The Fogera Plain, Ethiopia, is affected by recurrent flooding of the Ribb and Gumara Rivers. A large dam on the Ribb River is under construction for irrigation, but also to reduce flooding. We investigated the effects of the dam on the flood regime of the floodplain wetlands using a combination of hydrodynamic and a rainfall-runoff models. The model was calibrated based on inundation maps retrieved from Landsat images. Pre- and post-dam model comparison for 10 years shows that the dam will reduce the flooding extent by 11%, as it only regulates 23.8% of the upstream watershed. The flood extent and duration necessary to maintain ecologically significant water depths (≥ 0.5 m) show no notable changes. 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 terrain resolution and analysis of hydrological data.

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This study assessed the downstream river system adaptation in response to upstream damming on the Ribb River, Ethiopia, to irrigate 15,000 ha. It combined primary and secondary data, and the application of remote sensing and mathematical modeling. The predam morphodynamic trends of the Ribb River were analyzed for 59 years based on aerial photographs, satellite images, and newly collected field data. Three dam operation scenarios were developed to analyze the long-term hydro-morphological effects of the dam on the downstream river reaches. It also assessed the applicability of physics-based analytical equations (Equilibrium Theory) compared to a 1D numerical model (SOBEKRE) to determine the least-morphologically impactful dam operation scenario on the river reaches downstream of the dam. Moreover, a HEC-RAS 2D hydrodynamic model was developed to assess the effect of the dam on the flooding extent of the Fogera Plain. This was used to study the potential implications of hydrological alteration on the ecology of the floodplain wetlands, as they are the habitats of important fish and bird species. The results contribute to knowledge on the hydro-morphological and environmental impacts of dams on downstream river systems. The developed methodologies and findings may be used to study future hydro-morphological and ecological changes that may arise due to other dam operations or climate change.@en

This work investigates the applicability of an analytical method for quick assessments of the long-term morphological effects of different dam operations on downstream river reaches with the idea to apply the method in feasibility studies to identify the least morphologic-impacting operation scenario. The Ribb River (Ethiopia) is used as a study case. The analytical method estimates the idealized, new equilibrium of the river bed profile without considering the duration of the morphological evolution. We apply the analytical method distinguishing sand-bed from gravel-bed reaches. The outcome of the analytical method is compared to that of a calibrated one-dimensional river morphology computer model. The analytical method overestimated the morphological changes compared to the one-dimensional model. By establishing the upper limits of the impact, the analytical method identifies a theoretical maximum river bed degradation near the base of the dam. If all sediment is trapped in the reservoir, the method allows distinguishing the effects of different dam operation scenarios, but only for gravel-bed river reaches. However, the method can also be applicable for sand-bed reaches if there is sediment input from the upper reaches. Further research works should be done to validate both methods if they indeed allow to detect the least impacting scenario, considering that data showing the effects of long-term dam operations on the downstream river reaches are lacking.

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The meandering Ribb River flows in northwest Ethiopia to Lake Tana, the source of the Blue Nile River. The river has already undergone changes due to 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, causing adjustments to the morphology. Assessing the current morphodynamic trends is the first necessary step to study the future effects and find ways to mitigate them. This paper presents an analysis of the current and past river based on newly collected data, aerial photographs, SPOT and Google Earth images. The riverbed 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 the downstream direction, which has resulted in 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 to be minor.

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Dam construction affects the discharge regime and the sediment transport of rivers (e. g. Williams & Wolman, 1984). Dams reduce the annual peak discharges (Graf, 2006) and may store all sediment or a large part of it. The downstream river reach adjusts its morphology to the new conditions. This involves changes in planform, slope, width, depth and sediment characteristics through time. The Ribb River is located in the North Western part of Ethiopia where it drains to Lake Tana. A 73 m high dam and a diversion weir 30 km downstream are under construction to irrigate 15,000 ha of land (WWDSE & TAHAL, 2007). The river reach downstream of the dam is 77 km long. This part of the river is strongly affected by flooding, pump irrigation, sand mining and backwater effects from Lake Tana and will be even more impacted by Ribb Dam operations. Lake Tana level was regulated for hydropower production at a higher elevation between the years 1995 and 2001, at a lower elevation between the years 2001 and 2010 and then to a higher elevation again. Embankments have been constructed in the lower part of the river for flood prevention. The objective of this study is to analyse the effects of the dam on downstream river bed topography and channel alignment. This work presents the assessment of pre-dam morphological trends of the river. @en

The Ribb River is one of the components of the  Blue Nile River system located in the North  Western part of Ethiopia. It drains to Lake  Tana, the source of the Blue Nile River. The  Ribb has a length of 130 km, with a catchment  area of 1,812 km2. The average yearly rainfall  of the catchment is 1300 mm, with 80 %  occurring between the months of June and  September. The average and daily maximum  discharge of the river are 15 m3/s and 220  m3/s, respectively. A large dam and a diversion  weir 30 km downstream of the dam are under  construction to irrigate 15,000 ha of Fogera  flood plain (WWDSE and TAHAL, 2007).  Downstream of the dam location, the Ribb is a  meandering river with slope ranging from  0.18% to 0.03%. The river bed material is  dominated by sand with a gravel component in  its upper reaches. Intensive agriculture without  any natural resources conservation,  deforestation, dike construction, pump  irrigation and sand mining are the most  impactful activities in the Ribb watershed  (Tarekegn et al., 2010; Garede and Minale,  2014). The Lake Tana level is regulated since  1995 for hydropower production, which  enhances flooding along the lower river reach.  During the 2006 event, 45 people died, 30,000  persons were displaced and 5371 ha of  agricultural land were inundated (ENTRO,  2010). To prevent flooding, dikes have been  constructed in the lower reach of the river.  This study aims to describe current river  morphodynamic trends, including planimetric  changes for the definition of the pre-dam  conditions of the river. The first part of the work  is presented here with some preliminary  results, focusing on the river planimetric  changes. This paper describes the initial state  of the study.  @en

The performance of reservoirs designed to store river runoff is seriously threatened by sedimentation processes. The accuracy of the empirical methods to predict reservoir sedimentation, distribution and flushing process is low and applicability is limited for certain type of reservoirs only and due to the limitation of the use of empirical methods, the development of the mathematical modelling is clear needed for an accurate prediction and simulation of future changes. The paper presents the conclusion of the authors based on a number of case studies where mathematical models are used for simulation of the flushing process.@en