Abstract: This paper is to explore the relationship between the topographic changes of Ganjiang River network and the changes in its hydrodynamics in flood season under the influence of new water and sediment conditions and human activities. We developed one-dimensional river network mathematical models during 1998-2020 based on the river topographic changes in 1998, 2013 and 2020 respectively to simulate the 1998 flood in this river. The influence of topographic changes in different periods on hydrodynamic characteristics in flood season is studied. The results show that in this river network, the overall downcutting has caused the water level-discharge relationship to shift to the right, and the flood peak discharges in each branch increases, with an inflow increase of more than 50% under the same water level. During 1998-2020, the southern branch had a topographic downcutting depth greater than the other branches, and its uneven topographic downcutting resulted in a change in the distributary ratios at the river network nodes. On the whole, the distributary flow increased in the main branch and the southern branch, while it decreased in the middle and northern branches. Flow velocity in each branch changed dramatically, with an increase of more than 1 m/s in the downstream section of the main branch. This study is useful for river regulation planning, dam protection, and disaster prevention.

Key words: hydrodynamics, flood season, one-dimensional river network, terminal of Ganjiang River