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Person# Daniel Vito Papa

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Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained.

An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effe

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Christophe Ancey, Daniel Vito Papa

Braiding is a complex fluvial process in which sediment-laden water flows are split into multiple threads. Thread joining and splitting at the nodes occurs continuously in the floodplain. Understanding how braiding responds to external factors is a key issue. A number of braided-pattern intensity indices have been proposed to characterize the degree of braiding. Due to the inherent complexity of their dynamics braided rivers exhibit a complex morphology that is difficult to study using simple indices. In this study, we propose a series of experiments to investigate the influence of sediment supply in the evolution of the braiding patterns. To that end, we use a set of morphological indicators, including the index proposed by Redolfi et al. (2016). The experiments are carried out in a 4-m-long and 1-m-wide flume. The bed is made of moderately sorted sand grains whose median diameter is 1 mm. A classic braided pattern configuration composed of various channels, confluences and bifurcations is observed. The experiments satisfy the Froude similarity criterion. Each run starts from an initial straight channel with rectangular cross section. After a certain time (approximately 40 hours) the system reaches equilibrium. This equilibrium is then perturbed by suddenly increasing the sediment feeding until a new point of equilibrium is reached (this takes approximately 40 hours again). During the whole run, bed elevation and water height are measured optically using laser sheets projected on the bed surface. Cross sections are calculated at 2 cm intervals from each others. The entire morphology is then obtained from these cross sections every hour. Sediment is feeded at the flume inlet and sediment transport rates are measured every 15 minutes at the flume outlet.

2017Christophe Ancey, Daniel Vito Papa

Braided rivers are highly dynamical systems characterized by varying network-like structures even under quasi-steady conditions. Understanding their dynamics is crucial in geomorphology and river engineering (e.g., river restoration in Alpine and piedmond streams). Open questions about these dynamics include the definition and quantitative description of bed equilibrium. Here we propose to tackle this problem using a new method based on graph theory. This algorithm, called low-path allows one to extract the network structure of a braided river from its Digital Elevation Model (DEM). It is then possible to quantify and analyse the dynamics of the braided system, and not just the bed evolution, as has been done in earlier studies. To assess the dynamics and equilibrium of a braided river, we study two runs representing two distinct phases of the same braided river: the transition from a single channel to a braided river (run 1) and the equilibrium state of this river (run 2). A set of control parameters was used to characterise both runs and supplement the low-path method. We find that although a clear distinction can be made between straight channel and braided channel for both methods, it is more difficult to distinguish between transitional braided and equilibrium braided rivers. Finally we propose a set of dimensionless numbers that specify the braided network and can be used with numerical or stochastic simulations of a braided network. To illustrate their utility, we apply the Low Path method to a real Alpine braided river (the River Navisence, Wallis, Switzerland) and compare the results to our experimental data.

2020Braided rivers form some of the most fascinating fluvial patterns found on Earth. They are identifiable by their unique morphology of complex networks of intertwined channels that spread across wide floodplains. Detailed knowledge of their dynamics is needed to define proper river management strategies that can address both human needs (e.g. protection against floods, bank migration, etc.) and natural needs (e.g. the preservation of fauna and flora, river restoration, etc.).
Recently, the study of braided rivers has undergone significant progress. Developments in the areas of laboratory experiments, monitoring techniques and field surveys, in addition to new paradigms in the field of geosciences and mathematical modelling have greatly improved our understanding of braided rivers. However, many questions remain unanswered. Is it possible to predict the long-term evolution of a braided river under steady flow conditions? More fundamentally, where do the braided pattern emerge from? Does it grow out of an intrinsic flow instability? And, if this is the case, which one? The present work aims to fill two specific gaps in the current state of knowledge: the dynamics of braided river networks and the development of a morphodynamic model that uses a non-equilibrium bedload formula that can predict bedforms that ultimately produce braiding.
This thesis studied the dynamics of the braided networks experimentally. Two laboratory-scale experiments were performed from which we extracted and investigated the braided network's temporal evolution. A set of variables describing the network was determined -namely the number of nodes, the number of links and the network's total link length. These variables were shown to relate to the flow conditions. Moreover, the evolution of the braided network was described by identifying similar network configurations as modes. The modes' evolution was well captured by their probability. Using a Markov process, we were ultimately able to reproduce the probability of occurrence of those modes.
A morphodynamic model based on the shallow-water equations and a stochastic-based bedload transport formula was developed. Applying linear stability theory, we were able to obtain marginal stability curves that predicted the development of bedforms. Two types of bedforms were identified: two-dimensional bedforms (antidunes and dunes) and three-dimensional bedforms (bars). The results agreed well with the literature. The present work was the first morphodynamical model to predict the development of both dunes and bars within the same framework using shallow-water equations. Moreover, we were able to show, albeit qualitatively, the influence of particle diffusion-present in the bedload transport equation-in the development of bedforms.