Comparative hydraulic and sedimentologic study of ripple formation using experimental turbidity currents and saline currents

Abstract

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%.