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Topic

Flushed Away: How low and high magnitude turbidity currents affect the bathymetry, morphology and, sedimentation in the Nass Delta submarine channel

Department of Geography

Date & location

• Monday, January 13, 2025

• 9:00 A.M.

• Clearihue Building, Room B017

• and Virtual Defence

Reviewers

Supervisory Committee

• Dr. Eva Kwoll, Department of Geography, University of Victoria (Supervisor)

• Mr. Cooper Stacey, Department of Geography, UVic (Member/NRCan)

• Dr. Gwyn Lintern, Department of Geography, UVic (Member/NRCan) 

External Examiner

• Dr. Matthieu Cartingny, Department of Geography, University of Durham 

Chair of Oral Examination

• Dr. Ulrich Mueller, Department of Psychology, UVic

   

Abstract

Turbidity currents, a type of sediment density flow event, are one of the largest mechanisms of sediment transport in the ocean where they often incise a submarine channel that can confine flows and allow them to runout long distances. However, these flow events are poorly understood as they are difficult to monitor directly, and therefore questions remain about how different magnitudes of flow move sediment while interacting with seafloor morphology. The aim of this study was to address some of these knowledge gaps by investigation sediment change in the Nass Delta submarine channel system, a region that had not previously been studied. Here, repeat bathymetric surveys from 2020, 2021, and 2023 were used to calculate bathymetric change to quantify erosion and deposition in the submarine channel system caused by turbidity current flows. The results from the first bathymetric change interval show predominantly deposition at the head of the channel, an approximate balance of erosion and deposition throughout most of the length of the channel and minor deposition on the lobe, whereas during the second interval the channel was dominated by erosion, and the lobe complexes showed net deposition that was an order of magnitude higher than the previous interval. This was interpreted to represent three magnitudes of flows that were either dissipative with a short runout distance, in an auto suspension state with balanced erosion and deposition, or in an ignition state. These flows were observed to modify in channel morphology including inducing migration of crescentic bedforms, knickpoints, and the widening of channel bends. Knickpoints were shown to influence the highest rates of erosion in both bathymetric survey intervals. However, only during the ‘flushing’ event were knickpoints created or removed. The ‘flushing’ event also caused a channel avulsion which reactivated a distributary channel that had been previously blocked. Here, the term thresholding flows is proposed to describe these events that change the channel equilibrium state. Two facies were identified in sediment cores: Facies 1 which were interpreted as higher magnitude flows with the Ta, Tb, Tc, and Td Bouma intervals and Facies 2 which were interpreted as smaller magnitude flows with Td and Te Bouma intervals. These deposits were dated by creating an age-depth model using excess 210Pb activity and 14C ages. This gave a return interval of 21 years for Facies 1 and 9 years for Facies 2 in the mid-channel reaches and 85 years for Facies 1 and 15 to 40 years for Facies 2 in the distal basin. Although there were no direct measurements of turbidity currents, observations of river discharge, tidal predictions and mass wasting in the canyon indicate that the most likely triggers were either convective settling and remobilization of sediment on the delta or slope failures at the delta lip or canyon.