Abstract
Abstract. Rainfall-induced shallow landslides are common phenomena in many parts of the world, affecting cultivation and infrastructure and sometimes causing human losses. Assessing the triggering zones of shallow landslides is fundamental for land planning at different scales. This work defines a reliable methodology to extend a slope stability analysis from the site-specific to local scale by using a well-established physically based model (TRIGRS-unsaturated). The model is initially applied to a sample slope and then to the surrounding 13.4 km2 area in Oltrepò Pavese (northern Italy). To obtain more reliable input data for the model, long-term hydro-meteorological monitoring has been carried out at the sample slope, which has been assumed to be representative of the study area. Field measurements identified the triggering mechanism of shallow failures and were used to verify the reliability of the model to obtain pore water pressure trends consistent with those measured during the monitoring activity. In this way, more reliable trends have been modelled for past landslide events, such as the April 2009 event that was assumed as a benchmark. The assessment of shallow landslide triggering zones obtained using TRIGRS-unsaturated for the benchmark event appears good for both the monitored slope and the whole study area, with better results when a pedological instead of geological zoning is considered at the regional scale. The sensitivity analyses of the influence of the soil input data show that the mean values of the soil properties give the best results in terms of the ratio between the true positive and false positive rates. The scheme followed in this work allows us to obtain better results in the assessment of shallow landslide triggering areas in terms of the reduction in the overestimation of unstable zones with respect to other distributed models applied in the past.
Highlights
Shallow landslides can be defined as slope movements affecting a small thickness of superficial deposits
The assessment of shallow landslide triggering zones obtained using TRIGRSunsaturated for the benchmark event appears good for both the monitored slope and the whole study area, with better results when a pedological instead of geological zoning is considered at the regional scale
The difference in elevation between the top of the monitored slope and the valley floor is 132 m, while the difference in elevation between the position of the monitoring station and the Rio Frate Creek is 63 m. This slope was chosen as a test-site slope within the study area by considering a series of criteria: 1. the presence of triggering zones of shallow landslides that occurred in April 2009; 2. its position in areas with medium–high susceptibility to shallow landslides according to previous studies (Zizioli et al, 2013); 3. its representativeness of the whole study area in terms of the geomorphological and hydrogeological features; 4. the presence of access roads to reach the slope and install instrumentation; 5. its east-facing orientation, allowing for a good recharge of the photovoltaic panel of the station that supplies power to the devices
Summary
Shallow landslides can be defined as slope movements affecting a small thickness (generally lower than 2 m) of superficial deposits. The failure surface is often located along the interface between the soil and bedrock or between soil levels with differences in permeability These movements are very hazardous phenomena: they generally involve small volumes of soil, they can be densely distributed across territories as a consequence of intense and concentrated rainfalls (Howard et al, 1988; Montrasio and Valentino, 2008). To assess the occurrence of rainfall-induced shallow landslides in a certain area, three main aspects can be considered to be of prominent importance: (1) a detailed description of the physical– mechanical triggering mechanism in relation to the sitespecific characteristics of the involved soils and stratigraphy; (2) the choice of the more suited slope stability model to be applied at the local scale; and (3) the definition of a reliable
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