Catchment Sediment Budget Research Articles

Catchment-scale variability and driving factors of fine sediment deposition: insights from a coupled experimental and machine-learning-based modeling study

PurposeFine 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.MethodsFine 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.ResultsFine 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.ConclusionThis 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.

Gullies and badlands of India: Genesis, geomorphology and land management

AbstractGully erosion is a formidable land degradation process globally. It is omnipresent across India, wherein two of the largest badlands regions in the world also exist. However, despite being affected by widespread gullying, gully erosion research in India has been rather limited, with key aspects of gully formation, morphologies and dynamics remaining unknown. Through this comprehensive systematic review, we synthesise previous studies on gully erosion in India and in turn highlight pivotal knowledge gaps. The review starts with a discussion of the causal factors of gullying in India, which underlines how gully initiation in several regions was an aftermath of deforestation and overgrazing. Contrastingly, the badlands of Central and Western India have mainly developed in response to natural triggers like neotectonics and Holocene climate change. The section on mapping highlights how mapping methodologies have not only been dictated by the availability of imagery and/or means of data processing, but also the mapping purpose—that is, individual gully channels or entire badlands. Although a few studies applying concepts of fractal geometry to characterise badlands’ geomorphology are innovative and unique, the most striking research gaps we have identified also pertain to understanding and quantification of gully geomorphology and erosion dynamics in different regions of India. Our review reveals that gullies of peninsular India have been the least studied, followed by those of the Himalayan and Sub‐Himalayan region. Although the literature provides interesting examples of linkages between badlands development and the wider geomorphic evolution of the landscape, better chronological understanding is required to disentangle the relative importance of natural and anthropogenic drivers of landscape change in India's badlands. Large‐scale mapping of gully characteristics, quantification of gully morphologies, gully erosion rates and its share in catchment sediment budget across various physiographic regions or river basins of the country also constitute important areas for future research.

Shallow landslides and vegetation at the catchment scale: A perspective

Shallow, rainfall-triggered landslides are an important catchment process that affect the rate and calibre of sediment within river networks and create a significant hazard, particularly when shallow landslides transform into rapidly moving debris flows. Forests and trees modify the magnitude and rate of shallow landsliding and have been used by land managers for centuries to mitigate their effects. We understand that at the tree and slope scale root reinforcement provides a significant role in stabilising slopes, but at the catchment scale root reinforcement models only partially explain where shallow landslides are likely to occur due to the complexity of subsurface material properties and hydrology. The challenge of scaling from slopes to catchments (from 1-D to 2-D) reflects the scale gap between geomorphic process understanding and modelling, and temporal evolution of material properties. Hence, our understanding does not, as yet, provide the necessary tools to allow vegetation to be targeted most effectively for landslide reduction. This paper aims to provide a perspective on the science underpinning the challenges land and catchment managers face in trying to reduce shallow landslide hazard, manage catchment sediment budgets, and develop tools for catchment targeting of vegetation. We use our understanding of rainfall-triggered shallow landslides in New Zealand and how vegetation has been used as a tool to reduce their incidence to demonstrate key points.

Sediment sources, soil loss rates and sediment yields in a Karst plateau catchment in Southwest China

