OpenUCT :: Browsing by Author "Brundrit, Geoff"

Browsing by Author "Brundrit, Geoff"

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Open Access
An ADCP study of subtidal scale density-driven exchange in Saldanha Bay, South Africa
(2003) Stewart, Helen Frances; Monteiro, Pedro M S; Waldron, Howard; Brundrit, Geoff
An ADCP and water-column temperature study was conducted to determine the circulation aspect of subtidal-scale, density-driven exchange in Saldanha Bay, South Africa. Density-driven exchange conditions develop in response to synoptic-scale wind events in the southern Benguela region, even under light (<5m s-') wind conditions. During a density-driven exchange event, directionally opposing bi-level flow, similar to an estuarine system, develops in response to remote upwelling-favourable winds. The bi-level flow component occurs in two distinct bands, bayward at 0-9m height from bottom and seaward 15-20m height off bottom, and is very sensitive to changes in wind forcing. Observations of current behaviour are added to the four-phase conceptual model of density-driven exchange developed by Monteiro and Largier (1999). In addition, estimates of bay flushing based on ADCP current velocities and the four-phase conceptual model are calculated and implications of shelf water influx into Saldanha Bay are discussed.
Open Access
Antipodal receptions in global acoustics
(1997) Courtney, Jennifer Susan; Brundrit, Geoff
Global Acoustic Propagation is a recently developed scientific discipline within the study of long range underwater acoustic propagation. Acoustic propagation over extremely long ranges involves a combination of effects from earth curvature and the global distribution of oceanographic and geophysical features. Antipodal receptions, that is to ranges of the order of 20Mm (1 Megameter = 1 000 km), require underwater acoustic propagation to very long ranges and thus effects due to the form of the earth and the range dependence of the sound speed field within the ocean can not be ignored. The purpose of this thesis is to investigate the nature of antipodal receptions with reference to the form of the earth and horizontal sound speed variations within the ocean and thereby contribute to the new and specialized field of study, Global Acoustic Propagation. Close to an acoustic source acoustic energy diverges so that local signal strengths decrease with distance from the source. However, as the antipodal region is approached acoustic energy that has not been blocked by bathymetric features will refocus, counteracting the distance loss rule. Thus at antipodal sites there is a good prospect of receiving a focused signal. Even so, the ellipsoidal form of the earth and the horizontal variability within the sound speed field of the ocean means that the re-focusing will result in a region of enhanced signal rather than an exact antipodal point. The precise detail of the advantage of placing receivers in the antipodal region will depend upon the paths taken by the acoustic energy and the characteristics of the acoustic medium encountered along each path. The proposal for this thesis is to develop an algorithm to determine the nature of the antipodal region under certain physical assumptions. The physical assumptions are referred to as the geometric assumption, which refers to the form of the earth, and the refractive assumption, which refers to the horizontal variability of sound speed within the world oceans.
Open Access
Characteristics of extreme wave events and the correlation between atmospheric conditions along the South African coast
(2004) Van der Borch van Verwolde, Emile; Brundrit, Geoff; Moes, Hans
Characteristics of extreme wave events along the coast of South Africa were researched through a dataset obtained by CSIR wave recording network at four locations. The locations from west to east are Slangkop, FA-Platform (Agulhas bank), East London and Richards Bay. The longest dataset available was the 25-year dataset at Slangkop measured by accelerometer wave buoys. In the subsequent years the wave recording network along the South African coast was expanded to six locations at present.
