MS Plan B Defense: Analysis of XBeach non-hydrostatic model capability in processing SWAN spectral input

Malia Selman
Master’s Student
Department of Ocean and Resources Engineering
University of Hawai’i at Mānoa

**This defense will be held both in person (POST 723) and over Zoom**
Meeting ID: 918 3466 1004
Passcode: MaliaMS
https://hawaii.zoom.us/j/91834661004

Numerical models have become an industry staple for identifying and predicting coastal conditions. These models are used in engineering design and hazard mitigation with implication for public and infrastructure safety. Open-sourced model packages are attractive options as they promote collaboration between academics and working professionals in the development and improvement of the source code. Developed by the Delft University of Technology, the nearshore model XBeach has gained traction in both academic and professional communities for its ability to couple wave simulation with morphodynamic calculations. A common wave forcing option is provided by the Delft spectral model, SWAN (Simulated Waves Nearshore). The capabilities of the phase-resolving model, XBeach non-hydrostatic (XB-NH), in replicating SWAN spectral input was analyzed through a series of prevailing and annual wave conditions for a fringing reef study area on the southeast shore of Oahu. Wave conditions were identified at the CDIP 098 buoy located approximately 11 km from the study area. Control tests for both swell and wind-sea conditions were conducted to further investigate the influence of wave steepness and nonlinear effects. XB-NH model sensitivity to grid resolution was investigated for all conditions by testing the performance of three resolutions defined by set relationships between cell size and the modeled primary wavelength (L/15, L/20, and L/30). An initial objective of this study was to determine the “cut-off” resolution relationship that accurately captured the primary wavelength while maintaining a reasonable computation time to mitigate the effects of numerical dissipation for all conditions. Steep wave conditions yielded poorer agreement between the SWAN input and XB-NH output due to dissipation of overestimated super-harmonic components in XB-NH. Increased grid resolution mitigated these effects adequately with the level of improvement being directly related to relative wave steepness. This study provides guidelines for achieving adequate wave information transfer between SWAN and XB-NH for the one-way nesting method.