An introduction to multifractal geometry of wave sea states on the west and south-east coasts of South Africa

Thesis (PhD)--Stellenbosch University, 2015.ENGLISH ABSTRACT: All of the Ports along the South African coastline are subject to bound infragravity wave action to a greater or lesser degree, for example, at the Ports of Saldanha Bay on the west coast and Ngqura on the south-east coast. Saldanha Bay harbour principally services loose- and liquid-bulk carriers and the Ngqura harbour mainly services container ships. The long wave actions when severe cause moorings to be broken, ships to leave the quay and loading to stop. This research has confirmed that the sea surface is a multifractal structure characterised by many singularities ranging from highly irregular or rough features to smooth or calm features. Any wave train is comprised of the full range of these features to various degrees and in varying percentages of occupancy. Notwithstanding this problem, relatively little is known about them in the South African context due to the fact that they cannot be visually detected and specialised, sophisticated equipment is required to physically measure them. The country is currently planning the development of a new port and the expansion of others for larger ships. Under these circumstances this research is seen to be appropriate from the point of view of obtaining a new method for the characterisation of these hazardous wave conditions. The objective of the research was achieved. This was to identify a set of fractal dimensions that describe the surface geometry of a hazardous bound infragravity wave sea state. In order to achieve the objective, a set of fractal dimensions was firstly determined from video imagery of an open water wave field, by analysing a set of single point time series data derived from the imagery. This has been done in order to be able to visually compare the derived set of fractal dimensions with video imagery of the sea surface that they represent. It also has the advantage of proving that fractal methods of analysis are applicable for the study of sea surface single point time series data. Secondly, periods when long wave action occurs at both Saldanha Bay and Ngqura harbours were identified by the presence of their actions in the harbours. Thirdly, single point time series data recorded by the Council for Scientific and Industrial Research (CSIR) were obtained during the identified periods as well as two days before these times and fractal sets of dimensions for the periods were determined. This was achieved by means of the following methods of analysis: ● The rescaled range (R/S) method, ● The Multifractal Detrended Fluctuation Analysis (MDFA) method, ● The Power Spectral Density (PSD) method in both the Fourier and the wavelet domains, and ● The Wavelet Transform method. Fourthly, the fractal data sets from each harbour were compared to confirm that the sets of dimensions for the hazardous sea state are clearly different from those of the non-hazardous sea state and can be used to describe the condition. Finally, the fractal sets of dimensions for hazardous sea states at both harbours were compared to identify any variances between them. During the research it was found that a hazardous sea state could be profiled for identification purposes and for complementing the currently determined significant wave height and peak period details by means of fractal indices. These indices were identified by comparison with a similar set of indices for nonhazardous sea states at the same location, as part of a ‘calibration’ process and clearly identified shifts in the Holder exponents of the sea states enabled the unambiguous identification of the hazardous condition. Having completed the research and analysis work, the author has identified other areas of coastal engineering, besides the identification of hazardous bound infragravity wave sea states, where a study of multifractal geometry could be applied advantageously.AFRIKAANSE OPSOMMING: Alle hawens langs Suid Afrika se kuslyn is tot ‘n meerdere of mindere mate onderworpe aan gebonde infragravitasie golf aksie. Hierdie probleem is egter veral straf by die hawens van Saldanhabaai aan die weskus en Ngqura, aan die suidooskus. Saldanhabaai voorsien dienste hoofsaaklik aan massa draers van los stowwe en vloeistof, terwyl Ngqura hoofsaaklik houerskepe bedien. Die lang golf aksies veroorsaak dat ankertoue breek en die skepe die kaai verlaat, sodat laai van die skepe tot stilstand kom. In die loop van hierdie navorsing is gevind dat die seevlak ‘n multifraktale struktuur is, met singulariteitseienskappe wat wissel van hoogs onreëlmatig of rowwe eienskappe tot gladde, reëlmatige eienskappe. Enige golfreeks behels die volle omvang van hierdie eienskappe in verskillende grade en wisselende teenwoordigheids persentasies. Die navorsing is gefokus op die geometrie van gebonde infragravitasie golfaksie seetoestande, wat oral langs die Suid Afrikaanse kuslyn voorkom, en in twee hawens ‘n beduidende bedreiging vir vasgemaakte skepe is. Ondanks die probleem, is min bekend oor hierdie toestande in Suid Afrikaanse konteks, omdat hulle nie visueel bespeur kan word nie en spesiale gesofistikeerde gereedskap nodig is om hulle fisies te kan meet. Daar word tans beplan om ‘n nuwe hawe te ontwikkel, wat hierdie navorsing veral gepas maak, met die doel om ‘n meer volledige beskrywing van hierdie bedreigende golftoestande te weeg te bring. Die doel van die navorsing is om ‘n stel fraktale dimensies te identifiseer wat die oppervlakgeometrie van ‘n bedreigende gebonde infragravitasie golf-seetoestand omskryf. Hierdie dimensies kan dan gebruik word om ‘n indentifiserende profiel van die seetoestand te teken om die inligting tans beskikbaar oor beduidende golfhoogte en piektye, aan te vul. Om hierdie doel te bereik is ‘n stel fraktale dimensies eerstens bepaal deur middel van videobeelding van ‘n oopwater golfveld. ‘n Stel enkelpunt tydserie data, afgelei van die beelding, word dan ge-analiseer. Dit het visuele vergelyking tussen die afgeleide stel fraktale dimensies en die videobeelding van die seevlak wat dit verteenwoordig het, moontlik gemaak. ‘n Verdere voordeel is dat dit bewys het dat fraktale analisemetodes toepaslik is vir die bestudering van seevlak enkelpunt tydreeks data. Tweedens is die tye wanneer lang golfaksie teenwoordig was in die hawens by Saldanha en Ngqura, vasgestel deur die uitwerking daarvan in die hawens. Derdens is enkelpunt tydreeks data wat deur die WNNR aangeteken is vir die vasgestelde tydperke, sowel as twee dae voor elke tydperk, verkry en is fraktale dimensiestelle vir elke tydperk vasgestel. Vierdens is die fraktale datastelle van albei die hawens vergelyk om te bevestig dat die stelle dimensies vir bedreigende seetoestande duidelik verskil van die vir niebedreigende toestande, en dus geskik is om die seetoestand te beskryf. Ten slotte is die fraktale dimensiestelle vir bedreigende seetoestande in die twee hawens vergelyk om enige verskille tussen hulle te bepaal. Na voltooiing van die navorsing en analise is ander gebiede van kusingenieurswese behalwe die bepaling van bedreigende gebonde infragravitasie golf seetoestande, identifiseer waar multifraktale geometrie ook tot voordeel aangewend kan word

