Comparing models of debris-flow susceptibility in the alpine environment
Introduction
In the Alpine mountain belt debris flows are widespread and constitute one of the most dangerous gravity-induced surface processes that cause severe damage to dwellings, roads and other lifelines. Evaluation of the susceptibility requires the identification and the quantitative assessment of the factors leading to the initiation, propagation and deposition of debris-flows. The task is not easy and is complicated by the fact that the term “debris-flow” is commonly used to indicate a wide spectrum of slope-instability phenomena that may significantly differ in mechanical properties of the material (fine particles vs. rock boulders, etc.), geomorphological setting of the process (open slope, channel, etc.), and hydrological conditions (high or low water content, etc.).
We confined the investigation to develop models to predict debris-flow source areas, neglecting the runout of the phenomena. Indeed, most of the predicting geo-environmental variables, which can be cost-effectively acquired at regional scale (rock type, land-use, structural setting, etc.), are poorly suited to model the propagation of debris-flows. The latter task requires the collection of site-specific data generally not available over large areas. Despite the few attempts to model debris flow runout along a 3D topography with a spatially distributed approach at the regional scale (Iverson et al., 1998, O'Brien et al., 1993, Chau and Lo, 2004), we believe that the uncertainties relative to the parameters controlling runout and the complexity of the processes involved still hamper reliable and physically sound modelling of the runout susceptibility.
Confining the analysis to the problems related to model the initiation of debris-flows, we point out that local micro-topography and mechanical and hydraulic soil properties greatly control the processes responsible for triggering debris flows. As a consequence, “physically-based” models are widely used to assess debris-flow susceptibility (Montgomery and Dietrich, 1994, Wu and Sidle, 1995, Burton and Bathurst, 1998, Crosta and Frattini, 2003). Indeed, these models are more frequently applied than empirical and statistical approaches (Irigaray et al., 1999, Dai and Lee, 2001, Baeza and Corominas, 2001, Lee et al., 2002) that are commonly and successfully used for evaluating the occurrence of deep-seated landslides (e.g., Carrara, 1983, Yin and Yan, 1988, Carrara et al., 1995, Chung et al., 1995).
Although physically-based models may be suitable for modelling the hydrological conditions leading to debris-flow initiation, they have several limitations when applied to predict the spatial distribution of debris flows, i.e. to map debris-flow susceptibility. With the exception of slope morphology, in fact, the physical variables that control the spatial distribution of landslides within such models (i.e., physical and mechanical parameters of the slope and the failed material) cannot be acquired over large areas at reasonable cost.
Conversely, data-driven, statistical models are intrinsically based on the analysis of relations between the spatial distribution of “mappable” instability factors (i.e., the environmental factors that are directly or indirectly correlated with slope instability) and the observed distribution of landslides. Under the assumption that the factors that caused slope failures in a specific region are similar to those that will generate landslides in the future, statistical techniques may provide information on the future spatial distribution of landslides.
In the domain of geomorphology, land evaluation (Dymond et al., 1995, Gallant and Wilson, 2000, Pike, 2000) and mineral exploration (Isaaks and Srivastava, 1989), investigators have highlighted the relevance of the terrain unit (or mapping unit or homogeneous domain) in analysing and modelling environmental or geological phenomena. With a few exceptions, the topic has not received adequate attention in landslide literature. This is quite surprising since, when a given unit is selected, all subsequent works will refer to and treat each terrain unit as a spatially homogeneous domain in terms of both instability characteristics and the degree of susceptibility. In landslide susceptibility assessment, geomorphological units (Hansen, 1984, Meijerink, 1988), grid-cells, unique condition units (resulting from the overlay of categorical map; Bonham-Carter et al., 1989), stream tubes (Gallant and Wilson, 2000), terrain facets (Dymond et al., 1995), and slope units (Carrara et al., 1991) all have been used with different degrees of frequency and success (Carrara et al., 1995).
