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Depositional and erosional behaviour of cohesive sediments in estuarine environments
No full textCohesive muddy sediments are abundant across estuarine ecosystems and these sediments are typically composed of mineral grains and biological matter. The erosional characteristics of such sediments are impacted by micro-organisms providing bio-stabilisation. Deposition may be impacted by biochemical composition and turbulence properties, in turn influencing the flocculation of suspended particulate matter. Such flocculation processes impact settling velocity, porosity and density characteristics. Interactive processes between sedimentary dynamics and hydrodynamics are influenced by the various bio-physical attributes of these cohesive sediments, thus prediction of muddy sediment transport processes via numerical modelling is beneficial to improve the understanding of intertidal sediment dynamics. The accuracy of such models relies on quantitative erosional and depositional data for calibration.
Sediment erodibility in intertidal habitats was investigated utilising sediment stability data collected during the CBESS (Coastal Biodiversity and Ecosystem Service Sustainability) programme. This project collected landscape-scale field measurements for biodiversity and ecosystem service flows in coastal wetlands to quantify patterns between ecosystem service provision and biodiversity (Paterson et al., 2015). An additional case study on saltmarsh sediment stability was conducted using geotechnical samples from Tillingham Marsh, Essex (UK), investigating composition and stability variability of open coast marsh substrates (Brooks, 2021). The studies investigate within- and between-marsh variation in sediment erosion thresholds from a cohesive strength meter, including various sediment compositions for comparison (clay, silt, sand composition; colloidal carbohydrate concentration). The aim of this chapter is to provide a state-of-the-art review of key research on intertidal sediment dynamics, including sediment stability and erodibility (erosion thresholds), drawing on case studies and key research contributions for quantitative analysis. This presentation includes a background and literature review of such topics in intertidal habitats, drawing from key case studies using CBESS and geotechnical sediment sample data sets
Monitoring and modelling of sediment flushing: a review
No full textWith ever decreasing potential for suitable new dam sites, sustainable use of existing water reservoirs is of paramount importance. In absence of appropriate measures, reservoir storage is continually reduced due to sedimentation. One option to remove sediment deposits is hydraulic flushing. During the flushing operation, bottom outlets are open and water and sediment released. Whether flushing successfully removes sediment depends on a number of factors, such as bottom outlets capacity, reservoir shape and water availability. Modelling is often used to assess viability of flushing for sediment management in the reservoir, as well as to design the operations and optimize their scheduling. One-dimensional numerical models are preferred for long term simulations, assessments on of a large number of scenarios, and optimization studies. Two- and three-dimensional numerical models and physical models can be used, each on their own or in combination as hybrid models, to understand local scouring near the gates and other details of operation. Monitoring of flushing operations can help improving their efficiency while at the same time limit downstream impacts. General monitoring of the reservoir and its catchment can help understanding the sedimentation problem and thus facilitate preparation of efficient sediment management strategies. Live monitoring of sediment concentrations is possible with modern equipment though not without challenges, and reservoir survey can be performed faster. Earth observation techniques are also an attractive option, allowing to monitor large areas and areas of difficult access, as well as to provide historical information going back several decades. This paper reviews monitoring and modelling approaches published in the literature, as well as presents some previously unpublished analyses
Persistent reshaping of cohesive sediment towards stable flocs by turbulence
No full textCohesive sediment forms flocs of various sizes and structures in the natural turbulent environment. Understanding flocculation is critical in accurately predicting sediment transport and biogeochemical cycles. In addition to aggregation and breakup, turbulence also reshapes flocs toward more stable structures. An Eulerian–Lagrangian framework has been implemented to investigate the effect of turbulence on flocculation by capturing the time-evolution of individual flocs. We have identified two floc reshaping mechanisms, namely breakage-regrowth and restructuring by hydrodynamic drag. Surface erosion is found to be the primary breakup mechanism for strong flocs, while fragile flocs tend to split into fragments of similar sizes. Aggregation of flocs of sizes comparable to or greater than the Kolmogorov scale is modulated by turbulence with lower aggregation efficiency. Our findings highlight the limiting effects of turbulence on both floc size and structure
Hybrid PD-DEM approach for modeling surface erosion by particles impact
