ERDC/CHL CHETN-II-45
March 2002
The difference in calculated shoreline planforms demonstrates the improved reliability of a
variable Kt prediction. The variable Kt formulation produced as much as 70 m more shoreline
advance behind the spur than predicted with constant Kt. The primary reason for the difference
is the sensitivity of the prediction to water level and incident wave height, acting together with
directionality of the wave climate. Figure 9 plots the change in Kt versus wave height for
constant water level. As compared to Kt =0.85, the Ahrens formulation predicts higher Kt-values
for waves less than 2.25 m in height. That trend then switches, and the Ahrens method predicts
smaller Kt-values for higher waves. The Grays Harbor wave climate is characterized by higher
winter waves that approach from the WSW and drive sand to the north, in contrast to the more
prevalent smaller summer waves that approach from the WNW and drive sand to the south. The
large winter waves tend to erode the beach near the jetty as they transport sediment northward
with no supply possible through bypassing the inlet. Waves from the WNW transport sand
toward the inlet where it is impounded at the jetty and widens the beach or bypasses the jetty and
enters the inlet. Because longshore transport is approximately proportional to H5/2, an increase in
higher waves results in much greater change in transport than does a similar height differential
for smaller waves. The reduced transmitted wave heights predicted by the Ahrens formulation
for the large waves decreases the predicted erosion, not only maintaining beach width but also
promoting a shoreline orientation that reduces the southbound transport induced by the WNW
waves. The result is a wider beach in the lee of the spur.
CONCLUSIONS: Wave transmission at a detached breakwater is a leading parameter among
many variables controlling the response of the shoreline to the structure. Wave transmission
depends on the configuration and composition of the structure, wave height and period, and
water depth, and the forcing parameters are time dependent. In this study, predictive formulas
for wave transmission at detached breakwaters were critically evaluated, and the new Ahrens
(2001) formulation proved to provide reliable predictions for reef-type structures over a wide
range of geometric, water level, and wave conditions. The predictions of the Ahrens formulas
were confirmed for a case study through comparison to results from a numerical model and a
physical model.
Variable wave transmission was incorporated in the GENESIS shoreline change numerical
model, which was calibrated to represent shoreline change at Grays Harbor, WA. Predicted
shoreline response to a proposed submerged shore-parallel spur on the north jetty differed
considerably between the constant and the time-dependent wave transmission cases. Sensitivity
tests indicated that seasonal directionality and energy of the incident waves, combined with the
variable wave transmission, contributed to a significant difference in predictions between
constant- and variable-wave transmission. The combined working of wave direction and wave
transmission was unanticipated and demonstrates the functional utility of numerical simulation
models of shoreline change that can automatically account for a wide range of contributing
variables determining longshore sediment transport and shoreline response to structures.
ADDITIONAL INFORMATION: This CHETN is a product of the Inlet Geomorphology and
Channel Evolution and Inlet Channels and Adjacent Shorelines Work Units of the Coastal Inlets
Research Program (CIRP) being conducted at the U.S. Army Engineer Research and
Development Center, Coastal and Hydraulics Laboratory. Much of the original research upon
which it is based was conducted under the support of the U.S. Army Engineer District, Seattle.
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