Scott McDougall

Rapid, flow-like landslides, such as rock avalanches and debris flows, cause human and economic losses around the world. The hazards associated with these events are partly related to the spatial runout extent of the flows, and their depth and velocity at elements at risk, which are highly uncertain. This work details the development of methods to estimate the variability of spatial impacts and the impact intensity with statistical models, and by examining the ranges of outcomes in numerical models.Descriptive attributes and mapping techniques were described, and two datasets of rock avalanches and sediment mass flows associated with rock avalanches were compiled. These data were used to develop statistical models to estimate probabilities for a range of potential impacts. These methods provide a new way to assess rock avalanche runout, and a first method for preliminary prediction of mobilized sediment runout.Large rock avalanches or flowslides can generate signals that can be detected by seismometers. Seismic data were used along with aerial imagery to constrain numerical simulations of rock avalanche events considering multi-stage initiations, examining how variability in the model parameters and initiation conditions affected variability in the runout and intensity. This work revealed the modelled seismic signature of a landslide was highly sensitive to the initiation conditions, showing the possibility of multi-stage initiation is an important consideration in understanding landslide dynamics. Observations of debris flows show complex flow patterns, with surges of high discharge followed by periods of low discharge. This surging behaviour is well known, but has not been incorporated into numerical landslide runout models before. By examining the interactions between topography, model parameters and inflow conditions, it was found that all three of these factors interact and influence the simulation results. Substantial variation in the model results could be achieved by varying the model inflow with all other model inputs held constant. The observed variability demonstrates the importance of considering surging behaviour when estimating debris flow impacts. This thesis demonstrates new methods to estimate landslide impacts considering sources of variability that are observed in nature, and provides a framework for both professional application and future research.

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Rapid landslides pose a significant hazard worldwide, and there is currently no routine way of predicting the impact area and velocities of these catastrophic events. Increased development in marginal areas is changing the landslide risk in many parts of the world. There is an urgent need for practical methods to predict the motion of these tragic events to cope with this changing risk. Practical methods currently in use rely on simplified landslide statistics that have a high degree of uncertainty, and are often unable to predict landslide velocities. The focus of this thesis is on developing practical methods to reliably predict the motion of rapid landslides so that public safety in landslide prone areas can be improved.This thesis makes extensive use of runout modelling in order to analyse the motion of rock avalanches, debris avalanches and flowslides. The work presented here can be broadly divided into two categories; the development of new tools and techniques to model flow-like landslide motion, and the compilation and analysis of a database of case histories. The new tools include: 1) A new rheology appropriate for the simulation of liquefied materials; 2) A new dynamic model to simulate the initially-coherent motion of some rock and debris avalanches; 3) Two new calibration methodologies. These techniques were then applied to a database of rock avalanches, debris avalanches and flowslide case histories in order to infer movement mechanisms and give guidance for forward prediction. The main findings include: 1) The character of the path materials is a plausible explanation for the mechanism governing rock avalanche motion. Based on this, a probabilistic framework to predict rock avalanche motion was suggested; 2) A back-analysis of a fatal debris avalanche that occurred in British Columbia in 2012 revealed that this flow was likely moving in an undrained condition, which had significant implications for the analysis of its motion; 3) It was found that flowslides can occur in fine grained colluvium, and this material should be recognized as potentially liquefiable.

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