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A loading model for an OWC caisson based upon large-scale measurements
No full textWave energy is one of the most promising marine energy resources in terms of the scale of the resource, but there remains little technology convergence and costs remain at near-prohibitive levels. Of many wave energy converter (WEC) concepts that have been developed over the years, the oscillating water column (OWC) stands out for its simplicity and low maintenance cost. Quite some experience of actual OWC operation has been gained to date from small, stand-alone pilot schemes. One way to reduce costs is the integration of an OWC-WEC into a breakwater, enabling some degree of cost-sharing between energy and harbour or coastal defence functions. A major problem encountered during the design of an OWC-WEC scheme remains the uncertainty in the wave loads, with their critical influence upon capital cost. A model to estimate forces acting on an OWC chamber in a caisson breakwater is proposed in this paper. Horizontal forces on the front (curtain) wall and the rear (in-chamber) wall are predicted. In addition, and unlike a conventional caisson breakwater, vertical forces acting on the caisson chamber ceiling will have considerable effect on sliding and overturning characteristics of the breakwater structure. The proposed model enables the prediction of chamber pressures which in turn influence the chamber vertical force. The new model has been compared with results from large scale physical model measurements from tests carried out in the very large wave channel, GWK, in Hannover (Germany). Forces under both regular and irregular wave conditions were measured. The comparisons show that the model fits well with the test results to the factor of 1 ± 0.2 for the regular wave cases and to the factor of 0.8 ± 0.2 for irregular wave cases. This model will enable the structural design of caisson breakwater-integrated OWCs to be approached with uncertainties reduced to those comparable with conventional caisson design
Estimation of uncertainty in flood forecasts ‐ a comparison of methods
No full textThe scientific literature has many methods for estimating uncertainty, however, there is a lack of information about the characteristics, merits and limitations of the individual methods, particularly for making decisions in practice. This paper provides an overview of the different uncertainty methods for flood forecasting that are reported in literature, concentrating on two established approaches defined as the ensemble and the statistical approach. Owing to the variety of flood forecasting and warning systems in operation, the question ‘which uncertainty method is most suitable for which application’ is difficult to answer readily. The paper aims to assist practitioners in understanding how to match an uncertainty quantification method to their particular application using two flood forecasting system case studies in Belgium and Canada. These two specific applications of uncertainty estimation from the literature are compared, illustrating statistical and ensemble methods, and indicating the information and output that these two types of methods offer. The advantages, disadvantages and application of the two different types of method are identified. Although there is no one ‘best’ uncertainty method to fit all forecasting systems, this review helps to explain the current commonly used methods from the available literature for the non‐specialist
National Indicative Fluvial Mapping: applying and updating FSU data to support revised flood risk mapping for Ireland
No full textA first cycle National Preliminary Flood Risk Assessment (PFRA) for the Republic of Ireland, completed in 2012, identified areas at significant flood risk, and included the production of national indicative fluvial flood maps. The National Indicative Fluvial Mapping (NIFM) project has recently been completed; this has produced second generation indicative fluvial flood spatial data that are of a higher quality and accuracy to those produced for the first cycle PFRA. This project has covered 27,000 km of river reaches, separated into 37 drainage areas, consisting of 509 sub-catchments. The main project goals were to produce higher quality flood maps, improving upon the outputs of the first cycle PFRA, to take account of potential climate change impacts on flooding, and to produce mapping to improve risk assessments for areas not covered by the National Catchment Flood Risk Assessment and Management (CFRAM) Programme. The NIFM approach to this national-scale modelling project has combined intelligent automation of the repetitive processes, such as hydrological calculations and the burning of watercourses into the 2D model mesh, with rigorous quality assurance techniques.
The hydrological inputs to the 2D hydraulic models, notably the index flood (QMED) values and hydrograph shape, were derived from Flood Studies Update (FSU) node data. The FSU Physical Catchment Descriptors (PCDs) and methods were developed using a dataset that extended to 2012. Since then, additional recent years of observed data had become available, during which period some significant flood events had occurred (notably in December 2015 – the largest event of the observed record at several locations across Ireland). Furthermore, flood growth factors, which convert the index flood to floods of higher return periods, had also been developed on the dataset to 2012, under the National CFRAM studies. It was therefore necessary and timely to reflect the impact of the additional recent years of data on the index flood and growth curve calculations.
