Christo Rautenbach

A computationally inexpensive operational wave forecasting system taking wave‐current
interactions over the fast‐flowing Agulhas Current into account is explored. Spectral wave model
hindcasts at 1/16 of a geographical degree are employed using the Simulating Waves in the Nearshore
(SWAN) wave model. Unidirectional wave‐current coupling, with currents affecting waves, is performed
using both Operational Mercator 1/12 degree (ORCA12) and GLOBCURRENT 1/4 degree… Show more

A computationally inexpensive operational wave forecasting system taking wave‐current
interactions over the fast‐flowing Agulhas Current into account is explored. Spectral wave model
hindcasts at 1/16 of a geographical degree are employed using the Simulating Waves in the Nearshore
(SWAN) wave model. Unidirectional wave‐current coupling, with currents affecting waves, is performed
using both Operational Mercator 1/12 degree (ORCA12) and GLOBCURRENT 1/4 degree currents.
Comparisons between the coupled and uncoupled simulations indicate that wave amplification or lowering
is highly dependent on the current magnitude and wave incident angle. In opposing current scenarios, the
significant wave height can increase by 20–40% and by 60% in extreme cases. Model validation is performed using Jason‐3 satellite altimetry measurements and in situ directional wave buoy measurements. The nearshore wave directional dispersion is found to be significantly influenced by the core of the Agulhas Current. This is especially prominent in current‐following cases with wave‐current direction differences of up to 20°. The findings are all corroborated by simplistic, idealized models. Show less

Regional storm tidal levels of the South African coastline are investigated by means of a calibrated and validated numerical model. The model was developed utilizing the shallow water hydrodynamic model, Delft3D. This model was coupled (online) with a non-stationary spectral wave model (developed in the Simulating WAves in the Nearshore (SWAN) numerical code). A local, 4.4 km version of the Unified Model was applied as atmospheric forcing for the coupled system. The models presented in this… Show more

Regional storm tidal levels of the South African coastline are investigated by means of a calibrated and validated numerical model. The model was developed utilizing the shallow water hydrodynamic model, Delft3D. This model was coupled (online) with a non-stationary spectral wave model (developed in the Simulating WAves in the Nearshore (SWAN) numerical code). A local, 4.4 km version of the Unified Model was applied as atmospheric forcing for the coupled system. The models presented in this study form part of the operational marine forecasts of the South African Weather Service, Wave and Storm Surge model. The operational protocol and model calibration and validation are presented via statistical correlations with measured water levels at six South African coastal locations. The main calibration parameters and thus physical drivers were winds, atmospheric pressure and waves. The validated numerical model is used to provide an experimental physical description of South African storm surge characteristics, per coastline. The dominant driver of South African storm surge is winds associated with mid-latitude cyclones. Further novelty in the present study is the quantification of the relative contribution of extreme storm wave set-up to the South African storm surge signal. This wave set-up contributes approximately 20% of the total surge signal in the southwest, with winds contributing approximately 55%. The importance of the continental shelves is also elucidated concerning the frictional shoaling effects of the long surge wave propagation. Show less

Over the past three decades, marine resource management has shifted conceptually from top-down sectoral approaches towards the more systems-oriented multi-stakeholder frameworks of integrated coastal management and ecosystem-based conservation. However, the successful implementation of such frameworks is commonly hindered by a lack of cross-disciplinary knowledge transfer, especially between natural and social sciences. This review represents a holistic synthesis of three decades of change in… Show more

