Location: Watershed Physical Processes ResearchTitle: Morphodynamic trends of the Ribb River, Ethiopia, prior to dam construction Author
|Mulatu, Chalachew - Ihe Delft Institute For Water Education|
|Crosato, Alessandra - Ihe Delft Institute For Water Education|
|Moges, Michael - Bahir Dar University|
|Mcclain, Michael - Ihe Delft Institute For Water Education|
Submitted to: Geosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2018
Publication Date: 7/9/2018
Citation: Mulatu, C.A., Crosato, A., Moges, M.M., Langendoen, E.J., Mcclain, M. 2018. Morphodynamic trends of the Ribb River, Ethiopia, prior to dam construction. Geosciences. 8(7):255. DOI:10.3390/geosciences8070255.
DOI: https://doi.org/10.3390/geosciences8070255 Interpretive Summary: Dams on rivers impede the flow of water and sediment downstream, which may have adverse impacts on downstream channel geometry such as incision and widening. A dam and weir are being constructed on the Ribb River, Ethiopia, to provide irrigation water to the Fogera Plain, which is an important food producing region in Ethiopia. However, no quantitative, sediment transport analyses were performed during the design phase of dam and weir. Researchers from USDA, ARS, Oxford, MS, in collaboration with scientists from IHE Delft Institute for Water Education, The Netherlands, and Bahir Dar University, Ethiopia, assessed the current geomorphic trends of the Ribb River. Analysis of historic human interventions, discharge records, aerial imagery, and downstream Lake Tana water surface elevations showed a reduction of sediment transport capacity in downstream direction, which has manifested itself by sediment deposition downstream resulting in blockage of the lower river reaches. This study is a first, required step to determine the future effects of dam and weir construction on the Ribb River in order to find ways to mitigate them. The results are therefore pertinent to the Ethiopian Ministry of Water, Irrigation and Energy and the Ethiopian Road Authority.
Technical Abstract: The meandering Ribb River flows in North-West Ethiopia to Lake Tana, the source of the Blue Nile River. The river has already undergone several human interventions, such as embanking, sand mining, water extraction and lake level regulation for hydropower. At present, a dam and a weir are under-construction to store and divert water for irrigation. This will strongly alter both water and sediment discharges to the downstream river reaches, which will react by adjusting their morphology. Thus, assessing the current morphodynamic trends is the first necessary step to study the future effects and find ways to mitigate them. This paper presents the current river dynamics by analyzing present and past evolutions, based on newly collected data, aerial photographs, SPOT and Google Earth images. The river bed changes are derived from historical staff gauge height analysis. The effects of sediment mining and water extraction are assessed using the theory of morphodynamic equilibrium. The findings of the analysis show a reduction of sediment transport capacity in downstream direction, which has manifested itself by intense sediment deposition resulting in blockage of the Lower River reach and subsequent channel avulsion. The effects of Lake Tana level regulation on the observed processes appear as minor.