Header Image: Rice Field in Mekong Delta. Image by Thai Nguyen from Pixabay.
Saltwater intrusion in the Vietnamese Mekong Delta is driven more by anthropogenic activity than climate change.
A recent study by a team lead by Sepehr Eslami from Deltares, a water research institute in the Netherlands, shows anthropogenic-induced sediment deprivation has been the dominant driver in changes in salt water intrusion in the Vietnamese Mekong Delta, and not sea level rise and global climate change as previously thought.
The study shows that the construction of hydropower dams and intensive sediment mining lead to deepening river channels. This activates complex three dimensional processes that transport salt up the delta.
The Vietnamese Mekong Delta, which includes 12 provinces and Can Tho city, is the third largest delta in the world and is Vietnam’s largest rice, fruit and seafood producer. Salt water enters the rivers and canals, affecting regional land use and its future habitability.
“Salt water intrusion threatens the freshwater supply to agricultural land and the whole delta ecosystem,” says Nam Nguyen Trung, one of the paper’s authors.
Many rice fields have been damaged in the Tra Vinh province because salt water reaches the fields. Do Trung, Director of the Tra Vinh Irrigation Work Exploitation and Management One Member Limited Company estimates that 50% of the area could be completely damaged.
According to the provincial Department of Agriculture and Rural Development, more than 2,100 hectares of rice have been affected from salt water intrusion and drought in the Soc Trang province.
Detailed study of saltwater intrusion in Mekong Delta
Sepehr’s team studied salt water intrusion in the Vietnamese Mekong Delta in extensive detail.
They measured salinity, temperature and riverbed levels along two channels in the Vietnamese Mekong Delta during neap and spring tides and used the field data to set up a sophisticated numerical model of the whole delta.
“The complexity and three-dimensionality of salt transport mechanisms in this deltaic system demand a 3D model to capture the delicate temporal variations of upstream salt transport,” says Sepehr.
The model was run under various scenarios to see how surges in the ocean and changes in the tides, winds, river discharge and riverbed levels affect saltwater intrusion in the delta. The model results confirmed earlier findings that the effects of anthropogenic activity on the short-term trends in salt water intrusion outstrip the effects of climate change.
Hydropower dams built upstream alter the hydrological regime. In addition, they stop sediment from flowing downstream, and there’s intensive sand mining in the region. The sediment deprivation has lead to erosion of the river banks and a deepening of the channels. Sepehr says the deeper channels magnify the effect of three-dimensional processes which amplify the upstream transport of salt.
The delta itself has become far more vulnerable to even mild natural events, he says. Ocean surges can increase the salt intrusion to 10 km during the dry season, and the effects last twice as long as the surge event itself.
An earlier study carried out in 2019, also lead by Sephr, reports that while the offshore lunar semi-diurnal tidal amplitude increases by 1.2–2 mm per year due to sea level rise, the tidal amplitude within the delta is increasing by more than ten times this amount. Salinity in the channels is increasing by 0.2–0.5 PSU per year. These changes are related to 2–3 m bed level incisions in response to sediment starvation.
Preventative measures against saltwater intrusion
Measures are being taken in the Mekong Delta to reduce the effects of saltwater intrusion.
Lam Hoang Hiep, Vice Chairman of the Soc Trang People’s Committee, said the delta has implemented many irrigation projects that prevent saltwater intrusion and hold fresh water on a farming area of 300,000 hectares.
The delta’s provinces and Can Tho have dredged irrigation canals to store fresh water and upgraded and built irrigation systems. Do Trung’s company has operated 48 sluices to prevent saltwater intrusion and store fresh water.
According to the Ministry of Agriculture and Rural Development of the Socialist Republic of Vietnam, authorities in the delta instructed farmers not to grow the winter-spring rice in areas that could be affected by saltwater intrusion.
Sephr’s study has provided a new understanding of the physical processes that dominate salt water intrusion in the Mekong Delta. The hope is this understanding will help to implement more measures like these against salinisation in the delta.
References and Resources:
Eslami, S., Hoekstra, P., Kernkamp, H. W. J., Nguyen Trung, N., Do Duc, D., Nguyen Nghia, H., Tran Quang, T., van Dam, A., Darby, S. E., Parsons, D. R., Vasilopoulos, G., Braat, L., and van der Vegt, M.: Dynamics of salt intrusion in the Mekong Delta: results of field observations and integrated coastal–inland modelling, Earth Surf. Dynam., 9, 953–976, https://doi.org/10.5194/esurf-9-953-2021, 2021.
Eslami, S., Hoekstra, P., Nguyen Trung, N., Ahmed Kantoush, S., Van Binh, D., Duc Dung, D., Tran Quang, T., and van der Vegt, M.: Tidal amplification and salt intrusion in the Mekong Delta driven by anthropogenic sediment starvation, Sci. Rep.-UK, 9, 18746, https://doi.org/10.1038/s41598-019-55018-9, 2019.
https://www.mard.gov.vn/en/Pages/mekong-delta-takes-measures-to-reduce-saltwater-intrusion.aspx?item=16 (Accessed: 1-10-2021)
This article was first published in Brainstorm Magazine.
Paul Sterlini
