Experimental study of sequential high density turbidity currents and their deposits in response to a simultaneous slope break and loss of confinement

Results are presented from a series of scaled laboratory experiments to explore how sequential high-density turbidity currents (HDTCs) with different volume fluxes (Q0 = 10.2 - 13.5 m3/hr) and initial volumetric sediment concentrations (c0 = 0.12 - 0.16) respond to different slope break (SB) angles (5° - 9°) at a simultaneous loss of confinement (LOC). The study also investigates the impact of the SB-LOC on the stacking pattern of the basin deposits produced by the sequential HDTCs. Of particular interest is the effect that antecedent HDTCs and their deposits have on the subsequent HDTC dynamics and how this feedback drives the evolution of basin depositional features. Each experimental run consists of three or four sequential HDTCs that are scaled on non-dimensional parameters for the flow properties (i.e. densimetric Froude and Reynolds numbers) and sedimentary conditions (i.e. Shields and Rouse numbers) (e.g. de Leeuw et al., 2016). Results indicate that on reaching the SB-LOC, the peak flow velocities within the HDTCs range between 0.88 m/s and 1.29 m/s. Immediately downstream of the SB-LOC, the HDTCs collapse towards the bed and expand radially into the basin (i.e. flow relaxation), resulting in flow acceleration and higher shear velocities immediately downstream of the SB-LOC. This transition from two- and three-dimensional flow processes between the confined channel and unconfined basin causes the initial HDTCs to produce a channel-lobe transition zone (CLTZ) immediately downstream of the SB-LOC. Although these CLTZs are maintained over subsequent HDTCs, they typically shorten as the centroids of the individual lobe deposits occur within increasing proximity to the feeder channel and SB-LOC. Ultrasonic velocity profile measurements demonstrate that the depositional topography from initial HDTCs reduces the centerline velocity of sequential HDTCs, whilst deposit maps highlight how sedimentation is topographically steered towards the flanks of the evolving lobe deposit. Results therefore highlight the mechanisms that drive lobe deposition downstream of a SB-LOC and provide an insight into how HDTCs form and maintain CLTZs.