Abstract
The sedimentology of rock-avalanche deposits, mass movements that commonly cause natural dams is poorly understood. This paper presents the results of detailed research into the grain-size distribution and sedimentology of rock-avalanche deposits. These data show poorly sorted deposits composed of angular, highly-fragmented clasts with preserved source stratigraphy. Grain-size distributions segregate based on rock type and become finer with distance travelled from the source when topographic constraints do not dominate. The results indicate that the inverse grading noted at deposits is often a misconception, the main interior of deposits show fragmentation variation only due to lithology. A facies model is presented consisting of a surficial, coarse openwork carapace facies, a fragmented interior body facies and a basal facies of entrained substrate mixed with rock-avalanche debris. Grain-size prove fractal in nature in direct contrast to Weibull type distribution previously cited for rock-avalanche deposits and suggest confined comminution as the formational mechanism. An application of the data is presented using finite element/limit equilibrium techniques to a simplified rock-avalanche dam. The modelling shows that sedimentology is key to the failure sequence. An idealised homogenous dam took 17.75 days to fail; a more sedimentologically realistic dam failed within 48 h as the lake reaches the carapace facies and phreatic tonguing occurred.
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Acknowledgements
This research and attendance of the NATO Advanced Research Workshop in 2004 from which this paper has developed was funded by the International Landslide Centre, University of Durham. The work was carried out in association with Dr. Tim Davies, Canterbury, New Zealand, and Dr. Mauri McSaveney, Geological and Nuclear Sciences, New Zealand, their help is much appreciated. The 2004 comments of the editorial committee when an earlier version of this paper was submitted are gratefully acknowledged.
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Dunning, S.A., Armitage, P. (2011). The Grain-Size Distribution of Rock-Avalanche Deposits: Implications for Natural Dam Stability. In: Evans, S., Hermanns, R., Strom, A., Scarascia-Mugnozza, G. (eds) Natural and Artificial Rockslide Dams. Lecture Notes in Earth Sciences, vol 133. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-04764-0_19
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