Spatial Distribution of Drifted-wood Hazard following the July 2017 Sediment-hazards in the Akatani river, Fukuoka Prefecture, Japan

In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had increasing impacts on communities in Japan, because of climate change and structural immobilism in a changing and ageing society. Decreasing rural population lowers the human vulnerability in mountains, but hazards can still leave the mountain to the plains and sea, potentially carrying drifted-wood. The aim of the paper is to measure the distribution of wood-debris deposits created by the 2017 Asakura disaster and to rethink the distribution and spatial extension of associated disaster-risk zoning. For this purpose, the authors: (1) digitized and measured the distribution of drifted-wood, (2) statistically analyzed its distribution and (3) calculated the potential impact force of individual drifted timber as a minimal value. The results have shown that there is a shortening of the wood debris as they travel downstream and that the geomorphology has an important control over deposition zones. The result of momentum calculation for different stems’ length show spatially differentiated hazard-zones, which limit different disaster-risk potentials. From the present finding, we can state that we (1) need to develop separate strategies for sediments and wood debris (2) and for wood hazards, zonations can be generated depending on the location and the size of the deposited trees that differs spatially in a watershed

Les glissements de terrain des versants côtiers du Pays d'Auge (Calvados) : Morphologie, fonctionnement et gestion du risque.

The present study is focused on complex rotational-translationnal landslides occurring along the coastal slopes of Calvados. The landslides kinematics is characterized by slow and permanent activity ranging from a few millimeters to a few centimeters per year. The slow velocity can be affected by major accelerations explained by combined triggering factors, as prolonged rainfall. The complexity of these phenomena is linked to their temporal and spatial heterogeneity dynamics. Furthermore, their location in urban coastal environment induces high socio-economic issues. For better understanding of these complex landslides processes, the study focused on: * (1) the internal structure characterization, with the help of geophysical, geotechnical and geomorphological data; * (2) the hydrological and hydrogeological specificities of the slope with hydrodynamics characterization; * (3) the kinematics of the landslides measurement from monitoring network, to put in forward the spatial and temporal heterogeneities; * (4) the identification of critical piezometric and rainfall thresholds inducing acceleration; * (5) the assessment of consequences and the risk management linked to several acceleration crisis. And finally the research focused on the actual element at risk and landslide extension possibilities.Cette thèse porte sur l'étude de glissements de terrain rotationnels-translationnels complexes localisés en bordure littorale du Calvados. Ces glissements se manifestent par une activité régulière, avec des déplacements saisonniers variant de quelques millimètres à quelques centimètres par an. Ils revêtent un caractère plus spectaculaire lorsque les déplacements s'accélèrent brutalement sous l'action de facteurs de déclenchement divers et combinés. La complexité de ces processus hydro-gravitaires réside dans leur fonctionnement hétérogène dans le temps, et dans leur localisation dans un environnement littoral urbanisé. Ces secteurs sont donc des zones à risque à fort enjeux socio-économiques. Pour mieux comprendre le fonctionnement de ces glissements, les travaux se focalisent sur : * (1) la caractérisation de la structure interne des glissements à l'aide de données géophysiques, géotechniques et géomorphologiques ; * (2) les spécificités hydrologiques et hydrogéologiques du versant et la caractérisation des paramètres hydrodynamiques des corps aquifères qui composent ces glissements ; * (3) la caractérisation de la cinématique de versant en surface et en profondeur à partir d'un réseau de surveillance et la mise en évidence des variabilités spatiales et temporelles des déplacements actuels ; * (4) l'identification de seuils piézométriques ou pluviométriques qui expliqueraient le déclenchement des instabilités brusques et saisonnières ; * (5) l'évaluation des conséquences et la gestion du risque suite aux différentes crises d'accélération et l'évaluation du risque actuel à partir des conséquences potentielles et des scénarios d'extension des glissements

Fusion network with attention for landslide detection. Application to Bijie landslide open dataset”

Remote sensing techniques are now widely spread for the early detection of ground deformation, implementation of warning systems in case of imminent landslide triggering, and medium- and long-term slope instability monitoring. The large breadth of data available to the scientific community, associated with processing techniques improved as the data volume was increasing, has led to noticeable developments in the field of remote sensing data processing, using machine learning algorithms and more particularly deep neural networks. This arsenal of data and techniques is necessary for the present scientific challenges the community of researchers on landslides still have to meet. As landslides can be complex, for risk management and disaster mitigation strategies, it is necessary to have a precise idea of their location, shape, and size to be studied and monitored. The challenge aims to automate landslide detection and mapping, especially through learning methods. Machine learning methods based on Deep Neural Networks have recently been employed for landslide studies and provide promising efficient results for landslide detection. In this study, we propose an original neural network for landslide detection. More precisely, we exploit a fusion network dealing with optical images on the one hand and Digital Elevation Models on the other hand. To improve the results, attention layers [3] (able to stabilize the training and more precise results) as well as mix up techniques [4] (able to generalize more efficiently) are exploited. The model was trained and tested on the open Bijie landslide dataset. Keywords: Remote sensing for landslide monitoring and detection, landslide detection, deep neural networks, attention

La géomorphologie des bassins-versants sous l’angle de la connectivité: est-ce réinventer la roue ou changer de paradigme?

