Department of Earth Sciences

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Browsing Department of Earth Sciences by browse.metadata.advisor "De Clercq, W. P."

Now showing 1 - 4 of 4
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    Characterising and mapping of wind transported sediment associated with opencast gypsum mining
    (Stellenbosch : Stellenbosch University, 2008-03) Van Jaarsveld, Francis; De Clercq, W. P.; Rozendaal, Abraham ; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    This study aims to provide a practical tool for the prediction and management of dust generated by the activities of an opencast mining operation. The study was conducted on opencast gypsum mines in the semi-arid environment of the Bushmanland, 90 km north of Loeriesfontein in the Northern Cape Province from April 2000 to October 2007. The vertical and horizontal components of wind transported sediment were sampled and a dust settling model was designed to predict the settling pattern of dust generated by opencast mining operations. The model was applied to soil samples collected from an area surrounding a mine. The influence sphere of the mining operation was predicted by the application of the model and then mapped. Once the influence sphere is mapped, the dust influence can be managed with the aid of an onsite weather station. By further applying the predictions based on climatic data, the influence sphere can be modelled. The model is not only applicable to the planning phase of an opencast mine to plan the position of dust sensitive areas like the living quarters, office buildings and workshops etc., but also to indicate the historical impact that a mining operation had once a quarry on an active mine is worked out and rehabilitated or a mine is closed. The model application can also aid with the explanation and visual or graphic representation of the predicted impact of planned mining operations on communities or neighbouring activities to them and thus avoid later penalties.
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    Gully dynamics evolution under environmental change pressures
    (Stellenbosch : Stellenbosch University, 2024-03) Olivier, George; De Clercq, W. P. ; Van De Wiel, Marco; Fried, Jana; Mashimbye, Eric; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Gully erosion is a severe land degradation process, primarily impacting land resources on-site and water resources off-site. When active in a catchment, it can be the dominant driver of soil loss, causing significant environmental and socio-economic consequences. However, other soil erosion mechanisms remain at the forefront of research, which contributed to our inability to assess gully erosion on a catchment to regional scale. The current capability to model gully erosion on larger geographic extents remains limited due to the complexity of interactions of control factors and various sub-processes driving gully expansion. In this study, an approach to apply local case studies to inform on regional gully severity is introduced to address modelling shortcomings, and an initial scaled framework is provided, which could be implemented for future regional scale investigations and monitoring. South Africa has a long history of erosion problems and has been considered an area with high gully incidence. The “hotspot” perception, coupled with the diverse climatic and geo-environmental attributes exhibited in South Africa, motivated the use as the focal region for this study. Local case study sites were used to extract physiographic properties and gully severity to produce a susceptibility map for South Africa. Additional local sites were selected across the E-W climate gradient of South Africa to assess gully severity and to isolate climate and land use controls of gully erosion to provide clues on how environmental change may influence future gully erosion. The findings from the susceptibility map, which used secondary data from the literature, converged with the findings from primary data derived from sites located across the climate gradient of South Africa. Gully erosion severity increases eastwards towards the Grassland biome, in which gullying is most severe. Here, gully erosion resulted in soil losses of up to 17 t ha⁻¹ y⁻¹, which exceeds the baseline limit (27 times more) and is almost twice the sustainable limit calculated for South Africa when the upper thresholds for both these limits are used. Perceptions from landowners/ -users/ and -managers mostly align with gully concerns from the field sites, showing that their appraisals are concurrent with local gully severity. Remediation efforts are ongoing at several sites; however, measures focus on gully headcuts and do not consider vegetation establishment. Vegetation is considered critical, especially for long-term success rates of mitigation, and could be a reason for the lack of successful mitigation. The poor success rate is also disconcerting, as climate change will likely exacerbate gully erosion in South Africa. Although climate change is predicted to increase gully erosion due to larger storm magnitudes, the data presented here indicates that rainfall intensity is likely to play a secondary role in exacerbating gully erosion. Rainfall variability may be the principal driver of gully erosion. If climate change increases the frequency of El