We provide evidence for a large-scale geomorphic event in Cambodia's great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8 m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake's flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake's extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture- and base-level fall- induced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap lake to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin.

Floodplain soils provide an important link in the land-ocean aquatic continuum. Understanding microbial activity in these soils, which can be many metres deep, is a key component in our understanding of the role of floodplains in the carbon (C) cycle. We sampled the mineral soil profile to 3 m depth from two floodplain sites under long-term pasture adjacent to the river Culm in SW England, UK. Soil chemistry (C, nitrogen (N), phosphorus (P), soil microbial biomass (SMB), moisture content) and soil solution (pH, dissolved organic C (DOC) and N, nitrate, ammonium, water extractable P) were analysed over the 3 m depth in 6 increments: 0.0–0.2, 0.2–0.7, 1.0–1.5, 1.5–2.0, 2.0–2.5, and 2.5–3.0 m. 14C-glucose was added to the soil and the evolution of 14CO2 measured during a 29 d incubation. From soil properties and 14C-glucose mineralisation, three depth groups emerged, with distinct turnover times extrapolated from initial k1 mineralisation rate constants of 2 h (topsoil 0.0–0.2 m), 4 h (subsoil 0.2–0.7 m), and 11 h (deep subsoil 1.0–3.0 m). However, when normalised by SMB, k1 rate constants had no significant differences across all depths. Deep subsoil had a 2 h lag to reach maximal 14CO2 production whereas the topsoil and subsoil (0.2–0.7 m) achieved maximum mineralisation rates immediately. SMB decreased with depth, but only to half of the surface population, with the proportion of SMB-C to total C increasing from 1% in topsoil to 15% in deep subsoil (>1.0 m). The relatively large SMB concentration and rapid mineralisation of 14C-glucose suggests that DOC turnover in deep soil horizons in floodplains is limited by access to biologically available C and not the size of the microbial population.

River channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristics of river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea-level change). This paper investigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80% of large confluences are mobile over this 40-year window; conversely this figure is < 40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield.

AbstractChannel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.

Channel bank erosion processes are controlled by numerous factors and as such are both temporally and spatially variable. The significance of channel bank erosion to the sediment budget is difficult to quantify without extensive fieldwork/data analysis. In this study, the importance of key physical factors controlling channel bank erosion, including channel slope, upstream catchment area, channel confinement, and sinuosity, was explored using regression analysis. The resulting analysis can be used in practical studies to provide a first approximation of bank erosion rates (in catchments similar to those investigated). A data set of channel bank erosion rates covering eight contrasting river catchments across England and Wales, over a time period of up to 150 years, was created using a modified GIS methodology. The best predictors were found to be upstream area, channel confinement, and sinuosity with respect to dimensionless width-averaged retreat rates (m m−1 yr−1). Notwithstanding these relationships, the results highlight the variability of the magnitude of sediment production by channel bank erosion both within and between catchments.

The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually, with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping, and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being 'drowned' by rising relative sea levels. Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world's great deltas. We demonstrate that spatial variations in the Mekong's suspended sediment load are correlated (r = 0.765, P 

© 2015 the Authors. We illustrate the potential for using physics-based modeling to link alluvial stratigraphy to large river morphology and dynamics. Model simulations, validated using ground penetrating radar data from the Río Paraná, Argentina, demonstrate a strong relationship between bar-scale set thickness and channel depth, which applies across a wide range of river patterns and bar types. We show that hydrologic regime, indexed by discharge variability and flood duration, exerts a first-order influence on morphodynamics and hence bar set thickness, and that planform morphology alone may be a misleading variable for interpreting deposits. Indeed, our results illustrate that rivers evolving under contrasting hydrologic regimes may have very similar morphology, yet be characterized by marked differences in stratigraphy. This realization represents an important limitation on the application of established theory that links river topography to alluvial deposits, and highlights the need to obtain field evidence of discharge variability when developing paleoenvironmental reconstructions. Model simulations demonstrate the potential for deriving such evidence using metrics of paleocurrent variance.