OMAE2007-29732 MEASUREMENT AND ANALYSIS OF OCEAN WAVE FIELDS IN FOUR DIMENSIONS

ABSTRACT Ocean wave data comprises either instrumentally measured data or model-derived data, and the former type of data is preferred in the offshore industry. Instrumental data can be considered to be comprised of both directly measured sea surface displacement data and derived data, from the acceleration of buoys. It has been found that significant differences can occur between sea surface displacements, which are recorded in steep waves by fixed probes or lasers (Eulerian), or by free-floating buoys (Lagrangian). This has given rise to the situation where wave buoy data should not be used to estimate wave profiles in steep waves. Short crested and heaped waves, in moderate to high sea states, can also cause a problem when recording wave data at a fixed point, when it comes to determining the representivity of the results across a wave field. Recorded wave data is used as the basis for the development as well as the verification of all wave models and, given the above uncertainties, the authors propose a new wave measurement method, using the recently developed Automated Trinocular Stereo Imaging System (ATSIS), for the recording of three-dimensional surface wave displacements with respect to time. The ATSIS is a novel system, which measures the temporal evolution of three-dimensional wave characteristics for analysis. An oblique configuration for the system effectively increases spatial coverage, allowing observations of wave phenomena over a broad range of temporal and spatial scales. The details in the paper provide a solution of quantifying the behaviour of irregular, non-linear, and directionally spread (short crested waves), and provides an efficient method for developing better design criteria in the future

Drift of pancake ice floes in the winter Antarctic marginal ice zone during polar cyclones

High temporal resolution in situ measurements of pancake ice drift are presented, from a pair of buoys deployed on floes in the Antarctic marginal ice zone during the winter sea ice expansion, over 9 days in which the region was impacted by four polar cyclones. Concomitant measurements of wave-in-ice activity from the buoys are used to infer that the ice remained unconsolidated, and pancake ice conditions were maintained over at least the first 7 days. Analysis of the data shows (i) the fastest reported ice drift speeds in the Southern Ocean; (ii) high correlation of drift velocities with the surface wind velocities, indicating absence of internal ice stresses >100 km from the ice edge where remotely sensed ice concentration is 100%; and (iii) presence of a strong inertial signature with a 13 hr period. A Lagrangian free drift model is developed, including a term for geostrophic currents that reproduce the 13 hr period signature in the ice motion. The calibrated model provides accurate predictions of the ice drift for up to 2 days, and the calibrated parameters provide estimates of wind and ocean drag for pancake floes under storm conditions

