Page - 1259 - in Book of Full Papers - Symposium Hydro Engineering

3. SAFETY ASSESSMENT DURING THE FLOOD At the dam safety assessment during floods the maximum water level (MWL) reached while the check flood wave passes the reservoir is compared with the so- called “threshold water level” (TWL) at which the dam is still considered as safe enough (fig. 3). Fig. 3 Scheme depicting the water levels considered in dam safety assessment During the assessment the reservoir water level drawdown during remedial works is searched to satisfy the condition MWL ≤ TWL for given check flood (chapter 2) taking into account the chart flow of the reconstruction works (tab. 2). 4. MAXIMUM WATER LEVEL The MWL is determined via flood routing procedure for the check flood specified in chapter 2. It is obvious that the construction methods and their duration strongly affect dam safety and its failure probability during refurbishment. The flow chart of the construction steps should be therefore elaborated in variants in advance to enable specification and negotiation of necessary structural and non- structural arrangements and measures in early stage of the preparation of works. The procedure usually uses trial-and-error method for searching initial reservoir water level (representing the drawdown) during the construction and recommendations on the arrangements on manipulation with water and technical arrangements on appurtenant works. The flood routing through the reservoir should take into account all appurtenant works available during the reconstruction period. The relevant considerations should contain expected reliability and functionality of bottom outlets and available intakes including possible adaptable measures (e.g. forced opening) in hazardous situation. An example is the opening of the side wall of the emergency spillway at the Mostiště dam (fig. 4) which would attenuate arriving flood more efficiently. Operative measures can be also adapted when the reconstructed dam belongs to the dam cascade to make the flood routing through the system more efficient. 1259