Irrigation Tunnel for St. Mary Dam, St. Mary-Milk River Project, Alberta, Canada: Fid-Bau Portal

Irrigation Tunnel for St. Mary Dam, St. Mary-Milk River Project, Alberta, Canada

Model investigations of the irrigation tunnel for St. Mary Dam, St. Mary-Milk River Project J Alberta, Canada, were conducted to examine the over-all hydraulic performance of a tunnel designed to pass a maximum discharge of 3,200 cfs under heads ranging from 25 to 81 ft. The structures included four short, high-pressure intake conduits, flow through which was controlled by four gates the outside pair being 4 ft wide by 5 ft high and the inside pair 5 ft wide by 5 ft high. These conduits discharged into a horseshoe-shaped nonpressure tunnel 17 ft high and 2,500 ft long. A flared outlet portal on the downstream end of the tunnel was designed to give a maximum recovery of velocity head. Model tests (performed on two models built to scales of 1:25 and 1:15) verified the predicted. over-all performance of the structure in that the capacity was found to be approximately that computed, a hydraulic jump formed in the tunnel for all operating conditions, and flow lines in the tunnel were approximately as anticipated. However, tests demonstrated: (a) the need for increasing the length of the intake curves to the high-pressure conduits to prevent cavitation thereon; (b) the desirability of eliminating air vents proposed in the intake conduits except those immediately downstream from the gates; and (c) modification of the outlet portal to obtain better flow distribution into the exit channel.