Reconstructing floodplain sedimentation rates from heavy metal profiles by inverse modelling

The embanked floodplains of the lower River Rhine in the Netherlands contain large amounts of heavy metals, which is a result of many years deposition of contaminated overbank sediments. Depending on local sedimentation rates and changing pollution trends in the past, the metal pollution varies greatly between different floodplain sections as well as vertically within the floodplain soil profiles. Maximum metal concentrations in floodplain soils vary from 30 to 130 mg/kg for Cu, from 70 to 490 mg/kg for Pb and from 170 to 1450 mg/kg for Zn. In the present study these metals were used as a tracer to reconstruct sedimentation rates at 28 sites on the lower River Rhine floodplains. The temporal trend in pollution of the lower River Rhine over the past 150 years was reconstructed on the basis of metal concentrations in sediments from small ponds within the floodplain area. Using a one‐dimensional sedimentation model, average sedimentation rates over the past century were determined using an inverse modelling calibration procedure. The advantage of this method is that it uses information over an entire profile, it requires only a limited number of samples, it accounts for post‐depositional redistribution of the metals, and it provides quantitative estimates of the precision of the sedimentation rates obtained. Estimated sedimentation rates vary between about 0·2 mm/year and 15 mm/year. The lowest metal concentrations are found in the distal parts of floodplain sections with low flooding frequencies and where average sedimentation rates have been less than about 5 mm/year. The largest metal accumulations occur in low‐lying floodplain sections where average sedimentation rates have been more than 10 mm/year. Copyright © 2002 John Wiley & Sons, Ltd.     

Modelling floodplain sedimentation using particle tracking

Both climate change and river rehabilitation projects induce changes in floodplain sedimentation. Notably along the lower River Rhine, the sediment deposition patterns and rates are subject to change. To assess the magnitude of these changes, we developed the MoCSED model, a floodplain sedimentation model within a geographical information system for the lower Rhine River. We based MoCSED on the ‘method of characteristics’ (MoC), a particle tracking method that minimizes numerical dispersion. We implemented the MoCSED model in the PCRaster dynamic modelling language. The model input comprises initial suspended sediment concentrations, water levels, flow velocities, and longitudinal and transverse dispersivities. We used a combination of the Krone and Chen concepts to calculate the subsequent sedimentation (SED routine). We compared the model results with sediment trap data for the Bemmel floodplain along the Dutch Waal River during the 2003 inundation. This comparison showed that MoCSED was able to simulate the pattern of sediment deposition. In addition, the model proved to be an improvement in comparison with a conventional raster‐based floodplain sedimentation model for the lower River Rhine. In future, MoCSED may serve well to study the impact of a changing discharge regime due to climate change and floodplain rehabilitation plans on deposition of sediments. Copyright © 2006 John Wiley & Sons, Ltd.     

Impact of natural reforestation on floodplain sedimentation in the Dragonja basin,SW Slovenia

Changes in floodplain sediment dynamics have profound effects on riverine habitats and riparian biodiversity. Depopulation due to socio‐economic changes in the Dragonja catchment (91 km²) in southwestern Slovenia resulted in the abandonment of agricultural fields, followed by natural reforestation since 1945. This profoundly changed the water and sediment supply to the streams, as well as floodplain sediment deposition. This paper presents a reconstruction of the development of the Dragonja floodplain due to these land use changes during the last 60 years. The reconstruction is based on dating of floodplain sediments using ¹³⁷Cs profiles, measurement of actual sedimentation rates using artificial grass sedimentation mats, and linking this information to the present‐day hydrological behaviour of the river. The sedimentation mats showed that floodplain sedimentation was restricted to peak flows of considerable magnitude. Due to the reforestation, the return period of such high flows increased from 0·31 year in the period 1960–1985 to 0·81 year between 1986 and 2003, with commensurate changes in sedimentation rates. At the 1·5 m river terrace (formed about 60 years ago), ¹³⁷Cs‐based sedimentation rates (1960–1986) were roughly twice the rates inferred from the artificial grass mats (2001–2003). This finding matches the increase in the return period for larger peak events during the 1986–2003 period, which caused fewer major inundations at this level. Conversely, sedimentation rates determined for the lowest terrace at 0·5 m were similar for both techniques (and periods) because the return periods of the peak events responsible for sediment deposition at this lower level did not change much over the period 1986–2003. Copyright © 2006 John Wiley & Sons, Ltd.     

