The configuration of open-channel junctions controls local sedimentary processes, channel scour, and sidewall erosion through its influence on the flow patterns established as confluent streams compete for limited channel capacity. Entry of a tributary into a mainstream ensures detachment of flow from the channel sidewall immediately downstream from the junction, and flume experiments show well-defined relationships between the dimensions of the separation zone and discharge from the tributary. Other things equal, both the width and length of the separation zone increase systematically with an increase in confluence angle, though it is shown that values of width are much less than predicted by recent mathematical modelling. The zone grows at the expense of the proportion of the channel occupied by the immediate post-confluence flow with the consequence that near-bottom velocity increases by a factor of 1.3 as confluence angle is increased from 15°–90°. The implications for sediment entrainment are reviewed briefly.

Separation Zone at Open‐Channel Junctions

Flow Dynamics at River Channel Confluences: Implications for Sediment Transport and Bed Morphology

Although open-channel junctions are common in many hydraulic structures, no comprehensive data set has been compiled that describes the 3D flow field within the junction itself. This physical model study examined a 90°, sharp-edged, open-channel junction for channels of equal width. Depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region. The average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location. In addition, a 2D mapping of the water surface was performed on a 76.2 mm square grid throughout the channel junction. This paper presents the details of the experimental procedure and the general flow characteristics observed. The full data set generated during this experimental work is available for downloading on the Internet. Using a small portion of the data recorded, an evaluation of several previously proposed theori...

Experiments on flow at a 90° open-channel junction

Although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences, this height differential is largely ignored in many models of junction flow dynamics. Here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence. A difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows, resulting in fluid upwelling at the downstream junction corner. This upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel. These experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites.

Effects of Bed Discordance on Flow Dynamics at Open Channel Confluences

Although awareness of the habitat value of large woody debris in streams has promoted a more environmentally sensitive approach to its management, present guidelines are largely intuitive and do not contain advice for conducting quantitative hydraulic investigations. This review of the literature provides information to assist management, and highlights deficiencies in current knowledge. Hydraulically, debris act as large roughness elements that provide a varied flow environment, reduce average velocity, and locally elevate the water-surface profile. This can significantly increase flood travel time. The significance of debris is scale-dependent. For example, the hydraulic effects are often drowned out in a large flood on a large river. Some hydraulic models can be used to predict the effect of debris removal or reinstatement. A challenge for research is the development of a hydraulically and biologically meaningful definition of debris geometry that can be readily used in the field. When more is known ab...

Environmental Hydraulics of Large Woody Debris in Streams and Rivers

An analysis of the flow behavior at rectangular channel junctions with combining flow using the conformal mapping technique has been presented. Equations for the location of the stagnation point and the free-streamline for different flow conditions at rectangular channel junctions have been developed. These equations have been solved numerically by using Simpsons rule. It is observed that for each channel junction there is a unique value of discharge ratio corresponding to which the stagnation point coincides with the upstream corner of the junction. The value of the discharge ratio depends on the junction angle as well as on the bed-widths of the channels. It is also indicated that the zone of stagnant fluid between the free-streamline and the channel boundary depends on the junction angle and the location of the stagnation point.

Mohan M. Dandekar

Papers

Conformal Mapping for Channel Junction Flow

Discussion of “Resistance Equation for Large‐Scale Roughness” by James C. Bathurst, Ruh‐Ming Li, and Daryl B. Simons (December, 1981)