Browsing by Author "Pringle, Justin James."

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Investigation of XBeach cross-shore capacity under fixed model parameters.
(2020) Seyoum, Dawit Tilahun.; Stretch, Derek Dewey.; Pringle, Justin James.
An accurate, computationally efficient shoreline model is fundamental to understanding long term shoreline behaviour. Currently, one-line models can be used to perform this but are limited to either cross-shore or alongshore changes only. A simple, more robust model could be developed based on volumetric beach profile changes. However, this requires large empirical datasets to explore the relationships between wave and shoreline variables. In instances where such data is not available, process-based models such as XBeach are an attractive solution. Therefore, it is important to explore the ability of processes-based models such as XBeach to accurately predict shoreline changes under varying wave conditions. In this study two wave conditions associated with erosion (Hs =3m) and accretion ( Hs =1m) were used to evaluate the performance of XBeach to accurately predict shoreline changes. The east coast of South Africa was used as a case study site. The coastal areas of South Africa are known for their high energy wave climate resulting in a dynamic shoreline, with significant onshore and offshore sediment transportation. The effects of wave nonlinearities on sediment transport was estimated using XBeach’s built-in wave asymmetry and the skewness calibrating factors. The sensitivity of sediment transport to these parameters was investigated by running 180, 1D wave flume simulations. The 180 simulations were formulated by varying the wave asymmetry and skewness calibrating factors, sediment transport models, and approaching wave conditions. The results showed that increasing the magnitude of the calibrating factors increased onshore sediment transport. This was attributed to an increase in the advection velocity in the onshore direction. The investigations on the model capacity showed that the wave asymmetry and skewness related parameters control the cross-shore sediment movement direction. It was found that a single set of model input parameters were not able to produce both onshore sediment transport during low energy wave conditions and offshore sediment transport during high energy wave conditions. This study demonstrated that calibrating factors should depend on incoming wave energy. Currently, they are not implemented like this in XBeach.
Mixing and turbulence in floating flexible photobioreactors.
(2021) Naidoo, Kélan.; Stretch, Derek Dewey.; Pringle, Justin James.
Renewable energy initiatives aim to advance global efforts toward a sustainable bioeconomy. The power generation industry is the most crucial sector requiring strategic planning to achieve economic and environmental sustainability. Biodiesel production utilising microalgae as a feedstock yields a ± 400% larger production range compared to traditional feedstocks. Floating flexible photobioreactors (FPBs) aim to provide largescale microalgae enclosures and promote biofuel production's commercial application. FPBs are placed off-shore, where they naturally accelerate microalgae growth by harnessing wave energy, sunlight, waste carbon dioxide and wastewater discharge (as a nutrient source). Microalgae production requires explicit physical and chemical conditions to achieve optimal growth rates. In this study, a physical FPB model is constructed within a laboratory wave flume where the change in the internal fluid motion within the FPB is observed under an idealised wave pressure constriction. Under the influence of an idealised wave, the flexible tube experiences a reduction and expansion in its crosssectional area, which deforms the internal fluid flow. The fluid’s flow structure is analysed using particle image velocimetry (PIV). PIVlab was used to analyse the data. The fluid moves backwards (or towards constriction) when placed under direct tube constriction; however, under unrestricted tube expansion, the fluid is propelled forward in the original direction of the idealised wave, in the form of a bulge wave. Relative to the constriction, the fluid moves negatively on the left-hand side of the peak and positively on the righthand side. Regions of strong shear in which there was a rapid change in the velocity were identified as potential mixing zones.
Modelling the Agulhas Ocean Current: with a focus on the related shallow water hydrodynamics in and around the Durban Bay, South Africa.
(2021) Naidoo, Kemira.; Pringle, Justin James.; Stretch, Derek Dewey.; De Graaff, Reimer.
