ALBERT

Description: Abstract Long‐term records are needed to investigate the impact of extreme events in the current framework of global change. Sedimentary reconstruction with a high resolution remains difficult without conducting expensive, destructive and/or time‐consuming analyses. In this study, high resolution CT‐scan profiles (0.6 mm resolution) were used to investigate their potential for detecting flood deposits induced by heavy rainfall events. This method was applied to a sediment core dated with fallout radionuclides – covering a 120 year period – collected in a pond draining a small forested catchment (French Massif Central – Central France). Between 1960 and 2017, 28 layers were deposited. Seventy‐six percent of these deposits were correlated to the occurrence of heavy rainfall (〉50 mm) recorded during one or two consecutive days. The remainder of the deposits detected with the Computer Tomography (CT) scanner (n = 5) were not correlated to weather events. They mainly occurred in response to landscape management operations (for example, afforestation works as a result of the major 1999 storm). This period was indeed characterized by an increase in the delivery of 137Cs‐enriched sediment, demonstrating a greater topsoil contribution to sediment during major forest management operations. The intensity of detritical layers has significantly decreased throughout time after a major land use change that took place in 1948 and land abandonment. The frequency of heavy rainfall and associated detritical deposits has nevertheless increased by 60% and 75%, respectively, between the 1960 to 2017 period. These results outline the potential of CT‐scan for reconstructing long‐term flood deposits associated with heavy precipitation. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Influence of the rainfall regime on erosion and transfer of suspended sediment in a 905-km² mountainous catchment of the southern French Alps was investigated by combining sediment monitoring, rainfall data, and sediment fingerprinting (based on geochemistry and radionuclide concentrations). Suspended sediment yields were monitored between October 2007 and December 2009 in four subcatchments (22–713 km²). Automatic sediment sampling was triggered during floods to trace the sediment origin in the catchment. Sediment exports at the river catchment outlet (330 ± 100 t km -2  yr -1 ) were mainly driven (80%) by widespread rainfall events (long duration, low intensities). In contrast, heavy, local and short duration storms, generated high peak discharges and suspended sediment concentrations in small upstream torrents. However, these upstream floods had generally not the capacity to transfer the sediment down to the catchment outlet and the bulk of this fine sediment deposited along downstream sections of the river. This study also confirmed the important contribution of black marls (up to 70%) to sediment transported in rivers, although this substrate only occupies c . 10% of the total catchment surface. Sediment exports generated by local convective storms varied significantly at both intra- and inter-flood scales, because of spatial heterogeneity of rainfall. However, black marls/marly limestones contribution remained systematically high. In contrast, widespread flood events that generate the bulk of annual sediment supply at the outlet were characterized by a more stable lithologic composition and by a larger contribution of limestones/marls, Quaternary deposits and conglomerates, which corroborates the results of a previous sediment fingerprinting study conducted on riverbed sediment. Copyright © 2012 John Wiley & Sons, Ltd.

Description: ABSTRACT An excess of fine sediment (grain size 〈2 mm) supply to rivers leads to reservoir siltation, water contamination and operational problems for hydroelectric power plants in many catchments of the world, such as in the French Alps. These problems are exacerbated in mountainous environments characterized by large sediment exports during very short periods. This study combined river flow records, sediment geochemistry and associated radionuclide concentrations as input properties to a Monte Carlo mixing model to quantify the contribution of different geologic sources to river sediment. Overall, between 2007 and 2009, erosion rates reached 249 ± 75 t km −2 yr −1 at the outlet of the Bléone catchment, but this mean value masked important spatial variations of erosion intensity within the catchment (85–5000 t km −2 yr −1 ). Quantifying the contribution of different potential sources to river sediment required the application of sediment fingerprinting using a Monte Carlo mixing model. This model allowed the specific contributions of different geological sub-types (i.e. black marls, marly limestones, conglomerates and Quaternary deposits) to be determined. Even though they generate locally very high erosion rates, black marls supplied only a minor fraction (5–20%) of the fine sediment collected on the riverbed in the vicinity of the 907 km 2 catchment outlet. The bulk of sediment was provided by Quaternary deposits (21–66%), conglomerates (3–44%) and limestones (9–27%). Even though bioengineering works conducted currently to stabilize gullies in black marl terrains are undoubtedly useful to limit sediment supply to the Bléone river, erosion generated by other substrate sources dominated between 2007 and 2009 in this catchment. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Summer typhoons and spring snowmelt led to the riverine spread of continental Fukushima fallout to the coastal plains of Northeastern Japan and the Pacific Ocean. Four fieldwork campaigns based on measurement of radioactive dose rates in fine riverine sediment that has recently deposited on channel bed-sand were conducted between November 2011 and May 2013 to document the spread of fallout by rivers. After a progressive decrease in the fresh riverine sediment doses rates between 2011 and early spring in 2013, a fifth campaign conducted in November 2013 showed that they started to increase again after the occurrence of violent typhoons. We show that this increase in dose rates was mostly due to remobilization of contaminated material that was temporarily stored in river channels or, more importantly, in dam reservoirs of the region during the typhoons. In addition, supply of particles from freshly eroded soils in autumn 2013 was the most important in areas where decontamination works are under progress. Our results underline the need to monitor the impact of decontamination works and dam releases in the region, as they may provide a continuous source of radioactive contamination to the coastal plains and the Pacific Ocean during the coming years. Scientific Reports 4 doi: 10.1038/srep04574

