River antidunes and bars: new models and nonmodal analysis

The first part of this thesis deals with river alternate bars. Fluvial bars are regular widespread bed forms that are characterized by vertical and transversal scales which are comparable with the stream depth and width, respectively. Although well-established linear and weakly nonlinear stability analysis have already been performed, no non modal analysis has been proposed yet. We here demonstrate the remarkable non normality of the operator that governs bar dynamics in large regions of the parameter space in fair agreement with our tests in flume experiments. This entails the occurrence of dramatic transient growths in the evolution of bed perturbations. Such algebraic growths suggest a novel explanation, through a purely linear process, of the progressive increase in the dominant bar wavelength that is observed in flume experiments and real rivers during bar inception. The second part of this thesis deals with river antidunes. A supercritical free-surface turbulent stream flowing over an erodible bottom can generate a characteristic pattern of upstream migrating bedforms known as antidunes. This morphological instability, which is quite common in fluvial environments, has attracted speculative and applicative interests, and has always been modelled in 2D or 3D mathematical frameworks. However, in this work we demonstrate that antidune instability can be described by means of a suitable one-dimensional model that couples the Dressler equations to a mechanistic model of the sediment particle deposition/entrainment. The results of the linear stability analysis match the experimental data very well, both for the instability region and the dominant wavelength. The analytical tractability of the 1D modeling allows us (1) to elucidate the key physical processes which drive antidune instability, (2) to show the secondary role played by sediment inertia, (3) to obtain the dispersion relation in explicit form, and (4) to demonstrate the absolute nature of antidune instability

River antidunes and bars: new models and nonmodal analysis

The first part of this thesis deals with river alternate bars. Fluvial bars are regular widespread bed forms that are characterized by vertical and transversal scales which are comparable with the stream depth and width, respectively. Although well-established linear and weakly nonlinear stability analysis have already been performed, no non modal analysis has been proposed yet. We here demonstrate the remarkable non normality of the operator that governs bar dynamics in large regions of the parameter space in fair agreement with our tests in flume experiments. This entails the occurrence of dramatic transient growths in the evolution of bed perturbations. Such algebraic growths suggest a novel explanation, through a purely linear process, of the progressive increase in the dominant bar wavelength that is observed in flume experiments and real rivers during bar inception. The second part of this thesis deals with river antidunes. A supercritical free-surface turbulent stream flowing over an erodible bottom can generate a characteristic pattern of upstream migrating bedforms known as antidunes. This morphological instability, which is quite common in fluvial environments, has attracted speculative and applicative interests, and has always been modelled in 2D or 3D mathematical frameworks. However, in this work we demonstrate that antidune instability can be described by means of a suitable one-dimensional model that couples the Dressler equations to a mechanistic model of the sediment particle deposition/entrainment. The results of the linear stability analysis match the experimental data very well, both for the instability region and the dominant wavelength. The analytical tractability of the 1D modeling allows us (1) to elucidate the key physical processes which drive antidune instability, (2) to show the secondary role played by sediment inertia, (3) to obtain the dispersion relation in explicit form, and (4) to demonstrate the absolute nature of antidune instabilit

Thin-film-induced morphological instabilities over calcite surfaces

Precipitation of calcium carbonate from water films generates fascinating calcite morphologies that have attracted scientific interest over past centuries. Nowadays, speleothems are no longer known only for their beauty but they are also recognized to be precious records of past climatic conditions, and research aims to unveil and understand the mechanisms responsible for their morphological evolution. In this paper, we focus on crenulations, a widely observed ripple-like instability of the the calcite–water interface that develops orthogonally to the film flow. We expand a previous work providing new insights about the chemical and physical mechanisms that drive the formation of crenulations. In particular, we demonstrate the marginal role played by carbon dioxide transport in generating crenulation patterns, which are indeed induced by the hydrodynamic response of the free surface of the water film. Furthermore, we investigate the role of different environmental parameters, such as temperature, concentration of dissolved ions and wall slope. We also assess the convective/absolute nature of the crenulation instability. Finally, the possibility of using crenulation wavelength as a proxy of past flows is briefly discussed from a theoretical point of view