No full textPeridynamics (PD) theory is a promising technique for modeling solids with discontinuities. Short-range repulsive force models are commonly employed in PD impact event simulations. Despite their extensive usage, short-range force models do not take damping, friction, and tangential force components into account and hence are unable to effectively describe energy dissipation, leading to uncertainty in the calculation of contact forces. However, the accuracy of impact simulations using alternate contact models has not been extensively investigated in the context of PD impact simulations. The Discrete Element Method (DEM) has been proven to be the most reliable and effective approach to model collision processes between distinct solid objects. This work presents, a particle-based hybrid PD-DEM model to accurately predict the particle impact forces and the resulting damage to the target material. The present model brings together the unique capabilities of PD and DEM and has the potential to make use of the various DEM contact laws, which allow the development and adjustment of relevant contact forces in the normal and tangential directions. Furthermore, damping effects, friction, and intra-particle stiffness are incorporated into the simulations through DEM. The proposed method has been used for modeling material failure after being validated and verified for the contact parameters during the impact process. The predicted damage patterns and resulting material loss demonstrate good agreement with the experimental results reported in the literature
Flocs as vectors for microplastics in the aquatic environment
No full textFlocculation occurs as water turbulence promotes particle-particle collisions with flocs/aggregates increasing in size and changing hydrodynamic behaviors of incorporated suspended particulate matter (SPM). Microplastics (MPs) have become an increasingly important component of SPM in the aquatic environment, and their fate and behavior are significantly influenced by flocculation. Therefore, understanding and quantifying the flocculation of MPs in multiple aquatic systems is a critical step to better predicting the fate and transport of MPs. However, this is challenging given the wide-ranging physical and chemical properties of both MPs and the aquatic environment.
Here, we use a meta-analysis of existing data which included 2580 flocs and incorporated MPs from different aquatic environmental settings (marine, estuary, freshwater and engineered water), and an experimental approach that investigated the incorporation of MPs with ranging properties into more than 4000 flocs exploring the flocculation dynamics of MPs. A clear relationship between the floc size and the incorporated MP size was found, even with high complexities in both MP properties and environmental conditions. An empirical mathematical model was fitted based on this size relationship to classify MPs into two groups: 1) incorporated MPsfloc; and 2) not incorporated MPsnon-floc. This can then aid in modelling the fate and behavior of MPs in various aquatic environments and also contributes to a new classification of MPs into small (100 µm)
Biophysical flocculation reduces variability of cohesive sediment settling velocity
No full textBiophysical cohesion, introduced predominantly by Extracellular Polymeric Substances (EPS) during mineral flocculation in subaqueous environments, plays important role in morphodynamics, biogeochemical cycles and ecosystem processes. However, the mechanism of how EPS functioning with cohesive particles and affects settling behaviors remain poorly understood. We measure initial flocculation rate, floc size and settling velocity of mineral and artificial EPS (Xanthan gum) mixtures. Combining results from these and previous studies demonstrate coherent intensification of EPS-related flocculation compare with those of pure mineral and oil-mineral mixtures. Importantly, the presence of EPS fundamentally changes floc structure and reduces variability of settling velocity. Measured data shows that ratios of microfloc and macrofloc settling velocity for pure mineral flocs is 3.9 but greatly reduced to a lowest value of 1.6 due to biological EPS addition. The low variability of settling velocity due to EPS participation explains the seemingly inconsistent results previously observed between field and laboratory studies
Modelling fine scale route choice of upstream migrating fish as they approach an instream structure
No full textThis study used pattern-oriented modelling (POM) to investigate the space use and behavioural response of upstream migrating European river lamprey (Lampetra fluviatilis) to the two-dimensional hydrodynamic conditions created by an instream structure (triangular profile gauging weir). Passive Integrated Transponder (PIT) and acoustic telemetry were used to map the spatial-temporal distribution patterns of lamprey as they migrated upstream. Acoustic Doppler velocimetry and computer modelling were used to quantify the hydrodynamic environment. In adherence with the POM methodology, multiple movement models, incorporating increasingly complex environmental feedback mechanisms and behavioural rules were created and systematically assessed to identify which factors might reproduce the observed patterns. The best model was a spatially explicit Eulerian-Lagrangian Individual Based Model (IBM) that included two simple behaviours: 1) tortuous non-directed swimming when in low flow velocity (< 0.1 m s−1) and 2) persistent directed (against the flow) swimming in moderate to high flow velocity (≥ 0.1 m s−1). The POM indicated that flow heterogeneity was an important influence of lamprey space use and that simple behavioural rules (i.e. two separate movement behaviours in response to flow velocity) were sufficient to reproduce the main movement pattern observed: avoidance of flow recirculating regions near the banks. The combination of field telemetry, hydrodynamic modelling and POM provided a useful framework for systematically identifying the key factors (hydrodynamic and behavioural) that governed the space use of the target species and would likely work well for investigating similar relationships in other aquatic species
