. Reef
breakwaters are rubble mounds of rock size similar to that found in the armor and first under-
layer of conventional detached breakwaters, and they are designed to adjust in cross section in
response to the waves and currents at the site. Reef breakwaters are broad crested in comparison
to conventional detached breakwaters. Detached breakwaters (hereafter also referred to as reef
breakwaters unless otherwise stated) can be either emergent or submergent, depending on the
depth of placement, crest elevation of the structure, and tidal range. Often, detached breakwaters
are designed with crest elevation close to mean sea level to reduce construction cost and allow
waves to pass over them to prevent accretion of the beach that reaches the structure, called a
tombolo. Detached structures built of prefabricated units are typically porous and likewise allow
transmittal of wave energy. Attached breakwaters (hereafter spurs) can also be submergent or
emergent and may be designed to protect the beach adjacent to an inlet and/or reduce sediment
bypassing and shoaling of the entrance channel. Thus, wave transmission properties can vary
significantly depending on structure configuration and composition. Wave transmission
properties also vary over different time scales as controlled by tidal variations, change in the
incident waves, and possibly longer-term change in water level such as occurs in the Great
Lakes.
The response of the shoreline to placement of a detached breakwater must be considered in the
design process. The transmission coefficient is a leading parameter in controlling beach
response to detached breakwaters (Hanson and Kraus 1990). GENESIS (Hanson and Kraus
1989) has been applied to model shoreline change both in the field and in movable-bed physical
model experiments based on its capability of representing combined wave diffraction, refraction,
and transmission at multiple detached breakwaters (e.g., Hanson and Kraus 1989, 1990, 1991a,
1991b; Hanson, Kraus, and Nakashima 1989; Rosati, Gravers, and Chasten 1992; Gravens and
Rosati 1994; Herbich et al. 1996). Previously the GENESIS model only represented a constant
transmission coefficient (Kt ) for detached breakwaters. It is desirable to have the capability of
predicting shoreline response to detached breakwaters for a wide range of engineering
conditions. To achieve this goal, an expression for the wave transmission coefficient must be
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