Sediments sources typology and mapping based on remote sensed data analysis

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Before studying the sedimentary cascade from the point of view of connectivity, it is necessary to identifyand characterise the functioning of its components and elementary (de)couplings from the source zones.For this purpose, the open-air laboratory represented by the geomorphological disaster of 5-6 July in theAsakura region (Kyushu) allows us to study the sedimentary sources with an estimated sample of morethan 1500 mass movements triggered by the heavy rainfall during this High-Magnitude / Low-Frequency(HMLF) event.This work follows three stages: i) identification and mapping of source zones, ii) characterisation of theirnature and iii) estimation of their contribution to the sedimentary cascade.The mapping of source zones was carried out using multispectral satellite remote sensing combined witha machine-learning method. The images used are from the Pleiades sensor, which provides a very highresolution (VHR) spatial (0.50 m with Pansharpening) and spectral (RGB PIR). The semi-supervisedclassification was performed by the Random Forest learning algorithm using the Dzetsaka extension inQGIS. Bare soils on slopes are interpreted as instability zones subject to lateral transfer. The typology ofthe source areas was carried out in two steps. Firstly, the nature of the mass movements and therefore ofthe source zones was characterised using expert analysis methods associated with morphological criteria(length/width ratio). Secondly, the contribution of the source zones to the sedimentary cascade wasestimated using expert analysis methods and geomorphological criteria (e.g. types of contact, obstacles tosedimentary continuity, etc.).The mapped sedimentary sources represent 6.7 km² and 3.5% of the study area (193 km²). There is aspatial concentration of source areas in the southwest of the study area. The machine error of theclassification is about 97% and the operator error is 58%. The nature of the sedimentary sources ischaracterised by a broad spectrum of hydro-gravity processes that can be grouped mainly betweenslides/slope failures and flow-like landslide (e.g. debris avalanche, mud flow). These sedimentary sourcesactively contribute to the sedimentary cascade through generally efficient lateral and longitudinalcouplings despite local difficulties of sediment export due to decouplings or anthropogenic /environmental obstacles to sediment continuity.