The Val di Fassa (Trento Province, Eastern Italian Alps) is famous for its landscape of very high cliffs carved in thick dolostone or limestone sequences that frequently rise to over 1000 m. The steep slopes and channel networks of these spectacular mountains are, however, a major source of debris flows that seriously threaten the inhabitants of the valley, their dwellings, local roads, and the other lifelines.
As part of a long-term project aimed at assessing the susceptibility from both shallow fast-moving (debris flows, rockfalls, etc), and deep-seated (slides, rock and earth flows, etc.) landslides in Val di Fassa, we carried out a systematic inventory of debris flows through aerial photo interpretation and field work (Carrara et al., 2004). Owing to the wealth of data acquired, and taking advantage of previous experience on susceptibility assessment (Carrara, 1983, Carrara et al., 1995, Carrara et al., 2003a), we developed different predictive models of debris-flow source areas by applying statistical and physically-based approaches.
In this paper we illustrate and compare statistical and physically based models designed to assess debris-flow occurrence. We discuss the application of statistical models to debris flows using different terrain units, and critically evaluate the results in terms of data quality and potential application of the resulting susceptibility maps in land management and planning. Lastly, we examine and discuss the verification and evaluation of these models.
Section snippets
Study area
The study area, which is almost 300 km2 in size and includes the upper drainage basin of the Avisio river (Val di Fassa, Trento Province), lies in the Dolomites that are part of the Southern Calcareous Alpine belt (Fig. 1). Bedrock geology is made up by different rock types that witness the complex history of the region (Bosellini et al., 2003). A basal effusive igneous (rhyolite) complex, which crops out in the southern part of the area, underlies a sedimentary succession that ranges from
Debris-flow inventory and thematic maps
Debris flows are common in Val di Fassa, especially on the upper slopes, where glacial talus and colluvial deposits supply large amounts of detrital material for the initiation and propagation of the process (Fig. 1). Historically, debris flows have reached the alluvial fans along the main valley where villages are located, causing damage to human life and property. The last significant event dates from 1989, when three small channels were invaded by debris flows (Dona and Udai creek) or
Models of debris-flow susceptibility
To predict location of the debris-flow source areas in Val di Fassa, we designed and implemented five models that differ in method and type of terrain unit of reference (Table 1). Among the various indirect methods and techniques applied in the domain of landslide assessment (empirical, statistical, neuronal networks, deterministic, physically-based, etc.), we selected two multivariate procedures (discriminant and logistic regression) and a physically-based (SHALSTAB) approach.
Over the last
Model comparison
Having developed different models of debris-flow source areas, we faced the issue to compare them to both highlight their respective limitations/potentials, and establish which would be most suitable for planning soil conservation measures and setting forth land use regulations by local administrators. Comparison was accomplished through traditional, GIS-based, map overlay operations, and by calculating receiver operating characteristic (ROC) curves of each model.
For discriminant (fSU_DIS) and
Factors leading to debris-flow initiation
The modelling strategies for debris-flow susceptibility mapping implemented and tested here for the Val di Fassa area differ in terms of method (statistical vs. deterministic and type of terrain-unit of reference (slope unit vs. grid cell; Table 2). However, all share the same input dataset: the debris-flow inventory map and the thematic layers describing the geo-environmental characteristics of the study area. This uniformity makes it possible to compare the different methods and identify the
Conclusions
Developing and comparing five different models for predicting the occurrence of debris-flow source areas in Val di Fassa study area led to the following results:
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Models based on multivariate statistical techniques and a physically-based approach proved to differ significantly, as expressed by spatial mismatch, close to 25%, between the maps displaying areas predicted by each model as stable and unstable.
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A larger spatial discrepancy (over 30%) is observed between susceptibility maps generated by
Acknowledgements
We are grateful to Richard Pike, U.S Geological Survey-Menlo Park, for the critical review of the manuscript. We thank Paolo Campedel, Geological Survey of the Provincia Autonoma di Trento, and Marco Stefani, University of Ferrara, for having made available geological and land-use data for the Val di Fassa area. Elena Valbuzzi, Samuel Cucchiaro, and Sabrina Iannaccone are acknowledged for the contribution in the preparation of the landslide inventory map by aerial photographic interpretation.
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