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Grain size and magnitude of post-earthquake debris flows

Harvey, Erin L 2022. Grain size and magnitude of post-earthquake debris flows. PhD Thesis, Cardiff University.
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Post-earthquake debris flows pose significant hazards to recovering local communities and transport large volumes of co-seismic sediment from hillslopes into higher order channels. In the decade after the 2008 Wenchuan earthquake, several extremely large debris flows have occurred. These debris flows, hereafter referred to as catastrophic debris flows, bulked to volumes (>105 m3) much larger than their initiation volumes by rapidly entraining sediment. Controls on the runout and magnitude of these catastrophic debris flows are poorly constrained due to the lack of in-field measurements and the infrequent and unpredictable nature of large debris flows. In this thesis I used field investigations of debris flow grain-size distributions (GSDs) and modelling to infer controls on the runout length of catastrophic debris flows. I first compared five different methods to identify the most accurate approach to measure mass movement deposit GSDs. Based on these results, I then used three methods (sieving, manual photo counts and automated tool, pyDGS) to collect high-resolution GSDs for two post-earthquake debris flows in Wenchuan. Both debris flows were triggered from co-seismic sediment in the same storm event but had vastly different runout lengths. The debris flows deposited similar GSDs in terms of width and maximum size, suggesting that grain size was not the primary control on runout length in these locations. Using a multi-temporal inventory, I then analysed controls on debris flow magnitude regionally. I found that catastrophic debris flows were more frequent than estimated when assuming a single magnitude-frequency distribution for all debris flows. Finally, I used the runout model Massflow to demonstrate that bed saturation, basal friction angle and triggering volume all controlled the runout of catastrophic debris flows. Magnitude-frequency distributions based on simulated debris flows, where parameters were described using field and remotely sensed data, also underestimated the frequency of catastrophic debris flows. This thesis used local and landscape-scale datasets to infer that catastrophic debris flows should be considered as a separate process. By currently considering all post-earthquake debris flows on a single continuum we may be underestimating the risk posed by debris flows to infrastructure and life.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Funders: NERC GW4+ DTP
Date of First Compliant Deposit: 2 May 2023
Last Modified: 03 May 2023 09:21

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