Florian Pohl, Joris T. Eggenhuisen, Matthieu Cartigny, Mike Tilston

Turbidity currents flowing across the ocean floor encounter changes of the local bathymetry including abrupt reductions in slope gradient also known as slope breaks. Turbidity currents flowing across a slope break will change their flow dynamics and may start to deposit as a consequence. Previous experiments on turbidity currents crossing a slope break have indeed observed abrupt changes of flow dynamics by the formation of a hydraulic jump, i.e., the transformation from Froude super- to subcritical flow. However, in these previous experiments the link between the flow dynamics and the onset of deposition by the flow downstream of the slope break is rather unclear due to the overall depletive and highly depositional character of the turbidity currents. In this paper, Shields-scaled turbidity currents were used to observe the flow dynamics of none-depositional supercritical flows that only started to deposit after passing a slope break. Hydraulic jumps only occurred in experiments where rapid deposition of sediment created an adverse slope downstream of the slope break, which resulted in significant deceleration and chocking of the flow. All flows crossing a slope break showed a thickness increase of the wall-region (i.e. the portion of the flow below the velocity maximum) resulting in a shear velocity decrease, which reduced the sediment suspension capacity. Estimated capacity parameters were below unity suggesting capacity-driven deposition. However, the calculated capacity parameters underestimated of the flow capacity. Because capacity-driven deposition is independent of grain size, the resulting deposits should reassemble the sediment characteristics at the flow base. The deposits in the experiments were, however, coarser and better sorted than the sediment suspended at the flow base. This discrepancy implies that both flow capacity and grain-size (competence) controlled the deposition downstream of a slope break.