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Is the North Atlantic oscillation affecting the longshore drift rate in South-East England?
No full textCoastal erosion at an artificially maintained headland on the Southeast of England continues to be a problem, one of the main concerns being that the outflanking of the seawall here would increase the chances of erosion or flooding of the hinterland. Over the years, it has been speculated the movements and changes of the offshore banks, the mobile ness on the beach nearby or the defences themselves have been accountable for this erosion. This paper shows the relationship between the longshore drifts in the area, caused by two main nearly-opposite wave directions, and the NAO (North Atlantic Oscillation) index, a measure for interannual variability in the atmospheric circulation. The NAO could intuitively be considered to affect the western coastline of the UK, although in principle it would have been less expected in Suffolk. The increased erosion observed in the area since 2013 is linked to two high positive NAO index years following a high negative NAO index in 2013
Modelling engineering applications using OpenFOAM at the riverine, coastal and marine environment
No full textHistorically, the design performance of hydraulic, coastal and marine structures was assessed and improved using empirical tools and physical modelling studies. Whilst these approaches are still indispensable in the design process, numerical models are increasingly used for confirming and verifying the structure general dimensions during early stages and for optimising and finalising the design during final stages, usually in combination with physical modelling. The design of coastal and hydraulic structures is mostly influenced by physical processes that involve complex three-dimensional flows, so the use of computational fluid dynamics (CFD) models is generally recommended for these applications. These models are considered computationally expensive, due to the numerical solution of large systems of transport equations in 3D domains. The rapid advances in computer technology have nevertheless made their use parietal for engineering applications over the recent years.
Currently, there is a wide range of CFD platforms suitable for simulating fluid interaction with hydraulic, coastal and marine structures, including both commercial solutions, such as ANSYS®-Fluent [1], Flow-3D® [2] or Star-CCM® [3] and open-source software, such as OpenFOAM® [4], Reef-3D [5] and Proteus [6]. Currently, OpenFOAM® is without doubt the standard regarding the open-source software and during the last years is increasingly established as the standard for CFD applications both in academic and commercial organisations. In addition to the lower cost, OpenFOAM is generally efficient in terms of accuracy and speed, flexible as it allows expert developers to further improve of the software, and it is accompanied with reasonable documentation and support from the online community.
In this work, we will present a portfolio of test cases, comprising validation studies performed internally in HR Wallingford and results from consultancy projects, where OpenFOAM was used as a supporting tool to verify and improve the design of the structure. The objective of the presentation is to demonstrate that OpenFOAM® is a reliable CFD tool for modelling realistic scenarios and to present new developments currently being pursued in HR Wallingford
Threshold uncertainty for multivariate extreme value models for coastal flood risk analysis
No full textFor the past few decades, extreme value statistical models have been widely used in the field of coastal flood risk analysis. In particular, the class of multivariate extreme value / extremal dependence models, such as Heffernan and Tawn (2004), have become one of the main approaches for simulating multivariate extreme coastal conditions as part of a joint probability study, e.g. simulating jointly the wave height, skew surge, wind speed and direction, etc. during storms. These variables serve as the boundary condition to other sophisticated physical or numerical models to ultimately generate the flood extent and depth. Such analysis enables the users to obtain a probabilistic view of possible flooding events as well as to understand the consequence of rarely observed or unobserved coastal conditions.
The conditional model from Heffernan and Tawn (2004) is reasonably parametrised, so that fitting the model is relatively easy, even for high dimensional problems where other models may suffer. It can also account for both asymptotic independence and dependence, and in an asymmetric way if necessary, which is often observed in the coastal condition data. In a threshold-based setup, the proposed model is theoretically justified in the limiting case where the threshold tends to infinity. However, the fitting of the model to a given dataset is almost always performed at a high but finite threshold, assuming the bias induced is negligible. This, together with the threshold choice for the marginal distribution with a fitted Generalised Pareto (GP) tail, leads to variability in the parameter estimation, which in turn affects the probability estimation.
