The response of high density turbidity currents and their deposits to an abrupt channel termination at a slope break: Implications for channel–lobe transition zones

Abstract

ABSTRACTThe transition between the slope and basin floor is typically marked by a slope break, in some cases causing channels to terminate and turbidity currents to undergo a loss of confinement. It is thus essential to understand how these slope breaks and losses of confinement influence the hydrodynamic evolution of turbidity currents and impact their depositional variability within natural scale channel mouth settings. Flume experiments, utilizing Shields scaling, are conducted to study how channel slope angle (3°, 6° and 9°) and initial suspended sediment concentrations (12 to 18% by volume) impact the hydrodynamics and deposit geometries of high density turbidity currents, subject to a simultaneous break of slope and loss of confinement. Measured velocity and concentration profiles indicate that turbidity currents are supercritical, with mean velocities between 0.80 m s−1 and 1.04 m s−1 and depth‐averaged basal concentrations between 9.2% and 23.9%, yielding bed shear velocities between 0.050 m s−1 and 0.064 m s−1. Upon encountering the slope break and loss of confinement, turbidity currents exhibit increases to their densimetric Froude numbers and shear velocities. This is due primarily to two factors: firstly, turbidity currents continue to accelerate during an initial period of velocity lag as their residual momentum gradually dissipates; and, secondly, expansion via flow relaxation collapses their structure towards the bed. The corresponding depositional geometries of these processes reveal that turbidity currents produce elongate channel–lobe transition zones that disconnect channel and basin deposits. The length to width ratios of channel–lobe transition zones decrease as the initial sediment concentrations of turbidity currents increase, while a reduction in the channel slope break angle reduces their length to width ratios. Corresponding, lobe elements are observed to increase in length, width and thickness with increasing initial sediment concentrations, while a reduction in channel slope break angle reduces their dimensions due to enhanced slope deposition.