On 5-6 July 2017, an unstable atmospheric condition caused an unusual concentration of rainfall a... more On 5-6 July 2017, an unstable atmospheric condition caused an unusual concentration of rainfall above the Northern part of Kyushu Island, triggering a set of hydro-meteorological hazards. Within the affected area, the mountainous subwatershed of the Akatani River was significantly impacted by numerous landslides combined with debris flow and floods. National and local agencies deployed a plan of reconstruction to restore the floodplain and protect inhabitants. Regarding the hydrosystem in the Akatani watershed, this reconstruction project mainly focuses on the restoration of damaged protection systems and the construction of new infrastructures. Thus, this paper aims to explain the restoration plan of the Akatani River in terms of the strategic Japanese River System Sabo and then as a model of a national-scale spatial plan. It draws on (i) a literature review based on the historical evolution of Japanese protection systems and the River Sabo System; (ii) field surveys in 2019, 2022 and 2023, in conjunction with (iii) interviews with local, regional, and national officials; and (iv) a Geographical Information System analysis of previously and newly built protection systems through aerial photograph interpretation and geospatial data. Sabo works implemented in the Akatani watershed illustrate the engineering vision of Japanese river management. They also constitute a comprehensive system and include a downstream-upstream logic which echoes that of the River System Sabo. In addition, the disaster of July 2017 and the government's response emphasize the continuous adaptation and improvement of the Japanese disaster management system, which mitigates severe disasters.
In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had in... more 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 zon...
In the frame of the SAMCO project designed for mountain risk assessment, our goal is to estimate ... more In the frame of the SAMCO project designed for mountain risk assessment, our goal is to estimate specific form of vulnerability for communities living in the Upper Guil catchment in order to provide useful documentation for a better management of the valley bottom and the implementation of adequate mitigation measures. Here we present preliminary results on three municipalities of the upper Guil catchment: Aiguilles, Abries, and Ristolas. We propose an empirical semi-quantitative indicator of potential hazards consequences on element at risk (based on GIS) with an application to different (local and regional scale) scales. This indicator, called Potential Damage Index, enable us to describe, quantify, and visualize direct (physical injury, structural and functional damage on buildings, lifelines and land cover) and indirect (socio-economic impacts) consequences. The method allows estimating the possible damage caused by torrential and gravitational hazards by combining weighted indi...
In the frame of the SAMCO project designed for mountain risk assessment in a context of global ch... more In the frame of the SAMCO project designed for mountain risk assessment in a context of global change, we developed a systemic approach to assess three specific components of vulnerability – physical, social and institu- tional – for the six municipalities of the Upper Guil catchment (Ristolas, Abries, Aiguilles, Château-Ville-Vieille, Molines-en-Queyras and St-Veran). Physical vulnerability, which represents total potential consequences of hazards on stakes, was estimated and mapped using a GIS model based on an empirical semi-quantitative indicator, the Potential Damage Index (PDI). This index allowed us to quantify and describe both direct (physical injury, structural and functional damage on buildings, network and land cover) and indirect consequences (socio-economic impacts) induced by hazards, by combining weighted parameters (age, state, material, function, etc.) reflecting the exposure of elements at risk. At least 1890 buildings, 367 km2 of land cover and 902 km of linear inf...
In mountainous areas, especially in large river catchments with torrential tributaries, the produ... more In mountainous areas, especially in large river catchments with torrential tributaries, the production and sediment transport significantly increase flood impacts in the valley bottoms. For a better understanding of Alpine river catchments considered as complex systems, our research focuses on the Guil River catchment (Queyras, Southern French Alps). This catchment is largely prone to catastrophic summer floods (June 1957 (> R.I. 100 yr), June 2000 (R.I. 30 yr)...) whose impact is amplified by the strong hillslope-channel connectivity, inducing important volume of sediment supply into the main channel network. Consequently to these last flood events, serious damages to infrastructure and buildings were caused; mostly at confluences with debris flow prone tributaries, but also along some specific reaches constrained by protection structures. In the frame of SAMCO (ANR 12 SENV-0004 SAMCO) project, for mountain hazard mitigation, one of our purposes is to understand the hydro-geomor...