Intensive agricultural activities have accelerated soil erosion and rocky desertification in karst regions of southwest China. Knowledge of sediment sources and soil erosion rates can be used to target soil conservation measures and to improve calibration and validation of process-based soil erosion and sediment delivery models for scenario analyses. Due to the complexity of karst environments, however, catchment scale information on these components of sediment budgets has rarely been assembled, meaning there continues to be an evidence gap. Within this context, this study selected Chenqi catchment, given its appropriate research infrastructure, to investigate sediment sources and soil loss rates in a typical karst agroforestry landscape. We estimated the relative contributions from three sources: surface soil, subsurface soil and clastic rock, using a composite fingerprinting procedure combining 137Cs and magnetic susceptibility and a frequentist un-mixing model with Monte Carlo uncertainty analysis. Suspended sediment samples were taken at an hourly interval during seven rainfall events in 2017–2018 to characterize and quantify the sediment exported in both surface and underground drainage. The overall average median contributions (with 5th-95th percentile uncertainty ranges) from the sources to the suspended sediment samples from the surface drainage outlet were in the order: 62% (0−99%) subsurface soils, 25% (0−91%) surface soils and 13% (0−45%) clastic rock. For the sediment samples collected from the underground drainage catchment outlet, the corresponding estimates were in the order: 68% (0−97%) subsurface soils, 25% (0−53%) clastic rock and 7% (0−44%) surface soils. Plot scale soil loss rates were highest on cropland (0.70 Mg km−2) and pasture land (0.48 Mg km−2). The average (2017 and 2018) annual suspended sediment load exported through the surface outlet was 4.64 Mg km−2 compared with 1.20 Mg km−2 through the underground outlet. The broader implications of this study are that subsurface and clastic rock sources represent a significant component of the catchment sediment budget, meaning erosion control measures targeting hillslope surface soils alone may have limited impact on suspended sediment export at landscape scale.

Distributed water erosion modelling at fine spatial resolution across Denmark

Water erosion on agricultural land and sediment delivery to streams are a major threat to soil productivity and surface water quality. Climate change and different national and international societal drivers now require Denmark to take action to protect soil and water resources. In this study, we adapted the spatially distributed sediment transport model WaTEM using the best data available at national scale. To calibrate and validate the model, sediment yield data from 31 catchments and 189 slope units in Denmark were compared with the model output, which was produced at a fine spatial resolution of 10 × 10 m.Residual analysis and cross-validation were used to identify potential catchment outliers and assess model robustness. We obtained a median Nash & Sutcliffe model efficiency range of 0.06–0.6 for the Danish environment. Based on the equifinality concept, an ensemble of 100, NSE-weighted model realisations for acceptable transport capacity coefficients was used to assess model uncertainty. The comparison between rill survey data and predicted values indicates that although the model captures well the spatial variability in erosion, it may underestimate the long-term average of soil erosion. Based on the modelling, 71% of the agricultural land in Denmark is mapped as stable in terms of the amount of erosion and deposited material. Overall, 6.1% of the farmland is estimated to have unsustainable erosion and 0.9% of the farmland exceeds erosion rates of 7.5 t ha−1 a−1. Sediment export into surface water in Denmark equals 92,000 t a−1, corresponding to an average sediment yield of 2.7 t km−2.The performance of WaTEM is considered satisfactory in this study. Importantly, modelled water erosion exceeds the perceived erosion risk in Denmark. Strengthened by distributed uncertainty assessment at national scale, our study provides an important national knowledge base for engaging land users and regulators in the process of targeted erosion mitigation planning that is required to comply with national and EU regulation. Future investigations concerning the deviation between predicted and observed data and specific catchment parameters for the non-behavioural catchments are required as well as studies that include the establishment of catchment sediment budgets.

Predicting spatial patterns in riverbank erosion for catchment sediment budgets

ABSTRACTRiverbank erosion is an important component of catchment sediment budget models but remains one of the least well-understood processes, particularly at large spatial scales. Here, we develop and test a new bank migration model in New Zealand for large catchment applications that (1) better represents spatial variability in factors influencing bank erosion and (2) improves predictive performance. We represent bank migration rates as a function of reach-scale stream power, channel sinuosity, soil texture, valley confinement, riparian woody vegetation and channel protection works. The new model significantly improves prediction compared to the SedNetNZ model. Comparison of measured bank migration rates with individual variables shows percent silt + clay derived from soil maps exhibited the strongest correlation, whereas other variables were non-significant. The model results demonstrate that improved prediction can be achieved by combining spatial representation of multiple factors over large areas, despite low correlation between individual variables and bank migration rates.

