Abstract
Abstract 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 (Frd) 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 Frd. 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.
Highlights
Gravity flows are a type of flow whose movement takes place because of relative differences in the density between flow and ambient water
With the purpose of contributing to knowledge about bedform generation by density currents in marine environments, the present study developed an experimental methodology for the generation of saline density currents and investigated the associated bedforms developed from the interaction between these flows and the mobile bed
Dunes (1.35 < Froude number (Frd) < 1.66), and in some cases ripples (1.07 < Frd < 1.70), were identified in supercritical flows conditions, unlike the expected for fluvial bedforms but already observed in other experimental studies with density currents (Hand 1974, Fedele et al 2016, Koller 2016)
Summary
Gravity flows are a type of flow whose movement takes place because of relative differences in the density between flow and ambient water. Turbidity currents may be correlated with the formation of submarine canyons and fans, and the sediment deposits created by these flows can form important hydrocarbon reservoirs (Meiburg & Kneller 2010, Talling et al 2012). The interaction between these currents and mobile beds, through erosion, transport and deposition of sediments, may result in the generation and development of bedforms. They can be developed under subcritical, critical and supercritical hydraulic conditions, established by the Froude number (Simons & Richardson 1961, Kennedy 1969, Engelund & Fredsφe 1982, Van Rijn 1984), which considers the role of inertial and gravitational forces in a flow
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