Decisions and attitude of decision makers facing hydrological risk

Hydrological risk is analysed as a two-component concept related to probability and to adverse consequences. Studying simplified examples shows how the decision maker's attitude is important in the face of risk. In particular, this attitude has consequences on his behaviour in relation to uncertainties. One has to insist on the necessary accounting by the hydrologist for an expression, even simplified, of actual damaging consequences provided by the decision maker or by potential victims of the risk; and on the interaction between the value of the information and the related risk aversion. The analysis is based on Bayesian principles which allow a whole and quantified rational evaluation of every aspects of the dialogue between the hydrologist and the decision maker.