Open Access
Design sea levels for Southern Africa : a probabilistic approach
(1993) Wijnberg, Allan Richard; Kilner, F A; Brundrit, Geoff
This thesis describes the development of a methodology for the quantitative assessment of design sea levels for southern Africa. In order to achieve this objective it was necessary to establish which ocean processes affected sea level in the sub-continent and develop a probabilistic model for the combination thereof. The methodology, is used to characterize regional design sea levels in terms of the west, south and east coasts. A site specific application is undertaken to demonstrate the model's capabilities with regard to the design of depth limited structures. The ultimate objective of this study is to provide a practical approach to the quantification of the sea level component of loading in the full probabilistic design assessment. Data analysis considers all available sea level and wave data for three ports around the coast. A three parameter threshold analysis technique is used to define independent identically distributed events. The distinction between the major processes affecting sea levels in southern Africa may be related to the differences in both the time and space scales of their response to the forcing mechanisms. The data analysis procedure is used to defined the primary statistical characteristics of the observed events in each data set as they relate to sea level. A stochastic simulation model is developed which reproduces a synthetic hourly sea level record displaying the same statistical characteristics as the observed data. Annual maximum values are extracted from the model output with a view to estimating extreme sea levels. The model may be run over any number of periods until satisfactory convergence in the results is obtained. The theoretical basis of the model is described and the results compared with the Gumbel method. A regional assessment of design sea levels for southern Africa indicated that the south coast experienced larger fluctuations in the stochastic component of sea level than the east and west coasts. Sea levels throughout the sub continent are primarily affected by tide, shelf waves, wind waves and edge waves. These processes were found to be statistically independent of one another for the areas evaluated. Design sea level would appear to be determined by a combination of a number of moderate magnitude events rather than one single process. The application of the model illustrates the importance of considering both wave height and sea level conditions as stochastic variables for the design of depth limited structures. The relative influence of stochastic sea level is shown to increase from deep to shallow water.
Open Access
The effect of quick freezing and fast thawing on stored seawater nutrient concentrations of dissolved inorganic phosphate, nitrate and ammonium
(2004) Wedeinge, Josef; Brundrit, Geoff; Waldron, Howard
In an attempt to improve the quality of preserving seawater nutrients for delayed analyses, a sample of unfiltered and aged nutrient-depleted seawater was obtained from Marine and Coastal Management (M&CM) and spiked with small different volumes of standard nutrient solutions. These were prepared into 100 mL glass bottles to constitute the lowest, intermediate and highest levels of dissolved inorganic phosphate (0.5, 1.5, 3.0 μM), nitrate (5, 15 and 30 μM) and ammonium (0.5, 1.5, 2.5 μM) that are found in the southern Benguela region. After spiking, the samples were analysed to determine their "true" initial concentrations and immediately quick frozen using liquid nitrogen. A time series analysis (up to seven weeks) was performed by first fast-thawing the samples using a microwave oven and fixing up the nutrients within 20 minutes. Variations in nutrient concentrations did not exceed 7% for phosphate, 4.0% for nitrate at all levels and 8% for ammonium at the intermediate and highest levels, but varied by 19.2% at the lowest level. Statistical tests for significance of the means were made at a = 0.05 level and no difference was found at any level for the nitrate, and the lowest level phosphate. The higher levels of phosphate, and all ammonium were found to be significantly different. Hence, the experimental conditions employed in this paper would not be recommended for the all ammonium and the higher levels of phosphate concentrations. These variations were however not large or regular in pattern but erratic, which suggested that they were not due to microorganisms and optimizing those conditions may well to yield improved results.
Open Access
Environmental requirements for optimal naval operational efficiency
(2004) Uys, R L; Brundrit, Geoff
In this thesis the applicability of ocean environment modelling as a part of optimal naval operational efficiency, and thus military oceanography, is evaluated. To be able to do this, the principles of knowledge-based warfare and the ability to make rapid environmental assessments, are introduced. These then form part of optimal efficiency. Modelling of the environment implies knowledge-based warfare and accommodates the ability to make a rapid environmental assessment. After an overview of past and current ocean modelling (specifically wave modelling), the third generation SWAN (Simulating WAves Nearshore) model is selected to model a small component of the military oceanographic spectrum viz. waves. The selected area, includes an area where the US Navy conducted an amphibious landing exercise (Operation Laurel) during October 2001. Three case studies are considered for modelling. These included an extreme wave condition, a mode wave condition and the conditions during Operation Laurel. Data were obtained from the wave rider buoy at Slangkop near Cape Town and analysed for a specific period during 2001. In all three cases the wave dissipation and maximum energy transfer areas were determined from the model. From these, certain deductions could be made regarding the influence the environment under these three different conditions could have on naval operations in the littorals. It is concluded that ocean modelling should form an integral part of naval operational efficiency and its contribution as force multiplier should be taken into consideration.
