Quick Assessment Method to Establish Long-Term Effects of Dam Operations on Downstream Rivers
Abstract
Dam construction affects the discharge and sediment transport regime of the downstream river by reducing and shifting the annual peak flows and storing the incoming sediments. Dam construction thus results in changes of river planform, width, depth, slope and sediment characteristics through time. These long-term changes may depend on released discharge and sediment and, the geological formation. Hence, there is no general mechanism to predict dam induced impacts as the drivers and process are site specific. This study deals with the effects of dam construction and its operation on the morphology of the Ribb River, Ethiopia, combining analytical and numerical investigations. The aim is to establish which method allows selecting the least impacting dam operation during the preliminary phases of dam planning.
The Ribb River, located in the North Western part of Ethiopia, flows with a meandering pattern to Lake Tana, the source of Blue Nile River (Mulatu et. al., 2018). A dam is in construction to store 234 Million m3 of water and a diversion weir is in construction for the purpose of irrigation (BRLi and MCE, 2010). To assess the long-term effects of different future dam operations, we investigated the applicability of an analytical method, known as Equilibrium Theory (Jansen et al, 1979). The method relates the initial reach-scale morphodynamic equilibrium to the final state. It was applied for the sand- and gravel-bed reaches to describe the new idealized reach-scale morphodynamic equilibrium characteristics. We also applied a 1D morphodynamic model (SOBEK-RE) for comparison and to estimate the required time to reach the new equilibrium. Model simulations covered 1,500 years assuming long enough to obtain a new reach-scale morphodynamic equilibrium. Both methods show that the dam operation scenario which releases the highest peak discharges will also have the strongest morphological impacts. The river bed degradation obtained near the dam toe is overestimated by the Equilibrium Theory, on average by 16%, compared with the 1D model. This is due to the over-simplification of the method. Generally, the result allow us to apply the Equilibrium Theory which does not require much data and time than the 1D morphological model to quickly identify the least impacting dam operation scenario for the preliminary dam design phases.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP33D2386M
- Keywords:
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- 1808 Dams;
- HYDROLOGY;
- 1857 Reservoirs (surface);
- HYDROLOGY;
- 1861 Sedimentation;
- HYDROLOGY;
- 1880 Water management;
- HYDROLOGY