Sediment Mobility Research Articles

Wave-induced sediment mobility on a morphologically complex continental shelf: eastern Brazilian shelf

Wave-induced sediment mobility and transport are important issues regarding seabed stability and sediment balance on continental shelves. This study compares the near-bed velocity induced by waves with the critical orbital velocity for sediment mobility to determine the threshold exceedance for non-cohesive sediment motion on a portion of the eastern Brazilian continental shelf. Based on a 13-year wave time series, dominant wave climate has been used as boundary conditions in a wave propagation model. The region presents complex continental shelf morphology, varying north to south from a narrow shelf with coastal parallel isobaths to a wide shelf with irregular isobaths. The wave orbital velocities, which vary from 0 to 1.4 m s−1, are capable of mobilizing sediment across almost the entire continental shelf area, since the critical bottom sediment velocity required for potential sediment mobility varies from 0.11 to 0.13 m s−1 Such findings are of great relevance to gain a better understanding of the sediment dynamics in this area that hosts several reef banks, and that has been recently affected by a large environmental disaster through the failure of a tailing dam that resulted in an increase in contaminated sediments that have spread into the ocean.

A comprehensive understanding of enhanced Pb mobilization in sediments caused by algal blooms.

A good understanding of lead (Pb) mobilization in eutrophic lakes is a key to the accurate assessment of Pb pollution. In this work, dissolved and labile Pb was determined by both high resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) in sediment-water profiles of the hyper-eutrophic Meiliang Bay of Lake Taihu on a monthly basis during one year. The drinking water standards for dissolved Pb of the World Health Organization (10μg/L) and those of China were exceeded in the overlying water (20.79-118.5μg/L). Out of which, a total of five months even exceeded the fisheries water quality limitation (50μg/L) in China. The algal blooms created an anaerobic environment in the surface sediments in July. The reductive conditions led to the dissolution of Fe/Mn and this caused the release of Pb, followed by organic matter complexation. This was supported by the coincident changes of dissolved Pb with dissolved organic matter (DOM) in sediments under anaerobic incubation. Algae residue decomposition in October caused another distinct release of Pb, but this process should be considerably suppressed by increased sulfide precipitation and pyrite adsorption of Pb ion. These results indicated that Pb mobilization in sediments can be significantly enhanced by algal blooms in eutrophic lakes, indicating that further attention should be paid to Pb pollution in waters with harmful algal blooms.

High-resolution bathymetry of the Alderney Race and its geological and sedimentological description (Raz Blanchard, northwest France)

ABSTRACT We present a high-resolution 1:15,000 bathymetric map (Main map) of Alderney Race located offshore of northwestern France, with the strongest currents in Europe. We use this map, underwater video transects and Shipek grabs to improve geological maps previously published. We distinguished Proterozoic crystalline rocks, Paleozoic and Cretaceous sedimentary rocks on the present-day sea floor. Some structures as faults and folds are also mapped. We identified a Quaternary cover made of pebbles, boulders and blocks interpreted as corestones resulting in differential erosion and alteration of the substratum. This cover is commonly encrusted by fixed fauna, such as bryozoans and barnacles. Finally, we describe the present-day mobile sediment cover characterized by sand patches and pebble dune fields (up to 10 m in height). Our videos show the presence of mobile fine-grained sediment patches under the resolution of our map lying between the cobble and pebble cover. We summarize our interpretations on a non-exhaustive geological-sedimentary map.

The Impact of Coarse‐Grain Protrusion on Near‐Bed Hydrodynamics

In steep rivers, sediment is often transported over immobile cobbles and boulders. Previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains. Here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine‐sediment bed, modeled as a patch of hemispheres, modify the local hydrodynamics near the fine‐sediment bed. Five protrusion levels defined by P=k/R= {0%, 20%, 40%, 60%, 80%} were investigated, where k is the protrusion height and R the radius. For small protrusion (P=20%), enhanced shear stress and turbulence intensity on the fine‐sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine‐sediment bed. Moreover, sweep events generated near the top of the hemispheres reach the fine‐sediment bed. For large protrusions (P ≥ 60%), the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops. The remaining turbulence near the fine‐sediment bed, although enhanced by the wakes generated by the hemispherical caps, is quasi‐isotropic. The transition between these two distinct near‐bed flow regimes is found to be around P=40%, corresponding to the protrusion levels observed by Grams and Wilcock, https://doi.org/10.1002/2013JF002925) above which the erosion of fine sediment ceases.

