Feasibility and geotechnical design of subsurface dams in dry ephemeral rivers for the augmentation of shallow groundwater supply
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: Recent droughts and projected climate changes in South Africa make it an essential priority to augment water availability, particularly in the most vulnerable rural communities. The Molototsi River, a torrential tributary of the Letaba River in the Limpopo Province was selected as case study. The research proposes new methods for determining the feasibility and design of subsurface dams in dry ephemeral rivers. The methods are predominantly based on the physical factors affecting the decision-making for constructing subsurface dams. This is typical geotechnical engineering approach which involves an applied evaluation of parameters obtained from site visits, field surveys, geological data, geophysical data, precipitation and runoff data, laboratory testing, desktop studies, numerical modelling, irrigation requirements, abstraction demand, and construction costs. The specific objectives of the project are: 1) to identify possible sites to construct subsurface dams; 2) to investigate the geological and geotechnical characteristics; 3) to undertake a hydrogeological and hydrological assessment; and 4) to design and determine the technical feasibility of subsurface dam. The results of the Molototsi River (quaternary catchment B81G and B81H) case study indicates that the assessment of these physical factors are critical prior to construction decision-making. The geological and geotechnical investigation, including GIS applications, proved effective for finding the most favourable subsurface dam sites. The hydrogeological assessments found satisfying yields and are evidently one of the most important aspects of the study as the reservoir yield is directly related to the specific yield and porosity of the riverbed sand. The hydrological modelling also confirms that subsurface dams can significantly increase water availability throughout the dry season. The geotechnical design of this study, according to the estimations made, were deemed adequate and safe against overturning. The cantilever retaining wall also proved to be the most robust and cost effective structure to build, therefore found feasible. It is envisaged that such technology, if feasible, could mitigate water shortages in the rural communities across South Africa, reduce evaporation losses, and contribute to the conservation of fresh water resources, influencing the livelihood of the population directly.
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