Formation of a cohesive floodplain in a dynamic experimental meandering river

Field studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers. However, it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering. This is needed because only then does a dynamic equilibrium exist and channels maintain meandering with low width–depth ratios. Our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering. In this study we present two experiments with a self‐forming channel in identical conditions, but to one we added cohesive silt at the upstream boundary. The effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2. The experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels. In contrast, in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel. Overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed: first, overbank vertical‐accretion of silt, e.g. levee, overbank sedimentation or splays; and second, lateral point bar accretion with silt on the scrolls and in the swales. The first style led to a reduction in bank erosion, while the second style reduced excavation of chutes. We conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering. Copyright © 2013 John Wiley & Sons, Ltd.     

Reconstruction of floodplain sedimentation rates: a combination of methods to optimize estimates

Reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics, and thereby provides a basis for efficient and sustainable floodplain management. We compared the results of four independent reconstruction methods – optically stimulated luminescence (OSL) dating, caesium‐137 (¹³⁷Cs) dating, heavy metal analysis, and flood bed interpretation – applied at three embanked floodplain sites along lower Rhine River distributaries in the Netherlands. All methods indicate significant sedimentation rates on the floodplains, varying between 2–7 mm/a in the distal zones and 3–9 mm/a in the proximal zones. On a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm/a were found. Except for some minor inconsistencies in ¹³⁷Cs dating results, all methods show decreasing sedimentation rates with increasing distance from the river channel. Intercomparison of the results of the different dating methods revealed the potential errors associated with each method, particularly where disagreement among the results were found. Uncertainties may arise due to (1) grain‐size dependent downward migration of ¹³⁷Cs, (2) smoothing of the vertical heavy metal and ¹³⁷Cs profiles, (3) delayed sediment‐associated input of ¹³⁷Cs in addition to direct atmospheric fall‐out, (4) overestimation of the burial age in OSL dating due to incomplete resetting of the OSL signal, or (5) non‐linear relationships between sediment deposition and flood magnitude in the count‐from‐the‐top correlation between sediment lamination and past observed flood records. Still, taking the uncertainties associated with each method into account, the results are generally in good agreement. Using the results we indicate the optimal spatial range of application of each method, depending on sediment texture and sedimentation rate. The optimal spatial and temporal ranges differ for each method, but show significant overlap. A combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale. Copyright © 2010 John Wiley & Sons, Ltd.     

Enlightenment and the GM floodplain

In the last 400 years floodplains in England and Wales have changed drastically. This has been steered by changes brought about through diverse human activities including river regulation for transport, water abstraction and power generation; mining, industrial and urban pollution; the spread of buildings and transport link construction; land drainage; minimization of flood risk through engineering; floodplain gravel extraction; and environment redesign for recreation and conservation. Adding to the evolving complexity of floodplains, a sequence of post‐Enlightenment impacts from the earliest of industrial societies provides an interesting precursor for other transforming global systems. Historical and sedimentological evidence for this history is available, despite limited quantitative monitoring data. A four‐phase floodplain transformation model is presented for the period. Novel patterns of erosion and sedimentation (in location and quality) have emerged as geomorphological processes have continued in ‘genetically‐modified’ form. Problems building up for the future are likely to rest particularly with more extreme events. Understanding the last four centuries of floodplain history can aid enlightened remedies and adaptations. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluation of floodplain variability considering impacts of climate change

