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

Abstract

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. 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.

Rapidly Migrating Secondary Bedforms Can Persist on the Lee of Slowly Migrating Primary River Dunes

“…The 50th ( D 50 ) and 90th percentile sediment diameter ( D 90 ) were determined following the approach of de Ruijsscher et al. (2020a) and are shown in Figure 1 ( bs 1 through bs 5).…”

“…The RWS data were acquired with an interval of six days, on October 9 and October 15. At the location of the field site, information on the bed grain size distribution is also available, based on samples that have been taken on November 24, 2017 using a Hamon grab (de Ruijsscher et al, 2020b;Eleftheriou & Moore, 2013). The 50th (D 50 ) and 90th percentile sediment diameter (D 90 ) were determined following the approach of de Ruijsscher et al (2020a) and are shown in Figure 1…”

3D upstream passability of novel river training structures by migratory fish in the river Waal

“…These LTDs are river training structures 3-4 km long parallel to the river's shoreline that divide the river into a main channel (230 m wide) and shore channels (80 m) protected from the effects of navigation on river banks and biodiversity (Eerden, 2013;Collas et al, 2018). One of the LTD shore channels was estimated to carry 18-27% of the total river discharge (De Ruijsscher et al, 2020). These structures were constructed with boulders, have an inflow with adjustable boulder sills that are generally fully submerged and an outflow into the main channel of the river.…”

Coupled Morphodynamics of River Bifurcations and Confluences

“…They also serve as key elements in the restoration of pristine multithread patterns, as in the case of the Drau River in Austria (Figure 1c), where a former side‐channel was reopened with the aim of improving the habitat conditions, stabilize the river bed, and ensure flood protection through channel widening (Formann et al., 2007; Habersack & Piégay, 2007). A further example comes from a novel interest of river restoration practices, namely the construction of the so‐called longitudinal training dams to mitigate the effects of shipping on riverine habitat biodiversity and functionality and, at the same time, guarantee the fluvial navigation (Collas et al., 2018; de Ruijsscher et al., 2020) (Figure 1d).…”

“…comes from a novel interest of river restoration practices, namely the construction of the so-called longitudinal training dams to mitigate the effects of shipping on riverine habitat biodiversity and functionality and, at the same time, guarantee the fluvial navigation (Collas et al, 2018;de Ruijsscher et al, 2020) (Figure 1d).…”