Browsing by Author "Habtamu Itefa Geleta"

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Watershed sediment yield modeling for data scarce areas
(2011) Habtamu Itefa Geleta; Wieprecht, Silke (Prof. Dr.-Ing.)
The sustainability and service life of reservoirs depends on the amount of sediment storage. Reservoir sedimentation is a critical problem in reducing the service life of dams. The sedimentation problem is the consequence of watersheds sediment supply to the river networks and then to the reservoir. River bank sediment deposition is another consequence of excess sediment supply from the upstream watersheds. The deposition of sediment on bank of a river causes change in flow regime and as result flooding may happen to the adjoining areas. The degree of severity of river bank sediment deposition or reservoir sedimentation can be analyzed with the availability of information on the sediment load from the upstream watersheds. Physically based models are appropriate tools for sediment modelling and prediction at the outlet point of a watershed. Nevertheless, the applicability of the existing physically based models is limited to data availability which restricts their application to data scarce areas. Therefore, this research has been undertaken to analyze the fundamental watershed soil parameters, geomorphologic parameters and hydrologic parameters and thereafter suggest an alternative sediment yield estimation method that can be applied to data scarce areas. A Soil erodibility estimation equation has been derived from soil data of the Upper Awash basin in Ethiopia and has been evaluated for its applicability to the FAO (1998) world soil database. The evaluation has been made with reference to the equation of Williams et al. (1984). According to the evaluation result, for 80 % of the World Soil Database the relative error of the proposed equation as compared to the equation of Williams et al. (1984) has been estimated to be less than 20 %. This indicates that with easily measurable soil parameters like the percentage of sand, silt and clay, a reasonable soil erodibility factor can be predicted. SWAT2005 model has been applied to the Awash basin and two selected subbasins (Fincha and Gudar) of the Blue Nile basin. The model has been calibrated to measured sediment data available at different locations in the selected river basins. After the model calibration the spatial pattern of soil loss rates and erosion risk area has been identified. The erosion risk has been found to increase in areas with highest annual rainfall depth and agriculture dominated areas. Moreover, areas with steep slope have shown more vulnerability to erosion. The geomorphologic parameters of the watersheds have been combined with the soil erodibility factor, peak surface runoff and stream flow elements to formulate an alternative empirical model of sediment yield estimation. The empirical model has been derived from the Upper Awash basin based on the SWAT2005 model results. Two equations have been formulated, i.e. an equation which can predict sediment outflow from a single watershed and a routing equation which can predict net sediment flux from multiple and interconnected watersheds. The scope of applicability of the developed empirical models has been evaluated for different watersheds in various climatic zones and satisfactory results have been obtained. The findings of this research provide an alternative solution for the estimation of soil erodibility and net sediment outflow from various watersheds. Moreover, the flexibility of the sediment outflow equation is a special aspect of the research findings that makes the formulated empirical model successfully applicable to different watersheds. Therefore, in the absence of measured sediment data sufficient for the application of physically based models, the formulated empirical model can be used for sediment prediction.