Academic literature on the topic 'River processes'

Rivers’ behavior is increasingly of interest to wide engineering and scientific communities. In fact, fluvial dynamics have an impact on infrastructure and anthropic environments as well as on natural habitats. In addition, the economic values of river as routes of commerce is great, as is the importance of precious minerals deriving from fluvial sedimentary structures. One of the more fascinating river’s peculiarity is the wide range of space and time scales that impact on the overall fluvial dynamics: from the small scales typical of turbulence that are responsible for the motion of single bed’s grains to wider scales typical of fluvial catchment and climate changes. Such a variety of impacting scales firstly accounting for the difficulties of fluvial investigations. Moreover, a second source of difficulty comes from the wide interactions between the fluvial scales: typical hydrodynamics scales influences and being influenced by the scales of sediment transport and bed deformation. It follows that non-linear and non-trivial behavior characterizes the river dynamic. Notwithstanding, great improvements in the fluvial knowledges have been done up to now. Nowadays, fluvial engineer and researchers can take advantage of field studies, laboratory experiments and hydrodynamic models to improve and deepen the study of the various fluvial phenomena. A rough chronology of fluvial advances dates back to 50s the development of field studies and laboratory experiments, and to 80s the growth of mathematical theories and numerical models. All these investigation methods are nowadays being improved and each result to be fundamental to the others. Field study represents the only way to study the real fluvial system, but imply two disadvantages: firstly, they are difficult to be performed (mainly during flood periods) and expensive, and secondly are site-sensitive (i.e., it is difficult to drawn general laws and lessons from a specific fluvial environment). Laboratory experiments are useful to operate in a reductionist manner, since they allow to isolate and separate physical issues that in nature are merged and somehow hidden. Being a strong simplification of complex natural phenomena, experiments must be carefully designed and run in order to be a realistic reproduction of what happens in nature. Mathematical theories and physically-based numerical models are a strongly quantitative way to face to fluvial behavior. Nowadays, we can see an increasingly number of theoretical models and this is due to the increase of computing power of computers and to advances in mathematical modeling. At the same time, the 4 a) b) d) e) f) g) c) Figure 1.1: Examples of various river channel patterns. From a) to g): Brahmaputra River, India (10 km wide braid plain), Rakaia River, New Zealand (1.7 km widebraid plain), Allier River, France (0.8 km wide meander belt), Koyukuk River, Alaska (10 km wide meander belt), Columbia river, Canada (2.1 km wide fluvial valley), Escalante River, Utah (60 m wide channel) and Nanedi Valles, Mars (2 km wide channel) (after Kleinhans (2010)). Introduction 5 theoretical advances risk to provide models that, even though mathematically refined, are not useful to resolve practical fluvial problems. It follows that a correct mix of field observation, experiments, and theories can be the only way to face the fascinating and still not completely discovered fluvial world. The consequence of all the physical processes that characterize rivers is the wide and fascinating range of planimetric patterns that a river can exhibit. On the basis of the channel form on the horizontal plane, rivers are traditionally classified as straight, braiding or meandering (e.g., Leopold and Wolman, 1957). Each of these planforms present different mobility on the floodplain and different degree of pattern stability. Moreover, every river planform has its peculiar natural habitat, in terms of different riparian vegetation, geochemical characteristics, and fauna. Figure 1.1 shows a good example of the different channel patterns that are present in nature. The planimetric patterns shown from panels 1.1a to 1.1g are qualitatively characterized by a decreasingly stream power versus bank strength ratio. Another peculiar feature of rivers, common to all the planimetric patterns, is the intrinsic bed instability. Once the motion threshold is exceeded the water flow over a granular bed leads the bed to presents transversely oriented wave-like features, for example ripples and dunes. This bedforms travel beneath the current, take part to sediment transport, and increase the hydraulic resistances. Dunes are of the order of magnitude of water depth, presenting in nature typical wavelength of 100-102m. Figure 1.2 shows an example of a train of dunes obtained in laboratory. Figure 1.2: Dunes in an our laboratory run. Flow was from right to left. For scaling, channel width was 50 cm. Major sizes are typical of another ubiquitous bedforms, called bars. River bars are 6 longitudinal sedimentary accumulation, submerged and moved only during high flows. Bars can assume a classical alternate configuration with respect to channel axis (alternate bar) or be present on the inner side of bends (forced bars). Bars play different morphodynamics role on different river planform: e.g., they can trigger and enhance bend evolution in meandering river and separate the single channels of braiding network. Figure 1.3 reports two alternate bar in our experiments on the pseudo-meandering pattern. Figure 1.3: Alternate bars in an experimental channel. Flow was from left to right. For scaling, the distance between the triangular markers was about 3 m. In our work we study both issues about fluvial planforms and bedforms, investigating some connections between planimetric and bed deformation. In particular we face with a planimetric configuration called "pseudo-meandering". The pseudomeandering pattern exhibits several features of both meandering rivers (alternate bars, migrating bends and asymmetrical cross-sections) and braiding rivers (flow diversion and tendency to create secondary channels due to the development of a chute channel between the inner side of the bar and the bank) which coexist in the same reach. Thanks to an experimental approach and some field observation we demonstrate how such pattern is strictly influenced and determined by the water discharge variability. Fluvial planforms were also focused in experiments reproducing some pattern changes (from braiding to single-thread) that are induced by strong sediment supply decline, that mainly happens caused by anthropic activities and infrastructures. Bedforms issue are instead presented by coupling experiments and a mathematical models, with the aim to investigate and clarify the initial stages of alternate bar formation. In particular we present results showing how our model is able to predict the wavelength selection typical of alternate bar. A great part of the experimental runs presented in this work has taken advantage Geomorphological background 7 of a new instrument that is able to profile the flume bed during the run and in a non– invasive way. We underwent this newly-developed device to several trials to test its accuracy. The maximum errors in the bed’s elevation measurement resulted to be less than 1 mm in hydraulic conditions that are typical of morphodynamics runs. The present thesis is organized as follows. Chapter 2 presents a general introduction about fluvial geomorphology, introducing the various river planforms that are present in nature and the bedforms typical of river’s beds. In chapter 3, the novel instrument used to scan in a non–invasive way the flume bed is described. The following chapters represent the core of the experimental researches: experiments regarding the influence of a varying discharge on a pseudomeadering channel are reported in chapter 4, chapter 5 is devoted to elucidate the transition from multi to single-thread fluvial patterns, and in chapter 6 a new theory and its experimental verification is developed to explain the wavelength selection typical of alternate bars.

Border ice is one of many ice freeze-up processes, but it is discussed only to a limited extent in the literature. Border ice formation can be a precursor for ice jam formation that may restrict navigation and lead to flooding. This master’s thesis is mainly devoted to the research on the border ice on the Saint Lawrence River from Montréal to Québec City. This reach stays artificially open all winter because commercial ships are continuously preventing a full ice cover to form. The traffic also limits the extent of border ice. This study provides key information on ice formation and decay. Through analysis of Environment Canada’s historical data (ice charts from 2004 to 2009), the areal coverage of border ice is analyzed during freeze-up, winter and breakup periods. The historical information of ice coverage is collected in order to find out the factors which influence its formation and its spatial limits. Border ice growth and decay rates are also discussed. The thesis shows that border ice coverage has three stages including the rapid growth period at the beginning of the winter, the relatively stable period in the mid-winter and the breakup period as March progresses. During the mid-winter period, the border ice coverage sometimes drops sharply if the air temperature rises above 0 °C and/or if there is some rain. It was also found that the maximum border ice spatial limits are quite similar over the five winter seasons. Based on the analysis of the ice charts, a number of empirical laws regarding the formation and decay of border ice are proposed. Along the river flowing direction, the border ice is formed easily when there are obstacles particularly at the downstream end. The obstacles could include river bends, ice booms, shoals, artificial islands, bridge piers and so on. Thus, the obstacle influences the flow velocity, which is an important factor for ice formation and also provides an object against which the ice can become fast and initiate its formation. On average, border ice reaches 20% of its maximum coverage when the accumulated freezing degree days (AFDD) reaches 124 °C-D. This is followed by a rapid growth period that ends when the ice cover reaches about 80% of its maximum cover corresponding to AFDD equal to 247 °C-D. Border ice coverage usually reaches the maximum value when the average AFDD is 551 °C-D corresponding to the end of January. The winter period is characterised by a stable ice cover (>90% of max) upstream of Trois-Rivières except in the event of a mid-winter thaw. Downstream of Trois-Rivières there is no stable period as the decay begins very soon after the ice reaches its maximum value. Breakup is a gradual process that normally begins on about Feb. 15th downstream of Trois- Rivières and about March 1st upstream. Most ice has normally gone by March 31st. Moreover, the river flow velocity, river depth and Froude number along the limits of border ice once it reaches its maximal areal coverage are evaluated and analyzed. The flow velocity is almost always less than 1.0 m/s; the maximum Froude number is normally 0.1 at Lake Saint-Pierre and 0.2 in the Montréal to Sorel reach; river depth at the ice edge can vary widely. Through numerical modelling, it was found that border ice increased the current velocity by 0.1 m/s in the Lake Saint-Pierre reach and raised water levels by 14 cm in the Montréal to Sorel reach.<br>La glace de rive est un des nombreux processus de formation des couverts de glace sur les rivières. Cependant peu d’articles dans la littérature traitent de ce sujet malgré que la formation de la glace de rive peut-être un précurseur de l’apparition d’embâcles qui peuvent entrainer des inondations. Ce mémoire de Maitrise porte sur l’étude de la glace de rive le long de la portion du fleuve Saint-Laurent allant de Montréal à Québec. Du fait qu’il y a de la navigation commerciale toute l’année, le fleuve reste ouvert (libre d’un couvert de glace entier) artificiellement pendant tout l’hiver. Ce trafic limite aussi l’extension de la glace de rive. Cette étude fournit des informations clés sur la formation et la désagrégation de la glace de rive. À partir des données historiques d’Environnement Canada (cartes des glaces de 2004 à 2009), la répartition superficielle de la glace de rive est analysée pour les périodes de formation, de stabilité et de rupture de la glace. Les informations historiques sur les couvertures de glace sont collectées afin de déterminer les paramètres qui influencent la formation et les limites spatiales de ce type de glace. Les taux de croissance et de décomposition de la glace de rive sont aussi abordés. Il est montré que l’évolution de la structure propre à la couverture de la glace de rive se fait en trois étapes. Une période de formation rapide (début hiver), suivie d’une période stable (milieu d’hiver) et enfin une période de rupture (pendant le moi de mars). Pendant la période stable, la glace de rive se rompt partiellement parfois lorsque la température de l’air monte au dessus de zéro °C et surtout lorsque le redoux est accompagné de pluie. Il a été trouvé aussi que les limites spatiales maximales des glaces de rive sont très semblables sur 5 hivers de la période d’étude. À partir de l’analyse des cartes des glaces, un certain nombre de relations empiriques sont proposées. Ces relations caractérisent la formation et la désagrégation des glaces de rive. Le long de la direction de l’écoulement la glace de rive est formée facilement en présence d’obstacles, et particulièrement lorsqu’elles sont à l’extrémité aval. Parmi ces obstacles on peut citer les méandres de rivière, les bancs, les estacades, les iles artificielles, les piliers de ponts. Ainsi, les obstacles influencent la vitesse d’écoulement qui est un paramètre important dans la formation de la glace et peut aussi effectuer un apport d’objets sur lesquels la glace peut s’attacher et initier son accroissement. En moyenne la glace de rive atteint 20% de sa couverture maximale lorsque son le nombre de degrés jours accumulés (DJA) atteint 124 °C-j. Ceci est suivi d’une période d’accroissement rapide qui prend fin lorsque la couverture de glace atteint 80% de son maximum qui correspond à un DJA de 247 °C-j. La couverture de glace de rive atteint son maximum lorsque le DJA atteint 551 °C-j; ce qui correspond normalement à la période de fin janvier. La période d’hiver est caractérisée par une couverture de glace stable (supérieure à 90% de son maximum) en amont de Trois-Rivières, sauf pendant les périodes de dégel mi hivernales. À l’aval de Trois-Rivières, il n’y a pas de période stable, vu que la désagrégation commence très tôt après que la glace ait cru à son étendu maximal. La rupture est un processus graduel qui normalement commence vers le 15 février en aval de Trois-Rivières et vers le premier mars en amont. La grande majorité de la glace disparait généralement avant le 31 mars. Par ailleurs, la vitesse d’écoulement de la rivière, ainsi que sa profondeur et son nombre de Froude le long des limites de la glace de rive sont évalués. Ceci dans la condition où la glace de rive a atteint sa répartition superficielle maximale. La vitesse est presque toujours inférieure à 1 m/s, le nombre de Froude maximal est normalement de 0,1 au dans le Lac St Pierre et de 0,2 sur le tronçon Montréal-Sorel. La profondeur de la rivière à la limite de la glace peut varier largement. À partir d’une modélisation numérique, il a été calculé que la glace de rive cause une augmentation de la vitesse de 0,1 m/s dans le chenal maritime du Lac St Pierre et du niveau d’eau de 14 cm dans le tronçon Montréal-Sorel.