Impact of Vegetation on Bed Load Transport Rate and Bedform Characteristics

AbstractThe impacts of aquatic vegetation on bed load transport rate and bedform characteristics were quantified using flume measurements with model emergent vegetation. First, a model for predicting the turbulent kinetic energy, kt, in vegetated channels from channel average velocity U and vegetation volume fraction ϕ was validated for mobile sediment beds. Second, using data from several studies, the predicted kt was shown to be a good predictor of bed load transport rate, Qs, allowing Qs to be predicted from U and ϕ for vegetated channels. The control of Qs by kt was explained by statistics of individual grain motion recorded by a camera, which showed that the number of sediment grains in motion per bed area was correlated with kt. Third, ripples were observed and characterized in channels with and without model vegetation. For low vegetation solid volume fraction (ϕ ≤ 0.012), the ripple wavelength was constrained by stem spacing. However, at higher vegetation solid volume fraction (ϕ=0.025), distinct ripples were not observed, suggesting a transition to sheet flow, which is sediment transport over a plane bed without the formation of bedforms. The fraction of the bed load flux carried by migrating ripples decreased with increasing ϕ, again suggesting that vegetation facilitated the formation of sheet flow.

Assessment of Climate Change Impacts in the North Adriatic Coastal Area. Part II: Consequences for Coastal Erosion Impacts at the Regional Scale

Coastal erosion is an issue of major concern for coastal managers and is expected to increase in magnitude and severity due to global climate change. This paper analyzes the potential consequences of climate change on coastal erosion (e.g., impacts on beaches, wetlands and protected areas) by applying a Regional Risk Assessment (RRA) methodology to the North Adriatic (NA) coast of Italy. The approach employs hazard scenarios from a multi-model chain in order to project the spatial and temporal patterns of relevant coastal erosion stressors (i.e., increases in mean sea-level, changes in wave height and variations in the sediment mobility at the sea bottom) under the A1B climate change scenario. Site-specific environmental and socio-economic indicators (e.g., vegetation cover, geomorphology, population) and hazard metrics are then aggregated by means of Multi-Criteria Decision Analysis (MCDA) with the aim to provide an example of exposure, susceptibility, risk and damage maps for the NA region. Among seasonal exposure maps winter and autumn depict the worse situation in 2070–2100, and locally around the Po river delta. Risk maps highlight that the receptors at higher risk are beaches, wetlands and river mouths. The work presents the results of the RRA tested in the NA region, discussing how spatial risk mapping can be used to establish relative priorities for intervention, to identify hot-spot areas and to provide a basis for the definition of coastal adaptation and management strategies.

Fluvial incision and coarse gravel redistribution across the modern Dead Sea shelf as a result of base‐level fall

AbstractGlobal eustatic lowstands can expose vast areas of continental shelves, and occasionally the shelf edge and the continental slope. The degree of fluvial connectivity to receding shores influences the redistribution of sediments across these emerging landscapes. Shelf and slope emergence in the Dead Sea since the middle of the 20th century, offers a rare opportunity to examine evolution of stream connectivity in response to continuous base‐level decline. We characterize the connectivity evolution of two streams, using high‐resolution time series of aerial imagery and elevation models, field mapping, and grain‐size analyses. Our rich spatiotemporal dataset of evolving channel geomorphology, sediment transport conditions, and sediment redistribution, allows calculating potential coarse sediment mobility in response to base level decline. Following shelf emergence, alluvial fans first prograded onto the low‐gradient shelf under unfavourable conditions for transporting coarse sediment to the regressing shoreline. Then, with shelf and slope emergence, the two adjacent streams evolved differently. The smaller, more arid watershed still maintains its highstand delta progradation on the shelf and is practically disconnected from the receding lake. The larger catchment, heading in wetter environments and having a narrower shelf, has incised the shelf and renewed and gradually intensified the sediment transport from the highstand to the lowstand delta. Sediment mobilization to lowstand shorelines is controlled by the evolution of the channel profile and by the average speed of gravel transport (10s‐100s m yr‐1). These findings from the Dead Sea are relevant to fluvial processes operating on continental shelves during glacial maxima. Streams would have commonly stored high proportions of their coarse sediment on the continental shelves rather than efficiently connecting with the lowstand level. Additionally, differences in sediment routing patterns should exist among nearby streams, primarily due to continental margin geometry and watershed hydrology. © 2019 John Wiley & Sons, Ltd.

