Abstract:
Hydraulic structures such as dams have inevitable effects on their downstream that
requires investigation. This study addressed the impacts of land use change and
hydraulic structures on river morphology in Shina watershed, Lake Tana basin,
Ethiopia. Objectives of this thesis were to assess the impacts of land use change in
view of river characteristics, to assess Shina river planform changes and to
investigate vertical change of Shina river downstream from the dam. Satellite images
of the year 1986, 2001 and 2017 were used for land use/ land cover classification and
river planform analysis. To compare the vertical adjustment of the Shina River,
current river cross-sections were taken at different intervals and the past cross
sections were developed from the old topographic map of the study area. For the data
preparation and analysis image processing software (ENVI 5.1) and Arc GIS 10.2
were used.
From the result of land use /land cover classification, there is a decrease in forest
land by 70%, the Bushes and shrubs by 48% and accompanied by increase in
agriculture land by 23% and built up area by 26%. The anthropogenic impacts such as
farming, deforestation, resettlement and animal free grazing have also aggravated the
change of the planform in the downstream part of the river. The overlaying analysis
showed that for the considered reach (6km length), starting from the dam for a length of
about 3 km the general trend of the river banks were shifted to right and after that the
river banks shifts left. Comparisons of cross-sections at junctions of spillway outlet and
Shina river, at a distance of 210 m and 410 m from the dam axis to the downstream
showed that the river bed degraded by 0.38 m,0.94 m and 0.42 m for the period 2007and
2017 respectively.
The conclusion of this thesis showed that Shina river morphology has been changed due
to land use / cover changes and hydraulic structure. Therefore, upstream catchment
treatment and downstream river restoration should be done.