Browsing by Author "Olivier, George"

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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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    Gully erosion in the Sandspruit catchment, Western Cape, with a focus on the discontinuous split gully system at Malansdam
    (Stellenbosch : Stellenbosch University, 2013-12) Olivier, George; De Clercq, W. P.; Schloms, B. H. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Gully erosion is a major environmental problem not only having direct influences on site but also indirect influences felt further down the catchment. Combating gully erosion has proven to be elusive due to the difficulty in finding the causal factors and developing mechanisms involved. Soil is the medium in which gully erosion occurs yet few research have investigated it as a driving factor behind gully development and those that have has done it in a very elementary way. The first aim of this project was to physically and chemically characterise and classify the discontinuous gully system at Malansdam to establish the relationship between landscape hydrology and geomorphologic gully development with a focus on control factors. This was done by field observations, physical measurements and spatial and hydrological analyses with a Geographical Information System (GIS). The Malansdam gully system was the first ever recorded Strahler stream order (SSO) 5 classical gully system with the most active region being in the upper reaches where a steeper slope is experienced. Although piping was observed the V-shape channels and SAR data from traditional wet analysis indicated runoff to be the dominant formation process. A duo of factors, consisting of one anthropogenic and one natural factor respectively, was found that the major control factors behind the gully formation. A unique anthropogenic factor that has never been published beforehand was found to be the anthropogenic driving factor namely the ploughed contour cultivation technique employed by the farmers in the Sandspruit catchment. The ploughed contours act as channels firstly collecting and secondly moving water that would have drained naturally downwards in the valley to one exit point in the gully system. This allows increased erosive energy because of the larger volumes of water entering one single point in the upper reaches of the gully system where a steeper slope is experienced. The driving factor in the natural group was determined to be weak soil structure due to an abundant amount of exchangeable Mg2+ cations occupying the exchange sites on the clay fraction. This would cause soil to disperse in the presence of water even with a low amount of exchangeable Na+. Combat methods would accordingly exist in the form of rectifying the soil structure and finding an alternative to the ploughed contour system currently employed, but also planting vegetation especially grass or wheat in the gully channels. The second aim of this project was to determine the capability of Near Infrared (NIR) spectrometry, with wavenumbers 12 500 – 4 000 cm-1, to predict indicators used in soil science to establish the dispersive nature of a soil. These indices included the Exchangeable Sodium Percentage (ESP), Sodium Absorption Ratio (SAR), Magnesium Saturation Percentage (MS%), Electrical Conductivity (EC), Potential Hydrogen (pH) as well as the four main exchangeable cations namely calcium (Ca2+), potassium (K+), sodium (Na+) and magnesium (Mg2+). Surface and subsurface soil samples were collected from active gully heads. These samples were minimally pre-processed thus only dried, milled and sieved. Thereafter it was subject to NIR analysis making use of the Bruker multi-purpose FT-NIR Analyser (MPA; Bruker Optik GmbH, Germany) with a spectral range of 12 500cm-1 to 4000cm-1 which is. Partial Least Square Regression (PLSR) models were built for each index and the exchangeable cations making use of QUANT 2 utility of OPUS 6.5 (MPA; Bruker Optik GmbH, Germany) software. Five different regrssion statistics namely the coefficient of determination (r2), Root Mean Square Error of Cross Validation (RMSECV), Ratio of Performance to Deviation (RPD), Bias and the Ratio of Performance of Quartiles (RPIQ) were used to assess the legitimacy of each PLSR model. Upon validation all the PLSR models performed in line with previously published work and in certain cases better. The only exception was MS% which would require further investigation. NIR thus possess the capability to predict a soil’s dispersive nature in a fast, reliable, inexpensive and non- destructive way, thus implying whether or not it contributes to gully erosion at a significant level or only minimally.