This study presents an attempt to model and simulate the hydraulic-type solutions of air-water stratified flow in a sloped pipe with a large-scale diameter and restricted ventilation through scattered manholes along main. Four dry-weather scenarios and corresponding wet weather cases are considered in the urbanized catchment area to model hydraulically the air-water stratified flow inside the underground pipe, focusing on a hydraulic-gradient dependent formation of air cavity. For this purpose, the Manning-formula based solutions are developed in frames of the CoCoViLa platform application. The model is constructed using primitive building blocks as pipes and manholes, inlets and tanks, and enables computing the water and air flows in the system using standard hydraulic formulae for water flow and complemented parameterised air flow tables. This study gives an insight into the complexity of air-water stratified flow in a poorly ventilated water collection system, which reveals naturally the quasi-steady flow due to the distinct diurnal flow pattern that is characteristic for a large-city type water consumption profile, and the coastal-region stormwater events.