In this study, an approach is presented for handling hydraulic uncertainties in the prediction of floodplain. Different factors affect river flood characteristics. Furthermore, the high changeability of flooding conditions leads to high variability of the inundation. River morphology is one of the most effective factors in river flood characteristics. This factor is influenced by sedimentation and erosion in the river cross sections, which affects the discharge variation. The depth and the width of the river cross section lead to an increase or decrease in the river flow path. This results in changes in the extent of the floodplain based on the generated rainfall. The inundated region boundaries are determined by utilizing the mean first‐order second‐moment analysis. The proposed method is applied to the Kajoo River in the south‐eastern part of Iran. Determination of floodplain uncertainty is a damage‐reduction policy in this region. Also, it is useful to prepare the necessary activities for overcoming the flood hazards. Climate change is the second effective factor on the floodplain uncertainties. Climate change affects the magnitude, extent and depth of inundation and it may intensify the flood problem. Therefore, the future rainfall pattern of the study area under climate change is simulated to evaluate its impacts on the river flow characteristic. Subsequently, a hydraulic routing model is used to determine floodplain. Finally, the copula function is used to estimate the joint probability of the changes in the inundation area due to changes in river morphology and the rainfall changes due to impacts of climate change. Results show that the uncertainties of the extent of floodplain are affected by climate change and river morphology, leading to noticeable changes in the magnitude and frequency of floods. Evaluating these impacts and estimating corresponding river discharges will help in the study of river dynamics, and will also contribute towards devising effective mitigation and management strategies. Copyright © 2010 John Wiley & Sons, Ltd.     

The use of multivariate statistics to elucidate patterns of floodplain sedimentation at different spatial scales

Floodplains are depositional features of riverine landscapes that display complex sedimentation patterns that are amenable to multi‐scale approaches. We examined sedimentation in the Lower Balonne floodplain, Queensland, Australia, at three different spatial scales: the channel (10³ km), floodplain process zone (10 km) and geomorphic unit (10² m) scales, and compared scale‐related patterns evident from stratigraphy with those evident from quantitative multivariate analysis. Three stratigraphic sequences were found in the Lower Balonne floodplain: generally fining upward, episodic fining upward, and mud‐dominated. Stratigraphical analysis revealed the detailed character of sedimentary sequences embedded within the scale patterns derived from multivariate analysis. Multivariate statistical analyses of a range of textural and geochemical data revealed different patterns of floodplain sedimentation at each scale. At the channel scale, sediment texture and geochemistry were more heterogeneous in the Culgoa River than in Briarie Creek. At the floodplain process zone scale clear patterns of sediment texture and geochemistry were observed along the upper, mid and lower floodplain process zones of Briarie Creek, but not along the Culgoa River. At the geomorphic unit scale, clear patterns of sediment texture and geochemistry were observed among the bank, buried channel and flat floodplain units of the Culgoa River, but were not as clear in Briarie Creek. Recognition of rivers as hierarchically organized systems is an emerging paradigm in river science. Our study supports this paradigm by demonstrating that different sedimentation patterns occur at different scales to reveal a hierarchically organized floodplain environment. Copyright © 2006 John Wiley & Sons, Ltd.     

Channel and floodplain sediment dynamics in a reach of the tropical meandering Rio Beni (Bolivian Amazonia)

The study investigates interactions, water and sediment exchanges, between a rapidly migrating meander and its associated floodplain at fine temporal and spatial scales. The Beni River, an Amazonian free meandering river, makes the transition between Andean ranges and Amazonian lowlands. For the period 2002–2006, an assemblage of tools and methods (water and sediment discharges, topometric and bathymetric surveys, sedimentation rate estimations from unsupported ²¹⁰Pb and sediment trapping system) was used to jointly analyse the influence on the sediment budget of external factors (mainly water and sediment discharge) and the inherent behaviour of the system. The main issue addressed is the investigation of the complex relationship between ‘morphological conditioning’ of fluvial landform and process. The first part of the study was undertaken with the aim of linking erosion–deposition in an active meander with water and sediment fluxes. The three inter‐annual evolutions are characterized by very unequal sediment budgets; the first two intervals underwent predominant erosion, and the latter slight accumulation. Digital elevation models, evaluated for the active meander, demonstrate that sedimentation on the point bar depends more on external factors than erosion of the concave bank, which fluctuates slightly. The second part of the study, focusing on water and sediment exchanges between active bend and floodplain, examines the respective parts played by overbank flow and by an abandoned channel on the diffusion and sequestration of sediment. The association of short‐ and long‐term estimation of sedimentation rates suggests that floodplain construction is associated with two different processes and rhythms of sediment transportation. Finally, a sediment budget is proposed for the Beni River in the upper part of the Amazonian lowlands. Copyright © 2010 John Wiley & Sons, Ltd.     