Open Access
A fast procedure for generating random numbers by a modification of the Marsaglia-Maclaren method
(1974) Ioannou, Ioannis Elias; Brundrit, Geoff
Marsaglia and Maclaren combined two linear congruential generators in order to produce a pseudo random number sequence uniformly distributed in the range [0,2³⁵]. Their method is a considerable improvement compared with the primitive linear congruential method at the cost of greater generation time. In this thesis, a simple modification of the Marsaglia-Maclaren method is presented in which there is an alleviation of the increased generation time, and a slight further increase in randomness. The modified generator is tested extensively in a variety of statistical tests and simulation problems.
Open Access
Influences on the hydrology of the Cape Columbine/St. Helena region
(1985) Waldron, Howard Neil; Brundrit, Geoff
The overall objective of the thesis is to investigate and interpret hydrological events occurring at the Cape Columbine upwelling site and the adjacent coastal waters, with special reference to St. Helena Bay. The first step in the study involves monitoring the meteorological occurrences which resulted in the prevailing hydrology and thus give a general background to the observed structure. It became necessary to establish two sub-systems in the area which can be distinguished on the basis of the time scales within which they operate. Generally speaking, outside St. Helena Bay the waters have a 3-5 day synoptic variability governed by meteorological conditions, but within the semi-closed system of the Bay itself a longer time span of around 25 days governs the processes in this separate but inextricably linked water body.
Open Access
An intensive study of the currents and general hydrology of an anomalous upwelling area off South West Africa
(1981) Boyd, Alan John; Brundrit, Geoff
The currents, winds and hydrology of a coastal site of approximately 120 square miles off South West Africa were studied intensively for two weeks during June/July 1978. Currents were measured by means of drogues tracked by RADAR from a central station at 45m of water depth. Classical inertial motion was observed at several depths during a calm period implying a short relaxation time for pressure gradient forces and the existence of different Velocity layers within the water column. Diurnal sea breezes and landbreezes controlled the currents at 2 and 5m depth with a response time of a few hours but the deeper currents were not directly affected by wind. Average surface current speeds were between 0,2 and 0,3 kts. Sustained southerly winds caused a deepending of the upper layer and were accompanied ~y slower currehts. Selected data recorded during the diurnal wind regime yielded the relationship: Surface current speed = 0,017 x Windspeed. A meandering poleward under-current with an average velocity of 0,07 kts was detected at 30m depth whilst onshore flow of similar average speed existed at 20m. Using the results of the experimental work as a basis, the upper layer of the sea to 8m depth was modelled as a slab, which was affected by wind, friction and the coriolis force.
Open Access
The interaction of periodic surface gravity waves with slowly varying water currents
(1982) Bleach, Gordon Phillip; Brundrit, Geoff
The governing equations for interactions between surface gravity wavetrains and slowly-varying water currents are derived and the incorporation of Vocoidal water wave theory into this framework is discussed. The emphasis throughout is on the derivation of the general form of the governing equations plus a detailed discussion of the qualitative physical behaviour implied by the equations. Particular solutions are usually given only where they serve to clarify the general method or some physical feature of the analysis. The thesis proper is introduced by a derivation of wave kinematics on still water. A review of the kinematics and dynamics of an inviscid and irrotational fluid follows. The wave and fluid properties are then combined via the definition of wave integral properties. A derivation of the Airy and Stokes O(a2) wave theories is given and used to illustrate a number of points. Water currents (following or opposing the waves) are introduced via their influence on the wave-kinematics. The wave/current dynamics are then presented in two ways: firstly using a wave energy approach and secondly by introducing the wave action concept. Wave action is more convenient because it is a conserved quantity unlike wave energy. The general equations for two dimensional wave/current interactions are derived and discussed. At this point three topics are reconsidered: group velocity, momentum density in wave motion and Lagrangian mean forms of averaging. The general equations for wave/current interaction are shown to be compatible with the Vocoidal water wave theory and applications of the theory to wave/current problems are discussed.
