Masters Degrees (Civil Engineering)

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The application of the numerical wind wave model SWAN to a selected field case on the South African coast
(Stellenbosch : University of Stellenbosch, 2002-03) Van der Westhuysen, A. J.; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: In this study the numerical short wave model SWAN is evaluated for application to a selected coastal region in South Africa. The aim of this study was to evaluate the degree of accuracy with which SWAN can simulate prototype nearshore wave spectra and wave parameters (e.g. wave height, mean wave direction and mean wave period) for an Algoa Bay field case. Algoa Bay represents a typical deep, sheltered embayment on the South African south coast, which is exposed to high-energy swell. Sensitivity analyses on various wave-related processes were also done, with the aim of establishing the dominant physical processes and appropriate model setup for the Algoa Bay field case. With the dominant wave-related processes and appropriate model setup for the Algoa Bay field case established, selected final runs were performed to determine the degree of accuracy with which SWAN can simulate prototype conditions, by comparing its results with available field recordings. This study comprises a review of the SWAN evaluation work conducted to date by others, an overview of South African coastal conditions, and numerical model simulations. The model simulations, which represent the main focus of this study, were conducted for a selection of available offshore wave conditions (at 85 m water depth) observed during the Algoa Bay field case and were compared to available nearshore observations (at 17 m water depth). Environmental conditions of waves, wind and currents were included in these simulations. The study focuses on model application and sensitivity analysis, rather than model development, and includes evaluation of all relevant processes, without focussing on any specific model aspect. The results of this study show that SWAN simulations correlated well with observations at the nearshore station in Algoa Bay, both in wave spectral shape and its associated parameters. Dominant processes identified for the field case were depth-induced refraction, bottom friction and directional spreading. This finding agrees with those of previous evaluations of SWAN and previous modelling experience by others. It is shown that high-energy swell is relatively more sensitive to the choices of model setup than wind sea. Based on the simulation results of high-energy swell, it is concluded that the calculation of depth-induced refraction in SWAN seem to contain a degree of inaccuracy. It is also concluded that the findings of this study could be used as a guideline to SWAN modelling studies along the South African south coast.
Artificial aeration on stepped spillways with piers and flares to mitigate cavitation damage
(Stellenbosch : Stellenbosch University, 2017-12) Koen, Jaco; Basson, G. R.; Bosman, D. E.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Stepped spillways have been used for approximately 3500 years and, with the recent technical advances in Roller Compacted Concrete construction, these spillways have received a renewed interest over the past few decades. However, because of the possibility of cavitation damage to the spillway chute at higher discharges, the maximum discharge that these spillways can safely handle has been limited. A pre-emptive measure to combat cavitation damage is to introduce flow aeration at the pseudo-bottom. In order to aerate the flow, various crest pier aeration structures were investigated to ultimately increase the maximum safe unit discharge capacity of stepped spillways. Different aeration structures were investigated, on two types of spillways (Type A and Type B), with the aid of two physical hydraulic models. The Type A spillway was a 1:15 scale, USBR stepped spillway with transitional crest steps and a constant step height of 1.5 m. The spillway performance of each aeration structure was determined by measuring the air concentration at the pseudo-bottom and the minimum pressure at the step riser. Experiments on the Type A spillway were carried out at a prototype unit discharge of 30 m²/s for the investigation of different pier configurations near the spillway crest. The crest pier configurations comprised two pier nose shapes, two pier lengths and the addition of a flare to the pier. The pier configuration results were compared with the performance of an unaerated stepped spillway. The maximum safe unit discharge capacity of an unaerated stepped spillway had previously been determined by Calitz (2015) to be 25 m²/s. The implementation of the bullnose, short pier, increased the maximum safe unit discharge capacity to 30 m²/s, by eliminating the risk of cavitation damage in the vicinity of the natural aeration inception point. The Type B spillway was a WES stepped spillway, with a smooth ogee crest and constant step height of 1 m, which was used to evaluate the Chinese developed Flaring Gate Pier (FGP) design. The model scale for this spillway was 1:50. The design of the model was based on the Dachaoshan Dam (China), which has a design unit discharge of 165 m²/s. The FGP designs consisted of an X-Shape and a Y-Shape FGP, together with a slit-type flip bucket. The performance of these aerators was compared to an unaerated stepped spillway for prototype unit discharges of 50 m²/s to 200 m²/s. The most notable improvement was the increase in the maximum safe unit discharge capacity to 50 m²/s in the case of the X-Shape FGP. In summary, the addition of a short, bullnose crest pier on low head/velocity stepped spillways increased the maximum safe unit discharge capacity to 30 m²/s. In the case of a high head/velocity stepped spillway, while the X-Shape FGP improved the maximum safe discharge capacity to 50 m²/s.
