Calibration and assessment of process-based numerical models for beach profile evolution in southern California

Abstract

Cross-shore profile response to energetic wave events is challenging to predict because the physics are poorly understood and wave and topographic data are often sparse or unknown. Six events with varying duration, wave intensity and beach slopes are used to calibrate and assess two process-based cross-shore models, CShore and XBeach, at two southern California beaches. Model performance is quantitatively evaluated using high resolution temporal-spatial survey observations along with Brier Skill Scores (BSS) and a novel Bulk Shoreline Change Error (BSCE) metric that considers alongshore-averaged upper beach volume. XBeach is tested with default and site-calibrated parameters. Site calibration improved XBeach model skill, however the XBeach skill scores are still often low and in no case was the offshore bar correctly predicted. Notably XBeach calibration is sensitive to depth extent and produces significantly different model skill for upper beach and full profile predictions. For CShore, the better performing of two existing sets of parameter values is used. CShore and XBeach predict profile change with limited skill. In their present forms, CShore and XBeach are unable to accurately predict beach profile change on these typical southern California beaches, but when calibrated may provide qualitatively useful estimates of bulk shoreline erosion.

Simulating Destructive and Constructive Morphodynamic Processes in Steep Beaches

“…The calibration of XBeach in steep profiles and under low to moderate wave conditions is a challenging task, as calibrated simulations of XBeach under such conditions generally show low model skill (e.g., [48]). For calibrating the post-Emma storm recovery in Faro Beach, a crucial obstacle was the underappreciation of wave runup heights with the gradual steepening of the profile.…”

Beach nourishment has complex implications for the future of sandy shores

“…Current state-of-the-art cross-shore process-based models perform best for predominantly offshore directed morphological development on time scales of days, such as the large erosion of nourished profiles during a storm 99 . When applied to natural profiles and moderate waves, model skill is significantly reduced up to the point that a simulated development, when compared to observed changes, can be worse than a no-change prediction 100 .…”

Process-Based Model Prediction of Coastal Dune Erosion through Parametric Calibration

“…Wave dissipation in the nearshore is caused by the bed friction, breaking, and surface roller. WTI settings and several studies use wave dissipation (alpha, α) [25,29], which is related to breaking, and the breaker slope coefficient (beta, β) [29], which is related to the surface roller, as calibration factors. The Xbeach model calculates the effect of wave surface roller using Equation (1) [27] with those two coefficients, contributing to the shoreward kinetic energy and resulting in radiation stresses and wave-induced forces so that they would affect the prediction of wave height and runup in the surf and swash zone.…”