Convective-absolute nature of ripple instabilities on ice and icicles

Film hydrodynamics is crucial in water-driven morphological pattern formation. A prominent example is given by icicle ripples and ice ripples, which are regular patterns developing on freezing-melting inclined surfaces bounding open-channel flows. By a suitable mathematical model based on conservation principles and the use of the cuspmap method, in this paper we address the convective-absolute nature of these two kinds of instabilities. The obtained results show that icicle ripples, which develop at inverted (overhang) conditions, have subcentimetric wavelengths which are unstable when the Reynolds number of the liquid flow (Re) is small and the supercooling is intensive. With the increase in Re, the instability switches from absolute to convective. Ice ripples instead exhibit the opposite dependance on Re and are highly affected by the surface slope. In addition, the evaluation of the so-called absolute wave number, which is responsible for the asymptotic impulse response, suggests a different interpretation of some recent experiments about ice ripples

Effect of river flow fluctuations on riparian vegetation dynamics: Processes and models

Several decades of field observations, laboratory experiments and mathematical modelings have demonstrated that the riparian environment is a disturbance-driven ecosystem, and that the main source of disturbance is river flow fluctuations. The focus of the present work has been on the key role that flow fluctuations play in determining the abundance, zonation and species composition of patches of riparian vegetation. To this aim, the scientific literature on the subject, over the last 20 years, has been reviewed. First, the most relevant ecological, morphological and chemical mechanisms induced by river flow fluctuations are described from a process-based perspective. The role of flow variability is discussed for the processes that affect the recruitment of vegetation, the vegetation during its adult life, and the morphological and nutrient dynamics occurring in the riparian habitat. Particular emphasis has been given to studies that were aimed at quantifying the effect of these processes on vegetation, and at linking them to the statistical characteristics of the river hydrology. Second, the advances made, from a modeling point of view, have been considered and discussed. The main models that have been developed to describe the dynamics of riparian vegetation have been presented. Different modeling approaches have been compared, and the corresponding advantages and drawbacks have been pointed out. Finally, attention has been paid to identifying the processes considered by the models, and these processes have been compared with those that have actually been observed or measured in field/laboratory studies

REAL-TIME MEASUREMENT FAULT DETECTION AND REMOTE CONTROL IN A MOUNTAIN WATER SUPPLY SYSTEM

This work presents an algorithm for real-time fault detection in the SCADA system of a modern water supply system (WSS) in an Italian Alpine Valley. By means of both hardware and analytical redundancy, the proposed algorithm compares data and isolates faults on sensors through the residual analysis. Moreover, the algorithm performs a real- time selection of the most reliable measurements for the automated control of the WSS operations. A coupled model of the hydraulic and remote-control system was developed to test the effectiveness of the proposed algorithm. Simulations showed that error detection and measurement assessment are crucial for the safe operation of the WSS

Recovery times of riparian vegetation

Riparian vegetation is a key element in a number of processes that determine the ecogeomorphological features of the river landscape. Depending on the river water stage fluctuations, vegetation biomass randomly switches between growth and degradation phases and exhibits relevant temporal variations. A full understanding of vegetation dynamics is therefore only possible if the hydrological stochastic forcing is considered. In this vein, we focus on the recovery time of vegetation, namely the typical time taken by vegetation to recover a well-developed state starting from a low biomass value (induced, for instance, by an intense flood). The analytical expression of the plot-dependent recovery time is given, the role of hydrological and biological parameters is discussed, and the impact of river-induced randomness is highlighted. Finally, the effect of man-induced hydrological changes (e.g., river damming or climate changes) is explored

Fault detection in level and flow rate sensors for safe and performant remote-control in a water supply system

International audienceno abstrac

Water disinfection by orifice-induced hydrodynamic cavitation

Abstract Hydrodynamic Cavitation (HC) is considered as a promising water-disinfection technique. Due to the enormous complexity of the physical and chemical processes at play, research on HC reactors is usually carried out following an empirical approach. Surprisingly, past experimental studies have never been designed on dimensional-analysis principles, which makes it difficult to identify the key processes controlling the problem, isolate their effects and scale up the results from laboratory to full-scale scenarios. The present paper overcomes this issue and applies the principles of dimensional analysis to identify the major non-dimensional parameters controlling disinfection efficacy in classical HC reactors, namely orifice plates. On the basis of this analysis, it presents results from a new set of experiments, which were designed to isolate mainly the effects of the so-called cavitation number ( σ v ). Experimental data confirm that the disinfection efficacy of orifice plates increases with decreasing σ v . Finally, in order to discuss the significance of the results presented herein and frame the scope of future research, the present paper provides an overview of the drawbacks associated with dimensional analysis within the context of HC