Browsing by Author "Rautenbach, Christo"

Now showing 1 - 4 of 4
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    Analysing modelled nearshore wave climate variability and change as relevant to the traditional handline fishery of the South African South Coast
    (2018) Lyttle, Casey Tara; Jarre, Astrid; Backeberg, Björn; Rautenbach, Christo
    The South Coast traditional handline fishing communities of South Africa are integrated into a complex ecosystem where human and natural components interact and overlap on many different spatial and temporal scales. The South Coast marine ecosystem, on which the fishers depend, already suffers from depleted fish stocks. The South Coast handline fishery is therefore vulnerable to added stresses such as those induced by climate change. While fishers have noted that deteriorating sea state and a declining number of sea days caused by shifts in wind patterns are affecting their livelihood, applicable scientific research and data on scales relevant to the fishers is insufficient. Insight into the complexities involved in climate change and local-scale responses of these highly integrated social-ecological system therefore remains sparse. While South Coast nearshore winds have been the subject of recent research, the wave climate aspect of the nearshore sea state has not. In a recent project conducted by the Department of Environmental Affairs and the Council for Scientific and Industrial Research, Simulating WAves Nearshore model outputs spanning 17 years (from 1997 to 2014) were produced for the South African coastline, including the South Coast. Wind (speed and direction) and swell (significant wave height, peak wave direction and period) outputs from the WaveWatch III model (provided by National Centre of Environmental Prediction, US) were used as boundary conditions. The present study uses these wave model outputs to conduct an investigation into the nearshore local-scale wave climate of four traditional handline fishing towns of the South Coast: Witsand, Still Bay, Gouritz and Mossel Bay. Results suggest that the shape and bathymetry of the coastal sites influence average significant wave height, peak wave directions, and seasonal variability of the approaching swell waves. This is due to the nearshore processes of refraction, bottom friction and sheltering by headlands from the approaching swell, driven by the offshore swell. Additionally, the presence of low peak period waves depended on the focussing of waves that were generated by easterly winds during summer (i.e., wind-waves, which are shorter period waves compared to swell) driven by the synoptic-scale winds. While summer afternoon waves remained higher than morning waves from 1997 to 2014, the significant wave height difference did not change over time; however, variability increased post-2006, particularly for sites more exposed to approaching swell. A regime shift in mean significant wave height was detected for 2006 across the South Coast, from lower to higher waves. The more exposed study sites showed a strong seasonality (higher waves during winter than summer), where the duration of summer conditions lengthened post-2006 during the period of higher significant wave heights. Significant wave height increased significantly from 1997 to 2014 across the South Coast. Since swell dominates across the South Coast, the observed regime shift (including interannual variability) and trend is likely to be attributed to offshore swell. The recent increase in wave height variability is in line with fishers observations where increase in climate variability has been observed. The increase in wave height is also in line with fishers' observations which state that the sea state has deteriorated, and sea days have decreased. Additionally, the lengthening duration of summer conditions in waves was also observed by fishers in terms of winds. This analysis of South Coast wave climate contributes to bridging the gap between the first hand observations of fishers and conclusions drawn from coarse resolution scientific data.
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    Short-term sea level forecasting using machine learning techniques: A case study for South Africa
    (2022) Ingreso, Maria Kristina; Smit, Albertus; Rautenbach, Christo; Vichi, Marcello
    Seawater levels along the South African coastline are investigated with the use of machine learning techniques. In this study, data-driven methods, which are more computationally efficient in comparison to numerical models, are applied to predict seawater levels. The open-loop NARX model was developed using the Neural Net Time Series application from the Deep Learning Toolbox 14.0 provided by MATLAB® (Mathworks, 2020). A total of five inputs (atmospheric pressure, mean wave period and direction, wind speed and direction) and a single output of seawater level was fed into the neural network where 70 % of the data was used for training, 15 % was used for validation and the remaining 15 % was used to test the model. Three separate storm events that occurred along the coast of South Africa were used for the final model validation. Model performance was measured using the correlation coefficient (R), the root mean square error (RMSE), the bias and the Willmott indices of correlation. It was found that, through principal component analysis (PCA), atmospheric pressure, wind speed and direction and mean wave period and direction are important physical drivers of sea level. The overall model performance was better when all five met-ocean variables were included as inputs to the model than when one or two were excluded, with R and RMSE values ranging from 0.85 to 0.99 and 4.344 to 100.5 mm, respectively. The study presented here clearly shows an effective methodology to not only demonstrate the high accuracy the model has on seawater level predictions, but also able to further investigate the importance of what each oceanic and atmospheric variable has on the seawater level. The model performance may be affected by frictional shoaling, coastally trapped waves, bathymetry and the local dynamics contributed by Agulhas Current, which were not taken account for in this study and could be incorporated in the model for future research.
