Sediment transport and bed morphology at river channel confluences

Abstract

River channel confluences form important morphological elements of every river system, being points at which rapid changes in flow, sediment discharge and hydraulic geometry must be accommodated. This article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence.Bed morphology at channel confluences is characterized by three distinct elements : avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone formed at the downstream junction corner. These elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels. As confluence angle and discharge ratio increase, the sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transported around rather than through the centre of the confluence. This segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence, an increase in the scour depth, a change in the orientation of the scour and an increase in the size of the separation zone bar. Field measurements closely replicate the flume simulation.A model of sediment transport and bed morphology links these features to the fluid dynamics of these sites. An understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record.

Field investigation of three‐dimensional flow structure at stream confluences: 1. Thermal mixing and time‐averaged velocities

“…Data on W and V were used to develop vector plots depicting velocity magnitude and orientation parallel to the plane of each cross section. (Table 1), actually has a slightly larger momentum flux than the major tributary (Table 2) [Best, 1988] and with past studies of formative events at this site Kenworthy, 1995, 1998;Rhoads, 1996 [Allen, 1982]. The slope along the leading edge of the sediment wedge at KRTMS is slightly steeper (21 ø) than the slope at KRCS but still considerably less than the angle of repose.…”

“…The body of research focusing on confluences includes laboratory experiments [Mosley, 1976;Best, 1986Best, , 1987Best, , 1988 A particular shortcoming of field investigations that have attempted to reconcile this controversy has been a reliance on two-dimensional measurements of velocity within a CHZ characterized by highly three-dimensional fluid motion. McLelland et al [1996] and DeSerres et al [1999] obtained velocity measurements in three dimensions, but these data are only quasithree-dimensional in the sense that either (1) two twodimensional (2-D) electromagnetic current meters with rather large sampling volumes relative to the total flow depth were placed close together in the flow and oriented orthogonally to one another or (2) two sets of measurements were obtained at the same point in the flow at successive time intervals with a single 2-D electromagnetic meter.…”

Form roughness and the absence of secondary flow in a large confluence–diffluence, Rio Paraná, Argentina

“…Research investigating the processes of flow at river channel confluences has highlighted the possible importance of secondary flows and their controls on channel geometry, channel dynamics, sediment transport and flow mixing (e.g. Ashmore et al, 1992;Best, 1986Best, , 1987Best, , 1988Best and Reid, 1984;Best and Roy, 1991;Biron et al, 1993Biron et al, , 1996Bradbrook et al, 1998Bradbrook et al, , 2000Bradbrook et al, , 2001Lane et al, 2000;McLelland et al, 1999;Mosley, 1976;Rhoads and Kenworthy, 1998;Rhoads and Sukhodolov, 2001). The classical model of flow at a confluence involves divergence between near-bed and nearsurface flows as a result of the interaction between pressure gradient forcing and bed topographic steering.…”

Hidrodinâmica da Confluência dos Rios Cuiabá e Paraguai, Pantanal Mato-grossense

“…De acordo com a descrição das zonas hidrodinâmicas em confluências proposta por Rhoads e Sukhodolov (2001) Em relação aos parâmetros hidráulicos, a seção 13 no rio Paraguai, anterior a confluência, apresentou menor potência de canal específica (entre 0,01 W/m 2 e 0,11 W/m 2 ) (Tabela 2). A maior potência de canal específica foi registrada na seção 16 após a confluência que, segundo Best (1987Best ( , 1988, compõem a zona de aceleração de fluxo. Uma das hipóteses previamente levantadas era que o rio Cuiabá possuía capacidade de descarga suficiente para dominar o fluxo na confluência.…”