Critical Shear Stress For Erosion Research Articles

Application of the tidally averaged equilibrium cohesive sediment concentration for determination of physical parameters in the erosion-deposition fluxes

A puzzling fact for the Partheniades-Krone formula is the adoption of very contrasting values for the empirical parameters (e.g., the erosion coefficient and the critical shear stresses) but satisfactory reproduction of the suspended sediment concentration (SSC). Here, a theoretical equation for the tidally-averaged equilibrium (TAE) condition is derived by setting the integrated erosion flux throughout a tidal period equivalent to the integrated deposition flux throughout the same tidal period, leading to a new constraint condition on these parameters if values of the TAE-SSC are available. A preliminary check using the measured data of the Yangtze Estuary shows that a larger critical shear stress for erosion would require a smaller critical shear stress for deposition, both of which must be enhanced if the erosion coefficient further increases. Contrasting sets of parameter values derived by this constraint are fed into a coupled hydro-sediment-morphodynamic model to simulate cohesive sediment transport in the Yangtze Estuary. The results confirm that very contrasting parameter values can indeed lead to satisfactory reproduction of the temporal variations of the SSC (e.g., those in July 2016, including both spring tide and neap tide). This is further demonstrated by the numerical modelling of cohesive sediment transport in an idealized symmetrical estuary. These results indicate that the TAE condition can be used as a constraint for determining the erosion coefficient and the critical shear stresses for erosion and deposition. In addition, two simplified equations of the bed sediment mass conservation (no erosion version and the no deposition version) are derived as complements for obtaining a unique set of the three parameter values, provided that the simultaneous measured time series of SSC, bed shear stress and bed level can be collected at the same gauging station.

Impact of Salinity on the Erosion Threshold, Yield Stress, and Gelatinous State of a Cohesive Clay

AbstractClay is the main component that contributes to sediment cohesiveness. Salinity impacts its transport, which controls the electrochemical force among the sediment grains. Here, we quantify the impacts of salinity on the erosion threshold, yield stress, and the microstructures of a fluorescently labeled smectite clay, laponite, by combining flume experiments, rheometer measurements, and macro‐ and microscopic imaging. We show that the critical shear stress for clay erosion, τb,crit, increases by one order of magnitude with increasing salinity when salinity <1.5 ppt and slightly decreases when salinity >1.5 ppt showing a weaker dependency upon salinity. We further show that the yield stress, τy, of the clay remains roughly a constant at salinity less than 1.5 ppt and decreases by over one order of magnitude at salinity larger than 1.5 ppt. This change in the dependency of τb,crit and yield stress on salinity corresponds to a change in the gelatinous state of clay, from gel‐like structures to phase‐separated structures as salinity increases. Our results provide a quantitative characterization of the dependency of clay erosion threshold and yield stress on salinity and highlight the importance of the clay gelatinous state in controlling clay transport.

Surface sediment erosion characteristics and influencing factors in the subaqueous delta of the abandoned Yellow River Estuary

Estuarine deltas play a critical role in the interaction between the ocean and land and serve as the primary location for the deposition of terrestrial organic carbon and pollutants. However, many deltas are increasingly at risk of erosion due to natural processes and human activities. In particular, abandoned estuarine deltas are extremely prone to erosion in the absence of sediment replenishment. The erosion of sediment in these deltas is crucial in comprehending the evolution of delta erosion as it directly impacts coastal geomorphology, ecological environments, geochemical cycling, and carbon sequestration dynamics. Therefore, determining accurately the erosion characteristics of estuarine deltaic sediments is essential for assessing and managing these evolving coastal systems. Therefore, this study conducted erosion experiments on undisturbed sediment samples collected from the subaqueous delta of the abandoned Yellow River Estuary (Old Qingshuigou Estuary) utilizing the UMCES-Gust Erosion Microcosm System. The aim was to reveal the spatial distribution characteristics of sediment erodibility and explore the erosion process, characteristics, and influencing factors of the surface sediments in the area. Erosion characteristics exhibited a spatial distribution pattern of increased erodibility in nearshore sediment and reduced erodibility in offshore sediments. The critical erosion shear stress of surface sediments ranged from 0.2 to 0.68 N/m2, while the erosion rate increased linearly with an increase in erosion shear stress and reached a maximum value of 2.119 g/m2 s, suggesting that nearshore sediments exhibited strong erodibility. Furthermore, the erosion characteristics exhibited a strong correlation with the type of sediment. Silt exhibited surface erosion and Type I erosion (depth-limited erosion), whereas silty sand primarily underwent volume erosion and Type II erosion (steady-state erosion). Particle size characteristics played a crucial role in influencing erosion. Additionally, our results revealed that the effective particle size of sediment aggregates and the scale effect of turbulence were also important factors that affected sediment erosion characteristics. These findings not only enhance our understanding of the erosion characteristics of silty sediments but also provide valuable parameters for sediment transport models in large river deltas such as the Yellow River Delta, thus facilitating the release of buried organic carbon and heavy metals from estuaries.