In practice, such threshold uncertainty is not always explicitly accounted for as well as the other sources of uncertainty. Quite often the choice is introduced as a fixed input to the whole analysis procedure, assuming the impact would be small. Through this study, we will demonstrate with real data the potential influence of the threshold choice over the joint probability estimation. We will also introduce several model fitting methods that account for the threshold uncertainty and compare their effectiveness in quantifying the threshold uncertainty in a typical coastal flood risk analysis
Enhancing resilience to coastal flooding from severe storms in the USA: international lessons
No full textRecent events in the USA have highlighted a lack of resilience in the coastal population to coastal flooding, especially amongst disadvantaged and isolated communities. Some low-income countries, such as Cuba and Bangladesh, have made significant progress towards transformed societies that are more resilient to the impacts of cyclones and coastal flooding. To understand how this has come about, a systematic review of the peer-reviewed and grey literature related to resilience of communities to coastal flooding was undertaken in both countries. In both Cuba and Bangladesh the trust between national and local authorities, community leaders and civil society is high. As a consequence evacuation warnings are generally followed and communities are well prepared. As a result over the past 25 years in Bangladesh the number of deaths directly related to cyclones and coastal flooding has decreased, despite an increase of almost 50 % in the number of people exposed to these hazards. In Cuba, over the course of eight hurricanes between 2003 and 2011, the normalized number of deaths related to cyclones and coastal floods was an order of magnitude less than in the USA. In low-income countries, warning systems and effective shelter/evacuation systems, combined with high levels of disaster risk-reduction education and social cohesion, coupled with trust between government authorities and vulnerable communities can help to increase resilience to coastal hazards and tropical cyclones. In the USA, transferable lessons include improving communication and the awareness of the risk posed by coastal surges, mainstreaming disaster risk reduction into the education system and building trusted community networks to help isolated and disadvantaged communities, and improve community resilience
Damage to the Mutriku OWC breakwater – some lessons from further analysis
No full textIn a bold experiment, the Basque government constructed an innovative Oscillating Water Column (OWC) power station breakwater outside the small fishing port of Mutriku during 2007-2008. While the main structure was a rubble mound, the OWC section of the breakwater consisted of 16 chambered vertical wall units formed by stacked concrete layers. Before the power station could be completed however, storms hit the breakwater in December 2007 and March 2008). A further storm in January 2009 result in severe structural damage to a number of the OWC cells. Previous papers have explained some of the contributions to damage, but have not described detailed flows, loads and consequent responses. In order to understand the causes of the damage, this paper will describe recent numerical model (CFD) experiments to calculate flows, pressures and resulting loads at critical positions within the OWC cells
Thrown in at the deep end: modelling sediment plumes 1000 m under water
No full textSeamounts are underwater mountains caused by volcanic activity. They are of great oceanographic interest because of: (i) their local and basin-scale influence over ocean systems, (ii) their often unique ecosystems (e.g. Figure 1), and, (iii) because they are associated with the formation of ferromanganese (Fe-Mn) crusts (Figure 2).
These crusts are of particular interest to scientists because they are rich in some of the rare elements increasingly required for development of renewable technologies.
When the mining such seamounts is considered, an inter‑disciplinary study of seamount hydrodynamics, crust formation and ecosystem sensitivity is required. MarineE-tech is a multi-disciplined project commissioned to address these different aspects in order to improve understanding of the viability and desirability of mining Fe-Mn crusts
Open data from physical model tests: Lessons learned from related initiatives
Get PDFThe HYDRALAB network of European physical model laboratories (www.hydralab.eu) has a range of facilities that includes flumes, basins, ice facilities, rotating tanks and environmental facilities. Each institution had its own data collection system, there are many proprietorial data formats, a shortage of meta-data and no central effort to curate or preserve this data in a findable, accessible, interoperable and reusable (FAIR) way. HYDRALAB+ (2015-2019) is a European Commission Horizon 2020 project to support this network, which requires FAIR data management. HYDRALAB is reviewing the steps taken to make data openly accessible in related disciplines, so that lessons learned can be applied to HDRALAB+. The chosen communities were: (i) the University of Hull’s digital repository, (ii) EMODnet Baltic Checkpoint, (iii) OpenEarth and (iv) the FP7 projects PEGASO and MEDINA and the EU MED project COASTGAP. It is clear that no one solution can deal with all situations: different data types and requirements can best be dealt with by different approaches. Standards for meta-data should be applied, but no existing standard covers the range of situations faced by HYDRALAB. All can be extended in a bespoke manner (which can potentially be included in an update of the standard) but it is highly likely that more than one standard (and none) will be used in such a diverse community. This is perfectly acceptable, so long as the standard is published. There is also a clear need for guidance on the development of repositories where large volumes of data are collected and an understanding of how much needs to be made available on-line. Although there can be conflicts of interest between institutions that are developing policies for data management and projects that want a uniform approach to data management across all partners, systems today can generally accommodate this
Sandscaping the coast: two approaches to large-scale beach nourishment in the UK
No full textThe Sand Motor is an inspirational 21.5 million m3 Dutch beach nourishment, that was constructed in 2011. The possibility of applying the principles of the Sand Motor in the UK been explored since then by a team consisting of The Crown Estate, Royal HaskoningDHV, HR Wallingford, van Oord and Arup. We refer to this as sandscaping: coastal management that uses large volumes of sediment to achieve economies of scale, work with natural processes and provide multiple benefits.
