Carles Ferrer-Boix

Influence of the sediment supply texture on morphological adjustments in gravel‐bed rivers

The role played by the texture of the sediment supply on channel bed adjustments in gravel-bed rivers is poorly understood. To address this issue, an experimental campaign has been designed. Flume experiments lasting 96 h in a 9 m long, 0.60 m wide have been performed with different sand-gravel mixtures as feed textures. The response of the surface texture has been found to be highly dependent on the grain size distribution of the feed. When the feed texture included gravel, the finest fractions of the sediment supply infiltrate beneath the surface. Conversely, sand remains on the surface when the feed texture lacks gravel. This different textural response becomes obscured when water discharge increases. Further, the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture. When a small proportion of gravel is part of the feed texture, bed load transport rate asymptotically approaches the feed rate. However, when a significant fraction of gravel is part of the feed grain size distribution, bed load transport rate approaches the feed rate by following an oscillatory path. These findings have been verified in terms of a one-dimensional numerical model. This modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture, the more evident is the nonasymptotic transient trend toward equilibrium.

Channel evolution after dam removal in a poorly sorted sediment mixture: Experiments and numerical model

Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: (a) how does an initial channel excavated into the deposit evolve, and (b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection-like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width, and bed grain-size distribution, which when solved numerically, describe the observed channel processes.

Channel adjustments to a succession of water pulses in gravel bed rivers

Gravel bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply. The evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers. However, the role of successive floods on the surface texture evolution is still poorly understood. An experimental campaign in an 18 m-long 1 m-wide flume has been designed to study these issues. Eight consecutive runs, each one consisting of a low-flow period of variable duration followed by a sudden flood (water pulse) lasting 1.5 h, have been conducted. The total duration of the experiment was 46 h. The initial bed surface was created during a 280 h-long experiment focused on the influence of episodic sediment supply on channel adjustments. Our experiments represent a realistic armored and structured beds found in mountain gravel bed rivers. The armor surface texture persists over the duration of the experiment. The experiment exhibits downstream fining of the bed-surface texture. It was found that sorting processes were affected by the duration of low-flow between flood pulses. Since bed load transport is influenced by sediment sorting, the evolution of bed load transport is impacted by the frequency of the water pulses: short interpulse durations reduce the time over which fine material (transported as bed load) can be winnowed. This, in turn, contributes to declining reduction of the bed load transport over time while the sediment storage increases.

Principles of Bedload Transport of Non-cohesive Sediment in Open-Channels

This text addresses the particular case of motion and causes of motion of granular material as bedload in the fluvial domain. The aim is to perform and overview of key concepts, main achievements and recent advances on the description of the processes involved in erosion, deposition and transport of sediment in open-channels. The theoretical functional relations describing both the initiation of motion and the sediment transport are introduced. The classical problem of the initiation of motion of particles is treated at grain and at reach scales, accounting for the stochastic nature of flow. Concepts of granular kinematics and methods for quantifying the sediment transport rate in rivers are presented. The latter results from the interactions between the flow and the particles on the bed surface. The sediment transport rate, which has been shown to have a stochastic behaviour, is converted to a lumped statistic distribution. Finally, some field and laboratory techniques for measuring sediment transport, accounting for its inherent fluctuations, are introduced.

Geomorphology and river restoration: dam removal monitoring in Gipuzkoa

espanolSe presenta la metodologia disenada para el seguimiento geomorfologico del derribo de azudes, asi como los resultados obtenidos hasta el momento en dos casos concretos: las presas de Mendaraz (rio Urumea) e Inturia (rio Leitzaran) en Gipuzkoa. Se han realizado secciones transversales, abundantes mediciones de procesos sobre testigos, analisis de los nuevos depositos generados y muestreos granulometricos y morfometricos. En el caso de Mendaraz se han registrado rapidas movilizaciones de sedimentos e importantes cambios geomorfologicos aguas arriba y abajo del obstaculo. Este proceso de recuperacion de la dinamica fluvial natural y de regularizacion del cauce fue favorecido y acelerado por la crecida extraordinaria de noviembre de 2011. Ademas de sus beneficios para el estado ecologico, se ha constatado que el derribo de presas es una medida eficaz para la restauracion de la dinamica geomorfologica en cauces fluviales. El seguimiento geomorfologico es una labor fundamental para cuantificar y valorar la dinamica generada a raiz del derribo de la presa y toda su evolucion posterior. EnglishA methodology for dam removal monitoring and the results of two case studies is presented: Mendaraz dam (Urumea River) and Inturia dam (Leitzaran River), both located in Gipuzkoa. This monitoring is conducted by river survey cross-sections, measurements of processes and granulometrical analysis. Fast sediment erosion and sedimentation together with geomorphological adjustments were detected after dam removal. In Mendaraz an extraordinary flood favored these processes. The recovery of fluvial dynamics shows benefits not only from the ecological point of view but also for the restoration of natural river dynamics. Geomorphological monitoring is a key tool to quantify and assess river evolution and dynamics after dam removal.

A numerical approach for analysing the performance of a sewage screening chamber

The flow within a sewage screening chamber can be highly complex due to high-flow conditions and the geometrical constraints imposed by the screening chamber itself. This paper presents an application of a two-dimensional depth-averaged shallow flow model for flow simulations in a sewage screening chamber. For this purpose, the shallow flow model is improved by including a simple turbulence model. The improved model is able to reproduce the main features of the flow field, as confirmed by comparing with the measurements resulting from a scale-down laboratory model. The potential of numerical model for analysing the performance of the screening chamber is then demonstrated by testing a new design of including flow dissipating blocks to (i) re-distribute more evenly the flow through different flow passages installed with screens and (ii) reduce maximum mean flow velocity approaching the screens, so as to reduce the risk of damaging the screens during high-flow conditions.