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We try to explain the variations of the sediments flux ("daily production") measured at a water intake on the French Loire River during "10 years". Over the last decade, this intake channel has experienced a change in its siltation dynamic, resulting in changes of sediments type and volume settled at its bottom. These changes may be explained, either by the variation of sediments production rates (soil erosion) on the upstream watersheds or by the fluvial dynamics of the Loire River, in particular, by the storage and remobilization of fine sediments in the complex morpho dynamics of the riverbed.In this context, we will evaluate the variation of sediments production rates (soil erosion) in the upstream watersheds, over an area of approximately 35,000 km². The aim is to determine how the produced fluxes evolve in space (spatial distribution) through the principle of sediment yield but also to study the changes in temporal dynamics and to identify its potential causes, for instance lithology, landuse, rainfall events…

In the current context of raising concerns related to nuclear accidents and warfare, the lessons learnt from the Fukushima accident in 2011 are of particular interest. Indeed, the Japanese authorities implemented an ambitious decontamination program, which strongly differs from the strategy adopted in Chernobyl where the most contaminated area remains closed to the population nowadays. However, the impact of this strategy on the dispersion of radioactive contaminant fluxes across mountainous landscapes exposed to typhoons remains to be quantified. Based on the unique combination of river monitoring and modelling in a catchment representative of the most impacted area in Japan, we could demonstrate for the first time that decontamination only led to a decrease of 17% of the radionuclide fluxes in the river system. Furthermore, we calculated that 67% of the initial radiocesium remains stored in forests and may contribute to radiocesium dispersion in river systems in response to future erosive events. As the current research was conducted in an area representative of the 1,117 km²-area where remediation was completed early in 2017, it raises questions about the overall sustainability and cost-benefit effectiveness of such a remediation program that generated 9,100,000 m3 of waste for a cost of ~12 billion USD. Only a limited proportion of the initial population returned to their hometown (~30% by 2019), which remains a major challenge for the future of this region, although the primary goal of authorities to decrease the radiation dose rates in the inhabited areas was achieved.

Purpose Fine sediment deposition is an important component of the catchment sediment budget and affects river morphology, biology, and contaminant transfer. However, the driving factors of fine sediment deposition remain poorly understood at the catchment scale, limiting our ability to model this process. Methods Fine sediment deposition and river reach characteristics were collected over the entire river network of three medium-sized (200–2200 km2) temperate catchments, corresponding to 11,302 river reaches. This unique database was analyzed and used to develop and evaluate a random forest model. The model was used to predict sediment deposition and analyze its driving factors. Results Fine sediment deposition displayed a high spatial variability and a weak but significant relationship with the Strahler order and river reach width (Pearson coefficient r = −0.4 and 0.4, respectively), indicating the likely nonlinear influence of river reach characteristics. The random forest model predicted fine sediment deposition intensity with an accuracy of 81%, depending on the availability of training data. Bed substrate granularity, flow condition, reach depth and width, and the proportion of cropland and forest were the six most influential variables on fine sediment deposition intensity, suggesting the importance of both hillslope and within-river channel processes in controlling fine sediment deposition. Conclusion This study presented and analyzed a unique dataset. It also demonstrated the potential of random forest approaches to predict fine sediment deposition at the catchment scale. The proposed approach is complementary to measurements and process-based models. It may be useful for improving the understanding of sediment connectivity in catchments, the design of future measurement campaigns, and help prioritize areas to implement mitigation strategies.

International audience; The detrimental impacts of surface runoff and soil erosion, particularly in cultivated areas, call for the use of distributed runoff and soil erosion models with a view to supporting adapted catchment management strategies. However, runoff model parameterization remains challenging in agricultural catchments due to the high spatial and seasonal variability of soil properties. Data acquisition is demanding and may not always be feasible. Therefore, model parameterization in such environments have been the subject of numerous research efforts. The combined analysis of land use management and soil surface state was proposed in literature to address this issue and demonstrated its potential for runoff analysis and modelling. However, these research findings were related to specific rainfall sequences and/or soil surface state. In this study, existing knowledge on soil surface state and its application to runoff model parameterization were synthetized and included in an easy-to-use parameterization software (PREMACHE), providing a framework for modelers lacking of means and/or data for modelling complex agricultural catchments. To develop and evaluate the software, a dataset was acquired over 9 years on more than 110 plots in a 1045 ha agricultural catchment, including crop types, soil surface state, rainfall and runoff time series. Soil surface state dynamics was modeled based on crop types and daily rainfall. It was evaluated in the experimental catchment and validated in a nearby catchment. Soil hydrodynamic properties (e.g. infiltration capacity) were deduced from this framework and literature data at a daily time step, for each plots. Moreover, runoff events were measured when the modeled infiltration capacity was low, indicating that the parametrization adequately captured its temporal dynamics. The software developed in this study, as well as setup values deduced from the monitoring campaigns are provided with the manuscript for application in other ungauged catchments and explore their impact on agricultural catchment hydrological dynamics.

International audience; Soil erosion on agricultural land is associated with deleterious off-site impacts including the siltation and the pollution of the receiving water bodies. To better manage this situation, local/regional water agencies need spatially-distributed information to compare the sensitivity to erosion of the areas draining into these water bodies and supplying the vast majority of sediment and associated pollutants leading to this water quality impairment. These soil erosion hazard maps are now often included in the latest versions of the basin management plans that must be designed to meet the water quality objectives of the EU Framework Directive. However, the resolution of these maps is often too coarse to meet the practical needs of these agencies. Accordingly, the current research used the latest input databases to improve the MESALES model outputs in one of the largest French River basins (Loire, 117,000 km2), with the implementation of three main modifications. First, the seasonal variations of land cover were incorporated into the model through a revised set of expert rules based on the agricultural census data. Second, the discrimination of the soil textures was improved within the infiltration and erodibility module of the model. Third, variations in rainfall erosivity across the study area were described taking into account the latest erosivity map available at the European scale. Then, the model results obtained with the updated model version were compared with those generated by the previous version. Overall, the simulated soil erosion hazard changed for 35% of the pixels of the Loire River basin, with a significant increase of the lowest hazard classes during all seasons except summer. When aggregating the results at the scale of water bodies, the simulated erosion hazard changed for 49% of these management units. Although 28% of these water bodies were associated with a lower hazard, 23% of the river systems were attributed a higher erosion risk. The implications of these model/map revisions for the local decision makers were discussed, taking the strategy of concentrating the management budget on those water bodies associated with the highest erosion risk as an example. In the future, this model could be used to compare the soil erosion hazard in contrasted regions of Europe and to simulate the impact of management plans designed to decrease this risk to support the decisions of water agencies.

