Hydrological Modelling of the Rietvlei Wetland, Western Cape, South Africa : Identifying possible Environmental Impacts

Abstract

Conceptualizing the hydrological processes governing wetland environments is only possible when the defining attributes of the wetland are considered and understood. In this case, the groundwater level fluctuations, wetland hydroperiod (or periods of saturation), wetland residence time, groundwater-surface water connectivity and soil texture distribution of a complex coastal wetland are conceptualized to enhance the understanding of the hydrodynamics of the system. The Rietvlei Wetland has been significantly impacted by alterations to its geomorphology and hydrodynamics due to anthropogenic stressors. Thus, by considering and quantifying these aspects in the wetland’s current environment, the environmental impacts and the significance of these impacts to enact change upon the wetland may be identified. Soil texture provides insight into the runoff/infiltration relationship and potential sub-surface preferential flow pathways within the wetland. This in return governs the groundwater movement and distribution within the wetland, as the characteristics of the porous media would determine the magnitude and direction of groundwater and surface water flow, as well as control the groundwater-surface water interactions based on the ability to conduct water horizontally and vertically. As the nature of wetland environments is defined by a shallow water-table, it makes the identification of extended periods of saturation in the current environmental setting all the more important. Identifying these events would contribute to the conceptualization of the historic impacts from and on the surrounding environment, and provide a reasonable indication of hydropatterns within the wetland. Once identified, the wetland residence time was considered, as the flow through the wetland determines the extent of its capacity to filter and flush water within the wetland environment, especially within the central Marsh/Vlei region. Geostatistical and conceptual modelling techniques were utilized to quantify and conceptualize these aspects within the Rietvlei Wetland. Through the use of these methods - which include spatial modelling, interpolation and numerical integration - the wetland attributes were successfully conceptualized and where relevant quantified; and through the incorporation of historic data and existing literature, the significance of the impact of the identified aspects was determined by way of a qualitative environmental impact assessment.