Seite - 1539 - in Book of Full Papers - Symposium Hydro Engineering

above considerations. A minimum height of 2 m is necessary. For mechanized handling of excavated material a minimum section of 2.5m x 2.5 m is required. Approach Channel for diversion tunnel- The dimensions and alignment of the excavated approach channel upstream from the river to the tunnel and of the discharge channel to convey the water back to the river downstream are determined by the hydraulic design requirements and the stable slopes for the material excavated 4. THE TUNNEL SLOPE, LENGTH The tunnel longitudinal slope, S, has no typical value as it depends on the geometry of the river and the valley but generally is of the order of the riverbed slope at the site. The effect of the slope is to add LxS to H. Therefore, if the slope increases, the discharge capacity of the tunnel increases and the required cofferdam height decreases. When S is significantly different from the average value assumed, the effect on the cofferdam height estimated from guides in this study should be accounted for. A circular diameter (D) of the tunnel is used to represent and compute the tunnel cross-sectional area available for the flow. In the case of the horseshoe section, the equivalent circular section should be employed. L depends on the extent of the site needed to remain dry and is typically a few tens of meters more than the base width of the dam. It is generally greater for embankment dams than for concrete dams. In any given project, L is fairly fixed and is not subject to optimization here. In the optimization attempt, D and H are inter-related and vary together. Greater H requires smaller D and vice versa.Longer tunnels are generally associated with smaller optimum diameter- Doptimum regardless of Q and W and the support type. When the tunnel is very long, an overly large tunnel must be avoided and heightening of the cofferdam should be examined instead. It is prudent that some who follow the tradition of determining D only by Q and W often overlook the simple fact of dependency of Doptimum on L. Reservoir depletion tunnel comprises three sub-structures: Inlet Structure, Tunnel body and Outlet Structure. Section of a depletion tunnel studied at Central Water and Power Research Station is shown in Figure 4 1539