Le ruissellement est un phénomène épisodique et localisé, donc difficile à mesurer. Il résulte de l'interaction de nombreux facteurs, dont les caractéristiques intrinsèques et de surface du sol, la morphologie du bassin versant, mais également les caractéristiques pluviométriques. Afin d'améliorer la compréhension de la genèse et de la propagation du ruissellement au sein de bassins versants de tête, cinq observatoires du ruissellement sont présentés, incluant 11 bassins versants. Pour chacun d'entre eux, la pluie et le ruissellement sont mesurés sur le terrain à haute fréquence (≤ 15 min) pendant des durées longues (1,5 à 11 ans). 1640 évènements pluvio-ruisselants sont extraits de ces chroniques en continu et statistiquement analysés, afin de cerner les facteurs influant significativement le ruissellement. La réponse ruisselante de chaque bassin versant est extrêmement complexe mais permet d'identifier des comportements communs. L'importance des caractéristiques des sols est montrée, ainsi que celle de leur occupation, de leurs états de surface et état hydrique. La dispersion de la relation pluie-débit observée au sein de chacun des observatoires permet par ailleurs de montrer et de quantifier l'impact des caractéristiques pluviométriques sur la lame d'eau ruisselée. Les leçons tirées de chacun des observatoires et leurs conséquences sur la compréhension et la modélisation du ruissellement dans différents contextes sont discutées.

The present paper contributes to bridging the gaps in modelling post-fire impact and mitigation measures on soil erosion. The specific aims were to predict the effects of forest fires and post-fire mitigation measures on runoff and specific sediment yield (SSY) in a river basin (Celone, S-E Italy). The Soil and Water Assessment Tool model, calibrated with field observations, was used to evaluate runoff and SSY for the current land use (baseline) and six post-fire scenarios. From 1990 to 2011, at the basin scale, the average annual SSY was 5.60 t ha- 1yr-1 (SD = 3.47 t ha-1yr-1). 20% of the total drainage area showed a critical value of SSY (>10 t ha-1 yr-1). The effects of different fire-severity levels were predicted for one year after the fire, acting on a limited area (2.3% of the total basin area). At the basin scale, the post-fire effect on surface runoff was negligible for all scenarios (< 0.4%), and the impact on SSY increased from 5.86 t ha-1 yr-1 up to 12.05 t ha-1 yr-1. At the subbasin scale, the post-fire logging scenario showed the highest increase of soil loss (SSY increased from 9.48 t ha-1 yr-1 to 57.40 t ha-1 yr-1). Post-fire mitigation treatments like straw mulching and erosion barriers effectively reduced soil erosion in high- and moderate-severity fires (19.12 t ha-1yr-1 and 20.93 t ha-1 yr-1, respectively). At the hydrological response unit level, the SSY estimated for the forest in the baseline ranged from 1.18 t ha- 1yr-1 to 2.04 t ha-1 yr-1. SSY increased more than one order of magnitude for the high-severity fire scenarios and ranged from 4.33 to 6.74 t ha-1 yr-1 in the very low-severity fire scenario, underlining the scale effect from the HRU to the basin scale.

Artificial radionuclides including radiocesium (134Cs and 137Cs) and radiosilver (110mAg) were released into the environment following the Fukushima Dai-ichi nuclear power plant accident in March 2011. These particle-bound substances deposited on soils of north-eastern Japan, located predominantly within a ∼3000 km2 radioactive fallout plume and drained by several coastal rivers to the Pacific Ocean. The current dataset (Evrard et al., 2021), which can be accessed at https://doi.org/10.1594/PANGAEA.928594, compiles gamma-emitting artificial radionuclide activities measured in 782 sediment samples collected from 27 to 71 locations across catchments draining ∼6450 km2 during 16 fieldwork campaigns. These campaigns were conducted in Japan between November 2011 and November 2020 in river catchments draining the main radioactive plume. This database may be useful to evaluate and anticipate the post-accidental redistribution of radionuclides in the environment and for the spatial validation of models simulating the transfer of radiocesium across continental landscapes.

Requiring spatial and temporal quantified information on landslide hazard over a large area is a prerequisite to forecast them. However, in many cases, the quantification remains partial, because of a lack of information on the phenomena, on predisposing and triggering factors or because the scientific approaches used in research domain are complex to apply in a regulatory framework. Thus, in this context, for many sites and end-users, the documents produced by empirical methods are used, without quantification of hazards.In 2019, a collaboration between the DIMENC Geological Survey Service of New-Caledonia (South-Pacific) and the BRGM planed the development of a global methodology of landslide hazard assessment at the 1:25,000 scale of work according to the recommendations of the JTC-1. Indeed, landslide hazard in New Caledonia is insufficiently assessed and few taken into account in land-use planning. However, this large mountainous island is regularly affected by different type of instabilities (i.e. rock-falls; rock-slides; slides; debris-flows) due to intense rainfalls. The consequences can be material and human, as in 2016 for the municipality of Houaïlou, where debris-flows occurred, inducing 5 deaths, 3 missing persons, 8 injuries along with large material damages. Few heuristic landslide hazard maps based on expert opinion are available, but the methodology is not homogeneous and harmonized. Therefore, even if these maps constitute a solid base of knowledge, their valorization for land use planning remains difficult.To overcome these shortcomings, the methodology chosen is quantitative, taking into account the susceptibility of the territory (i.e. spatial probability of phenomena occurrence with discrimination of initiation and run-out), the temporal probability of occurrence (i.e. from diachronic analyses) and the phenomena intensity (i.e. through the considered velocity of runout and the potential of induced damages). The methodology is declined by type of phenomena and is based on a comprehensive inventory. Six main steps are defined with:An inventory of the events by visual remote sensing and field observations; Discriminated mapping of bedrock and surficial formations (i.e. regolith: weathered formations and gravitational deposits); Computation of each landslide initiation susceptibility by a bivariate method; Integration of the temporal occurrence probability; Computation of the phenomena runout by a numerical approach taking into account the reach angle; Integration of the intensity of the phenomena according to the estimated volumes and/or velocity to quantify landslide hazard. The classes of spatial and temporal probabilities are based on the JTC-1 agreement and allow obtaining quantified hazard maps. The validation of the results is performed by a field validation, by phenomena not used for the computations, and by statistical tests. The method is tested in the municipality of Mont-Dore (643 km²), which was heavily impacted in 1988 by cyclone 'Anne'. Beyond the fact that the methodology will be applied throughout the territory in an operational framework and will allow the adaptation of local planning, the project allows the improvement of:Knowledge of the different kind of landslides in a volcano-sedimentary and metamorphic context strongly weathered; Knowledge of the regolith, which newly integrated for this type of analysis for the island’s municipalities.

Several studies have shown that global changes have important impacts in mountainous areas, since they affect natural hazards induced by hydrometeorological events such as landslides. The present study evaluates, through an innovative method, the influence of both vegetation cover and climate change on landslide hazards in a Pyrenean valley from the present to 2100. We first focused on assessing future land use and land cover changes through the construction of four prospective socioeconomic scenarios and their projection to 2040 and 2100. Secondly, climate change parameters were used to extract the water saturation of the uppermost layers, according to two greenhouse gas emission scenarios. The impacts of land cover and climate change based on these scenarios were then used to modulate the hydromechanical model to compute the factor of safety (FoS) and the hazard levels over the considered area. The results demonstrate the influence of land cover on slope stability through the presence and type of forest. The resulting changes are statistically significant but small and dependent on future land cover linked to the socioeconomic scenarios. In particular, a reduction in human activity results in an increase in slope stability; in contrast, an increase in anthropic activity leads to an opposite evolution in the region, with some reduction in slope stability. Climate change may also have a significant impact in some areas because of the increase in the soil water content; the results indicate a reduction in the FoS in a large part of the study area, depending on the landslide type considered. Therefore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization. The increasing rate of areas prone to landslides is higher for the shallow landslide type than for the deep landslide type. Interestingly, the evolution of extreme events is related to the frequency of the highest water filling ratio. The results indicate that the occurrences of landslide hazards in the near future (2021–2050 period, scenario RCP8.5) and far future (2071–2100 period, scenario RCP8.5) are expected to increase by factors of 1.5 and 4, respectively.