The characterisation of South African sea storms
(Stellenbosch : University of Stellenbosch, 2006-12) MacHutchon, K. R.; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
This research provides and overview of sea storms around the South African Coast in terms of weather types, characteristic wave statistics, storm processes and wave energy. Sea Storm Profiles are unique to the particular storm events causing them, but they can be associated with Equivalent Wave Energy (EWE) Storm Profiles, which are representative of them and have a linear, symmetric, “Capital Lambda” ( Λ ), shape. The actual storm profile and the EWE Profile are equivalent in wave energy, and the benefit of the EWE is that it is regular and can be can be readily compared with another EWE Storm Profile for analysis, and for the comparison of impacts. The ability to compare the impacts of Sea Storms from different areas, on the basis of characteristic Equivalent Wave Energy (EWE) Storm Profiles within the South African Coastal Regions, is considered to be advantageous. This will allow Engineers to apply the knowledge gained in one area to another with a similar EWE Storm Profile, with more confidence. There will always be the need for site-specific investigations, data recording, data analysis and interpretations in Coastal Engineering Work, but one needs to start with an understanding of the general nature of the coastal region in which one is working. This research adds to the background “Body of Knowledge” relating to the character of the sea storms in the Regions around South Africa. The study is based on a literature survey of atmospheric weather, sea wave theory and wave climates, as well as the analysis of weather and sea state data at selected recording stations around the South African Coastline.
The cross-shore distribution of grain size in the longshore transport zone
(Stellenbosch : University of Stellenbosch, 2009-12) Soltau, Christoph; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Calculation of longshore sediment transport rates is a typical part of coastal engineering work. One of the important inputs to such calculations is the sediment grain size. A single, representative grain size is typically required. The inter-tidal beach is the most convenient and common area from which grain size data can be obtained. Yet only a fraction of the longshore transport occurs at the beach, with the bulk of the transport occurring in the surf zone, where sampling is difficult. Sediment transport calculations can be improved if the representative grains size is also characteristic of this area. A better understanding how the grain size in the longshore transport zone compares to the beach grain size is required. A review of relevant literature indicates that limited attention has been given to quantifying the grain size in the longshore transport zone. No previous investigations were found that tried to link the longshore transport zone grain size to that found on the beach. A comprehensive analysis of beach and longshore transport zone grain sizes was therefore undertaken and is described in this thesis. Beach grain sizes were compared to those in the longshore transport zone for three different locations around the world: Published grains size information, together with detailed wave and profile data, was obtained from the US Army Field Research Facility at Duck; a second data set was obtained from measurements done at Bogenfels in Namibia; a third dataset was compiled from sampling undertaken by the author in Table Bay, South Africa. A total of 189 samples were collected at four sampling lines in Table Bay between September 2005 and September 2006. Samples were collected across the entire profile from the primary dune to a water depth of 10 m. Samples were collected by grab in the offshore, and by swimming and diving in the surf zone. The location of the four Table Bay sampling lines was chosen so as to obtain data from beaches with different wave and grain size characteristics. Together with the Duck and Bogenfels data, data from six different beaches was therefore available for study. A settling tube was used to determine the grain sizes. Verification of the settling tube analyses against conventional sieving indicated a good comparison. However, the settling tube proved unsuitable for processing of samples with coarse to very coarse material, for which sieving was conducted instead. The grain size at the mid-tide level has been used to characterise the beach. The limits of the longshore transport zone were defined by calculating the cross-shore distribution of longshore transport with the Unibest model. Simpler methods, such as the depth of closure, either overestimated or underestimated it, depending on which wave condition was used in the depth of closure formula. It was found that the beaches with steeper mid-tide beach slopes, such as Bogenfels and northern Table Bay, had coarser median grain sizes than more gently sloping beaches such as found in the south of Table Bay. On energetic beaches, the mid-tide beach grain sizes were significantly coarser than those in the surf zone, by more than twice. At less exposed locations, such as Duck and the central Table Bay beaches, this difference was less. At sheltered locations, such as the southern sampling lines in Table Bay, the mid-tide beach grain sizes are virtually the same as those found in the surf zone. The surf similarity parameter was used to compare the characteristics of the different sites. This parameter was defined using the average wave height seaward of breaking, and the mid-tide beach slope. The ratio between the longshore transport zone grain size and the mid-tide beach grain size was found to be similar to the inverse of the surf similarity parameter for the six beaches that were studied. These findings have led to an improved understanding of the grain size in the longshore transport zone and allow a better characterisation of the representative grain size to use for sediment transport calculations.