Interaction between meander dynamics and floodplain heterogeneity in a large tropical sand‐bed river: the Rio Beni,Bolivian Amazon

The evolution of meandering river floodplains is predominantly controlled by the interplay between overbank sedimentation and channel migration. The resulting spatial heterogeneity in floodplain deposits leads to variability in bank erodibility, which in turn influences channel migration and planform development. Despite the potential significance of these feedbacks, few studies have quantified their impact upon channel evolution and floodplain construction in dynamic settings (e.g. locations characterized by rapid channel migration and high rates of overbank sedimentation). This study employs a combination of field observations, geographic information system (GIS) analysis of satellite imagery and numerical modelling to investigate these issues along a 375 km reach of the Rio Beni in the Bolivian Amazon. Results demonstrate that the occurrence of clay‐rich floodplain deposits promotes a significant reduction in channel migration rates and distinctive styles of channel evolution, including channel straightening and immobilization of bend apices leading to channel narrowing. Clay bodies act as stable locations limiting the propagation of planform disturbances in both upstream and downstream directions, and operate as ‘hinge’ points, around which the channel migrates. Spatial variations in the erodibility of clay‐rich floodplain material also promote large‐scale (10–50 km) differences in channel sinuosity and migration, although these variables are also likely to be influenced by channel gradient and tectonic effects that are difficult to quantify. Numerical model results suggest that spatial heterogeneity in bank erodibility, driven by variable bank composition, may force a substantial (c. 30%) reduction in average channel sinuosity, compared to situations in which bank strength is spatially homogeneous. Copyright © 2015 John Wiley & Sons, Ltd.     

Floodplain sedimentation rates in an alpine watershed determined by radionuclide techniques

Vertical profiles of the activities of ¹³⁷Cs and ²¹⁰Pb were measured on floodplain sediment cores and upland soil cores along the Soda Butte Creek and the Yellowstone River to determine floodplain sedimentation rates. The position of mine tailings from a 1950 impoundment failure was used as a stratigraphic marker to estimate the sedimentation rates and to make comparisons with rates provided by radionuclide‐based methods. Mass accumulation (sedimentation) rates calculated from the position of the mine tailings ranged from 0·00 to 0·17 g cm^?2 yr^?1 and were in good agreement with sedimentation rates calculated from the inventories of ¹³⁷Cs and ²¹⁰Pb. Sedimentation rates calculated from the position of the ¹³⁷Cs peak generally overestimated the sedimentation rates, probably because of increased downward migration of ¹³⁷Cs caused by the low pH of water moving through the mine tailings or the high permeability of floodplain sediments relative to upland reference soils. This study demonstrates that the ¹³⁷Cs and ²¹⁰Pb inventory methods for determining sedimentation rates can be applied to an alpine floodplain where sedimentation events are episodic and where orographic effects on precipitation generate strong downstream gradients in the delivery of atmospheric radionuclides. Copyright © 2007 John Wiley & Sons, Ltd.     

Methods to calculate sedimentation rates of floodplain soils in the middle region of the Elbe River

This study presents different methods to quantify the historic and recent sedimentation of floodplain soils along the Elbe River. These methods include the comparison of surface elevations, the quantification of sedimentation with the aid of anthropogenic and geogenic tracers, sediment trap studies, and the calculation of load balances. Selected results from sites at the lower section of the middle Elbe River are presented and verified. The results show that several methods are suitable. In future work it should be possible, depending on the available soil and sediment data, to calculate sedimentation in Elbe River floodplain and the loss of retention volume for larger areas.     

