Idealized Model for the Deflection of Sediment Into Lateral Branches of Lowland Rivers

Abstract

The division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers and distributary delta channel networks. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for man‐made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents cause especially water from the lower parts of the water column to be diverted into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. However, a systematic mathematical analysis of the parameter space has not yet appeared in the literature. Here, we present a comprehensive analysis by way of an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability.

Diversion of Flow and Sediment Toward a Side Channel Separated From a River by a Longitudinal Training Dam

“…Both the stationary behavior of the dividing streamline and the form of the pattern resemble an analytical potential flow pattern for side channel outflow as described by equation () (Kästner, 2019 and Kästner & Hoitink, 2020). When applying the analytical model with the spatial dimensions of the current case study, the resulting flow pattern is qualitatively similar to what is observed in the field (Figure 8).…”

“…Qualitatively, the depth-averaged flow direction over the side weir is well captured by the potential flow model of Kästner (2019) and Kästner and Hoitink (2020) based on superposition of uniform main channel flow and lateral outflow. The underestimation of the flow angle into the side channel is likely caused by underestimation of the parameter = v 0 ∕u 0 from field observations.…”

“…The analytical potential flow model for side channel outflow as discussed by Kästner (2019) and Kästner and Hoitink (2020) is used in the present study. They assumed a rectangular cross section of the main channel with uniform depth and represented the diversion as a rectangular section with the same width as the main channel.…”

“…Dashed lines denote upstream and downstream ends of the sill. (b) Same as (a) but including the estimate of over the sill from a potential flow model(Kästner, 2019 andKästner &Hoitink, 2020). (c) Flow angle (blue) and water level z w (red) versus time at 1 m above the LTD sill from H-ADCP data at x = 75.5 m (dotted line in a and b).…”

Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

“…Due to the strongly asymmetric planform of the bifurcations, the division of sediment is likely influenced by the Bulle effect. At an idealized lateral diversion with similar geometry as the Kapuas bifurcations, about 1.5 as much bedload as water is diverted (Kästner & Hoitink, ). The sediment‐to‐water division ratio of the suspended sediment is closer to unity (Meijer & Ksiazek, ) but still strongly influenced by the Bulle effect (Dutta et al., ).…”