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

Creators: Eslami, Sepehr; Hoekstra, Piet; Kernkamp, Herman W. J.; Nguyen Trung, Nam; Do Duc, Dung; Nguyen Nghia, Hung; Tran Quang, Tho; van Dam, Arthur; Darby, Stephen E.; Parsons, Daniel R.; Vasilopoulos, Grigorios; Braat, Lisanne; van der Vegt, Maarten

Abstract

On the list of challenges facing the world largest deltas, increased saline water intrusion (SWI) in the surface water system and its role in jeopardizing freshwater supply are often ranked very high. Yet, detailed process-based studies of SWI at the whole delta scale are limited, and the trends are regularly associated with 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 SWI at the scale of the Mekong Delta in extensive detail. While many studies downscale the SWI problem to a topic within an estuary, we show that the physical processes on the continental shelf, 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 SWI. The delta's estuarine system is also more sensitive than many other systems to variations of river discharge. Furthermore, spring–neap variability plays a key role in SWI 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 SWI in the delta. The study also showcases the fact that riverbed incision in response to anthropogenic sediment starvation in the last 2 decades has increased stratification and activated or magnified 3D salt transport subprocesses that amplify upstream salt transport. With all the external forces on the delta, namely climate change and an altered hydrological regime by the upstream dams, due to deeper estuarine channels (driven by sand mining and upstream impoundments) compared to its near past, the delta itself has become 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 SWI.

Additional Information

© Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 14 December 2020 – Discussion started: 5 January 2021; Revised: 10 May 2021 – Accepted: 15 June 2021 – Published: 12 August 2021. We would like to show our gratitude to Le Quan amongst other colleagues at SIWRR for their sincere support during the field campaign and our stay in the city of Dai Ngai. Without their support, this study would not be possible. Special thanks to the personnel of the Southern Institute for Water Resources Planning (Ho Chi Minh City, Vietnam) for openly supporting the project during its development. We are grateful to Jasper RFW Leuven for his support through the toughest times of the field campaign. We sincerely appreciate how DHI supported our research by generously providing us with wind, water level, and velocity data in the coastal waters. This research is part of the "Rise and Fall" project, funded by NWO-WOTRO (W 07.69.105), Urbanizing Deltas of the World-1 (UDW1). The 2018 bathymetric data were acquired with support from NERC under two distinct projects referenced as NE/S002847/1 and NE/P014704/1. Data availability: DFlow-FM is an open-source numerical model available at https://oss.deltares.nl/web/delft3dfm (Deltares Open Source Softwares, 2021). The underlying gauge data (observed water level, discharge, and salinity) provided by the SIWRP, following the organizational policy, can be provided upon request for non-commercial use. The wind data originally developed by NCEP/NOAA can be downloaded from the DHI repository at https://www.metocean-on-demand.com/#/main (DHI metocean data portal, 2021). The geometry data on the network of primary and secondary channels can only be provided in direct communication with the SIWRP. The 2008 bathymetry data are partly received from the Mekong River Commission at https://portal.mrcmekong.org/data-catalogue (Mekong River Commission (MRC) data portal, 2021) and partly received from the SIWRP; the 2018 contemporary bathymetry data can be downloaded from https://hydra.hull.ac.uk/resources/hull:17952 (Vasilopoulos et al., 2018). Nevertheless, all the data and models that cannot be directly accessed by the public can be provided to the reviewers for any validation or reproduction. Author contributions: SE coordinated efforts of various parties, carried out formal analysis, set up the salinity measurement field campaign and models, pre-processed, post-processed, and visualized the data, and led the paper draft. PH and MVdV were responsible for funding acquisition, supervision, and review of the research and paper. SEA, together with MVdV and PH, conceptualized the study. HWJK and AvD supported software application and conducted the required numerical model development and update at various stages. HWJK contributed to the writing and reviewed the paper, and AvD reviewed the final draft. GV carried out the field measurement of bathymetry, prepared the bathymetry for application in the modelling exercise, and reviewed and edited the final paper. DDD provided resources and reviewed the findings and the writing, while NNT and TTQ supported the investigation, analysis, and model set-up, as well as reviewing the findings and the writing. LB assisted during the field campaign and pre-processing and post-processing of the data, as well as leading the video abstract development and reviewing the final paper. HNN facilitated and coordinated the salinity and bathymetry measurement field campaigns and reviewed the final paper. SED and DRP facilitated the funding acquisition for the bathymetry field campaign, supervised the bathymetry development, and contributed to the writing of the final paper. The authors declare that they have no conflict of interest.

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