Browsing by Author "Wasyihun Degu"

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    Hydrological and Hydraulic Modeling of Rib Reservoir and Ribb River for Predicting Downstream Morphological Changes
    (Addis Ababa University, 2026-04-01) Wasyihun Degu; Yilma Seleshi
    Dams apply profound impacts on downstream river systems, primarily through significant hydrological alterations and a substantial reduction in sediment transport. By blocking the movement of sediment produced in the upstream watershed, dams drastically reduce the sediment input to the downstream river. The aim of this study was HEC-HMS Erosion and Sediment transportation modeling to estimate reservoir sediment deposition.and.to determine the extent of downstream river bed degradation and bank erosion through the application of HEC-RAS. The Ribb River is located in Nile River basin of Amhara regional state. The dam and diversion weir are located in South Gonder zone and East of Lake Tana, on Ribb River, on the side of Woreta Debretabor road. Hydrologic modeling, calibration and validation were performed at Addis zemen gauging station. To simulate the precipitation-runoff model, this consists of developing a hydrological model operating the Soil moisture accounting method as loss method, Clark unit hydrograph model as transform method and erosion model using MUSLE in the application of HEC-HMS. Elevation data, river cross-sections, Manning’s roughness coefficients, and reach lengths were used as key inputs to simulate steady and unsteady water surface flow profiles, as well as unsteady sediment transport, using HEC-RAS. Additionally, controlled flow hydrographs generated from HEC-HMS were imported via HEC-DSS and applied as boundary conditions, enabling integrated hydrologic–hydraulic modeling for more realistic representation of flow behavior and sediment dynamics in the river system. The sedimentation analysis indicates that the rate of silt deposition in the study area result shows that the siltation will be 367 ton/ km2/ year. In the downstream steady 30 kilo meters Ribb River cross section data collected from field surveying. River channel contour was generated and exported to Auto CAD ROB, LOB and Channel measurements were taken from the polyline. Unsteady flow sediment transport analysis was performed using new HEC-RAS 5.0.1. The magnitude of the morphological changes are felt more just downstream of the dam 7m of scouring and 33% of width reduction, as compared to other locations far from the dam; which indicates these changes come due to the influence of the dam.