Tide-induced currents in a phase-resolving wave model

Abstract

Alongshore currents are currents parallel to the coastline. In coastal areas they are driven by a combination of wave and tidal forcing. In this study, the validity of the non-hydrostatic, wave-flow model SWASH to predict these currents is investigated. This is done by comparing model predictions with measurement observations. Observations were obtained from the COAST3D data-set at the gently sloping barred beach near Egmond. It contains six weeks of wave and velocity measurements for a wide range of conditions. An initial study was performed to investigate model settings and parameter sensitivity for correctly capturing wave dynamics. From this study, it can be concluded that two terrain-following layers and 75 grid cells per wave length lead to accurate results for surf zone wave modelling. Although some discrepancies in mean water level set-up were encountered, this is assumed not to affect alongshore current predictions. The present SWASH model does not allow for the inclusion of alongshore tidal currents. Therefore, this study presents a method to include tidal forcing, which was implemented in the source code. This method enables the user to include the tide by imposing a water level gradient, which is assumed to be time averaged and spatially uniform. These assumptions are reasonable as spatial and temporal model scales are small compared to a tidal wave. Validation results demonstrate the capability of SWASH to correctly represent wave transformation in a surf zone. Furthermore, modelled velocity predictions are in good agreement with observations for cases with waves from southwest and northward flood tidal currents, which were not disturbed by instabilities. Cases with different forcing were influenced by instabilities due to a bug in the source code. Two cases, which were least influenced by the instabilities, have been analysed in more detail as well. Some discrepancies are observed between model and observations. Further study is required to evaluate whether this is caused by the instabilities, introduced by the bug in the source code, or whether other physical phenomena play a role in the mismatch. Inertia of a tidal wave and bottom friction and processes which could potentially be the cause of the discrepancies.