Seite - 1545 - in Book of Full Papers - Symposium Hydro Engineering

the tunnel, economics, construction limitations, etc. It is generally preferred that a hydraulic model study is conducted to arrive at an efficient but economic transition. tunnel. When a circular tunnel flowing partly full empties into a chute, the transition from the tunnel section to one with a flat bottom may be made in the open channel downstream from the tunnel portal or it may be made within the tunnel so that the bottom will be flat at the portal section. Ordinarily, the transition should be made by gradually decreasing the circular quadrants from full radius at the upstream end of the transition to zero at the downstream end. For usual installations the length of the transition can be related to the exit velocity. An empirical rule which may be used to design a satisfactory transition for velocities up to 6 m/s is as follows: F 1 tan   (10) Where L = length of transition in m, v = exit velocity in m/s, and D = tunnel diameter in m 6.1. OUTLET STRUCTURE The return diverted flow, through the river diversion, can be made through direct connection to the river or by deploying outlet structure. The deployment of the restitution structure may have two objectives, the dissipation of excess energy or the recovery of part of the kinetic energy. The need to implant a restitution structure is associated with the fact that the flow in the tunnel that is supercritical or if the downstream section of the tunnel is elevated compared to the river bottom, hindering establishment subcritical conditions in the restitution area. The outlet structure can match the deployment of a transition structure or deployment of stilling basin. Diversion tunnel discharge can be led to directly in to river depending upon gradient of river. To avoid bank erosion some form of bank protection is provided. In case of depletion tunnel exits, where high velocity may occur, energy dissipation arrangement has to be provided as shown in fig: 6.It works well as part jet is lifted to 350 splitting and spreading before plunging in to river pool. 1545