In order to investigate the impact of sand excavation within Shimeitang and Chenjiazui mining areas on channel hydrodynamics of the lowest Lishui River and Songhu River,we adopt data of riverbed terrain and hydrology in 2021 and 2022 to build numerical model of planar 2D flow motion,and calculate hydrodynamics changes in the two channels for different hydrological conditions before sand excavation and after.The results show that the initial sand excavation scheme does not endanger Lishui channel and Songhu channel,but water level during dry season maximally declines by 0.16 m within the Lishui channel as the isolation beach of Shimeitang mining area entirely collapses after sand excavation,endangering the navigation safety.Among all optimized schemes,water-level drop within the Lishui channel decreases with mining-area contractions during dry season for the extreme situation of isolation beach collapse and water level no longer declines under the recommended scheme.However,the maximum water-level differences between the two channels reach 0.20 m and 0.30 m during water flood and recession stages,and the two mining areas produce a maximum flow velocity of 1.00 m/s on adjacent beaches and result in scour of beaches.During flood season,high flow velocities occur in mining areas and vicinities,and deposition may happen as sediment is carried into the shipping channels.Therefore,regular monitoring is advised to be implemented on riverbed of channels,and protection project is also needed to prevent beaches to collapse.