© 2015. American Geophysical Union. All Rights Reserved. The three-dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion. Although submerged slump blocks and associated large-scale bank roughness elements have both previously been proposed to divert flow away from the bank, direct observations of the interaction between eroded bank material and the 3-D flow field are lacking. Here we use observations from multibeam echo sounding, terrestrial laser scanning, and acoustic Doppler current profiling to quantify, for the first time, the influence of submerged slump blocks on the near-bank flow field. In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may also deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion. We use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

This paper examines processes of chute channel formation in four tropical sand-bed meandering rivers; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics-based modelling work. GIS analyses indicate that bend widening may be driven by a variety of mechanisms, including scour and cutbank bench formation at sharply-curving bends, point bar erosion due to cutbank impingement against cohesive terrace material, rapid cutbank erosion at rapidly extending bends, and spontaneous mid-channel bar formation. Chute channel initiation is observed to be predominantly associated with two of these widening mechanisms; i) an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension, and ii) bank erosion forced by spontaneous mid-channel bar development. The work extends previous empirical analyses, which highlighted the role of bend extension (elongation) in driving chute initiation, with the observation that the frequency of chute initiation increases once bend extension rates and/or widening ratios exceed a reach-scale threshold. A temporal pattern of increased chute initiation frequency on the Ok Tedi, in response to channel steepening and mid-channel bar development following the addition of mine tailings, mirrors the inter- and intra-reach spatial patterns of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power and sediment load are associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy, reminiscent of slope-ratio thresholds in river avulsion. Furthermore, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation may also be rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy (and sediment load), predominantly affecting sites of rapid channel elongation, and thereby mediating the river response. © 2013.

To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre-scale bars vary within a multi-kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre-scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná - Río Paraguay confluence, where a significant volume of fine-grained suspended sediment is introduced into the network. Bar-scale cross-stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar-scale sets are found on top of finer-grained ripple-laminated bar-trough deposits. Bar-scale sets make up as much as 58% of the volume of the deposits in small, incipient mid-channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of the sedimentary structures found in the Río Paraná is similar in scale to those found in much smaller rivers. In other words, large river deposits are not always characterized by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small-scale structures, such as ripple sets, to be grouped into thicker cosets, which indicate river scale even when no obvious large-scale sets are present. The results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the Río Paraguay. Relative to other controls on downstream fining, the tributary input of fine-grained suspended material from the Río Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ca 5 m of mid-channel bars shows: (i) an increase in the abundance and thickness (up to metre-scale) of laterally extensive (hundreds of metres) fine-grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar-trough deposits and a corresponding decrease in bar-scale cross-strata (

The processes of hillslope runoff and erosion are typically represented at coarse spatial resolution in catchment-scale models due to computational limitations. Such representation typically fails to incorporate the important effects of topographic heterogeneity on runoff generation, overland flow, and soil erosion. These limitations currently undermine the application of distributed catchment models to understand the importance of thresholds and connectivity on hillslope and catchment-scale runoff and erosion, particularly in semi-arid environments. This paper presents a method for incorporating high-resolution topographic data to improve sub-grid scale parameterization of hillslope overland flow and erosion models. Results derived from simulations conducted using a kinematic wave overland flow model at 0.5m spatial resolution are used to parameterize the depth-discharge relationship in the overland flow model when applied at 16m resolution. The high-resolution simulations are also used to derive a more realistic parameterization of excess flow shear stress for use in the 16m resolution erosion model. Incorporating the sub-grid scale parameterization in the coarse-resolution model (16m) leads to improved predictions of overland flow and erosion when evaluated using results derived from high-resolution (0.5m) model simulations. The improvement in performance is observed for a range of event magnitudes and is most notable for erosion estimates due to the non-linear dependency between the rates of erosion and overland flow. © 2013 John Wiley & Sons, Ltd.

Physically-based modelling of rivers has advanced in recent decades by developing separate approaches for representing single-thread and multi-thread channels. This paper reports on a new morphodynamic model developed with the goal of simulating river and floodplain co-evolution within a general framework suitable for investigating diverse fluvial styles. Simulations illustrate the potential for representing meandering, braided and anabranching channels using this model. Moreover, by adopting relatively simple parameterizations of many processes, this work provides insight into what may constitute sufficient (minimal) model complexity, and highlights uncertainties that should be addressed by future research. © 2013 John Wiley & Sons, Ltd.

Recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers. However, measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach-length scales that can be monitored. Similar constraints exist for computational fluid dynamics (CFD) studies of large rivers, requiring maximization of mesh- or grid-cell dimensions and implying a reduction in the representation of bedform-roughness elements that are of the order of a model grid cell or less, even if they are represented in available topographic data. These "subgrid" elements must be parameterized, and this paper applies and considers the impact of roughness-length treatments that include the effect of bed roughness due to "unmeasured" topography. CFD predictions were found to be sensitive to the roughness-length specification. Model optimization was based on acoustic Doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths. This proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness-length treatments. However, the general spatial flow patterns are effectively predicted by the model. Changes in roughness length were shown to have a major impact upon flow routing at the channel scale. The results also indicate an absence of secondary flow circulation cells in the reached studied, and suggest simpler two-dimensional models may have great utility in the investigation of flow within large rivers. © 2012. American Geophysical Union. All Rights Reserved.

Depth-averaged velocities and unit discharges within a 30. km reach of one of the world's largest rivers, the Rio Paraná, Argentina, were simulated using three hydrodynamic models with different process representations: a reduced complexity (RC) model that neglects most of the physics governing fluid flow, a two-dimensional model based on the shallow water equations, and a three-dimensional model based on the Reynolds-averaged Navier-Stokes equations. Flow characteristics simulated using all three models were compared with data obtained by acoustic Doppler current profiler surveys at four cross sections within the study reach. This analysis demonstrates that, surprisingly, the performance of the RC model is generally equal to, and in some instances better than, that of the physics based models in terms of the statistical agreement between simulated and measured flow properties. In addition, in contrast to previous applications of RC models, the present study demonstrates that the RC model can successfully predict measured flow velocities. The strong performance of the RC model reflects, in part, the simplicity of the depth-averaged mean flow patterns within the study reach and the dominant role of channel-scale topographic features in controlling the flow dynamics. Moreover, the very low water surface slopes that typify large sand-bed rivers enable flow depths to be estimated reliably in the RC model using a simple fixed-lid planar water surface approximation. This approach overcomes a major problem encountered in the application of RC models in environments characterised by shallow flows and steep bed gradients. The RC model is four orders of magnitude faster than the physics based models when performing steady-state hydrodynamic calculations. However, the iterative nature of the RC model calculations implies a reduction in computational efficiency relative to some other RC models. A further implication of this is that, if used to simulate channel morphodynamics, the present RC model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations. These observations illustrate the trade off between model realism and efficiency that is a key consideration in RC modelling. Moreover, this outcome highlights a need to rethink the use of RC morphodynamic models in fluvial geomorphology and to move away from existing grid-based approaches, such as the popular cellular automata (CA) models, that remain essentially reductionist in nature. In the case of the world's largest sand-bed rivers, this might be achieved by implementing the RC model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales. © 2012 Elsevier B.V.

Meander bends of many large, sand-bed meandering rivers are partitioned by chute channels that convey permanent flow, and co-exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely-spaced scrolls with intervening sloughs that are positively aligned with primary over-bar flow. The rivers plot in order of increasing chute activity on an empirical meandering-braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single-thread meandering and braided.

Results are presented from a new cellular model of braided river dynamics that simulates flow, sediment transport, morphological change and the effects of braidplain vegetation. This model is used to investigate the effect of changes in upstream sediment supply on braided river systems over simulation periods of 200 years. Modelled changes in channel morphology, associated with both aggradation and degradation, were seen to be consistent with those reported in the literature. In addition, simulation results allowed the identification of diagnostic characteristics of aggrading and degrading reaches, in the form of relationships between the age, extent and relative elevation of fluvial surfaces. Interpretation of spatial patterns of valley floor surface characteristics in the Avoca River, New Zealand, on the basis of these relationships, allowed the identification of channel reaches that appear to be experiencing either aggradation or degradation. These inferences are shown to be consistent with independent evidence of spatial patterns of sediment supply to the main valley floor, derived from aerial photographs and an existing sediment source inventory. These results illustrate the potential for using cellular models to develop an improved understanding of natural river behaviour. © 2007 Elsevier B.V. All rights reserved.

This paper outlines the principles of cellular modelling in fluvial geomorphology and addresses issues of model formulation and validation in the context of numerical modelling more generally. In doing so it seeks to highlight the prospects for using cellular approaches to develop an improved understanding of both rivers and models. Copyright © 2005 John Wiley & Sons, Ltd.

