The Central Trunk Diversion Structure (CTDS) is a sewage conveyance system in King County, WA. The CTDS ends at a Vortex Drop Structure (VDS) that discharges to the East Portal Drop Structure. The field data indicate that the CTDS does not provide sufficient diversion capacity during high flow events. Flow Science has performed a two-phase CFD analysis of the flow in the CTDS and VDS. The purpose of the analysis was to determine the hydraulic grade line in the VDS to aid analysis of possible flow restrictions. The analysis was performed for a range of anticipated flow rates from 10 MGD to 120 MGD. The modeling results show that the hydraulic jump inside the VDS can potentially cause the top of the pipe to be choked when the flow rate exceeds 90 MGD. In addition, the air core area ratio in the vertical drop shaft is predicted to drop below the recommended minimum value of 25% when the flow rate exceeds 60 MGD, reducing the overall capacity of the VDS.