Soil slips and debris flows on terraced slopes

Abstract

Abstract.Terraces cover large areas along the flanks of many alpine and prealpine valleys. Soil slips and soil slipsdebris flows are recurrent phenomena along terraced slopes. These landslides cause damages to people, settlements and cultivations. This study investigates the processes related to the triggering of soil slip-debris flows in these settings, analysing those occurred in Valtellina (Central Alps, Italy) on November 2000 after heavy prolonged rainfalls. 260 landslides have been recognised, mostly along the northern valley flank. About 200 soil slips and slumps occurred in terraced areas and a third of them evolved into debris flows.Field work allowed to recognise the settings at soil slipdebris flow source areas. Landslides affected up to 2.5 m of glacial, fluvioglacial and anthropically reworked deposits overlying metamorphic basement. Laboratory and in situ tests allowed to characterise the geotechnical and hydraulic properties of the terrains involved in the initial failure. Several stratigraphic and hydrogeologic factors have been individuated as significant in determining instabilities on terraced slopes. They are the vertical changes of physical soil properties, the presence of buried hollows where groundwater convergence occurs, the rising up of perched groundwater tables, the overflow and lateral infiltration from superficial drainage network, the runoff concentration by means of pathways and the insufficient drainage of retaining walls.

The application of numerical debris flow modelling for the generation of physical vulnerability curves

“…Selvetta is located in a north-facing slope of the valley. The slopes are mainly composed of metamorphic rocks (gneiss, mica schist, phyllite, and quartzite) and intrusive rock units, with subordinate sedimentary rocks (Crosta et al, 2003). On less steep parts, two Pleistocene glacial terraces could be distinguished at the height of about 560-760 m a.s.l.…”

“…A total of 25 000 evacuated persons, 53 casualties and 2000 million euros (C) of damages were recorded (Luino, 2005). From 14 to 17 November 2000, prolonged and intense rainfalls triggered about 260 shallow landslides in Valtellina (Crosta et al, 2003). About 200 soil slips and slumps took place in the terraced slopes, and one third of them evolved into debris flows .…”

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

“…A homogeneous erodible soil depth of 1.5 m was selected since there was no detailed soil depth map available (this depth was also assumed to quantify the released initial volume). This assumption was done after analyzing the descriptions of the geological and morphological characteristics of the area and past events by Cancelli and Nova (1985), Crosta (1990) and Crosta et al (2003). In terms of the computational time parameters, as used in the Barcelonnette study case, a calculation time step of 1 s was selected and the simulations had a total time elapsed of 500 s. For the simulation numerical stability control the values for the CFL superior limit used was 0.6 and lower limit was 0.3 with a maximum of 124 loops.…”

Climate change impacts on the Alpine ecosystem: an overview with focus on the soil

“…Landslides and floods affect the surface soils developed on hillslopes, provoking the loss of highly productive soils (loss of organic matter in soils, degradation of the ecological functions of soils) and the partial or complete destruction of crops planted in steep terrains. An example of the consequences of landslide development for crops in steep terrains of the Alpine region is the destruction of terraced slopes with vineyards and the subsequent economic losses for the intense rainfall event of 14-17 November 2000 in the Valtellina area (Central Alps, Italy) (Crosta et al 2003).…”