Transport of suspended sediments under the influence of bank macro‐roughness

Abstract

River restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro-roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier-fed streams and very low-gradient reaches of large catchment rivers, these lateral cavities may trap these sediments. Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. Herein, we analyse the influence of these macro-roughness elements on the transport of fine sediments in the main channel. Laboratory tests with uniform flow charged with sediments in a channel with banks equipped with large-scale rectangular roughness elements were carried out. The laboratory experiments covered a wide range of rectangular cavity geometrical configurations and shallowness ratios. The influence of key parameters such as flow shallowness, geometric ratios of the cavities and initial sediment concentration was tested. Surface particle image velocimetry, sediment samples and temporal turbidity records were collected during the experiments. The amount of sediments captured by the cavities, the temporal evolution of the concentration of sediments in suspension and the flow hydrodynamics are cross-analysed and discussed. It is shown that the trapping efficiency of the macro-roughness elements is a clear function of the channel geometry and the shallowness of the flow.

The Origin of Fine Sediment Determines the Observations of Suspended Sediment Fluxes Under Unsteady Flow Conditions

“…The suspended sediment yields at the channel inlet and outlet were possible to calculate since both water level and SSC were measured simultaneously and continuously. The SSC measurements were made at a distance from the bottom and were empirically verified to provide a value corresponding to the depth-averaged SSC (approximately 12 mm from the bottom of the channel; Juez et al, 2017Juez et al, , 2018. Furthermore, by assuming that the SSC measurements were equivalent to the section-averaged values, and assuming that the water level was constant across the channel, it is possible to integrate the section averaged SSC over the section and obtain an estimation of the total sediment yield for the upstream (T1) and downstream (T2) sections of the reach under analysis.…”

“…Additionally, to isolate sediment sources in the field under varying sediment supply conditions is difficult. This point is extremely important in water bodies that undergo an alteration in their sediment continuum cycle by the effect of reservoirs (Schleiss et al, 2016), during hydropeaking (Meile et al, 2011) and in restored rivers with different prerestored sediment dynamics conditions (Juez et al, 2017(Juez et al, , 2018. Episodic and varying sediment supplies cause the channel to alternate over time between supply-limited and transport-limited situations, leading to an impact on channel morphology (Hassan et al, 2005).…”

Numerical Modelling as a Support Tool for River Habitat Studies: An Italian Case Study

“…Nowadays, the use of numerical models for evaluating the effects of multiple natural and anthropogenic stressors in impacted ecosystems is becoming paramount and widely diffused also because of the improvements in computational capacity. In fact, several modelling tools, both commercial and freeware, are now available and can be applied to evaluate the dynamics of river ecosystems at the appropriate spatial and temporal scales [2,3,17], as well as to assess the benefits deriving from small-and large-scale river restoration projects [18].…”

Influence of lateral embayments on suspended sediment transport under unsteady flow conditions

“…In a previous study, [6], the influence of the lateral cavities on the transport of suspended fine sediments in the main channel was studied considering a constant flowrate. Depending on the geometrical design of the lateral embayments configurations, cavities were filled up partially or.…”