Numerical modeling of wave dynamics at Ulupa‘u crater: Validation of SWAN and XBEACH with field observations

Abstract

Accurately modeling wave conditions is essential for assessing the impact of artificial coastal structures on wave dynamics and coastal resilience. Before predicting the effects of proposed modifications, it is critical to validate numerical models against existing site conditions to ensure reliability. This thesis focuses on the validation of two wave models, SWAN and the nonhydrostatic version of XBeach, by comparing modeled wave transformations with field data collected offshore of Ulupaʻu Crater, Oʻahu, Hawaiʻi. The primary objective is to assess the accuracy of SWAN in simulating deepwater to transitional water wave transformations by comparing model output to Acoustic Doppler Current Profiler (ADCP) data collected at the site. This validation will determine if nesting SWAN with XBeach is an effective approach for wave modeling over large spatial domains. Additionally, the thesis aims to calibrate the XBeach model with the existing reef environment by validating wave transformations within the proposed artificial reef deployment site using pressure sensor data. The validation process involved integrating site-specific bathymetry and offshore wave conditions into the XBeach and SWAN models. Model parameters, including bottom friction and nonhydrostatic effects, were adjusted to best replicate the observed wave transformations. Spectral analysis and time-series comparisons were used to assess model performance, identifying key processes such as energy dissipation across the existing reef system. Successfully calibrating these models to represent existing site conditions establishes a foundation for future simulations for hybrid reef installations such as those being pursued under the Rapid Resilient Reefs for Coastal Defense (R3D) project. Future simulations will incorporate these artificial structures to assess their effectiveness in wave attenuation and coastal protection. This research provides a framework for using numerical modeling to inform coastal engineering decisions, ensuring that artificial reef designs are optimized for both ecological and hydrodynamic benefits.