Wind-wave interaction in the nearshore environment
Zachry, Brian Charles
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The flux of momentum between the atmosphere and ocean, described in terms of an aerodynamic drag coefficient (CD), is required to accurately forecast hurricane track and intensity; to predict hurricane storm surge, ocean waves, and currents; to define wind load standards; and to generate hurricane risk models. Recently, a significant effort has been made to advance our understanding of air-sea momentum exchange in hurricane winds via the development, refinement, and implementation of novel instrumentation such as the Global Positioning System (GPS) dropwindsonde and the â€˜Bestâ€™ Aircraft Turbulence (BAT) probe. However, these platforms are best suited for measurements in an open ocean (deep water) environment. As deep water datasets continue to grow, it is apparent that data collected near the coast remain inadequate. Ironically, the coastal region is where the accuracy of hurricane models and building code provisions are most severely tested. This vacuity is exacerbated by the continued increase in wealth, infrastructure, and population along the hurricane-prone coast. Whether the nearshore drag coefficient differs from deep water observations or from historic linear formulations with wind speed has yet to be determined and is the focus of this dissertation. To help fulfill the need for a better understanding of nearshore air-sea momentum flux, a combination of laboratory and full scale studies are presented. Since it is exceptionally difficult to conduct in-situ measurements over shoaling waves (waves that are in shallow-enough water to interact with the sea floor and undergo transformation processes), an innovative wind tunnel study was designed. The atmospheric boundary layer wind tunnel facility at Texas Tech University (TTU) was used to characterize the wind flow and determine the drag coefficient over a statistically valid train of fixed shoaling wave models. Methodologies employed in this analysis were tested in a pilot experiment utilizing a train of fixed sinusoid waves (this work also provided general limits for CD). It was determined that fixed wind tunnel waves can be compared to propagating ocean waves, provided that the latter are in strong winds and limited by fetch (e.g., hurricane waves). Results indicated that the drag coefficient over shoaling waves is approximately 50% higher than those observed in deep water hurricane conditions. Wind tunnel values are in relative agreement with preliminary estimates of CD over shoaling waves in hurricane conditions. This study also found that the air flow does not separate over shoaling waves (the flow remains attached) and that a pronounced speed-up region is present over the wave crest. Once validated by shoaling wave datasets, this methodology can be effectively used to estimate the drag coefficient and examine the flow over other critical wave shapes. To complement the laboratory results, a joint field campaign during the 2008 Atlantic Hurricane Season collected valuable nearshore wind and wave data as Hurricane Ike made landfall near Galveston, TX. Coastal drag coefficient behavior was similar to that found in deep water, where CD increased with wind speed, reached a limiting value, and decreased thereafter. Crucially, at wind speeds below the limiting value, drag coefficients were significantly higher than those previously measured in deep water, in lakes, or in wind/wave laboratory studies. Based on this analysis, storm surge models using a deep water wind speed dependent drag coefficient are likely to underestimate hurricane storm surge, and additional parameterizations are needed. Coastal roughness lengths computed from these data provide evidence that the American Society of Civil Engineers (ASCE) wind load code should prescribe Exposure D (smoother) rather than Exposure C (rougher) along hurricane prone coastlines.