Numerical models have the capability of representing the response of the shoreline to structures
exposed to time-varying forcing. In this study, published empirical formulas for the wave
transmission coefficient were incorporated in the GENESIS shoreline change model to calculate
time-dependent wave transmission and shoreline response. The functional utility of time-dependent
wave transmission was assessed running simulations for a single detached breakwater of varying
crest heights and forced with a range of wave conditions. Results indicate that the time-dependent
Kt enters centrally for all wave climates and plays a major role for submerged and emergent near-
surface structures. The application of a time-dependent wave transmission calculation entered most
significantly for submerged structures, for which shoreline position predictions may change by 80%
or more. The time-dependent calculation appears to be necessary to represent emergent near-surface
structures because application of a constant Kt may over-predict shoreline advance by as much as
30%. These percentages apply to the wave climates and structural configurations tested. Greater
changes may be possible for a given application as illustrated by the Grays Harbor case study.
The functional utility of time-dependent wave transmission was further examined in application
to a submerged spur being studied as a possible sediment-control measure for the north jetty 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.
The combined working of wave direction and wave transmission demonstrates the complex
interaction of forcing parameters that cannot be anticipated by a constant transmission coefficient for
design applications.
Predictive capability of numerical shoreline response models such as GENESIS can be improved
by incorporating a time-dependant wave transmission calculation for detached breakwaters. In the
present study, Kt is represented in a more realistic manner than previously, and results indicate that
the time-dependant nature of wave transmission is a major factor in determining shoreline response
for submerged and emergent near-surface structures. In practice, detached breakwaters are often
submerged to reduce cost, produce moderate shoreline change, and to minimize dangerous
diffraction currents. Therefore, the improved simulation capability in GENESIS is expected to have
wide applicability.
ACKNOWLEDGEMENTS
This work was performed as part of a study commissioned by the U.S. Army Engineer District,
Seattle, with additional support provided by the Inlet Channels and Adjacent Shorelines Work Unit
and Inlet Geomorphology and Channels Work Unit of the Coastal Inlets Research Program, U.S.
Army Corps of Engineers (USACE). We appreciate guidance by study managers Messrs. Hiram
Arden and Robert Parry of the Seattle District through the course of this work. The authors thank
Ms. Julie Dean Rosati for helpful review. Permission was granted by Headquarters, U.S. Army
Corps of Engineers to publish the information contained in this paper.
REFERENCES
Ahrens, J. P. 2001. "Wave transmission over and through rubble-mound breakwaters." Draft
Contract Report submitted to the U.S. Army Engr. Res. and Development Center, Coastal and
Hydraulics Lab., Vicksburg, MS.
Wamsley et al
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