Which Triggers Produce the Most Erosive, Frequent, and Longest Runout Turbidity Currents on Deltas?

Abstract

Subaerial rivers and turbidity currents are the two most voluminous sediment transport processes on our planet, and it is important to understand how they are linked offshore from river mouths. Previously, it was thought that slope failures or direct plunging of river floodwater (hyperpycnal flow) dominated the triggering of turbidity currents on delta fronts. Here we reanalyze the most detailed time‐lapse monitoring yet of a submerged delta; comprising 93 surveys of the Squamish Delta in British Columbia, Canada. We show that most turbidity currents are triggered by settling of sediment from dilute surface river plumes, rather than landslides or hyperpycnal flows. Turbidity currents triggered by settling plumes occur frequently, run out as far as landslide‐triggered events, and cause the greatest changes to delta and lobe morphology. For the first time, we show that settling from surface plumes can dominate the triggering of hazardous submarine flows and offshore sediment fluxes.

Seeking the Shore: Evidence for Active Submarine Canyon Head Incision Due to Coarse Sediment Supply and Focusing of Wave Energy

“…Considering the predicted sediment budget for the Spanish Flats littoral cell, many transport events likely occurred during the surveyed interval, resulting ultimately in deep incision of a gully at the canyon head and migration of crescent-shaped bedforms characteristic of supercritical flow conditions at the bases of a coarse-grained turbidity currents (Covault et al, 2017;Hage et al, 2018). Due to the canyon's distance from the outlet of a major stream, these events were unlikely to have been triggered by river-generated hyperpycnal currents (Mulder et al, 2003) or plume settling at a river outlet (Hizzett et al, 2018). Instead, wave modeling suggests that a strong longshore current ends abruptly above the canyon head due to canyon-induced sheltering, trapping, and temporarily accumulating sediment in the canyon headwall region.…”

Direct Monitoring Reveals Initiation of Turbidity Currents From Extremely Dilute River Plumes

“…(a–c) Mechanisms triggering turbidity currents at river mouths proposed in the literature. Percentage of flows triggered in Squamish by each mechanism are based on Hizzett et al (). References for given examples are 1: Piper & Savoye, ; Mulder et al, .…”

What controls submarine channel development and the morphology of deltas entering deep‐water fjords?

“…Hizzett et al . () show that these flows generated by settling plume events also tend to coincide with peaks in river discharge.…”

“…However, Hizzett et al . () note that the size of the collapse does not necessarily influence the size of the sediment flow generated, with larger delta‐lip collapse (up to 150 000 m 3 ) often having relatively short runout distances. Therefore it is likely that the increased frequency and erosive nature of sediment flows generated by settling of sediment from surface plumes tend to have the greatest influence on channel system morphology within these fjords.…”

“…As river discharge increases, the frequency of turbidity current events also increases leading to significantly greater turbidity current activity in Knight and Bute Inlets. These flows, most likely generated by settling plume events, also tend to have the longest runouts and therefore are most important in terms of channel extension (Hizzett et al, ). The increased frequency of erosive, channel‐forming flows in Bute and Knight Inlets leads to the enhanced development of submarine channels.…”