Transformation of Multidirectional Sea Field and Computational Study

A computational model based on the evolution equation for water waves (Li, 1994b) derived from the original Berkhoffs (1972) "Mild Slope Equation" is tested against multidirectional sea data. The model accounts for reflection as well as diffraction-refraction processes, which is important for applications involving coastal structures. The accuracy and convergence of the numerical solution, as well as the possibility of the implementation of an adaptive numerical scheme are investigated and implemented. The model was firstly tested using laboratory measurements (Briggs et al, 1995) of random directional wave diffraction around a semi-infinite breakwater on a flat bottom. These tests confirm the need to use "directional modelling" (using the principle of linear superposition) for the prediction of wave heights behind the breakwater. The model was then tested using directional wave data records, which were chosen from 4500 wave records collected in the field campaign, by the University of Plymouth and the University of Brighton, at Elmer - Sussex, UK from September 1993 - January 1995. The results showed that the representation of the measured random sea by monochromatic wave runs can introduce a significant error in wave height predictions shoreward of the breakwaters in the diffraction region, thus confirming the importance of directional modelling for random wave simulation. Evidence strongly suggests that non-linear wave effects have a significant influence (40-60%) on the accuracy of the model. Consequently, further tests are needed, which should also consider the influence of wave-current interaction, wave breaking, bed fiiction and transmissive boundaries. Over all the model predictions are more accurate for the controlled environment in the laboratory (5-13%), than for field conditions where the directional modelling accuracy varied from 8 - 32%. A summary of the data, collected by the author as a member of the University of Plymouth Research Team and a database of spectral and directional parameters is also presented in this thesis. Field validation of the numerical model required accurate estimates of measured data. Emphasis was placed on identifying a suitable directional analysis method, which accurately predicted direction and directional spread in the far-field from structures where reflection is still present. The non-phase-locked (hfPL) methods developed for a homogeneous sea are found to be appropriate. The anzdysis of two NPL methods, the Maximum Likelihood Method (MLM) and Bayesian Directional Method (BDM), directional estimates for simulated data shows that both methods can predict accurate incident wave height and direction. Both methods tend to overpredict directional spread and give non-accurate reflection estimates. The MLM method is easier to implement than the BDM method, which is sensitive to the chosen starting value of the hyperparameter u. As the difference between estimates of the two methods for numerical data is small, the MLM method's estimates were chosen for model testing

HVAC-based cooperative algorithms for demand side management in a microgrid

The high penetration of renewable power generators and various loads have brought a great challenge for dispatching energy in a microgrid system. Heating ventilation air conditioning (HVAC) system, as a household appliance with high popularity, can be considered as an effective technology to alleviate energy dispatch issues. This paper presents novel distributed algorithms based on HVAC to solve the demand side management problem, where the microgrid system with HVAC units is considered as a multi-agent system (MAS). The approach provides a desirable operating frequency signal for each HVAC based on the power mismatch value occurring on each local bus. It utilizes demand response of the HVAC units to minimize the supply-demand mismatch, thus reducing the quantity and capacity of energy storage devices potentially to be required. Compared with existing approaches focusing on the distributed algorithms under a fixed communication network, this paper addresses a consensus problem under a switching topology by using the Lyapunov argument. It is verified that a jointly strong and connected topology is a sufficient condition in order to achieve an average consensus for a time-varying topology. A number of cases are studied to evaluate the effectiveness of the algorithms by taking into account its power constraints, dynamic behaviors, anti-damage characteristics and time-varying communication topology. Modelling these system interactions has demonstrated the feasibility of the proposed microgrid system

Evolution of local scour around a collared monopile through tidal cycles

This paper presents the results of an experiment designed to assess the time-development of scour around an offshore wind turbine collared monopile over a number of tidal cycles. One collar shape and location was investigated. The scour developed more slowly and the scour depth was shallower than for the case of a smooth monopile throughout the majority of the first half-cycle. This difference reduced quite rapidly during the second half-cycle and the scour depth at the end of two tidal cycles was essentially the same as for the smooth monopile. The time development of the scour was compared with results from existing empirical models for the time-development of scour under unidirectional flow. As expected, these models give a much smoother evolution of scour and different scour rates than those measured. Time variation in scour depth was better reproduced with a simplified approach for prediction of the time-varying development of scour. This also highlighted a problem with estimation of the time scale for the development of the equilibrium scour depth. Further investigations are needed before this alternative scour protection is completely rejected

