Hydrodynamic modelling of a meso-tidal river using in-situ and SWOT measurements in a data-scarce region
Creators: name: Rodrigues do Amaral, Francisco
Publisher: name: Université Grenoble Alpes, UGA
Year: 2024
Subjects: subject: Coastal and estuarine hydraulics
subject: Satellite technology
subject: Hydrology
subject: Vietnam
subject: tidal river

description: Coastal environments have historically supported thriving societies due to their rich food resources, fertile soils, and strategic locations as transportation hubs, fostering urban development and economic growth. This trend continues today, with many of the world’s most densely populated cities situated in Low-Elevation Coastal Zones (LECZs). Currently, approximately 896 million people, nearly 11% of the global population in 2020, reside in LECZs, and this number is projected to exceed one billion by 2050. Coastal megacities face substantial risks from climate change due to their low elevation, climate-sensitive physical and ecological characteristics, and high societal exposure and vulnerability. Despite these risks, LECZs are poorly monitored globally, especially in under-developed and developing countries.

This thesis focuses on the poorly gauged LECZ of Ho Chi Minh City (HCMC) in South Vietnam. HCMC, a densely populated megacity, has urban center areas with up to 30,000 inhabitants per square kilometer. The city is traversed by the tidal Saigon River, which includes a network of about 800 km of watercourses and canals. With 90% of HCMC’s urban area being impermeable, the hydrological cycle is significantly impacted, leading to frequent flooding from coastal, riverine, and rainfall runoff sources.Achieving a sustainable future for HCMC relies on our ability to monitor both hydrological and oceanographic variables concurrently to better understand the physics of these systems. To enhance our understanding of this estuarine system, two key variables are of interest: river and coastal ocean water levels, and river discharge. These variables can be effectively monitored using a combination of in-situ and satellite measurements.

The general objective of this thesis is to achieve a better understanding of the hydrological-to-ocean continuum in the LECZ of Ho Chi Minh City. To do so we use a trinity of tools: in-situ measurements, remote sensing measurements and hydrodynamic modelling. In specific, we are interested in assessing the potential of the new Surface Water and Ocean Topography (SWOT) satellite mission over this region.

Chapter 1 introduces the subject area of the thesis.

Chapter 2 assesses the impact of Typhoon Usagi, which made landfall in HCMC in 2018. The typhoon did not create a significant storm surge but caused extreme rainfall, leading to significant inland water level surges. Prolonged flooding in the city, especially in Thao Dien, resulted from the combination of a high river spring tide and  impervious, low-lying streets. Coastal tidal forcing was the main driver of river discharge during this event. A general insight into the the behaviour of the Saigon river was also found: seasonal coastal storm surges from East Asian summer monsoon wind patterns, rather than precipitation, were found to control river water levels.

Chapter 3 presents data from a two-month field campaign in HCMC, where high-resolution river water level and discharge measurements were obtained. This campaign was managed and supervised by me, with assistance from Mr. Tin Nguyen Trung of the Center for Asian Research on Water (CARE). 

Chapter 4 details the implementation, calibration, and validation of a 1D hydrodynamic model of the Saigon and Dongnai rivers, addressing calibration challenges in this data-scarce region. The model, coupled with a modified Manning-Strickler (MS) law, was developed in collaboration with Dr. Benoit Camenen of INRAE. The best calibration strategy was found to involve directly measured water levels and indirectly measured discharge data, despite challenges from dynamic tidal conditions and lack of reliable upstream discharge data. Validation against independent measurements showed promising results, demonstrating the effectiveness of the modified MS equation in  enhancing discharge estimates.

Chapter 5 provides a statistical study of the potential of SWOT satellite data to improve discharge estimations in the Saigon River, conducted before real SWOT data was available. A methodology using simulated SWOT products at a 200-meter node resolution showed improved discharge estimates by reducing mean relative RMSE and increasing R2 between true and estimated discharge.

Chapter 6 explores the application of real SWOT data in one of the most challenging coastal environments this satellite mission could encounter: a datascarce, tropical region characterized by flat topography, a heavily urbanized environment, and a river with a weak water surface slope that fluctuates daily around zero due to strong coastal tidal forcing. To do so, we compare SWOT’s reach level products of river water level and slope against in-situ measurements. The results show that about 50% of SWOT’s water level measurements were found to be valid compared to in-situ data, showing a good accuracy despite the challenges posed by this region. However, SWOT’s slope measurements lack accuracy and are unreliable without prior validation against in-situ data. Despite these limitations, SWOT provides high-spatial resolution observations that can offer new insights into dynamic hydrological phenomena when combined with in-situ data and modeling efforts.

Chapter 7 concludes the thesis and offers perspectives for future work.