Deformation ‘boundary front’ movements in subglacial tills—A microsedimentological perspective from till sequences near Pine Point, NWT, Canada

AbstractInvestigations of glacial sediment exposures in a former open pit in the Pine Point Mining District, Northwest Territories, reveal evidence of changes in stress magnitude as exhibited by the movement of deformation ‘fronts’ within subglacial tills, as evident through micromorphological analysis. Indications of deformation, driven by stress fluctuations, were observed throughout a set of vertical subglacial till samples associated with the Laurentide Ice Sheet. These tills exhibit multiple sub‐lithofacies variations in concert with marked changes in microshear orientation, which can be directly related to changing stress levels within a subglacial deforming bed. The evidence of changes in till stress conditions is typical of subglacial tills occurring beneath an active temperate ice mass within a soft deforming bed. As differing ‘layers’ of till that form a mosaic‐like pattern become immobilized and ‘detached’ subjacent to the deforming bed sediments, a signature of this action is recorded within the tills as sequences of specific microstructures. These microstructures are part of a spectrum of micro‐forms that develop as consequence of changes in stress, porewater content and clay content within the ‘fault‐gouge’—like environment formed beneath an active temperate ice sheet. A model is developed that uses data from Pit O‐28 demonstrating where deformation ‘boundary front’ evidence in the form of deformation bands has developed as basal driven stress levels have penetrated active deforming sediments. The evidence from Pine Point indicates that till acts as an excellent palaeo‐strain record of past active glacial processes, subglacial basal thermal conditions and sediment mobility.

A two-phase SPH model for massive sediment motion in free surface flows

Massive sediment motion in water with a free surface is an important kind of geophysical flows such as hyper-concentrated sediment laden river flows discharging into estuarine delta and turbidity currents generated by subaqueous landslides. One of the key and common characteristics of such flows is that interactions between water and sediment as well as those among sediment particles are equally important in affecting the sediment motion and the fluid flow. This paper presents a numerical model that builds on and extends an earlier two-phase SPH model based on a continuum formulation of solid-liquid mixtures (Shi et al., 2017) to provide a unified description of massive sediment motion in free surface flows. In the model, a constitutive law based on the rheology of dense granular flow is introduced to express the intergranular stresses while the interphase drag force is determined by combining the Ergun equation for dense solid-fluid mixtures and the power law for dilute suspensions. The proposed model is firstly applied to the study of collapse of loosely or densely packed granular columns submerged in water. The computed surface profiles of the granular column are found to be in good agreement with the experimental data. It shows that the loosely packed and the densely packed columns behave rather differently due to the differences in water-sediment interaction processes. The model is then used to simulate a dam-break flow over a mobile sediment bed. The computed configurations of the flow and the movable bed also agree well with the measured data. The predicted position on the leading edge of the flow has a mean error of 0.8% while the mean error for the maximum bed height is 12.9%. To further identify the dynamic processes involved, effects of water-sediment interactions on the motion of bed materials are investigated by examining the spatial and temporal variations of pressure and flow velocity. As shown in the applications, the proposed two-phase SPH model can successfully represent both the gravity-driven underwater granular flows and the shear flow driven intense sediment transport, implying its potential use in practical scenarios in which the two kinds of flows exist simultaneously, such as landslides triggered by storm in shallow sea and flows resulted in barrier or dam breaks.

High-resolution imaging of phosphorus mobilization and iron redox cycling in sediments from Honghu Lake, China

Phosphorus (P) pollution is a significant environmental problem in lakes, especially in lakes highly impacted by human activities, e.g., Honghu Lake. Years of farming activities have led to a complex occurrence of P in the sediment which has not been studied in detail. Iron redox cycling is a key mechanism controlling P adsorption–desorption behavior, and its study is an effective means of assessing P dynamics, which could provide basic theoretical data for the control of internal phosphorus in such lakes. The high-resolution ZrO-Chelex DGT (ZrO-Chelex diffusive gradients in thin films) and HR-Peeper (high-resolution dialysis sampler) techniques were used to assess labile P, labile Fe, and soluble reactive P (SRP) in five sediment samples from four functional areas of Honghu Lake. The temperature, pH, and dissolved oxygen levels of water samples were measured in situ using a portable water quality analyzer, and the dissolved organic carbon (DOC) concentration was analyzed with a TOC analyzer after 0.45 μm membrane filtration. Total Fe concentrations were determined using the phenanthroline colorimetric approach. Sediment porosity was calculated based on weight loss after drying for 6 h at 105 °C. One-dimensional changes in labile P and Fe in different functional areas of the lake were quite heterogeneous, and there was a significant linear correlation between labile P and Fe near the sediment–water interface (SWI). Sediment samples HH-2 and HH-5 exhibited positive P diffusion flux (F) and tended to be a P source, while sites HH-1, HH-3, and HH-4 tended to take in P from overlying water. The ratio (R) of labile P to SRP was used as an indicator of potential soluble reactive P release from sediments into the surrounding water and initially increased at four sampling sites, then decreased deeper in the sediment except at site HH-4. The results indicated that the tendency of P released from sediments or uptake from overlying water was distinct in different functional areas and that iron redox cycling was a key mechanism controlling P dynamics in the sediments of Honghu Lake. Thus, the iron–phosphorus relationship can provide an accurate assessment of P release processes in a surface water environment.

