Development of a methodology for extreme flood estimations in alpine catchments for the verification of dam safety

Dams are critical infrastructures which may be endangered by natural hazards as extreme foods. The water release structures especially the spillways must be designed in such a way that they can ensure the safe passage of extreme floods. Uncontrolled overtopping of the dam cannot be accepted since it may result in its failure. The estimation of the design and the safety flood to take into account for the determination of the spillway capacity is a complex and difficult task in alpine regions. Dr. Fränz Zeimetz proposes a new and validated methodology for the estimation of extreme floods in alpine catchment based on several scientific developments which could close some relevant gaps of knowledge. The first question addressed by Dr. Zeimetz concerns the temporal rainfall distribution based on the existing Swiss PMP maps. The candidate could show by a comprehensive analysis that a unique rainfall mass curve (RMC) can be adopted for the entire territory of Switzerland. Another open question is the temperature gradient in the atmosphere to take to into account for extreme flood simulations. Regarding a coherent combination of temperature and extreme precipitations, Dr. Zeimetz could develop linear relations between the duration of the precipitation event and the zero-degree isothermal altitude. When using hydrological modelling, the influence of the initial conditions for extreme flood simulations has to be assessed. Based on an extensive analysis of the model state variables, Dr. Zeimetz could develop methodological recommendations for the choice of the initial conditions for extreme flood simulations. Furthermore, he could define the maximum admissible area of the catchment area for which the PMP events derived from the Swiss PMP maps can be still applied. Dr. Zeimetz could also show that the combination of the simulation results with the approach of upper bounded statistical extrapolations has the advantage that the sample sensitivity can be reduced and that the plausibility of the extrapolations is enhanced compared to conventional statistical distributions. Finally, Dr. Zeimetz could formulate a new holistic methodology for extreme flood estimations which he illustrated with three application examples. Based on the PMP-PMF approach the methodology allows to estimate extreme flood hydrographs using hydrological simulation and to attribute a return period to the simulated peak discharge using deterministically determined upper discharge limit.