Abstract: Since the Xinglong Hydrojunction Project on Hanjiang River was put into operation in 2013,the water level in the middle and dry seasons downstream near the dam site has decreased to varying degrees compared to that before the construction of the dam. By 2018,the dry water level has dropped by 1.67 m to the maximum,resulting in insufficient water depth of the ship lock threshold and affecting the smooth passage of ships. Having analyzed the measured hydrology,geology,and riverbed topography data of the river segment,we looked into the causes and preventive measures of the water level drawdown. Results reveal that the downward erosion and widening of flat riverbed induced by the discharge of clear water and the easily eroded riverbed sand are main causes of the drop of middle and low water levels in the downstream of dam site. We propose some engineering measures such as protecting the bottom of flat beach channel in the downstream near the dam site and constructing submerged dams to prevent riverbed from erosion and to appropriately raise the low water level of the river channel. In the meantime,we recommend to construct water diversion project from the Yangtze River to the Hanjiang River as soon as possible to increase the flow of water in the middle and lower reaches of the Hanjiang River in low water period in the purpose of fundamentally solving the problem of ships passing the gate and the adverse impacts caused by the water level drawdown.

Key words: water level, downstream near the dam site, river bed scour, preventive and control measures, Xinglong Hydrojunction Project