Assessment of flood risk accounting for river system behaviour

In this paper "river system behaviour" is defined as the complex interaction between river flow and the flooding of flood prone areas. A basic aspect of river system behaviour is that a local dike breach may affect hydraulic loads and hence dike failure probabilities at other locations. Important aspects in river system behaviour are discussed as well as the fact that effects of river system behaviour on flood risk may be both beneficial as well as adverse. This paper presents a conceptual approach to quantify effects of river system behaviour on probabilities of dike breach and flood risk. It was successfully applied to two example river configurations. The results of these examples are discussed. It is concluded that for proper flood risk assessment all relevant failure mechanisms, uncertainties as well as all proposed safety improvement measures are to be jointly taken into account. The conceptual approach enable all this. In the authors' views, there is a need for developing models that account for effects of river system behaviour on flood risk. Such models can serve as a tool for policy makers in evaluating the effects that (regional) safety improvement measures have on the flood risk in the entire river basin.