Proglacial sediment dynamics from daily to seasonal scales in a glaciated Alpine catchment (Bossons glacier, Mont Blanc massif, France)

AbstractThe sediment yields of Alpine catchments are commonly determined from streamload measurements made some distance downstream from glaciers. However, this approach indiscriminately integrates erosion processes occurring in both the glacial and proglacial areas. A specific method is required to ascertain the respective inputs from (i) subglacial and supraglacial sediments, (ii) proglacial hillslopes and (iii) proglacial alluvial areas or sandurs. This issue is addressed here by combining high‐resolution monitoring (2 min) of suspended sediment concentrations at different locations within a catchment with discharge gauging and precipitation data. This methodological framework is applied to two proglacial streams draining the Bossons glacier (Mont Blanc massif, France): the Bossons and Crosette streams. For the Bossons stream, discharge and suspended load data were acquired from June to October 2013 at 1.15 and 1.5 km from the glacial terminus, respectively upstream and downstream from a small valley sandur. These hydro‐sedimentary data are compared with the Crosette stream dataset acquired at the outlet of the Bossons glacier subglacial drainage system. A fourfold analysis focusing on seasonal changes in streamload and discharge, multilinear regression modelling, evaluation of the sandur flux balance and probabilistic uncertainty assessment is used to determine the catchment sediment budget and to explain the proglacial sediment dynamics. The seasonal fluctuation of the sediment signal observed is related to the gradual closing of the subglacial drainage network and to the role of the proglacial area in the sediment cascade: the proglacial hillslopes appear to be disconnected from the main channel and the valley sandur acts as a hydrodynamic sediment buffer both daily and seasonally. Our findings show that an understanding of proglacial sediment dynamics can help in evaluating paraglacial adjustment and subglacial erosion processes. Copyright © 2017 John Wiley & Sons, Ltd.

Proglacial erosion rates and processes in a glacierized catchment in the Swiss Alps

AbstractIn the Swiss Alps, climatic changes have not only caused glacier retreat, but also likely increased sedimentation downstream of glaciers. This material either originates from below the glacier or from periglacial environments, which are exposed as glaciers retreat, and often consist of easily erodible sediment. Griesgletscher's catchment in the Swiss Alps was examined to quantify erosion in the proglacial area, possible hydrological drivers and contributions of the sub‐ and periglacial sources. Digital elevation models, created from annual aerial photographs, were subtracted to determine annual volume changes in the proglacial area from 1986 to 2014. These data show a strong increase in proglacial erosion in the decade prior to 2012, coincident with increasing proglacial area size. However, examination of the gradient between discharge and sediment evacuation, and modeled sediment transport, could suggest that the proglacial area began to stabilize and sediment supply is limited. The large influx of sediment into the proglacial reservoir, which is roughly 2.5 times greater than the amount of sediment eroded from the proglacial area, demonstrates the importance of subglacial erosion to the catchment's sediment budget. Although far more sediment originates subglacially, erosion rates in the proglacial area are over 50 times greater than the rest of the catchment. In turn, both sub‐ and periglacial processes, in addition to constraining sediment supply, must be considered for assessing future sediment dynamics as glacier area shrinks and proglacial areas grow. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

The complexities of measuring fine sediment accumulation within gravel‐bed rivers

AbstractFine sediment storage within gravel beds is a key component of catchment sediment budgets and affects the health of benthic and hyporheic habitats. Here, we assess the performance of two substrate infiltration traps for the characterization of fine sediment (<2 mm) accumulation. One design, the vertically extending sediment trap, permits both lateral and vertical exchange in the sediment column, whereas the second type, a more traditional fixed‐area sediment trap with impermeable side walls, permits only vertical exchange. Traps were deployed at three sites on the River Tame, Birmingham (UK), over varying installation periods (14–401 days). Results indicate that the facilitation of multiple pathways of exchange within the vertically extending sediment traps (vertical and lateral) resulted in a significantly greater amount of fine sediment being accumulated than in adjacent fixed‐area sediment traps. This suggests that lateral transport is an important component contributing to fine sediment accumulation. However, there are notable and inherent problems associated with the use of different types of sediment trap and in the way the data should be presented and interpreted. This paper discusses the practical implications of the study findings and reflects on the complexities of undertaking accurate sediment deposition measurements in the field.

