Sediment yield in Europe: Spatial patterns and scale dependency

Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management

Both from a scientific and environmental management perspective, there is a large need to assess the magnitude and controlling factors of human impacts on catchment sediment yield. Quantifying this impact is difficult, since it requires knowing both the actual sediment yield (SY a , [t km − 2 y − 1 ]) as well as the corresponding “pristine” value of a catchment (SY p , [t km − 2 y − 1 ]; i.e. the sediment yield that can be expected if the catchment was not affected by humans). Here we address this problem by comparing measured SY a values for 165 European catchments that were unaffected by dams or reservoirs with their corresponding SY p , which were predicted using a recently developed regression model. The ratio between these two values is expected to reflect the degree of human impact on catchment sediment yield (HIF). Correlation and partial correlation analyses showed that spatial variability in HIF is mainly explained by differences in land use (i.e. the fraction of arable land) and catchment area. The effect of these two factors was clearly linked in western and central Europe: whereas SY a can be easily 40 times higher than SY p in intensively cultivated small (≤ 1 km 2 ) catchments, the difference is negligible for large (> 1000 km 2 ) catchments with the same land use. While, this concurs with our knowledge that the effects of land use (change) on erosion rates can be buffered at the catchment scale, this study provides a first robust quantification of this effect. Apart from a potential climatic effect (i.e. a correlation between HIF and the average annual air temperature) no other factors could be identified that are significant in explaining observed differences in HIF. This indicates that HIF is mainly controlled by catchment scale and land use, while other factors may be only of secondary importance at an intra-continental scale. Nonetheless, more accurate quantifications of these HIF values and more refined characterizations of the catchments in terms of (historical) land use, soil types/lithology, weather conditions and topography may reveal additional trends.

Quantifying human impacts on catchment sediment yield: A continental approach

Current models aiming to simulate contemporary sediment yield (SY) implicitly assume that tectonic effects are either irrelevant or are reflected by catchment topography. In this study we analyse t...

Moderate seismic activity affects contemporary sediment yields

Soil erosion and suspended sediment (SS) pose risks to chemical and ecological water quality. Agricultural activities may accelerate erosional fluxes from bare, poached or compacted soils, and enhance connectivity through modified channels and artificial drainage networks. Storm-event fluxes dominate SS transport in agricultural catchments; therefore, high temporal-resolution monitoring approaches are required, but can be expensive and technically challenging. Here, the performance of in situ turbidity sensors, conventionally installed submerged at the river bankside, is compared with installations where river water is delivered to sensors ex situ, i.e. within instrument kiosks on the riverbank, at two experimental catchments (Grassland B and Arable B). The in situ and ex situ installations gave comparable results when calibrated against storm-period, depth-integrated SS data, with total loads at Grassland B estimated at 12 800 and 15 400 t, and 22 600 and 24 900 t at Arable B, respectively. The absence of spurious turbidity readings relating to bankside debris around the in situ sensor and its greater security make the ex situ sensor more robust. The ex situ approach was then used to characterise SS dynamics and fluxes in five intensively managed agricultural catchments in Ireland which feature a range of landscape characteristics and land use pressures. Average annual suspended sediment concentration (SSC) was below the Freshwater Fish Directive (78/659/EEC) guideline of 25 mg L−1, and the continuous hourly record demonstrated that exceedance occurred less than 12 % of the observation year. Soil drainage class and proportion of arable land were key controls determining flux rates, but all catchments reported a high degree of inter-annual variability associated with variable precipitation patterns compared to the long-term average. Poorly drained soils had greater sensitivity to runoff and soil erosion, particularly in catchments with periods of bare soils. Well drained soils were less sensitive to erosion even on arable land; however, under extreme rainfall conditions, all bare soils remain a high sediment loss risk. Analysis of storm-period and seasonal dynamics (over the long term) using high-resolution monitoring would be beneficial to further explore the impact of landscape, climate and land use characteristics on SS export.

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Investigating suspended sediment dynamics in contrasting agricultural catchments using ex situ turbidity-based suspended sediment monitoring

In the UK, fine sediment is viewed increasingly as a diffuse pollutant due to its role as a vector for the transport of potential contaminants, and in causing siltation, which may have adverse effects on river and estuarine habitats. There is a need, therefore, for river managers to have reliable information on sediment budgets in order to plan measures that will achieve good status under the EU Water Framework Directive. As part of a wider sediment-budget investigation in the EU-funded Cycleau Project, detailed records of fine sediment yield over the 10-year period from 1994-2003 have been analysed for the Exe Basin (1500 km 2 ), a principal river system of southwest England. The longer-term average yields in the three major tributaries of the Exe Basin are discussed and results of monitoring of sediment loads at a site near the tidal limit over a one-year period confirm the importance of the River Exe in contributing sediment to the Estuary.

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Anna Harlow

Papers

Sediment yields in the Exe Basin: a longer-term perspective