Investigation of rapid filling of poorly ventilated stormwater storage tunnels

Below-grade stormwater storage tunnels are designed to provide relief for collection networks during intense rain events. Air must be evacuated as the tunnel fills and, depending on the system geometry, the air may become pressurized. This research presents an investigation on various ways that the air pressurization influences the water flow. The investigation used an experimental apparatus consisting of a horizontal 14.8 m long, 94 mm diameter acrylic pipeline with various degrees of air ventilation. The experiments were primarily conducted to explore two features of the flow termed pre-bore motion and the interface breakdown. Experimental measurements include flow velocity, air and water pressure, and flow depth. The experimental results were compared to the predictions of a numerical model based on the Saint-Venant equations that handles the flow regime transition and the possibility of air pressurization. The numerical predictions agree well with the experimental observations and provide an explanation for the interface breakdown occurrence.