Modeling the hydrodynamics and morphodynamics of sandbar migration events

Abstract A number of models are available for science and engineering purposes that numerically simulate nearshore hydrodynamics and the corresponding morphological evolution. However, the models include adjustable coefficients in parameterizations for physical processes that need to be calibrated, and thus there remains room for improvement by including additional physics. One such model is XBeach, which can simulate erosion during storms with proper calibration based on observations. The modeled sediment transport, especially in the cross-shore direction, is sensitive to the adjustable coefficients, with preferred values that are site and event specific. Here, the skill of XBeach is investigated by comparing 1-dimensional (cross-shore) depth-averaged simulations with observations of waves, currents, and sandbar migration across an Atlantic Ocean beach. Calibration of coefficients improved the agreement of the computed results with observed wave heights, offshore-directed mean currents (undertow), the wave-orbital-velocity third moments (skewness and asymmetry), and onshore/offshore sandbar migration although the proposed coefficient values depend on the parameterizations used. For example, including a variable breaking-wave roller energy model resulted in more skillful predictions of undertow than using the default constant coefficient value. Using the calibrated roller coefficients and corresponding undertow, XBeach simulated the observed offshore migration of the sandbar. Onshore transport in XBeach is driven by non-sinusoidal wave-orbital velocities, and proposed values for coefficients depend on the parameterization used to estimate skewness and asymmetry and the associated transport, as well as on incident wave conditions. XBeach calculations of cross-shore sediment transport rates were compared with those estimated by a commonly used sediment transport formula based on laboratory experiments. The inter-comparison suggests that using a wave-induced onshore transport parameter 3 or 4 times larger than the default value may at least in part compensate for the lack of bottom-boundary-layer-streaming-driven-onshore transport in XBeach.