Coupling numerical models of deltaic wetlands with AirSWOT, UAVSAR, and AVIRIS-NG remote sensing data

Abstract. Coastal marsh survival relies on the ability to increase elevation and offset sea level rise. It is therefore important to realistically model sediment fluxes between marshes, tidal channels, and bays as sediment availability controls accretion. Traditionally, numerical models have been calibrated and validated using in situ measurements at a few locations within the domain of interest. These datasets typically provide temporal information but lack spatial variability. This paper explores the potential of coupling numerical models with high-resolution remote sensing imagery. Products from three sensors from the NASA Delta-X airborne mission are used. Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) provides vertical water level change on the marshland and was used to adjust the bathymetry and calibrate water fluxes over the marsh. AirSWOT yields water surface elevation within bays, lakes, and channels, and was used to calibrate the Chezy bottom friction coefficient. Finally, imagery from AVIRIS-NG provides maps of total suspended solids (TSS) concentration that were used to calibrate sediment parameters of settling velocity and critical shear stress for erosion. Three numerical models were developed at different locations along coastal Louisiana using Delft3D. The coupling enabled a spatial evaluation of model performance that was not possible using simple point measurements. Overall, the study shows that calibration of numerical models and their general performance will greatly benefit from remote sensing.

Assessing geomorphic and hydraulic changes related to an unplanned reconnection of the middle Mississippi River and its floodplain near Miller City, Illinois, USA

The 2016 Len Small Levee breach and the decision not to repair this breach provides a unique opportunity to study the geomorphic and hydraulic changes associated with floodplain reconnection along a large (~30 km long) meander of the Mississippi River known as Dogtooth Bend. This study evaluates recent geomorphic, hydraulic, and sediment-transport capacity changes related to the breaching of the Len Small Levee and the development of cutoff channels across the neck of Dogtooth Bend. We also assess actions undertaken to mitigate further development of cutoff channels across Dogtooth Bend. To address these goals, we perform geomorphic change detection (GCD) analysis and hydrodynamic modeling to assess the geomorphic and hydraulic changes within both the Mississippi's river-connected floodplain and channel.GCD analysis using digital elevation models (DEMs) of the floodplain from pre (2011) and post (2020) levee breach showed net deposition of 23 million tons (MT) of sediment across the reconnected portion of Dogtooth Bend floodplain alone and an additional 34 MT within the batture (the land between the channel's low-water elevation and the flood-mitigation levee). Converting these deposition masses to deposition rates indicates ~5.7 MT yr−1 of deposition within the river-connected floodplain, which is equivalent to ~6 % of the average annual suspended load (~89 MT yr−1) showing this floodplain area is currently a substantial sink for river-transported sediments.In 2021, the U.S. Army Corps of Engineers constructed a grade-control structure down gradient of the levee breach to inhibit the development of cutoff channels across the neck of Dogtooth Bend. Our modeling indicates the addition of this grade-control structure could decrease peak discharge by up to ~15 % and annual flow volume by half assuming channel conveyance remains similar to 2020 conditions. However, sediment-transport-capacity modeling revealed the decrease in flow across Dogtooth Bend attributed to the grade-control structure did not translate into an increase in sediment-transport capacity within the main channel. Assessment of critical shear stress of erosion for fine-grained cohesive materials across Dogtooth Bend suggests conditions favorable to scouring along the actively forming cutoff channels even with the grade-control structure in place. These observations taken together suggest the current river management efforts may not be sufficient in preventing a meander cutoff over the long-term.

