1 - Alaska Energy Data Inventory

1.05 LAKE TAPA. Genera 1. The 1 ake tap is referred to in the Pol arconsu It report(Exhibit 4) as the "open system/wet tunnel type. II It is called thisbecause just prior to the final blast the entire tunnel between the laketap and the gate structure is filled with water brought in through the gatestructure which is open to th~ atmosphere. The configuration of the tap iss imil ar to the one used in the OKSLA Hydro-power project in Norway. Thelake tap consists of an entrance orifice, a large rock trap and atransition to the 11 ft diameter modified horseshoe tunnel. This opensystem/wet tunnel configuration is a preferred type of lake piercingarrangement because it reduces the blast forces on the service gate anda 1 so reduces the amount of post blast rubb 1 e in the tunnel downstream ofthe primary rock trap. Details of the tap are shown on Plates 11 and 12.B. Orifice.(1) The orifice is a short tube, 12 ft in diameter and 10 ft inlength. The discharge coefficient is 0.81 (reference 21) resulting in amaximum velocity at the vena contracta of 5.7 ft/s for the maximum expecteddischarge of 518 ft 3 /s. The top of the orifice is at approximateelevation 801. The minimum lake elevation was set at 820. The vortexsubmergence is calculated from the equation: S = (O.4)(V)(D)1/2(EM 1110-2-1602) where:S = Required submergence in ft = 7.9 = round to B.OV = Velocity at vena contracta in ft/s = 5.7D = Diameter of entrance conduit in ft = 12.0This minimum lake elevation allows for a requiredB.O ft, 6.0 ft of ice cover and a 5 ft safety factor.vortex submergence ofBl-41