The use of erosion models to predict the influence of land use changes on urban impoundments

Gilau, Andriaan Nicolaas Petrus
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The impact of soil loss from urban erosion processes is a major problem confronting decision makers on a national and local level. One such resource is the Boksburg Lake in the Eastern Service Delivery Region of the Ekurhuleni Metropolitan Municipality of the Gauteng Province, South Africa. The purpose of the study was to quantify what impact soil erosion, as a result of changes in land-use, had on the urban impoundment. There is a close relationship between how land is managed and the impact erosion may have on in-stream health. Increased erosion as a result of catchment changes increases the loads of phosphorus introduced into streams (Croke, 2002) and subsequently increases the occurrence of eutrophication. The management of sediment levels combined with reduced catchment phosphorus load is viewed as the most viable option in eutrophication abatement. Available soil erosion models and methods were compared and the most suited selected for the study. The study used a modified approach of the Universal Soil Loss Equation and the Soil Loss Estimation Model for Southern Africa. These were adjusted for urban conditions. Various simulation models were run and the results presented. Results from five of the models yielded results within 15%, or 85% confidence, of the measured results. Four of these models are however not generally accepted methods and can only be used as indication. The USLE method utilizing the Vanoni SDR equation is the preferred method and was applied in subsequent modelling. The simulation results of the phosphorus loading, although not within a 10% accuracy, relates to the observed loadings of 2008. By observing a similar trend as the sediment loadings, as a result of the development, it was concluded that the phosphorus loadings relate to the soil loss models which was related to changes in the catchments as a result of changes in land usage (imperviousness as indicator).