Design of Side Weirs in Subcritical Flow

Abstract

Side weirs are flow-diversion and flood control devices widely used in urban drainage, land drainage, and irrigation systems. The De Marchi equation is widely used for computation of side-weir flow. However, in this equation the variation of flow depth with longitudinal distance is expressed in implicit form. Many investigators focused on the relation between the coefficient of discharge and the upstream Froude number and other flow and/or geometric parameters. Presented herein is a general design method for side weir. Also presented is a concept that for subcritical flows over a side weir the flow characteristics are governed by downstream flow conditions and the geometric parameters of the channel and the side weir. The study is based on De Marchi's equation. The analysis is conducted through non-dimensionization of the parameters in the equation. The study results indicate that the side weir flow is a function of the downstream flow depth and Froude number, and the ratios of weir height to downstream depth and channel width to the product of weir length and discharge coefficient. Charts were produced and procedures were given for engineering design.