Abstract: Since the piezometer heads in open channels and pressurized waters are of the same expression in Saint-Venant equations, the unified description of free-surface and pressurized mixed flow is obtained. A unified mixed flow model suitable for open channel alternating with sinking stream in real Karst areas is established by describing the irregular closed cross-section of sinking stream with triangular closed subsections. The pressure gradient term is discretized using semi-implicit method, while the advection term is solved by the Eulerian-Lagrangian method, and the coupling of branches in dendritic and loop channel networks is solved by prediction-correction method. The proposed model is checked by using the hydrological process of Jiayan Reservoir in 2000. Results reveal an error of flow calculation within 1.6%. Moreover, the variation rules of velocity along the longitudinal direction of the open channel alternating with sinking stream and the division rules of artificial tunnels in front of the sinking reach are investigated. In the conditions of peak flood inflow and normal water level in front of the dam, the flow velocity in open channel segment is generally smaller than 0.25 m/s, whereas the velocity in sinking stream reaches 1.0-3.0 m/s, which is beneficial to the maintenance of sinking stream. During the simulation of hydrological process from 1971 to 1990, the division ratio of artificial tunnels in front of big-medium sized overbridge is 40.3%-42.8%, and the division ratio of artificial tunnels in front of small overbridge is 76.8%-78.6%.

Key words: open channel, sinking stream, mixed flow, hydrodynamic model, Karst area, Jiayan reservoir