The effect of artificial reef configuration on wave breaking intensity relating to recreational surfing conditions
(Stellenbosch : University of Stellenbosch, 2009-03) Johnson, Craig Michael; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
Multi purpose reefs are a relatively new concept that incorporate functionalities of beach stabilization, breakwater/seawall protection, biological enhancement and recreational amenity. Economic benefits increase their attractiveness. There is, however, some degree of uncertainty in design guidelines as to the predictability of each of these aspects. With regards to recreational amenity enhancement, one such uncertainty exists in the ability to predict the reef configuration required to give a certain degree of surfability of a reef, and more specifically, to predict the shape of a plunging wave. An extensive survey of the relevant literature has been conducted to provide a background on multi purpose reefs and the uncertainties in predicting the success of multi purpose reefs in achieving their design objectives. A study of wave breaking has been done, along with an analysis of existing breaker height and breaker depth formulae. The effects of bottom friction, refraction, shoaling, winds currents and varying water level on wave breaking has been addressed. Surfability aspects were reviewed including a definition of breaking intensity which is defined by the wave profile in terms of vortex shape parameters, and other surfability parameters that influence the surfability of a reef. Background on numerical modelling methods has been given, along with a description and some trial runs of a new and promising method, Smooth Particle Hydrodynamics. Numerical models were run using the open source SPHysics package in order to assess the applicability of the package in measuring vortex shape parameters. The SPHysics package is, however, still in a stage of development, and is not yet suitable for reef studies with very long domains and with high numbers of particles (required for sufficient resolution in the plunging vortex). A theoretical examination was done on the relevant literature in order to gain an insight into the dynamics affecting the development of the plunging vortex shape. A case study of a natural surf reef was carried out in order to give qualitative estimation of the wave dynamics and reef structure required to give good quality surfing waves and high breaking intensity. The West- Cowell surfing reef factor was used as a tool in predicting wave focusing effects of a naturally occurring reef. Extensive two dimensional physical model laboratory studies were conducted in order to quantify the effects of the reef configuration and wave parameters on breaking intensity. Design guidelines were developed in order to assist in the prediction of breaking intensity for reefs constructed with surfing amenity enhancement as one of their design objectives. The results show that large underwater topographic features can significantly affect the shape and size of incoming waves. Refraction, focusing and shoaling can transform ordinary waves into waves deemed suitable for surfing. The West-Cowell surfing reef factor gives reasonable results outside its applicable range. The 2D physical model laboratory tests show significant variations in vortex shape parameters due to interactions between broken and unbroken waves in a wave train and also to the reflections developed in the flume. Results show that the predicted trends agree with the observations. The results also show that the junction between the seaward reef slope and the horizontal crest may have an effect on the wave shape in the form of a secondary crest between the primary crests. Design guidelines based on the results are presented, and show that breaker height formulae for smooth planar slopes show good agreement with the values of breaker heights measured in the physical model tests, and that existing breaker depth formulae show average agreement. The design guidelines could assist with more effective design of artificial reefs for surfing purposes.
