How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study

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Vol17 No.4: 753-772

Title】How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study

Author】Bruno MAZZORANA1,2,3; Elisa GHIANDONI4; Lorenzo PICCO2,3,4

Addresses】1 Universidad Austral de Chile, Faculty of Sciences, Instituto de Ciencias de la Tierra, 5090000 Valdivia, Chile; 2 Universidad Austral de Chile, RINA – Natural and Anthropogenic Risks Research Center, 5090000 Valdivia, Chile; 3 Universidad Austral de Chile, Laboratory of Hydromorphology, Faculty of Forest Sciences and Natural Resources, 5090000 Valdivia, Chile; 4 University of Padova, Department of Land, Environment, Agriculture and Forestry, 35020 Legnaro, Italy

Corresponding author】Bruno MAZZORANA

Citation】Mazzorana B, Ghiandoni E, Picco L (2020) How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study. Journal of Mountain Science 17(4). https://doi.org/10.1007/s11629-019-5788-x

DOI】https://doi.org/10.1007/s11629-019-5788-x

Abstract】Alluvial fans are among the most privileged settlement areas in many mountain regions. These landforms are particularly dynamic being episodically affected by distributary processes generated by extreme flood events. Addressing risk assessment entails determining hazard exposure and unravelling how it might be related to process loading and to process dynamics once the flow becomes unconfined on the surface of alluvial fans. By following a ‘similarity of process concept', rather than by attempting to scale a real-world prototype, we performed a set of 72 experimental runs on an alluvial fan model. Thereby, we considered two model layouts, one without a guiding channel and featuring a convex shape and the other one with a guiding channel, a bridge, and inclined but planar overland flow areas as to mirror an anthropic environment. Process magnitude and intensity parameters were systematically varied, and the associated biphasic distributary processes video recorded. For each experiment, the exposure was detected by mapping the exposed area in a GIS, thereby discerning between areas exposed to biphasic flows and the associated depositional phenomena or to the liquid flow phase only. Our results reveal that total event volume, sediment availability and stream power in the feeding channel, as well as depositional effects, avulsion, and channelization on the alluvial fan concur to determine the overall exposure. Stream process loading alone, even when rigorously defined in terms of its characterizing parameters, is not sufficient to exhaustively determine exposure. Hence, further developing reliable biphasic simulation models for hazard assessment on settled alluvial fans is pivotal.

Keywords】Alluvial fan; Hazard; Exposure; Flood risk; Experimental model; Process similarity