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In the context of elevated concerns related to nuclear accidents and warfare, the lessons learnt from the Fukushima Daiichi Nuclear Power Plant accident in 2011 are important. In particular, Japanese authorities implemented an ambitious decontamination program to reduce the air dose rate in order to facilitate the return of the local inhabitants to previously evacuated areas. This approach contrasts the strategy adopted in Chernobyl, where the most contaminated areas remain off limits. Nonetheless, the effectiveness of the Japanese decontamination strategy on the dispersion of radioactive contaminant fluxes across mountainous landscapes exposed to typhoons has not been quantified. Based on the unique combination of river monitoring and modeling in a catchment representative of the most impacted area in Japan, we demonstrate that decontamination of 16% of the catchment area resulted in a decrease of 17% of sediment-bound radioactive fluxes in rivers. Decontamination operations were therefore relatively effective, although they could only be conducted in a small part of the area due to the dominance of steep forested slopes. In fact, 67% of the initial radiocesium contamination was calculated to remain stored in forested landscapes, which may contribute to future downstream radiocesium dispersion during erosive events. Given that only a limited proportion of the initial population had returned in 2019 (~30%), it raises the question as to whether decontaminating a small percentage of the contaminated area was worth the effort, the price, and the amount of waste generated? [ABSTRACT FROM AUTHOR]

While soil erosion and runoff physical processes and socio-environmental impacts are widely addressed in the literature, few studies have focused on the economical dimension. However, it is essential to consider this dimension to conduct appropriate land use management policies. Erosion and runoff are known to result into on-site and off-site impacts. A fully exhaustive analysis of erosion and runoff economic costs may be difficult and ambitious due to the low availability of the data and considering that some issues are still unevaluated in the literature (cost of public policies to prevent erosion and runoff impacts, prohibition of drinking water due to turbidity, etc.). In this study, we chose to analyze the main off-site economic costs induced by these processes in two specific areas located in the northwestern European loess belt (Normandy, France). We quantified avoidance and social damages over the last 25 years through a global and retrospective analysis of financial databases provided by regional or local authorities (water agencies, departmental councils, reinsurance, drinking water companies, transport infrastructures managers) and literature review (scientific articles and technical reports). Our analysis suggested that from 1995 to 2017, the total damages cost ranged from 611 to 721 M€. Off-site avoidance damage costs accounted for almost 2/3 of the total expenditure. In the Seine-Maritime area, the mean cost was evaluated to 4 319 € yr−1 km−2 and to 868 € yr−1 km−2 in the Eure area. Even if we tried to be as exhaustive as possible some off-site economic costs remained unknown. It appeared that more research is necessary for the scientific community to get a full picture of off-site economic costs induced by erosion and runoff. • Off-site economic costs of runoff and erosion were quantified through an analysis of public expenditures. • The total avoidance damages cost ranged from 375 to 485 M€. • The total social damages cost reached 236 M€. • The mean cost of erosion and runoff off-site impacts was estimated to 868 and 4319 € yr−1 km−2. [ABSTRACT FROM AUTHOR]

A major challenge in geomorphology is reconciling the disparity between runoff coefficients and erosion rates from the field to the catchment scale. In the European loess belt, a decrease of runoff coefficients and erosion rates occurs with increasing spatial scales. Indeed, it is important to understand the scale dependency of run-off and erosion to manage the off-site impacts of accelerated soil erosion. Accordingly, a continuous simulation of the scale dependency of runoff and erosion, from the field scale to the catchment scale (100 – 1000 km²) was conducted with the Water and Sediment (WaterSed) model for two catchments (Andelle-756 km², and Austreberthe-214 km²) over 12 years (>1000 events). Scale effects were evident with a 100-fold decrease in runoff coefficients and sediment delivery 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 were high. The inter-annual variability of runoff and erosion was closely linked to the number of extreme events per year 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 of the soil surface and of the landscape to retain overland flows are largely exceeded. The seasonality of soil surface characteristics 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 governed by hortonian overland flow. When saturation flows are generated, topography and soil depth become the dominant factors. Understanding runoff and sediment response at different scales should focus on the location and amount of runoff and sediment production within the catchment and the capacity of the downstream flow path to retain or transfer overland flow and sediment. Furthermore, this study demonstrates the need to implement model that represent both hortonian and saturation flows when simulating erosion events at the catchment scale. [ABSTRACT FROM AUTHOR]