Integrity assessment procedure for buffer dune systems on the Cape South Coast, South Africa
(Stellenbosch : University of Stellenbosch, 2011-03) Barwell, Lauriston; Toms, G.; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The hypothesis postulated in this research, namely that the effectiveness of natural and constructed buffer dune systems can be assessed by a set of indicators that defines the integrity of the dune system and triggers informed management decisions, was evaluated and proved to be essentially true. Two key objectives, namely (1) the identification of key indicators that define the buffer dune integrity; and (2) the development of a scientifically defendable and practical checklist-based method of gathering qualitative information on the identified key indicators so as to guide decision-making at municipal level formed the core of the study. The six dune integrity indicators that collectively define the risk profile of a particular site along the Southern Cape coastline are (1) the degree of protection from prevailing wave energy, (2) the characteristics of the dominant winds and sand supply during the dry season, (3) the relative height of the foredune, (4) the degree of pressure on the buffer dune due to humans, (5) the vulnerability of the type of coastline to erosion, and (6) the coastline stability considering the prevailing coastal processes. The first two indicators relate to the natural (permanent) characteristics of the site and can be defined by experts and presented in the form of a risk and vulnerability atlas layer for direct use by non-experts. The third and fourth indicators relate directly to the implementation of proactive assessment and appropriate management actions to ensure a high level of buffer dune integrity. The last two indicators allow for management intervention to reduce the vulnerability but may entail costly engineering solutions and require expert input. A conceptual risk profile assessment procedure and a decision support guideline incorporating these indicators were developed and evaluated for relevance and practicality through a series of workshops with municipal officials along the south coast of South Africa. It was seen that although some initial basic training may be required, carrying out rapid assessments of the environmental status of key components of an identified human–nature system, such as a buffer dune, is practical and achievable by non-experts.
Investigation of marine components of large direct seawater intake & brine discharge systems for desalination plants, towards development of a general design approach
(Stellenbosch : University of Stellenbosch, 2010-03) Le Roux, Maria; Bosman, D. E.; Toms, G.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: This investigation focused on the marine components of large direct seawater intake and brine discharge systems for seawater desalination plants, with the main aim to provide an overall design approach for these components. Due to its complexity, an overall and systematic design approach, addressing all the components (feedwater requirements, plant technology, marine structures and environmental issues) is required to ensure an optimum design. A literature review was done on the various desalination technologies, the main components of a seawater desalination plant, as well as the physical, hydraulic, operational and environmental issues regarding seawater extraction facilities, marine pipelines and discharge structures (diffuser). In order to obtain practical input to the development of an overall design approach, information regarding the marine structures of ten of the largest existing seawater desalination plants throughout the world were obtained and compared with each other and the available technologies. By way of example, the recently designed marine components of a new seawater reverse osmosis desalination plant in Namibia were reviewed and, as part of this thesis, alternative conceptual concepts which will include two additional components (sump and brine reservoir) were designed. The alternative design was compared with the actual design in order to determine the feasibility of the alternative in terms of operation and cost and subsequently provide input to the overall design recommendations. Furthermore, from the literature review it seems that there are still significant uncertainties regarding the required performance of a brine (dense) outfall and this required more attention in terms of environmental and hydraulic performance. Based on the Namibian plant, the diffuser configuration was analysed in terms of its hydraulic and environmental performance and subsequently some general guidance with specific respect to a brine diffuser was developed, which in turn formed part of the overall design approach for the marine components. Finally, the design approach for seawater intake structures, brine outfalls and the connecting marine pipelines is provided in the form of flow diagrams.
An investigation of the wave energy resource on the South African Coast, focusing on the spatial distribution of the South West coast
(Stellenbosch : University of Stellenbosch, 2008-03) Joubert, J. R.; Bosman, D. E.; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
This thesis is an investigation of the wave power resource on the South African coast, focusing on the spatial distribution of wave power of the coastal region exposed to the highest wave power. The study’s main objective is to provide a detailed description of the spatial distribution of wave power to assist in the selection of locations for deployment of Wave Energy Converter (WEC) units in this zone. The study methodology employed to achieve this main objective entails an analysis of measured wave data recorded at wave recording stations distributed along the South African coast. The analysis provided a general description of wave power at locations for which wave data exist. From this analysis it was found that the South West Coast is exposed to the highest wave power, with an average wave power of approximately 40 kW per meter wave crest. The rest of the South African coast is exposed to average wave power between approximately 18 kW/m to 23 kW/m. The wave power characteristics on the South West Coast region (from Cape Point to Elands Bay) were therefore the focus of this thesis. The study objective was achieved by transferring deep sea wave data into the nearshore South West Coast study area with the Simulating WAves Nearshore (SWAN) wave model. The deep sea wave data was obtained from a 10 year period of available hindcast data. A simplified simulation procedure was required in order to make the study practically feasible. A sensitivity analysis was carried out to determine the validity of the simplified simulation procedure and it was found that the procedure slightly overestimate wave power in the shallower water regions due to the underestimation of energy dissipation processes. This overestimation was deemed acceptable for the dominant wave conditions and the simplified model was therefore applied in the study. An appropriate programming system was developed and used to transfer the available 10 year deep sea wave data into the selected South West Coast region. From this exercise spatial distribution of wave power and related statistical parameters were obtained for the study area. The accuracy of the modelled output was investigated by directly comparing it to wave data recorded during the overlapping recording period. It was found that the model slightly overestimates the monthly wave power resource compared to the measured data with a maximum overestimation of 9%; which is sufficiently accurate for the purpose of the study. The results of this investigation can be used for the identification of areas of high wave power concentration within the study area for the location of WEC units. Further numerical modelling is required for the detailed design of wave farms, especially if potential sites are located in shallow water (shallower than approximately 50 m).