Open Access
An investigation into the use of the method of spherical currents for the determination of scattered wave fields
(1976) Du Plessis, N M; Brundrit, Geoff
In this thesis an approximation method, the method of spherical currents, is advanced for the determination of scattered fields (acoustic and electromagnetic) from convex bodies for plane harmonic incident waves. The method is worked out for the scalar (acoustic) case and applied to sound soft and sound hard obstacles of a given shape. The surface fields and backscattering cross sections of these obstacles are calculated according to the method and the results are compared with those of other methods. For the vector (electromagnetic) case only the basic theory is worked out for a perfectly conducting convex body.
Open Access
Investigation of the nearshore, episodic poleward current in the southern Benguela : a numerical modelling approach
(2006) Viljoen, Anél; Shillington, Frank; Brundrit, Geoff
Fisheries are of great economic importance on the South African West Coast (the Southern Benguela). The St Helena Bay region is a key nursery habitat for these fisheries because of its retention, upwelling and stratified water column characteristics. However, these characteristics also result in other outcomes such as hypoxia and harmful algal blooms (HAB's) which impact on the habitat suitability character of the system. A nearshore, episodic poleward current has been observed in this region, and it is believed that this current plays an important role in the incidence of HABs as well as hypoxia events. The drivers and dynamics of this nearshore, episodic poleward current have not been clearly understood, nor thoroughly investigated, due to the complexity of the scales and processes. However, the importance of this current in transporting harmful algae from the north into St Helena Bay and its role in habitat hypoxia has emphasized the need to understand its dynamics.
Open Access
Mechanisms for the wet and dry years over Eritrea during the summer rainfall season
(2003) Kidanemariam, Mussie T; Rouault, Mathieu; Brundrit, Geoff
The economy of Eritrea depends on rain-fed agriculture, which is highly sensitive to seasonal and inter-annual variations of rainfall. Adequate amount of rainfall is required every year for adequate agricultural production and drinking water. Therefore, the distribution of seasonal rainfall in space and time is critical to the country's economy. Drought has great influence on the socio-economic aspect and livelihood of the rural population. Drought occurs in Eritrea due to anomalous regional atmospheric circulation. This study focuses on the summer rainy season of June, July and August (JJA). It examines atmospheric and oceanographic mechanism patterns associated to summer rainfall anomalies. The investigation mainly concentrates on wet and dry spells that occurred over Eritrea during the rainy seasons (June to August) of 1950's-1990's. Rainfall data was available from the Climatic Research Unit (CRU) Hulme dataset for a period of 1950 -1996 and this together with rainfall data from the Africa Data Dissemination Service was used to determine the characteristics of wet and dry spells. Finally, NCEP-NCAR reanalysis climate data was used to analyse the atmospheric circulation associated with wet and dry years. It has been found that the pattern of the SST along the eastern Pacific and the Indian oceans seem to influence the occurrence of the anomaly rainfall years. Correlation analysis shows that Eritrean summer rainfall is more strongly correlated with the eastern Pacific and Indian Oceans SST. Thus, the drought records in Eritrea are mainly associated with EI Nino events.
Open Access
New identities for Legendre associated functions of integral order and degree
(1973) Schach, Stephen Ronald; Brundrit, Geoff
In the solution of the boundary value problems of mathematical physics in a separable 3-dimensional coordinate system, the shape of the boundary of the space may be such that the Green's function of the second order differential operator can be expanded as an infinite series of orthogonal functions. In many coordinate systems (such as the spherical, spheroidal and some cyclidal systems) these expansions are given in terms of Legendre associated functions of integral order and degree. Starting with Dougall's identities for Legendre associated functions of non-integral degree, new identities for infinite series of Legendre associated functions of integral degree are derived. Uniform convergence of each new identity is investigated in detail. The direct applicability of these identities is demonstrated by using them to verify theorems satisfied by the Dirichlet Green's function of the infinite half-space and of the interior of the prolate hemispheroid. The results and techniques are then generalized, and a sufficient condition found under which a generalized orthogonal function which satisfies Dougall's identity will also satisfy the new identity. This theorem is applied to the Legendre associated function, the generalized Legendre associated function and to the Jacobi function.