In risk analyses, two components are taken into account: (1) hazard analysis, including susceptib... more In risk analyses, two components are taken into account: (1) hazard analysis, including susceptibility and temporal occurrence, and (2) consequence analysis, including characterization of elements at risk (EaRs) and their vulnerability. This study focused on characterization of EaRs, in which items are spatially displayed and impacted by natural events. Several methods can assess these EaRs through expert or engineering approaches. Among the expert approaches, the multicriteria method is more flexible and allows integration of a wide range of information in order to characterize and discretize different EaRs. Traditionally, the mapping and criteria accuracy of EaRs is the same at all spatial analysis scales, while the hazard accuracy changes according to the spatial scales. Therefore, we propose an approach based on the selection of different information/criteria among several private or open access multiple geographical databases to adapt the mapping and criteria accuracy of each EaR according to the hazard analysis spatial scale. After harmonizing the different databases and merging them under GIS, a single database per work scale is created through a specific procedure with interoperability of results between scales. Thus, the number of criteria used to describe these EaRs will depend on the scale of work and the spatial scale of the analysis. To develop and test the transposability of this method, three experimental coastal study sites subject to several hazards (multirisk) have been selected in Normandy (France) with error estimations ranging between 10% and 20%. Subsequently, these data can be integrated into risk and multirisk analyses.
Coastal environment with high interaction between nature and societies is subject to multihazard ... more Coastal environment with high interaction between nature and societies is subject to multihazard interaction such as landslides, flood or cliff retreat. These territories are characterized by numerous elements at risk located in valley bottoms, front sea or at the outlets of small dry watershed. The aim is to quantify the potential consequences of EaR by integrating multiple hazards exposure at various scale analyses. To quantify the element at risk, three steps have been required. First, an initial rank has been attributed to each class of element at risk at three different scales analysis. Second, the potential consequences are weighted according to environmental dimension. Third, the consequences are combined with a linear combination of criteria in GIS environment. At medium-scale analysis, element at risk highlighted is built-up areas, national road, railway, lifeline and urban centers. At large-scale analysis, consequences concern any kind of house, apartment and complex located on multiple exposure areas. At local scale, consequences concern buildings located on multiple exposure areas with one floor in mixed materials and built after 1980. Thus, this method proposes an approach with multiple scales analysis and by integrating multiple exposure areas to quantify potential consequences. With the environmental dimension in element at risk analysis, it is an intermediate step to traditional risk analysis and, more specifically multirisk analysis without considering in this case the spatial and temporal dimension of hazards.
Du fait d’un certain nombre de facteurs predisposants (lithologie schisteuse, forte connectivite ... more Du fait d’un certain nombre de facteurs predisposants (lithologie schisteuse, forte connectivite versant – chenal), le bassin versant du Haut-Guil (317 km²) est particulierement sensible au risque de crues torrentielles. Liees a des episodes meteorologiques de haute magnitude/basse frequence, ces crues entrainent d’importants remaniements de stocks sedimentaires dans les drains elementaires. Au cours de la crue trentennale de juin 2000, l’un des principaux affluents du Guil, le Peynin (15 km²), a fourni jusqu’a 22 500 m³ de materiaux deplaces par les laves torrentielles. Afin d’ameliorer les connaissances sur la dynamique de ces stocks sedimentaires, 320 transpondeurs passifs RFID (PIT tags) ont ete implantes dans les galets et blocs du lit du torrent de la Peyronnelle. Depuis deux ans, ces galets et blocs equipes ont fait l’objet d’une surveillance reguliere. Le 9 aout 2015, un orage estival (14 mm en 3 h) a declenche des laves torrentielles dans deux sous-bassins du Peynin (Ravin de la Peyronnelle ; Ravin des Trois arbres), mettant en mouvement plus de 90 % des galets et blocs equipes. Les distances maximales parcourues ont depasse 700 m. Pour cet episode, un volume total de 9 000 m³ de sediments a ete mobilise le long du bassin de la Peyronnelle et plus d'un tiers des galets et blocs equipes a rejoint le Peynin. Le taux de retour (~ 30 %) des galets et blocs equipes ainsi que les distances parcourues mettent en avant la rapide recharge sedimentaire des drains de premier ordre dans les bassins elementaires.
Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Norman... more Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Normandy, France) but no study has yet focused on the functioning of these phenomena at a detailed scale. This study aims to identify the structure and mechanical properties of a representative case study in the region. The main objective is to understand landslide dynamics and behaviour in order to assess triggering conditions and quantify triggering thresholds. The results will help complement the regional landslide hazard mapping based on landslide statistical susceptibility mapping and quantification of triggering thresholds. The landslide morphology and internal structure were identified using a multi-method approach. A morphodynamic map was produced in the field using cartographic GPS to depict the surface morphology and map the estimated landslide activity. These field measurements were completed by an analysis of all available aerial-photo images from the French National Geographic Institute (IGN) to identify the occurrence dates and possible landslide reactivations. The landslide structure was defined using multiple electrical tomography profiles, boreholes, augerings and penetration tests. Despite the overall low electrical resistivity of the landslide materials (i.e. ± b 80 Ω•m), the electrical profiles showed good agreement with the interpreted structure based on direct observations (augerings and penetration tests). The landslide slip surface, internal morphology and palaeotopography were identified. A finite slope model was used to calculate the landslide safety factor based on the internal structure and geotechnical data. The evolution of this safety factor according to the rainfall and the groundwater levels shows that the landslides are more likely to occur after long episodes of high cumulative precipitations with an important role being played by the preliminary conditions and the rise of the surficial groundwater table level.
The ‘Cirque des Graves’ and the ’Fosses du Macre’ landslides in Normandy (France) are slowmoving ... more The ‘Cirque des Graves’ and the ’Fosses du Macre’ landslides in Normandy (France) are slowmoving rotational and translational coastal landslides characterized by surface displacement of a few cm per year. The permanent landslide activity generates important economical and physical damages (building perturbations and destruction) in this area where the land pressure associated to coastal tourism is increasing. For these reasons, the landslides have been progressively monitored since 1984. Previous researches have demonstrated the efficiency of traditional instrumentation and the use of Global Positioning System (GPS) techniques to detect very low amplitude movements. Surface displacements correlated with rainfall and groundwater level analysis allowed to identify hydro-climatic thresholds initiating seasonal movements. A continuous monitoring at several points on the landslide allows specifying: 1) the slope kinematics, 2) the relationship between rainfall, groundwater level and velocity, 3) the temporal evolution of the different landslide sectors/areas. To establish a near real-time warning system, the monitoring network is based on the combination of data collection on the triggering factors and on the slope response in terms of kinematic. The state of the recent monitoring network is detailed and some preliminary results on the kinematics and on the triggering factors are presented.
The coastal and continental parts of the Pays d'Auge plateau (Normandy, N-W France) are highly af... more The coastal and continental parts of the Pays d'Auge plateau (Normandy, N-W France) are highly affected by slope instabilities. The landslides are one of the most important issues regarding to the regional natural hazards and associated risk management. This communication proposes a synthetic assessment of the landslide triggering thresholds at regional scale combining monitoring based and regional statistical time series analysis. The assessment of the potential evolution of these thresholds is based on modelled rainfall data integrating climate change tendencies for two different scenarios.
Along 4,5 km between Houlgate and Villers-sur-Mer (Calvados, Lower-Normandy), the cliffs called &... more Along 4,5 km between Houlgate and Villers-sur-Mer (Calvados, Lower-Normandy), the cliffs called 'Les Vaches Noires' evolve under the action of various sub-aerial weathering and marine processes. The main scarp formed by a thick layer of limestone recedes by rotational landslides and rockfalls, while down, the slope present a chaotic morphology subjected to mudflows flowing between pinnacles reinforced at the top by limestone strata. This study aims (1) at global scale, to analyze the historical and morphological evolution of these slopes, and (2) at local scale, to study a large landslide located at 'Ermitage' (Auberville) and occurred in the first part of the XX e century (the triggering date is not well known). To analyze the historical and morphological evolution of the slopes, we used a diachronical method to quantify the displacements and the changes of the coastline position since 1759. The analysis is based on numerous historical documents (archives) and spatial geodata (maps, cadastral surveys, air-photo images from the French national geographic institute -IGN-, GPS) which uncertainties have been quantified. Following the nature of the materials at the toe, the toe erosion rate varies from 10 to 15 cm per year. Also, at the human scale, the toe erosion appeared like relatively very low or null; but at numerous places, the sea action is balanced by the materials provided by the active mudflows and the 'toe coast line' is temporally in progression (during the necessary time for the sea actions to put off the loosed and softy materials). At local scale, for the 'Ermitage landslide', morphodynamic maps were performed in the field using cartographic GPS in order to define the surface morphology and the estimated landslide activity. These field measurements were completed with the analysis of all available air-photo images, to identify the occurrence dates and the possible landslides reactivations.
