Performance assessment of urban drainage systems using a river–pipe–surface coupled modeling approach

Study Region: The HUA district represents a typical low-lying urbanized floodplain within a coastal megacity, where a high-density municipal pipe network directly intersects a dynamic river system, making it a regional model for studying compound waterlogging under the nonlinear interaction of localized precipitation and fluctuating downstream river boundaries. Study Focus: The research develops an integrated evaluation framework combining a river–pipe–land two-way dynamically coupled hydraulic model with a multi-level Analytic Hierarchy Process (AHP). Remote sensing data is utilized to refine surface parameters, while a newly proposed Drainage Efficiency Indicator (DEI) system evaluates performance across four dimensions: system capacity, drainage impact, overall efficiency, and backwater suppression. The study analyzes 12 distinct scenarios to quantify the sensitivity of urban drainage to varying rainfall intensities, river stages, and outlet configurations. New Hydrological Insights: Analysis of the study area reveals that precipitation intensity is the primary determinant of drainage efficiency, exhibiting a strong non-linear negative correlation with the DEI score. A critical process-based finding is the peak-on-peak interaction, where the synchronization of the river flood peak and the maximum pipe discharge triggers transient backwater suppression, significantly extending the drainage recession time by up to 425% as return periods increase from 5a to 50a. Prescribed-stage experiments show that high receiving-river levels restrict drainage, while low levels enhance performance under the same local rainfall. The results also reveal an “integrated hydraulic limit” in the urban catchment: lowering outlet elevations reduces backflow risk, but marginal efficiency gains diminish during extreme events, indicating that downstream boundary conditions ultimately become the dominant constraint on system recovery.