Abstract: An integrated hydrological and hydrodynamic model is developed and applied to the watershed and river network system of the lower Xiangjiang River, coupled with high-frequency operations of the Changsha navigation-hydropower project. It is configured, validated and verified using detailed riverbed topography, high-frequency data collected from nearly 80 stations at the time interval of 5 to 60 min during 2016-2021, and land use data with a resolution of 30 m. And the hydrodynamics and inundation processes with and without the project are calculated and compared. The results show the calculated water levels and flows in the rivers agree well with the high-frequency measurements, and the Nash Sutcliffe efficiency (NSE) is greater than 0.91 from the verification of Xiangtan, Langli, and other stations. The project operation and scheduling raised the water level and reduced the average flow velocity significantly in the backwater sections of the main river and its tributaries in the dry season, so that small-scale fluctuations in water level and flow were intensified and even reverse flows occurred in certain sections of the lower Liuyang and other rivers. For the incoming hourly flow at the frequencies of 1%, 50% and 99%, the calculations of water level and flow velocity in the river network reveal that the project scheduling increased the frequency of inundation in the sections of sand shoals and point bars in the upstream of the main river and its tributaries in the normal and dry seasons. This study demonstrates a new method and its application for quantitative assessment to the impacts of high-frequency scheduling of dams on the river system.

Key words: grid distributed hydrological model, hybrid river networks, operation of navigation-hydropower project, high frequency hydrological processes, submergence frequency analysis