Niño Southern Oscillation events, gully erosion severity may increase and even reactivate previously stabilised gullies due to more intense rainfalls after periodic droughts. Continuous assessment and monitoring of gully extents are crucial to assessing where gullies are of concern and whether there is a change in severity. Manually digitising gullies or solely relying on fieldwork will not sufficiently address a need for monitoring via temporal data. Semi-automated detection strategies which are scaleable and transferrable would enable the extraction of gully dimensions unbiasedly and would allow to quantitively assess gully expansion (or contraction) by subtracting polygon- or raster-based output. A semi-automated approach that uses gully morphology to extract gully dimensions is developed and tested with datasets from South Africa, Namibia, Spain, and Australia. Initial assessment shows positive results, accurately predicting > 75.4% of the gullied area when scaling between small gullies (planimetric area of 1619 m²) to large gullies (planimetric area of 70246 m²). Regarding transferability to benchmark areas where other land uses were practised and where different spatial resolution data were used as input, the variance between 1.4% and 14.8% was determined, with producer accuracies above 84.5% and 70.6%. The semi-automated method has some shortcomings, with the requirement for manually digitising gully headcuts being the most pertinent. As a framework, regional assessments and monitoring should implement a scaled approach. The initial step should produce a susceptibility map using key variables associated with gullying. Following that, more computationally intensive detection strategies could be implemented, constrained to areas of most concern defined by susceptibility. Lastly, representative field sites can be identified from the detected gullies, where primary data can be retrieved to quantify gully processes, severity, and implications. Continued work is required to refine this framework, for example, refining semi-automated approaches to increase accuracy and increasing localised field sites in different geo-environments to improve trend analysis and better our understanding of how various controls interact to steer gully evolution. Lastly, this new information should yield data that can be used to build and calibrate models; such gully evolution modelling currently needs to be improved and is pivotal to further our understanding of how gully networks will react to climate and land-use changes.
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    A hydrogeological study of the Kasteelberg Mountain aquifer, Western Cape, South Africa
    (Stellenbosch : Stellenbosch University, 2020-03) Pieters, Bernardus Lambertus; De Clercq, W. P.; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The availability of freshwater is one of the major factors that are limiting South Africa’s development. With this in mind the area under investigation in this study forms part of the Sandspruit catchment, which is about 100 km north-east of Cape Town near the town of Riebeek Kasteel. The climate is semi-arid with a Mediterranean landscape. This study forms part of multiple studies that were initiated to assist in alleviating the crisis brought about by the continuing drought in the Western Cape Province. This study investigated the possibility of utilising the Kasteelberg Mountain, located near the town of Riebeek Kasteel, as an additional source of freshwater. The regionally fractured sandstone aquifer was the focus during the modelling, volume and porosity calculations in this hydrogeological research of the Kasteelberg Mountain Aquifer. This resulted in an estimated water reserve that can be sustainably extracted. Sustainable development is needed to protect the sensitive ecosystems against anthropologic and climate-driven impacts. The study started with analysing the responses from water level loggers that were installed in boreholes in the study area to monitor the water fluctuations during the seasons so as to utilise this resource sustainably. During the study, the physical geology of the area was characterised. Geographic Information Systems (GISs) were used to generate maps and derive volumetric information needed to estimate water volumes, and this included the delineation of the watershed, elevation and the spatial maps of the boreholes that were monitored. A cascade model was created by using climate data collected from local weather stations and the physical character of the local sandstone to study the waterflow through the mountain. The cascade model was used to appraise its potential in runoff. Some common features between the proposed model and HYDRUS-1D runoff model are also discussed. Data was also used in the HYDRUS-1D model where the results generated were compared with the cascade model results and the measured results from fieldwork studies. The study therefore reflected on the volume of water present in the mountain aquifer and despite the area experiencing its worst drought in a century, this excess water was available for extraction.
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    Using distributive surface water and groundwater modelling techniques to quantify groundwater recharge and baseflow for the Verlorenvlei estuarine system, west coast, South Africa.