Temporal variability of contemporary floodplain sedimentation in the Rhine–Meuse delta,The Netherlands

It is often believed that extreme but infrequent events are most important in the development of landforms. When evaluating the overall effect of large floods on floodplain sedimentation, quantitative measurements of both high- and low-magnitude events should be considered. To analyse the role of flood magnitude on floodplain sedimentation, we measured overbank sedimentation during floods of different magnitude and duration. The measurements were carried out on two embanked floodplain sections along the rivers Rhine and Meuse in The Netherlands, using sediment traps made of artificial grass. The results showed an increase in total sediment accumulation with flood magnitude, mainly caused by enhanced accumulation of sand. At low floodplain sections the increase in sediment deposition was smaller than expected from the strong increase in suspended sediment transport in the river. Spatial variability in sediment accumulation was found to depend both on flood magnitude and duration. Deposition of sand on natural levees mainly takes place during high-magnitude floods, whilst low floods and slowly receding floods are important for the deposition of silt and clay in low-lying areas, at greater distance from the main channel. © 1998 John Wiley & Sons, Ltd.     

Elevated sedimentation in lake records linked to agricultural activities in the Ishikari River floodplain,northern Japan

The purpose of this study was to examine the historical change in sedimentation rates in lakes that have been impacted by river regulation and agricultural activities in the Ishikari River floodplain. We dated sediment cores using caesium‐137 (¹³⁷Cs) dating and tephrochronology, and we estimated sediment sources from ¹³⁷Cs concentrations in the topsoil of representative land covers. We found that, between 1739 and 1963, the distance between the lake and the main river channel and whether or not the lake was connected to the river affected the sedimentation rates. After 1963, agricultural drainage systems were established in the Ishikari River floodplain. The average sedimentation rate before and after the construction of drainage ditches varied between 1–66 and 87–301 mg cm^–2 a^–1, respectively. The increase in the sedimentation rate after 1963 was caused by the construction of a number of drainage networks, as well as extensive cultivation activity and/or fragmentation of the swamp buffers surrounding the lakes. The ¹³⁷Cs activities at the surfaces of the lake as well as the catchment‐derived ¹³⁷Cs contributions and ¹³⁷Cs inventory in the lake profiles were used to examine the sediment influx from the various drainage areas after the establishment of the drainage system. Our results indicate that the majority of the lake sediments were derived from cultivated areas, and therefore the catchment‐derived ¹³⁷Cs contribution in the lakes was strongly correlated with the sedimentation rate. The ¹³⁷Cs inventory across all of the lake profiles was also significantly greater than the atmospheric fallout. We identified a negative correlation between the ¹³⁷Cs lake profile inventory and the sedimentation rate. This is because the sediment originating from the drainage areas contained low ¹³⁷Cs concentrations, which diluted the overall concentration of ¹³⁷Cs in the lake sediment. Copyright © 2009 John Wiley & Sons, Ltd.     

Estimating sedimentation rates and sources in a partially urbanized catchment using caesium-137

While there has been increased interest in determining sedimentation rates and sources in agricultural and forested catchments in recent years, there have been few studies dealing with urbanized catchments. A study of sedimentation rates and sources within channel and floodplain deposits of a partially urbanized catchment has been undertaken using the ¹³⁷Cs technique. Results for sedimentation rates showed no particular downstream pattern. This may be partially explained by underestimation of sedimentation rates at some sites by failure to sample the full ¹³⁷Cs profile, floodplain erosion and deliberate removal of sediment. Evidence of lateral increases in net sedimentation rates with distance from the channel may be explained by increased floodplain erosion at sites closer to the channel and floodplain formation by lateral deposition. Potential sediment sources for the catchment were considered to be forest topsoil, subsurface material and sediments derived from urban areas, which were found to be predominantly subsurface material. Tracing techniques showed an increase in subsurface material for downstream sites, confirming expectations that subsurface material would increase in the downstream direction in response to the direct and indirect effects of urbanization. © 1998 John Wiley & Sons, Ltd.     

