ABSTRACT The inefficiency and poor performance of drainage system is increasing considerable among causes of urban floods. This study focuses on the influence of the aperture ratio on drainage capacity through the 170 local numerical models and 46 physical model tests. Four regions of water characteristic were obtained: Open Channel Flow Non-Overload (OC-NOL), Full Flow Overload (F-OL), Full Flow Overload Affected (F-OLe) and Full Flow Overflow (F-OF). The findings were applied in a practical case, which showed that: pipe diameter had an evident effect than manhole diameter. As the aperture ratio increased, the flood areas with low water depth grades evolved faster towards higher grades, while the drainage capacity decreased. Manhole overflow seriously increases the risk of flood when the pipe diameter was below 0.4 m or aperture ratio was above 2.00. This paper explains the water characteristic from the perspective of aperture ratio, which provides supplementary information for flood risk assessment.