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.

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.

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.

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.