1642 research outputs found
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Risk-based approach for scour susceptibility for offshore wind farms
No full textThis paper presents a methodology for the application of erosion test results for scour prediction. Erosion tests on field samples were performed to understand the erodibility and erosion rates of the soil samples. The soil characteristics that affect susceptibility to erosion include: Particle Size Distribution (D50) and Fines Content. These are employed alongside the normalised Soil Behaviour Type (SBTn) expressed by the index Ic, derived from Cone Penetration Tests (CPT), correlated to the erosion test results, to define parameters where the onset of erosion is observed. Threshold values of the D50, Fines Content, percentage of clay and the Ic values are determined based on the erosion test results to inform the likelihood of scour occurrence in-field. These threshold parameters were then assigned weight factors based on their criticality and applied on soil samples acquired from a field in the North Sea to assess the likelihood of scour susceptibility and its duration. Based on the results, a risk-based approach was developed and implemented to plan and deploy scour protection around the foundations of an Offshore Wind Farm in the North Sea following foundation installation
A Modeling framework for flocculated cohesive sediment transport in the current bottom boundary layer
No full textCohesive 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
Enhancing understanding of breach processes using computational fluid dynamics and laboratory tests
No full textAccurate 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
Breach modeling flume tests
No full textFrom previous flume tests conducted in the framework of the OVERCOME Project, intended to investigate the effect of different soil materials, overflow discharges, and upstream face watertightness on the breaching processes of overflowed levee sections, we determined the need for further investigation on the effect of test repeatability, content of fines and armouring caused by the coarser particles. This document presents the results of this new set of tests which have shown an impact of these variables on the breach macro erosion processes, erosion rates and breach dimensions
How plausible are severe and extreme climate model droughts? A water supply perspective
No full textThe plausibility of severe and extreme droughts outside the range of those experienced historically is critical to adapting water supply systems to climate change. Current planning practices in England and Wales take stochastically generated droughts, perturbed by mean monthly change factors from climate models to create these droughts, however there are notable drawbacks to this approach, including the plausibility of these events. Moving beyond change factors, what information from climate models and how much of it to include within climate change assessments, is integral to improving climate change assessments. The aim of this study is to quantify the plausibility of severe and extreme water supply droughts within climate models using weather types, using a system-led approach. A stochastic weather generator is developed using a Markov process, trained on the historical Met Office weather types, with sequences run through hydrological and water supply system models to determine the weather type sequences that result in severe and extreme water supply droughts. The difference between transitional probabilities in the historical sequence and the generated sequences is used as a measure of the plausibility of the generated sequences. The resulting envelope is then compared with the weather patterns contained within the United Kingdom Climate Projections 2018 Global Circulation Model ensemble to understand the plausibility and severity of the droughts within current and future time periods, generated by climate models. The results of this study could be used to improve climate change risk and vulnerability assessments, aid in adaptive planning, within bottom-up climate change assessments, and in screening climate models for inclusion within assessments, based on the pressure fields within the models, rather than simply the climate variables alone
A dual-horizon peridynamics–discrete element method framework for efficient short-range contact mechanics
No full textShort-range forces enable peridynamics to simulate impact, yet it demands a computationally expensive contact search and includes no intrinsic damping. A significantly more efficient solution is the coupled dual-horizon peridynamics–discrete element method approach, which provides a robust framework for modeling fracture. The peridynamics component handles the nonlocal continuum mechanics capabilities to predict material damage and fracture, while the discrete element method captures discrete particle behavior. Whereas existing peridynamics–discrete element method approaches assign discrete element method particles to many or all surface peridynamics points, the proposed method integrates dual-horizon peridynamics with a single discrete element particle representing each object. Contact forces are computed once per discrete element pair and mapped to overlapping peridynamics points in proportion to shared volume, conserving linear momentum. Benchmark sphere-on-plate impact demonstrates prediction of peak contact force, rebound velocity, and plate deflection within 5% of theoretical results found in the literature, while decreasing neighbour-search cost by more than an order of magnitude. This validated force-transfer mechanism lays the groundwork for future extension to fully resolved fracture and fragmentation
Rethinking Impact-based Forecasts and Warnings (IbFW) for multi-risks
No full textImpact-based Forecasts and Warnings (IbFWs) are crucial for disaster risk reduction; however, most systems focus on single hazards, overlooking the complex interactions of multi-risk events. This siloed approach can underestimate impacts, especially when hazards occur simultaneously or sequentially. Developing multi-risk IbFW systems requires interdisciplinary collaboration, improved modelling, more impact data and clearer terminology. Analysis of historical disasters shows that multi-hazard events cause disproportionate economic losses. Advancing IbFW systems demands pragmatic innovation, robust datasets, and inclusive strategies to better reflect real-world hazard complexity
Time evolution models for scour burial of isolated objects on a granular seabed
No full textUneXploded 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
Marine snow as vectors for microplastic transport: Multiple aggregation cycles account for the settling of buoyant microplastics to deep-sea sediments
No full textMany studies have reported the paradoxical observation of high concentrations of low-density microplastics (plastic particles < 5 mm) in deep-sea sediments despite their buoyancy. The incorporation of buoyant microplastics into marine snow has been observed to enhance microplastic settling. Previous studies on the vertical movement of buoyant microplastics have been unable to theoretically account for these ocean observations and no study has comprehensively elucidated microplastic transport pathways in the ocean from the surface to seafloor. Here, we establish a one-dimensional theoretical model, that embraces key elements of the flocculation process, to explain how marine snow acts as a vector to transport buoyant microplastics to deep water and the ocean bottom. Microplastics reach the ocean floor through multiple cycles of aggregation, settling, and disaggregation between marine snow and microplastics. Each settling cycle results in a net settling of 200–400 m. We demonstrate that microplastics with different sizes show distinct vertical settling behaviors and only microplastics less than 100 μm in diameter can reach the ocean bottom. This theoretical model refines our ability to predict and understand the global and long-term fate, transport, and inventory of microplastics in the ocean interior, the influence of microplastics on the biological carbon pump and the efficacy of plastic management policies
Numerical modelling for scour near cofferdams using eulerian two-phase flow model
No full textCofferdams play a vital role in water-based construction projects, of-fering a secure environment by isolating structures from water. However, scour around cofferdams poses significant risks to structural stability, necessitating ac-curate predictions to guide design and mitigate construction costs. Traditional methods for estimating scour, primarily based on physical experiments and em-pirical formulas, often fail to capture the complexities of sediment-water interac-tions. This study presents a Computational Fluid Dynamics (CFD) model using a two-phase Eulerian approach to simulate scour around cofferdams.
The model, implemented in OpenFOAM v2012, integrates advanced inter-gran-ular stress models - Kinetic Theory of Granular Flows and μ(I) rheology provid-ing accurate representations of sediment transport under varying flow conditions. Simulations were conducted to analyze the impact of cofferdam geometry and flow velocity on scour depth and sediment dynamics. Results were validated against experimental data from HR Wallingford’s General Purpose Flume, demonstrating the model's reliability in predicting scour depths.
Key findings indicate that scour depth is highly influenced by local hydrodynam-ics controlled by the shape of the structure, highlighting the importance of precise modeling in designing resilient cofferdam structures. This research advances the state-of-the-art in scour prediction by bridging experimental observations with robust numerical methods