Preprints
https://doi.org/10.5194/esurf-2020-109
https://doi.org/10.5194/esurf-2020-109

  05 Jan 2021

05 Jan 2021

Review status: this preprint is currently under review for the journal ESurf.

Dynamics of salt intrusion in the Mekong Delta; results of field observations and integrated coastal-inland modelling

Sepehr Eslami1,2, Piet Hoekstra1, Herman W. J. Kernkamp3, Nam Nguyen Trung4, Dung Do Duc4, Hung Nguyen Nghia5, Tho Tran Quang4, Arthur van Dam3, Stephen E. Darby6, Daniel R. Parsons7, Grigorios Vasilopoulos7, Lisanne Braat8, and Maarten van der Vegt1 Sepehr Eslami et al.
  • 1Department Physical Geography, Faculty of Geoscience, Utrecht University, Utrecht, 3584 CB, the Netherlands
  • 2Marine and Coastal Systems Unit, Deltares, Delft, 2629 HV, the Netherlands
  • 3Software Department, Deltares, Delft, 2629 HV, the Netherlands
  • 4Southern Institute for Water Resources Planning (SIWRP), Ho Chi Minh City, Ward 3 72710, Vietnam
  • 5Southern Institute of Water Resources Research (SIWRR), Ho Chi Minh City, 656 Võ Văn Kiệt, District 5, Vietnam
  • 6Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK
  • 7Department of Geography, Environment and Earth Sciences, University of Hull, Hull HU6 7RX, UK
  • 8Department of Geology and Planetary Sciences, California Institute of Technology, CA 91125, USA

Abstract. In the list of challenges facing the world largest deltas, increased salt intrusion and its role in jeopardizing freshwater supply is often ranked very high. Yet, detailed process-based studies of salt intrusion at the whole delta scale are limited and the trends are regularly associated to global sea level rise. Here, using field measurements and a sophisticated 3D model that integrates the riverine, rural, estuarine, and coastal dynamics within one numerical domain, we study salt intrusion at the scale of the Mekong Delta in extensive detail. While many studies down-scale the salt intrusion problem to a topic within an estuary, we show that the continental shelf is an intrinsic component of the delta, and its physical processes, such as monsoon-driven ocean surge, directly influence salinity dynamics within the delta. Typical values of 20–40 cm surge over the continental shelf contribute to up to 10 km of further salt intrusion. The delta's estuarine system is also more sensitive than many other systems to upstream discharge variations. Furthermore, spring-neap variability plays a key role in salt intrusion in the delta. The estuarine variability from a stratified to a mixed system between neap and spring tides develops 3D processes such as estuarine circulation and tidal straining that become the main upstream salt transport mechanisms. The 3D nature of salinity dynamics, and the role of upstream and downstream processes, suggests that compromising on dimension or extent of the numerical domain, can limit the accuracy of predictions of salt intrusion in the delta. The study also showcases that riverbed incision in response to anthropogenic sediment starvation in the last two decades, has increased stratification, and activated or magnified 3D salt transport sub-processes that amplify upstream salt transport. With all the external forces on the delta namely climate change and altered hydrological regime by the upstream dams, due to deeper estuarine channels (driven by sand mining and upstream impoundments), the delta itself is far more vulnerable to even mild natural events. This exemplifies the fundamental importance of preserving the sediment budget and riverbed levels in protecting the world's deltas against salt intrusion.

Sepehr Eslami et al.

Status: open (until 16 Feb 2021)

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Sepehr Eslami et al.

Data sets

Salinity measurements in the Mekong Delta Sepehr Eslami, Maarten van der Vegt, and Piet Hoekstra https://doi.org/10.5281/zenodo.4395277

Sepehr Eslami et al.

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Short summary
Increased salt intrusion jeopardizes freshwater supply to the Mekong Delta, and the current trends are often inaccurately associated to sea level rise. Using observations and models, we show that salinity is highly sensitive to ocean surge, tides, water demand and upstream discharge. We showcase that anthropogenic riverbed incision has significantly amplified salt intrusion, exemplifying the gravity of preserving sediment budget and riverbed levels to protect the deltas against salt intrusion.