Uncertainty in runup predictions on natural beaches using XBeach nonhydrostatic

Abstract

The wave time series that forces phase-resolving models is a source of model uncertainty that can propagate into wave runup predictions when the wave phase information is unknown. The effect of beach morphology on the propagation of this intrinsic uncertainty related to wave randomness (Ui) is largely unexplored. Here, we quantify the importance of uncertainty in wave runup at a dissipative (TS), intermediate (DU) and reflective (DE) beach using the phase-resolving model XBeach nonhydrostatic. Intrinsic uncertainty is evaluated with respect to the effect of free model parameters and of an evolving bed. Uncertainty contributions due to model equations and numerical methods are not assessed. The model is forced at each beach with 100 different wave time series corresponding to the same storm condition. Computed uncertainty metrics reveal that Ui is most important at TS, where extreme runup (R2%) of a single simulation can deviate up to 31% from the 100-simulation ensemble average. Intrasite differences in R2% can be attributed to the predominance of low-frequency swash (SLF), which is more sensitive to Ui than high-frequency swash and setup. Additional simulations using the suggested range of breaking and bed friction coefficients demonstrate that Ui produces more variability in SLF than the two most important model parameters, whereas the parametric range is more important for high-frequency wave height. Furthermore, morphodynamic simulations show that Ui produces a larger variability in R2% and SLF than an evolving bed under a 7-hr storm. Not surprisingly, Ui causes larger variability in morphologic than hydrodynamic predictions due to morphologic feedback. Trends can reverse from swash zone accretion to erosion (TS) by using a wave time series with differently distributed phase. Ensemble simulations are recommended for forecasts based on wave time series with randomly distributed phase.