WAVE ENERGY TRANSFORMATIONS IN A COMPLEX REEF ENVIRONMENT; OBSERVATIONS, MODELING AND APPLICATIONS

Abstract

Wave-driven runup events are severely impacting West Maui’s (WM) coastline with episodic inundation and chronic erosion. A combination of background sea level, tides and wave driven components, such as setup and infragravity (IG) wave energy, contribute to the level of runup experienced at the shore. The setup, swash and IG wave responses under different sea and swell forcing conditions are highly variable along the WM coastline, due to complex nearshore bathymetry. Simulating the setup, swash, and IG wave energy responses to large swell events at different locations along the WM coast is necessary for accurate calculation of runup, enabling forecasting of, and community preparation for, these coastal inundation events. The PacIOOS Coastal Hazards Group has implemented a two-dimensional, fully-nonlinear and weakly dispersive, phase resolving numerical wave model (Boussinesq Ocean and Surf Zone; Roeber and Cheung, 2012), using high resolution bathymetry and topography of WM. The model simulates the cross- and along-shore transformations of gravity and IG wave energy for simulation of runup. The main objective of this study is to validate the model for the WM domain against observations of swell events. Nearshore sea level observations are derived from bottom water pressure records collected at different depths (1-13 meters) and locations along the West Maui coastline, from November 2018 - June 2020. Comparisons between in situ observations and co-located virtual stations within the model reveal a high degree of agreement in both the sea and swell and IG period bands, between 8 seconds and up to 10 minutes. Spectral analysis is used in the comparisons to investigate the spatial variability of the wave energy. A series of sensitivity tests using variable model resolutions shows that the choice of a 5 x 5 meter grid is optimal for this domain. Observations and model comparisons are discussed for both a North swell and a South swell event at six different locations along the WM coastline, including two relatively compact arrays. The simulation for the North swell is in better agreement with observations on the northern coast of WM (at Oneloa, Nāpili and Kahana). Similarly, the simulation for the South swell is in better agreement with observations in the southern area of WM (at Puamana and Olowalu). The comparisons reveal the high IG amplitude variability alongshore resulting from complicated IG wave patterns, which are generally well simulated in the model. At both arrays the comparisons of coherence phase and amplitude of the observations and model reveal the model is simulating well the frequency-dependent, spatial variability of the nearshore wave-driven phenomena that contribute to runup. The few occasions where the simulation is unimpressive suggest that ways to improve the model methodology should be investigated.