Coastex wave calculation — A cost-efficient method for computing high-resolution wave conditions in coastal regions

Abstract

Accurate and reliable information about ocean surface waves is crucial for marine activities in coastal regions. In areas with complex coastlines, where fjords, islets and strong bathymetry gradients affect the wave field, a high-resolution computational grid is needed to generate model results that agree with observations. For traditional wave modelling this requires enormous computational power, often resulting in insufficient spatial resolution at the most complex fjord and coastal regions. We present an alternative method for coastal wave calculation, the Coastex Wave Calculation, where the wave field is calculated from a set of predefined sea states given by wave properties at an offshore position and the local wind field. We assess four different versions of the Coastex method, using wave observations from five measurement sites at the western coast of Norway, and two control runs using a traditional model setup on a high-resolution computational grid. For our study area we find that deviations between the results of the Coastex calculations and the control runs are small: At the most exposed location the mean error, the mean absolute error and the root mean square error of the significant wave height are in the range 0.06 m to 0.10 m, 0.26 to 0.28 m and 0.34 m to 0.36 m, respectively (Coastex) and 0.08 m to 0.10 m, 0.24 m to 0.26 m and 0.31 m to 0.33 m for the control runs. At the most sheltered location the mean error of the Coastex hindcast ranges from −0.07 m to −0.02 m, compared to −0.05 m for the control runs, the mean absolute error is 0.08 m and 0.09 m (Coastex) and 0.07 m (control runs) and the root mean square error is 0.11 m and 0.12 m (Coastex) and 0.10 m for the control runs.