Simon Charles Warder , Kevin Horsburgh, Matthew Piggott

Numerical storm surge models are essential to forecasting coastal flood hazard and informing the design of coastal defences. However, such models rely on a variety of inputs, many of which will carry uncertainty, and an awareness and understanding of the sensitivity of the model outputs with respect to those uncertain inputs is necessary when interpreting model results. Here, we use an unstructured-mesh numerical coastal ocean model, Thetis, and its adjoint, to perform a sensitivity analysis for a hindcast of the 5th/6th December 2013 North Sea surge event, with respect to the bottom friction coefficient, bathymetry and wind stress forcing. The results reveal spatial and temporal patterns of sensitivity, providing physical insight into the mechanisms of surge generation and propagation, and can also be used to estimate the uncertainty in skew surge model predictions due to uncertainty in each model input. Our results demonstrate the power of adjoint methods to gain relevant insight into a storm surge model, providing information complementary to traditional ensemble uncertainty quantification methods.