Hydraulic Model Tests for Mayfield Power Plant

This report describes the results of the hydraulic model tests for the forebay structure of the Mayfield hydroelectric power plant conducted at the St. Anthony Falls Hydraulic Laboratory during the period May 1973 through March 1974. There are three existing power generating units, each of which is fed by an 18-ft-diameter penstock. Provisions have been made for an additional unit to be installed in the near future. Water is taken from Mayfield Lake, conveyed through a 37-ft-diameter tunnel, and discharged into a forebay. There are four intake structures, one for each penstock, at the downstream end of the forebay. Presently, the operation is hampered at high loading conditions. It is generally believed that unstable hydraulic conditions in the forebay are the cause of the present poor operating conditions. Strong vortices at the intakes have been observed in the field. The flow in the forebay is rough and turbulent. There are large shock waves at the pier noses, causing the water surface to fluctuate and become higher at units 42 and 43 and lower at unit 44. It is planned to add the fourth unit in the immediate future. Because an additional unit would mean an increased flow requirement, it is probable that the problems stated above would worsen. For this reason, a model study has been conducted to assess the problems for the expanded operation and find means of alleviating them. A 1:36 scale model was constructed and tested. After calibration, the model satisfactorily reproduced the existing flow conditions. The model indicated an increase in flow distortion and in water surface instability for the planned expansion. Air-entraining vortices were observed for all flows tested without modification of the existing facilities. On the basis of the experiments with various alternatives, the following schemes are recommended to provide good flow conditions: (a) two 18-ft-wide submerged guide vanes to be installed near the tunnel portal to redistribute the flow and improve the water surface profile and (b) a V-shaped vortex suppressor to be installed at each intake bay to eliminate the air-entraining vortices. A documentary motion picture showing the highlights of the model tests has been made as part of this test program.