Physically based and distributed rainfall intensity and duration thresholds for shallow landslides
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Roberto J. Marin
Physically based and distributed rainfall intensity and duration thresholds for shallow landslides
Abstract For landslide assessment in a specific small area of interest, regional thresholds may not be applicable and the application of empirical-statistical methodologies could be limited due to a lack of required databases (landslide inventories and triggering rainfall events). Rainfall thresholds for landslides have been recently derived from the implementation of physically based methodologies that usually provide a power-law equation for the study area (e.g. region or a specific basin). In this work, a new methodology for defining rainfall intensity and duration thresholds using a physically based model (TRIGRS) is presented and implemented in a tropical mountain basin of the Colombian Andes. Multiple calculations of the factor of safety for different (synthetic) rainfall events permitted the fitting of power-law equations to the sets of critical intensity and duration conditions, causing failure in each grid cell. Maps of the equation parameters (scale and shape) and the range of applicability (initial and final duration) of the grid cell thresholds were analysed. The results show that the hydraulic and mechanical properties of the soil affected the threshold curve characteristics, comparing sandy soil with silty loam; the higher strength characteristics of sandy soil meant that most extreme rainfall conditions are required to reach their thresholds. This methodology could improve landslide early warning systems in terms of analysing the specific (local) areas that are highly susceptible to landslide occurrence. Keywords Rainfall thresholds . TRIGRS . Shallow landslides . Intensity . Duration Introduction Worldwide, the regions with the most hazardous landslide areas (with many events registered annually, causing hugely damaging loss of human life and property) do not have a landslide early warning system (LEWS) (Guzzetti et al. 2020). Even though the activities related to LEWS have been increasingly considered by researchers, authorities, and risk managers, developing a LEWS involves complex structure and participation from experts working in different fields (Intrieri et al. 2013). A very important factor, that makes it difficult, is the uncertainty, in scientific (e.g. threshold definition) and technological (e.g. sensor operation) terms. An organized decision making processes is essential, in order to generate an alert or to educate the population (Guzzetti et al. 2020). Specifically for landslide forecasting, rainfall thresholds (the triggering conditions above which, when reached or exceeded, landslides are expected) are the most used tool for estimating a future state relating to landslide occurrence, around the world (Guzzetti et al. 2008; Segoni et al. 2018). Differences between the threshold type (global, regional, or local) can lead to better understanding the suitability for a LEWS. Regional thresholds are supposed to be applicable for LEWS (Chae et al. 2017), but local thresholds
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