An investigation of controlling variables of riverbank erosion in sub-tropical Australia

A large proportion of the uncertainty surrounding catchment sediment budget modelling has been attributed to sediment supplied from riverbank erosion. Some of the variables influencing riverbank erosion are bend curvature, specific streampower, riparian vegetation, and in some instances sand and gravel extraction. The empirical relationship between these variables and observed riverbank erosion across 78 km of the Upper Brisbane River, Australia was investigated. No significant relationship was found between curvature, specific streampower and riverbank erosion. The role of riparian vegetation relative to sediment supply from riverbank erosion varied with spatial location, susceptibility of a reach to erosion, and human disturbance such as sand and gravel extraction. Despite not having data on substrate type the model described approximately 37% of the variation in observed riverbank erosion. It appears that inclusion of a management practice factor in riverbank erosion models is justified, where appropriate, and may improve model performance.

Validating the Use of 137Cs Measurements to Derive the Slope Component of the Sediment Budget of a Small Rangeland Catchment in Southern Italy

AbstractThe sediment budget is a key concept and tool for characterizing the mobilization, transfer and storage of fine sediment within a catchment. Caesium‐137 measurements can provide valuable information on gross and net erosion rates associated with sheet and rill erosion that can be used to establish the slope component of a catchment sediment budget. However, there is a need to validate the use of 137Cs measurements for this purpose, because their reliability has sometimes been questioned. The study reported focuses on a small (3·04 ha) steepland (mean slope 37%) catchment in Southern Italy. It exploits the availability of information on the medium‐term sediment output from the catchment provided by the construction of a reservoir at its outlet in 1978 and the existence of estimates of soil redistribution rates derived from 137Cs measurements made on 68 replicate soil cores collected from the slopes of a substantial proportion of the catchment in 2001, to validate the use of 137Cs measurements to construct the slope component of the catchment sediment budget. An additional 50 replicate soil cores were collected from the catchment slopes for 137Cs analysis, to complement the data already available. Nine cores collected from the area occupied by the reservoir were used to estimate the mean annual sediment input to the reservoir. In the absence of evidence that the poorly developed channel system in the catchment was either a significant sediment source or sink, it was possible to directly compare the estimate of net soil loss from the catchment slopes (7·33 Mg ha−1 y−1) with the estimate of sediment output from the catchment provided by the reservoir deposits (7·52 Mg ha−1 y−1). Taking account of the uncertainties involved, the close agreement of the two values is seen as providing a convincing validation of the use of 137Cs measurements to both estimate soil redistribution rates and as a basis for constructing the slope component of the sediment budget of a small catchment. Copyright © 2015 John Wiley & Sons, Ltd.

Evaluation of slopes delivery to catchment sediment budget for a low‐energy water system: a case study from the lingèvres catchment (normandy, western france)

Quantification and characterization of hydrological and sedimentary transfers are now major challenges to preserving uropean soils and water resources (aw on ater, ater ramework irective). In northwestern rance, stream water quality is currently reduced by suspended sediment. However, in these low‐energy water systems, sediment origin and dynamics into the catchment remain difficult to analyze from a local to a regional scale because of intermediary traps caused by landscape structure (hedgerow areas) and land use organization on the slopes. To provide a better understanding of sediment dynamics in these systems we proposed to analyze runoff dynamics on slopes and to assess slopes' contribution to the sediment budget of a catchment. This study is based on sediment connectivity analysis, used to accurately determine runoff dynamics between producing areas and the outlet of the catchment. This work is focused on the Lingèvres catchment (15 km2), a typical feature of the low‐energy water system located in the western part of rance, in between open‐field and bocage landscapes. The results demonstrate that in a hedgerow landscape, known for its protective role against runoff connection to the river, sediment delivery from slopes is more important and efficient than in an open‐field area where no obstacles are observed. This is explained by diverse drainage densities and spatial organization of roads, and ditch and plot entries which induced differentiated sediment delivery from agricultural plots and the river. However, they also underlined the weakness of sediment delivery from slopes compared with sediment fluxes exported from the catchment. Conversely, they highlight the fundamental role of erosion processes due to agricultural practices and landscape organization in headwater systems, which seem to be the main functional area at the sub‐catchment scale.