International audience; Intensification of agricultural practices during the second half of the 20 th century has accelerated of soil erosion around the world. Although this phenomenon has been widely investigated through a combination of monitoring or modelling at short timescales (

International audience; Soil erosion, runoff and sediment connectivity are strongly impacted by anthropogenic features in lowland agricultural catchments. Among these landscape features, the role played by tile drainage on water and sediment transfers and hillslope-to-river connectivity in drained catchments remains poorly understood. This study quantified water and sediment transfers in a tile drained catchment of central France by combining high frequency rainfall, discharge and sediment concentration measurements at the outlet of a set of 10 tile drained plots (34 ha) and at the medium-sized (120 km2) catchment scale. Over the monitoring period, including a dry and a wet year compared to average conditions (one year with 112% of the mean annual rainfall and one year with 64% of the mean annual rainfall), 36 rainfall-flood events were recorded and analyzed. The high frequency analysis of water and sediment transfers in tile-drained plots showed a high seasonal variability and the occurrence of two transfer pathways in the soil column including the slow drainage of saturated soils and the occurrence of preferential flow pathways through the soil column. Indeed, 13 of the 36 recorded flood events showed hydrographs with two components, reflecting these two pathways: slow transfers in the soil columns and fast transfers through soil macropores and/or cracks. Indeed, at the beginning of the flood event, a high-magnitude peak overlaid on the hydrograph. On average, this fast peak contributed 15% of the water and sediment fluxes. The sediment dynamics in tile drains was suggested to depend on sediment storage and exhaustion processes occurring in the tile drain network.

Several studies have shown that global changes have important impacts in mountainous areas, since they affect natural hazards induced by hydro-meteorological events such as landslides. To estimate the capacity of mountainous valleys to cope with landslide hazard under global change (climate change as well as climate- and human-induced land use change), it is necessary to evaluate the evolution of the different components that define this type of hazard: topography, geology and geotechnics, hydrogeology and land cover. The present study evaluates, through an innovative methodology, the influence of both vegetation cover and climate change on landslide hazard in a Pyrenean valley from the present to 2100. Once the invariant features of the studied area, such as geology and topography, were set, we first focused on assessing future land use changes through the construction of four prospective socioeconomic scenarios and their projection to 2040 and 2100. These inputs were then used to spatially model land use and land cover (LUCC) information to produce multi-temporal LUCC maps. Then, climate change inputs were used to extract the water saturation of the uppermost layers, according to two greenhouse gas emissions scenarios. The impacts of land use and climate change based on these scenarios were then used to modulate the hydro-mechanical model to compute the factor of safety (FoS) and the hazard levels over the considered area. The results demonstrate the influence of land use on slope stability through the presence and type of forest. The resulting changes are significant despite being small and dependent on future land use linked to the socioeconomic scenarios. In particular, a reduction in human activity results in an increase in slope stability; in contrast, an increase in anthropic activity leads to an opposite evolution in the region, with some reduction in slope stability. Climate change may also have a significant impact in some areas because of the increase in the soil water content; the results indicate a reduction in the FoS in a large part of the study area, depending on the landslide typology considered. Therefore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization. These changes are not uniform over the area and are particularly significant under the most extreme climate scenario, RCP 8.5. Compared to the current period, the size of the area that is prone to deep landslides is higher in the future than the area prone to small landslides (both rotational and translational). On the other hand, the increase rate of areas prone to landslides is higher for the small landslide typology than for the deep landslide typology. Interestingly, the evolution of extreme events is related to the frequency of the highest water filling ratio. The results indicate that the occurrences of landslide hazards in the near future (2021–2050 period, scenario RCP 8.5) and far future (2071–2100 period, scenario RCP 8.5) are expected to increase by factors of 1.5 and 4, respectively.

The increasing evolution of urban areas in landslide-prone regions and the modification of precipitation due to global climate change have led to a growing need for landslide hazard assessment (LHA) for local land management and security services. LHA is based on the definition of the spatial and temporal probability of landslide occurrences and is split into three steps: (i) a phenomenon inventory, (ii) a landslide susceptibility analysis, and (iii) a landslide hazard analysis. Nevertheless, in practice, essential information is often missing, and LHA corresponds most of the time to landslide susceptibility assessment acquired by expert knowledge. This article presents a physical-based model (ALICE – Assessment of Landslides Induced by Climatic Events) that can integrate different types of landslides (shallow and deep) with different sizes. This tool can be used at the meso or broader scale (1:25,000 to 1:10,000) of work according to expert knowledge. However, a calibration must be carried out to fit observations and to take uncertainties into account. Thus, a specific strategy is applied to the model on a study site located in the French South Alps where numerous different landslide types were observed and analyzed. After calibration on a small representative area, the best-fitted parameters are applied to a larger area for different types of landslides. Three landslide susceptibility maps (shallow translational landslides, rotational landslides and complex landslides) are produced, and the different susceptibility classes are compared with a landslide susceptibility map produced by a geomorphological approach. The different susceptibility envelopes computed by the model are well integrated into the high-susceptibility envelopes defined by the so-called expert geomorphological approach. Thus, the tool makes it possible to improve the mapping of potentially unstable areas by taking into account parameters, such as the groundwater level in different surficial formations or the geotechnical values of landslide-prone formations, that experts are not always able to integrate. ALICE is therefore proven to be a useful tool for mapping potential landslide-prone areas if the tool is well calibrated, taking into account the expert vision of the field, different landslide types, and different triggering factors, including rainfall and seismic acceleration.