The beaver meadow complex revisited – the role of beavers in post‐glacial floodplain development

We evaluate the validity of the beaver‐meadow complex hypothesis, used to explain the deposition of extensive fine sediment in broad, low‐gradient valleys. Previous work establishes that beaver damming forms wet meadows with multi‐thread channels and enhanced sediment storage, but the long‐term geomorphic effects of beaver are unclear. We focus on two low‐gradient broad valleys, Beaver Meadows and Moraine Park, in Rocky Mountain National Park (Colorado, USA). Both valleys experienced a dramatic decrease in beaver population in the past century and provide an ideal setting for determining whether contemporary geomorphic conditions and sedimentation are within the historical range of variability of valley bottom processes. We examine the geomorphic significance of beaver‐pond sediment by determining the rates and types of sedimentation since the middle Holocene and the role of beaver in driving floodplain evolution through increased channel complexity and fine sediment deposition. Sediment analyses from cores and cutbanks indicate that 33–50% of the alluvial sediment in Beaver Meadows is ponded and 28–40% was deposited in‐channel; in Moraine Park 32–41% is ponded sediment and 40–52% was deposited in‐channel. Radiocarbon ages spanning 4300 years indicate long‐term aggradation rates of ~0.05 cm yr^‐1. The observed highly variable short‐term rates indicate temporal heterogeneity in aggradation, which in turn reflects spatial heterogeneity in processes at any point in time. Channel complexity increases directly downstream of beaver dams. The increased complexity forms a positive feedback for beaver‐induced sedimentation; the multi‐thread channel increases potential channel length for further damming, which increases the potential area occupied by beaver ponds and the volume of fine sediment trapped. Channel complexity decreased significantly as surveyed beaver population decreased. Beaver Meadows and Moraine Park represent settings where beaver substantially influence post‐glacial floodplain aggradation. These findings underscore the importance of understanding the historical range of variability of valley bottom processes, and implications for environmental restoration. Copyright © 2011 John Wiley & Sons, Ltd.     

Modelling sediment deposition and phosphorus retention in a river floodplain

Phosphorus (P) is one of the major limiting nutrient in many freshwater ecosystems. During the last decade, attention has been focused on the fluxes of suspended sediment and particulate P through freshwater drainage systems because of severe eutrophication effects in aquatic ecosystems. Hence, the analysis and prediction of phosphorus and sediment dynamics constitute an important element for ecological conservation and restoration of freshwater ecosystems. In that sense, the development of a suitable prediction model is justified, and the present work is devoted to the validation and application of a predictive soluble reactive phosphorus (SRP) uptake and sedimentation models, to a real riparian system of the middle Ebro river floodplain. Both models are coupled to a fully distributed two‐dimensional shallow‐water flow numerical model. The SRP uptake model is validated using data from three field experiments. The model predictions show a good accuracy for SRP concentration, where the linear regressions between measured and calculated values of the three experiments were significant (r² ≥ 0.62; p ≤ 0.05), and a Nash–Sutcliffe coefficient (E) that ranged from 0.54 to 0.62. The sedimentation model is validated using field data collected during two real flooding events within the same river reach. The comparison between calculated and measured sediment depositions showed a significant linear regression (p ≤ 0.05; r² = 0.97) and an E that ranged from 0.63 to 0.78. Subsequently, the complete model that includes flow dynamics, solute transport, SRP uptake and sedimentation is used to simulate and analyse floodplain sediment deposition, river nutrient contribution and SRP uptake. According to this analysis, the main SRP uptake process appears to be the sediment sorption. The analysis also reveals the presence of a lateral gradient of hydrological connectivity that decreases with distance from the river and controls the river matter contribution to the floodplain. Copyright © 2014 John Wiley & Sons, Ltd.     