In mountainous areas, especially in large river catchments with torrential tributaries, the produ... more In mountainous areas, especially in large river catchments with torrential tributaries, the production and sediment transport significantly increase flood impacts in the valley bottoms. For a better understanding of Alpine river catchments considered as complex systems, we focused our research on the Guil River catchment (Queyras, Southern French Alps-317km 2). This catchment is prone to catastrophic summer floods (June 1957 (> R.I. 100 yr), June 2000 (R.I. 30 yr)...) characterized by considerable sediment transport from tributaries to downvalley, very much facilitated by strong hillslope-channel connectivity (≈12,000 m3 volume of sediment aggraded during the June 2000 flood event). During the last flood events, several infrastructures and buildings were seriously damaged. Most of them were located at confluences and, at specific reaches such as canyon reaches constrained by infrastructures. For risk mitigation some protection equipments were built after the 1957 event, but most ...
The Guil River catchment (Queyras, Southern French Alps) is prone to hydro-geomorphic hazards rel... more The Guil River catchment (Queyras, Southern French Alps) is prone to hydro-geomorphic hazards related to catastrophic floods, with an amplification of their impacts due to strong hillslope-channel connectivity such as in 1957 (> R.I. 100 yr), and more recently in 2000 (R.I. 30 yr). In both cases, the rainfall intensity, aggravated by pre-existing saturated soils, explained the immediate response of the fluvial system and the subsequent destabilisation of slopes. This resulted in serious damages to infrastructure and buildings in the valley bottom, mostly along some specific reaches and confluences with debris flow prone tributaries. After each event, new protective structures are built. One of the purposes of this study, undertaken in the frame of the SAMCO (ANR) project, was to understand the hydro-geomorphological functioning of this upper Alpine catchment in a context of hazards mitigation and sustainable management of sediment yield, transfer and deposition. To determine the ...
On 5-6 July 2017, an unstable atmospheric condition caused an unusual concentration of rainfall a... more On 5-6 July 2017, an unstable atmospheric condition caused an unusual concentration of rainfall above the Northern part of Kyushu Island, triggering a set of hydro-meteorological hazards. Within the affected area, the mountainous subwatershed of the Akatani River was significantly impacted by numerous landslides combined with debris flow and floods. National and local agencies deployed a plan of reconstruction to restore the floodplain and protect inhabitants. Regarding the hydrosystem in the Akatani watershed, this reconstruction project mainly focuses on the restoration of damaged protection systems and the construction of new infrastructures. Thus, this paper aims to explain the restoration plan of the Akatani River in terms of the strategic Japanese River System Sabo and then as a model of a national-scale spatial plan. It draws on (i) a literature review based on the historical evolution of Japanese protection systems and the River Sabo System; (ii) field surveys in 2019, 2022 and 2023, in conjunction with (iii) interviews with local, regional, and national officials; and (iv) a Geographical Information System analysis of previously and newly built protection systems through aerial photograph interpretation and geospatial data. Sabo works implemented in the Akatani watershed illustrate the engineering vision of Japanese river management. They also constitute a comprehensive system and include a downstream-upstream logic which echoes that of the River System Sabo. In addition, the disaster of July 2017 and the government's response emphasize the continuous adaptation and improvement of the Japanese disaster management system, which mitigates severe disasters.
In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had in... more 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 zon...
In the frame of the SAMCO project designed for mountain risk assessment, our goal is to estimate ... more In the frame of the SAMCO project designed for mountain risk assessment, our goal is to estimate specific form of vulnerability for communities living in the Upper Guil catchment in order to provide useful documentation for a better management of the valley bottom and the implementation of adequate mitigation measures. Here we present preliminary results on three municipalities of the upper Guil catchment: Aiguilles, Abries, and Ristolas. We propose an empirical semi-quantitative indicator of potential hazards consequences on element at risk (based on GIS) with an application to different (local and regional scale) scales. This indicator, called Potential Damage Index, enable us to describe, quantify, and visualize direct (physical injury, structural and functional damage on buildings, lifelines and land cover) and indirect (socio-economic impacts) consequences. The method allows estimating the possible damage caused by torrential and gravitational hazards by combining weighted indi...