This paper reports the application of a two-dimensional hydraulic model to a braided reach of the Avoca River, New Zealand. Field measurements of water surface elevation, depth and velocity obtained at low flow were used to validate the model and to optimize the parameterization of bed friction. The main systematic trends in the measured flow variables are reproduced by the model. However, field data are characterized by greater spatial variability than model output reflecting differences in the scale of processes measured in the field and represented by the model. Additional model runs were conducted to simulate flow patterns within the study reach at five higher discharges. The purpose of these simulations was to evaluate the potential for using two-dimensional hydraulic models to quantify the reach-scale hydraulic characteristics of braided rivers and their dependence on discharge. Changes in flow depth and velocity with increasing discharge exhibit trends that are consistent with the results of previous field investigations, although the tendency for the wetted area of the braidplain within particular depth and velocity categories to remain fixed as discharge rises, as has been noted for several braided rivers in New Zealand, was not observed. Modelled shear stress frequency distributions fit gamma functions that incorporate a distribution shape parameter, the value of which follows clear systematic trends with rising discharge. These results illustrate both the problems of, and potential for, using two-dimensional hydraulic models in braided river applications. This leads to something of a paradox in that while such models provide a means of generating hydraulic information that would be difficult to obtain in the field at an equivalent spatial resolution, they are, due to the problems inherent to data collection, difficult to validate conclusively. Despite this limitation, the application of spatially distributed models to investigate relationships between discharge and reach-scale form and process variables appears to have considerable potential. © 2003 John Wiley and Sons, Ltd.

This article presents results from an investigation of the hydraulic characteristics of overbank flows on topographically-complex natural river floodplains. A two-dimensional hydraulic model that solves the depth-averaged shallow water form of the Navier-Stokes equations is used to simulate an overbank flow event within a multiple channel reach of the River Culm, Devon, UK. Parameterization of channel and floodplain roughness by the model is evaluated using monitored records of main channel water level and point measurements of floodplain flow depth and unit discharge. Modelled inundation extents and sequences are assessed using maps of actual inundation patterns obtained using a Global Positioning System, observational evidence and ground photographs. Simulation results suggest a two-phase model of flooding at the site, which seems likely to be representative of natural floodplains in general. Comparison of these results with previous research demonstrates the complexity of overbank flows on natural river floodplains and highlights the limitations of laboratory flumes as an analogue for these environments. Despite this complexity, frequency distributions of simulated depth, velocity and unit discharge data closely follow a simple gamma distribution model, and are described by a shape parameter (α) that exhibits clear systematic trends with changing discharge and floodplain roughness. Such statistical approaches have the potential to provide the basis for computationally efficient flood routing and overbank sedimentation models. Copyright © 2002 John Wiley and Sons, Ltd.

Growing awareness of the pressure on land resources emphasises the need to understand the full range of processes operating in human-impacted agroecosystems. In such systems one of the greatest threats to long-term sustainability is the erosion and depauperation of soil, which, until recently, was attributed almost entirely to water erosion. This study builds on recent awareness of the significance of tillage erosion and presents the results of an experimental investigation of tillage erosion due to mouldboard ploughing. Aluminium cubes were used to trace soil translocation as a result of a single pass of the plough perpendicular to the contour in downslope and upslope directions. In common with others studies, translocation was found to be directly proportional to slope tangent for downslope tillage and unrelated to slope for upslope tillage. The influence of non-topographic variables on the relationship between translocation distance and slope was partially filtered out by using the ratio of translocation distances in the tillage direction and perpendicular to tillage. Shallow plough depths of 0.17 m produced tillage detachment of only 230 kg/m2; however, a high tillage translocation coefficient of 1.16 m/pass resulted in a soil flux coefficient of 265 kg/m.pass. The high tillage translocation coefficient is probably partly due to the loose nature of the regularly cultivated loessic soil, however, on the basis of comparison with other published studies, it is suggested that the high tillage speed of 7 km/h is the principal control on the magnitude of the coefficient. Analysis of the available data suggests that a 30% reduction in tillage erosion intensity could be obtained by reduction of the tillage speed to 4 km/h; nevertheless, more experimental work is needed to test this suggestion. On the transect studied, a pair of opposing passes of the mouldboard plough would produce erosion rates as high as 5.1 kg/m2.year (51 t/ha.year) from shoulder slope elements and as high as 1.9 kg/m2.year (19 t/ha.year) over half of the slope length. This pattern matched closely the distribution of 137Cs-derived erosion rates documented previously for a nearby field, suggesting that for this environment, as for many mechanised agricultural systems, tillage erosion is the dominant soil redistribution process and the greatest threat to long-term sustained on-site productivity. Reduction of tillage erosion should, therefore, be seen as a priority in the development of sustainable land management strategies.

