CHANNEL STABILITY ANALYSIS OF WOIBO RIVER, SOUTHERN ETHIOPIA

Abstract

ABSTRACT Rivers dynamically respond to changes in the discharge and sediment supply of catchments. A change of land use and other natural factors in catchments result in changes on sediment supply and discharge. And this process of change of river geomorphology seeking for its equilibrium may affect people and property especially in urban areas located adjacent to rivers. Areka town, situated on flat land plains in the middle course of the Woibo River is facing a danger of flooding and destruction of properties due to the instability of the river reaches crossing the town. To prevent the loss of lands and properties resulting from the changes in the characteristics of those river reaches, stabilization measures must be employed. For proper design and implementation of any stabilization measures, analysis on stability of these channel reaches must be carried out. So this thesis deals with analysis of the stability of the channel reaches by evaluating their capacity and stability in response to discharge and sediments that are supplied from the catchments. Field investigation and surveying to collect data for channel cross sections, bed & bank material, and the vegetation covers is carried out. Gradations of bed and bank materials are determined in laboratory using sieve analysis. Design discharges of different return periods and quasi unsteady flow data are analyzed from measured maximum daily instantaneous discharge data. Using such flow and existing field data as input, HEC-RAS 5.0.3 beta version is used for simulation of flow and mobile bed sediment transport Based on the model results which are analyzed using the Meyer Peter Muller and Yang’s sediment transport formula, deposition is observed in almost all of the stations of the Woibo River. And there is deposition at some of the downstream stations in the tributary reach and degradation in the rest. It is also analyzed that the material on the banks is less resistance to the minimum shear induced by the flow, besides much of the stations in the tributary reach are unstable. Over flooding still is the problem on upper and lower reaches of Woibo which is resulted from the complex processes of gradational problems. And stabilization measures are proposed and should be implemented with further engineering, economical and hydraulic analysis for effective and sustainable stabilization of the rivers.