In the frame of the SAMCO project designed for mountain risk assessment in a context of global ch... more In the frame of the SAMCO project designed for mountain risk assessment in a context of global change, we developed a systemic approach to assess three specific components of vulnerability – physical, social and institu- tional – for the six municipalities of the Upper Guil catchment (Ristolas, Abries, Aiguilles, Château-Ville-Vieille, Molines-en-Queyras and St-Veran). Physical vulnerability, which represents total potential consequences of hazards on stakes, was estimated and mapped using a GIS model based on an empirical semi-quantitative indicator, the Potential Damage Index (PDI). This index allowed us to quantify and describe both direct (physical injury, structural and functional damage on buildings, network and land cover) and indirect consequences (socio-economic impacts) induced by hazards, by combining weighted parameters (age, state, material, function, etc.) reflecting the exposure of elements at risk. At least 1890 buildings, 367 km2 of land cover and 902 km of linear inf...
In mountainous areas, especially in large river catchments with torrential tributaries, the produ... more In mountainous areas, especially in large river catchments with torrential tributaries, the production and sediment transport significantly increase flood impacts in the valley bottoms. For a better understanding of Alpine river catchments considered as complex systems, our research focuses on the Guil River catchment (Queyras, Southern French Alps). This catchment is largely prone to catastrophic summer floods (June 1957 (> R.I. 100 yr), June 2000 (R.I. 30 yr)...) whose impact is amplified by the strong hillslope-channel connectivity, inducing important volume of sediment supply into the main channel network. Consequently to these last flood events, serious damages to infrastructure and buildings were caused; mostly at confluences with debris flow prone tributaries, but also along some specific reaches constrained by protection structures. In the frame of SAMCO (ANR 12 SENV-0004 SAMCO) project, for mountain hazard mitigation, one of our purposes is to understand the hydro-geomor...
In risk analyses, two components are taken into account: (1) hazard analysis, including susceptib... more In risk analyses, two components are taken into account: (1) hazard analysis, including susceptibility and temporal occurrence, and (2) consequence analysis, including characterization of elements at risk (EaRs) and their vulnerability. This study focused on characterization of EaRs, in which items are spatially displayed and impacted by natural events. Several methods can assess these EaRs through expert or engineering approaches. Among the expert approaches, the multicriteria method is more flexible and allows integration of a wide range of information in order to characterize and discretize different EaRs. Traditionally, the mapping and criteria accuracy of EaRs is the same at all spatial analysis scales, while the hazard accuracy changes according to the spatial scales. Therefore, we propose an approach based on the selection of different information/criteria among several private or open access multiple geographical databases to adapt the mapping and criteria accuracy of each EaR according to the hazard analysis spatial scale. After harmonizing the different databases and merging them under GIS, a single database per work scale is created through a specific procedure with interoperability of results between scales. Thus, the number of criteria used to describe these EaRs will depend on the scale of work and the spatial scale of the analysis. To develop and test the transposability of this method, three experimental coastal study sites subject to several hazards (multirisk) have been selected in Normandy (France) with error estimations ranging between 10% and 20%. Subsequently, these data can be integrated into risk and multirisk analyses.
Coastal environment with high interaction between nature and societies is subject to multihazard ... more Coastal environment with high interaction between nature and societies is subject to multihazard interaction such as landslides, flood or cliff retreat. These territories are characterized by numerous elements at risk located in valley bottoms, front sea or at the outlets of small dry watershed. The aim is to quantify the potential consequences of EaR by integrating multiple hazards exposure at various scale analyses. To quantify the element at risk, three steps have been required. First, an initial rank has been attributed to each class of element at risk at three different scales analysis. Second, the potential consequences are weighted according to environmental dimension. Third, the consequences are combined with a linear combination of criteria in GIS environment. At medium-scale analysis, element at risk highlighted is built-up areas, national road, railway, lifeline and urban centers. At large-scale analysis, consequences concern any kind of house, apartment and complex located on multiple exposure areas. At local scale, consequences concern buildings located on multiple exposure areas with one floor in mixed materials and built after 1980. Thus, this method proposes an approach with multiple scales analysis and by integrating multiple exposure areas to quantify potential consequences. With the environmental dimension in element at risk analysis, it is an intermediate step to traditional risk analysis and, more specifically multirisk analysis without considering in this case the spatial and temporal dimension of hazards.
