Effect of downstream water level on operation features of a dropshaft-–tunnel system

• Under free outflow conditions, the air pressure in the dropshaft is related to the flow rate but changes little when the outflow is submerged. • As the downstream condition transitions from free outflow to submerged outflow, the flow pattern changes from wavy flow to blocked flow with air pockets. • Downstream water levels affect the drainage system by altering the linkage between the dropshaft and tunnel and the air release resistance from the downstream side. This study investigated the operation features of a dropshaft-tunnel system under varying downstream water levels through a large-scale physical model. In the experiments, the air pressure distribution in the system was measured, and the flow pattern was recorded by cameras. The results revealed that the air pressure in the dropshaft increased with increasing water flow rate under free outflow conditions but changed little when the outflow was submerged, even when the flow rate further increased. Additionally, there was a wavy flow in the tunnel under free outflow conditions, whereas plug flow with air pockets occurred under submerged outflow conditions. The downstream water level was found to affect the system through changing the linkage between the dropshaft and tunnel and the resistance to air release downstream. The findings of this study contribute to a more comprehensive understanding of the operation of deep tunnel systems.