Analysis of runout prediction methods for debris flows

The term runout length of debris flows refers here to the depositional part or terminal flow path downstream of a defined point; for larger debris flows in the Alps deposition usually occurs on the fan. Although runout prediction of debris flows is essential for hazard assessment, no well established methods are available. The project aims at testing existing and developing new methods. The following methods will be considered for runout estimation: Empirical-statistical approaches refer to methods where the runout distance is typically related to morphometric parameters of the flow path and to the event volume. Analytical methods are those based on a simplified mechanical description of the flow behavior for which analytical solutions are available. Continuum-based mechanical simulation models for debris flows require numerical procedures to solve the differential equations. In the past, similar methods were proposed for the estimation of the runout distance of snow avalanches and landslides/rock falls. Some of these methods were used quite successfully for predicting runout of snow avalanches where sufficient data were available to develop or calibrate the methods. It is expected that similar methods can be developed for runout estimation of debris flows. The emphasis of the project is on larger events occurring in the Alpine region, i.e. debris flows with event magnitudes larger than about 5000 to 10`000 m3. As a basis for the project, existing information on past debris flow events in Austria will be compiled and supplemented by additional information. The newly developed methods for runout prediction of debris flows can be an important and useful tool supporting practical hazard assessment in torrent catchments.

Although debris-flow events occur less frequent related to similar natural hazards, these processes often endanger human living in Alpine regions. The disastrous effects of debris-flow events are based on the combination of precipitation events with the availability of transportable sediments. Beside the knowledge of the triggering of a debris flow and the basic disposition, it is therefore also important to estimate the potential runout of a debris flow, in terms of a debris-flow hazard analyses. In a first step, a database on historical torrential events in the Alps has been established. Within this database we compiled about 17 fluvial sediment transport events, 10 debris-flood events as well as 106 debris-flow events based on documentations from Austria, Switzerland and South-Tyrol (Italy). Based on the database, existing methods to classify different process types were applied and discussed. Additionally a new method was proposed to classify deposition patterns of debris-flow events. Further analysis of the compiled data confirm the semi-empirical approach to describe the mobility of a debris flow based on the relationship between observed deposition areas and deposition volumes. This analysis results in a new, quantitative method to describe a potential runout of debris-flow events on the fan. By empirical evidence of the compiled geomorphologic data, a correlation between the average fan slope and the average channel slope was found to calculate a dimensionless "mobility-coefficient". This coefficient can be used to estimate the potential runout of debris-flow events on the fan. For the practical application of the concept of the mobility coefficient, a new simulation tool, called TopRunDF, was developed. This program uses the semi-empirical area-volume relationship and is based on a D8-algorithm to estimate potential flow paths. TopRunDF further combines the single flow path algorithm (D8) with a Monte-Carlo technique to simulate lateral spreading depending on the fan topography. In a further step a one-dimensional analytical approach to estimate the runout of a debris flow on the fan was tested combined with a sensitivity study of the needed input parameter. Based on the results of this study, a second simulation tool (TopFlowDF) was developed to estimate the runout as well as the maximum velocity of a potential debris-flow event on the fan. Both simulation tools (TopRunDF, TopFlowDF) have been evaluated based on observed events and can be downloaded for free by www.debris-flow.at.