Page - 1118 - in Book of Full Papers - Symposium Hydro Engineering

Hazard scenarios and load cases In order to start with the structural safety assessment, the specialist engineer defines the operational and hazard scenarios (load cases) according to the Enforcement Guideline: usual, unusual (1000-year return period flood while one weir opening blocked for maintenance) and extreme (probable maximum flood and 5’000-years return period earthquake respectively). Other hazard scenarios to be evaluated either quantitatively or qualitatively are erosion due to high velocity flows, abrasion due to debris transport, impact due to floating debris, sediment deposition on the gates, jamming of gates due to floating debris, rapid water level changes due to rapid closure of the turbines or of the control technology. Flood safety assessment The safe passage of a 1000-year return period design flood has to be guaranteed with sufficient freeboard and under the assumption that the gated discharge work of largest capacity is blocked (“n-1 rule”). In this case no damage and no uncontrolled overflow must occur. The “n-1 rule” also applies to side embankments or dams in the vicinity of the installation. For Flumenthal and Ruppoldingen, 1’400 m3/s and 1’550 m3/s are considered respectively. The probable maximum flood (PMF) must pass the dam without causing it to surpass its critical water level, i.e. level above which stability may be impaired. Overflow and uncritical damages of the installation are allowed. The PMF is evaluated 20% higher than the corresponding 1000-year design flood for each plant. Seismic safety assessment (SSA) The seismic evaluation earthquake (SEE) for these large run-of-river installations is assessed on the basis of a 5’000-year return period for all water- retaining and safety-critical appurtenant structures of the installation as evaluated during visual inspection. The main objective of the SSA is to protect the population against loss of life and property damage. To fulfill this goal, the main and sole minimal requirement for the SEE is that no failure with uncontrolled release of water occurs (in accordance to ICOLD 2016 [5]), i.e. non safety-critical damages, such as permanent deformations or structural sliding are allowed if the main objective is fulfilled. The SSA applies also to the gates, that could suffer partial or total failure with uncontrolled water loss. Their functionality, which has to be rapidly restored after the earthquake, has to be verified. The 1’000-year return period earthquake applies to side embankments or dams in the vicinity of the installation. 1118