The Agulhas Ocean Current is a powerful and persistent western boundary current that flows along the continental shelf edge off the eastern coast of South Africa in a southerly direction. In addition to the tide- and wind-induced currents, the Agulhas Current influences the nearshore currents in Durban Bay, for example, the Durban Eddy. eThekwini Municipality and Deltares proposed that Durban be used as a pilot study to investigate the capabilities of developing a Delft3D Flexible Mesh (D-Flow FM) model that integrates the combined forcing of ocean currents, tide and wind in a coastal model domain, which once successfully achieved, is to be integrated within eThekwini’s Forecast Early Warning System. The aim of this study is to use D-Flow FM to accurately model the Agulhas Current and analyse its effects on the nearshore waters of Durban. Output from global ocean models, such as the NEMO models operated by E.U. Copernicus Marine Service Information (CMEMS) in the Global Reanalysis Multi-Model Ensemble Product (GREP), include the Agulhas Current but are not suitable for coastal applications due to their relatively coarse resolution and absence of tidal forcing. For this reason, model output from the GREP was downscaled to a coastal scale using a D-Flow FM model of KZN with appropriate boundary conditions and evaluating the use of a new Delft3D nudging technique. The final D-Flow FM model that was developed applied a relatively high-resolution grid on top of the continental shelf. A stable ocean current was simulated by forcing the model with a full set of 3D ocean boundary conditions, including currents, salinity, temperature, sea surface anomalies. In addition, the model was forced with tide and wind. Model tests showed that the nudging technique was not required when applying a suitable model extent. Realistic currents seem to develop along the edge of the shelf in the D-Flow FM model as observed from literature and measurements. The final coastal model output was compared with Acoustic Doppler Current Profiler (ADCP) data from ACEP (African Coelacanth Ecosystem Program) and data from a SADCO (Southern African Data Centre for Oceanography) database off Durban. The results of the D-Flow FM coastal model correlated better with these measurements when statistically compared to data from the GREP. The D-Flow FM coastal model was used to reproduce the Durban Eddy and analyse its modelled characteristics. The modelled duration of the eddy was between 8 to 10 days, which included the formation and dissipation of the eddy. A monthly average in agreement with previous observational studies of 1.66 eddies was seen from model outputs, with the reversal of currents along the Durban coast whenever an eddy was present.
Modelling the influence of varying sediment sources on coastlines.
(2019) Deoraj, Vibhav Atish.; Stretch, Derek Dewey.; Pringle, Justin James.
Coastal erosion is of concern to developed shorelines worldwide and has largely intensified due to anthropogenic influences. Sea-level rise, reductions in sediment supply and changes to wave behaviour due to changes in climate were identified as potential causes of chronic erosion. With climate change expected to increase the frequency and intensity of storms, coastline management and planning will require greater attention. A major obstacle of coastal planning is the lack of available models for predicting long-term changes. Furthermore, reliable long-term wave data are often unavailable or unreliable. Predicting long-term changes is essential for effective management of coastal defence schemes. One-line models present a reduced-physics and reduced dimension approach and provide an efficient and viable alternative to 2D and 3D models while being less computationally intensive. The long-term impacts of varying sediment inputs on the stretch of coastline between uMhlanga and the uMngeni River mouth in Durban are explored using a one-line model. Site selection was based on ongoing erosion and known operations of sand-mining, damming and a sand-bypass scheme. Existing models are used as a framework to develop a coastline model that uses statistically modelled wave climates as the input source of wave data. Results indicated that a minimum longshore sediment supply (460,961 m3/year) required to maintain beach volume in the study region exceeds the estimate by Corbella & Stretch (2012) of 418,333 m3/year. Observed beach erosion by eThekwini Municipality indicated a current longshore sediment supply of 410,276 m3/year. Furthermore, volume conservation did not ensure beach width conservation along the entire coastline, with a minimum sediment influx of 596,183 m3/year required for beach width and beach plan area conservation. Shore nourishment behaviour were analysed in the form of alongshore sand waves with results showing that multiple, smaller nourishments results in more realistic sand wave amplitudes that are required for diffusion dominant waves. Smaller nourishments allow for more diffusive effects while maintaining a diffusive state whereas larger nourishments tend to become advection dominant following rapid diffusion. vii An investigation of the advection-diffusion relationship of river sediment discharges inferred that sand waves along the Durban coastline are advection dominated. A critical aspect ratio of between 0.037 and 0.041 represented the equilibrium point between advection and diffusion. River sediment discharges of this aspect ratio are potentially significant in preventing erosion given the relatively high diffusive rate and slow advection speed associated with the value. Furthermore, extreme river discharges exceeding 200,000 m3 remained in coastal systems for between 3 and 4 years and are potentially important mechanisms behind coastline recovery after storms.
