Tracing the development of coastal ecosystems through satellite imagery: a case study from Ho Chi Minh City, Vietnam

Like many other coastal megacities, Ho Chi Minh City in Southern Vietnam is regularly affected by flooding from torrential rainfalls and storm surges. Global climate heating will exacerbate these natural hazards, but rapid urbanization has a share in the intensification of urban disaster risk as well. Proceeding surface sealing, uncontrolled land subsidence and the urban heat island effect are only a few of the factors increasing exposure to flooding, which often hits the most vulnerable parts of the population. To defend local residents and reduce flood-induced losses, classic protection measures like embankments, flood gates and large-scale pumping stations are implemented across Ho Chi Minh City. But as low-level inundations continue to cause frequent disruptions to the local economy, ecosystem-based solutions or so-called blue-green infrastructure are more and more seen as promising means to complement existing management strategies and ensure sustainable flood resilience.

In our study, we investigate whether the role of coastal ecosystems in mitigating floods in Ho Chi Minh City is already reflected in recent urban developments. Specifically, we use multi-spectral Sentinel-2 imagery to calculate the Normalized Difference Vegetation Index (NDVI), a dimensionless indicator to describe terrestrial vegetation. The obtained annual NDVI maps have a spatial resolution of ca. 1 arc second and cover the years 2016 to 2022; special attention was given to the composition of cloud-free images. On this basis, temporal trends were determined that specify the (qualitative) development of bio-activity across the urban and suburban districts of the province. These local trends are complemented by annual histograms, which describe the relative frequency of specific NDVI ranges and thus allow estimations of ecosystem density. Unsurprisingly, preliminary results suggest constant ecosystem density for the Can Gio biosphere reserve in the South of the HCMC province, except for slight changes along its boundaries. For the urban districts of HCMC, however, ongoing urbanization can be traced by decreasing ecosystem density according to our assessment of NDVI values. The employed method provides promising results, yet currently still lacks a decent validation through ground truth data.