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    Simulating the Coastal Ocean Circulation Near the Cape Peninsula Using a Coupled Numerical Model
    (2021-03-26) de Vos, Marc; Vichi, Marcello; Rautenbach, Christo
    A coupled numerical hydrodynamic model is presented for the Cape Peninsula region of South Africa. The model is intended to support a range of interdisciplinary coastal management and research applications, given the multifaceted socio-economic and ecological value of the study area. Calibration and validation are presented, with the model reproducing the mean circulation well. Maximum differences between modelled and measured mean surface current speeds and directions of 3.9 × 10−2 m s−1 and 20.7°, respectively, were produced near Cape Town, where current velocities are moderate. At other measurement sites, the model closely reproduces mean surface and near-bed current speeds and directions and outperforms a global model. In simulating sub-daily velocity variability, the model’s skill is moderate, and similar to that of a global model, where comparison is possible. It offers the distinct advantage of producing information where the global model cannot, however. Validation for temperature and salinity is provided, indicating promising performance. The model produces a range of expected dynamical features for the domain including upwelling and vertical current shear. Nuances in circulation patterns are revealed; specifically, the development of rotational flow patterns within False Bay is qualified and an eddy in Table Bay is identified.
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    Towards Rogue Wave Characterization in False Bay, South Africa
    (2019) Salonen, Nicholas M; Vichi, Marcello; Rautenbach, Christo
    Rogue waves are extreme oceanic waves which exceed twice the significant wave height of the wave record. In False Bay, South Africa, a shallow sea mount called rocky bank was suggested by Shipley (1964) to be the cause for increased wave energy along the eastern periphery of False Bay, where it is not uncommon for coastal users, particularly rock fishermen, to be washed off of the rocky coastline. The shallow bathymetry associated with rocky bank refracts the incoming open ocean swell and is thought to focus the wave energy toward sections of this coastline. This investigation aims to understand which offshore wave conditions could be conducive to rogue wave development along the eastern periphery of False Bay, South Africa. To do so, the phase-averaging spectral wave numerical model SWAN was used to investigate various wave conditions which are known to enter False Bay. The focusing effect which rocky bank supposedly has on the incoming wave energy was investigated by deepening and shallowing rocky bank's depth. This study showed that rocky bank does cause a varying degree of wave focusing which appears to be most dependent on the wave period, with higher wave periods leading to greater extents of refraction. According to this study, the average conditions which impact the south-western coastline of South Africa are conducive to wave focusing by rocky bank and despite the rarity of rogue wave events, coastal users are still affected by them. Calm seas have the ability to mislead users that make use of these dangerous rocky coasts, when the event of a rogue wave in small seas can still be sufficient to wash a person into the sea. A study by de Vos & Rautenbach (2019) suggested that users are at a higher risk, for a variety of coastal activities, during (seemingly) good conditions due to the increased human exposure. Since this study only simulated monochromatic wave conditions, one would expect the results to be significantly different if multiple swell systems were simulated concurrently, as this introduces the possibility of the dispersive focusing mechanism for swells that travel at different speeds and thus increasing the likelihood of a rogue wave. The knowledge acquired from this study is a great step towards understanding the wave climate of False Bay. This information can be used to implement possible warning systems which would hopefully be respected by the community and ultimately lessen the lives lost to extreme waves along False Bay's 'Death coast'.