Downstream impacts during the dam construction: A case of Büyükkumla dam

This study aims to determine the morphological effect of a dam construction process on the downstream part of the riverbed. For this purpose, morphological measurements were taken at the downstream of the dam in six-month periods during the construction of Büyükkumla Dam in the Marmara Region (2014-2020). Cross-section changes and sediment movements in the downstream riverbed were evaluated with morphological measurements. In addition, spatial maps of morphological change were created with geographic information systems. Hydrometeorological changes and result of excavation and filling activities effect on morphological changes were also examined. The changing bed structure was evaluated with grain-size analysis (May 2017, 2019, 2021). The flow-sediment relationship was revealed by the monthly measuring the amount of suspended sediment during 2020, and the natural sediment transport of the stream with the effect of the dam process on this situation were examined. As a result, the stream bed form in the downstream has changed significantly due to excavation and filling activities during the dam construction period has been observed. The riverbed reached the state of equilibrium approximately 2 years after the excavation activities of the dam construction were completed. Coarse-grained material was accumulated in the downstream region were intense during the dam excavation activities. The riverbed reached a state of equilibrium including a finer and uniformly dispersed material in the later period. The morphological form was mostly changed as a result of excavation and filling activities on the construction site, and there was no clear relationship between hydrometeorological data and morphological changing.