International audience; Excessive inputs of sediment and acceleration of primary production have been observed worldwide in a large number of water bodies. Human-environment interactions were recognized as one of the main drivers of this evolution during the 20th century with the occurrence of major landscape changes and a greater use of agricultural inputs.In this study, we used paleo-production proxies such as chlorophyll-a, organic matter properties (TOC and TN concentrations, δ13C and δ15N) measured in sediment cores dated with fallout 210Pbex and 137Cs activities for reconstructing changes in accumulation rates and sources of organic matter during the recent period of agricultural intensification (1920–2020). In order to record these changes at the regional scale, sediment cores were collected at the outlet of several headwater catchments (n = 9), covering a wide range of land covers / land uses across the Loire River basin (117,000 km²), France.The rates of sedimentary organic matter deposition in the studied water bodies accelerated from 1950 onwards (+48 %). Between 1950 and 1970, the signature of sedimentary organic matter indicates a dominant contribution of soil-derived inputs. This period corresponds to major landscape modifications across the basin (land consolidation, stream re-design, implementation of tile drains) driving a general acceleration of erosion rates. Then, from 1960 onwards, chlorophyll-a and C/N proxies indicate an increase in primary production coupled with a decrease of terrigenous supply in agricultural catchments. These proxies were strongly correlated to the agricultural inputs during the 1955–1990 period (e.g., r = 0.9 between chlorophyll-a content and N inputs), suggesting a progressive eutrophication of these reservoirs driven by increasing fertilizer use. During these 35 years, sedimentary organic matter deposition rates increased on average by 30 %. During the 1990s, despite a slight reduction in fertilizer use (−13 %), the paleo-production proxies (e.g. chlorophyll-a) still indicate a positive trend suggesting the contribution of another driving factor such as climate warming or again nutrient release from soils and sediments.In the absence of long term geochemical and sediment input monitoring, paleolimnological reconstructions provide a powerful tool to reconstruct past agricultural pressures in rural environments. This study illustrates the impact of intensive farming on water body siltation driven by varying sources of organic material during the 20th century. In addition, these results suggest that eutrophication processes of these reservoirs with contrasting land uses started during the 1960–1970 period and are still ongoing nowadays.

Numerous studies worldwide have reported a dramatic increase in soil erosion rates following the development of agriculture. In Western Europe, food security issues led to an intensification of agricultural practices after World War II. A profound modification of the landscapes was operated that translated into an increase in hydrosedimentary connectivity and a decrease in soil cover in winter. Related on-site soil degradation and off site societal and environmental detrimental effects rapidly started to call for the implementation of conservation measures. Since 2000, the French water agencies, through the European water framework directives, started to fund the implementation of soft hydraulic conservation measures, such as vegetated filter strips or linear vegetation barriers. These measures have the advantage to be easily implemented and to be visible in the landscape without compromising the intensive agriculture production system. After twenty years of funding of soft hydraulic conservation measures, soil erosion is still an issue. Are these solutions just a plaster on a wooden leg or are they really effective? Recent efforts consisting in catchment scale monitoring programs and modelling exercise tend to show that soft hydraulic conservation measures may be usefull for local mitigation actions but may have a limited impact in terms of floods and muddy floods. On the basis of simulation exercises in contrasting environments we will discuss the advantages and limitations of such measures.

Fire may have a severe impact on soil and water as fire-related contaminants are delivered with suspended sediment to the river and may lead to a depletion of water quality with consequences for aquatic ecosystems precluding the achievement of the Water Framework Directive objectives. The general aim of this study is to define a procedure to assess the impact of fire on hydrology, soil losses, and sediment transport to a river network. Specific objectives are to: (i) assess the adequacy of the SWAT model to predict post-fire impacts at the basin and subbasins scale in order to be used as a post-fire management tool, and (ii) estimate potential changes in hydrological regime and soil erosion rates. The study focuses on a high temporal and spatial scale for a management perspective.

International audience; Better understanding the pathways through which future socioeconomic changes might influence land use and land cover changes (LULCCs) is a crucial step in accurately assessing the resilience of societies to mountain hazards. Participatory foresight involving local stakeholders may help building fine-scale LULCC scenarios that are consistent with the likely evolution of mountain communities. This paper develops a methodology that combines participatory approaches in downscaling socioeconomic scenarios with LULCC modelling to assess future changes in mountain hazards, applied to a case study located in the French Pyrenees. Four spatially explicit local scenarios are built each including a narrative, two future land cover maps up to 2040 and 2100, and a set of quantified LULCC. Scenarios are then used to identify areas likely to encounter land cover changes (deforestation, reforestation, and encroachment) prone to affect gravitational hazards. In order to demonstrate their interest for decision-making, future land cover maps are used as input to a landslide hazard assessment model. Results highlight that reforestation will continue to be a major trend in all scenarios and confirm that the approach improves the accuracy of landslide hazard computations. This validates the interest of developing fine-scale LULCC models that account for the local knowledge of stakeholders.

International audience; Rural landscapes were extensively modified by human activities in Western Europe since the beginning of the 20 th century. Cultivated areas expanded in wetlands traditionally used as grassland. These expansions were associated with extensive anthropogenic modifications of the landscape, for instance through the implementation of artificial tile drainage networks, increasing water drainage from soils. These networks increase the water and sediment connectivity from plots to rivers, creating new transfer pathways beneath the soil surface. As traditional soil conservation measures, such as grass strips, were designed to limit transfers by surface runoff, tile drain make sediment particles bypass these measures. Moreover, these transfers mobilize small particles (in the order 10-5-10-6 m) with a high adsorption potential, making them possibly detrimental for the environment. It is therefore crucial to understand the water and sediment transfer dynamics occurring through tile drainage, and their importance at both the plot and catchment scales. Although tile drainage was studied at the plot scale, for instance to analyze nutrients transfers, the high frequency behavior of sediment transfer from tile drainage, and its potential impacts at the catchment scale, is still poorly understood. Our research quantified sediment fluxes with a high frequency (10 minutes time step) during three years at five nested stations including a tile drain outlet across a small (25 km 2) agricultural catchment (Loire River basin, France) representative of lowland cultivated environments. This catchment was largely modified by anthropogenic features: streams were redesigned and more than 67% of the fields are equipped with tile drainage networks. Flows from the drain were measured during 11.4% of the monitoring time. The nested tile drainage-catchment monitoring network allowed quantifying both the sediment fluxes from tile drain, an often overlooked process, and its proportion in the associated catchment sediment fluxes. Most of sediment fluxes (79 ± 9%) were measured during flood events (n = 44), mostly occurring in winter (75%) and spring (20%). Seasonality therefore controlled most of the variations of sediment fluxes in this context, meaning that the hydrosedimentary response of the catchment should be studied during long observations periods.

International audience; A major challenge in geomorphology is reconciling the disparity between runoff coefficients and erosion ratesfrom the field to the catchment scale. In the European loess belt, a decrease of runoff coefficients and erosion ratesoccurs with increasing spatial scales. Indeed, it is important to understand the scale dependency of run-off anderosion to manage the off-site impacts of accelerated soil erosion. Accordingly, a continuous simulation of the scaledependency of runoff and erosion, from the field scale to the catchment scale (100 – 1000 km2) was conducted withtheWater and Sediment (WaterSed) model for two catchments (Andelle-756 km2, and Austreberthe-214 km2) over12 years (>1000 events). Scale effects were evident with a 100-fold decrease in runoff coefficients and sedimentdelivery ratios between the field scale and the catchment scale. In spite of a low variability of the annual rainfall(19%), the inter-annual variability of the runoff volume (37%) and erosion rates (92%) at catchment outlets werehigh. The inter-annual variability of runoff and erosion was closely linked to the number of extreme events peryear and their distribution through the year, in particular during periods with highly crusted soil surface states.For these high magnitude events, a complex distributed modelling approach was not necessary as the ability ofthe soil surface and of the landscape to retain overland flows are largely exceeded. The seasonality of soil surfacecharacteristics also affected the scale dependency of runoff and erosion, from the field scale to the catchment scale.However, this is only observed up to a certain spatial extent, i.e. where hillslope erosion processes are governedby hortonian overland flow. When saturation flows are generated, topography and soil depth become the dominantfactors. Understanding runoff and sediment response at different scales should focus on the location and amountof runoff and sediment production within the catchment and the capacity of the downstream flow path to retainor transfer overland flow and sediment. Furthermore, this study demonstrates the need to implement model thatrepresent both hortonian and saturation flows when simulating erosion events at the catchment scale.

