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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 caesium-137 (137Cs) 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 137Cs contamination was investigated through the calculation of 137Cs enrichment ratios in sediment relative to nearby soils. Rainfall erosivity (EI30) 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 EI30 ranged from 262 MJ mm ha−1 h−1 for typhoon Jelawat (in 2012) to 1695 MJ mm ha−1 h−1 for typhoon Roke (in 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 137Cs 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 radiocaesium in Fukushima Prefecture and their temporal evolution, which will help with ongoing decontamination efforts. Copyright © 2016 John Wiley & Sons, Ltd. [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]

Abstract Indices of connectivity are critical means for moving from qualitative to (semi-)quantitative evaluations of material (e.g., water, sediment and nutrients) transfer across the building blocks of a terrestrial system. In geomorphology, compared to closely related disciplines like ecology and hydrology, the development of indices has only recently started and as such presents opportunities and challenges that merit attention. In this paper, we review existing indices of sediment connectivity and suggest potential avenues of development for meeting current basic and applied research needs. Specifically, we focus on terrestrial geomorphic systems dominated by processes that are driven by hydro-meteorological forcing, neglecting seismically triggered events, karstic systems and environments controlled by eolian processes. We begin by setting a conceptual framework that combines external forcings (drivers) and system (intrinsic) structural and functional properties relevant to sediment connectivity. This framework guides our review of response variables suitable for sediment connectivity indices. In particular, we consider three sample applications concerned with sediment connectivity in: (i) soil studies at the plot scale, (ii) bedload transport at the reach scale, and (iii) sediment budgets at the catchment scale. In relation to the set of response variables identified, we consider data availability and issues of data acquisition for use in indices of sediment connectivity. We classify currently available indices in raster based, object or network based, and indices based on effective catchment area. Virtually all existing indices address the degree of static, structural connectivity only, with limited attention for process-based, functional connectivity counterparts. Most recent developments in indices of sediment connectivity deal, to some extent, with different styles of anthropogenic and hydro-meteorological forcings and with the temporal variability of sediment connectivity, by incorporating additional variables and parameters in existing indices. We believe that, in order to use structural connectivity as explanatory or predictive tool, indices need to be interpretable in relation to geomorphic processes, material properties, and forcing styles and magnitude-frequency spectra. Improvements in this direction can be made through studies shaped to constrain structural-functional correlations across a range of hydro-meteorological scenarios, for example employing field-based techniques such as particle tracking and sediment provenance analysis, as well as numerical simulations. We further consider existing indices in relation to spatial and temporal scales. The latter have immediate implications on the distinction and application between indices and models of sediment connectivity. In this context, we suggest that sediment connectivity over millennial or longer time scales should be dealt with models, as opposed to indices. [ABSTRACT FROM AUTHOR]

In recent decades, stream valleys have been profoundly modified by the construction of weirs and dams and by channelisation. Channelisation modifies the morphology of streams and induces changes in their energy regime and sediment transport capacity. These types of changes in the channel morphology have to be quantified to allow the implementation of management strategies to regulate sediment transfer. However, studies over an entire stream using historical comparisons remain scarce, and the associated uncertainties have not yet been resolved. In this study, the sedimentary response to channelisation on a medium time scale (42 years) of a French river known as the Ligoire is investigated. This river is the main channel of a small rural headwater catchment that has been channelised over 21 km. We have used the historical cross sections before and after channelisation and the current ones, and the objectives of this study were as follows: (1) to develop a methodology of cross section superposition and the associated uncertainties; (2) to quantify the erosion and aggradation processes in the bed and on the banks along the bed profile; and (3) to calculate a sediment budget for the entire stream and determine the relative contributions of the banks and the streambed to this budget. A comparison of the cross sections before and after the channelisation shows that the morphology of the stream has been completely altered: the main channel length was reduced by 10%, the bankfull width was increased on average by 63%, and the slopes were smoothed. A total of 60,000 m 3 of sediments was excavated during the channelisation works. Our results indicate that erosion is the dominant process: over 63% of its length, the streambed was incised by 0.41 m on average; and over 60% of its length, the banks were eroded by 0.20 m on average. The successive patterns of erosion and deposition along the stream are the result of the cumulative effects of channelisation and of the presence of weirs and artificial knickpoints in the Ligoire channel. The vertical uncertainty of the elevation of the historical cross section is an important parameter for controlling the areas and sediment budget values. Using Monte Carlo methods, we found that 1000 sediment budgets from different profile shiftings are necessary to obtain a variation coefficient below 0.1%. The overall mean stream sediment budget for the period 1970–2012 is -9358 ± 412 m 3 , with 66% originating from the banks and 34% from the streambed. Relative to the Ligoire watershed surface, the stream sediment yield is 2.71 ± 0.12 m 3 .km − 2 .y − 1 . The approach developed in this study is easily replicable and relatively cheap and provides an integrated quantified, overview of the morphological adjustments after channelisation works on a stream. [ABSTRACT FROM AUTHOR]