Exploring heavy-tailed behavior of flood frequency curves: an analysis of dike overtopping effects

Introduction Flood Frequency Analysis (FFA) plays a crucial role in the design of hydraulic structures and flood risk management. Conventional estimates of flood exceedance quantiles are highly dependent on the underlying flood frequency distribution, especially its upper tail, which is particularly difficult to estimate from observed data. Methods We use a simulation-based approach to investigate the transformation in upper tail behavior of flood peak distributions when dikes are overtopped in the large and complex river network of the River Rhine. We account for the effects of dike overtopping, floodplain storage, upstream-downstream interactions and interplay of tributaries. A coupled hydrologic-hydraulic modeling system, driven by long synthetic time series of meteorological fields from many realizations of a stochastic weather generator, is used to simulate dike overtopping and inundation. Results Overtopping results in a consistent reduction of the shape parameters of the Generalized Extreme Value distribution at mainstream and tributary gauges. At the same time, the variability of shape parameters between model realizations increases in most cases Discussion On average, reaches with large overtopping volumes and storage capacities show stronger reduction of shape parameters, whereas, in individual model realizations, the timing of overtopping and the rate at which overtopping volumes are generated seem to control shape parameter reduction.