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Evaluating the role of draghead positioning in dredging performance using 3D CFD, mud sampling, and rheometry
No full textThis 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
Modelling the combined influence of offshore wind farm infrastructure, sea level rise and climate change
No full textInfrastructure associated with OWFs modify local hydrodynamics and the associated sediment dynamics (i.e. sediment mobility and seabed composition). Alongside the effect of OWF infrastructure, sea level rise (SLR) and climate change will also affect currents and waves in the future. It is unclear how the combined influence of OWF infrastructure, SLR and climate change will impact hydrodynamics and the seabed in shelf sea environments.
The Eastern Irish Sea has experienced rapid OWF development with nine existing OWFs and a combined capacity of 2905 MW. Three additional wind farms are proposed under the current leasing round. The Eastern Irish Sea is a data rich environment with observations of currents and waves, and sediment sample data collected during the pre- and post-construction surveys. It has also hosted extensive oceanographic and geophysical research historically. As such, it is an ideal environment in which to assess the combined influence of SLR, climate change, and OWF infrastructure.
As part of the NERC ECOWind-ACCELERATE project, fully coupled TELEMAC2D-TOMAWAC-GAIA models were developed to represent past, present-day, and future scenarios of OWF development in the Eastern Irish Sea. For the future scenarios, the tidal forcing applied to the model boundary was extracted from NEMO models of the UK continental shelf (~1.5 km resolution, AMM15 configuration) including UKCP18 RCP8.5 SLR projections. The wave spectra used to force the model boundary were extracted from Wave Watch III models of similar extent and resolution future periods chosen from the HADGEM3 RCP8.5 climate projection dataset. The wind and atmospheric pressure fields associated with these events were applied across the model domain. In the TELEMAC-TOMAWAC-GAIA models, a hybrid bed shear stress method was used to appropriately resolve turbulence generated bed shear stresses in the wakes of the OWF monopiles.
The results highlight the effects of existing and proposed OWF infrastructure on tidal currents, bed shear stresses, and sediment transport. SLR acts to decouple bed shear stresses from the bed, but the higher water levels allow more wave energy to propagate further inshore than under present-day conditions. In the vicinity of the OWF developments, the decoupling of bed shear stresses associated with SLR is dominated by the wake-generated bed shear stresses of the monopiles. The wake generated bed shear stresses are enhanced by the increased storm intensity associated with the future climate scenarios
The prediction of infragravity waves within ports and harbours
No full textInfragravity waves are known to give rise to mooring problems for large ships, notably including container ships. With the increased development of container ports that are more exposed to long period swell and the associated infragravity waves, the need to predict infragravity waves has become crucial in understanding and predicting port operability. Using the TELEMAC- SYSTEM wave model ARTEMIS, a method is developed to predict a long term hindcast of infragravity waves in harbours and is validated against measurements. A relationship between swell waves at a given port configuration and the corresponding infragravity waves is explored
Experimental coefficient of discharge for leaky woody dams in clear water conditions
No full textNatural flood management (NFM) uses natural features and processes to manage flood risk. Although natural processes in river flow are well known, their use to manage floods has been only deeply analyzed since recent years, and the hydraulic behavior and performance of some measures is not yet fully understood. Leaky woody dams (LWD) are one example of the application of NFM, which is very complex to understand, and many uncertainties are still unresolved. In practical applications, the effect of LWD is modeled with different assumptions (Manning's n, geometry changes, porosity, etc.), but none of them represent the real physics of the problem. This paper presents novel lab experiments that attempt to simulate LWD in a river channel. The experiments were developed in a straight research flume. The performance of the LWD in terms of outflow capacity and the effect in terms of increase in water levels upstream and velocities downstream have been analyzed. The orifice + weir model has been proposed as the more realistic model to simulate the flow through LWD, and empirical coefficients of discharge for applications in analytical methods and in numerical models have been obtained. The results help to understand the hydraulic behavior of LWD, and the coefficients of discharge obtained can be useful to reduce uncertainties in numerical modeling for practical applications
Hindcast Modeling of Morphodynamic Changes and UXO Burial Caused by Hurricane Matthew 2016, Fort Pierce, Florida
No full textWe 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
Looking back and looking forward (again) (Editorial)
No full textFor this editorial, journal colleagues suggested that I should reflect on my time with the Journal of Flood Risk Management, as my role as Editor in Chief of the Journal has now passed to Nigel Wright and David Proverbs.
I will start over 20 years ago by recalling that, as project Coordinator, at the first project team meeting of the EU funded FLOODsite1 research consortium in 2004, I outlined my understanding that flood risk and its management is an entirely human problem. I wanted to emphasise to our team of European researchers who were drawn from many disciplines and backgrounds that the project was not to focus on my own area of expertise (computational modelling) but was to be fully interdisciplinary. I argued that flooding of land is a natural process whether it arises from rainfall, rivers or the sea. Risk, however, is human concept and flood risk arises when the natural process of inundation conflict with people, possessions and property in the path of water. Put simply, ‘no people then no risk!
