Froude supercritical geophysical flows: Their related bedforms and frontal structure

Abstract

Sediment transport around the globe is dominated by rivers and turbidity currents. While rivers shape the land, turbidity currents shape the ocean floor. These flows can pose hazards to infrastructure placed in their paths. The sedimentary deposits left behind by these flows are key to understanding how our planet evolved over geological timescales. This thesis aims to enhance the general understanding of these geophysical flows by tackling specific areas of flow dynamics and their deposits that have so far remained poorly understood.
Rivers transport most of their sediment in rare flooding events. A specific type of bedform can occur during these floods, called cyclic steps. Little is known about these cyclic steps as their occurrence is rare and observations during such powerful floods are difficult to make. Here, cyclic steps are reproduced in a numerical model to constrain bedform morphyodynamics. Additionally, the model is used to predict how the occurrence of cyclic steps can be deduced from studying the deposits that are left behind by floods. The numerical modelling shows that the relation between flow properties and the occurrence of erosion deviate from existing models. The deposits arising from cyclic steps depend on how fast sediment is deposited. Cyclic steps that deposit rapidly form a series of upstream-dipping laminations bound by erosion surfaces. Cyclic steps that deposit slowly, however, form amalgamated concave-up erosion surfaces that are infilled with laminations that dip upstream and downstream. These results now allow geologist to reconstruct river floods throughout geological time with more confidence.
In contrast to rivers, where cyclic steps are rare, similar bedforms are abundant in submarine channels, which are major conduits of sediment transport to the deep ocean. These bedforms are an important building block for submarine channels; just as dunes are for rivers. Unlike rivers, the formative controls on these bedforms in submarine channels are presently not well understood. Here I tested three hypotheses on the formation of the bedforms, again using a numerical model. (1) These bedforms only form under fast and thin flow, (2) the bedforms only form under stratified flow and (3) the bedforms only form under flows strong enough to erode sediment from the seafloor. The results of the study showed that (1) not all fast and thin flows create the bedforms, (2) only stratified flows, but not all, create the bedforms and (3) only flows that exceed a threshold of erosion created the bedforms. These results show that the bedforms form in submarine channels that fall within a “sweet spot” of particular grain-size and slope, and thus explains their abundance, but also their local absence.
Finally, this thesis presents some direct observations of turbidity currents on the seafloor. These observations confirm the importance of stratification already noticed in the previous numerical modelling work. The observations show that the fronts of stratified turbidity currents are wedge-shaped, and host the fastest and densest part of the flow. These observations contrast the bulbous and dilute front found in unstratified turbidity currents seen in most modelling studies. The frontal structure of stratified turbidity currents shows remarkable similarities to that of stratified pyroclastic density currents and powder-snow avalanches. A fast and dense front on turbidity currents poses substantially larger hazards for any submarine infrastructure such as pipelines and telecommunication cables.

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