Over the past three decades, marine resource management has shifted conceptually from top-down sectoral approaches towards the more systems-oriented multi-stakeholder frameworks of integrated coastal management and ecosystem-based conservation. However, the successful implementation of such frameworks is commonly hindered by a lack of cross-disciplinary knowledge transfer, especially between natural and social sciences. This review represents a holistic synthesis of three decades of change in the oceanography, biology and human dimension of False Bay, South Africa. The productivity of marine life in this bay and its close vicinity to the steadily growing metropolis of Cape Town have led to its socio-economic significance throughout history. Considerable research has highlighted shifts driven by climate change, human population growth, serial overfishing, and coastal development. Upwelling-inducing winds have increased in the region, leading to cooling and likely to nutrient enrichment of the bay. Subsequently the distributions of key components of the marine ecosystem have shifted eastward, including kelp, rock lobsters, seabirds, pelagic fish, and several alien invasive species. Increasing sea level and exposure to storm surges contribute to coastal erosion of the sandy shorelines in the bay, causing losses in coastal infrastructure and posing risk to coastal developments. Since the 1980s, the human population of Cape Town has doubled, and with it pollution has amplified. Overfishing has led to drastic declines in the catches of numerous commercially and recreationally targeted fish, and illegal fishing is widespread. The tourism value of the bay contributes substantially to the country’s economy, and whale watching, shark-cage diving and water sports have become important sources of revenue. Show less

A calibrated and validated, regional tidal model for South Africa is presented.
Comparisons between the new model, TPXO 7.2, 8 and in situ measurements are made.
Semi-diurnal coastal resonance is identified and quantified.
Tidal characterization is performed via tidal constituent mapping.
The phase lags of the mixed progressive and standing tidal waves are mapped.

Highlights

•During austral summer a negative (positive) SAM results in bigger (smaller) waves.
•During austral summer a negative ENSO results in smaller, more southerly waves.
•During austral summer a positive ENSO results in bigger, more westerly waves.
•Waves during austral winter do not respond coherently to either SAM or ENSO.
•A shift to smaller and more southerly waves occurred in the early 1990s.

South Africa hosts a multitude of interests along its coastline and in its coastal waters. These range from recreational and small-scale commercial activities to those related to tourism and large-scale industry. The associated need for robust coastal risk management is key in securing both economic interests and safety of life. Despite widespread intuitive appreciation of the relationship between weather and coastal safety, objective analysis in this regard is lacking. This study strives to… Show more

South Africa hosts a multitude of interests along its coastline and in its coastal waters. These range from recreational and small-scale commercial activities to those related to tourism and large-scale industry. The associated need for robust coastal risk management is key in securing both economic interests and safety of life. Despite widespread intuitive appreciation of the relationship between weather and coastal safety, objective analysis in this regard is lacking. This study strives to address this gap to assist, for example, coastal management,
the efficient deployment of search and rescue (SAR) resources and investment in safety infrastructure.
Further, we used statistical relationships between weather and incident-occurrence to develop a basic risk characterisation framework for different coastal areas and activities. Results from our investigation revealed varying sensitivities to coastal marine meteorological parameters. For activities in which individuals are more inherently vulnerable (e.g. swimming), incidents were most frequent during Good conditions. For activities involving small personal water craft (e.g. kayaks), incidents were most frequent during Marginal conditions. Incidents involving small vessels (e.g. rigid-inflatable boats) were most numerous during Bad conditions, with no clear pattern in respect of larger, commercial vessels (e.g. fishing trawlers). Finally, we present empirically derived risk coefficients, showing the relationship between risk, user vulnerability and user exposure for given weather severity scenarios. Show less

The Durban Bight beaches, on the KwaZulu-Natal coast of South Africa, have been the topic of several studies in the past, because of their recreational and economic value. These beaches are mainly nourished via a sand-pumping scheme, which has been the case since the 1980s. The present study investigated the longer-term dynamic stability of the Durban Bight beaches using a one-dimensional numerical shoreline model. The wave conditions that drive the longshore transport of sand were simulated… Show more

The Durban Bight beaches, on the KwaZulu-Natal coast of South Africa, have been the topic of several studies in the past, because of their recreational and economic value. These beaches are mainly nourished via a sand-pumping scheme, which has been the case since the 1980s. The present study investigated the longer-term dynamic stability of the Durban Bight beaches using a one-dimensional numerical shoreline model. The wave conditions that drive the longshore transport of sand were simulated using the numerical spectral wave model Simulation WAves in the Nearshore (SWAN). The shoreline model was calibrated and validated against existing historical data. The calibrated model was then employed to simulate possible future scenarios. These included climate change phenomena and the influence of anthropogenic changes. Each of the simulations predicts a twenty-year period, with results extracted at five-year intervals. The results are presented and discussed, and the emphasis is placed on the importance of maintaining the required sand-bypassing rates and the awareness of the possible effects of climate change on shoreline maintenance and management. Show less

