Model Analysis of Spillway and Stilling Basin of Porcupine Dam

Introduction: The Engineering Experiment Station at Utah State University was engaged by the Utah Water and Power Board to make a model analysis of the spillway and stilling basin of the Porcupine Dam. This dam will be located on the East Fork of Litle Bear River, just east of Avon, Utah. It will be approximately 650 feet long and 160 feet high, composed of zones earth fill with rock rip-rap facing. The spillway will have a reinforced concrete inlet section and the remainder of the chute and stilling basin will be excavated out of the rock and leck unlined. The design-flow and the elevation of the spillway crest were both determined by the Water and Power Board. The purpose of the model study was to design an economical spillway that would carry the design-flow with a minimum of head over the crest. Also, studies were made to determine whether or not there were danger of the toe of the dam being undermined by the swirling waters from the stilling basin. Several radically different designs were tested as were a number of variations in each design. Only part of these are detailed in this report. Pictures of the final design are included