Uncertainty in Nearshore Sand Bar Migration.

A coupled nearshore hydrodynamics and sediment transport model with adjoint‐based uncertainty quantification has been applied to sand bar morphodynamics in the Duck94 field experiment (Duck, NC). The model, which is based on previously established physical process formulas, is capable of producing accurate predictions of sand bar migration during the experiment. However, reasonable perturbations to the model inputs—namely initial conditions, boundary conditions, and calibration coefficients—can produce a range of model predictions. This range is taken to represent the potential for uncertainty of the model, for scenarios where one or more model inputs are poorly constrained. Experiments are presented that quantify the relative impact of different input types on uncertainty, and the results are contrasted for accretive versus erosive wave conditions. During accretive conditions (shoreward bar migration), uncertainty is dominated by the growth of errors in initial conditions, owing to positive feedbacks in the model. In contrast, during erosive conditions (seaward bar migration), there is a smaller impact of errors in initial conditions, and uncertainty is instead dominated by errors in sediment transport coefficients. The results thus point to qualitative differences in the nature of nearshore morphodynamic uncertainty under different wave conditions, with a greater role of internal error growth (positive feedbacks) during accretive conditions and errors in model physics being more important during erosive conditions. Plain Language Summary: Sandy wave‐exposed beaches are in a constant state of change due to sediment transport by waves and wave‐driven currents. The relevant physical processes are mostly concentrated within 1–10 cm above the seabed and vary on timescales of order 0.1–10 s—these scales are several orders of magnitude smaller than those of practical interest for nearshore morphodynamics, hence simplified approximations are needed to represent subgrid‐scale physical processes. In addition, the interplay between morphology and sediment transport can trigger feedbacks, such that even small model errors may become amplified over time. Both these factors (process errors and feedbacks) are generally understood to cause considerable uncertainty in model predictions, but their relative importance is not well known. The present work quantifies effects of feedbacks and model parameters, showing their relative importance changes depending on whether the beach is accreting or eroding. By better understanding the nature of uncertainty, these results can inform future efforts into predicting nearshore coastal change at storm‐to‐seasonal timescales. Key Points: Uncertainty quantification in a nearshore morphodynamic model using an adjoint‐based approachMorphodynamic uncertainty is dominated by positive feedback during accretive conditions and by model parameter error during erosive onesNearshore sediment transport formulas are highly sensitive to uncertainties in wave hydrodynamics [ABSTRACT FROM AUTHOR]