Modern day tomographs enable the research community to investigate the internal flow behavior of a fluidized bed by non-invasive methods that partially overcome the opaque nature of a dense bubbling bed. Each tomographic modality has its own limitations and advantages and in the present study two modern day tomographic systems were evaluated with respect to their performance on a cold dense fluidized bed. The two tomographs investigated are an electrical capacitance tomography (ECT) tomograph… Show more

Modern day tomographs enable the research community to investigate the internal flow behavior of a fluidized bed by non-invasive methods that partially overcome the opaque nature of a dense bubbling bed. Each tomographic modality has its own limitations and advantages and in the present study two modern day tomographic systems were evaluated with respect to their performance on a cold dense fluidized bed. The two tomographs investigated are an electrical capacitance tomography (ECT) tomograph and a time-resolved X-ray tomography tomograph. The study was performed on spherical glass particles with various particle size distributions that could mainly be classified as Geldart B or D particles. Two experimental towers were employed, one with a diameter of 10.4 cm and the other 23.8 cm while compressed air was used as fluidizing fluid during all of the experiments.

Results obtained with both systems are provided in comprehensive figures and tables and some first results are obtained with the time-resolved X-ray tomography system. The bubble size measurements of both tomographs are compared with several theoretical correlations via the root mean square error of the predictions (RMSEP). With the results it was also concluded that a small amount of small particles can noticeably alter the fluidization hydrodynamics of a powder. The bubble frequencies are also presented to aid in understanding the hydrodynamic behavior of the powders investigated. A comprehensive summary of the two tomographic modalities is also provided.
Show less

Fluidization experiments were performed using several particle size distributions of spherical glass particles, ranging from Geldart B to D. An Electrical Capacitance Tomography (ECT) tomograph was utilized in the present study and its usefulness as a diagnostic tool is illustrated. During the experiments a 10.4 cm diameter column was utilized and the column was operated at atmospheric pressure and room temperature (cold fluidized bed). Statistical analyses were performed on the average solid… Show more

Fluidization experiments were performed using several particle size distributions of spherical glass particles, ranging from Geldart B to D. An Electrical Capacitance Tomography (ECT) tomograph was utilized in the present study and its usefulness as a diagnostic tool is illustrated. During the experiments a 10.4 cm diameter column was utilized and the column was operated at atmospheric pressure and room temperature (cold fluidized bed). Statistical analyses were performed on the average solid fraction data obtained using the ECT tomograph. Using the time domain skewness and kurtosis the time series could be characterised and the quality of fluidization is determined at different superficial gas velocities (Azizpour, H., Sotudeh-Gharebagh, R., Zarghami, R., Abbasi, M., Mostoufi, N., Mahjoob, M., 2011. Characterization of gas–solid fluidized bed hydrodynamics by vibration signal analysis. International Journal of Multiphase Flow, 37, 788–793). Statistical analysis is also used to characterise the influence of small particles on the bed hydrodynamics.
Show less

The aim of this work is to study flow behaviour in a fluidized bed with different mixtures of particles.
Mathematical simulation is an alternative way to study the fluidization behaviour.
Experiments are performed in a cylindrical bed with a uniform air distribution.
Spherical glass particles with different mixtures of particles are used in the experiments.
The pressure and void fraction variations are observed for two different powders and mixtures of the powders.
The commercial… Show more

The aim of this work is to study flow behaviour in a fluidized bed with different mixtures of particles.
Mathematical simulation is an alternative way to study the fluidization behaviour.
Experiments are performed in a cylindrical bed with a uniform air distribution.
Spherical glass particles with different mixtures of particles are used in the experiments.
The pressure and void fraction variations are observed for two different powders and mixtures of the powders.
The commercial CFD code Fluent 6.3 is used for the corresponding simulations. Eulerian-Eulerian model are used to simulate a multiphase bubbling fluidized bed.
The influence of particle size distribution on the bubble size, pressure and void fraction variation along the bed has been investigated computationally.
The computational results are compared to the experimental data and the discrepancies are discussed. Show less