SHORELINE RESPONSE TO BREAKWATERS WITH TIME-DEPENDENT WAVE TRANSMISSION

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Ty V. Wamsley 1, Nicholas C. Kraus1, and Hans Hanson2

Abstract: Wave transmission is a leading parameter determining the response of the shoreline to a

detached breakwater, reef, or spur attached to a jetty. To improve the predictive capability of the

shoreline response numerical model GENESIS, published empirical formulas for the wave

transmission coefficient were incorporated to calculate time-dependent wave transmission and

shoreline response. Simulations for different structural configurations, wave climates, and water

levels demonstrate the functional utility of time-dependent wave transmission on shoreline response

predictions. Results indicate that variable wave transmission is of significance for modeling the

response of the beach to submerged and emergent near-surface structures. Predictions of the model

are examined in application to a functional design of a submerged spur being studied as a possible

sediment-control measure for the north jetty at Grays Harbor, WA. Results show that for design

applications, beach response under time-varying forcing cannot be anticipated with a constant

transmission coefficient. The improved capability is expected to have wide applicability.

INTRODUCTION

Detached breakwaters and breakwaters attached to jetties are constructed parallel to shore to

serve as a shore protection measure. The planform response of the shoreline to the placement of a

breakwater must be considered in the design process. The response can take the form of a tombolo

that extends from shore to attach to the structure, a salient or cusp in the shoreline that extends

partially to the structure, or a null response. Herbich (1999) reviews available empirical and

numerical predictive capabilities for the functional design of detached breakwaters.

Hanson and Kraus (1989, 1990) identified 14 parameters controlling beach response to detached

breakwaters. They performed extensive numerical simulations over a wide parameter range,

validated by reference to performance of structures in the field, to produce the following predictive

expression for beaches with 0.2 mm median grain size:

≤ N (1 - Kt ) 0

(1)

where X = length of breakwater, L = wavelength at the breakwater located in depth D, Kt = wave

transmission coefficient, H0 = deepwater wave height, and N = empirically determined coefficient

distinguishing response of the beach to the structure. Their work demonstrated that wave

transmission is a leading parameter determining shoreline response. It is not reliable to apply an

1) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry

Road, Vicksburg, MS 39180, USA. Ty.V.Wamsley@erdc.usace.army.mil;

2) Department of Water Resources Engineering, Lund University, Box 118, S-221 00, Lund, Sweden.

Hans.Hanson@tvrl.lth.se.