Fine‐suspended sediment and water budgets for a large, seasonally dry tropical catchment: Burdekin River catchment, Queensland, Australia

AbstractThe Burdekin River catchment (∼130,400 km2) is a seasonally dry tropical catchment located in north‐east Queensland, Australia. It is the single largest source of suspended sediment to the Great Barrier Reef (GBR). Fine sediments are a threat to ecosystems on the GBR where they contribute to elevated turbidity (reduced light), sedimentation stress, and potential impacts from the associated nutrients. Suspended sediment data collected over a 5 year period were used to construct a catchment‐wide sediment source and transport budget. The Bowen River tributary was identified as the major source of end‐of‐river suspended sediment export, yielding an average of 530 t km−2 yr−1 during the study period. Sediment trapping within a large reservoir (1.86 million ML) and the preferential transport of clays and fine silts downstream of the structure were also examined. The data reveal that the highest clay and fine silt loads—which are of most interest to environmental managers of the GBR—are not always sourced from areas that yield the largest total suspended sediment load (i.e., all size fractions). Our results demonstrate the importance of incorporating particle size into catchment sediment budget studies undertaken to inform management decisions to reduce downstream turbidity and sedimentation. Our data on sediment source, reservoir influence, and subcatchment and catchment yields will improve understandings of sediment dynamics in other tropical catchments, particularly those located in seasonally wet‐dry tropical savannah/semiarid climates. The influence of climatic variability (e.g., drought/wetter periods) on annual sediment loads within large seasonally dry tropical catchments is also demonstrated by our data.

Patterns of topographic change in sub-humid badlands determined by high resolution multi-temporal topographic surveys

Badlands are highly erodible landscapes with sparse vegetation and rapid runoff responses. Badland surfaces experience high erosion rates that may have a direct and marked influence on river channel networks and catchment scale sediment budgets. Erosion rates from badlands have been widely estimated by discrete observations of topographic change measured by erosion pins or profile meters. Recent developments in survey instrumentation provide the opportunity to build high-resolution topographic models over multiple spatial scales at sub-centimeter accuracy. In this paper we demonstrate how reliable estimates of topographic variables and temporal change can be derived for badlands by repeat Terrestrial Laser Scanning (TLS) surveys undertaken at multiple temporal (event to annual) scales.A total of seven TLS-surveys were obtained in an experimental sub-humid badland located in the Central Pyrenees. Data analyses were conducted on two temporal scales: (a) five rainfall events and (b) three long term (spaced over an annual cycle) scales. Our results show a clear erosional pattern for most of the badland at the annual scale (i.e. annual net change at around −6cmyr−1). Aspect, surface roughness and slope were significant predictors of topographic change, although the sign and magnitude of the change differed at the event scale. Net topographic change at the event scale varied from −1.8 to 1.4cm. Although these patterns could be also affected by swell-shrink processes, surface roughness was more important at controlling badland geomorphological processes at the event-scale than the annual scale. Our observations suggest that longer-term studies may underestimate the importance of surface roughness as a control on badland geomorphology. At the annual scale the effect of aspect in the sign of the topographic change was removed and slope becomes more important. Erosion proved to be dominant on slopes higher than 100%. Overall, the results indicate that coupling of appropriately-scaled spatial and temporal data is critical to understand topographic changes and their drivers on badlands. Although the approach outlined in this paper was only applied to a relatively small area, with careful survey design and application of new survey technology, it could readily be upscaled to cover entire badland systems. Such surveys would permit detailed analysis of controls of sedimentological connectivity across badlands and their influence on channel networks and catchment sediment budgets.

Can control of soil erosion mitigate water pollution by sediments?

