Nearshore and Surf Zone Wind Stress

Abstract

Wind stress on the water surface plays an important role in generating storm surges. Predicting the wind shear stress and drag coefficient over the ocean is therefore at the heart of storm surge modelling. The wind setup is inversely proportional to the water depth. As such, it is important to use the wind stress and the wind drag coefficient over the nearshore region and the surf zone for modelling the storm surge, which is a nearshore phenomenon. However because of limited data in the nearshore zone, engineers typically use the wind drag coefficients which are obtained from measurements over other parts of the ocean in order to model storm surges. Two major field campaigns were conducted in the present study to investigate the wind shear stress on the surf zone and surrounding coastal water. Coastal wind velocity profiles were measured during the first campaign, namely the North Stradbroke 2011 experiment, and the wind profile method was used to study the wind shear stress. The eddy correlation technique was adopted for the second field campaign, namely the North Stradbroke 2012 experiment. The wind drag coefficients measured using the eddy correlation technique were found to be banded with respect to the stability. The conventional treatment of stability, using exiting empirical stability functions, was found inadequate to rectify the stability banding of the data. This was explained based on interrelated tilt-stability effects and land-sea thermal interactions and their influence on thermal fluxes and empirical stability functions. The wind drag coefficients in near-neutral conditions were found to systematically change with the wind angle of approach relative to the shoreline. The idea of an apparent wave steepness was developed to explain this behaviour. Results of the Stradbroke 2012 experiment suggest that the drag coefficients during longshore wind were on average CDN10=1.25 x10-3 and were found to be much smaller than those during onshore wind. The drag coefficients over the surf zone during onshore wind and near-neutral conditions were found to be on average CDN10=2 x10-3 for u10=5-11 m/s. This was almost twice the values predicted for the same wind speed by existing linear drag coefficient formulations based on observations outside the surf zone. The differences were attributed to different wave celerity and wave shapes in the nearshore region. The observed Charnock coefficient was on average 0.110 which was an order of magnitude larger than open ocean values. It was found that a wave celerity of the order of those observed in the inner surf zone is required to explain the observed large roughness and drag coefficients in the present study using existing wave-age dependent parameterisations. Coastal wind profile measurements during the Stradbroke 2011 experiment suggested that the measured apparent drag coefficients were correlated with the tide. A detailed change of terrain analysis was conducted to examine this behaviour and rectify the internal boundary layer effects. The evaluated wind drag coefficients over the surf zone using the profile method, for u10=9-14 m/s, were on average CD10=3.2 x10-3 The corresponding surf zone roughness was on average z0=8.8mm. Measured wind drag coefficients over the surf zone were found to be more than double the values previously found in other parts of the ocean for the same wind speed. Therefore, datasets validate the hypothesis of large drag coefficients over the surf zone. It was suggested that a rougher and wider surf zone during the Stradbroke 2011 study, as a result of larger offshore wave heights, was the reason for observed larger drag coefficients than those measured during the 2012 study.n