IMPROVED WIND DRAG FORMULATION FOR NUMERICAL STORM WAVE AND SURGE MODELING

Abstract

The study proposes an enhanced method of computing wind drag coefficient (Cd) for extreme hurricane wind conditions which can be applied to precisely estimate the hurricane induced storm wave and surge. Thirty different tropical cyclone/hurricane conditions were chosen and modeled using the enhanced Wave Boundary Layer Model (WBLM) to estimate Cd based on air-sea interaction concept and are then subjected to polynomial regression with wind speed as an independent and Cd as the dependent variable. An improved wind drag (IWD) method (cubic function) that matches the WBLM's pattern at all simulated wind speeds was established. To evaluate the performance of the IWD method, it was used to simulate storm wave and surge levels for Hurricanes Katrina, Rita and Michael, along with two other widely used wind drag methods ( Garratt, 1977 and Zijlema et al., 2012 ). Katrina, Rita and Michael were chosen for their extreme wind intensity, the surge that they induced along their landfall regions and also the field data availability. The simulations were carried out using a coupled model to compute the wave heights and water levels, which were further compared and validated with the field measurements. A detailed analysis of the modeled results conveys the fact that the proposed IWD method performs better than the other two methods in accurate estimation of significant storm wave heights and peak surge levels. Hence, the study recommends the IWD method for computing Cd at extreme wind conditions up to 90 m/s.