Description: Influence of the rainfall regime on erosion and transfer of suspended sediment in a 905-km² mountainous catchment of the southern French Alps was investigated by combining sediment monitoring, rainfall data, and sediment fingerprinting (sediment geochemistry and radionuclide concentrations). Suspended sediment yields were monitored between October 2007 and December 2009 in four subcatchments (22–713 km²). Automatic sediment sampling was triggered during floods to trace the sediment origin in the catchment. Sediment exports at the river catchment outlet (330 ± 100 t km -2  yr -1 ) were mainly driven (80%) by widespread rainfall events (long duration, low intensities). In contrast, heavy, local and short duration storms, generated high peak discharges and suspended sediment concentrations in small upstream torrents. However, these upstream floods had generally not the capacity to transfer the sediment down to the catchment outlet and the bulk of this fine sediment deposited along downstream sections of the river. This study also confirmed the important contribution of black marls (up to 70%) to sediment transported in rivers, although this substrate only occupies ca. 10% of the total catchment surface. Sediment exports generated by local convective storms varied significantly at both intra- and inter-flood scales, because of spatial heterogeneity of rainfall. However, black marls/marly limestones contribution remained systematically high. In contrast, widespread flood events that generate the bulk of annual sediment supply at the outlet were characterised by a more stable lithologic composition and by a larger contribution of limestones/marls, Quaternary deposits and conglomerates, which corroborates the results of a previous sediment fingerprinting study conducted on riverbed sediment. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e., the conventional approach based on radionuclide and geochemical concentrations and the alternative Diffuse Reflectance Infrared Fourier Transform Spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterised by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterised by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way to prioritise the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Sediment connectivity characterizes the physical transfer of sediment through different geomorphic compartments in catchments due to sediment detachment, transport and deposition. Quantifying and modelling sediment connectivity is therefore a key prerequisite to improving our understanding of the dispersion of particle-borne contaminants, especially in catchments exposed to highly erosive climates. The objective of this study is to provide novel insights into typhoon impacts on sediment connectivity from hillslopes to rivers. The dispersion of particle-bound cesium-137 ( 137 Cs) was investigated in two coastal catchments draining the main contamination plume from the Fukushima Daiichi Nuclear Power Plant accident. Five sampling campaigns were carried out from November 2011 to November 2015, after each typhoon season. The spatial and temporal evolution of 137 Cs contamination was investigated through the calculation of 137 Cs enrichment ratios in sediment relative to nearby soils. Rainfall erosivity (EI 30 ) associated with the main typhoons that occurred prior to each sampling campaign were computed, mapped, and finally used to improve a topographic-based Index of Connectivity. From 2011 to 2015, mean contamination levels in Mano and Niida catchments decreased from 11.9 kBq kg -1 to 3.3 kBq kg -1 and from 34.1 kBq kg -1 to 8.0 kBq kg -1 , respectively. Regional mean EI 30 ranged from 262 MJ mm ha -1 h -1 for typhoon Jelawat (2012) to 1695 MJ mm ha -1 h -1 for typhoon Roke (2011). Typhoons Roke (2011) and Etau (2015) showed the highest connectivity from contaminated sources to the rivers, and induced a significant export of sediment to the ocean. In 2013 a slight increase in 137 Cs levels in river sediments occurred, likely resulting from initial decontamination works and the occurrence of two consecutive typhoons. Importantly, this research provides new insights into the connectivity of the main sources of sediments contaminated with radiocesium in Fukushima Prefecture and their temporal evolution, which will help with ongoing decontamination efforts. This article is protected by copyright. All rights reserved.

Description: Measurement of radioactive dose rates in fine sediment that has recently deposited on channel bed-sand provides a solution to address the lack of continuous river monitoring in Fukushima Prefecture after Fukushima Dai-ichi nuclear power plant (FDNPP) accident. We show that coastal rivers of Eastern Fukushima Prefecture were rapidly supplied with sediment contaminated by radionuclides originating from inland mountain ranges, and that this contaminated material was partly exported by typhoons to the coastal plains as soon as by November 2011. This export was amplified during snowmelt and typhoons in 2012. In 2013, contamination levels measured in sediment found in the upper parts of the catchments were almost systematically lower than the ones measured in nearby soils, whereas their contamination was higher in the coastal plains. We thereby suggest that storage of contaminated sediment in reservoirs and in coastal sections of the river channels now represents the most crucial issue. Scientific Reports 3 doi: 10.1038/srep03079