Abstract: During high-flow operations,significant detached vortex streets and abnormal water level fluctuations can be frequently observed at the outlets of water conveyance structures along the Middle Route of the South-to-North Water Diversion Project(MRSNWDP).The five typical inverted siphons were taken as the research objects along the MRSNWDP.Combining the prototype observations,physical flume model tests,and 3D numerical modeling,the influences of various factors were explored on the inverted siphons outlet hydraulic characteristics.A rapid quantitative formula was proposed for the water level fluctuations.The results show the followings:(1)Under the design and increased flow conditions,water level fluctuations occur at the inverted siphon outlets ranging from 0.5~0.8m.The Karman vortex streets are obvious behind the gate piers.The vortex quantity and the water level fluctuations are correlated in the gate chamber.The vortex street is the primary cause of water level fluctuations.(2)The fluctuation amplitude is directly proportional to the discharge and inversely proportional to the water depth and diffusion angle;the greater the deviation of the flow distribution from equal allocation among the four chambers,the larger the water-level fluctuation amplitude.The proposed quantitative formula is validated by prototype observation data with an average error within ±0.02m.The findings provide technical support for the inverted siphons engineering design and operational scheduling under high-flow operations.