Wind, waves, and surface currents in the Southern Ocean:Observations from the Antarctic Circumnavigation Expedition

The Southern Ocean has a profound impact on the Earth's climate system. Its strong winds, intense currents, and fierce waves are critical components of the air-sea interface and contribute to absorbing, storing, and releasing heat, moisture, gases, and momentum. Owing to its remoteness and harsh environment, this region is significantly undersampled, hampering the validation of prediction models and large-scale observations from satellite sensors. Here, an unprecedented data set of simultaneous observations of winds, surface currents, and ocean waves is presented, to address the scarcity of in situ observations in the region-https://doi.org/10.26179/5ed0a30aaf764 (Alberello et al., 2020c) and https://doi.org/10.26179/5e9d038c396f2 (Derkani et al., 2020). Records were acquired underway during the Antarctic Circumnavigation Expedition (ACE), which went around the Southern Ocean from December 2016 to March 2017 (Austral summer). Observations were obtained with the wave and surface current monitoring system WaMoS-II, which scanned the ocean surface around the vessel using marine radars. Measurements were assessed for quality control and compared against available satellite observations. The data set is the most extensive and comprehensive collection of observations of surface processes for the Southern Ocean and is intended to underpin improvements of wave prediction models around Antarctica and research of air-sea interaction processes, including gas exchange and dynamics of sea spray aerosol particles. The data set has further potentials to support theoretical and numerical research on lower atmosphere, air-sea interface, and upper-ocean processes.

Three-dimensional imaging of waves and floes in the marginal ice zone during a cyclone

The marginal ice zone is the dynamic interface between the open ocean and consolidated inner pack ice. Surface gravity waves regulate marginal ice zone extent and properties, and, hence, atmosphere-ocean fluxes and ice advance/retreat. Over the past decade, seminal experimental campaigns have generated much needed measurements of wave evolution in the marginal ice zone, which, notwithstanding the prominent knowledge gaps that remain, are underpinning major advances in understanding the region’s role in the climate system. Here, we report three-dimensional imaging of waves from a moving vessel and simultaneous imaging of floe sizes, with the potential to enhance the marginal ice zone database substantially. The images give the direction–frequency wave spectrum, which we combine with concurrent measurements of wind speeds and reanalysis products to reveal the complex multi-component wind-plus-swell nature of a cyclone-driven wave field, and quantify evolution of large-amplitude waves in sea ice

Effects of an explosive polar cyclone crossing the Antarctic marginal ice zone

Antarctic sea ice shows a large degree of regional variability, which is partly driven by severe weather events. Here we bring a new perspective on synoptic sea ice changes by presenting the first in situ observations of an explosive extratropical cyclone crossing the winter Antarctic marginal ice zone (MIZ) in the South Atlantic. This is complemented by the analysis of subsequent cyclones and highlights the rapid variations that ice-landing cyclones cause on sea ice: Midlatitude warm oceanic air is advected onto the ice, and storm waves generated close to the ice edge contribute to the maintenance of an unconsolidated surface through which waves propagate far into the ice. MIZ features may thus extend further poleward in the Southern Ocean than currently estimated. A concentration-based MIZ definition is inadequate, since it fails to describe a sea ice configuration which is deeply rearranged by synoptic weather

Exploring South Africa's southern frontier : a 20-year vision for polar research through the South African National Antarctic Programme

Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’. Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change. A growing body of evidence now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change. For example, a recent study indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators. When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.http://www.sajs.co.zaam2017GeneticsMammal Research InstituteZoology and Entomolog

README for "Development of a Multi-tier System for the Analysis of Ice Crystallography of Antarctic Shelf Ice" Masters Dissertation

  This README provides comprehensive information about the datasets collected during the 2021/2022 Antarctic expedition on the Fimbule Ice Shelf. These datasets comprise a multitude of ice core data points, integral to the Master's research project titled "Development of a Multi-tier System for the Analysis of Ice Crystallography of Antarctic Shelf Ice". The document elucidates the structure, content, and labelling system used for the datasets, ensuring seamless navigation and comprehension for users. The data was collected and analyzed during the 2021/2022 Antarctic summer expedition, with additional analysis performed in the Computational Continuum Mechanics Research Group's laboratory.</p