Impacts of urbanization and climate change on surface runoff in Thu Duc City, Vietnam

Thu Duc City (TDC), a sub-city of Ho Chi Minh City (HCMC), Vietnam, has undergone rapid transformation in land use - land cover (LULC) over the past few decades. Flooding is one of the biggest challenges to the sustainable development of the city. Quantifying excess runoff induced by urbanization and climate change is essential for developing an efficient flood risk management strategy. This research aims to evaluate the effects of the impervious surface expansion (a key indicator of urbanization) and the rainfall increase caused by climate change on the excess runoff depth in TDC. The dataset derived from LANDSAT satellite images was used to explore the spatial-temporal dynamics of impervious surface areas from 1985 to 2020. The EPA-SWMM urban rainfall-runoff model was employed to investigate the effects of increased impervious surface and climate change on excess runoff depth. The satellite data showed that TDC has 24% of the impermeable surface area currently, much lower than 75.6% in areas located in the central districts such as Nhieu Loc - Thi Nghe (NL-TN) basin, and that impermeable areas have expanded rapidly in the north-south and west-east directions since 2010. The simulated results indicated that impervious surface development increased excess runoff depth by 14.0mm between 1985 and 2020. It also showed that increasing rainfall due to climate change could raise the average excess runoff depth by 18.2%, 64.0%, and 49.4%, respectively, at the beginning, the middle, and the end of the century compared to the current stage. It suggests mainstreaming stormwater retention and detention systems into urban development plans for adapting to the future uncertainties of urbanization and climate change. It also recommends that the impervious surface areas must be maintained within design thresholds to control runoff and mitigate floods in practice. The research findings could assist policymakers and stakeholders in formulating a long-term vision to propose an efficient urban stormwater management strategy in TDC and similar growing cities.