Distributed bioretention under tropical rainfall and tidal backwater: a SWMM-LID assessment for a catchment in Ho Chi Minh City

Green infrastructure (GI) is widely promoted for urban flood control, yet the performance figures most often quoted come from temperate cities and are a poor guide for tropical, tide-affected deltas. The aim of this study is to quantify, with a locally-parameterised model, how much a distributed bioretention retrofit can deliver in Ho Chi Minh City (HCMC) and what limits it. In terms of materials and methods, we built a hydrodynamic SWMM model of a real sixteen-conduit, six-kilometre trunk sub-catchment (~48 ha) extracted from the surveyed municipal sewer network of the former District 8, draining to a single outfall on the tidal Kênh Đôi canal; the model was forced with design storms of 15–180 min built from the national-standard intensity–duration–frequency (IDF) relation for HCMC (TCVN 7957:2023, 10-year return period, cross-checked against a peer-reviewed 33-year station IDF), bounded downstream by a measured Phú An spring-tide series, and calibrated against the surveyed street-flood depths of the 6 August 2020 storm (Nash–Sutcliffe efficiency (NSE) 0.77, RMSE 0.03 m, PBIAS −10%).+ The main results are that bioretention on 5% of the catchment reduced the outfall peak by only 0.6–2.1% for storms of 60 min and longer (41.2% for the short 15-min burst), because the flat, surcharged, tidally-controlled network is capacity-limited; the substantial and robust benefit is instead a reduction in flood (street-ponding) volume of 33% for the 60-min design storm and up to 84.7% for short bursts, together with a 6.4–19% runoff-volume reduction and, over a continuous wet season, a 12.7% volume and 33.9% flooding reduction, while a coincident spring tide raised flooding by 15–56%. In discussion, these findings reframe distributed GI in HCMC as a flood-volume and flood-frequency measure for frequent, short convective storms, to be deployed at several percent of the drained area and integrated with the grey and tidal-control system rather than used as a substitute for it. Future research should obtain a gauged outfall hydrograph and locally-measured bioretention-media and shallow-groundwater data to tighten the absolute estimates. The study offers a first flood-calibrated benchmark and a reusable framework for GI planning in tropical, tide-affected deltas such as HCMC.