Bedforms are sedimentary features that can be generated on the seafloor by the interaction between density currents and mobile beds. Developing knowledge about the hydraulic and sedimentary processes involved during these events is in the interest of research groups and oil companies. Because of the magnitude of the density currents in its natural environment and the challenge in collecting data, studies in laboratory are of great value. We present results of 29 experiments focusing in the bedform development generated by saline currents, testing two different sediment types and three grain sizes: melamine (245 μm-plastic) and sands (206 and 485 μm). We analyzed the current velocity and fractional density profiles as well as pictures taken during and after each run. Results have showed classical velocity and concentration profiles, for 8 subcritical and 21 supercritical currents, with densimetric Froude numbers (Fr d) between 0.5 and 2.2. Some correlations were identified, such as the decrease of the velocity peak height and increase in the mean velocity (with consequent reduction of the current thickness), due to an increase of the concentration and/or flume slope. The occurrence of bedforms was more likely for high discharge and concentration rates of current density, which directly influence the Fr d. Bedforms were classified according to the shear stresses values applied by the current to the bed, resulting in the generation of lower plane bed, ripples and dunes. Dunes and ripples were observed in supercritical flow conditions, which is a hydraulic scenario of bedforms generation not predicted by fluvial models. Thus, this study demonstrated the existence of differences in generation and, consequently, the classification approach for density current bedforms, compared to those generated by river flows. To this fact is attributed the hydrodynamic (velocity and concentration profiles) and sediment transport differences between fluvial flows and density currents. Further studies may be carried out in order to constructing new concepts of bedforms generation by density currents.

Density currents, whose movement takes place by the density difference between the flow and the ambient fluid around it, can interact with the substract generating bedforms similar to the fluvial environments. However, there are no specific bedform phase diagrams capable to predict this type of phenomenon. This study aims to compare the prediction of fluvial bedforms phase diagram with those generated by experimental saline currents. Bedforms were generated in two-dimensional tilting plexiglass flume submerged in a larger tank filled with water with three different mobile beds and varied values of discharge and salt concentration. It was observed three types of bedform (lower plane bed, ripples and dunes), which, with the concomitant calculation of hydrodynamic parameters (mean velocity, energy and mobility) allowed the use of the phase diagram. It was observed that the fluvial phase diagrams did not present good predictions for bedforms generated by density currents. This fact is associated to the hydrodynamics differences (velocity and concentration profiles) and the limitation of the dimensional parameters in the extrapolation of results. Therefore, it is indicated the need to draw up a proper phase diagram to density currents. Keywords: Density current; Bedforms; Physical modeling; Mobile bed; Bedform phase diagram. RESUMOAs correntes de densidade, cujo movimento ocorre pela diferença de massa específica entre o escoamento e o fluido ambiente ao seu redor, podem interagir com o substrato gerando formas de fundo, similares às encontradas em ambientes fluviais. Entretanto não existem diagramas de previsão específicos correspondentes para esse tipo de fenômeno. Assim, este trabalho visa comparar a ocorrência das formas de fundo fluviais previstas nos diagramas de previsão com aquelas geradas por correntes de densidade salinas obtidas experimentalmente. As formas de fundo foram geradas em um canal bidimensional de declividade variável, preenchido por água, com três composições de leito móvel e diferentes valores de vazão, massa especifica e inclinação. Três formas de fundo foram identificadas (leito plano inferior, ondulações e dunas), as quais, juntamente com o cálculo de parâmetros hidrodinâmicos permitiram a utilização dos diagramas fluviais. Verificou-se que os diagramas fluviais não apresentaram boas previsões das formas de fundo geradas por correntes de densidade. A esse fato são atribuídas as diferenças hidrodinâmicas dos escoamentos (perfis de velocidade e concentração) e, também, à limitação dos parâmetros dimensionais na extrapolação dos resultados. Dessa forma, indica-se a necessidade de se elaborar um diagrama de previsão próprio adaptado a estas correntes.Palavras-chave: Corrente de densidade; Formas de leito; Modelagem física; Leito móvel; Diagrama de previsão.

