Alteration of the Fogera Plain flood regime due to Ribb Dam construction, Upper Blue Nile Basin, Ethiopia

Authors: MULATO, CHALACHEW - Delft University; CROSATO, ALESSANDRA - Delft University; Langendoen, Eddy; MOGES, MICHALE - Bahir Dar University; MCCLAIN, MICHAEL - Delft University

Submitted to: Journal of Applied Water Engineering Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2021
Publication Date: 7/1/2022
Citation: Mulato, C.A., Crosato, A., Langendoen, E.J., Moges, M.M., Mcclain, M. 2022. Alteration of the Fogera Plain flood regime due to Ribb Dam construction, Upper Blue Nile Basin, Ethiopia. Journal of Applied Water Engineering Research. 10(3), 175–196. https://doi.org/10.1080/23249676.2021.1961618.
DOI: https://doi.org/10.1080/23249676.2021.1961618

Interpretive Summary: 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, it is unknown how the dam operation will impact the annual flooding of the Fogera Plain, which sustains wetlands that are critical spawning sites for various fish species. 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 change in flooding extent, depth, and velocities using a combination of hydrologic and hydrodynamic computer models. Model results show that dam operation could reduce the maximum expected flooding extent by about 11%, but the flood duration period is not significantly affected. The latter finding is critical to sustain long-term ecosystem services. The approach and findings of the work can be used as a baseline to study similar river basin planning efforts, specifically in the Upper Blue Nile Basin, where data for such studies are sparse.

Technical Abstract: The Fogera Plain, located on the eastern shore of Lake Tana, Ethiopia, is affected by recurrent flooding of two major rivers, the Ribb and the Gumara. A large dam in the headwaters of the Ribb River is under construction primarily to provide irrigation water, but also to reduce flooding. We investigate the effects of the dam on floods and the ecology of the Fogera Plain wetlands using a two-dimensional hydrodynamic model with discharge time series generated by a rainfall-runoff model. The model was calibrated based on inundation maps retrieved from Landsat images using the Google Earth Engine. The dam is found to reduce the flooding extent by 11%, as it only regulates 23.8% of the combined watershed of the Ribb and Gumara Rivers. Maximum water depth and velocity remain less than 1 m and 0.6 m/s, respectively, indicating that the life of the villagers is not at risk if warning strategies are implemented. The wetlands’ flooding extent and duration for a water depth of at least 0.5 m do not show any ecologically significant changes. However, the construction of irrigation structures in the Fogera Plain may influence the free movement of water to the wetlands, thereby potentially impacting fish migration and other ecological processes. The developed hydrologic and hydrodynamic models can be used to analyze other dam operation and climate change scenarios even though there are uncertainties related to, among others, terrain resolution and analysis of hydrological data.