Carmelo Juez

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

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.

Environmental heterogeneity promotes spatial resilience of phototrophic biofilms in streambeds

The loss of environmental heterogeneity threatens biodiversity and ecosystem functioning. It is therefore important to understand the relationship between environmental heterogeneity and spatial resilience as the capacity of ecological communities embedded in a landscape matrix to reorganize following disturbance. We experimented with phototrophic biofilms colonizing streambed landscapes differing in spatial heterogeneity and exposed to flow-induced disturbance. We show how streambed roughness and related features promote growth-related trait diversity and the recovery of biofilms towards carrying capacity (CC) and spatial resilience. At the scale of streambed landscapes, roughness and exposure to water flow promoted biofilm CC and growth trait diversity. Structural equation modelling identified roughness, post-disturbance biomass and a ‘neighbourhood effect’ to drive biofilm CC. Our findings suggest that the environment selecting for adaptive capacities prior to disturbance (that is, memory effects) and biofilm connectivity into spatial networks (that is, mobile links) contribute to the spatial resilience of biofilms in streambed landscapes. These findings are critical given the key functions biofilms fulfil in streams, now increasingly experiencing shifts in sedimentary and hydrological regimes.

Modeling sediment pulses on an armored channel

Dams trap sediments in the upstream reservoir, thus the continuity in sediment transport along the river is interrupted. Several negative aspects related to the lack of sediment supply were observed downstream many dams and the artificial replenishment of sediment was applied as a method to restore natural morphological conditions. The replenishment technique may be seen as a pulse of sediments added to the channel. Once the pulse-like volume of sediments is added into the reach, it is seen like an alteration of the channel geometry producing cross section narrowing and flow acceleration lengthwise the river. The river geometry modification together with the water flow allow erosion and transportation of the replenished material. In this work, the response of the flow to the material added in the channel is studied by means of the 2D shallow water equations model in combination with the Exner equation for the sediment continuity equation. Computational outcomes are compared with experimental data obtained from several replenishment configurations studied in the laboratory. It is observed how the bed fining provokes a larger spread of the material without aggradation within the armored channel.