The deep tunnel system is increasingly applied for urban flood control. By constructing a large-scale physical model of dropshaft-tunnel system, this study investigated the air-water flow characteristics in the tunnel under the effects of air entrainment by the upstream dropshaft. Under different conditions of various downstream water levels and inflow rates, the development of air-water flow features along the tunnel were recorded and analyzed. The pressure fluctuations along the tunnel were also measured and studied. The experimental results showed that as the downstream water level rose, four air-water flow patterns could be observed in the tunnel, accompanied with different ways of air release. The distribution of pressure fluctuations along the tunnel also varied under different air-water flow patterns, but the most significant pressure fluctuations were all observed at the air release points. Additionally, as the inlet flow rate increased, the pressure fluctuations in the tunnel got enhanced. This study can contribute to a better understanding of the operation processes of a deep tunnel system.
