Seite - 1258 - in Book of Full Papers - Symposium Hydro Engineering

warning time and efficient activities in case of emergency situation. Therefore, the probability may be assumed within the “tolerable” risk zone in the diagram in fig. 2. It can be seen that failure probability fits the range 10-3 to 10-5 which corresponds to 1000 to 100 000 years return period. Fig. 2 F-N curves showing situation in case of 10 expected fatalities [4] For given dam the maximum tolerable failure probability should be specified taking into account previous two aspects – material losses and loss of lives. The above mentioned procedure gives the guidance for the determination of appropriate probability of the flood for the dam safety assessment during the reconstruction (hereinafter called as “check flood”). The specified probability has to be adapted to the period of the hazard situation, e.g. decreasing of the dam crest level, dismantling the spillway entrance, etc. It reads: 𝑃 𝑎 𝑑 𝑗 𝑢 𝑠 𝑡 𝑒 𝑑 =1−(1− 1 𝑁 ) 𝑇 [3] where: T is the length of hazardous phase during reconstruction; N is the return period of the flood wave related to one-year occurrence. It is evident that in case of T < 1 year the required return period may be shorter. In tab. 2 the required return periods N are related to the hazardous period T for originally determined return period N1year = 1 000 years. Table 2 Approximate required return periods N corresponding to hazardous phase duration T T (months) N (years) 12 1000 6 500 2.4 200 1.2 100 1258