International audienc

Des instabilités de versant aux sources sédimentaires : étude de la catastrophe géomorphologique du 5-6 juillet 2017 dans le bassin-versant du Chikugo (Kyūshū, Japon)

Le 5-6 juillet 2017, sur l’île de Kyūshū (Japon), des pluies torrentielles ont déclenché plus de 1 500 mouvements de terrain dans les montagnes au nord d’Asakura à l’origine d’une catastrophe hyro-sédimentaire majeure (41 victimes). Cet événement représente un laboratoire à ciel ouvert pour améliorer la compréhension des cascades sédimentaires des hydrosystèmes montagnards influencées par des sources sédimentaires variées (e.g., mouvements de terrain). Pour comprendre les relais de processus lors de ce type d’évènement, (i) les zones sources ont été cartographiées, (ii) leur nature et origine définies, et (iii) une typologie des modalités de (dé)couplages dans ces hydrosystèmes a été construite. Les zones d’instabilité cartographiées représentent environ 6,7 km² soit 3,5 % de la surface totale de la zone d’étude (193 km²) avec une erreur comprise entre 58 % et 97 %. Les sources sédimentaires sont principalement des glissements de terrain et des coulées de débris. Les couplages latéraux versant-chenal sont majoritaires, en particulier dans les petits bassins versants torrentiels (vallée d’ordre 0). Les découplages latéraux prennent la forme d’une absence de contact entre les sources sédimentaires et les cours d’eau causée parfois par les plaines alluviales qui constituent des zones tampons. Les barrages sédimentaires (SABO dams) et les rares grands glissements favorisent les découplages longitudinaux. Les volumes sédimentaires produits par cette catastrophe sont estimés entre 3,9 et 13,4 millions de m³. Les différentes modalités de (dé)couplages produisent des disparités dans les apports sédimentaires des zones sources avec des influences anthropiques encore ambiguës.On the 5th and the 6th July 2017, heavy rainfalls triggered more than 1,500 landslides in the mountains north of Asakura in the north of Kyūshū Island (Japan), resulting in 41 casualties and a geomorphological “disaster”. This event represents an open-air laboratory to improve the understanding of sedimentary cascades in these mountain hydrosystems from sedimentary sources (e.g., landslides). For this purpose, (i) the source zones were mapped, (ii) their nature and origin defined, and (iii) a typology of (de)coupling modalities was constructed. The mapped instability zones represent about 6.7 km², i.e., 3.5% of the total surface area of the study area (193 km²) with an error between 58% and 97%. The sedimentary sources are mainly slides/slope failures and debris avalanche/mudflow. Lateral hillslope-channel coupling is particularly prevalent in small torrential catchments (0-order valley). Lateral decouplings take the form of a lack of contact between sedimentary sources and rivers, sometimes caused by alluvial plains which constitute buffer zones. Sedimentary dams (SABO dams) and rare large landslides favour longitudinal decoupling. The sedimentary volumes produced by this “disaster” are estimated to be between 3.9 and 13.4 million m³. The different cases of “(de)coupling” have produced disparities in the sediment supply from the sedimentary sources with still ambiguous anthropogenic influences

Geomorphic analysis of catchments through connectivity framework: old wine in new bottle or efficient new paradigm?

1. A trendy concept for old questions 1.1. A recent and spectacular scientific upturn “Old wine in new bottle”, here is Bracken's skepticism (Bracken et al., 2015) regarding the concept of hydro-sedimentary connectivity at the conclusion of a special issue of Earth Surface Processes and Landforms, dealing specifically with connectivity. This skepticism is due to the recent and spectacular infatuation of geomorphologists for this concept, particularly perceptible in the rhythm of publications ..

