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2D coupled CFD Model of an oscillating water column using Proteus
No full textFloating Oscillating Water Columns (OWC) are one of many potential Renewable Energy Devices under consideration as part of the global drive towards clean energy sources. In order to reach deployment, these devices first exist in the form of computational models. These models allow the devices to be tested under a range of conditions and mooring configurations. However, the design of systems often fails to account for the moorings at an early stage, which can result in disappointing performance at later stages of design and deployment.
In this study, Proteus is used to produce a 2D model of an OWC that demonstrates its effectiveness as a tool for modelling floating marine structures. The model solves the Reynolds Averaged Navier Stokes equations in the fluid domain, using a Volume of Fluid-Level Set approach for defining the air-water interface. Proteus is coupled with the Chrono library to solve the rigid body motions, and the response of the mooring lines. The model is validated against both experimental and other computational models. The effect of a mooring system is shown on the water column response.
This study provides a launch pad for more complex studies. Proteus has 3D modelling capabilities, and as a result, 3D OWC, other wave energy devices and floating wind turbines are all potential devices that Proteus has the capability to recreate in high fidelity models
Foundation scour as a geohazard
Get PDFCarrying out a hazard assessment for offshore structures can entail the consideration of a number of different factors. Scour hazard assessments are routinely undertaken, and scour development at offshore structures should be considered a time-varying process. However, scour may take place within a morphologically dynamic environment, the combination of which will impact on the soil–structure–fluid response. This paper presents the analysis of an unique data set that shows the partial collapse of a scour hole at a large monopile foundation within a morphologically active site. The collapse suggests a slope failure mechanism, resulting in the movement of around 450 m3 of material within a period of about 75 min. The paper analyzes the processes involved regarding formation and development of the collapse
Uncertainty and sensitivity analysis of a coastal flood risk modelling chain
No full textThis paper describes the application of uncertainty and sensitivity analysis techniques to a coastal flood risk modelling chain to a site on the south coast of England. The modelling chain comprises multivariate extreme value modelling of sea conditions. Whilst this technique is now well-established, it is well-known that significant uncertainties arise when extrapolating historical datasets to extremes. Whilst these uncertainties can, to a certain extent, be evaluated through the statistical model fitting process, the resulting confidence limits are rarely utilised in practice.
The analysis described here evaluates the uncertainty associated with the statistical extrapolation to extremes. This uncertainty is then propagated through a modelling chain that comprises: Wave transformation; Wave overtopping; Flood inundation; Economic damage.
Each model component within a model chain has uncertainty associated with it. This includes uncertainty relating to the input data and the formulation of the model component itself, sometimes referred to as model structural uncertainty. To date, however, the overall uncertainty associated with the output of a chain of coastal flood models is not well understood. In this study the uncertainty associated with the multivariate extreme value model has been combined with uncertainty from these other model components, to provide estimates of uncertainty on flood risk.
Sensitivity analysis is related to uncertainty analysis. The objective of the sensitivity analysis undertaken in this context is to gain an insight into which sources of uncertainty (both model components and data) within the modelling chain are most important in terms of contributing to the overall output uncertainty. So, for example, at the site analysed, it is possible to answer questions like “is the uncertainty associated with the multivariate extrapolation to extremes more influential than the uncertainty associated with the wave transformation model?” This information can then be used to support decisions relating to prioritisation of data collection and model component improvement activities. A generic technique, Variance Based Sensitivity Analysis (VBSA), has been applied to the modelling chain. The analysis shows that at this site the uncertainty associated with the wave overtopping model dominates all other sources
Regional sediment transport study in Poole and Christchurch Bays, UK
No full textThe beaches of Poole and Christchurch Bays in the south of England are important to their local communities for protection against erosion and flooding as well as the recreational space they provide. These beaches are extensively managed through beach nourishment or recycling, combined with construction and maintenance of groynes and seawalls. Three local councils are jointly considering the management of beaches between Swanage and Hurst Spit as a whole. They recognise the complexity of sediment transport within their shared area and the benefits in identifying how that resource could be managed holistically to optimise beach management. The councils have commissioned a detailed numerical model of sediment transport in Poole and Christchurch bays to facilitate a joined approach.
Previous studies of sediment transport and erosion along this frontage have tended to use one-line models of wave-driven beach plan-shape elevation (such as Beachplan or Genesis). These models allow the user to run a long time series of wave conditions from an established hindcast simulation. The use of a long time series allows seasonality and inter-annual variations in longshore transport and sediment budget to be determined. However these models only simulate wave-driven longshore transport along gently-curved beaches.