The three different future scenarios showed an increase in mean temperature for all months between 0.5-2.4°C and a reduction in precipitation (by 4-7%) for the period 2030-2059 (MPI, KNMI, SMHI). The results of the present work show that climate change will bring a reduction of water resource availability and some alterations in the hydrological regime. The SWAT model, which proved to be a valuable operational tool for evaluating the potential impact of climate change on water resources, estimates a reduction of total water yield and a shift of the flow regime towards drier conditions, although the river type classification will probably remain essentially unvaried. A sever reduction of snowfall in the mountainous part of the basin was also estimated that is expected to impact the flow regime. However, it is important to take into account that several sources of uncertainties, which depend both on the used hydrological models and on the climate change scenarios, affect the predictions of the hydrological response of a river basin under climate change. In addition, some of the assumptions made (i.e. that land use does not change in the future) could be incorrect as climate change could also result in a significant alteration of land cover. Hence, we have to consider projections not as a predictive method, but as a tool that may be used to assess changes in process dynamics.

International audience; Soil erosion and sediment transfer are extensive in lowland agricultural catchments. In these environments, high-frequency datasets of both water discharge and suspended sediment concentrations are often lacking. In particular, the impact of tile drainage networks on sediment fluxes in these catchments is poorly understood. Our research quantified sediment fluxes between 2013 and 2016 at five nested stations including a tile drain outlet across a small (25-km(2)) agricultural catchment (located in the Loire River basin, France) representative of lowland cultivated environments of the middle part of this basin. Sediment fluxes varied from 1 to 38 t km(-2) yr(-1) across the monitored subcatchments. Most of sediment fluxes (79 +/- 9%) were measured during flood events (n = 44), mostly occurring in winter (75%) and spring (20%). Seasonality controlled most of the variations of sediment fluxes. Sediment transfers in tile drains occurred episodically. Flows were measured during 11.4% of the monitoring time. A mean time lag of 80 min was recorded between the peak discharge at the drain outlet and the downstream river station. At the event scale, sediment fluxes exported from the tile drain varied between 1.1 x 10(-4) and 2.5 t km(-2) and remained of the same order of magnitude as those measured at the downstream river station, although the latter were on average 53% lower. Results suggest significant storage of sediment material in the river channel and its export during the most intense flood events. Our research emphasizes the need for continuous monitoring of water and sediment transfers in agricultural catchments equipped with tile drains.

International audience; Soil erosion is recognized as one of the main processes triggering the physical and chemical degradation of water bodies. Although erosive dynamics have been widely described in morphogenic areas, much less is known about this process in lowland agricultural catchments.To address this issue, a multidisciplinary study was carried out at the scale of a small agricultural lowland catchment (Louroux catchment, France, 25 km (2)) to reconstruct the evolution of the soil erosion rates and sources, from the hillslopes to the deposition areas in the water bodies. This study's goal is specifically to understand the impact of changes in land use and agricultural practices after 1950 (field re-design, ditch creation, drainage) on sediment dynamics For this purpose, sediment cores were collected in a medieval pond (870AD) located at the outlet of the catchment. These cores have been dated and used to reconstruct the temporal variations in sediment fluxes over time (from 1954 to 2013). In addition, sediment sources were identified using fallout radionuclide measurements (cesium-137 and beryllium7 / excess lead-210 ratios). These results will contribute to the implementation of effective management measures to reduce the transfer of sediments from the hillslopes to the hydrosystems.; 14 15 L'érosion des sols est reconnue comme étant l'un des principaux processus à l'origine de la dégradation physico-16 chimique des masses d'eau. Les phénomènes érosifs, largement décrits en contexte morphogène, restent pourtant 17 peu étudiés en contexte de plaine. 18 Pour combler ce manque, une étude pluridisciplinaire a été menée à l'échelle d'un petit bassin versant (25 km²) 19 de plaine agricole drainée pour reconstruire l'évolution des taux et des sources d'érosion, depuis les zones 20 d'accumulation de matière jusqu'aux zones sources. Cette étude vise ainsi à comprendre l'impact des 21 aménagements intervenus dans le bassin après 1950 (remembrements, création de fossés, drainage) sur la 22 dynamique sédimentaire actuelle et passée. 23 À cette fin, des carottages ont été réalisés dans un étang médiéval (870AD) localisé à l'exutoire du bassin versant 24 du Louroux (France). Les carottes ont été datées et ont permis de reconstruire les variations temporelles du flux 25 sédimentaire (de 1954 à 2013), mais aussi de les associer avec les changements d'usage des sols. Par ailleurs, les 26 sources de sédiments ont été identifiées à partir de mesures de radionucléides (césium 137 et rapport béryllium 27 7/plomb 210 en excès). Ces résultats vont permettre de guider la mise en place d'aménagements pour lutter contre 28 le départ de matière sur les versants. 29 MOTS CLEFS : érosion, sédiment, colmatage, archive sédimentaire, traçage 30 31 Increase of erosion source contributions to rivers and lakes (1950-2010): 32 the case of the Louroux Pond (Central France) 33 Soil erosion is recognized as one of the main processes triggering the physical and chemical degradation of water 34 bodies. Although erosive dynamics have been widely described in morphogenic areas, much less is known about 35 this process in lowland agricultural catchments. 36 To address this issue, a multidisciplinary study was carried out at the scale of a small agricultural lowland 37 catchment (Louroux catchment, France, 25 km²) to reconstruct the evolution of the soil erosion rates and sources, 38 from the hillslopes to the deposition areas in the water bodies. This study's goal is specifically to understand the 39 impact of changes in land use and agricultural practices after 1950 (field redesign , ditch creation, drainage) on 40 sediment dynamics 41 For this purpose, sediment cores were collected in a medieval pond (870AD) located at the outlet of the 42 catchment. These cores have been dated and used to reconstruct the temporal variations in sediment fluxes over 43 time (from 1954 to 2013). In addition, sediment sources were identified using fallout radionuclide measurements 44 (cesium-137 and beryllium7 / excess lead-210 ratios). These results will contribute to the implementation of 45 effective management measures to reduce the transfer of sediments from the hillslopes to the hydrosystems. 46 KEY WORDS: erosion, sediment, siltation, lake deposits, sediment fingerprinting 47 48 49

