This study investigates the hydraulic and airflow dynamics of a section of the urbanised area combined sewerage system located in a highly populated area through modelling and field-based validation. A detailed hydraulic model was developed using the US EPA Storm Water Management Model (SWMM), incorporating the physical characteristics, operational conditions, and boundary inputs. Simulated flow rates and discharge hydrographs were generated for a range of observed rainfall and dry-weather conditions. To evaluate model reliability, the results were systematically compared with field measurements collected within the Engineering Tools of Stratified-Flow Processes in the Built Environment project, which provided high-resolution discharge data under real-world operating conditions. The comparison enabled the identification of both systematic and event-specific discrepancies. Beyond hydraulic flows, the study also examined air-water interactions within the sewer system. Air volumes and airflow velocities are derived for the modelled network segment.