This study investigates the reduction in stormwater drainage capacity and the exacerbation of urban waterlogging caused by river-stage backwater and submerged outfalls in urbanized catchments within low-gradient river networks during extreme rainstorms. It aims to reveal the differences in drainage performance and river hydraulic responses of coupled sewer-river systems under free and submerged outflow conditions

thereby providing a basis for waterlogging mitigation and sluice-gate and dam operation. The Xiaotaihou River urbanized catchment in Beijing was selected as the study area. A coupled hydrological-hydrodynamic model

integrating surface rainfall-runoff processes

stormwater sewer conveyance

outfall discharge

and river routing

was developed using InfoWorks ICM. The model was calibrated and validated using four observed rainfall-discharge events. Six 3-h design storms with return periods of 2

5

10

20

50

and 100 years were simulated. With rainfall forcing

underlying-surface conditions

and sewer and river parameters held constant

two idealized river boundary scenarios

namely free outflow and fully submerged outflow

were established to quantify the effects of high river-stage backwater. During both the calibration and validation periods

the Nash-Sutcliffe efficiency (NSE) exceeded 0.70 and the coefficient of determination (

R

²) exceeded 0.80

indicating satisfactory model performance. For return periods of 20 years or less

the difference in inundated area between the two scenarios was below 2%. The 50-year storm was identified as the critical return period for catchment hydraulic response. Compared with free outflow

submerged outflow increased the inundated area with depths of 0.5–1.0 m from 3.30 km² to 4.06 km²

representing an increase of 23%

and enlarged the area with depths of 0.15–0.5 m by approximately 1.30 km². Peak discharges at repr

esentative upstream

middle

and downstream outfalls decreased by 8.42%

3.21%

and 1.57%

respectively. In addition

submerged outflow advanced the critical return period for abrupt water-level rises in the upper and middle reaches from 50–100 years under free outflow to 20–50 years.River-stage backwater has pronounced nonlinear

threshold-dependent

and spatially heterogeneous effects on catchment drainage and waterlogging control. For return periods of 20 years or less

the system retains sufficient conveyance and storage capacity

and outflow conditions have limited effects on waterlogging. Under the 50-year storm

submerged outflow substantially reduces sewer discharge capacity

expands moderate-to-deep inundation

and advances the threshold for abrupt water-level rises in the upper and middle reaches. Upstream outfalls are more sensitive to backwater effects and may experience drainage obstruction or reverse flow during extreme rainfall. Therefore

drainage modelling should couple river stages with outfall conditions. Pre-storm gate-dam drawdown to create storage can alleviate backwater effects and improve waterlogging control.