How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows

Abstract

Subaqueous sediment density flows are one of the volumetrically most important processes for moving sediment across our planet, and form the largest sediment accumulations on Earth (submarine fans). They are also arguably the most sparely monitored major sediment transport processes on our planet. Significant advances have been made in documenting their timing and triggers, especially within submarine canyons and delta-fronts, and freshwater lakes and reservoirs, but the sediment concentration of flows that run out beyond the continental slope has never been measured directly. This limited amount of monitoring data contrasts sharply with other major types of sediment flow, such as river systems, and ensure that understanding submarine sediment density flows remains a major challenge for Earth science. The available monitoring data define a series of flow types whose character and deposits differ significantly. Large (N 100 km 3 ) failures on the continental slope can generate fast-moving (up to 19 m/s) flows that reach the deep ocean, and deposit thick layers of sand across submarine fans. Even small volume (0.008 km 3 ) canyon head failures can sometimes generate channelised flows that travel at N 5 m/s for several hundred kilometres. A single event off SE Taiwan shows that river floods can generate powerful flows that reach the deep ocean, in this case triggered by failure of recently deposited sediment in the canyon head. Direct monitoring evidence of powerful oceanic flows produced by plunging hyperpycnal flood water is lacking, although this process has produced shorter and weaker oceanic flows. Numerous flows can occur each year on river-fed delta fronts, where they can generate up-slope migrating crescentic bedforms. These flows tend to occur during the flood season, but are not necessarily associated with individual flood discharge peaks, suggesting that they are often triggered by delta-front slope failures. Powerful flows occur several times each year in canyons fed by sand from the shelf, associated with strong wave action. These flows can also generate up-slope migrating crescentic bedforms that most likely originate due to retrogressive breaching associated with a dense near-bed layer of sediment. Expanded dilute flows that are supercritical and fully turbulent are also triggered by wave action in canyons. Sediment density flows in lakes and reservoirs generated by plunging river flood water have been monitored in much greater detail. They are typically very dilute (b 0.01 vol.% sediment) and travel at b 50 cm/s, and are prone to generating interflows within the density stratified freshwater. A key objective for future work is to develop measurement techniques for seeing through overlying dilute clouds of sediment, to determine whether dense near-bed layers are present. There is also a need to combine monitoring of flows with detailed analyses of flow deposits, in order to understand how flows are recorded in the rock record. Finally, a source-to-sink approach is needed because the character of subm...

Key Future Directions For Research On Turbidity Currents and Their Deposits

“…4;Talling et al 2013). However, it is the longer-runout flows which reach submarine fans that form much of the rock record, and their character may differ significantly from shorter and more frequent events.…”

“…The challenges of selecting locations where turbidity currents are frequent and designing instruments that can make the necessary measurements are well known (Inman et al 1976;Talling et al 2013). The flows evolve significantly, such that source-to-sink data are needed.…”

“…such as breaching (Van Den Berg et al 2002;Mastbergen and Van Den Berg 2003;Talling et al 2013) or liquefied slumps (Paull et al 2010a;their fig. 12) may be involved.…”

“…Sediment concentration is the critical parameter controlling such flows, yet it has never been measured directly for flows that reach and build submarine fans. Indeed, studies in only six locations (Xu et al , 2013(Xu et al , 2014Xu 2011;Hughes Clarke et al 2012, 2014Cooper et al 2012;Khripounoff et al 2003Khripounoff et al , 2012Vangriesheim et al 2009;Ayranci et al 2012; Lintern and Hill, personal communication; see Talling et al 2013) have estimated sediment concentrations for turbidity currents in shallow water, typically using backscatter values from acoustic Doppler current profilers (ADCPs). This indirect technique for measuring sediment concentration (backscatter is also affected strongly by additional parameters including grain size) has large uncertainties and cannot penetrate into high sediment concentrations at the base of the flow.…”

Insights into Submarine Geohazards from Breaks in Subsea Telecommunication Cables

“…This paper reports on a succession of cable-damaging events that disrupted a key part of the modern fiber-optic network off southernmost Taiwan (Hsu et al, 2008;Carter et al, 2012;Gavey, 2012;Su et al, 2012;Talling et al, 2013). Information from this network provides further insights into the triggers, frequency, and behavior of sediment density flows, a general term applied here to cover various forms of sediment-laden currents, including hyperpycnal flows, debris flows, and turbidity currents, whose movements are controlled by the density of the sediment contained in their waters (for a more detailed description of these flows, see Talling et al, 2013).…”