Integrated coastal-river water surface elevation datasets derived from SWOT to improve compound flooding simulations over the Mekong Delta

River deltas are home to more than 400 million people worldwide, being fundamental centers for industry, and ecosystems of great ecological and economic importance. Some of the most densely populated rural and urban areas are in low-lying deltaic regions, such as the Mekong Delta. These areas are highly vulnerable to the impacts of climate change on coastal-river floods, which are driven by several factors, such as sea level rise, extreme river flows or storm surges. To mitigate these effects, accurate integrated coastal-river hydraulic models are essential for enhancing predictive capabilities for compound flooding events and developing effective contingency plans. However, the accuracy of hydraulic models is often limited by the quality of available observations. Developing reliable datasets for coastal-river domains involves addressing several challenges, including a) the high spatial and temporal variability of coastal-estuary dynamics, b) the complex morphology of delta regions characterized by extensive floodplains, braided river channels, and man-made structures, and c) the lack of continuous coastal-river datasets.

Traditional in-situ monitoring provides data only at widely spaced stations, which limits coverage. As a results, satellite Earth Observation (EO) has emerged as a solution to generate datasets with large spatial coverage and high spatial resolution. The Surface Water and Ocean Topography (SWOT) mission is the first dedicated mission to monitor surface water, while also providing ocean height measurements, making it ideal to overcome the monitoring challenges in coastal-river domains. The SWOT mission, with a 120 km wide swath, offers large spatial coverage that can deliver water surface elevation (WSE) and surface water extent observations for rivers as narrow as 50 meters. Additionally, the mission offers a revisit time of 21 days, delivering 2-6 observations in each cycle.

In this study we utilize SWOT observations over the Mekong Delta to generate continuous datasets that span from the river to the ocean. These datasets are used to inform and validate an integrated coastal-river hydraulic model of the Mekong Delta. The SWOT L2_HR_Raster product is exploited at a 100-meter resolution, to derive coastal and estuarine WSE time series and surface water extent. This dataset has the capability to map complex river morphological structures at a temporal resolution previously unattainable by satellite EO missions. It can also capture the effects of ocean tides and storm surges on river water levels, as well as the impact of high river flows on coastal domains. Moreover, the 2D nature of the L2_HR_Raster product can deliver not only river-ocean WSE profiles, but also coastal longitudinal ocean height, to better understand the effect of high river flows in near-coastal areas.

The results provide continuous coastal-river datasets mapping the interplay between near coastal and estuarine dynamics, as well as the complex morphology of the Mekong Delta region. The datasets are used to calibrate and validate a hydraulic model of the Mekong Delta that integrates river and coastal zones to accurately simulate WSE and surface water extent in deltaic regions. The integrated model supports better prediction capabilities for compound flooding simulations and the impacts of climate change on the coastal and estuarine environments.