On weather and waves : applications to coastal engineering.
(2015) Pringle, Justin James.; Stretch, Derek Dewey.
Shoreline erosion in response to extreme wave events can be severe. The reduction in beach width leaves development within the hinterland exposed and vulnerable to future wave attack. Wave climates are a fundamental driver of coastal erosion and changes to wave height, direction and period can severely impact a coastline. These changes are directly linked to changes within the principle drivers of wave climates namely synoptic scale atmospheric circulation. The links are complex and if they can be clarified they can be used to provide insight into wave climates and improve the evaluation of future climate scenarios. The coupling between atmospheric circulation and wave climates provides a tool for risk assessment that is strongly based on fundamental physical processes. This study is focused on exploring this relationship and its effect on coastal vulnerability. A statistical classification algorithm is utilized to explore the relationship between synoptic scale circulation patterns and regional wave climates. The algorithm is fully automated and discrete atmospheric patterns are derived through an optimization procedure. It is driven to an optimal solution through statistical links between regional wave climates and atmospheric circulation patterns (CPs). The classification is based on the concept of fuzzy sets and differs from standard classification techniques. It employs a "bottom–up" approach as the classes (or CPs) are derived through a procedure that is guided by the wave climate. In contrast existing classification techniques first explore the atmospheric pressure space while links to the variable of interest are only made post classification. The east coast of South Africa was used as a case study. Wave data off the Durban coastline were utilized to evaluate the drivers of the wave climate. A few dominant patterns are shown to drive extreme wave events. Their persistence and strong high– low coupling drive winds toward the coastline and result in extreme wave events. The sensitivity of the algorithm to key input parameters such as the number of CP classes and temporal resolution of the data was evaluated. The Shannon entropy is introduced to measure the performance of the algorithm. This method benefits from incorporating the link between atmospheric CPs and the wave climate. A new stochastic wave simulation technique was developed that is fundamentally based on the CPs. This technique improves the realism of stochastic models while retaining their simplicity and parsimony relative to process-based models. The simplicity of the technique provides the framework to evaluate coastal vulnerability at site specific locations. Furthermore the technique was extended to evaluate changes in wave behaviour due to climate change effects.
Smoothed particle hydrodynamics (SPH) modelling of nearshore breaking waves.
(2021) Mahomedy, Ayesha.; Stretch, Derek Dewey.; Pringle, Justin James.
Breaking waves drive sediment transport in the nearshore zone of coastal regions and directly govern beach transformation. Accurate coastal modelling of breaking waves is essential to predict sediment transport accurately. Efficient and sustainable management of natural coastal systems and urban coastal developments relies on accurate sediment transport predictions. This study proposes a mesh-free, Lagrangian, smoothed particle hydrodynamics (SPH) model to simulate nearshore breaking waves in two-dimensions. This study emphasises using SPH to evaluate the wave field parameters (local velocities, vorticities, and shear stresses) that can be used to predict coastal phenomena, namely sediment transport in nearshore, wave-driven environments. This study showed that a two-dimensional SPH model could replicate the free surface of nearshore breaking waves and accurately predict the flow characteristics beneath breaking waves. However, the accuracy of the results can vary depending on the position of the breaking wave in the surf zone. Furthermore, SPH applications must choose between accuracy and computational efficiency. The key SPH calibration parameters identified were the artificial viscosity coefficient (α), the dimensionless smoothing length ratio (hSPH/dp), and the particle resolution (H/dp). Extensive comparative analysis was performed between simulated results and measured data to obtain suitable parameter values for a plunging solitary wave. A suitable choice of α=0.1, hSPH/dp=3, and H/dp=90 were selected based on the results. Furthermore, the results suggested that a suitable choice of model parameters depends on the viscosity treatment method (artificial/sub-particle scale viscosity approach) and the type of wave breaking simulated (plunging/spilling). Thus, α=0.1, hSPH/dp=3, and H/dp=90 were only deemed suitable when the standard SPH artificial viscosity approach is used to simulate