HR Wallingford has undertaken numerical modelling of potential sandscaping solutions at two contrasting sites on the east of England:
• Slaughden: a narrow shingle (sand and gravel) barrier with wave-dominated sediment transport.
• Bacton: a soft cliff and eroding shore platform with partial sand cover concentrated in the upper beach. Progressive tides produce relatively fast current speeds close to the shoreline.
The contrasting nature of these coastal sites has required different modelling approaches to be undertaken in order to assess the required size and shape of possible sandscaping options.
Slaughden lies just south of Alborough (Suffolk, UK) at the narrowest point of the shingle ridge between the river Alde and the sea. The area has suffered persistent erosion in recent decades, mitigated by re-cycling shingle from the south. The one-line model Beachplan has been employed to simulate the wave-driven evolution of the beach plan-shape, due to variations in longshore drift rates. The performance of a number of shingle nourishments, of varying volume and dimensions has been examined assuming a range of possible variation in the future wave climates. The results indicate that a Shingle Engine of approximately 1.2Mm3 would be required to provide a reasonable level of defence at the end of the 50year design life.
The morphodynamic evolution of five beach nourishment options, intended to defend Bacton (Norfolk, UK) from erosion and supply sediment to down-drift villages, has been modelled using the TELEMAC coastal area modelling suite. A fully-coupled wind, wave, flow and sediment transport model was used as beach sediment is lost offshore at this location and a one-line model does not simulate these processes. At first order, the amount of protection provided is determined by the spread of the nourishment. A wide nourishment initially protects more coastline from erosion and leads to lower losses offshore, as there is less exposure to strong tidal currents, leaving more material available to be distributed along the shoreline.
Different modelling approaches were applied to reflect the important physical processes at each site. A one-line model is relatively quick to run, so many variations can be explored, but simulates wave-driven longshore transport only (although sediment losses or gains can be imposed). In contrast, the coastal area model is computationally intensive, yet still does not include all relevant processes (such as undertow or the onshore transport of sand by waves) so still requires interpretation. The paper will explore the differences in the approaches and how the results were interpreted
Agent-based models: How can they reduce the risk posed to people by extreme flood events?
No full textAs the effects of major flood events continues to increase around the world, driven at least in part by climate change, the importance of considering a wide range of response measures has never been more pressing. Over the past 10 to 20 years there has been a paradigm shift from flood defence to flood risk management, and that use of non-structural measures can be more cost-effective, particularly when dealing with residual flood risk. One management response is to plan for evacuation of affected communities so as to reduce risk to people and with it loss of life. For this to be effective and safe, emergency plans need to be based on realistic assumptions of human behaviour, and to have been tested for a range of event scenarios.
Agent-based modelling of human responses before and during flood events is a powerful tool for considering risk to people, and its use has grown in the past couple of decades. This paper describes the application of the Life Safety Model in many countries and for different flood types, including dam failure in Malaysia, river flooding in Australia and coastal surges in the eastern USA. The paper considers how the model attempts to capture the physical and behavioural rules that may apply during evacuation, and the uncertainty inherent in any modelling approach.
The LSM software combines a widely-used transport module with physical and behavioural rules that are in part informed by limited observations. So the impact of floodwater on loss of life and injury, in terms of its depth and velocity, is derived from laboratory experiments that tested human stability over a range of conditions. Even so, this testing does not cover all age groups or physical characteristics, and so there remains uncertainty in the LSM simulations. Similarly, the model can specify the behaviour of people during major floods, such as time taken to start the evacuation and mode of evacuation. The model is therefore a powerful tool in being able to consider how such assumptions affect loss of life and evacuation times. But further research and experiential evidence would help in informing such behaviour to include in the model.
The LSM is currently being used in several flood applications for emergency management planning, to assess risks to people and to try and impose a response on the affected communities to minimise fatalities and injuries. The paper will provide examples to illustrate this. Key issues are the assumptions on response times to the warnings, and the effect this has on the road usage. The case studies will illustrate how the location of warning centres and safe havens affects the loss of life.
The LSM is an established and powerful agent-based tool that permits the simulation of behaviour before and during a flood, that can be used in emergency management. It uses standard physical rules on the stability of people, and allows a range of behavioural decisions to be tested
Caisson design for high storm conditions
No full textThis preliminary investigation by HR Wallingford examines how the overall crest level of a caisson
structure used as a seawall to front a reclamation in relatively exposed conditions may be lowered,
and how the resultant overtopping discharge can be controlled and efficiently drained back to sea. The
study pre-selects a cross section and test conditions in order to perform 2D physical hydraulic model
testing in order to measure overtopping performance and examine the influence on results on the
positioning of crest elements. For the highest incident waves the crest level on the front face of the
caisson is seen to govern the trajectory of the resultant overtopping flow