The bathymetry here is too complicated for such an approach to cover the entire region. Moreover, offshore sediment transport pathways and sinks are of interest as potential sources of sand for beach nourishment. Therefore the model developed is a fully-coupled coastal area model of wind, tide, waves and sediment transport, created using the open source, finite element TELEMAC modelling suite. It has a high resolution of 5m in the surf zone, expanding out to 5,000m outside the bays. The model has been set up using the latest data on bathymetry and seabed composition. It has been calibrated and validated against different periods of deployment of nine measurement frames, which recorded waves, currents and suspended sediment concentrations over periods of two months. A representative year and a 20-year simulation have been run and provide information on sediment pathways and transport rates. A morphological speed-up factor was used for the 20-year simulation.
This approach simulates many more processes than a one-line modelling approach and is much more computationally demanding. The model was run on 108 cores and took several weeks to run. A huge amount of information is available from the model at each timestep, but shorter durations can be simulated than with a one-line model.
This presentation will discuss the model set-up and calibration, the simulations undertaken the and information from them (including contributions)
Using seasonal forecasts to inform the management of water resources during drought
Get PDFWater is considered to be one of the main mechanisms through which people will experience climate change, with the number of people estimated to become exposed to water scarcity projected to increase sharply in the future. Water resource managers in the UK have access to a range of meteorological and hydrological indicators of drought. However these data are limited in their utility to directly forecast how systems should be managed to reduce impacts on water users. At present there is no generically applicable method to provide such an outlook.
We are working with practitioners and regulators in the UK water industry to demonstrate the use of seasonal forecasts to support decision-making during drought. The work is funded by Copernicus through the European Centre for Medium-Range Weather Forecasts (ECMWF) with the aim of showing how Copernicus Climate Change Service (C3S) data can be used in sectoral contexts. C3S data are combined with both operational practices and the latest UK water resources planning developments to provide metrics of value tailored to the needs of water resource managers. National and industry stakeholders have been fully engaged from the outset, co-creating a tool to evaluate, visualise, and communicate the potential impact of emerging droughts in a meaningful way.
The tool reads water supply system information presented by water companies including drought response surfaces, operational decisions, and demand. The water supply system is simulated using seasonal forecasts, and an assessment of drought likelihood and vulnerability is provided along with an estimate of the associated uncertainty. Impacts are presented in terms of consequences for stakeholders and contextualised in terms of system vulnerabilities and the historic record. This tool supports operational decision-making, in particular when deliberating the timing of supply and demand-side interventions as a drought develops, and communicating such risks to stakeholders
Big coastal physical models – controlling variable wave boundaries
No full textIn coastal physical modelling, when bathymetries are complex and sea-states vary locally, it becomes necessary to combine all natural features in a single 3D physical model. This leads to physical models requiring long wave generation boundaries over which wave heights and directions could vary significantly. HR Wallingford has developed a novel wave generation method by which the wave height and direction along the paddle can be varied to match target wave parameters along the generation boundary. A recent large physical modelling study of a new port development in Chile, had a wave generation boundary of 5.2 km. The local bathymetry in the model needed to include a deep submarine canyon, which extended several km offshore and reached depths of over 150 m, which lead up to the breakwater of the new port. Due to the canyon, the wave heights along the wave generation boundary varied by 30-40 %. As part of the study, an investigation was carried out to determine the optimal calibration method; whether using variable wave heights and / or variable directions along the paddle front best matched the predicted design conditions at the structure toe. An ARTEMIS numerical model was used as part of the validation and calibration process
Distribution of and hydrographic controls on ferromanganese crusts: Tropic Seamount, Atlantic
No full textHydrogenetic ferromanganese crusts are likely to be exploited as resources for critical metals in the near future, yet the processes controlling where and how they grow are poorly understood. Using detailed mapping of seafloor outcrop and well constrained hydrographic modelling alongside scanning electron microscope imagery of samples from the Tropic Seamount, a star-shaped guyot located in the Tropical East Atlantic, we investigate the relationship between currents, ferromanganese crustal texture and the locations and intensity of crustal erosion. Here, we report the distribution of FeMn crusts and explore factors controlling their growth and erosion. We find that just over 35% of the summit plateau of the guyot exposes some form of ferromanganese crust mineralisation, with the rest variably covered by plains of mobile sediment and slim cliff exposures of carbonate. The steep flanks of the guyot largely expose ferromanganese crust both in situ and as debris flows. The strongest currents are located on the upper flanks of the guyot, the central part of its eastern limb, and across the summit plateau. Three categories of surface morphologies are identified; from pristine botryoidal surfaces to flat areas that have been completely polished by the erosive action of currents and sediment. The relationship between the outcrop of crusts, their erosional states and the hydrographic current regime to which they are exposed is complicated. There is a general correlation between the degree of erosion and location across the seamount, with the least eroded being found on the flanks below 2000 m water depth and the most heavily eroded crusts largely restricted to the summit area. Furthermore, the pristine samples all reside in areas that rarely experience current magnitudes over 0.2 m/s, suggesting that above this the currents have the ability to erode ferromanganese crust. However, there is a strong overlap between the measured current magnitudes at the locations of partially and completely eroded crusts, as well as partial overlap with the current magnitudes measured at pristine crust locations. This complexity is likely due to the presence of cliffs and plateaus increasing current magnitudes and turbidity at a scale smaller than the model resolution
Estimating flood forecast performance using inundation data in Soroti, Uganda
Get PDFThis work explores the question “How can data on flood extents derived from Earth Observations (EO) be used to assess the performance of a global flood forecasting model in the ungauged catchment of the Okere and Okok Rivers in Uganda?”. The Global Flood Awareness System (GloFAS), jointly developed by the European Commission and the European Centre for Medium-Range Weather Forecasts (ECMWF), is a global hydrological forecast and monitoring system. In many parts of sub-Saharan Africa the performance of GloFAS has not been assessed. GloFAS is being used in some parts of Uganda to forecast floods.
