Anticipating Transient Problems during the Rapid Filling of Deep Stormwater Storage Tunnel Systems

Abstract

Deep storage tunnels are an alternative to mitigate combined sewer overflows, but they may develop transient flow problems during rapid filling conditions. Such issues include excessive surges, formation of pipe-filling bores, entrapped air pockets, and uncontrolled releases of air. Initial decisions on stormwater tunnel designs are generally based on steady-flow considerations but can have a significant impact on the transient hydraulic performance. However, tunnel designers rarely have an opportunity to systematically assess the interplay between geometric characteristics (e.g., tunnel diameter, alignment, and junction areas) and inflow hydrographs. This work aims to provide an examination of the relationship between key design parameters and the performance of stormwater tunnels undergoing rapid filling. A flow regime transition model was used to simulate 216 different scenarios of rapid filling of tunnels, considering air–water interactions such as air pocket entrapment, compression, and expansion. Results indicate that peak surges and air outflows though vertical shafts correlated well with design parameters such as junction shaft plan areas and inflows at the time of tunnel pressurization. Other variables, such as entrapped air pocket volume, did not correlate well with geometric or inflow parameters selected in this study. While these results are not directly applicable to any existing stormwater tunnel geometry, the findings should help designers to understand potential transient flow issues and the input conditions that may lead to the most severe transient conditions.