Page - 968 - in Book of Full Papers - Symposium Hydro Engineering

EVENT TREE CONCEPT FOR DESCRIBING THE RELIABILITY OF GATED WEIRS AND SPILLWAYS Markus AUFLEGER Head of the Unit Hydraulic Engineering, UNIVERSITY OF INNSBRUCK Barbara BRINKMEIER PostDoc at the Unit Hydraulic Engineering, UNIVERSITY OF INNSBRUCK AUSTRIA 1. INTRODUCTION The safety and reliability of dams and weirs depend – among other things – on the functionality and reliability of the gates, which are used to control the flow. Various design approaches exist in different countries. In Germany and Austria and numerous other countries, the so called (n-1) – condition has to be met when designing the gates of dams and weirs [1]. This deterministic design concept aims at a safe discharge of the design flood, assuming one gate is out of operation. Typically, the return period of the design flood is of the order of 100 years for weirs and 1.000 to 5.000 years for the spillways of large dams. In addition to the above mentioned design flood, which has to be discharged through (n-1) gates, typically a much higher ´check flood´ has to be passed through the weir or spillway in a safe way. In Germany this ´second design flood´ (or ´check flood´) has a return period of 1.000 years for large weirs and 10.000 years for large dams, whereby all available openings and outlets can be used for flood discharge. The specific design rules for the safe passage of the ´check flood´ vary significantly between different countries. In many cases the (n-1) concept ensures a very robust and safe hydraulic design of weirs and spillways weirs. The probabilistic approach behind this concept is that the coincidence of a blockage of two or more gates with a design flood is very unlikely. Anyhow, there remains a number of aspects which cannot be taken into account with this very clear and simple (n-1) - approach, among other things - the number of gates - the type of gates - the revision concept - the steering units (inter alia reliability and type) - the power supply (inter alia redundancy). In many cases existing weirs and less frequently existing spillways do not fulfil the (n-1)-criteria. Many of them were carefully investigated and upgraded with additional measures (e.g. additional lifting units). For proofing the reliability of 968