Delft, The Netherlands, 17 Dec 2021

Damming effects: Chalachew Abebe Mulatu earns PhD for research on how a dam in Ethiopia should be operated to reduce harmful downriver consequences

Following PhD research at IHE Delft, Mr. Chalachew Mulatu presented and successfully defended his PhD thesis and was awarded with a Doctoral degree on 16 December 2021. Professor Michael McClain is his promotor and Dr. Alessandra Crosato his co-promotor. Dr. Mulatu shared a few insights as he embarks on a new chapter of his life.

My thesis in a nutshell

In my PhD research, I studied what consequences the damming of the Ribb River, in Ethiopia’s Lake Tana basin, has on the downstream river system. I looked at the hydrological, morphological, and ecological consequences – that is, how the damming affects the river flows, the shape of the riverbed and the river’s path, and the eco-system in and around the river.

The Ribb Dam irrigation project - a dam and a diversion weir 77 km and 50 km upstream of the Ribb River mouth – will irrigate 15,000 ha of the Fogera Plain, which is home to cattle and cash crop farms, as well as wetlands that provide important habitats for fish and bird species.

I first analysed the changes in the river bed level and the path taken by the river  before the dam was built, using aerial photographs, satellite images, and field data that covered the past 59 years. I used this analysis to study how future dam operations would affect the river and assessed what consequences different ways of operating the dam would have, and then I selected the operation approach that causes the least damage. I also studied how the Ribb Dam affects the seasonal flooding of the Fogera Plain’s wetlands and the wildlife habitat it provides.

My results aim to help authorities in charge of the dam – the Ministry of Water Resources and Development, and the Amhara Water Resources Bureau - choose the best operational approach. They may use the results to operate the dam to get sustainable development with less side effects. The findings may also be used to operate other planned dams in the area.

On a global level, the results contribute to knowledge on the hydro-morphological and environmental impacts of dams on low-land river systems. The hydrological and hydrodynamic models I developed can be applied elsewhere to study future hydro-morphological and ecological changes that may arise due to other dam operations or climate change, and my findings provide insights into what changes might be expected.

Challenges during my PhD studies

I found that proposal development was the most challenging. After collecting and analysing field data, writing scientific journal articles that explained my findings succinctly and clearly were other challenges.

The influence of my PhD research and degree

In an ideal world, my PhD work would lead to sustainable water resources development, in particular in the Lake Tana Basin of Ethiopia. Projects for irrigation, hydropower, and water supply strongly affect rivers as water resources and their sediment transport and ecological conditions. However, not enough attention is given to these projects’ effects on river basins.  How will river systems downstream of dams be affected? What will be the hydrological and ecological consequences? My PhD research findings can be considered as a starting point for studies of the hydrological, morphological, and ecological consequences of dam construction.

Next steps

The current conflict in my country, Ethiopia, does not allow anyone to plan for tomorrow. However, I hope that this will change. If we have a normal situation, in the next few years, I plan to do more research at the watershed level. I will study the impacts of dam breach analysis on the river’s morphological changes and the flooding of areas downstream of the dam. I plan to continue to teach and supervise undergraduate and postgraduate students. If possible, I hope to do post-doc research work in my field.

If I could advise myself I started my PhD…

I would advise myself not to read too many articles when preparing the PhD proposal – instead, discuss with senior PhD fellows to get an idea on how they started their PhD research work and to get tips on how to use time effectively. Also, start preparing journal articles on time as the review process may take a long time.

Research summary

Thesis title: Downstream consequences of Ribb River Damming, Lake Tana Basin, Ethiopia

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 pre-dam 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 (SOBEK-RE) 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.

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