Mass, Energy, and Momentum Flux Analysis of a Bendway Weir

Bendway weirs are hydraulic control structures found in watercourses that range in scale from small streams to large rivers. They are designed to be submerged, broad-crested weirs, typically placed in groups on the outer bank of river bends. They are angled upstream, in order to redirect the flow that passes over them away from the bankline and toward the center of the channel. Although there exists a wealth of valuable guidance concerning best practices for bendway weir design, this guidance is derived mostly from engineering experience, and from physical and numerical model studies. To date, there exists no fundamental theoretical development of the mechanism by which bendway weirs function. Therefore, in order to provide this theoretical foundation, this paper presents a formal mass, energy, and momentum flux analysis of a bendway weir. The analysis yields a detailed description of the basic functionality of a bendway weir and suggests a means of reconciling the results from several extant modeling studies that appear to conflict. In addition, the paper provides design recommendations resulting from the analysis for both individual bendway weirs and for bendway weir groups. (C) 2017 American Society of Civil Engineers.