Abstract
Rock avalanches can trigger destructive associated hazards following the initial collapse and fragmentation of a rock slope failure. One of these associated hazards occurs when the material derived from the initial collapse of the source zone impacts and mobilizes a mass flow composed of sediment from along the travel path. These mass flows can be grouped into radial impact areas that occur on relatively flat, open terrain (typically a floodplain), and more linear impact areas that occur in channelized terrain. Rock avalanche-generated sediment mass flows are an important consideration because they can significantly increase the area impacted by an event, thereby increasing the hazard area, especially in valley bottoms where there are likely more elements at risk. Existing runout prediction methods do not consistently account for the increase in the impact area from rock avalanche-generated sediment mass flows. Thus, there is a need for a simple data-supported method for estimating the extent of mass flow impacts resulting from an initial rock avalanche event with sediments along the potential travel path. This paper presents data from 32 rock avalanches and 23 rock avalanche-generated sediment mass flows from around the world, described using a consistent set of quantitative and qualitative attributes. A wide range of mass flow impacts were observed, with the sediment mass flow impact area or runout length exceeding the impact of the coarse, rocky debris in some cases. The area and length impacted by the coarse, rocky debris is estimated using multiple linear regressions considering the event volume and topographic features. The sediment mass flow dataset is used as input to develop an exponential distribution of the area or runout length of the sediment mass flow over that of the coarse, rocky debris. A decision tree framework is presented for estimating the extent of potential rock avalanches and potential rock avalanche-generated sediment mass flows for hazard and risk analysis, which is demonstrated by comparing the stochastic empirical predictions to those from numerical runout modelling.
Highlights
Rock avalanches are mass flows that can initiate complex hazard cascades
The potential for rock avalanches to generate mass flows of sediments has been recognized for a long time, but quantitative methods to estimate the impacts of these events have been lacking
We have proposed a methodology for consistently describing rock avalanches and the mass flows resulting from them, and compiled a dataset using the new methodology
Summary
Rock avalanches are mass flows that can initiate complex hazard cascades. They are defined by their dominant characteristics: high velocity flow-like motion of a large volume (typically greater than 1 million m3) of fragmenting rock that can travel several kilometers and run up opposing valley slopes (Hermanns, 2013; Hungr et al, 2014). When rock avalanches encounter sediments, open water or snow/ice along their path, a relatively fluid and mobile flow of these materials can be generated that extends beyond the margins of the coarse rocky flow. We refer to this process generally as a rock avalanche-generated mass flow (Figure 1).
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