Mobility and Bioavailability of Sediment Phosphorus in Urban Stormwater Ponds

AbstractStormwater ponds can serve as retention hotspots for phosphorus (P) moving out of the urban environment. This retention may be reduced by P speciation that reduces the bioavailability of P to primary producers and alters its mobility in sediments. Here we examined the mobility and fate of dissolved P in urban stormwater ponds with a set of complementary field measurements and short‐term laboratory and field experiments. We measured the types and amount of P in water column and sediments of urban stormwater ponds. We further assessed the mobility of different P types in pond sediments in the field and rates of P release from sediment cores maintained under laboratory conditions. Finally, we assessed P uptake rates by pond algal communities using short‐term bioassay experiments. We found that dissolved organic P was highly prevalent in urban pond water and sediments and that this type of P was mobile within sediments and could be released under high or low O2 conditions. We also found highly variable P demand by algae among stormwater ponds and that algal growth responses to P was correlated to water column N:P ratios. Altogether, our results indicate an important role for organic phosphorus cycling in urban stormwater ponds, which likely constrains the overall retention efficiency in these aquatic ecosystems.

Role of multiple flow stages over submerged structure

This study presents the results of an experimental investigation on the turbulence characteristics of flow over a submerged structure at three different submergence ratio. The experiments were performed in a water channel and time series of velocity data were collected by an acoustic-Doppler velocimeter (ADV). The recorded velocity data were analysed in the framework of time-averaging and profiles and contours of time-averaged velocity components, turbulent intensity components, and Reynolds stresses, are presented and discussed at different vertical and horizontal planes. This study also addressed the flow separation and reattachment region and the turbulent events associated with burst-sweep cycles. With decreasing submergence ratio marked variation in diving and lifting nature of the flow is observed at the wake region of the cube that has important implications for local sediment mobility.

Sediment mobility and connectivity in a catchment: A new mapping approach

In fluvial basin analysis, sediment connectivity is an important element for defining channel dynamics. Nevertheless, although several approaches to quantify this concept have been trialed, there is considerable discussion about ways to measure and assess sediment connectivity. The present study investigates sediment connectivity through the definition of a new index, aiming to integrate functional aspects within a structural component. Our objective is to produce a sediment flow connectivity index (SCI) map, directly applicable to monitoring and management activities. Our SCI is defined as the result of the gradient-based flow accumulation of a sediment mobility index, which is in turn a simple function of rainfall, geotechnical properties of soil and land use. This method is here applied to the Vernazza basin (eastern Liguria, Italy), producing a sediment connectivity map that shows good performance in predicting the positions and accumulation paths of mobilized deposits detected on the ground after the October 25th, 2011, flood event. A further evaluation of the proposed index is performed through a comparison of the maps derived using the SCI and connectivity index (IC) developed by Cavalli et al. (2013), which highlights comparable quantitative overall performances, together with a slightly better qualitative identification of subtle sediment flow paths by the SCI. In spite of current limitations due to, e.g., the local nature of the final index, the availability of input information through open global datasets promises the potential application of this method to larger-scale assessments, paying attention to properly addressing upscaling and standardization issues.

Peak Chromium Pollution in Summer and Winter Caused by High Mobility of Chromium in Sediment of a Eutrophic Lake: In Situ Evidence from High Spatiotemporal Sampling.