Results from a series of numerical simulations of two-dimensional open-channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open-channel flow over two contrasting gravel beds. Boundary roughness effects are presented using both the conventional wall function approach and a random elevation model that simulates the effects of supra-grid-scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at ylh = 0.1 -0.3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra-grid-scale roughness also allows a reduction in the height of the near-bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near-bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel-bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh-independent solutions. Copyright © 2001 John Wiley and Sons. Ltd.

The acoustic Doppler velocimeter (ADV) measures three-dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single-point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright (C) 2000 John Wiley and Sons, Ltd.

This paper outlines an approach for estimating the annual bedload yield of a braided channel. This procedure is based on the extension of theory of flow and sediment transport in braided rivers recently presented by Paola (1996). The revised approach accounts explicitly for the relationship between increasing discharge and varying channel hydraulics, and is suitable for use in obtaining bedload transport rate estimates over a range of discharges. Integration of such estimates using flow-duration data allows annual bedload yield to be determined. Model parameterisation is achieved using topographic survey data for the Waimakariri River. New Zealand. Comparison of modelled bedload yield with values estimated from repeated topographic surveys indicates that the model is able to accurately predict both the medium-term (c. 30 years) mean annual bedload yield of the Waimakariri at Crossbank (the section 17.8 km upstream of the river mouth), and also short-term fluctuations in bedload yield associated with varying annual flow statistics. Streamwise patterns of volumetric erosion and deposition determined for a 45-km length of the Waimakariri using the model are also in broad agreement with trends identified in topographic survey data for the period 1961-1997. However, significant deviations between modelled and surveyed volumes of cut and fill are evident at some locations. Comparison of model performance with conventional applications of bedload transport equations, which tend to underestimate transport rates for braided channels, suggests that the approach presented here may represent a significant improvement. This is the case because the model quantifies the relationship between braid intensity and spatial variability in flow hydraulics at a cross-section. Output from the model suggests that braided rivers may transport a significant proportion of their annual bedload at lower discharges than those indicated by earlier theoretical approaches. Results also provide quantitative support for the argument that increased intensity of braiding may promote higher rates of bedload transport in gravel bed rivers. © 2000 Elsevier Science B.V. All rights reserved.

Results are presented from a numerical simulation of three-dimensional flow hydraulics around a mid-channel bar carried out using the FLUENT/UNS computational fluid dynamics (CFD) software package. FLUENT/ UNS solves the three-dimensional Reynolds-averaged form of the Navier-Stokes equations. Turbulence closure is achieved using a RNG k-ε model. Simulated flow velocities are compared with measured two-dimensional velocities (downstream and cross-stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the flow structures observed in the field. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r = 0.88) and a mean difference of 0.09 m s-1. Deviations between simulated and measured velocities may be identified that are both random and systematic. The former may reflect a number of factors including subgrid-scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh configuration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities. These results illustrate the potential for using CFD software to simulate flow hydraulics in natural channels with complex configurations. They also highlight the need for detailed spatially distributed datasets of three-dimensional flow variables to establish the accuracy and applicability of CFD software.

A numerical model is presented for the prediction of floodplain inundation extents, flow depths and velocities, and patterns of suspended sediment dispersion and deposition. The model employs a simplified and novel treatment of fluid flow and a conventional convection-diffusion mass balance relationship for suspended sediment transport and deposition. These relationships are solved over a finite difference grid with a relatively fine nodal spacing, which enables a detailed representation of complex floodplain topography. The model is applied to a 600 m reach of the River Culm, Devon, UK. Continuous records of river stage and susepnded sediment concentration monitored at the upstream boundary of the study reach provide the data needed to operate the model. These data are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. Model predictions of overbank flow depths and velocities and patterns of suspended sediment concentrations and deposition amounts are presented. Predicted patterns of overbank deposition are described and compared with field measurements obtained with the aid of astroturf sedimentation traps are also examined and compared with model predictions. The results of both the field measurement programme and modelling work are shown to have significant implications for the representation of floodplain topography and suspended sediment particle size and settling properties in numerical simulations of overbank processes.