Study on the geomorphological changes of deep troughs under the influence of reclamation in the Caofeidian

Reclamation projects generally change the hydrodynamics and thus affect the topography of the seabed, which is the main threat to the safety of marine engineering. The Caofeidian Reclamation Project, the largest reclamation project in China to date, involved the reclamation of coastal regions of the Hebei Province. However, the project also created several issues and flaws due to the size of the reclaimed area. Through the comparative analysis in the Caofeidian sea area of multi-phase bathymetric topography, shallow stratigraphic changes, and numerical simulation of sediment dynamics, the characteristics of surface sediments and spatial distribution of erodibility were summarized. New marine geological hazards were discovered, such as submarine landslides and erosion depressions. It was concluded that after the reclamation of the Caofeidian, the south of the deep trough was still in an erosion state, and the erosion rate increased slightly. The shear stress formed by strong tidal currents in the deep trough was greater than the critical erosion shear stress of surface sediments, which caused the erosion transport of sediments. It was the primary dynamic mechanism leading to the long-term erosion state of the deep trough.

Enhancement of sewer sediment control and disruption of adhesive gelatinous sediment structure using low-dose calcium peroxide

Large quantities of sediments in urban sewer systems pose significant risk of pipe clogging and corrosion. Owing to their gel-like structure, sewer sediments have strong resistance to hydraulic shear stress. This study proposed a novel approach to weaken the erosion resistance of sewer sediments by destroying viscous gel-like biopolymers in sediments with low doses of calcium peroxide (CaO2). After treatment with 10–50 mg g−1 TS of CaO2, the critical erosion shear stress was significantly reduced by 25.7%–59.9%. The sediment aggregates gradually disintegrated into small diameter particles with increasing CaO2 dosage. Further analysis showed that the strong oxidizing and alkaline environment induced by CaO2 treatment led to cell lysis and changes in the composition and property of extracellular polymeric substances (EPS). After CaO2 treatment, aromatic proteins and humic acid-like substances associated with adhesion translocated from the inner EPS layers to outer layers while being disintegrated into small organic molecules. Concomitantly, CaO2 treatment disrupted the main functional groups (–OH, COO−, C–N, CO, and CN) in inner EPS layers, thus weakening EPS adhesion. Analysis of protein secondary structure and zeta potential reflected the reduced aggregation capacity of sediment microorganisms and loosening of sediment structure after CaO2 treatment. Thus, CaO2 treatment facilitated fragmentation and disaggregation of the gelatinous structure of sewer sediments. Such green strategy decreased the cost of sewer sediment disposal by 42.10–68.95% when compared to water flushing, and it would improve the self-cleaning capacity of sewer system and efficiency of dredging equipment.

Benthic sediment disturbances by episodic human-controlled discharge in an altered estuary

In the Geum River, a representative altered estuary, in-situ mooring and erosion experiments were conducted to reveal the effects of human-controlled discharge on benthic sediment disturbances. The strong discharge through the estuarine dam increased the current velocities within the benthic boundary layer (BBL) up to 79% and 153% during spring and neap tides, respectively. During the discharge period, the suspended sediment concentration in BBL was five times higher than that during the non-discharge period. During the flood phase, a critical shear stress for erosion (τce) was in the range of 0.1278 to 0.1391 Pa. Immediately after the ebb phase with strong discharge, τce increased to 0.1848 Pa, and the erosion rate (E) decreased from 33.73 to 14.81 mg m−2 s−1. The repetition of human-controlled discharge removed the erodible sediments within BBL, exposing the underlying consolidated sediment bed with low E and high τce. The results suggest that an altered estuary is vulnerable to benthic sediment erosion under combined natural and anthropogenic forcings.

Determination of Microplastics' Vertical Concentration Transport (Rouse) Profiles in Flumes.

The transport behavior of microplastics (MPs) in the fluvial environment is scarcely researched. Besides settling velocities and critical shear stress for erosion, only a few investigations aim at MPs' vertical concentration profile and the underlying theory required. Therefore, this paper's experiments investigate vertical concentration profiles of approximately spherical MP particles (d = 1-3 mm) with densities close to water (0.91-1.13 g/cm3) in flow channels, coupling them with fundamental theory for the first time. The experiments were conducted in a tiling flume (slope of 0-2.4%) at 67 and 80 mm water depth, with a turbulent flow, velocities ranging from 0.4 to 1.8 m/s, and turbulence kinetic energy from 0.002 to 0.08 m2/s2. The measured profiles confirm the assumption that the concentration profile shapes of settling plastics are similar to those of sediments and running reversed for buoyant plastics. Furthermore, the hypothesis of the Rouse formula's applicability for floating and sinking plastics could be confirmed for approximately uniform flows. Future studies tying in with this research should increase particle properties and hydraulic parameter variation.

