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

T V Ruijsscher,B Vermeulen,A J F Hoitink

doi:10.1029/2019wr026750

Abstract

AbstractA human‐made entrance to a side channel separated from the river by a longitudinal training dam can be considered a new, emergent type of river bifurcation. To understand the processes controlling the diversion of flow and sediment toward the side channel at such bifurcations, a comprehensive field monitoring program was performed in the Waal River, which is the main branch of the Rhine River in the Netherlands. Local processes govern the flow field in the bifurcation region. The angle between the main river flow and the flow into the side channel increases with decreasing lateral and longitudinal distance to the bifurcation point, which corresponds to the head of the training dam. The general flow pattern can be well reproduced with a uniform depth, potential flow model consisting of a superposition of main channel flow and lateral outflow. For submerged flow conditions over the sill, the side channel hydraulic conditions influence the exchange processes, yet free flow side weir theory describes the flow field at this bifurcation type qualitatively well. The vertical flow structure in the side channel, which governs the sediment exchange between the main channel and the side channel, is steered by the geometrical details of the sill. The presence of the sill structure is key to controlling the morphological stability of this type of bifurcation given its primary influence on bed load sediment import and exerts an indirect impact on suspended sediment exchange.

Highlights

Bifurcations in natural and man-made systems control the division of water and sediment over the downstream branches
A human-made entrance to a side channel separated from the river by a longitudinal training dam can be considered a new, emergent type of river bifurcation
We study a new, emergent type of river bifurcation with a side weir: the entrance of a side channel separated from the river by a longitudinal training dam (LTD) with a bifurcation angle close to zero and a rip-rap sill at the upstream end of the side channel (Figure 1)

Summary

Introduction

Bifurcations in natural and man-made systems control the division of water and sediment over the downstream branches. Bifurcations occur in alluvial fans, braided rivers, fluvial lowland plains, and deltas (Kleinhans et al, 2013), whereas flow division in man-made systems occurs in side-channel spillways (Bremen & Hager, 1989), in lateral outflow channels (Neary & Odgaard, 1993), and at the entrance of side channels (van Denderen et al, 2018). All these bifurcation systems have been extensively studied to address their morphological evolution, which depends on the water and sediment distribution over the two downstream branches. For typical bed shear stresses in sand bed rivers, symmetrical bifurcations are typically unstable, and one of the channels will eventually dominate (Bolla Pittaluga et al, 2015) as a result of the Bulle effect: The curved flow forces a disproportionally large amount of sediment into one of the channels, which is a self-enforcing effect (Bulle, 1926; Blanckaert et al, 2013; Dietrich & Smith, 1984; Dutta, 2017)

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