A Modeling framework for flocculated cohesive sediment transport in the current bottom boundary layer

Cohesive sediment transport, where its settling velocity is controlled by the flocculation process, is a crucial component in determining biochemical cycles, fate of pollutants, and morphodynamics in many aquatic ecosystems. In this study, a modeling framework is presented to investigate how flocculation influences cohesive sediment transport in the current bottom boundary layer in dilute conditions, consistent with the calibration range of the flocculation model. From a local analysis of floc dynamics in homogenous turbulence, we identify that the floc size distribution is mainly controlled by floc cohesion and yield strength. The uncertainty in fractal dimension plays a minor role for the floc size but it influences the resulting floc density and settling velocity. The transport analysis in the current boundary layer shows that the flocculation process alters the vertical distribution of the settling velocity and hence the sediment concentration with a strong dependence on cohesion, floc yield strength, and floc structure. When the flocs are more susceptible to breaking, a well-mixed concentration profile is obtained. In contrast, for flocs with higher cohesion or yield strength, higher concentration with a sharp gradient is observed close to the bed. Overall, the settling velocity exhibits a low vertical variability within 20 % of the depth-averaged value except near the bed. This suggests that using a depth-averaged settling velocity yields acceptable predictions of the sediment concentration profiles, especially for flocs with lower cohesion

Comparative analysis of floc measurement setups for characterising settling velocities and size distributions

Floc size distribution and settling velocities are crucial parameters for characterising cohesive sediments, as they influence how these sediments behave in various environmental settings. The accurate measurement of these properties is essential, with different methods available depending on the scope of the study. For long-term monitoring, in situ techniques based on laser diffraction are commonly used, while video microscopy techniques are preferred for shorter studies due to their ability to provide detailed information on individual particles. This study compares two high-magnification digital video camera setups, LabSFLOC-2 and FLOCCAM, to investigate the impact of particle concentration on settling velocity in flocculated sediments. Flocculated clay was introduced into settling columns, where both the size and settling velocities of the flocs were measured. The results obtained from both setups are in line with each other, even though the FLOCCAM was slightly more efficient at capturing images of small particles (of size less than 50 microns) and LabsFLOC-2 was better at detecting large size fraction particles (having a low contrast due to the presence of organic matter). Floc size and settling velocity measurements from both setups however exhibit mostly similar trends as a function of clay concentration and the same order of magnitudes for the recorded settling velocities

Time evolution models for scour burial of isolated objects on a granular seabed

UneXploded Ordnances (UXOs) and Discarded Military Munitions (DMMs) frequently appear in coastal and offshore regions, representing a threat for maritime engineering works and for the public. Hydrodynamic and morphodynamic forcings can cause these objects to bury and/or mobilise, making their detection challenging. Hence there is a need for reliable approaches predicting burial and mobilisation of UXOs to support the risk assessment of contaminated sites. The present work proposes two models to predict the time evolution scour burial of isolated objects on granular soils. The first model is referred to as DRAMBUIE 3.0. DRAMBUIE 3.0 couples recent empirical equations for the scour burial equilibrium depth with a time stepping approach. A new Artificial Neural Network (ANN) predictor for equilibrium burial depth was also developed and coupled with a time evolution model, making DRAMBUIE-ANN. Both models have been compared with small-scale and large-scale experiments, and with field measurements. Despite some discrepancies between the observations and predictions, both models showed the capability to predict UXO burial for a range of objects and hydrodynamic conditions. Future work should focus on further validating the models and extending their range of applicability. DRAMBUIE 3.0 and DRAMBUIE-ANN can be used to support site manager decisions for the remediation of UXO and DMM-contaminated sites

Shielding effects of neighbor particles on flocculation dynamics of cohesive sediment