A CSM-Based Procedure for Identifying Segments of Agricultural Drainage Ditches to Be Prioritized in Maintenance Work

Highlights Maintenance of agricultural ditches is important to secure food production. To reduce costs, ditch segments in most need of maintenance should be prioritized. CSM measurements can help prioritize maintenance of agricultural ditch segments. Abstract. Productive agricultural land is vital for food production. To help secure the productivity of drained agricultural soils, diches need to be intermittently maintained. Maintenance work is costly, however, so ditch segments in most need of maintenance should be prioritized. Here, we present a procedure for identifying drainage ditch segments likely to need maintenance based on susceptibility to soil erosion by water flowing in the ditch, evaluated using a cohesive strength meter (CSM). An important part of the procedure is to relate the pressure of the CSM jets to pressures acting on the soil. The relationship between CSM jet pressure and pressures at the soil surface was established based on measurements made with a pressure sensor plate and was applied to obtain several values for pressure at the surface, which were used in turn to estimate critical shear stress for erosion. The results showed that the CSM-based method was able to identify differences in critical shear stress for erosion for different field soils. This information can be useful in identifying drainage ditch segments that should be prioritized for maintenance work. Keywords: Drainage data acquisition, Drainage maintenance, Drainage performance evaluation, Soil erosion.

Measuring and modelling the dispersal of salmon farm organic waste over sandy sediments

Fish farm waste dispersal models are widely used but have only been directly validated to a limited extent. Two shallow (<20 m) Atlantic salmon farms (Bay of Meil and Quanterness) in Orkney, Scotland were studied. Bay of Meil has peak near-bed currents of 9.7 cm s-1 whereas Quanterness has flows up to 31.6 cm s-1. Sediment tray traps which allow resuspension to occur were deployed at each site. The patterns of particulate organic carbon (POC) deposition into the traps were in broad agreement with the observed water current directions and results from infaunal benthic monitoring. Despite the markedly different flow regimes at the 2 sites, most of the deposition occurred within 210 m of the cage perimeters. POC footprints were then modelled using the particle tracking model NewDEPOMOD. For Bay of Meil, a footprint was obtained using the recommended parameter defaults, but the spatial extent was too constrained compared to the sediment tray results. For Quanterness, all simulated particles were lost from the model domain and the critical erosion shear stress had to be increased to unrealistic levels to obtain a footprint. The failure to find a common set of parameter values applicable to both sites, despite their similar depths and sandy seabed, suggests that there remain unresolved issues, likely in how NewDEPOMOD handles waste resuspension. The sediment trays provided a direct method for quantifying the organic carbon deposition, facilitating direct validation of the dispersal model and demonstrating that further research is needed on fish farm waste dispersal at coarser sediment sites.

Threshold of surface erosion of cohesive sediments

Cohesive sediment is ubiquitous in aquatic systems, which often forms fractal aggregates due to cohesive and adhesive forces between particles and is generally eroded as aggregates at low bed shear stresses. The erosion of aggregates plays a significant role in cohesive sediment dynamics. However, the effects of fractal bed aggregation on the erosion threshold of sediment have not been well understood. The incipient motion condition of cohesive sediment is investigated, in which particle aggregation is taken into account by employing the fractal theory and the van der Waals force between particles. A formula for the critical shear stress for surface erosion of cohesive sediments composed of fractal aggregates is developed based on the balance analysis of momentums acting on an aggregate in the bed surface. The developed formula has been successfully applied to different kinds of cohesive sediment. The fractal dimension is found as a function of the solid volume fraction and the diameter of primary particles. The contribution rate of the effective weight of aggregate to the erosion threshold of cohesive sediment is quantified.

Comparison of Critical Shear Stress of Rill Erosion Estimated from Two Methods

Various disputes exist regarding the critical shear stress (τc) of rill erosion determined by linear regression. Alternatively, some researchers have attempted to measure critical shear stress (τo) of rill erosion by observing the start of soil particle detachment and showed promising results. However, few studies have been conducted to evaluate the reliability of the method determining τo. Thus, this study was conducted to identify the reliability of the method determining τo by comparing the values of τc and τo and their relationships with rill erodibility (Kr) and influencing factors, using 360 disturbed soil samples (standing for the freshly tilled condition) from six sampling sites along a 330 km transect in the Loess Plateau. Results indicated that the mean τc was 1.90 times greater than that of τo. No significant relationship was found between τc and Kr. An inverse changing trend between Kr and τo was observed from Yijun to Zizhou, while both Kr and τo exhibited increasing trends from Zizhou to Yulin. The τo of six sampling sites all demonstrated increasing trends with slope gradient (S), which contradicted the statement of the WEPP model that τc was independent of S. The relationships between τc and the measured soil properties were poor, whereas the τo increased exponentially with soil organic matter. Generally, τo did not present significantly better results than τc; the reliability of τo also showed some uncertainties, such as the subjective judgment of the beginning detachment of soil particles. This result has great importance for deepening our understanding of the rill erosion mechanism.