This project conducts an analysis of bank erosion processes on a large, monsoonaffected river, the Lower Mekong River in Laos. The methodological approach taken was to build integrated models of bank erosion processes at three study sites on the Lower Mekong River in Laos (Friendship Bridge, Ang Nyay and Pakse) to simulate processes of (i) groundwater seepage and pore water pressure evolution, (ii) the effect of this on mass-wasting (using the Geo-slope model) and, (iii) fluvial erosion (using a model adapted from Kean and Smith, 2006ab). In all cases the models were parameterised using measured bank geotechnical properties. Across the study sites, a total of 42 simulations were undertaken to represent a wide range of observed flow events. Specifically, 14 selected flow hydrographs (comprising three types: single peak, multiple peak and rapid fall) were evaluated at each of the study sites, such that the influence on bank erosion of the hydrological properties of different monsoon floods could be evaluated. The main findings indicate that although the Mekong is a big river, its dominant bank erosion process is one of slow, gradual, fluvial erosion. This research forms a partial contribution to understanding bank erosion processes operating in the Mekong. It was found that bank stability on the Mekong responses to variations in flood magnitude in ways that are similar to other rivers located within humid temperate areas. However, the Mekong has had the greater stability than these rivers due to its greater bank heights and more consolidated bank materials.

Postglacial rivers are part of the relatively young low-relief landscape system left behind by glaciers. Over time, postglacial rivers are susceptible to both minor and major channel planform changes as the Earth and its newly exposed rivers adjust to new isostatic and geomorphic equilibriums. Those planform changes result in topographic features that are well preserved among the largely unaltered landscape and offer opportunities to learn about the processes that create them. This work focuses on those minor and major planform changes and the resulting landforms, with a focus on processes effecting the glaciolacustrine Red River Valley. Here, three studies were conducted, two regarding minor planform changes and one focusing on major planform changes. Studies included in this work regard 1) the spatial distribution of meander cutoffs and meander cutoff relief on the Red River, 2), avulsion timing and length resulting from isostatic tilting and 3) mobile river ice and bank interaction frequency, locations, and erosion in meandering rivers. Results show that rivers develop meander cutoffs that faster in areas where geologic materials are more easily eroded and their relief shows a positive relationship with the rate of river incision. Major channel path changes (avulsions) in the presence of isostatic tilting were found to be most frequent soon after river establishment while rates of isostatic rebound are high enough to outpace channel incision. River ice was found to most frequently interact with the outer banks of channels with long, tight bends and high sinuosity, potentially contributing to the meandering process. From these results it can be interpreted that postglacial rivers were highly dynamic early in their history and have stabilized over time, with most of the changes occurring in areas with more erodible alluvium. Presently, rivers undergo most of their changes during the spring thaw when mobile river ice is impacting the banks, with sinuous river reaches impacted most frequently by mobile river ice.<br>North Dakota Water Recourses Research Institute (ND WRRI) Fellowship Program

Previous scientific research has documented channel narrowing on the Green River near Green River, Utah, but the exact timing, rates, and causal mechanisms of that narrowing have been the source of disagreement in the scientific literature. This thesis demonstrates that the Green River has narrowed in two separate periods during the last 100 years. The narrowing is driven primarily by changes in the hydrologic regime and not by the invasion of saltcedar. The channel narrowed between 1930 and 1938, when a shift from wetter than normal conditions to a period of draught led to a reduction in river discharge. Channel width then remained relatively stable until construction of Flaming Gorge Dam in 1962, despite the presence of saltcedar. Narrowing has occurred since dam construction. Detailed analysis of the formation of an inset floodplain deposit indicates that it formed by a process of vertical accretion, during incremental events. Inset bank deposits within the study area are composed primarily of particles smaller than 0.125 mm. Measurement of suspended sand distribution within the water column shows that particles of this size are carried in suspension by the 2-yr flood. Continued vertical accretion over time elevated the floodplain surface until inundation rarely occurs.

