Response of large-scale coastal basins to wind forcing: influence of topography

Wen L Chen,Henk M Schuttelaars,Tjerk J Zitman,Mohit Kumar,Pieter C Roos,Suzanne J M H Hulscher

doi:10.1007/s10236-016-0927-1

Wen L Chen, Henk M Schuttelaars + Show 4 more

Open Access

https://doi.org/10.1007/s10236-016-0927-1

Copy DOI

Abstract

Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect to the along-basin direction. Turbulence is represented using a spatially uniform vertical eddy viscosity, combined with a partial slip condition at the bed. The surface elevation amplitudes, and hence the vertical profiles of the velocity, are obtained using the finite element method (FEM), extended to account for the connection to the outer region. The results are then evaluated in terms of the elevation amplitude averaged over the basin’s landward end, as a function of the wind forcing frequency. In general, the results point out that adding topographic elements in the inner region (such as a topographic step, a linearly sloping bed or a parabolic cross-basin profile), causes the resonance peaks to shift in the frequency domain, through their effect on local wave speed. The Coriolis effect causes the resonance peaks associated with cross-basin modes (which without rotation only appear in the response to cross-basin wind) to emerge also in the response to along-basin wind and vice versa.

Highlights

Wind-driven set-up is the main contribution to extreme high water events, which may threaten coastal safety
The goal of the present study is to investigate the influence of large-scale topography on the wind-driven frequency response of large-scale coastal basins, measured in terms of the set-up at the coast, and paying particular attention to the role of the Coriolis effect and wind angle
We have developed an idealised process-based model to analyse the influence of specific topographic elements on the frequency response of semi-enclosed coastal basins subject to time-periodic wind forcing

Summary

Introduction

Wind-driven set-up is the main contribution to extreme high water events, which may threaten coastal safety. This is so when the combined characteristics of the wind forcing and the basin trigger resonance (Abraham 1960). A typical example is typhoon Winnie at the Korean coast of the Yellow Sea in 1997. The unusually strong and extensive coastal flooding was partly caused by resonant coupling of the Yellow Sea and the predominant period of the forcing (Moon et al 2003). The resonance properties of coastal basins can be affected by large-scale topographic elements.

Objectives

Results

Conclusion