Spatial variability of the erodibility of fine sediments deposited in two alpine gravel-bed rivers: The Isère and Galabre

In mountainous environments, high suspended sediment load during runoff or dam flushing events can lead to important amounts of fine deposits in gravel bed rivers. Fine sediment deposits may contribute to bar elevation, riparian vegetation growth and consequently to bar stabilization. Despite their contribution to the morphodynamic of mountain rivers, the erosion properties of fine sediments in this context is not fully understood.In order to investigate the dynamics of re-suspension of these deposits, field monitoring campaigns were performed to explore both the spatial variability and the controlling factors of the erodibility of fine deposits. A cohesive strength-meter (CSM), along with moisture, grain sizes, geographical position and elevation were used to evaluate both the critical bed shear stress for erosion and erosion rate of fine sediment deposits in two rivers of the French Alps: the Isère and Galabre.The results highlight a large variety of fine sediment deposition areas, which are discontinuous compared to those in estuaries and lowland rivers. A high spatial variability of erodibility was observed on the reach, the bar and the metric scale. While no upstream–downstream trend was observed at the scale of both studied reaches, the locations of the deposits, elevation from the river surface and their moisture were inter-related variables and with the highest correlations to erodibility. Measurements showed that both dry and humid deposits located at the highest and lowest elevation from the river surface respectively, were more easily eroded than intermediate deposits with medium moisture.

Review of a Semi-Empirical Modelling Approach for Cohesive Sediment Transport in River Systems

In this paper, a review of a semi-empirical modelling approach for cohesive sediment transport in river systems is presented. The mathematical modelling of cohesive sediment transport is a challenge because of the number of governing parameters controlling the various transport processes involved in cohesive sediment, and hence a semi-empirical approach is a viable option. A semi-empirical model of cohesive sediment called the RIVFLOC model developed by Krishnappan is reviewed and the model parameters that need to be determined using a rotating circular flume are highlighted. The parameters that were determined using a rotating circular flume during the application of the RIVFLOC model to different river systems include the critical shear stress for erosion of the cohesive sediment, critical shear stress for deposition according to the definition of Partheniades, critical shear stress for deposition according to the definition of Krone, the cohesion parameter governing the flocculation of cohesive sediment and a set of empirical parameters that define the density of the floc in terms of the size of the flocs. An examination of the variability of these parameters shows the need for testing site-specific sediments using a rotating circular flume to achieve a reliable prediction of the RIVFLOC model. Application of the model to various river systems has highlighted the need for including the entrapment process in a cohesive sediment transport model.

Influence Mechanism of Geomorphological Evolution in a Tidal Lagoon with Rising Sea Level

A tidal lagoon system has multiple environmental, societal, and economic implications. To investigate the mechanism of influence of the geomorphological evolution of a tidal lagoon, the effect of critical erosion shear stress, critical deposition shear stress, sediment settling velocity, and initial bed elevation were assessed by applying the MIKE hydro- and morpho-dynamic model to a typical tidal lagoon, Qilihai Lagoon. According to the simulation results, without sediment supply, an increase of critical erosion, deposition shear stress, or sediment settling velocity gives rise to tidal networks with a stable terrain. Such an equilibrium state can be defined as when the change of net erosion has little variation, which can be achieved due to counter actions between the erosion and deposition effect. Moreover, the influence of the initial bed elevation depends on the lowest tidal level. When the initial bed elevation is below the lowest tidal level, the tidal networks tend to be fully developed. A Spearman correlation analysis indicated that the geomorphological evolution is more sensitive to critical erosion or deposition shear stress than sediment settling velocity and initial bed elevation. Exponential sea level rise contributes to more intensive erosion than the linear or the parabolic sea level rise in the long-term evolution of a tidal lagoon.

Effect of potassium ferrate treatment on adhesive gelatinous biopolymer structure and erosion resistance of sewer sediments: Promotion or inhibition?

