Page - 1543 - in Book of Full Papers - Symposium Hydro Engineering

5.3. NUMBER AND SIZE OF TUNNELS The proper selection of number and size of diversion tunnels depends on hydraulic requirements, the maximum size of tunnel which it is practicable to drive in the rock encountered, operating flexibility, and economy of construction. In some instances it may be necessary to limit the size of tunnel in yielding rock to prevent excessive movement in the surrounding rock. Generally the larger the tunnel within practicable driving limits, the more economical it is per unit flow capacity. Also, the approach and discharge channels and the intake structure usually are more economical for fewer tunnels because the reduced width results in less channel excavation and a narrower intake structure. However, if it is necessary to use two parallel gates in each larger tunnel and only one in the smaller, the intake structure may not be reduced and economy may not be affected by use of larger tunnels. The most economical number of tunnels which will meet the other requirements should be used. A single tunnel is used when, as is frequently the case, the required diameter to handle the entire discharge is not over 20 ft to 25 ft. In order to provide reasonable operating flexibility, a total of not less than two service gates should be provided in the outlet works. In some cases, this requirement may influence the number of tunnels. Detailed design and construction procedures for tunnels are provided in EM 1110-2-2901, A Tunnels and Shafts in Rock[4]. 5.4. INFLUENCE OF DOWNSTREAM LEVELS FOR DIVERSION TUNNELS When dimensioning the river diversion tunnel it is important to study the influence of the downstream level into the flow conditions inside the tunnel, once it may not be guaranteed that the establishment of the uniform system within the diversion tunnel ensures that free surface flow occurs along its entire length. In order to proceed with the study of the downstream level influence, it is necessary to analyze the altimetry placement of diversion tunnel. Submerged flow considerably reduces the discharging capacity of the tunnel. A first stage begins by analyzing the total momentum inside the tunnel, comparing it with the natural bed total momentum. In the case of the natural bed total momentum overcome the values obtained in the tunnel, the hydraulic jump occurs inside the tunnel. Thus it 1543