breaking plunging and spilling waves on beach slopes milder than 1/10. The model sensitivity to α, hSPH/dp, and H/dp was also investigated based on the numerical wave energy dissipation and simulated wave surface of a plunging solitary wave in the space and time domain. When α was above or below the ideal value of 0.1 for a given hSPH/dp and H/dp, the numerical wave energy dissipation and wave height at breaking did not match the measured data. The choice of α was strongly related to H/dp, and a reduced α became more appropriate for a lower H/dp. The results also showed that the model was less sensitive to hSPH than the choice of α and H/dp in terms of the model performance. However, when hSPH/dp was less than 1, for any given α and H/dp, the numerical wave energy dissipation and wave height at breaking were under-predicted. The choice of H/dp was of principal importance and influenced the choice of the other model parameters. When H/dp was below the ideal value of 90, for any given α and hSPH/dp, the numerical wave energy dissipation and wave height at breaking did not match the measured data. Additionally, the breaking wave shape was poorly simulated. However, H/dp=90 becomes computationally expensive when simulating breaking waves in large numerical domains or with relative wave heights significantly less than 0.6. Hence, the available computing power limits the choice of H/dp. The performance of a two-dimensional SPH model was assessed by analysing the simulated flow field under several breaking waves. The local velocities, vorticities, and bed shear stresses were evaluated beneath two plunging solitary waves and a spilling solitary wave. Generally, the characteristics of the simulated flow field were fairly accurate during wave shoaling and wave breaking, less accurate during wave run-up, and inaccurate during wave run-down. The results also hinted at obliquely descending eddies occurring under the breaking plunging waves. However, the three-dimensional eddy structure beneath the breaking waves could not be investigated due to the limited two-dimensional nature of the model setup used in this study. A well-calibrated SPH wave and hydrodynamic model is an important coastal engineering tool. Thus, this study can serve as a physically based framework for using a two-dimensional SPH model to investigate coastal engineering problems that include wave-structure interactions, wave-run up on beach slopes and sediment transport in the surf zone over a wide range of scales and wave conditions.
Wind induced sediment re-suspension in a shallow lake.
(2011) Pringle, Justin James.; Stretch, Derek Dewey.
Wind induced turbidity within shallow lakes can greatly affect the biological functioning of a system in either a positive or negative manner. This research aims to understand and model the physical processes that cause sediment re-suspension. Lake St Lucia on the east coast of South Africa, a UNESCO World heritage site was used as a case study. Lake St. Lucia is a shallow water system which commonly experiences high levels of turbidity. Coupled with the naturally shallow depth of the lake, it is currently drought stricken, resulting in abnormally low water levels. A simple model has been developed which accounts for sediment re-suspension due to wind-driven waves and their associated bed shear stresses. The wave heights within a shallow lake such as St Lucia are controlled either by the fetch (for a large water depth), or the water depth (for a large fetch). When the wind is strong enough, the wind-driven turbulent mixing causes the water column to become fully mixed. When the wave-driven boundary layer becomes turbulent, sediment, being entrained within the water column increases significantly. The model also accounts for the effects of temporal consolidation on the re-suspension of sediments by setting a time scale for the erosion processes. It was found that the median of the monthly turbidity levels over the past ten years exceeded the average turbidity levels over the past 92 years. In all cases it was shown that mouth linkage with the uMfolozi resulted in lower turbidity levels than without any linkage due to the higher average water levels. The model was then developed to predict the spatial variation in turbidity within the Southern Lake. This was achieved through the use of existing bathymetric data for the Lake. This spatial model was then used to show how the turbidity varied for different wind and water depth conditions. Two conditions were considered, a NE and SW wind blowing at 8m/s for water levels of 0 EMSL and -0.5 EMSL. The spatial model showed that a decrease in water level increases the turbidity within the lake significantly. The wind directions appeared to yield similar results of sediment re-suspension. It was also shown that the high turbidity values were situated in the shallow depths even though the wave heights were small in comparison to those in deeper water.