The “modern” fluvial morphology, is the results of a series of events characterized by both natural and human dynamics. Recognizing the process responsible for particular morphology is not a simple analysis, it can be more difficult or impossible if the data collected have too low resolution or too high uncertainty in relation to the spatial and temporal scale assessed. This work aims to analyse and optimize different data and collection methods, derived from different time, space and resolution scales, with a good equilibrium between time-consuming and results at low uncertainty. Different gravel bed reaches were analysed as study area: Brenta, Piave, Tagliamento River (Italy) and Feshie River (Scotland). Three geomorphic analyses were applied at different spatial and temporal scale. A planimetric approach through a multitemporal analysis over the last 30 years on the Brenta River. A volumetric approach through a revised colour bathymetry; hybrid digital terrain models (HDTM) building and comparison of different digital elevation models (DoD) was used to study relevant flood events that occurred in the North-East Italian rivers (Brenta, Piave and Tagliamento). A highly detailed resolution, derived from Terrestrial Laser Scanner (TLS) to study its uncertainty, was applied on the Feshie River and to some laboratory experiments. Results show that on the Brenta River, lower active channel narrowing happened from 1981 to 1990 even if relatively important floods occurred. The active channel was likely at its minimum extent due to still relevant human impacts. Partial recovery of the active channel width was detected from 1990 to 2011 due to less gravel mining and human pressure. The proposed methodology for producing high-resolution Digital Terrain Models (DTMs) in wet areas has an uncertainty comparable to LiDAR (Light Detection And Ranging) data in dry areas. The bathymetric model calibration only requires a dGPS survey in the wet areas contemporary to aerial images acquisition. Detailed and automatic erosion - deposition analyses starting from a DoD are possible thanks to the “principal erosion deposition analyser” script developed. Density, angle of incidence and laser intensity seem to be the most uncertain influencing factors in DTMs building from TLS point clouds. A new TLS filter developed provides semi-automatic point cloud classifications to filter the vegetation. The geomorphic approaches presented provide an adequate topographical description of the rivers to explore channel adjustments due to natural and human causes at different spatial and temporal scales. The study represents a valuable tool for any fluvial engineering, river topography description, river management, ecology and restoration purposes.<br>La “moderna” morfologia fluviale, è il risultato di una serie di eventi caratterizzati da differenti dinamiche, naturali ed antropiche. Riconoscere i processi responsabili di una particolare morfologia, può divenire complesso se i dati disponibili presentano bassi livelli di risoluzione o eccessiva incertezza in funzione della scala temporale e spaziale analizzata. Questo lavoro si è focalizzato ad analizzare ed ottimizzare differenti tipi di dati e metodologie di rilievo in differenti tratti fluviali a fondo ghiaioso dell’Italia Nord-Orientale e della Scozia: Fiume Brenta, Piave e Tagliamento (Italia) e Fiume Feshie (Scozia). Tre differenti metodologie geomorfometriche sono state applicate a diverse scale spaziali e temporali. Un approccio planimetrico attraverso un’analisi multitemporale degl’ultimi 30 anni in un tratto del Fiume Brenta. Un approccio volumetrico attraverso una rivisitata applicazione di batimetria da colore, con costruzione di modelli digitali del terreno “ibridi” (HDTM) e comparazione di modelli di elevazione (DoD) per lo studio di un intenso evento di piena, avvenuto nei fiumi italiani considerati. Rilievi in laboratorio e nel Fiume Feshie ad alta risoluzione, tramite laser scanner terrestre (TLS), sono stati eseguiti per studiarne l’incertezza ed individuare metodologie di classificazione spaziale delle nuvole di punti. I risultati, mostrano che dal 1981 al 1990 nel Fiume Brenta persiste ancora un processo di restringimento dell’alveo attivo. L’impatto umano è ancora presente. L’alveo attivo presenta la sua minima estensione. Dal 1990 al 2011, sembra che un parziale recupero della larghezza dell’alveo attivo sia in atto. Minor pressione da estrazione di ghiaia e da impatto umano, caratterizzano questo periodo. La metodologia proposta per produrre DTM ad alta risoluzione in presenza di aree bagnate ha dimostrato un’incertezza comparabile con il LiDAR nelle aree secche. La calibrazione dei modelli batimetrici, richiede un rilievo dGPS nelle aree bagnate in “contemporaneo” con l’acquisizione delle foto aeree. Grazie allo script sviluppato (PrEDA), sono possibili più dettagliate e automatiche analisi dell’erosione e della deposizione. Densità, angolo di incidenza ed intensità laser sembrano essere i fattori che maggiormente influenzano l’incertezza nella realizzazione di modelli di elevazione da TLS. Il filtro sviluppato per nuvole TLS è in grado di fornire semi-automatici filtraggi della vegetazione. Gli approcci geomorfometrici presentati, forniscono adeguate descrizioni topografiche dei sistemi fluviali; utili ad esplorare aggiustamenti dei canali dovuti a cause naturali o antropiche in differenti scale spaziali e temporali. Lo studio proposto, può rappresentare un valido supporto alla topografia in ambito fluviale, alla progettazione di interventi di ingegneria fluviale, ad una adeguata gestione fluviale, considerando aspetti ecologici e di riqualificazione fluviale.