Debris flow hazard modelling on medium scale: Valtellina di Tirano, Italy

Abstract

Abstract.Debris flow hazard modelling at medium (regional) scale has been subject of various studies in recent years. In this study, hazard zonation was carried out, incorporating information about debris flow initiation probability (spatial and temporal), and the delimitation of the potential runout areas. Debris flow hazard zonation was carried out in the area of the Consortium of Mountain Municipalities of Valtellina di Tirano (Central Alps, Italy). The complexity of the phenomenon, the scale of the study, the variability of local conditioning factors, and the lacking data limited the use of process-based models for the runout zone delimitation. Firstly, a map of hazard initiation probabilities was prepared for the study area, based on the available susceptibility zoning information, and the analysis of two sets of aerial photographs for the temporal probability estimation. Afterwards, the hazard initiation map was used as one of the inputs for an empirical GIS-based model (Flow-R), developed at the University of Lausanne (Switzerland). An estimation of the debris flow magnitude was neglected as the main aim of the analysis was to prepare a debris flow hazard map at medium scale. A digital elevation model, with a 10 m resolution, was used together with landuse, geology and debris flow hazard initiation maps as inputs of the Flow-R model to restrict potential areas within each hazard initiation probability class to locations where debris flows are most likely to initiate. Afterwards, runout areas were calculated using multiple flow direction and energy based algorithms. Maximum probable runout zones were calibrated using documented past events and aerial photographs. Finally, two debris flow hazard maps were prepared. The first simply delimits five hazard zones, Correspondence to: J. Blahut (blahut@irsm.cas.cz) while the second incorporates the information about debris flow spreading direction probabilities, showing areas more likely to be affected by future debris flows. Limitations of the modelling arise mainly from the models applied and analysis scale, which are neglecting local controlling factors of debris flow hazard. The presented approach of debris flow hazard analysis, associating automatic detection of the source areas and a simple assessment of the debris flow spreading, provided results for consequent hazard and risk studies. However, for the validation and transferability of the parameters and results to other study areas, more testing is needed.

ASCHFLOW - A dynamic landslide run-out model for medium scale hazard analysis

“…The spreading and run-out estimates were based on basic probabilistic and energy calculations that allow them to define the maximal run-out distance of a debris flow. Blahut et al (2010a) used also Flow-R for a debris flow hazard assessment at medium scale in Valtellina di Tirano, Italy. Maximum probable run-out zones were calibrated using documented past events and aerial photographs.…”

“…This was done in order to test its use for medium scale debris flow susceptibility assessment. As mentioned in the introduction, two of the most recent studies in regional modeling of hazard have been done by Blahut et al (2010a) and Kappes et al (2011) with the Flow-R model. The results of the "AschFlow" model are compared with their results in order to evaluate the two modeling approaches and to make an assessment of the obtained modeling outputs.…”

“…The results of the "AschFlow" model in the Valtellina Valley simulating a major event using the mean values of the distribution were compared to the geometric results of Blahut et al 2010a where they calibrate the maximum probable debris flow run-out with the 19th July 1987 event and aerial photographs from 2001 (using the edge of alluvial fans where previous debris flows were observed). The modeled footprints obtained with "AschFlow" were compared with the modeled classes in the Flow-R model.…”

“…The central part of the valley in the neighborhood of Tirano town was chosen for a case study because a DEM with a resolution of 5 m was available for that area. Another reason to select that area was the availability of an inventory of past events and a susceptibility map of initiation areas (Blahut et al 2010a;Blahut et al 2010b) (Fig. 5).…”

“…Urbanized areas, water, quarries and orchards were excluded from the land use map. Thresholds values for the terrain factors and the exclusion of units from the land use and lithological maps were chosen based on the values used by Blahut et al (2010a) and Kappes et al (2011). A buffer zone linked to the flow accumulation of 50 m was created in order to include only the susceptible points inside these areas.…”