Turbidity current hydrodynamic properties are evaluated experimentally to understand the formation of turbidity current depositional lobes and the relationship with flow properties, in particular, flow rates. This study focused on the depositional behavior of unconfined turbidity currents through the analysis of three-dimensional experiments performed in a large-scale channel-basin tank without slope break. Three flow rates were simulated when flow velocities, both in longitudinal and transversal directions, were measured and resulting depositional features were evaluated. The three-dimensional physical experiments carried out in this work allowed the identification of two flow rate models with different hydrodynamic characteristics and two distinct lobes. Lower flow rates produced elongated lobate deposits, with characteristic lower flow regime plane bed on the surfaces, characteristic downstream sediment fining that resulted from lower flow velocities, and visibly less turbulent flows from less competent and waning turbidity currents. Higher flow rates showed a more characteristic radial and downstream fining sediment with lobe surfaces displaying ripples and dunes, generated by the higher flow velocities, presumably more turbulent, and more competent turbidity currents.

Saline currents (SCs) have commonly been used to model muddy turbidity currents (TCs) in a laboratory. However, little is known about the limitations of this proxying, in particular when concerning processes and products related to their sedimentologic and stratigraphic imprints. The present study is aimed at investigating experimental hydraulic and sedimentological conditions and processes involved in the generation and development of bedforms by both SCs and TCs, when similar input conditions are applied (discharge, slope, densimetric Froude number). In all performed runs reported herein, only ripples were observed to form, and were identified and classified using known criteria such as their dimensions, near-bed shear stresses (), shear velocities (), and grain Reynolds values (Re*). Turbidity currents were observed to deposit sediments carried by the flow predominantly in the upstream section of the flume, increasing bed slope and thus increasing , , and near-bed concentrations (cb). This resulted in longer-wavelength bedforms compared to those generated by SCs under similar input conditions in those sections of the flume. On the other hand, along the downstream sections of the flume, bed slopes were observed to remain similar for all experiments, and both types of currents showed similar vertical distribution of velocities, concentrations, and stable stratification. Measured bedform wavelengths and heights were slightly higher when generated by SCs, due to the coarser bed material observed in SCs (which influenced the increase in near-bed turbulent intensities). Moreover, TCs presented a slight decrease in turbulence intensities due to their observed high near-bed suspended-sediment concentration. Spatial and temporal changes in several hydraulic parameters in both SCs and TCs highlight the role of sediment suspension in modifying turbulent processes and vertical stratification of these flows, depending on their concentrations. Both hydraulic and sedimentologic observations of this study support in principle the assumption that SCs can be used experimentally as a surrogate for diluted TCs to reproduce bedforms classified as ripples, as long as both type of currents reach similar hydraulic and sediment-transport conditions, in particular for depth-averaged concentration Cvol < 1% and near-bed concentration cb < 2%.

Neste estudo, empregou-se uma abordagem dinâmica, baseada em modelo de balanço de massa, para avaliar fluxos afluentes e efluentes de cromo em segmentos selecionados dos rios Cadeia e Feitoria (RS, Brasil), visando identificar aportes a partir da atividade dos curtumes instalados na região. Empregando um traçador geoquímico de fontes naturais (ferro), em associação com o método de análise de risco da Agência de Proteção Ambiental dos Estados Unidos, a estratégia permitiu rastrear o incremento potencial do risco toxicológico à saúde humana em cada segmento avaliado, referente à ingestão de água contaminada com cromo de origem antrópica. Para aplicação do modelo numérico, usaram-se dados secundários de vazão e concentração de cromo, obtidos em dez pontos de amostragem, em quatro estações do ano – julho/1999, outubro/1999, janeiro/2000 e abril/2000. Embora os resultados indicassem teores abaixo do limite de detecção para a forma mais tóxica de cromo (hexavalente), assumiu-se um cenário crítico na estimativa do risco: ingestão de água bruta e possibilidade de conversão de todo o Cr3+ presente a Cr6+. Calculados os fluxos afluentes e efluentes de cada segmento, constatou-se uma exportação de cromo do rio Feitoria para o Cadeia e deste para o rio Caí, contribuinte do lago Guaíba. Todos os quocientes de risco obtidos foram inferiores à unidade, sugerindo ausência de possíveis efeitos toxicológicos adversos por ingestão de água contaminada com cromo. Por outro lado, a aplicação do modelo permitiu identificar segmentos fluviais e períodos do ano com incrementos de risco mais elevados, potencialmente decorrentes da contribuição de curtumes. O período de maior risco foi janeiro/2000, em condições de menor disponibilidade hídrica, que destacam a contribuição pontual dos curtumes. Os incrementos no fluxo de cromo antrópico e os correspondentes incrementos de risco foram maiores no trecho inferior do rio Feitoria, onde se concentram curtumes.