Flow regime transition mechanisms in rapidly filling stormwater storage tunnels

An issue in the design of combined sewer overflow storage tunnels is to avoid “geysering” which is an air/water mixture blowing up through vertical shafts connected to the tunnel. Studies indicate that the origin of this phenomenon is the entrapment of large air pockets as the rapidly filling tunnel undergoes a transition between free surface and pressurized flow. Commonly implemented numerical models are of the shock fitting type that tracks the location of a pipe filling bore. However, the flow regime transition does not have to occur through a pipe filling bore. Another possibility involves a free surface bore with a following gradual transition to a full pipe condition. Large air volumes may be trapped in this situation following the bore reflection off a tunnel transition if this reflection closes the flow cross-section. Experimental observations are presented to demonstrate both types of flow regime transition. Traditional shock fitting methods are ill-equipped to accurately simulate gradual flow regime transitions. The shock capturing method proposed by Vasconcelos et al. (J Hydraul Eng 132(6):553–562, 2006) is demonstrated to be capable of resolving both types of observed bores.