Modélisation de l'érosion sur deux bassins versants experimentaux des Alpes du Sud

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Novo modelo empírico de viscosidade para suspensões compostas utilizadas em experimentos de fluxos gravitacionais de sedimentos

Sediment gravity flows are natural flows composed by water and sediment in which the gravitational flow acts on the sediment. The distinct physical properties of the cohesive (clay) and non-cohesive (sand) sediment, and the interaction between these particles alter the ability of the flow to resist to the movement (rheology) along time and space, represented by the viscosity of a mixture suspension. Hence, we propose to study the rheological properties of those mixtures and calculate their relative viscosity when used in the physical simulation of turbidity currents. Rheological tests were performed with various mixtures composed by water, clay and/or coal. Two equations are proposed to estimate the relative viscosity as a function of volume concentration of each sediment, the maximum packing fraction and the percentage of clay present in the mixture. The results also show an error close to 20% comparing similar models from the literature, which are satisfactory. The results also demonstrate that caution should be exercised when generalizing the use of a single model to predict the relative viscosity of suspensions. The influence of density (ρ), grain shape, clay percentage (Cclay), volumetric concentration (φ) and maximum packaging fraction (φmax) should be considered in the formulation of the equations.Os fluxos gravitacionais de sedimentos são tipos de fluxos naturais compostos por água e sedimentos nos quais o fluxo gravitacional atua sobre o sedimento. As distintas propriedades físicas dos sedimentos coesivos (argila) e não coesivos (areia) e a interação entre essas partículas alteram a capacidade interna do fluxo em resistir ao movimento (reologia) ao longo do tempo e do espaço, capacidade representada pela viscosidade da suspensão de mistura. Assim, propomos estudar as propriedades reológicas dessas misturas e calcular sua viscosidade relativa utilizada na simulação física de correntes de turbidez. Foram realizados testes reológicos com várias misturas compostas por água, argila e/ou carvão. Duas equações são propostas para estimar a viscosidade relativa em função da concentração volumétrica de cada sedimento, a fração máxima de empacotamento e a porcentagem de argila presente na mistura. Os resultados também mostram um erro próximo a 20% na comparação de modelos semelhantes na literatura, o que é satisfatório. Os resultados também demonstraram a necessidade de cautela ao generalizar o uso de um único modelo para prever a viscosidade relativa das suspensões. A influência da densidade (ρ), formato do grão, porcentagem de argila (Cclay), concentração volumétrica (φ) e fração máxima de embalagem (φmax) devem ser considerados na formulação das equações

Caracterização geométrica de deltas dominados por rios simulados experimentalmente sob variação de condições de aporte fluvial e nível de base

Este trabalho visa caracterizar geometricamente depósito deltaico gerado em laboratório a partir de diversas condições de aporte fluvial (vazão e suprimento de sedimentos), bem como de alteração de nível de base da bacia, buscando entender quais fatores são mais significativos na formação dos deltas e como se dá o processo sedimentar (progradação, compensação lateral e retro/agradação). Para cumprir os objetivos propostos no trabalho foram realizadas quatro séries de experimentos em tanque tridimensional, buscando controlar e variar os seguintes parâmetros de entrada: vazão, concentração e nível de base. Ao todo, 150horas de simulação foram realizadas. Os resultados mostraram que a diminuição do nível de base e da concentração foram os principais fatores de mudança de geometria/forma dos deltas formados. O primeiro por criar mais espaço para a o avanço da frente deltaica, e o segundo, por provocar processos erosivos junto à planície fluvial e deltaica, transferindo esses sedimentos à jusante. Já a vazão fluvial também é importante, mas está mais condicionada à alta intensidade (capacidade de transporte) e curta duração (eventos catastróficos). A implicação dos resultados no estado da arte é discutida.This work aims to characterize geometrically the deltaic deposit generated at laboratory. Different fluvial conditions (sediment supply and discharge) and water-level change were varied in order to understand which parameters are most significant on sedimentary process of deltas (progadation, lateral migration and retro/aggradation). Four series of experiments in 3D-tank were carried out with variable input parameters as follows: flow rate, concentration and water-level base. At total, 150 hours of simulation were performed. Those experiments evidenced that both water-level fall (creating more space for the propagation of the deltaic front) and volumetric concentration decrease (causing erosion along the river and deltaic plain, transferring sediment downstream) were main factors of change in geometry/shape of the deltas. Yet, fluvial discharge plays an important role, but is related to high intensity (capacity of transport) and short duration (catastrophic events). The implications of the results in the state of art are discussed

Sedimentology and stratigraphy of a deltaic deposit generated by physical modelling from core samples

