Geometry, Kinematics & Energetics of Surf Zone Waves Near the Onset of Breaking Using Remote Sensing

Abstract

The surf zone is the shallow nearshore region where waves break due to depth-limitations. These breaking waves drive important nearshore processes, including alongshore and cross-shore circulation, sediment transport, and air-sea gas and particle exchange. Measuring and modeling such processes to gain better physical understanding of surf zone dynamics and predict future coastal change is of great interest to the scientific community, as well as to the global community that relies on the coastal region and resources for security, economic stability, and recreation. Wave breaking in the surf zone is effected by bathymetry changes, currents, tides and weather, all of which result in strong spatial and temporal gradients and pose a challenge to measuring and modeling wave breaking. In this thesis, thermal infrared imagery and line-scanning LIDAR are used to measure surf zone waves near the onset of breaking. These remote sensing methods provide broad spatial coverage and high spatio-temporal resolution, which enable investigation of breaking parameters and wave energy dissipation at the onset of breaking, a time of rapid wave change that has been prohibitively challenging to accurately measure in the field. Over 4200 waves are analyzed from data collected at the USACE Field Research Facility in Duck, NC, including over 2600 non-breaking waves, 414 spilling breakers, 110 plunging breakers, and 1139 breakers whose initial type could not be determined. Wave height is measured using a spatio-temporal method for wave tracking that preserves the true sea surface elevation maximum and is robust to instances when the wave trough is beyond the field of view of the LIDAR transect. Methods for estimating instantaneous wave speed are refined by fitting a skewed-Gaussian function to each wave profile and tracking the fitted wave form peak. Wave slope was estimated using a variety of fitting methods to the upper 20%, 50%, and 80% of the wave face. A linear fit to the upper 80% of the wave face provides the strongest correlation with geometric wave slope defined relative to mean sea level, and the maximum wave face slope achieved by the skewed-Gaussian fitted wave form is most robust to the wave shape changes near the onset of breaking. At the onset of spilling and plunging, critical breaking predictors are examined on a wave-by-wave basis. We find that γ, the ratio of wave height to water depth, peaks near the onset of breaking (0.7<γ_b<0.8 for plunging and 0.6<γ_b<0.7 for spilling) at values consistent with solitary wave theory (γ_sol=0.78) and critical γ_rms values previously observed at Duck and other beaches. Direct estimates of wave face slope and wave phase speed also peak at the onset of breaking. Wave face slope and γ are positively correlated and, when used together, strongly predict breaking and breaker type. A support-vector machine model is successfully used to identify or define the transition from non-breaking to breaking and from spilling to plunging. Finally, traditionally estimated wave energy flux gradients are compared with dissipation rates estimated using the bore model and the roller model. This analysis is pursued primarily on an ensemble-averaged basis, and the results are segregated by breaker type. We find that plunging breakers lose energy at a rate 40% greater than that of spilling breakers within the first 0.2 wavelengths after the onset of breaking. Following this interval, the rate of change of wave energy flux is approximately equal for spilling and plunging breakers. The bore model predicts maximum dissipation rate at the onset of breaking for both spilling and plunging breakers. For plunging breakers, high dissipation rate is concentrated very near the onset of breaking followed by a precipitous decrease, and for spilling breakers, the dissipation rate decreases gradually. Due to multipath reflections that can artificially augment the roller length, the roller model dissipation rates are inconclusive and require further research.