Page - 1544 - in Book of Full Papers - Symposium Hydro Engineering

is possible to determine the existence of hydraulic jump, and if there is, proceeding to its location. The occurrence of a hydraulic jump inside the diversion tunnel involves a correction of the upstream altimetry position. The calculation of the flow depths by defining curves backwater enables the calculation of the total momentum into the river diversion enabling comparison with the total momentum in the natural bed and the location of the hydraulic jump. 5.5. MULTIPLE DIVERSION TUNNELS For at least two reasons, the designer may choose multiple (mostly twin) tunnels to carry a large design flood: (I) large cross-section needed to divert large flood is associated with specific problems such as higher tunnel stability risks and expensive support measures, need for more complicated excavation or support equipment, and more risks brought about for the abutments specially when the overburden or the rock quality is not sufficient; (ii) the design flood may not happen during construction and the discharge in a great majority of construction days is much below the design flood. Therefore, twin tunnels may be constructed, one to carry the usual low and average discharges another with a higher invert elevation to carry the additional part of the design flood. When the second one is constructed sufficiently higher than the first, it may be mostly used as access road tunnel between upstream and downstream parts of the dam during construction, a double arch 180m high concrete dam under construction in southwest of Iran). The designer may choose to apply support but avoid RC lining in the second tunnel. 6. TUNNEL EXIT It is also essential to design the entry and exit points of the tunnel very carefully. Where the tunnel emerges out of the hill slope, a structure in the form of an arch is usually provided, which is called the portal. Since at this point the water leaves the tunnel, it is prone to hydraulic head loss and erosion. Proper transition shape has to be provided to keep the loss minimum and to avoid cavitations. The length and slope of the transition depends upon the velocity and flow conditions prevailing in 1544