International audience; In France, since the beginning of 20 th century, rural landscapes have been completely modified by human activities. These practices have resulted in profound sedimentary and morphological alterations (channel bed incision, deposition of fine sediment, bank erosion, etc.), detrimental to the achievement of good water status. Several research efforts have already investigated either global budgets at the river basin or continental scale or local detailed budget at the plot to the field scale. However, very few studies have tried to analyse the connectivity between fluxes and storages and to draw the links between temporal and spatial scales. In this broad context, the purpose of this study is to examine source-to-sink dynamic of the sediment cycle for the Loire River Basin. This project is broken down into two steps: the first step's aim is to understand poorly studied processes such as sediment production by agricultural drainage or bank erosion by catchment monitoring. The second step is to elaborate a distributed model of sediment connectivity from hillslopes to basin outlet. For the first step, two catchment sites are studied, using historical data or monitoring: the linear of two small streams (" La Ligoire ") and the Louroux lake catchment. Most of those two catchments are intensively cultivated and have been extensively submitted to subsurface drainage using drain tiles. The objectives of this part are threefold: quantify incision and deposition processes since the channelization of the streams, quantify in-channel deposition rates of fine sediments, and explain the spatial distribution of these deposits. For the second step, the modeling approach is based on the use of indicators to describe hillslope processes, potential downstream retention, attempting to link river basin characteristics to a prediction of sediment exports in rivers. It provides insight in the identification of the most influent sediment redistribution processes on the total sediment fluxes and on the differences between various basin typologies. The Ligoire study shows an important stream incision and the influence of water obstacles on sediment distribution. The Louroux monitoring permits to differentiate between the different sources of sediment and highlight the importance of surface erosion during flood events and of bank erosion during low flows. The increased export of the sediment is primarily due to the very high human-made connectivity of these landscapes that was originally created to evacuate the excess water during the humid seasons. The analysis of particulate sediment fluxes datasets at different spatial and temporal scales permits to identify some of the dominant processes, and also to distinguish natural from anthropogenic influences. Concerning upland physical soil surface erosion rates, we find that the average travel distance of eroded particles may be limited, implying a strong decrease in physical erosion rates when moving from the local scale to the river basin scale. The results also highlight the predominant role of surface connectivity to characterize the fraction of sediment exported out of river drainage areas by physical soil surface erosion.

International audience; Lowland agricultural landscapes underwent important changes since the second half of the XXth century such as hedges removal, implementation of drainage systems, stream redesign and land reallocation. It resulted in changes in sediment transfer processes, and in widespread morphological alterations of water bodies. However, little is known about the sediment dynamics in these environments. The Louroux catchment (25 km 2) is located in central France. It is a typical intensively cultivated and tile drained lowland catchment. The Xth century pond located at its outlet (52 ha) is undergoing large siltation, with a current sedimentation rate 60 fold higher than the pre-1950 period. Five monitoring stations, measuring water levels and turbidity at high frequency (15 mn and 1 mn respectively), combined with automatic samplers, were implemented in 2013. Three stations are located at the main tributaries outlets of the pond, one in a sub-catchment, and one at a tile drain outlet. 45 floods were observed during the three studied hydrological years. They occurred mostly between December and March (33 floods) and in May-June (8 floods). Specific sediment yields ranged from 0.02 to 0.38 t.ha-1.yr-1 depending on the monitoring site and the considered year. The vast majority of suspended sediment transfers occur during the winter floods. While large water volumes were also measured during spring floods, the sediment yields remained low. Suspended sediment yields present large inter-annual (ratio ranging between 2 and 6 depending on the monitoring station) and spatial variations, due to significant differences in total rainfall amounts during the winter season and variations in land use, respectively. The processes related to sediment transfers are most likely linked to soil saturation during winter despite the presence of a tile drainage network, with transfers occurring both at the soil surface and through the drainage system. While sediment transfer rates can be considered as low regarding values measured in other environments, they are sufficient to cause, in conjunction with eutrophication processes, a filling of the pond that should be complete in 50 to 100 years. Although lowlands show very limited morphogenic activity, it is crucial to determine appropriate management strategies to limit these sediment transfers to water bodies.

International audience; Following the shift towards more intensive agriculture in cultivated lowlands in Europe, field sizes have increased and stream valley meanderings have been suppressed and realigned along new straight field borders. These modifications have led to profound alterations of the hydromorphology of the streams. To test the importance of these modifications, the objective of this study is to assess the potential of using high resolution DTM (Digital Terrain Model) to quantify the current volume of small drainage ditches at catchment scales and to assess the evolution of these ditches using archival data. The method has been applied to a small agricultural catchment drained by an artificial stream network. A specific 1400 m long ditch was chosen to estimate the capacity of a DTM (0.5 m) and to evaluate the actual topography and volume of stream. Forty-four channel cross sections measured with a centimeter DGPS (Differential Global Positioning System) have been compared with the same profiles measured with the DTM. The average DTM error in estimating stream depth is approximately 13% and is less than 12% for stream width detection. Estimates of the ditch volume using DTM sections instead of DGPS sections produces a result of 3100 m(3). An average error of 11% can be ascribed to the difference in the estimated ditch volume between the DGPS and DTM approaches, which was principally caused by under-estimates of bottom ditch morphology by the DTM. This study highlights the ability of airborne instruments to quickly and robustly detect and estimate the volume occupied by small-width stream networks (1.5 to 4 m) over large areas. This approach has opened new perspectives for the study of current and past bank activities at catchment scales to quantify bank erosion contributions to the overall sediment budget.

International audience; The long-term and current volumes of sediment exported from stream banks were calculated as potential sources of sediment in a large pond located at the catchment outlet of a small agricultural lowland basin strongly affected by anthropogenic pressure in France. Bank erosion was measured over a short period using a network of erosion pins along a small stream (1400m long) to quantify the material exported during a single winter (2012-2013). The material exported by this same stream over the last 69years was quantified using an original approach involving the comparison of a compilation of three-dimensional historical stream redesign plans that date back to 1944 with the state of the banks in 2013 (differential global positioning system and LiDAR data). The results suggest that a global trend of material loss along the stream banks monitored by erosion pins, with an average erosion rate of 17.7mmyear(-1) and an average volume of exported material of 75tkm(-1). Over 69years, this same stream exported an average of 36tkm(-1)year(-1), and the average loss of material from the banks throughout the whole catchment was estimated to be 14tkm(-1)year(-1). The contribution of bank material to the filling of the pond over the last 10years is between 46% and 52% based on an extrapolation of erosion pin dynamics or between 27% and 30% based on the comparison of LiDAR data to the average historical profile extrapolated for the catchment. These results suggest that bank erosion represents a major source of sediment in degraded waters in traditionally understudied agricultural lowland catchments, where anthropogenic pressures are high.

Open image in new window Global changes would have impacts worldwide, but their effects should be even more exacerbated in areas particularly vulnerable. Mountainous areas are among these vulnerable territories. In order to estimate the capacity of such mountainous valleys to face global changes (climate, but also climate- and human- induced land-use changes), it is necessary to be able to evaluate the evolution of the different threats. The present work shows a methodology to evaluate the influences of both vegetation cover and climate on landslides activities over a whole valley until 2100, to propose adequate solutions for current and future forestry management. Firstly, the assessment of future land use is addressed through the construction of four prospective socio-economic scenarios up to 2040 and 2100, which are then spatially validated and modeled with LUCC models. Secondly, the climate change inputs of the project correspond to 2 scenarios of emission of greenhouse gases. The used simulations were performed with the GHG emissions scenarios RCP 4.5 and RCP 8.5. The impact of land use and climate change is then addressed through the use of these scenarios into hazards computations. For that we use a large-scale slope stability assessment tool ALICE which combines a mechanical stability model, a vegetation module which interfere with the first model, to take into account the effects of vegetation on the mechanical soil properties, and a hydrogeological model. The results demonstrate the influence of the forest on slope stability; the absence of the forest implies an increase of the probability of landslide occurrence, and at the contrary, the presence of forest has a local stability effect on the slope. The results also indicate some future evolution of the land use, leading to significant modifications of the stability of the slopes. Finally the climate change may have noteworthy impact on the occurrence of landslide with the increase of the water content of the soil when regarding future long periods; the results point out a reduction of the SF in a large part of the studied area. These changes are not uniform over the area, and are particularly significant for the worse scenario RCP 8.5.