The detrimental impact of sediment and associated pollutants on water quality is widely acknowledged, with many watercourses in the UK failing to meet the standard of ‘good ecological status’. Catchment sediment budgets show that hill slope erosion processes can be significant sources of waterborne sediment, with rates of erosion likely to increase given predicted future weather patterns. However, linking on-site erosion rates with off-site impacts is complicated because of the limited data on soil erosion rates in the UK and the dynamic nature of the source–pathway–receptor continuum over space and time. Even so, soil erosion control measures are designed to reduce sediment production (source) and mobilisation/transport (pathway) on hill slopes, with consequent mitigation of pollution incidents in watercourses (receptors).The purpose of this paper is to review the scientific evidence of the effectiveness of erosion control measures used in the UK to reduce sediment loads of hill slope origin in watercourses. Although over 73 soil erosion mitigation measures have been identified from the literature, empirical data on erosion control effectiveness are limited. Baseline comparisons for the 18 measures where data do exist reveal erosion control effectiveness is highly variable over time and between study locations. Given the limitations of the evidence base in terms of geographical coverage and duration of monitoring, performance of the different measures cannot be extrapolated to other areas.This uncertainty in effectiveness has implications for implementing erosion/sediment risk reduction policies, where quantified targets are stipulated, as is the case in the EU Freshwater Fish and draft Soil Framework Directives. Also, demonstrating technical effectiveness of erosion control measures alone will not encourage uptake by land managers: quantifying the costs and benefits of adopting erosion mitigation is equally important, but these are uncertain and difficult to express in monetary terms.

Cost‐Effectiveness of Soil and Water Conservation Measures on the Catchment Sediment Budget–The Laaba Watershed Case Study, Burkina Faso

AbstractIn the Sahelian region, the high precipitation intensity and the daily rainfall extreme values are currently the main cause of soil erosion and land degradation. In addition, solid transport often leads to reservoir siltation and reduction of the amount of water available for agriculture. To cope with these issues, Soil and Water Conservation (SWC) measures have been regularly employed in the Sahelian area. However, a proper cost‐effectiveness analysis of the impact of SWC interventions on the catchment sediment budget normally requires quantitative surveys on erosion and sedimentation processes. Where data for calibration and validation of models are scarce, an overall methodology to evaluate the economical sustainability of a proposed intervention can be of paramount importance. The study herein proposed aims to assess the monetary sustainability of SWC measures in limiting the reservoir siltation of the Laaba dam (Yatenga District, Northern Burkina Faso). In particular, the catchment sediment budget was estimated by means of morphological and pedologic parameters and dam sedimentation rates; a cost‐effectiveness analysis was then performed to assess the economic sustainability of a possible SWC intervention. The proposed methodology showed interesting potentials for land and water management in Burkina Faso, particularly when data and financial resources are limited and where the application of detailed process‐based models is not possible. Copyright © 2013 John Wiley & Sons, Ltd.

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Quantifying different riverbank erosion processes during an extreme flood event

ABSTRACTRiverbank erosion is a major contributor to catchment sediment budgets. At large spatial scales data is often restricted to planform channel change, with little information on process distributions and their sediment contribution. This study demonstrates how multi‐temporal LiDAR and high resolution aerial imagery can be used to determine processes and volumes of riverbank erosion at a catchment scale. Remotely sensed data captured before and after an extreme flood event, enabled a digital elevation model of difference (DoD) to be constructed for the channel and floodplain. This meant that: the spatial area that could be assessed was extensive; three‐dimensional forms of bank failures could be mapped at a resolution that enabled process inference; and the volume and rates of different bank erosion processes over time could be assessed. A classification of riverbank mass failures, integrating form and process, identified a total of 437 mass failure polygons throughout the study area. These were interpreted as wet flow mass failures based on the presence of a well defined scarp wall and the absence of failed blocks on the failure floor. The failures appeared to be the result of: bank exfiltration, antecedent moisture conditions preceding the event, and the historic development of the channel. Using one‐dimensional hydraulic modelling to delineate geomorphic features within the main boundary of the macrochannel, an estimated 1 466 322 m2of erosion was interpreted as fluvial entrainment, occurring across catchment areas from 30 to 1668 km2. Only 8% of the whole riverbank planform area was occupied by mass failures, whilst fluvial entrainment covered 33%. A third of the volume of material eroded came from mass failures, even though they occupied 19% of the eroded bank area. The availability of repeat LiDAR surveys, combined with high‐resolution aerial photography, was very effective in erosion process determination and quantification at a large spatial scale. Copyright © 2013 John Wiley & Sons, Ltd.

