Is the likelihood of getting a flood increased by higher than usual flows in the previous months?

Abstract:

How long a river remembers its past is still an open question. Perturbations occurring in large catchments may impact the flow regime for several weeks and months, therefore providing a physical explanation for the occasional tendency of floods to occur in clusters. The research question explored in this paper may be stated as follows: can higher than usual river discharges in the low flow season be associated to a higher probability of floods in the subsequent high flow season? The physical explanation for such association may be related to the presence of a higher storage at the beginning of the high flow season, which may induce lower infiltration rates and therefore higher river runoff. It is well known that larger catchments have longer memory and therefore we focus on two European river basins with large extension: the Po River at Pontelagoscuro and the Danube River at Bratislava, whose catchment areas amount to 70.000 km² and 131.331 km², respectively. We look at the stochastic connection between average river flows in the pre-flood season and the peak flows in the flood season, by using a bivariate probability distribution. We found that the shape of the flood frequency distribution is significantly impacted by the river flow regime in the low flow season, with different dynamics depending on the physical properties of the catchment and the human impact on the river flows. The proposed technique, which can be classified as a data assimilation and Bayesian approach, may allow one to reduce the uncertainty associated to the estimation of the flood probability.