A Case Study of Nearshore Drag Coefficient Behavior during Hurricane Ike (2008)

Over the past decade, numerous field campaigns and laboratory experiments have examined air-sea momentum exchange in the deep ocean during moderate to strong wind conditions. These studies have changed our understanding of drag coefficient behavior in hurricane force winds, with a general consensus that a limiting value is reached. Near the shore, where wave conditions are markedly different than those in deep water due to wave shoaling and breaking transformation processes, only very limited data exist. Knowledge of wind stress in this region is critical for storm surge forecasting and informing the wind load standard along the hurricane prone coastline.

During the 2008 Atlantic Hurricane Season, a field campaign, coordinated among Texas Tech University, the University of Florida, and the University of Notre Dame, collected valuable wind and wave data during the passage of Hurricane Ike. A TTU StickNet platform obtained wind measurements in true marine exposure with a fetch across the Houston ship channel at Fort Travis, and three UF/UND wave and surge gauges collected water level and shoaling wave data adjacent to landfall in the Gulf of Mexico. These data are used to evaluate air-sea momentum exchange and sea surface roughness at the immediate coastline.

As Ike made landfall, the mean wind flow was across the ship channel at the StickNet platform. To obtain a better understanding of the complex wave conditions in the ship channel and the surge level at the StickNet location, a SWAN+ADCIRC coupled simulation was conducted. The simulated waves were indicative of a fetch-limited condition with maximum significant wave heights reaching 1.4 m and with peak periods of approximately 4 s. Using the measured wind data and simulated wave conditions, this work examines the drag coefficient dependence on wind speed and wave conditions. Findings indicate that the drag coefficient reached a limiting value at wind speeds near hurricane force, and at slower wind speeds the drag coefficient was higher than over deep water.