Headcut Development in Vegetated Earth Spillways

Abstract

Vegetated earth (soil and rock) auxiliary or emergency spillways have been used extensively on flood control reservoirs within the United States. Despite their widespread use, the processes by which these spillways erode during extreme events are only imperfectly understood, and there is a need for procedure to better predict spillway performance and safety. Therefore, research utilizing both laboratory and field data was undertaken to improve criteria for design and analysis of these spillways. For computational purposes, it was found that erosion of vegetated earth spillways could be divided into three phases. These phases are vegetal cover failure, concentrated flow erosion, and headcut advance. A computational procedure is developed for predicting the time associated with the first two of these phases. The procedure combines simplified flow and detachment rate relations in a form intended to minimize data requirements while allowing application to a broad range of conditions. Results of applying the procedure to predict headcut formation are shown to be generally consistent with available field data. This procedure may, therefore, be used to estimate the time of headcut formation for given flow and channel surface conditions.