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DEPOSITIONAL CHARACTERISTICS AND RUNOUT OF ALPINE DEBRIS FLOWS
Christian Scheidl1, Dieter Rickenmann1,2 INTRODUCTION
The immense destructive impact caused by debris flows endangers human lives and infrastructure facilities all times. Due to their disastrous effect in the fan area (Figure 1), there is a need to better understand and describe the depositional characteristics and runout behaviour of debris flows. The occurrence of such phenomena in steep channels within small catchments often interferes with other natural hazard processes, like rock avalanches, landslides or debris floods and fluvial sediment transport in torrent channels. Several approaches have been developed to specify the magnitude of the depositional area of debris flows.
Fig. 1: Debris flow events, left: Zirknitzbach, Döllach-Austria, Archiv WLV Kärnten (1965), right: Glyssibach, Brienz-Switzerland, Schweizer Luftwaffe (2005).
In the laboratory experiment, Tsai (2006) studied the geometric development of debris-flow fans, and used the maximum length L, width B, and thickness Z of debris-flow fans as the characteristic parameters in the analysis of morphological similarity. In geomorphological studies, the mobility of small and large landslides has been related to the volume and angle of reach (Corominas, 1996; Legros, 2002). Similarly, Crosta and Agliardi (2003) tested the semi¬empirical approach of lahars (Iverson et al., 1998) with a larger number of observations for coarse debris flows. In principle hydraulic simulation models of debris flows can provide the most accurate description of the depositional process; however, these models are sometimes difficult to apply and generally require a "calibration" of "friction" parameters by back- calculation of past events (Rickenmann, 2005; Rickenmann et al., 2006).
The delineation of endangered areas is one of the most important tasks of an effective hazard zone mapping. For debris flows, snow and rock avalanches, runout prediction methods are important tools for natural hazard assessment. For torrential processes, such as debris floods
1 Institute of Mountain Risk Engineering (IAN), University of Natural Resources and Applied Life Sciences, Peter-Jordan-Strasse 82, 1190 – Wien
2 Swiss Federal Research Institute (WSL), Mountain Hydrology and Torrents, Zürcherstrasse 111, CH-8903 Birmensdorf
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and debris flows, there is no simple and universal runout prediction method. Due to improved event documentations over the last years within the alpine region, more data about debris flow events are now available. The objective of this study is to test the existing methods and develop new runout prediction methods for debris flows, also using data of recent debris events.
METHODS AND FIRST RESULTS
A first step of the investigation is the acquisition of data about past debris flow and debris flood like events. The main focus is on the deposition volume, deposition area and the shape of the debris-fan. For complementation also topological parameters and descriptions of geomorphologic features of debris events have been collected. Altogether 39 debris flows, 7 debris floods and 15 fluvial sediment transport events of Austria and Switzerland have been selected for further work. For some Swiss debris flow events of 2005, high resolution digital terrain models derived from airborne LiDAR data are available to provide more accurate estimates of event volumes and depositional geometry.
We have examined relations between topological parameters of the catchment, deposition area and type of process. This analysis is the basis for improving existing classification schemes to distinguish between different torrential flow processes. The new methodology, mainly based on the observed depositional characteristics, should help to differentiate debris flow events from debris floods and fluvial sediment transports. Our data also confirm that there is an empirical relationship between the deposition volume and deposition area, showing a similar trend as the data of Iverson et al. (1998) and Crosta and Agliardi (2003).
REFERENCES
COROMINAS J. (1996): The angle of reach as mobility index for small and large landslides, Can. Geotech. Journal, 33, 260-271
CROSTA, G. B. and AGLIARDI, F. (2003): A methodology for physically-based rockfall hazard assessment, Natural Hazards and Earth System Sciences, 3, 407-422
IVERSON, R. M., SCHILLING, S. P., VALLANCE J. W. (1998): Objective delineation of lahar-inundation hazard zones, GSA Bulletin, v.110, no.8 , 972-984
LEGROS F. (2002): The mobility of long-runout landslides, Engineering Geology 63, 301–
331RICKENMANN, D., LAIGLE, D. MCARDELL, B.W., HÜBL, J. (2006): Comparison of 2D debris-flow simulation models with field events. Computat Geosci, 10, 241-264.
RICKENMANN, D. (2005): Runout prediction methods. In: Jakob, M., Hungr, O., Debris- flow Hazards and Related Phenomena, 305-324; Springer, Chichester.
TSAI Y.-F. (2006): Three-Dimensional Topography of Debris-Flow Fan, Journal of Hydraulic Engineering, Vol. 132, 3, 307-318