Du fait d’un certain nombre de facteurs predisposants (lithologie schisteuse, forte connectivite ... more Du fait d’un certain nombre de facteurs predisposants (lithologie schisteuse, forte connectivite versant – chenal), le bassin versant du Haut-Guil (317 km²) est particulierement sensible au risque de crues torrentielles. Liees a des episodes meteorologiques de haute magnitude/basse frequence, ces crues entrainent d’importants remaniements de stocks sedimentaires dans les drains elementaires. Au cours de la crue trentennale de juin 2000, l’un des principaux affluents du Guil, le Peynin (15 km²), a fourni jusqu’a 22 500 m³ de materiaux deplaces par les laves torrentielles. Afin d’ameliorer les connaissances sur la dynamique de ces stocks sedimentaires, 320 transpondeurs passifs RFID (PIT tags) ont ete implantes dans les galets et blocs du lit du torrent de la Peyronnelle. Depuis deux ans, ces galets et blocs equipes ont fait l’objet d’une surveillance reguliere. Le 9 aout 2015, un orage estival (14 mm en 3 h) a declenche des laves torrentielles dans deux sous-bassins du Peynin (Ravin de la Peyronnelle ; Ravin des Trois arbres), mettant en mouvement plus de 90 % des galets et blocs equipes. Les distances maximales parcourues ont depasse 700 m. Pour cet episode, un volume total de 9 000 m³ de sediments a ete mobilise le long du bassin de la Peyronnelle et plus d'un tiers des galets et blocs equipes a rejoint le Peynin. Le taux de retour (~ 30 %) des galets et blocs equipes ainsi que les distances parcourues mettent en avant la rapide recharge sedimentaire des drains de premier ordre dans les bassins elementaires.
Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Norman... more Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Normandy, France) but no study has yet focused on the functioning of these phenomena at a detailed scale. This study aims to identify the structure and mechanical properties of a representative case study in the region. The main objective is to understand landslide dynamics and behaviour in order to assess triggering conditions and quantify triggering thresholds. The results will help complement the regional landslide hazard mapping based on landslide statistical susceptibility mapping and quantification of triggering thresholds. The landslide morphology and internal structure were identified using a multi-method approach. A morphodynamic map was produced in the field using cartographic GPS to depict the surface morphology and map the estimated landslide activity. These field measurements were completed by an analysis of all available aerial-photo images from the French National Geographic Institute (IGN) to identify the occurrence dates and possible landslide reactivations. The landslide structure was defined using multiple electrical tomography profiles, boreholes, augerings and penetration tests. Despite the overall low electrical resistivity of the landslide materials (i.e. ± b 80 Ω•m), the electrical profiles showed good agreement with the interpreted structure based on direct observations (augerings and penetration tests). The landslide slip surface, internal morphology and palaeotopography were identified. A finite slope model was used to calculate the landslide safety factor based on the internal structure and geotechnical data. The evolution of this safety factor according to the rainfall and the groundwater levels shows that the landslides are more likely to occur after long episodes of high cumulative precipitations with an important role being played by the preliminary conditions and the rise of the surficial groundwater table level.
The ‘Cirque des Graves’ and the ’Fosses du Macre’ landslides in Normandy (France) are slowmoving ... more The ‘Cirque des Graves’ and the ’Fosses du Macre’ landslides in Normandy (France) are slowmoving rotational and translational coastal landslides characterized by surface displacement of a few cm per year. The permanent landslide activity generates important economical and physical damages (building perturbations and destruction) in this area where the land pressure associated to coastal tourism is increasing. For these reasons, the landslides have been progressively monitored since 1984. Previous researches have demonstrated the efficiency of traditional instrumentation and the use of Global Positioning System (GPS) techniques to detect very low amplitude movements. Surface displacements correlated with rainfall and groundwater level analysis allowed to identify hydro-climatic thresholds initiating seasonal movements. A continuous monitoring at several points on the landslide allows specifying: 1) the slope kinematics, 2) the relationship between rainfall, groundwater level and velocity, 3) the temporal evolution of the different landslide sectors/areas. To establish a near real-time warning system, the monitoring network is based on the combination of data collection on the triggering factors and on the slope response in terms of kinematic. The state of the recent monitoring network is detailed and some preliminary results on the kinematics and on the triggering factors are presented.