Open Access
New mathematical models of inert gas transport through biological tissue in hyperbaric environments
(1973) Hennessy, Thomas Richard; Brundrit, Geoff; Van Zyl, J J W
The thesis is concerned with a fundamental mathematical analysis of inert gas transport through biological tissue at a raised ambient partial pressure. Three basic time-scales of transport in tissue are defined and their relationship examined and compared with existing models, which e.re shown to be usually inadequate in one or more ways. As a result three new mathematical models are proposed and solved both asymptotically and numerically. The first is applied to experimental data for non-perfused tissue which yields an improved value of the intracellular diffusion coefficient for nitrogen. An expression is also derived which should be useful in evaluating this constant and the volume fraction of extracellular fluid. The second embraces a number of current models and is applicable to perfused tissue. It should be useful in interpreting inert gas uptake curves. The model is applied to experimental data, and a source of possible error is discovered in using experimental non-asymptotic time constants. The third is a model which claims to resolve the controversy between the diffusion and perfusion theories of gas transport in tissue. The result is that in the large, diffusion is more important than perfusion, except in muscle tissue where they interact. Three different methods of numerical inversion of the Laplace Transform are compared and one is shown to be the most useful for solving gas uptake problems. The main result of the thesis is a contribution to the establishment of a mathematical basis for gas transport in various situations in the biological sphere.
Open Access
Regional estimates of potential new production in the southern Benguela upwelling system
(1996) Waldron, Howard Neil; Brundrit, Geoff; Probyn, Trevor A
The estimation of annual potential new production at a regional scale for the decade of the 1980s provides the main focus of this thesis. New production is the proportion of total primary production which relies on the uptake of NO₃-N by the phytoplankton while potential new production assumes that all the NO₃-N available to primary producers is assimilated. The Redfield ratio between carbon and nitrogen in the marine environment was used to express rates of potential new production in terms of carbon. In order to arrive at annual estimates of potential new production it was necessary to progress from the event-scale to the annual scale. This was achieved by the adoption of two novel methods of approach. Event-scale estimates of potential new production were made by assessing the amount of NO₃-N present in the nominal euphotic zone as a result of upwelling. Remotely-sensed images of sea surface temperature (SST) coincident with or shortly after upwelling events were used in combination with a derived relationship between SST and NO₃-N concentrations integrated over the depth range of the productive surface layer in the southern Benguela region. In this way it was possible to overcome the main shortcoming of passive satellite imagery by quantifying a sub-surface variable. The event-scale estimates of potential new production were extrapolated to the annual-scale by taking into account the dynamics of the system. Sea level fluctuation at the coast preceding upwelling events was found to be related to the event-scale estimates of introduced NO₃-N and hence used as a proxy for upwelling. Using a ten year record of sea level, estimates of annual potential new production, spanning the period June to May inclusive, were made for the period 1980/81 to 1989/90. Bibliography: pages 92-108.
Open Access
SOFAR signal shaping : influence of topography and relative source depth
(2000) Krige, Leon; Brundrit, Geoff
The shaping of SOFAR signals by topographic interference and the depth of the source relative to the axis of the sound channel is investigated. Eight shot signals from five different sites near Cape Town were received at three widely spaced hydrophones at Ascension over a number of 4.4 Megametre lines. These signals show the effects of (1) sources at different depths relative to the axis of the sound channel, (2) blocking and partial blocking of direct rays by seamounts and narrow undersea ridges and (3) a sound speed profile displaced downwards by a filament of warm surface water. Comparisons with ray trace and normal mode models show that some of the received signals consist of a combination of slower, refracted rays and faster rays reﬂected off Walvis ridge. The reflected energy starts earlier and has a greater amplitude than would be expected from ray trace modelling. This energy will act as noise and make ATOC (Acoustic Thermometry of Ocean Climate) determinations more difficult.