(2017). From hillslopes to rivers: erosion, clogging and silting. La Houille Blanche: Vol. 103, No. 6, pp. 5-6.

International audience; Modelling water and sediment transfer in lowland catchments is complex as both hortonian and saturation excess-flow occur in these environments. Moreover, their dynamics was complexified by the installation of tile drainage networks or stream redesign. To the best of our knowledge, few models are able to simulate saturation runoff as well as hortonian runoff in tile-drained catchments. Most of the time, they are used for small scale applications due to their high degree of complexity. In this context, a model of intermediate complexity was developed to simulate the hydrological and erosion processes at the catchment scale in lowland environments. This GIS-based, spatially distributed and lumped model at the event scale uses a theoretical hydrograph to approximate within-event temporal variations. It comprises two layers used to represent surface and subsurface transfers. Observations of soil surface characteristics (i.e. vegetation density, soil crusting and roughness) were used to document spatial variations of physical soil characteristics (e.g. infiltration capacity). Flow was routed depending on the local slope, using LIDAR elevation data. Both the diffuse and the gully erosion are explicitly described. The model ability to simulate water and sediment dynamics at the catchment scale was evaluated using the monitoring of a selection of flood events in a small, extensively cultivated catchment (the Louroux catchment, Loire River basin, central France; 25 km$^2$). In this catchment, five monitoring stations were equipped with water level sensors, turbidity probes, and automatic samplers. Discharge and suspended sediment concentration were deduced from field measurements. One station was installed at the outlet of a tile drain and was used to parameterize fluxes supplied by the drainage network. The selected floods were representative of various rainfall and soil surface conditions (e.g. low-intensity rainfall occurring on saturated soils as well as intense rainfall occurring on dry soils in spring). The model was able to reproduce the runoff volumes for these different situations, and performed well, especially in winter (the relationship between observed and modeled values has R2=0.72) when most of the sediment are transferred. Therefore, future work will evaluate the model ability to reproduce the erosion and sediment dynamics in this catchment in order to provide a tool for sediment management in these lowland environments draining agricultural land where river siltation is problematic.

Physically based model may be used to assess landslide susceptibility over large areas. However, majority of case studies are applied for complex phenomena for a one event, a little site or over large areas when landslides have simple geometry and environmental conditions are homogeneous. Thus, assessing landslide prone areas for different type of landslides with several geometries and for large areas needs some specific strategies. This work presents an application of a specific procedure based on a physically based model for one complex area with several landslide types. By different steps, it is demonstrated this is possible to improve susceptibility map and take into account of different slope failure with different depths. This first attempt encourages us to continue on this path in order to improve the existing susceptibility maps in this area.

International audience; Abandonment of agricultural land is widespread in many developed countries. These surfaces are projected to increase significantly worldwide during the 21th century. Identifying potential relationships between land abandonment and soil erosion dynamics over the long term (100 years) is therefore essential for predicting the environmental consequences of this extensive land use change. Accordingly, sediment cores were collected in two highland catchments of central France in order to reconstruct the change of sediment delivery during the last century. The results showed a substantial decline (71–78%) of rural population in both sites since 1900. This decrease occurred simultaneously with a sharp decline (85–95%) of the surface of arable land: previously cultivated areas were mainly converted into forests as the result of natural and anthropogenic processes. Consequently, sediment deliveries significantly decreased (75–99%) in both catchments. These trends were nevertheless interrupted by the implementation of afforestation works between 1945 and 1970 in one of the catchments. During these works, erosion rates increased three-fold because of extensive soil disturbance, and sediment delivery stabilized only 15 years after the onset of these management operations. Overall, this study demonstrates the long-term effect of land abandonment on soil erosion, which supplements the more widely reported acceleration trend of soil erosion because of agricultural intensification.

International audience; Rural landscapes have been extensively modified by human activities in Western Europe since the beginning of the 20th century in order to intensify agricultural production. Cultivated areas often expanded at the expense of grassland and wetlands located in lowland areas (de Groot et al., 2002). Therefore, large modifications were made to the agricultural landscapes: stream redesign, land consolidation, removal of hedges, and installation of tile drainage networks to drain the hydromorphic soils. These changes modified sediment processes and resulted in large morphological alterations (e.g. channel bed incision, deposition of fine sediment, channel bank erosion). Accordingly, these alterations threaten water quality and prevent to meet the requirements of the European directives. Improving water quality requires a clear understanding of the hydrosedimentary dynamics in these lowland cultivated catchments. However, few studies were conducted in drained environments. To fill this research gap, a pilot study was started in cultivated catchment of the Loire River basin, France, where tile drain densities are very high (> 1.5 km/km²). Six hydro-sedimentary monitoring stations were installed in the Louroux catchment (24 km²). One of them was specifically dedicated to measuring water/sediment fluxes from tile drains. Water level and turbidity were continuously monitored and sediments were sampled during floods and low stage periods. Samples were measured for particle size distribution, and sediment tracing studies are currently being developed to quantify the contribution of potential sources (e.g. surface vs subsurface, lithologies) to river sediment. Hydro-sedimentary fluxes were quantified and modelled for some selected events. The catchment hydrosedimentary fluxes and their properties were shown to be impacted by tile drain sediment transport, especially regarding particle size distribution, with the dominant export of very fine particles (< 2 µm) from tile drains. This study demonstrates that tile drain transport should be included when developing and running hydro-sedimentary models in catchments, and particularly in lowland/wetland environments.

International audience; Rural landscapes have been extensively modified by human activities in Western Europe since the beginning of the 20th century in order to intensify agricultural production. Cultivated areas often expanded at the expense of grassland and wetlands located in lowland areas (de Groot et al., 2002). Therefore, large modifications were made to the agricultural landscapes: stream redesign, land consolidation, removal of hedges, and installation of tile drainage networks to drain the hydromorphic soils. These changes modified sediment processes and resulted in large morphological alterations (e.g. channel bed incision, deposition of fine sediment, channel bank erosion). Accordingly, these alterations threaten water quality and prevent to meet the requirements of the European directives. Improving water quality requires a clear understanding of the hydrosedimentary dynamics in these lowland cultivated catchments. However, few studies were conducted in drained environments. To fill this research gap, a pilot study was started in cultivated catchment of the Loire River basin, France, where tile drain densities are very high (> 1.5 km/km$^2$). Six hydro-sedimentary monitoring stations were installed in the Louroux catchment (24 km 2). One of them was specifically dedicated to measuring water/sediment fluxes from tile drains. Water level and turbidity were continuously monitored and sediments were sampled during floods and low stage periods. Samples were measured for particle size distribution, and sediment tracing studies are currently being developed to quantify the contribution of potential sources (e.g. surface vs subsurface, lithologies) to river sediment. Hydro-sedimentary fluxes were quantified and modelled for some selected events. The catchment hydrosedimen-tary fluxes and their properties were shown to be impacted by tile drain sediment transport, especially regarding particle size distribution, with the dominant export of very fine particles (< 2 $\mu$m) from tile drains. This study demonstrates that tile drain transport should be included when developing and running hydro-sedimentary models in catchments, and particularly in lowland/wetland environments.