The coastal and continental parts of the Pays d'Auge plateau (Normandy, N-W France) are highly af... more The coastal and continental parts of the Pays d'Auge plateau (Normandy, N-W France) are highly affected by slope instabilities. The landslides are one of the most important issues regarding to the regional natural hazards and associated risk management. This communication proposes a synthetic assessment of the landslide triggering thresholds at regional scale combining monitoring based and regional statistical time series analysis. The assessment of the potential evolution of these thresholds is based on modelled rainfall data integrating climate change tendencies for two different scenarios.
Along 4,5 km between Houlgate and Villers-sur-Mer (Calvados, Lower-Normandy), the cliffs called &... more Along 4,5 km between Houlgate and Villers-sur-Mer (Calvados, Lower-Normandy), the cliffs called 'Les Vaches Noires' evolve under the action of various sub-aerial weathering and marine processes. The main scarp formed by a thick layer of limestone recedes by rotational landslides and rockfalls, while down, the slope present a chaotic morphology subjected to mudflows flowing between pinnacles reinforced at the top by limestone strata. This study aims (1) at global scale, to analyze the historical and morphological evolution of these slopes, and (2) at local scale, to study a large landslide located at 'Ermitage' (Auberville) and occurred in the first part of the XX e century (the triggering date is not well known). To analyze the historical and morphological evolution of the slopes, we used a diachronical method to quantify the displacements and the changes of the coastline position since 1759. The analysis is based on numerous historical documents (archives) and spatial geodata (maps, cadastral surveys, air-photo images from the French national geographic institute -IGN-, GPS) which uncertainties have been quantified. Following the nature of the materials at the toe, the toe erosion rate varies from 10 to 15 cm per year. Also, at the human scale, the toe erosion appeared like relatively very low or null; but at numerous places, the sea action is balanced by the materials provided by the active mudflows and the 'toe coast line' is temporally in progression (during the necessary time for the sea actions to put off the loosed and softy materials). At local scale, for the 'Ermitage landslide', morphodynamic maps were performed in the field using cartographic GPS in order to define the surface morphology and the estimated landslide activity. These field measurements were completed with the analysis of all available air-photo images, to identify the occurrence dates and the possible landslides reactivations.
In mountainous areas, especially in large river catchments with torrential tributaries, the produ... more In mountainous areas, especially in large river catchments with torrential tributaries, the production and sediment transport significantly increase flood impacts in the valley bottoms. For a better understanding of Alpine river catchments considered as complex systems, we focused our research on the Guil River catchment (Queyras, Southern French Alps-317km 2). This catchment is prone to catastrophic summer floods (June 1957 (> R.I. 100 yr), June 2000 (R.I. 30 yr)...) characterized by considerable sediment transport from tributaries to downvalley, very much facilitated by strong hillslope-channel connectivity (≈12,000 m3 volume of sediment aggraded during the June 2000 flood event). During the last flood events, several infrastructures and buildings were seriously damaged. Most of them were located at confluences and, at specific reaches such as canyon reaches constrained by infrastructures. For risk mitigation some protection equipments were built after the 1957 event, but most ...
The Guil River catchment (Queyras, Southern French Alps) is prone to hydro-geomorphic hazards rel... more The Guil River catchment (Queyras, Southern French Alps) is prone to hydro-geomorphic hazards related to catastrophic floods, with an amplification of their impacts due to strong hillslope-channel connectivity such as in 1957 (> R.I. 100 yr), and more recently in 2000 (R.I. 30 yr). In both cases, the rainfall intensity, aggravated by pre-existing saturated soils, explained the immediate response of the fluvial system and the subsequent destabilisation of slopes. This resulted in serious damages to infrastructure and buildings in the valley bottom, mostly along some specific reaches and confluences with debris flow prone tributaries. After each event, new protective structures are built. One of the purposes of this study, undertaken in the frame of the SAMCO (ANR) project, was to understand the hydro-geomorphological functioning of this upper Alpine catchment in a context of hazards mitigation and sustainable management of sediment yield, transfer and deposition. To determine the ...
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Papers by Candide Lissak