Page - 969 - in Book of Full Papers - Symposium Hydro Engineering

these structures despite the missing possibility of passing the design flood through (n-1) gates a more sophisticated method using the (n-a)-concept [2] is being used since decades. The parameter ‘a’ considers both the revision and the malfunction of the gates. Under favourable conditions ‘a’ can accommodate values lower than 1.0. The event tree concept presented below can be considered as a base for a future alternative way for determining and proving the reliability of gated weirs and spillways if the (n-1)-criteria cannot be fulfilled. It should be understood as a useful supplement to the very robust (n-1) design method. The event tree concept does not strive to replace any check flood design cases. 2. EVENT TREE CONCEPT 2.1. BASICS A new design approach, that has the potential to replace respectively to complement the (n-1) condition, was developed at the University of Innsbruck in cooperation with different partners. It is based on the idea, that the required level of safety is determined at the beginning of the design process. For example, it is accepted for weirs, that the maximum operation water level in the reservoir is exceeded with a probability of 0.01 each year (return period of 100 years). The design has to take into account four variables: the number of gates, the total capacity of the weir respectively the spillway of the dam, the duration of ‘critical works’, where the gates or their drive systems are under revision and not available for flood discharge, and the probability of the malfunction of one gate. Empirical values for the latter two variables have to be defined carefully. Within the scope of this critical step of the conceptual work for the event tree concept data gained from literature, experiences of the operators and fundamental considerations concerning the overall safety level have to be taken into account. The present paper will explain the basic design approach in detail. Detailed recommendations for the numerical values of the related variables for the duration of the critical works and the probability of the malfunction of on gate will be given after a period of detailed investigations, comprehensive discussions and sound verifications. Generally, the event tree concept can be applied for all kind of weirs and spillways with very different gated and not gated openings. Anyhow, within the scope of the present work the presentations, relations and equations are related to gated weirs and spillways with an optional number of gates of the same size and type. 969