Page - 1260 - in Book of Full Papers - Symposium Hydro Engineering

Fig. 4 Opened side spillway during the dam reconstruction 5. THRESHOLD WATER LEVEL The TWL is determined for each construction phase according the actual state of the dam and its appurtenant works. The vulnerability of the dam has to be assessed during the construction. Mostly the effect of surface erosion is considered. This is generally represented by the overtopping of the dam body and erosion of subsoil at foundation of the spillway or chute. The critical phases may be as follows: − the removal of the breakwater or parapet wall from the dam crest; − decreasing the dam crest due to increase of clayey core; − necessary stabilisation of upstream slope/face; − opening of the spillway chute; − reconstruction of the chute and stilling basin adjacent to the dam body, etc. During the construction period, temporarily arrangements improving (heightening) the TWL may be installed. Theses consist of the following elements: − cofferdams, fusegates or protecting walls are increasing TWL and protecting the construction site against raising of water level in reservoir (fig. 5); − wavebreaking arrangements like riprap fill; − sandbags located along the dam crest to protect it against overtopping (fig. 6). 1260