Beryllium‐7: The Cinderella of fallout radionuclide sediment tracers?

AbstractCurrent concern for the sustainable management of soil resources and the many adverse effects of increased sediment loads on streams and river systems has generated a need for more information on rates and patterns of soil loss and an improved understanding of catchment sediment budgets. Traditional measurement techniques are frequently unable to meet the requirements and fallout radionuclides have been shown to offer considerable potential for tracing the mobilization, transfer, redistribution and storage of soil and sediment within catchments and river systems. Caesium‐137 and lead‐210 have been the most widely used fallout radionuclide sediment tracers, but their half‐lives mean that the information that they provide is in most instances limited to the medium term (e.g. 25–120 years). The need for information on shorter timescales has directed attention to the potential for using beryllium‐7, which has a considerable shorter half‐life, for tracing soil and sediment redistribution over short timescales (e.g. days–weeks). To date, however, there have been relatively few attempts to exploit this potential. This situation is likely to be in part a reflection of the emphasis of early work on 137Cs and 210Pb and the more recent nature of work using 7Be. However, it may also reflect the limitations and constraints imposed by the nature of 7Be fallout and the short half‐life of this radionuclide. This contribution explores recent work in using 7Be as a sediment tracer and the constraints imposed by its origin and behaviour. Attention is directed to fallout inputs, the application of 7Be to tracing soil and sediment redistribution on slopes and on river floodplains, its use as a source fingerprint and other related applications. The likely future potential of 7Be as a sediment tracer is assessed. Copyright © 2012 John Wiley & Sons, Ltd.

Importance of bank erosion for sediment input, storage and export at the catchment scale

The importance of bank erosion was quantified during three periods (October 2006–April 2007, May 2007–April 2008 and May 2008–April 2009) in the 486 km2 catchment area of River Odense, Denmark. A catchment sediment budget was established including other sediment sources such as tile drains and surface runoff, in-channel and overbank sinks and storage and the resulting bed load and suspended sediment load exported from the catchment. Bank erosion and sedimentation were measured using ca. 3,000 erosion pins established in 180 pin plots, each consisting of three vertical lines of pins. Thirty-six representative reaches, each with a length of 100 m, were selected by a stratified random procedure in GIS. Bed load and suspended sediment export from the catchment were measured using a bed load sampler and from continuous measurements of turbidity at the outlet gauging station. The gross sediment input from bank erosion during the three study periods amounted to 21,100–25,200 t in the River Odense catchment, which is considerably higher than the estimated input of sediment from tile drains and surface runoff, which amounted to 220–500 t and 0–100 t, respectively. The measured bed load (20–490 t) was five to 60 times lower than the suspended sediment export from the catchment (1,240–2,620 t) during the three study periods, with the largest difference occurring in the driest year. Sediment sinks and storage were of high importance for the catchment sediment budget as the measured in-channel storage of sediment on stream banks was as high as 16,200–20,100 t, and the overbank sediment sink was estimated at 360–3,100 t. Bank erosion was the dominant sediment source (90–94 %) in the River Odense catchment during the three study years. In-channel and overbank sediment sinks and storage dominated the sediment budget as 79–94 % of the sediment input from all sources was not exported from the catchment during the three study years. Such a large attenuation of sediment in river channels and on floodplains is extremely important for fluvial habitats and ecology. Moreover, it has strong implications for attempts to document changes in sediment export following implementation of mitigation measures.