To study the mechanisms of chromium (Cr) mobilization in sediments of eutrophic lakes, monthly sampling was performed in the Meiliang Bay of Lake Taihu, China, combined with laboratory experiments. High-resolution dialysis and diffusive gradients in thin film (DGT) sampling techniques were used. Results indicated that in July 2016 and January 2017, the concentrations of soluble Cr and DGT-labile Cr(VI) in the overlying water exceeded both drinking and fishery water quality standards, resulting from the high mobility of Cr in sediments. In July (summer), the high concentration of soluble Cr (134.04 ± 7.20 μg/L) detected in the anaerobic sediments was primarily caused by the complexation of Cr(III) with dissolved organic matter (DOM). This mechanism was supported by an observed simultaneous increase of soluble Cr and DOM under simulated anaerobic conditions. In January (winter), the high concentrations of soluble Cr (97.55 ± 9.65 μg/L) and DGT-labile Cr(VI) (25.83 ± 1.25 μg/L) in aerobic sediments were primarily caused by reoxidation of Cr(III) by Mn(III/IV) oxides as evidenced by the lowest concentrations of soluble and DGT-labile Mn(II). This study sheds light on the full-year variation and mechanisms of Cr mobilization in eutrophic lake sediments and suggests the urgent need for remediation of Cr pollution especially for winter.

Assessing Mercury Mobility in Sediment of the Union Canal, Scotland, UK by Sequential Extraction and Thermal Desorption.

The mobility of mercury (Hg) was assessed in sediment from the Union Canal, Scotland, UK. Samples collected from the vicinity of a former munitions factory that manufactured mercury fulminate detonators were subjected to sequential extraction followed by cold vapor atomic absorption spectrometry (CVAAS) and direct analysis using thermal desorption (TD). The sequential extraction indicated that > 75% of mercury (up to 429mgkg-1) was in mobile forms, with < 12% semimobile and < 23% nonmobile species. In the TD method, > 67% of the total Hg content was desorbed in the temperature range 100-250°C consistent with species weakly attached to the mineral matrix [tentatively identified as an iron (oxy)hydroxide-associated species]. This predominance of mobile mercury species may arise from a lack of association between Hg and either organic matter or sulfur in the sediments. Further investigation of Hg mobilization, transport, and assimilation/biomagnification is required both to determine whether there is a need for remediation of the sediment and to improve understanding of the biogeochemical cycling of Hg in shallow, oxic, freshwater systems.

Influence of Grain Size on Sediment Transport during Initial Stages of Horizontal Dam Break–Type Flows

The influence of grain size on sediment transport during the initial stages of dam break-type flows over horizontal mobile sediment beds was investigated through dimensional analysis and experiments. A new dimensional analysis reconciles the unbalanced dimensions in Bagnold's formulations describing the grain size influence on steady-flow sediment transport and indicates that the sediment transport is inversely proportional to the square root of the grain size, consistent with Bagnold's empirical correlation. Total transport rates were measured for quartz grains ranging from 0.22 to 2.65 mm in diameter. Regression between the dimensionless variables indicated that the mobility number collapses the sediment transport data better than the Shields number and indicated that the total sediment transport is approximately inversely proportional to the square root of the grain size and the velocity to the fourth power. The Meyer-Peter Muller transport model based on the Shields number requires transport coefficients for the finest sand that are significantly larger than the conventional values suggested in the literature, between 30 for k = 2.5D and 40 for k = D . The values for other sediment sizes are closer to conventional values, within the usual range of 10-12 for k = 2.5D , but somewhat higher at 16-22 for k = D . The occurrence of sheet flow in a suspension regime appears possible, which may explain the significantly larger transport coefficients required for the finest grain size.

Arsenite and arsenate binding to ferrihydrite organo-mineral coprecipitate: Implications for arsenic mobility and fate in natural environments

Arsenic mobility in soils, sediments and groundwater systems is strongly controlled by adsorption occurring at iron oxide/water interfaces, and the extent of this adsorption may be influenced by the presence of natural organic matter (NOM). This study aims to investigate the adsorption of As(III) and As(V) onto coprecipitates made with ferrihydrite (Fh) and humic acid (HA) with two organic carbon (OC) loadings of 5 and 15 wt% OC. We show that the coprecipitation of HA with Fh can significantly reduce the retention of both As(III) and As(V) over a wide pH range (4–11), and with increased OC loading, there is reduced arsenic adsorption. On pure Fh, As(III) is adsorbed to a greater extent than As(V) at pH > 6.5 (the crossover pH), whereas the crossover pH shifts to more acidic pH in the presence of HA, implying that the binding of As(III) is more favorable than As(V) in the presence of NOM. Both As(III) and As(V) are complexed with the ferric hydroxyl functional groups, and no ternary Fh-HA-As complexes are detected. We observe that ∼40% of the adsorbed As(III) is oxidized to As(V) on pure Fh, compared to only ∼29% of As(III) oxidation on the Fh-HA coprecipitate, indicating that NOM hinders As(III) oxidation on iron (hydr)oxide. The results of this study suggest that NOM interacts with arsenic in ways that promote arsenic mobility and especially promote the mobility of arsenate relative to arsenite, which is of great significance for evaluating the migration and bioavailability of arsenic in both natural and contaminated environments.