A dynamic 2DH flocculation model for coastal domains
No full textA dynamic two-dimensional depth-averaged (2DH) parameterization for flocculation of cohesive sediments is proposed based on the kinetic model by Winterwerp (J Hydraul Res 36:309–326, 1998). The aim is to achieve a realistic representation of the suspended sediment field by accounting for flocculation, also taking into consideration its dependence on advection, turbulent diffusion, and turbulent shear. This formulation is evaluated in a sand-mud model of the Belgian Coast and the Western Scheldt. Results indicate that it can reproduce known sediment transport patterns: modelled floc size and suspended sediment concentrations are in the range of measurements. When evaluating the model results spatially, the extent and shape of the coastal sediment plumes are similar to the observed suspended particle matter (SPM) maps from the PROBA-V satellite. Therefore, the use of the presently proposed flocculation model has added value to improve sediment transport calculations in coastal areas
A system for in-situ, wave-by-wave measurements of the speed and volume of coastal overtopping
No full textWave overtopping of sea defences poses a hazard to people and infrastructure. Rising sea levels and limited resources mean accurate prediction tools are needed to deliver cost-effective shoreline management plans. A dearth of in-situ data means that the numerical tools used for flood forecasting and coastal scheme design are based largely on data from idealised flume studies, and the resulting overtopping predictions may have orders of magnitude uncertainty for complicated structures and some environmental conditions. Furthermore, such studies usually only provide data on the total volume of overtopping water, and no data on the speed of the water. Here we present WireWall, an array of capacitance-based sensors which measure the speed and volume of overtopping water on a wave-by-wave basis. We describe the successful validation of WireWall against traditional flume methods and present results from the first trial deployments at a sea wall in the UK. WireWall results are also compared with numerical predictions based on EurOtop guidance. WireWall technology offers an approach for reliable acquisition of the data needed to develop resilient coastal protections schemes
Geospatial techniques applied to the Texas regional sediment budget modeling study
No full textHR Wallingford and Freese & Nichols have been working for the Texas General Land Office (TGLO) to develop sand transport models for the entire Texas coast in support of the Texas Coastal Resiliency Masterplan. The overarching aim of the project is to refine our understanding of sand transport pathways and develop sediment budgets for the gulf facing beaches of the Texas Coast. Sediment budgets for Regions 1 and 4 have been completed and the associated technical reports can be downloaded from the TGLO website. This presentation will provide an overview of the Regions 1 and 4 project, the datasets developed and how these are being made available to the wider coastal community to assist with the early planning stages of future coastal projects along the Texas Coast.
One of the first tasks of the sediment budget project was to generate a long term timeseries of nearshore wave data along the coast. This resulted in nearshore wave climates at 67 locations along the coast spanning the period 1979 to 2019. These were used to calculate seasonal and interannual variability in longshore transport potential and to identify a representative year for the regional modeling.
Nearshore bathymetry used in the regional model was based upon the recently updated ADCIRC model used by the USACE for the Coastal Texas Study and has been supplemented offshore with GEBCO data to the model boundary. After calibration of the wave and flow models, sediment grain size data from past projects and online databases were used to develop sediment characteristic maps for the sediment transport model which was run, fully coupled with the wave and flow model for the representative year.
Bathymetric surveys and longer term dredge records were used as verification targets to confirm that the sediment transport model was predicting reliable shoaling quantities in the navigation channels. Net and gross sand transport fluxes were then calculated from model results in order to compute the resulting sediment budget as well as annual and monthly curves which show the cross-shore distribution of longshore transport at 67 transect locations along the coast. These transport curves will be very useful for planning any coastal work in these areas as the study found that the transport has a northwards or southwards direction depending on the time of the year, the position alongshore, and the position across-shore.
The following datasets from the study will be made available on the GLO’s updated Texas Coastal Sediments Geodatabase (TxSed) geospatial viewer to support other coastal projects in the Region.
Sediment budget shapefiles and corresponding annual sand transport fluxes; Littoral depth of closure points; Nearshore wave timeseries from 1979 to 2019; Bathymetry data
• Sediment characteristic maps; Annual sand transport curves accompanied by the net, gross and cumulative transport at 10- yard intervals; Monthly sand transport curves accompanied by the net, gross and cumulative transport at 10- yard intervals