    (Stellenbosch : Stellenbosch University, 2018-12) Watson, Andrew. P.; Miller, Jodie A.; De Clercq, W. P.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: For effective management of groundwater resources, recharge rates and baseflow volumes need to be quantified to determine sustainable abstraction regimes and to quantify the ecological reserve, the amount of water needed to maintain the natural environment. While a variety of methods have been used to estimate groundwater recharge, estimates vary due to method, temporal and spatial resolution used. In rainfall/runoff modelling where potential recharge is determined by calculating the amount of water that percolates through the unsaturated zone, aquifer components are usually lumped resulting in over or under estimation of recharge. In contrast, groundwater models which include distributive aquifer components are commonly setup to lump climate and surface variables, thereby neglecting seasonal and climatic variability. In this study, a combined rainfall/runoff and groundwater modelling approach was used to determine the net recharge and baseflow in the RAMSAR-listed Verlorenvlei sub-catchment on the west coast of South Africa. This sub-catchment is an important biodiversity hotspot but is also an important agricultural region, hence there is competition for water resources. To understand the water dynamics within the catchment a four-phase approach was taken to determine the baseflow and ecological reserve requirements. This involved firstly, determining the limits of the sub-catchment boundary. Although the Verlorenvlei lake is supported by the Verlorenvlei sub-catchment which is itself fed by four main tributaries (the Hol, Krom Antonies, Kruismans, and Bergvallei), previous research has indicated that only one of these tributaries (the Krom Antonies) played an important role in the delivery of fresh water to the lake system. Initially the catchment boundary was thus modelled on the Krom Antonies tributary, although the understanding gained by the delineation was applied to the entire sub-catchment. To include spatial and temporal variability in groundwater recharge estimates, a rainfall runoff model was used to determine potential recharge using regionalised climate and assumptions regarding aquifer hydraulic conductivity. The potential recharge estimates within the sub-catchment exceeded previous studies (30 % higher), with the daily timestep nature of the J2000 model (Krause, 2001) assumed to account for this difference. To determine whether aquifer hydraulic conductivity could impact groundwater recharge rates, a groundwater model (MODFLOW) was constructed for the main assumed freshwater source of the Verlorenvlei, the Krom Antonies. The groundwater model included distributive aquifer hydraulic conductivity, although the input recharge was lumped which reduced climate seasonality and daily variability. The resultant output from the groundwater model was net recharge (0.3-11.4 % of rainfall) and average baseflow (14, 000 - 19, 000 m3.d-1), with the model suggesting that the baseflow from the Krom Antonies was not enough to meet evaporation demands (90, 000 m3.d- 1) and that there must be another much larger source. To incorporate daily climatic fluctuations and seasonality in baseflow estimates, the groundwater components of the J2000 were distributed using the net recharge and aquifer hydraulic conductivity from the Krom Antonies. By distributing the groundwater components within the J2000 model, the proportion of interflow to recharge was improved allowing for comprehensive estimates of runoff and baseflow from each tributary. While the model was calibrated using streamflow measurements from the gauging structure on the Kruismans, the measurements were particularly hindered by the DT limit (discharge table) of the station (3.675 m3.s-1), which resulted in reduced confidence in modelling high flow events. To incorporate the limited resolution of the station as well as limited length, an Empirical Mode Decomposition (EMD) was applied to the runoff data and water levels measured at the sub-catchment outlet. The results of the model adaption was that the Krom Antonies was not in fact the main freshwater source, with the Bergvallei supplying the majority of groundwater (49 %) as well as a large contributor of streamflow (29%). While the Hol was initially believed to be a minor contributor, the tributary had the largest ratio of baseflow (0.56), which acted to reduce its flow variability. While the Krom Antonies and Bergvallei is comprised of highly conductive sandstones and quaternary sediments, the Hol which is mainly comprised of shales, resulted in a larger groundwater flow attenuation which reduces its susceptibility to drought and climatic variability. The results of this study highlighted that on average the streamflow (20, 500 m3.d-1) from the feeding tributaries was not able to meet the evaporative demand of the Verlorenvlei and that the lake was mainly supplied by low occurrence high flow events. With the Verlorenvlei under threat due to continued agricultural expansion, it is likely that the lake will dry up more frequently in the future, especially if flows are hampered during wet cycles, when ecosystems regenerate.