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Probabilistic modelling of debris flow travel distance using empirical volumetric relationships Wise, Michael Paul


Debris flows can occur on both forested (natural) and clearcut (logged) hillslopes in coastal British Columbia. Prediction of the travel distance of a potential debris flow event prior to clearcut harvesting is important to accurately assess the risk to downslope environmental resources. The travel distance is the distance from the point of initiation of a debris flow to the point of terminal deposition at the end of the flow path. Forensic data from 449 debris flow events in the Queen Charlotte Islands are used to charactrerize debris flow events in terrain where clearcut logging has been carried out. From these data, a subset of 131 events are used for the development of regression equations to calculate entrainment volume and deposition volume along distinct reaches of a debris flow event path. Slope morphology and geometry along the path, as well as the flow volume entering the reach, are used as input parameters for the regression equations. The regression equations are applied in an empirical-statistical model which uses the cumulative debris flow volume along the event path as a basis for determining the travel distance of debris flow events. The cumulative flow volume is defined as the volume of the flow as the event travels down the path, with the entrainment of debris material increasing the flow volume and the deposition of material decreasing the flow volume. Back-analyses of debris flow events were carried out for 20 independent events in the Queen Charlotte Islands and 17 events in other areas of coastal British Columbia. The model showed reasonable agreement with the peak cumulative flow volume, and the travel distance, of debris flow events reported from observations and surveys in the field. An observed variability in the forensic data was incorporated to create the empirical-statistical model UBCDFLOW. Variations in initial volume, as well as flow width, are used repeatedly to simulate the cumulative debris flow volume along a potential travel path. The probability of an event reaching a point along the path is determined based from the travel distances of these simulated flows. A comparative study of different the scenarios using UBCDFLOW illustrates that for confined flow events in gully channels, the initiation location is an important factor in determining travel distance, whereas the size of the initial volume is an important factor for unconfined flows on open slopes.

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