Recently Africa Risk Capacity has been developing a pan-African flood model for use in underpinning parametric flood insurance. The African Flood Extent Depiction Model (AFED) is a daily depiction of temporarily flooded areas everywhere in Africa over the past 20 years. The AFED uses satellite remote sensing from microwave sensors to map floods. The AFED data set was used to assess the performance of GloFAS for two rivers in Uganda. The AFED flood data consists of a flooded fraction per pixel which ranges from 0 to 1. This is not directly comparable to the river discharges produced by the GloFAS flood forecasting model. In order to compare both datasets and assess GloFAS’s performance, the following steps were taken:
Extracting the flooded fraction of the Okok and Okere Rivers. Five methods were explored: Flooded fraction of the most downstream pixel; Catchment average flooded fraction for all non-zero pixels; Maximum flooded fraction in catchment; Number of pixels that are non-zero in the catchment; Sum of flooded fraction of all the pixels in the catchment.
Comparing the recorded floods derived from newspaper articles with the EO data to establish if the AFED captures the flooding of the Okok and Okere Rivers.
Establishing the range of the flood fraction that signifies flooding in recorded events.
Extracting flood events using the peaks of the AFED data and the range of flooding from step 3.
Assessing the performance of GloFAS and calculating its skill scores using this extended flood events.
Results show that AFED data successfully identifies flooding for the two rivers and can be used to assess GloFAS’s performance
Advanced wave generation systems for numerical modelling of coastal structures
No full textAccurate generation of wave climates in the context of numerical models (and in particular CFD models) is a challenging problem, as these are increasingly used to provide design support to coastal engineering projects. In this paper we will briefly present a technique that addresses the generation (and active absorption) of non-repeating wave sequences for modelling storm events in a meaningful manner. This technique includes a spectral window preprocessing method that is used to reduce the computational costs associated with wave generation algorithms. These can be particularly cumbersome for generating storm events. It was demonstrated that numerical cost can be reduced by about 40 times by using O(101) frequencies for wave reconstruction, rather than O(104) which current methods would need to accurately reproduce long wave series, without any noticeable difference in terms of the generated wave signal. The technique is already in use within the context of the computational toolkit Proteus (https://github.com/erdc/proteus) and is it is combined with both the CFD and shallow water module of the model. The methodology is also fit with a 2nd order correction for generating nonlinear random wave series.
Case studies are also presented that prove i) the capability of the technique to reproduce meaningful sea states in the context of numerical modelling of coastal structures and ii) the improvement of computational cost, when compared to currently available techniques. These case studies comprise modelling of random waves in a numerical wave tank to acquire wave statistics by using both CFD and shallow water models, as well as modelling coastal structures such as a low-crested levees and a caisson breakwater using random sea states
Minimising the risk of tailings dams failures through the use of remote sensing data
No full textTailings dams are earth embankments used to store toxic mine waste and effluent, often constructed with steep
slopes and using the tailings to save on costs. Their failure, estimated to be more than two orders of magnitude
than for water dams, can cause loss of life, irreversible damage to ecosystems and large economic damages. In
countries with limited resources, it is challenging for the authorities to be able to effectively monitor this type of
infrastructure, especially when located in remote areas.
We are developing a system for a sustainable and cost effective way of remotely monitoring tailings dams.
We are measuring the displacement of the structures using earth observation technologies such as Interferometric
Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) technologies, combined with
real-time in-situ devices. Data analysis, weather forecasting tools and assessment of consequences support the
monitoring, allowing the issue of alerts for unusual behaviour or weather conditions that could lead to failure.
We are working with mining companies, local governments and private stakeholders in Peru to test our approach
on a number of sites. The project focuses in the mining region of Cajamarca, where in 2015 there were at
least 9,000 inactive mining facilities registered.
The system contributes to a sustainable management of tailings storage facilities, reducing the risk and the
consequent damage to population and ecosystem services downstream, upon which many vulnerable communities
rely for both their source of water and livelihoods