Sediment transport regime in the area of the East London harbour entrance
(Stellenbosch : Stellenbosch University, 2004-12) Theron, Andre K.; Bosman, D. E.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: East London is situated on the south-eastern, Indian Ocean, coast o f South Africa. The sediment transport regime at East London is quite unlike the regimes at other ports in South Africa. A major ocean current (the Agulhas) flows exceptionally close to the coastline in this area, thus significantly affecting nearshore sediment movements. The proximity o f a strong ocean current opposed to the net longshore drift (wave driven transport) creates an anomalous sediment transport regime in comparison with that found at most coastlines throughout the world. Furthermore, the Port o f East London (Figures 1.1 and 1.2) is the only major river harbour in South Africa, which all results in a very intricate pattern o f sediment movement in the area, making it o f particular interest. The aims o f this thesis are to study the littoral sediment transports at East London, achieve a holistic understanding o f this complex sediment transport regime and to quantify the various transport rates as far as possible. The study area includes the coastal zone between the Goda and Nahoon Rivers (Figure 1.3) with the main focus on the Hood Point to Orient Beach area (Figure 1.4). The offshore marine environmental conditions are also considered as they have a strong influence on nearshore currents, waves and sediment transport patterns. The prerequisite to understanding the sediment transport processes is a thorough investigation o f the relevant coastal processes. Thus, an in-depth study is undertaken o f the physical environmental data/information, nearshore processes and coastal dynamics that determine or topography changes, the wave regime, the Agulhas Current, the nearshore current regime, the continental shelf sediment dynamics, sediment characteristics, seabed features, the Buffalo River, the tides, the wind regime, and coastal morphodynamics. The specific contribution of each aspect of the environmental data/information to the qualitative understanding of the overall sediment transport regime is determined, and the various modes of sediment transport in different areas are quantified. All o f this information is then synthesised into an expose' o f the sediment transport regime at East London, as briefly described in the conclusions (Section 5 and Table XI) and illustrated in Figure 4.30: There is a net longshore transport (wave driven) of about 250 000 m3 to 300 000 m3 per year on average from the Foreshore area towards the head of the main breakwater, with the total transport into the main sand trap and entrance channel areas from the south-west estimated at 275 000 m3/year. In the offshore zone, large amounts of sediment are transported downcoast by means of the strong Agulhas Current, which also has a significant influence on nearshore currents and sediment transport in the harbour entrance area. About 75 000 m3/year of sand is transported into the “Bar” area (the seaward part of the entrance channel) from the north-east with downcoast flowing nearshore currents, which is the predominant current direction. The riverine input into the harbour has been estimated at less than 10 000 m3/year of sand. In this thesis, therefore, the sediment transport regime (and sediment transport balance) is derived fo r East London, providing for the first time a holistic understanding o f the complex sediment transport regime. It is recommended that more field data should be collected on specific aspects of this study and that the longer term effects of major weather systems should be investigated further. The numerical wave modelling should be extended and the integration of field measurements and numerical modelling to predict sediment transport and resultant bottom changes should be assessed in detail. Ultimately, the information contained in this thesis should feed into a wider regional investigation, with the aim of drawing up a sediment budget for the entire “regional macro sedimentary cell” of which the present study area forms a part.

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Browsing Masters Degrees (Civil Engineering) by browse.metadata.advisor "Bosman, D. E."

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