Annual maxima data were used to calculate adjustment factors to the QMED values, and these were applied using the pivotal site approach. A pivotal site is the gauging station that is considered most relevant to a particular flood estimation problem at the subject site, ideally, lying a short distance upstream or downstream from the subject site at which the flood estimation is required. In this project, pivotal sites were selected as the nearest downstream gauge on the same river; in the circumstances where no downstream gauge existed, the nearest gauge was used. Traditional alternative approaches to this method, such as the use of analogue catchments, can be potential sources of error; furthermore, the selection of analogue catchments is subjective and therefore difficult to implement on a widespread, automated basis such as the approach being used in this project.
CFRAM growth factors had been derived on a range of different bases for different Units of Management (UoM). Growth curves for each UoM were recalculated for the extended data series. The change in the growth factors as a result of the additional years of flow data was calculated for each OPW gauge. The growth curves for each UoM based on CFRAM growth curves were then scaled to account for the additional years of flow data
The effect of low frequency noise on the behaviour of juvenile Sparus aurata
No full textAnthropogenic activities are causing increased noise levels in the marine environment. To date, few studies have been undertaken to investigate the effects of different noise frequencies on juvenile fishes’ behaviour. In this study we evaluate the behavioural changes of juvenile gilthead seabream (Sparus aurata) exposed to low frequency noise for 7 hours. The emitted sounds were white noise filtered at 1/3 octave band centred at 63, 125, 500 and 1000 Hz (Sound Pressure Level: 140-150 dB re 1µ?a). For each frequency (plus control with no sound) three independent trial replicates were performed. The dispersion (group dispersion), motility and swimming height of the fish were analysed before and during the acoustic emission. Gilthead seabreams were found to be more sensitive to the lowest frequencies but exhibited different response and habituation behaviours depending on the particular frequency
A probabilistic approach to tropical cyclone modelling
No full textTropical cyclones are highly variable and, in many areas of the world, are the main cause of extreme wind and associated waves , surge and current conditions. At a given location, cyclones that cause a significant impact are relatively rare but severe events, which means that the number of historical events for which data are available is often quite small. In addition, the effects, particularly surge, can be relatively localized and affected by the local bathymetry and topography. This causes considerable difficulty in making quantitative predictions of extreme events for design of offshore or coastal structures in areas affected by tropical cyclones.
A new probabilistic method has been developed to increase the sample of tropical cyclones by producing 10,000 years of synthetic cyclone tracks with a range of paths, intensities and sizes based on Hall and Jewson [1] and Casson and Coles [2]. From this set of synthetic tracks, those tropical cyclones most
likely to affect the site of interest are modelled using time varying wind fields based on the Holland model [3] with surge, current and waves then modelled using the hydrodynamic model TELEMAC 2D coupled to the SWAN wave model. As it is impractical to model 10,000 years of tropical cyclones, a Gaussian process emulator is employed to relate the resultant conditions to parameters defining the cyclones, such as track position, heading, intensity and radius to maximum wind.
The result is a synthesized 10,000 years of cyclone events from which design conditions for a range of return periods can be predicted with a greater degree of certainty than by extrapolating from historical events
Sediment modelling for Poole and Christchurch Bay
Get PDFPoole and Christchurch Bays have narrow beaches which attract many visitors to the area and are therefore essential for the local economy. However, the beaches suffer from ongoing erosion, making regular beach nourishments to maintain them necessary. Sea level rise may cause an increase in the required frequency of these nourishments. If one can determine where the sediment sinks are, i.e. the location where the eroded beach material ends up, it may be possible to recycle the material, by dredging in the area of the sediment sinks and returning this material to the area of the beach. For this purpose, a numerical model is developed that computes the sediment transport pathways in the area between Swanage and the Isle of Wight. This model will be the basis for future projects to determine the changes due to the proposed works, either in large scale engineering or beach nourishments.
The model has been calibrated and validated against measurements taken at 9 locations using AWAC systems. The model fits well with the observations and even reproduces most of the observed sediment transport.