Conclusions from the performance assessment of industry applicable internal erosion initiated breach prediction models
No full textThe EDF funded Performance Assessment of Industry Applicable IE Initiated Breach Prediction Models project was initiated with a workshop at HERU in Stillwater in October 2019. In the following 4 years a team of modelers comprising both model developers and industry model users analyzed the performance of 6 different breach modelling codes against a range of different data sets, including hypothetical, field and real dam failure case studies. Each of the data sets reflected a dam or levee failure initiated by internal erosion, and which subsequently developed into an open breach. Modelers were required to simulate breach formation from the initial ‘pipe’ flow through to open breach and the associated dam or levee failure. Four phases of modelling work were undertaken, with the initial Phase 0 focusing on hypothetical data to test the team approach. Phase 1 focused on assessing performance against sets of field data, whilst Phase 2 focused on assessing performance against observed Lawn Lake and Big Bay dam failure data. The final Phase 4 work took a different approach by focusing on data uncertainty (both case study and modelling data uncertainty) to see how the observed and predicted conditions potentially overlapped. This final stage of work allowed us to determine whether a model might recreate the observed conditions given the correct combination of modelling parameters.
This paper presents a brief overview of the project, followed by a summary of findings from each of the model assessment phases, along with the overall project conclusions
Alignment and anisotropy of stresses in disordered granular media
No full textCharacterizing the degeneracy of local stress states is a central challenge in obtaining the complete statistical mechanics of disordered media. Here, we introduce a minimal force-balance model for isolated granular clusters to probe the structure of the stress space through principal stress orientation and stress anisotropy. We further show that when complemented by physically motivated pairwise constraints, the model produces predictions for the stress alignment in packings of repulsive hard spheres. We compare these predictions against simulation data for grains in hopper and simple shear flows, finding qualitative agreement. This demonstrates the promise of modeling bulk athermal disordered systems through the combinatorics of few primitive geometric motifs
Regulating factors and spatiotemporal patterns of terrestrial dissolved organic carbon adsorption in a high-turbidity estuary
No full textThe adsorption of terrestrial dissolved organic carbon (tDOC) onto surfaces of suspended sediment plays a fundamental role in regulating carbon fluxes across the river-estuary-ocean continuum. It is an important process that modulates carbon transport, transformation, and long-term carbon storage, influencing regional and global carbon budgets. However, the role of suspended sediment is frequently neglected in related coastal and estuarine studies due to the complex interplay of physical and biogeochemical processes. To elucidate the relationship between suspended sediment and tDOC and quantify the adsorption process, this study developed a tDOC-adsorption-floc-population model that integrates floc behavior with tDOC adsorption processes. Taking the Changjiang Estuary as an example, the model quantified tDOC removal through adsorption and examined the key mechanisms governing this process. Results indicate that approximately 12.8
1 % of DOC is removed via adsorption when passing through the turbidity maximum zone (TMZ). The dominant mechanism of tDOC adsorption is governed by floc size, with Brownian motion and differential sedimentation alternating as the primary mechanism, whereas fluid shear exerts a relatively minor influence. The adsorption process is spatially aligned with the TMZ, but its influence, driven by the hydrodynamics, can extend into adjacent areas. These findings highlight the need for incorporating suspended sediment dynamics into regional and global carbon cycle models to enhance predictions of carbon transport and transformation in estuarine and coastal systems
Estuarine floc mass distributions from aggregation/disaggregation and bed sediment exchange
No full textEstuarine benthos, among other lifeforms of interest to water quality, can be sensitive to size-distributed suspended cohesive flocs. In such a context, tide-dependent floc mass distributions in the Tamar Estuary in the UK are revisited. At the field site close to maximum turbidity, time-series of the water level, current velocity, salinity, and suspended sediment concentration (SSC) were recorded in 1998 over several tidal cycles. Concurrently, at selected times and elevation, floc mass distributions were derived from in situ observations of the SSC, floc diameters, and settling velocities. A previously developed time-dependent model, revised to account for both multiclass floc aggregation/disaggregation and bed sediment exchange by erosion and deposition, is applied to simulate mass distributions during ebb/flood cycles on 24 June and 5 August. Although the model does not account for the density effects of salinity or sediment advection, limited comparisons between simulated and observed mass distributions indicate generally good agreement in median diameter prediction on both days. This concurrence is due to the primary role of suspended floc dynamics and only a secondary contribution from bed sediment exchange in governing floc properties. For a better prediction of the SSC variation with the tide, the effects of salinity and advection can be incorporated by coupling the modeled floc dynamics with a suitable multi-dimensional hydrodynamic code