Channel Stabilty Analysis -the Case of Borkena River (Ethiopia)
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Date
2008-03
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Addis Ababa University
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. The
Kombolcha town, situated on flat land plains in the middle course of the Borkena River is
facing a danger of flooding and destruction of properties due to the instability of the Borkena
and Berberie 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 4.0 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 Borkena
River. And there is deposition at some of the down stream 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
geotechnically unstable. Over flooding still is the problem on upper and lower reaches of
Borkena which is resulted from the complex processes of aggradational 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.
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Keywords
Berberie; River