We present data from a proglacial river in Iceland that exhibits very different sedimentological characteristics when compared to its alpine counterparts. The braidplain is characterised by coarse outburst gravels that inhibit sediment transport and channel change. Bedload transport is restricted to the movement of fine-grained gravels that pass through the channel system without promoting significant changes in channel geometry. Bar forms are erosional features, inherited from the last major peak flow, rather than depositional in nature. On the basis of our observations we conclude that braidplain morphology is controlled by low frequency, high magnitude flow events, possibly associated with glacial outburst floods. This is in marked contrast to process-form relationships in more dynamic alpine proglacial channels that are characterised by high rates of sediment transport and channel change.

A numerical model of overbank deposition of fine-grained suspended sediment is presented and employed to predict spatial patterns of mean annual floodplain sedimentation within a small reach of the River Culm, Devon, UK. The model incorporates a deposition function which is calibrated with the aid of a dataset of measured deposition amounts obtained from nine flood events over a 13 month period. Model predictions of mean annual deposition rates are then compared against an independent dataset comprising estimates of longer-term (ca. 35 years) average sedimentation rates derived from caesium-137 measurements on 200 floodplain sediment cores collected from two floodplain locations. The numerical model is found to adequately reproduce the spatial patterns of overbank sedimentation documented by the 137Cs data. However, where maximum rates of deposition occur, the model is found to seriously underpredict the 137Cs derived estimates. The simulation model employed here might be improved upon by incorporating more sophisticated 2D or 3D numerical treatments of flow hydraulics and suspended sediment transport and settling processes. Such numerical schemes may offer the potential to provide not only more accurate predictions of sedimentation rates, but also more realistic representations of overbank processes. While models of this type are now becoming more widely available, in order for the benefits associated with increased model complexity to be realised, in future, greater emphasis must be placed upon testing models against distributed datasets of hydraulic and sedimentological variables obtained through intensive field monitoring programmes. The use of Cs measurements appears to offer considerable potential in this respect.

Gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions. These can be quantified in several ways, giving wide differences in derived statistics and calculations. Problems associated with measuring, summarizing, and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition. No single index of bimodality serves all purposes; alternatives are suggested. Measured near-bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume; this has implications for the abrupt downstream transition often found from gravel to sand bed.

This paper outlines a numerical model for the prediction of floodplain inundation sequences, overbank deposition rates and deposit grain size distributions. The model has two main components: first, a simplified hydraulic scheme which predicts floodwater flow depths and velocities, and second, a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by connective and diffusive processes and sediment deposition as a function of particle settling rates. These relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains. The model is applied to a 600 m reach of the River Culm, Devon, U.K. using data derived from a range of field and laboratory techniques. Continuous records of river stage and suspended sediment concentration provide the models upstream boundary input requirements. These are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. The models sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period. The model is shown to predict complicated flood water inundation sequences and patterns of suspended sediment dispersion and deposition, which are largely a product of the complex topography of the floodplain. These results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry. © 1997 by John Wiley & Sons, Ltd.

The effects of suspended sediment aggregation on rates of overbank deposition and on the grain size composition of deposited sediment were investigated using a combined field measurement and numerical modelling approach. Measurements of the in situ grain size characteristics of the suspended sediment load (termed the effective size distribution) of the River Culm, Devon, were obtained using a custom-built water elutriation system. Samples of suspended sediment and of sediment deposited on the floodplain were analysed in the laboratory to determine the size distributions of their constituent discrete mineral particles (termed the ultimate size distribution). Interpretation of these field data was aided by the development of a two-dimensional finite difference model of flood hydraulics and suspended sediment dispersion and deposition. Field measurements and model predictions show that particle aggregation results in higher relative contributions of fine size fractions to overbank deposits. Aggregation may also provide a mechanism for explaining the poor agreement between theoretical and observed trends in relationships between mean deposit grain size and distance from the main channel. Suspended sediment aggregation is shown to cause significant increases in rates of floodplain deposition. Increases in deposition rates resulting from aggregation may, however, be less than expected because of the effects of aggregate shape and density. The latter may offset increases in particle size so that the settling velocities of the largest aggregates do not increase appreciably with particle diameter.