A Connectivity Approach to Agricultural Diffuse Pollution in Tropical Montane Catchments Dominated by Swidden Landscapes

Shifting cultivation is widely practiced in many tropical mountainous watersheds. Agricultural practices are changing with the intensification of activities and the development of industrial monocultures associated with increasing land use and the use of pesticides and fertilisers. These changes have consequences for the evolution of sediment transfers in watersheds, resulting in new vulnerabilities for the inhabitants. This article shows the evolution of structural connectivity over 5 years in the village of Houaylack-Vangven, located in northern Laos, and its potential links with agricultural diffuse pollution. To develop a structural source-to-sink model to understand sediment transfers, our method was based on open-access data and various geographical tools. Field surveys were conducted to identify areas vulnerable to erosion and sediment transfers. The sources and sinks were then located using remote sensing techniques and image interpretation to then characterise connectivity rates. Finally, the relationship between the sources and sinks was analysed by graph theory to explore the potentialities for assessing the connectivity and exposure of sediment sinks. The main results are twofold: sinks coincide with areas at risk of contamination by pesticides and fertilisers, and the structural connectivity increases with the increasing of the source surfaces (swidden plots) due to the ongoing agricultural transition

Des instabilités de versant aux sources sédimentaires : étude de la catastrophe géomorphologique du 5-6 juillet 2017 dans le bassin-versant du Chikugo (Kyūshū, Japon)

International audienceOn the 5th and the 6th July 2017, heavy rainfalls triggered more than 1,500 landslides in the mountains north of Asakura in the north of Kyūshū Island (Japan), resulting in 41 casualties and a geomorphological “disaster”. This event represents an open-air laboratory to improve the understanding of sedimentary cascades in these mountain hydrosystems from sedimentary sources (e.g., landslides). For this purpose, (i) the source zones were mapped, (ii) their nature and origin defined, and (iii) a typology of (de)coupling modalities was constructed. The mapped instability zones represent about 6.7 km², i.e., 3.5% of the total surface area of the study area (193 km²) with an error between 58% and 97%. The sedimentary sources are mainly slides/slope failures and debris avalanche/mudflow. Lateral hillslope-channel coupling is particularly prevalent in small torrential catchments (0-order valley). Lateral decouplings take the form of a lack of contact between sedimentary sources and rivers, sometimes caused by alluvial plains which constitute buffer zones. Sedimentary dams (SABO dams) and rare large landslides favour longitudinal decoupling. The sedimentary volumes produced by this “disaster” are estimated to be between 3.9 and 13.4 million m³. The different cases of “(de)coupling” have produced disparities in the sediment supply from the sedimentary sources with still ambiguous anthropogenic influences.Le 5-6 juillet 2017, sur l’île de Kyūshū (Japon), des pluies torrentielles ont déclenché plus de 1 500 mouvements de terrain dans les montagnes au nord d’Asakura à l’origine d’une catastrophe hyro-sédimentaire majeure (41 victimes). Cet événement représente un laboratoire à ciel ouvert pour améliorer la compréhension des cascades sédimentaires des hydrosystèmes montagnards influencées par des sources sédimentaires variées (e.g., mouvements de terrain). Pour comprendre les relais de processus lors de ce type d’évènement, (i) les zones sources ont été cartographiées, (ii) leur nature et origine définies, et (iii) une typologie des modalités de (dé)couplages dans ces hydrosystèmes a été construite. Les zones d’instabilité cartographiées représentent environ 6,7 km² soit 3,5 % de la surface totale de la zone d’étude (193 km²) avec une erreur comprise entre 58 % et 97 %. Les sources sédimentaires sont principalement des glissements de terrain et des coulées de débris. Les couplages latéraux versant-chenal sont majoritaires, en particulier dans les petits bassins versants torrentiels (vallée d’ordre 0). Les découplages latéraux prennent la forme d’une absence de contact entre les sources sédimentaires et les cours d’eau causée parfois par les plaines alluviales qui constituent des zones tampons. Les barrages sédimentaires (SABO dams) et les rares grands glissements favorisent les découplages longitudinaux. Les volumes sédimentaires produits par cette catastrophe sont estimés entre 3,9 et 13,4 millions de m³. Les différentes modalités de (dé)couplages produisent des disparités dans les apports sédimentaires des zones sources avec des influences anthropiques encore ambiguës

La géomorphologie des bassins-versants sous l’angle de la connectivité : est-ce réinventer la roue ou changer de paradigme ?

1. Un concept à la mode pour des questionnements anciens 1.1. Un essor récent et spectaculaire « Old wine in new bottle », autrement dit réinventer la roue, voici posé le scepticisme de Bracken et al. (2015) concernant la connectivité hydro-sédimentaire lors de la conclusion d’un numéro spécial de la revue Earth Surface Processes and Landforms, portant spécifiquement sur ce sujet. Ce scepticisme est notamment lié à l’engouement récent et spectaculaire des géomorphologues pour ce concept, part..

Sediments sources typology and mapping based on remote sensed data analysis

International audienceBefore 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