Insights into Pb(II) binding by Fe/Al hydroxide–microbe composite: XAFS spectroscopy and isothermal titration calorimetry study

Soil is a complex assemble of colloidal and particulate constituents, including minerals, organic matter and microbes. Minerals and organics are not separate entities; rather, they are constantly interacting with each other. Such mineral-organic interaction can greatly affect the sequestration of metallic elements at the soil-water interfaces. This study aims to explore the binding of Pb(II) in a model Fe/Al hydroxide-organo composite (Fe/Al-OM) made with Pseudomonas putida cells which, for the purposes of this study, can be considered to be an analogue of some natural soil microaggregates. We measure an enhanced Pb adsorption to the Fe/Al hydroxide-bacteria composite (Fe/Al-B) over pure Fe/Al hydroxide at pH < 5.5. We find that Fe/Al-B sorbs an intermediate amount of Pb between the end-member Fe/Al hydroxide and bacteria. We identify a bidentate edge-sharing Pb complex on the Fe/Al oxide surfaces with a Pb–Fe/Al bond distance of ~3.33 Å, as well as a monodentate carboxyl binding on the bacterial surfaces. We report that Pb binding is an exothermic, energetically favorable process accompanied by positive entropy. Moreover, Pb distribution shifts from the bacterial fraction to the mineral fraction in Fe/Al-B when pH increases from 4 to 6.5. Results from batch to spectroscopic and thermodynamic experiments will facilitate our understanding of Pb speciation, mobility and fate in soils and sediments.

Mobility of chromium in sediments dominated by macrophytes and cyanobacteria in different zones of Lake Taihu

To study the mechanisms of chromium (Cr) mobilization in sediments of lakes with different ecotypes, seasonal sampling was performed in the macrophyte-dominated East Taihu (MDET) and cyanobacteria-dominated Meiliang Bay (CDMB) in Lake Taihu. Concentrations of labile Cr(VI) and dissolved Cr were assessed using diffusive gradients in thin films (DGT) and high-resolution dialysis passive sampling devices, respectively. Results indicated that in pore water the dissolved Cr concentrations and in sediments total Cr and Cr fractions concentrations (dissolved, exchangeable and carbonate fraction (F1), Fe-Mn oxide fraction (F2), organic/sulfide fraction (F3)) were lower in MDET than in CDMB. However, the highly toxic DGT-labile Cr(VI) concentrations were higher in MDET sediments than in CDMB sediments. In the two lake zones, the seasonal variations in concentrations of dissolved Cr and DGT-labile Cr(VI) were significant, while total Cr and Cr fractions showed negligible changes (except F1 fraction). In summer, the high mobility of dissolved Cr (MDET: 103.42±10.82μg/L; CDMB: 108.99±4.24μg/L) were mainly caused by dissolved organic matter complexing with Cr(III). In winter the high mobility of dissolved Cr (MDET: 100.27±22.04μg/L; CDMB: 102.01±8.81μg/L) and DGT-labile Cr(VI) (MDET: 28.26±3.73μg/L; CDLZ: 25.82±2.26μg/L) were primarily caused by the oxidization of Cr (III) by Mn(III/IV) oxides. This study establishes the mechanisms for seasonal variation of Cr mobilization in different lake ecological zones, highlighting the urgent need for remediation of Cr pollution, especially in macrophyte-dominated lake zones.

Elevated floodplains and net channel incision as a result of the construction and removal of water mills

ABSTRACTAn often underrated but historically long-lasting human impact on river-floodplain systems is that of the presence of water mills. The objective of this study is to determine how the construction and later removal of water mills influence the longitudinal bed profile of a river and its floodplain sedimentation. The effects of a river-mill system were analyzed using physically-based equations of backwater effects and sediment mobility in combination with field measurements of channel slope and floodplain development pre- and postdating water mills in the Wurm River (Germany) and the Geul River (Netherlands). The results show that the construction and removal of water mills likely resulted in a net incision of the riverbed into the valley bottom to a level below the original bed, with reduced floodplain inundation rates and, consequently, reduced floodplain sedimentation.