Uncertainty analysis of river flooding and dam failure risks using local sensitivity computations

The potential of Local Sensitivity Analysis (LSA) for analysis of uncertainty with respect to two major risks in river hydrodynamics – flash flood and dam failure – is assessed. LSA, implemented as an equation-based method, is compared to a Global Uncertainty Analysis (GUA) consisting in running Monte Carlo simulations with a hydrodynamic model. For a given statistical distribution of the model input parameters, the mean and standard deviation of the output variables are estimated with the two methods. In all single or multiple parameter cases investigated, including as much as±80% relative variation, LSA provides similar results to GUA, while requiring only one simulation instead of several hundreds or thousands. Only within a few meters of the shock (flow discontinuity) generated by the breaking of a dam do the two methods depart. This paper shows that despite the non-linearity of river flow processes, the first order, local approach remains generally valid for uncertainty analysis of hydrodynamic risks, even in the case of large parameter uncertainty. The contrast in importance of the various parameters on both sides of a shock is also highlighted.