The shielding effects of neighboring particles on the flocculation dynamics of cohesive sediment in homogeneous isotropic turbulence is investigated using a two-phase particle-unresolved, but turbulence-resolved, Euler–Lagrange simulations. A coupled CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) framework was applied, in which the discrete element method model captures collisional interactions among particles. The high-resolution grid used in the CFD resolves all the turbulent scales. The primary particles are substantially smaller than the Kolmogorov length scale, therefore, flow around particles is not resolved and the fluid–particle interactions are modeled by force models. The present work employs the semiempirical force model of Kim and Lee (KL), in which the multibody interactions between the particles that makeup a floc are modeled as functions of pairwise interactions among particles. In comparison, the widely used free-draining approximation (FDA) uses Stokes drag of individual particles and completely ignores all inter-particle interactions within the floc. Most importantly, we observe that by allowing more accurate hydrodynamic interactions among the fractal floc members, the KL method predicts much larger flocs at equilibrium. By including the intra-floc shielding effects, the KL model predicts the floc settling velocity to substantially increase with floc size, in contrast to the FDA model. The aggregation and breakup kernels follow qualitatively similar patterns for both the FDA and KL models. For future work, a computationally efficient and accurate force model for fractal floc shapes needs to be developed for better predictions of the flocculation processes of cohesive sediment

A critical review of closure depth theories and uncertainties: implications for shoreline modelling and coastal management

The sustainability of coastal systems is being increasingly compromised as a result of climate-related coastal hazards and increasing human occupation of coastal zones. Shoreline models play an important role in predicting and understanding coastal systems behaviour, informing coastal resilience and adaptation strategies. A critical parameter in these models is the depth of closure ( ), which defines the seaward extent of significant cross-shore sediment transport and shoreline morphodynamics. There are considerable uncertainties associated with estimating and identifying the , which has implications for the accuracy of shoreline predictions and ensuing coastal management decisions. We, therefore, provide a critical literature review of existing theories and methods for defining and estimating the , highlighting the complexities, uncertainties, and challenges. We also explore the role of the in shoreline models, paying particular attention to their applicability across variations in timescale and coastal environments while considering the associated implications for coastal management decisions. Our findings highlight the need for standardised estimation methods and a better understanding of the to improve the reliability and applicability of shoreline models across coastal morphologies and tidal environments. Our findings also emphasise the need for a paradigm shift in practice – from continuing to develop and apply flawed shoreline models to addressing the uncertainties underpinning the formulation and specification of key model parameters, of which the is arguably the most critical. This shift is needed to enhance the predictive power and reliability of shoreline models, to better inform decision-making for coastal management and governance

Creating and testing an approach for upscaling climate services

Upscaling is the process of moving beyond pilots or prototypes to repeatable and/or transferable services that are accessible and useful to stakeholders and users. It may also refer to increasing the provision, reach, or impact of an existing service. Upscaling any service or product is a complex process, which – in the case of climate services – is exacerbated by serving an emerging market, and many projects taking place in academic settings with short term funding cycles. Climate service providers, their delivery partners and funders could benefit from increased reach and impact by explicitly considering how their services can scale and what the enablers and barriers to this may be. This could take the form of reviewing academic literature, applying structured frameworks, or learning from best practice examples. Here, we describe the process of creating, testing, and refining an upscaling approach for climate services. The resulting approach is presented, alongside case studies that helped update it and provide evidence for its usefulness and useability. This detailed study of upscaling climate services sets the foundation for effective and sustainable provision of climate services beyond pilots and prototypes and further development of upscaling frameworks and tools to this end

Hindcast Modeling of Morphodynamic Changes and UXO Burial Caused by Hurricane Matthew 2016, Fort Pierce, Florida

We simulate the burial and exposure of potential UneXploded Ordnance (UXO) during Hurricane Matthew 2016 on the Fort Pierce Naval Amphibious Training Base, Florida (USA). We used a large-scale model to simulate currents, waves, and morphodynamics, and coupled it to DRAMBUIE 3.0, a newly developed UXO burial model, to predict the vertical movement of UXOs in the near shore zone at depths between the depth of closure and the shoreline, considering both a gently sloping beach and a barred beach profile. The effects of Matthew took place mostly in a short window of 10 h before and after the passage of the storm. Within this window, large variability occurred in the significant wave height (from 1 to 4.8 m), near bed turbulence (KC numbers from 20 to 83) and sediment fluxes (up to 2 kg/m/s). Local bedforms influenced the evolution of waves from offshore to the beach, and these bedforms migrated offshore as their stoss side accreted and their lee side was eroded. In the model, exemplar UXOs were placed on the seabed at depths between 3 and 10 m below mean sea level. The model showed that the slope of the beach profiles and in particular the presence of a submerged bar affect the burial or exposure of the UXO. UXOs located in the breaker zone and on a submerged bar were buried the deepest (0.2 m) after the passage of the storm (more than the exemplar reference munitions diameter, 0.155 m). UXOs along the rest of the model domain finished at shallower burial depths, roughly equivalent to the objects’ diameter. The results of the research identified areas where UXOs are most likely to be buried after storms, which will help coastal managers to more efficiently clean up and make safe the near shore seabed

