Hydraulic Drag: A Meander-Initiating Mechanism

Abstract

Previous work has shown that meandering is an intrinsic property of stream flow that is essentially independent of the effects of sediment. This can be easily demonstrated on a simplified sediment-free stream “table” or plate composed of an inclined hydrophobic plane surface on which a small stream of water is allowed to flow. Development of a meander series from a straight and then sinuous stream is induced by increasing water flow. A study of water motion by injecting ink into the stream reveals a genetic relationship between sinuous water motion (meandering thalweg) of straight reaches and the bends of meandering streams. Dimensional ratios of wave length of meandering thalweg to stream width in the straight stream or stream width to meander length and radius of curvature in the meandering stream closely match those of larger streams in nature. Variations of water depth along straight or meandering experimental streams match, more or less as mirror images, the familiar riffle and pool undulations of the stream bed in straight reaches or the crossings and deeps of meandering streams in nature. Pools apparently are analogous to deeps, and riffles to crossings. Introduced silicon carbide granules form point bars spaced approximately two to three times the stream width in the straight stream and five to seven times the stream width in the meandering stream in accordance with field observations. Reversing helical flow can be demonstrated in the meander bends but this is an exaggeration of sinuous water motion or hydraulic drag already present in the straight stream. A physical model demonstrating the initiation of meandering is also described which apparently is, in essence, similar to the initiating mechanism responsible for production of beach cusps and other evenly spaced longitudinal current-formed structures.