Massive presence of sediments not only results in decreasing drainage capacity but also causes a series of severe environmental problems. Gelatinous biopolymer adhesive properties in sediments will probably affect the flushing and pipeline self-cleansing efficiency. Therefore, exploring the molecular mechanism of biopolymer enhancing anti-erosion from the seldom-concerned perspective of reducing adhesion is an important approach to subvert the traditional method of controlling sediments. Through the gradient disintegration of continuous flow sewer reactors cultivated sediments with environmentally-friendly oxidant potassium ferrate (PF) in short-term treatment, this study evaluated its potential applications and explored the molecular structure mechanisms mentioned above. The results showed that the sediments erosion capability was improved to 4 to 20 times than untreated sediments after different PF dosage treatment. The corresponding critical erosion shear stress of sediments also decreased though PF treatment. Gelatinous biopolymer did contribute adhesion of inorganic particles through the first atomic force microscope (AFM) application in sediments, and such adhesion promoted anti-erosion. Furthermore, the average adhesion could be weakened by up to 93.5 % under PF treatment. HA (humic acid), protein-like tyrosine and protein-like tryptophan exhibited strong correlation with adhesion. –CH3/–CH2, COO−, O–H, N–H and other functional groups were inextricably related to the adhesion properties through two-dimensional correlation spectroscopy (2D-COS) analysis. Aggregated strands, α-Helix and β-Turn exhibited significant correlation with adhesion. However, PF could destroy these structures and transfer small and medium molecules to external, enhancing electronegativity to cripple gelatinous agglomeration. The destruction of the gelatinous biopolymer structure led to weaken adhesion and further deteriorate anti-erosion of sediments. Application of PF contributed to enhance drainage pipeline self-cleansing capacity or improve flushing device efficiency to substantially decrease the controlling cost by at least 51%.

Seasonal particle responses to near‐bed shear stress in a shallow, wave‐ and current‐driven environment

AbstractNovel analysis of in situ acoustic and optical data collected in a shallow, wave‐ and current‐driven environment enabled determination of (1) particle characteristics that were most affected by near‐bed physical forcing over seasonal scales and (2) characteristic shear stress, τchar, at which the rate of change to particle characteristics was most pronounced. Near‐bed forcing and particle responses varied by season. Results indicated that moderate τchar values of 0.125 Pa drove changes in particle composition during summer. In winter, particle concentration effects were most affected at τchar of 0.05 Pa, suggesting dominance of fluff layer resuspension. Changes to particle size were most relevant during a biologically productive springtime period, with initiation of particle disaggregation occurring most commonly at τchar of 0.25 Pa. These results suggest that it may be more important to parameterize τchar, as opposed to critical shear stress for erosion, for sediment transport models.

Critical Shear Stress for Erosion of Sand-Mud Mixtures and Pure Mud

The erosion threshold of sand-mud mixtures is investigated by analyzing the momentum balance of a sand particle or a mud parcel in the mixture bed surface, and a formula for the critical shear stress of sand-mud mixtures is developed, which also applies for pure sand and mud. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture. The developed formula reproduces well the variation of the critical shear stress of sand-mud mixtures over mud content and can predict the critical shear stress of both sand-mud mixtures and pure mud in the process of consolidation. The developed formula promises to be convenient for application by relating the critical shear stress to mud content and the dry bulk density of sediment.

Deposition and erosion behaviour of cohesive sediments in the upper River Taw observatory, southwest UK: Implications for management and modelling

Fine sediment is a key driver of water quality degradation in rivers globally and improved knowledge of its source and transport is required to assess the efficacy of sediment control options and pollution abatement strategies. Here the transport characteristics (deposition, erosion and flocculation) of fine sediment were studied experimentally in an annular flume to provide parameters necessary to model fine sediment transport in the upper River Taw in southwest England. Deposition experiments were carried out using a sediment–water mixture in the flume and in-situ size distributions of sediment flocs were measured to assess flocculation and settling behavior of sediment. The influence of bed shear stress and bed consolidation were investigated in a series of erosion experiments. These data were used to calibrate and test a mathematical model of fine sediment transport in annular flumes (FLUME model) and the measured and modelled data were in good agreement. Transport parameters for cohesive sediments, such as the critical shear stresses for deposition and erosion as well as the particle density, porosity and settling velocity as a function of floc size were established. The density and settling velocity of the cohesive sediments were sensitive to the biological activity within the floc structure. Data from the flume experiments are necessary as input parameters to models such as RIVFLOC or SHETRAN that simulate fine sediment transport in the River Taw at the catchment scale and assess the efficacy of sediment control options and catchment management scenarios