How mangroves and past land-use change have affected compound flood events in the Vietnamese Mekong Delta

The Vietnamese Mekong Delta (VMD) is among the vastest low-lying areas in the world and particularly exposed to relative sea-level rise, pluvial, fluvial and coastal flooding. While new studies have shown how the impacts of climate change and land subsidence will further increase the vulnerability of the delta, current flooding characteristics are also shaped by land use and its changes over time, including the distribution of mangroves and urban sprawl. However, the implications of delta-wide land-use changes and the role of coastal habitats for driving flood dynamics in the VMD remain unknown. In addition, there is a lack of analyses that integrate all hydrometeorological forcings in a compound setting (pluvial, riverine and coastal) and use two-dimensional hydrodynamic modelling across the entire delta including the Ho-Chi-Minh-City province.

We address these shortcomings by applying a state-of-the-art two-dimensional hydrodynamic model (SFINCS) across the VMD, and incorporating latest digital elevation models and land-use data from 1985 and 2022. We touch upon difficulties in validating large-scale hydrodynamic models for vast low-lying delta regions and highlight the importance of high-quality digital elevation models (DEMs) for investigating the role of mangroves in nature-based coastal defense schemes by comparing the modelling results obtained for different DEMs (FABDEM vs DeltaDTM). Furthermore, we attribute characteristics of recent flood events to land-use land-cover change since 1985 and sea-level rise, and investigate the role of existing mangrove forests for flood risk mitigation. Preliminary results emphasize the contribution of land use change for compound flood dynamics and point towards the high value of mangroves as a natural surge buffer across the VMD, but specifically in the provinces Ca Mau and Ho-Chi-Minh-City.