Global changes would have direct impacts on landslide activities through the modifications of triggering events with the evolutions of climate forcing. Howev-er, some predisposing factors would also evolve. Indeed, forests are likely to be modified, either by anthropogenic interventions, natural ageing or adaptation to climate change. This evolution is likely to result in changes of slopes susceptibili-ties to landslides. In order to propose adequate solutions for current and future for-estry management, it is therefore necessary to properly estimate the influences of the vegetation on slope stabilities. In the present study, we develop a complementary module to our large-scale slope stability assessment tool to take into account the effects of vegetation on the mechanical soil properties (cohesion and over-load), but also on the slope hydrol-ogy (change in interceptions, run-off, and infiltration). Hence the proposed meth-od combines a mechanical stability model (using finite slope analysis), a hydro-logical model, and a vegetation module which interfere with both aspects. All these elements are interfaced within a GIS-based solution. Uncertainties on input data are propagated in the models through distributions laws. The method has first been applied to a Pyrenean Valley, a site which is part of the Observatoire Pyré-néen du Changement Climatique (OPCC). A second application is being under-taken on another Pyrenean Valley, for the ANR Project SAMCO.

4 pages; International audience; The aim of this work is to develop a methodology for integrating climate change scenarios into quantitative hazard assessment and especially their precipitation component. The effects of climate change will be different depending on both the location of the site and the type of landslide considered. Indeed, mass movements can be triggered by different factors. This paper describes a methodology to address this issue and shows an application on an alpine test site. The proposed approach is based on coupling a hydrological model (GARDENIA®) with a slope stability model (ALICE®), estimating safety factors spatially. From a DEM, land-cover map, geology, geotechnical data and so forth the program classifies hazard zones depending on geotechnics and different hydrological contexts varying in time. The methodology is applied to the Ubaye valley (France) using present and past climate conditions.

Estimating soils geomechanical behavior interest numerous applications dealing with natural hazards, landscape management or terrestrial vehicles mobility. These applications need most of the time high time reactivity in producing decision-making maps, a strong input data availability and a spatial resolution in the order of decametric scale. This work presents an simplified hydrologic model allowing estimation of soil moisture from a generic dataset. This model is based on qualitative indexes related to runoff and infiltration processes that are convolved with climatic data extracted from a global database. The link between the resulting moisture map and the soil geomechanical behavior can be established from field observations or laboratory measurements. This model is applied to two different cases history and is discussed in term of positive results and limitations. Resume - L'estimation du comportement geomecanique des sols ou du proche sous-sol rentre dans bon nombre d'applications dediees aux risques naturels, a l'amenagement du territoire ou a la mobilite des vehicules terrestres en tout-terrain. Ces applications necessitent souvent une forte reactivite dans la production des diagnostics, l'utilisation de donnees facilement accessibles et une resolution de l'ordre du decametre. Ce travail presente un modele hydrologique simplifie permettant d'estimer rapidement un indice d'humidite des sols a partir d'un jeu de donnees generique. Le modele est base sur le calcul d'indices qualitatifs - rendant compte respectivement des effets de ruissellement et d'infiltration - qui sont ensuite ponderes par des donnees climatologiques issus de bases de donnees globales. Le lien vers le comportement geomecanique des sols est realise grâce a des abaques d'evolution de la portance des sols en fonction du taux d'humidite ou grâce a des observations in situ. Les deux applications presentees montrent l'efficacite, mais aussi les limites, du modele developpe pour la production de carte de mobilite de vehicules ou de cartes de susceptibilite aux mouvements de terrain.

Shaly formations are the focus of many research programs and consortia sponsored by petroleum companies and/or waste management organizations; given that they act as seals for oil- or gas-bearing reservoirs or as host rock for underground waste disposals, their integrity (e.g., the possible presence of water-bearing fractures) is a critical factor in risk assessment. To model their rheological properties through time, the observed clastic injectites are used as markers of their mechanical evolution. Aptian–Albian marly formations of the Vocontian Basin (southeast France) are the basis of this study; massive turbidite systems associated with large-scale clastic injectite networks have been described in exceptional outcrops. Field data have permitted the identification of early fracturing in the host formation; the injection of sand is an early event, contemporaneous with the deposition of massive sand bodies. The paleocompaction curve has been calculated, and the porosity evolution of the sediments has been restored from sea floor to about 500 m (1640 ft) burial. Then, the original configuration of dikes can be reconstituted. Boundary conditions of various numerical modeling have been derived from this extensive reliable data set. Numerical static simulations of the behavior of marly formations are presented, testing the possible function of heterogeneous lithology, bedrock geometry, or loading by sudden massive sand deposition; they indicate that early fracturing is physically possible in the presented scenarios. The next step will be to simulate in dynamic conditions the opening and filling of some of these cracks by hydrofracturing. We dedicate this chapter to the memory of our colleague and friend, Stephen T. Horseman.

International audience; Soil erosion is hypothesized to have strongly accelerated after WWII in response to changes in land use and landscape patterns across Europe. However, there is a lack of data on this process in cultivated-flat-environments of Northwestern Europe where very few records are available to quantify these changes in soil erosion rates during the last several decades. The Loire River basin (117,500 km²), in France, provides a good example of cultivated environments that have been exposed to extensive land use changes after 1945. To quantify the impact of these changes on soil erosion rates, 10 ponds and lakes draining contrasted cultivated environments (dominance of cropland, vineyards, forests or grassland) were selected for sediment coring. Several cores were collected depending on the pond morphology, and the sediment profiles were characterized (magnetic susceptibility, CT-scan, spectrocolorimetry, particle size, XRF elemental geochemistry, C/N ratio, Rock-Eval pyrolysis, palynofacies) and dated using fallout radionuclides (Cs-137, excess Pb-210) analyzed by gamma spectrometry. Based on these results, the Mass Accumulation Rates and the corresponding soil erosion rates were calculated for the pre-and post-1945 periods. The results show strong variations in soil erosion rates depending on the local changes in land use and landscape (documented by agricultural statistics and aerial/remote sensing imagery). The impact of the occurrence of extreme weather events (e.g. storms) was also recorded in some of the sediment cores. In the future, sediment tracing techniques will be applied on these cores to quantify the occurrence of potential changes in sediment sources during this period. The calculated soil erosion rates will also be compared to the outputs of a soil erosion model in order to identify the main factors (climate vs. human impact) controlling these changes and to propose effective measures to combat soil erosion and reservoir/river siltation in this region.