An investigation of the spatial variability of the grain size composition of floodplain sediments

River floodplains have been widely recognized as important sinks for storing suspended sediment and associated contaminants transported by river systems. The grain size composition of floodplain deposits exerts an important influence on contaminant concentrations, and commonly exhibits significant spatial variability in response to the dynamic nature of overbank flow and sediment transport. Information on the spatial variability of the grain size composition of overbank deposits is therefore essential for developing an improved understanding of the processes controlling sediment transport on floodplains, and for investigating the fate of sediment-associated contaminants. Such information is also important for validating existing floodplain sedimentation models. This paper reports the results of a study aimed at investigating the spatial variability of the grain size composition of floodplain sediments at different spatial scales, through analysis of surface sediment samples representative of contemporary floodplain deposits collected from frequently inundated floodplain sites on five British lowland rivers. Significant lateral and downstream variations in the grain size composition of the sediment deposits have been identified in the study reaches. An attempt has been made to relate the observed spatial distribution of the grain size composition of the overbank deposits to the local floodplain geometry and topography. The importance of the particle size characteristics of the suspended sediment transported by the rivers in influencing the spatial variability of the grain size composition of the overbank sediments deposited on these floodplains is also considered. © 1998 John Wiley & Sons, Ltd.     

USE OF FALLOUT Pb-210 MEASUREMENTS TO INVESTIGATE LONGER-TERM RATES AND PATTERNS OF OVERBANK SEDIMENT DEPOSITION ON THE FLOODPLAINS OF LOWLAND RIVERS

The potential for using fallout (unsupported) Pb-210 (²¹⁰Pb) measurements to estimate rates of overbank sediment deposition on the floodplains of lowland rivers is explored. A model which distinguishes the contribution from direct atmospheric fallout and the catchment-derived input associated with the deposition of suspended sediment has been developed to interpret the fallout Pb-210 inventories at floodplain sampling sites and to estimate average sediment accumulation rates over the past 100 years. The approach has been successfully used to estimate rates of overbank sedimentation on the floodplains of the Rivers Culm and Exe in Devon, U.K. A detailed investigation of the pattern of longer-term sedimentation rates within a small reach of the floodplain of the River Culm indicated a range of deposition rates between 0.07 and 0.59 g cm⁻² a⁻¹, which was in close agreement with estimates of current sedimentation rates obtained using sedimentation traps.     

Modelling the decadal dynamics of reach-scale river channel evolution and floodplain turnover in CAESAR-Lisflood

Sedimentary deposits provide records of environmental change quantifying erosion fluxes conditioned by natural and anthropogenic disturbances. These fluxes are lagged by internal storage, particularly within floodplains, complicating reconstruction of environmental changes. The time sediment remains in storage underpins the interpretation of sedimentary records and accurate monitoring of pollutant fluxes. Turnover time is a measure of the timeframe to erode every floodplain surface. CAESAR-Lisflood is used to simulate fluvial evolution at reach scale, providing a basis for quantifying environmental changes on the timescales of sediment storage. We evaluate the accuracy of CAESAR-Lisflood simulations of channel changes and turnover times for alluvial floodplains using historical channel changes reconstructed for 10 reaches in northern England to quantify model accuracy in replicating mean annual erosion, deposition and channel lateral migration rates, alongside planform morphology. Here, a split-sample testing approach is adopted, whereby five of the reaches were calibrated and the resulting parameter values were applied to the other reaches to evaluate the transferability of parameter settings. The lowest overall integrated error identified the best-fit simulations and showed that modelled process rates were within ~25–50% of rates from historical reconstructions, generally. Calibrated parameters for some reaches are widely transferable, producing accurate geomorphic changes for some uncalibrated sites. However, large errors along some reaches indicate that reach-specific parameterization is recommended. Turnover times are underpinned by the assumption that areas of floodplain previously unvisited by the channel are reworked. This assumption has been challenged by studies that show floodplain (re)occupation rates vary spatially. However, this limitation is less important for the short-duration simulations presented here. The simulations reconstruct floodplain turnover times estimated by mapped rates mostly successfully, demonstrating the potential applicability of calibrated parameters over much longer timescales. Errors in the form of under-predicted erosion rates propagated, resulting in over-predicted turnover times by even greater magnitudes. © 2020 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.