Identifying higher-order interactions in wave time-series

Reliable design and reanalysis of coastal and offshore structures requires, amongst other things, characterisation of extreme crest elevation corresponding to long return periods, and of the evolution of a wave in space and time conditional on an extreme crest. Extreme crests typically correspond to focussed wave events enhanced by wave-wave interactions of different orders. Higher-order spectral analysis can be used to identify wave-wave interactions in time-series of water surface elevation. The bispectrum and its normalised form (the bicoherence) have been reported by numerous authors as a means to characterise three-wave interactions in laboratory, field and simulation experiments. The bispectrum corresponds to a frequency-domain representation of the third order cumulant of the time-series, and can be thought of as an extension of the power spectrum (itself the frequency-domain representation of the second order cumulant). The power spectrum and bispectrum can both be expressed in terms of the Fourier transforms of the original time-series. The Fast Fourier transform (FFT) therefore provides an efficient means of estimation. However, there are a number of important practical considerations to ensuring reasonable estimation. To detect four-wave interactions, we need to consider the trispectrum and its normalised form (the tricoherence). The trispectrum corresponds to a frequency-domain (Fourier) representation of the fourth-order cumulant of the time-series. Four-wave interactions between Fourier components can involve interactions of the type where f1 + f2 + f3 = f4 and where f1 + f2 = f3 + f4, resulting in two definitions of the trispectrum, depending on which of the two interactions is of interest. We consider both definitions in this paper. Both definitions can be estimated using the FFT, but it’s estimation is considerably more challenging than estimation of the bispectrum. Again, there are important practicalities to bear in mind. In this work, we consider the key practical steps required to correctly estimate the trispectrum and tricoherence. We demonstrate the usefulness of the trispectrum and tricoherence for identifying wave-wave interactions in synthetic (based on combinations of sinusoids and on the HOS model) and measured wave time-series

Morecambe Bay Timescapes:Drawing together coastal futures that will, may, or could

This paper considers the role of drawing and creative processes of visualizing possible coastal futures as a means for engaging young people in climate change research and coastal management processes. While predictive models show the impact of climate change in coastal areas around the globe, what will happen to individual places will largely depend on local strategies and interventions. Yet, the complexity of these phenomena as well as the high level of specialisms involved often tends to leave local communities, and young people in particular, unable to participate decision-making processes which will determine the future of the places where they live. In the Morecambe Bay Timescapes project, three secondary schools and one college across Morecambe Bay were involved in a programme of activities which combined fieldwork, archival research, climate modelling, and art practice which led to the design of visions of hyperlocal coastal futures. These visions were used as part of an interactive exhibition that brought together young people and experts in conversations about possible futures. This paper describes the role that drawing played in enabling such conversations, by providing a way for students to work through multiple layers of complexity and articulate their reflections

Shoreline Delineation in Complex Intertidal Environments using Sentinel-1 SAR Imagery.

The threats that climate-change-induced sea level rise pose to coastal communities can be assessed using coastal vulnerability models. In order to be effective, such models require accurate estimates of shoreline position. Such data are rarely available in developing countries. Here, we develop a simple approach based on image thresholding and subsequent vectorisation of Sentinel-1 SAR data for a complex intertidal zone in the Niger Delta, Nigeria, and show how shoreline position varies over space and time under different tidal conditions

Implementing an efficient beach erosion monitoring system for coastal management in Croatia

This paper proposes a coastal erosion monitoring system for beach erosion management, which we demonstrate on natural and artificial pocket gravel beaches in Croatia. The approach uses low-cost Structure-from-Motion (SfM) photogrammetric imaging and multi-view stereo (MVS) to produce high-resolution 3D beach models for detecting morphological changes and erosion occurrence. Coastal state indicators, such as the shoreline position and subaerial beach volume, are derived from the 3D models and used to quantify changes between surveys. The method is illustrated through two case studies and, to our knowledge, these are the first repetitive measurements taken on the Croatian eastern Adriatic Coast (CEAC). In case of the natural Brseč beach, beach rotation was found to be a response to natural forcing from waves of various incident directions. For the artificial Dugi Rat beach, which loses sediment every winter and is subsequently re-nourished every spring, monitoring showed that beach nourishment is of limited durability. Both case studies showed that the SfM-MVS technique is suitable for the rapid and frequent acquisition of 3D survey data, from which quantitative coastal indicators can be derived to inform future coastal management interventions. Significantly, this low-cost data acquisition has a great potential for regular beach management survey. The introduction of beach monitoring in Croatia is timely because emerging Integrated Coastal Zone Management (ICZM) practices will require data-based approaches. Moreover, rare natural pocket beaches and the ever-increasing number of artificial beaches are extremely vulnerable to natural and man-made changes. Adaptive beach management, based on systematic monitoring data, should be included in the ICZM, and we detail how SfM-MVS-based monitoring can be used at different levels of the ICZM. Implementing robust ICZM monitoring will require broad considerations and consultation with all stakeholders, so we propose that SfM-MVS beach surveys should be initially integrated into the existing monitoring practices for CEAC sea water bathing quality. Extension of the existing database with rapidly-gathered low-cost 3D beach survey data, from a number of targeted beaches, could be used to provide a crucial baseline for the ICZM and strategic coastal monitoring of the CEAC