Flushed away: How low and high magnitude turbidity currents affect the bathymetry, morphology and, sedimentation in the Nass Delta submarine channel
| dc.contributor.author | Parkinson, Felix | |
| dc.contributor.supervisor | Kwoll, Eva | |
| dc.date.accessioned | 2025-01-31T21:32:48Z | |
| dc.date.available | 2025-01-31T21:32:48Z | |
| dc.date.issued | 2025 | |
| dc.degree.department | Department of Geography | |
| dc.degree.level | Master of Science MSc | |
| dc.description.abstract | Turbidity currents, a type of sediment density flow, are one of the largest mechanisms of sediment transport in the ocean. They often incise submarine channels 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 investigating 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 currents. The results from the first bathymetric change interval (2020-2021) showed little net-change and instead sediment was ‘shuffled’ down system by turbidity currents. This was expressed as 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. During the second interval (2021-2023), the channel was dominated by erosion, similar to ‘flushing’ flows observed in other channel systems, 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 resulted in substantial changes to in-channel morphology including aggradation up to 7 m/year and migration up to 70 m/year of crescentic bedforms, up-channel knickpoint migration up to 371 m/year, and the widening of channel bends by up to 45 m/year. Knickpoints were shown to influence the highest rates of erosion in both bathymetric survey intervals. However, only during the erosive, or ‘flushing’, interval were knickpoints created or removed. The ‘flushing’ interval also caused a channel avulsion which reactivated a distributary channel that had been previously blocked. Here, the term ‘thresholding flows’ is proposed to describe turbidity currents that change the channel equilibrium state. 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. Two facies of turbidite deposits were identified in sediment cores: Facies 1 sand beds, which were interpreted as deposits from higher magnitude flows with the Ta, Tb, Tc, and Td Bouma intervals and Facies 2 sand laminations, which were interpreted as deposits from smaller magnitude flows with Td and Te Bouma intervals. These deposits were dated by creating an age-depth model using excess 210Pb activity, 137Cs activity and 14C ages. This gave a return interval of 25 years for Facies 1 and 10 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 meaning that high- and low-density turbidity currents travel up to 22 km from the Nass Delta. These results show that the Nass Delta submarine channel system experiences frequent turbidity current activity that is comparable to other delta submarine channel systems such as the Squamish Delta and Bute Inlet systems, both over short (~1 year) and long (100s years) timescales, and therefore presents both an excellent study site for investigating turbidity current dynamics and poses a considerable hazard to seafloor infrastructure. | |
| dc.description.scholarlevel | Graduate | |
| dc.identifier.uri | https://hdl.handle.net/1828/21046 | |
| dc.language | English | eng |
| dc.language.iso | en | |
| dc.rights | Available to the World Wide Web | |
| dc.subject | Turbidity currents | |
| dc.subject | Geomorphology | |
| dc.subject | Sedimentology | |
| dc.subject | Geohazards | |
| dc.subject | Submarine channels | |
| dc.subject | Marine | |
| dc.title | Flushed away: How low and high magnitude turbidity currents affect the bathymetry, morphology and, sedimentation in the Nass Delta submarine channel | |
| dc.type | Thesis |