Open Access
The South African tide gauge network and its contribution to maritime safety
(2021) Farre, Ruth Elizabeth; Hermes, Juliet; Brundrit, Geoff; Ansorge Isabel
The use of tidal information and its application to, and value within, the study of climate change, sea-level rise and the tides effects on marine life is well known. However, little work has been done on the more technical aspects of tidal measurements and the use within maritime safety in accordance with international standards. This thesis will address the key technical elements in the design and realisation of the national tide gauge network around the South African coastline and how the data collected can be used to increase maritime safety around the South African coast, as well as the additional benefits the data may have both nationally and internationally. The South African Navy's national tide database, both actual records and predictions, as well as current and historical records, was used in order to establish the requirements for a sustainable tide gauge network. In addition, various case studies and data analysis were carried out, as were personal communications with various naval personnel. Establishing and maintaining a sustainable tide gauge network requires research and testing of various tide gauges. Identification of the correct locations to install the tide gauges is key to its success. Although the current South African Tide gauge network is not complete it does provide adequate coverage and is vitally important in the production of navigational products. The manner in which the raw collected tide data is processed and used for creating predictions meets international requirements and standards. Tidal predictions are a necessity for shallow water maritime operations and shallow water navigation, under keel and masthead clearances. The tide gauge network provides data to various organisations for storm surge and tsunami warnings while contributing to the “blue economy” through projects like Operation Phakisa. This thesis provides a comprehensive overview of South Africa's tide gauge network, investigates and justifies the reasons behind the locations and gives in depth examples of the essential use of tidal information for maritime safety.
Open Access
Tropical cyclones in the South-West Indian Ocean : intensity changes, oceanic interaction and impacts
(2008) Mavume, Alberto Francisco; Rouault, Mathieu; Lutjeharms, Johann R E; Brundrit, Geoff
This study investigates the climatology, intensification and ocean atmosphere interaction in relation to the passage of tropical cyclones (TCs) in the South-West Indian Ocean (SWIO). A Climatology of TCs in the SWIO including landfall in the area of Mozambique and Madagascar was developed for the 1952-2007 and 1980-2007 periods.
Open Access
Variability and characterizations of wind events over the Cape Columbine region along the west coast of South Africa
(2012) Ramulifho, Aluwani Elijah; Reason, Chris; Brundrit, Geoff
Wind data over Cape Columbine is investigated over a 38 year long time series to determine whether there are noticeable changes or variability through time. The long period dataset is collected using an Automatic Weather Station. Diurnal and seasonal to annual characterizations of wind together with occurrences of related events are established. Other linkages with synoptic to near-global (e.g., ENSO) scale circulations are investigated by examining relationships with other meteorological variables. This is achieved by complimenting long term data series with data from the new technologically advanced Automatic Weather Station (AWS). From the new AWS, several relationships and connections between different atmospheric variables were developed. These include the relationship between Air Temperature and Humidity, Wind Speed and Atmospheric Pressure, and Wind Direction and Rainfall. The AWS is able to monitor the passage of synoptic (e.g., cold fronts) and mesoscale (e.g., coastal low) weather systems. Passages of cold fronts over time could easily be detected by utilizing all the different variables as recorded by the AWS (Air Temperature, Wind Speed and Direction, Relative Humidity, Atmospheric Pressure and Rainfall). The most significant atmospheric system that passed over Cape Columbine over the 2009/10 summer was a deep low pressure system, where a significant amount of rainfall, atmospheric pressure depression and drop in temperature were witnessed. There were 17 upwelling events during this six months study period. Several of them were short-lived at 4 to 6 day durations, and the longest upwelling event occurred on the 24th December 2009 and lasted until January 14th 2010. An investigation into the variability in characteristics and behavior of wind signals at Cape Columbine was conducted. The study investigated wind behavior for the period 1957 to 1995 with data collected from a point source Automatic Weather Station. Apart from the characteristics of wind over time period, linkages with large scale circulations, frequency and duration of these wind events occurrences were studied. These were achieved by partitioning the study period into different time scales; namely, the diurnal, seasonal, inter-annual and the decadal. On the diurnal time scale, a strong variability in wind strength over the course of each day due to the land/sea breeze existed. Seasonally, the wind regime in Cape Columbine was found to be driven by the latitudinal shifting of the semi-permanent South Atlantic Anticyclone. However, there are pre-dominant southerly winds throughout the year. The long record showed the possible influence of ENSO and the Benguela Niño on interannual and decadal variations in the winds at Cape Columbine.