The model works well as a tool to analyse the sediment transport pathways and assess where material eroded from the beaches is likely to end up. The model run of a whole year shows a clear pathway of sand along the beaches towards the east and back further offshore, south of Dolphin Bank and Dolphin Sands. Only part of the sand leaves the bay and is moved east along the Isle of Wight. The erosion deposition depends strongly on the coarser fractions in the bed composition in the model, but the results are promising for impact assessments of engineering works
Ship-current interactions with TELEMAC
Get PDFThe Navigation Simulator is often used to train pilots and test harbour or channel designs. It combines hydraulic modelling with ship manoeuvring models and a team of experts pilots, tug m asters, naval architects and navigation specialists ), all focused through a suite of real time simulators.
Currently, the hydrodynamic modelling is an input of the ship simulator, i.e. it is carried out beforehand, the flow fields are then used by the ship simulator. This study presents how the effects of ships have been considered in the hydrodynamic modelling in order to improve ship navigation simulations. Several hydrodynamic scenarios were investigated: Representing the ships in the hydrodynamic model by imposing the ship hulls as a pressure field; Live communication between the hydrodynamic model and the ship simulator. The position and orientation of the ship is controlled by the ship simulator and passed on to the hydrodynamic model. The resulting flow fields and water elevation are calculated by the hydrodynamic model and fed back to the ship simulator; Including propeller wash.
The new developments were used in recent real time navigation simulations. Feedback from pilots and tug masters is positive. Some instabilities in the hydrodynamic model remain and further development s are underway
Design and operation of small hydropower projects in Scotland
No full textIn recent years, government incentives have led to the development of numerous small hydropower projects in the Scottish Highlands, four of which are described in this paper. The paper describes the control systems and the choices made to optimize the projects, which all have complex control systems and are operated remotely. It reviews construction problems encountered, including those attributable to working in a harsh environment, and experience gained from operating the completed schemes
Stochastic generation of spatially coherent river discharge peaks for continental event-based flood risk assessment
No full textWe present a new method to generate spatially coherent river discharge peaks over multiple river basins, which can be used for continental event-based probabilistic flood risk assessment. We first extract extreme events from river discharge time series data over a large set of locations by applying new peak identification and peak-matching methods. Then we describe these events using the discharge peak at each location while accounting for the fact that the events do not affect all locations. Lastly we fit the state-of-the-art multivariate extreme value distribution to the discharge peaks and generate from the fitted model a large catalogue of spatially coherent synthetic event descriptors. We demonstrate the capability of this approach in capturing the statistical dependence over all considered locations. We also discuss the limitations of this approach and investigate the sensitivity of the outcome to various model parameters
The flocculation characteristics of freshly eroded aggregates
No full textIn Europe, 260,000 km2 of soils already suffer erosion by water. This worrying level of land degradation is expected to increase in the context of climate change, with situations particularly critical in mountainous environments. As any multi-use resources, there is also a growing human pressure on mountainous rivers. Data is required to obtain a better understanding of the cycles of deposition and erosion and of the respective contributions of natural and human-induced processes to fine, cohesive sediment fluxes. When placed in suspension in rivers, cohesive materials are principally transported as flocs. The quantity of suspended solids exported downstream depends mainly of their morphometric properties, including: size, shape and effective density. Once eroded on hillslopes, the conveyance of fine sediment particles on continental surfaces requires the maintenance of particles in suspension by nearbed turbulence and its advection downstream by the mean flow. Traditional Rouse profile relationships do not readily hold for highly concentrated riverine suspensions (~10 g/l), where processes such as flocculation, hindered settling and stratification interplay. This study aims at improving sediment transport parameterisation, by examining the kinetics of fine soil aggregates (size, settling velocity, density), once immersed in a turbulent flow. Particle properties of three Mediterranean materials (clay loam soil, black marl and molasse, all sampled in Badlands environments) were tested in the grid stirred experiment following the protocol previously used by Gratiot and Manning (2004). Hydrodynamic properties were monitored with ADV and turbidity sensors. For each soil, three sediment loads (1.5; 5; 10 g.l-1) representative of flood conditions were tested. Aggregate properties were obtained after sampling at four depths above the grid, using the LabSFLOC – Laboratory Spectral Flocculation Characteristics – technique (Manning, 2006) and laser techniques. These acquisition heights are associated with the corresponding turbulence dissipation rates G of 1.5, 3, 7 and 19 s-1. Flocculation rates and dynamics are reported and discussed