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Hydraulics of water, air-water and sediment flow in downstream-controlled sediment bypass tunnels

While most sediment bypass tunnels (SBTs) are gate-controlled at the intake, so that the flow regime downstream of the gate is free-surface, some SBTs are controlled at their downstream end, e.g. due to spatial restrictions hindering the arrangement of the gate at the intake. For downstream control, the flow regime may change from free-surface via transitional to pressurized, mainly depending on the gate opening, the bed slope and the available head at the intake. Because the flow regime has a considerable effect both on the sediment transport and the transport of air being potentially entrained at the intake, the hydraulic design is crucial for the safe and reliable operation of SBTs. A poor design may have severe consequences such as pressure fluctuations, air blow-up, gate vibrations and cavitation erosion, thus compromising the structural safety. This paper deals with the hydraulic model investigation of the rectilinear Patrind SBT, Pakistan, conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich, Switzerland. The SBT has the following key features (referring to the optimized design): downstream-control by a tainter gate, length of 181.5 m, archway section over length of 140.5 m with b x h = 7.5 x 8.5 m and maximum discharge capacity of some 900 m 3 /s at full supply level. The main foci of the present study are on problems with air entrainment, transport and detrainment as well as on sediment transport for downstream-controlled SBTs. Mitigation measures such as the placement of detrainment devices are discussed.