Page - 971 - in Book of Full Papers - Symposium Hydro Engineering

Fig. 1 Event Tree (Example for a weir with four gates and a design flood with a return period of 100 years) 2.3. STATISTICAL DESCRIPTION OF THE FAILURE PROBABILITY In a next step it is required to determine the probability pD of a hydraulic respectively hydrological ´failure´ by considering all scenarios shown in the set-up of the event tree (see above). Hereby ´failure´ is defined as the exceeding of the maximum operation water level in the reservoir. Practically this situation is far away from a structural failure of the concrete structure of the weir or the related embankment dams. The additional freeboard respectively the vertical distance between the maximum water level and the dam crest or the top edge of the sealing element will not be considered directly within the scope of the event tree concept. To enable a statistical description of the related probability pD it becomes necessary to determine the return period of the maximal possible flows passing through the weir respectively spillway in all relevant scenarios [Qn, Qn-1, potentially also Qn-2 and Qn-3] defined within the scope of the event tree. This can 971