Evaluating the role of draghead positioning in dredging performance using 3D CFD, mud sampling, and rheometry

This paper presents a framework for the modelling of dredging operations, by developing high-fidelity CFD simulations and recognising the highly complex rheology of mud. The goal is to understand the impact of draghead position on the dredging efficiency. The mud sampling procedure is described, followed by a detailed account of the rheometry experiments, rheological modelling, and statistical data reduction methods. In this work mud samples are obtained from 5 different locations for up to 4 different depths in Harwich Harbour in the United Kingdom. Various analyses are performed including analysis of constituents (sand, silt and organic matter) and bulk density. Rheometry tests are performed and after examining the existing empirical models a dual Bingham and Herschel-Bulkley model was chosen which fits the data well and accurately captures the observed mud’s flow behaviour. In addition, the dual Bingham and Herschel-Bulkley model is implemented in the OpenFOAM open-source CFD framework and thoroughly validated by simulating the rheometer cell and comparing the calculated torque directly with the experimental data. Then, large-scale CFD simulations are performed to investigate the flux of different mud layers as a function of the draghead operating depth, employing the dual Bingham and Herschel-Bulkley model. The assessment of various dredging strategies based on draghead depth is presented through analysis of CFD mud layer suction flux data for a stationary draghead. Based on these results, recommendations are made regarding the optimal operating depth relative to different mud layers, considering both economic and environmental factors, including fuel consumption

Enhancing understanding of breach processes using computational fluid dynamics and laboratory tests

Accurate prediction of dam breach processes is crucial for risk assessment and emergency response planning. Computational Fluid Dynamics (CFD) offers a powerful tool for simulating such complex phenomena. However, the reliability of CFD models hinges on their validation against experimental data. This paper compares numerical simulations by the open-source code, OpenFOAM, to laboratory flume experiments to assess the accuracy of its predictions of dam breach overtopping processes. OpenFOAM is first being used to model the flow characteristics of two key stages of a dam breach experiment that took place in a physical laboratory setup. The two stages are the surface erosion stage and the steep faced / stepped erosion stage. The flume dimensions, bed profiles and flow conditions are replicated with the CFD numerical models. The free water surface is modelled with the Volume of Fluid (VOF) method while the flow is modelled using the Reynolds-Averaged Navier Stokes (RANS) equations. CFD flow velocity results are validated against the flume measurements. A second idealised case is also presented to show the effect of mesh resolution on bed shear stress distribution. The findings of this study provide insights into the capabilities and limitations of CFD models for predicting dam breach flows. This research contributes to the development of more accurate and reliable tools for assessing the potential impacts of dam failures. This work is ongoing, and results presented here are preliminary

Tsunami boulder transport in coastal environments: insights from physical experiments and dimensional analysis

Coastal boulder deposits hold the potential to aid in the reconstruction of past extreme wave events. However, commonly used hydrodynamic equations for calculating wave heights from transported boulders can be inaccurate. New and alternative methods need to be explored in an interdisciplinary way to ensure a more complete picture of the phenomenon of boulder transport is achieved. Through the use of a physical experiment, this study aims to investigate the influence of different tsunami wave types, wave parameters and boulder shapes on boulder transport distance. The experimental results also allow for a novel application of dimensional analysis to enable comparisons with other experiments as well as a field case study. In the experiment an elongate irregularly shaped boulder showed transport distances up to 1 m farther than a cuboid shaped boulder under the influence of the same waves. The irregularly shaped boulder had a predominant transport mode of rolling, whereas the cuboid shaped boulder predominantly underwent sliding transport. Tsunami wave type also influenced boulder transport distances, with N-waves frequently showing greater transport than E-waves of a comparable wave steepness. Key offshore wave and boulder parameters were then compared through dimensional analysis using Buckingham's Pi Theorem, enabling comparisons to other datasets to be made. Data from another published experimental study and a field study in Settai, Japan, showed reasonable agreement, particularly for the shorter period field data. These findings emphasize the importance of incorporating boulder shape, wave type, and dimensional analysis into future studies, providing a foundation for more accurate reconstructions of past tsunami events