A análise de testemunhos de depósitos sedimentares é um método rotineiramente utilizado nos estudos sedimentológicos e estratigráficos. Propõe-se neste trabalho um método inédito para amostrar depósitos sedimentares gerados por de modelagem física em escala reduzida, adaptando o método de testemunhagem para descrever e interpretar depósitos deltaicos e, posteriormente, determinar o quão representativa é a simulação do ponto de vista geológico. O processo de amostragem inicialmente consiste na perfuração vertical do depósito, ou seja retirada do material sedimentar, e posterior preenchimento da cavidade por resina. A resina penetra horizontalmente alguns centímetros nas camadas sedimentares e, após a secagem, permite a retirada de um testemunho bastante preservado. No total foram descritos 30 testemunhos, gerando perfis verticais de fácies que posteriormente foram interpretados e correlacionados, usando modelos e conceitos de sedimentologia e estratigrafia. Nos testemunhos foram identificadas características sedimentológicas e geométricas similares àquelas encontradas em deltas naturais, o que permitiu interpretar as associações de fácies deltaicas e, juntamente com a correlação lateral dos perfis, mostrar a evolução estratigráfica do depósito.In natural ambient, the analysis of deposit cores is a method widely used in sedimentological and stratigraphic studies. It is proposed in this paper, a new method to sample sedimentary deposits generated by physical modeling in small scale. This method was adapted in order to extract additional details and information in terms of geological point of view of the deposit generated. At the lab, the sampling process, consisted in a vertical hole on the deposit and removal sedimentary material, and then, filling the hole with resin. The resin penetrates horizontally a few inches in the sedimentary layers along all vertical cores and, after impregnation, allows the removal of a well preserved core. A total of 33 cores was collected and described , producing vertical profiles of facies which are then interpreted and correlated by using the models and concepts of sedimentology and sequence stratigraphy. Similar geometries and sedimentary features related to natural deltas were observed . It was also possible evaluate the deltaic facies associations and established a longitudinal section of the deposit in order to understand its stratigraphic evolution

Modelagem Física de Correntes de Turbidez: Descrição do Processo e Implicações no Estudo dos Depósitos Turbidíticos

The turbidity currents are responsible for the formation of the major hydrocarbon reservoirs around the world; however the fundamentals of such currents, both in theory and practice, are still unexplored to exhaustion nowadays. The description of initialization, transportation and deposition mechanisms of this process is surrounded by uncertainty. These uncertainties had inspired the accomplishment of a three series of experiments, in order to investigate it using two different physicals models. It were analyzed the geometrical, dynamical and depositional features of a turbidity current attempting to match the results of physical modeling to those found in natural outcrop. Granular materials are tested and grain size ranges for simulation are evaluated. Density currents with different densities, grain sizes, fluid injection rate and volume were generated. Therefore density current evolution, current velocity, geometric features, and bed forms were registered. The results show that velocity increases for larger current densities or smaller grain sizes and that the height of the current head increases when the current density decreases. The deposition volumes present a general tendency of exponential decline; the grain size range of the deposits decreases towards the distal portion of the channel. The results also show that advancing velocity and bed forms (it was observed the successive development of planar upper flow regime surfaces, low sinuous-ripples, middle sinuous-ripples, meandering channels and linear channels) depend basically of the flow oscillations identified in the fluid injection rate. The simulations presented intended to collaborate with the understanding of the natural phenomena, relating the current behaviour (flow parameters) with the generated deposit (turbidites), as well as indicated the applicability of physical modelling on the field of deep-water sedimentation and its reliability as a tool for hydrocarbon reservoir prediction.The turbidity currents are responsible for the formation of the major hydrocarbon reservoirs around the world; however the fundamentals of such currents, both in theory and practice, are still unexplored to exhaustion nowadays. The description of initialization, transportation and deposition mechanisms of this process is surrounded by uncertainty. These uncertainties had inspired the accomplishment of a three series of experiments, in order to investigate it using two different physicals models. It were analyzed the geometrical, dynamical and depositional features of a turbidity current attempting to match the results of physical modeling to those found in natural outcrop. Granular materials are tested and grain size ranges for simulation are evaluated. Density currents with different densities, grain sizes, fluid injection rate and volume were generated. Therefore density current evolution, current velocity, geometric features, and bed forms were registered. The results show that velocity increases for larger current densities or smaller grain sizes and that the height of the current head increases when the current density decreases. The deposition volumes present a general tendency of exponential decline; the grain size range of the deposits decreases towards the distal portion of the channel. The results also show that advancing velocity and bed forms (it was observed the successive development of planar upper flow regime surfaces, low sinuous-ripples, middle sinuous-ripples, meandering channels and linear channels) depend basically of the flow oscillations identified in the fluid injection rate. The simulations presented intended to collaborate with the understanding of the natural phenomena, relating the current behaviour (flow parameters) with the generated deposit (turbidites), as well as indicated the applicability of physical modelling on the field of deep-water sedimentation and its reliability as a tool for hydrocarbon reservoir prediction

Modelagem física de correntes de turbidez : descrição do processo e implicações no estudo dos depósitos turbidíticos

The turbidity currents are responsible for the formation of the major hydrocarbon reservoirs around the world; however the fundamentals of such currents, both in theory and practice, are still unexplored to exhaustion nowadays. The description of initialization, transportation and deposition mechanisms of this process is surrounded by uncertainty. These uncertainties had inspired the accomplishment of a three series of experiments, in order to investigate it using two different physicals models. It were analyzed the geometrical, dynamical and depositional features of a turbidity current attempting to match the results of physical modeling to those found in natural outcrop. Granular materials are tested and grain size ranges for simulation are evaluated. Density currents with different densities, grain sizes, fluid injection rate and volume were generated. Therefore density current evolution, current velocity, geometric features, and bed forms were registered. The results show that velocity increases for larger current densities or smaller grain sizes and that the height of the current head increases when the current density decreases. The deposition volumes present a general tendency of exponential decline; the grain size range of the deposits decreases towards the distal portion of the channel. The results also show that advancing velocity and bed forms (it was observed the successive development of planar upper flow regime surfaces, low sinuous-ripples, middle sinuous-ripples, meandering channels and linear channels) depend basically of the flow oscillations identified in the fluid injection rate. The simulations presented intended to collaborate with the understanding of the natural phenomena, relating the current behaviour (flow parameters) with the generated deposit (turbidites), as well as indicated the applicability of physical modelling on the field of deep-water sedimentation and its reliability as a tool for hydrocarbon reservoir prediction

Padrões erosivos de escoamentos rasos em meio coesivo

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