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Elevation of sinking for Vietnam’s Mekong Delta incorrectly estimated for decades: researcher

Saturday, October 05, 2019, 18:07 GMT+7
Elevation of sinking for Vietnam’s Mekong Delta incorrectly estimated for decades: researcher
Dr.Philip Minderhoud near a ground water pump in the Mekong Delta Vietnam where subsidence has caused the ground to sink by 20-30 cm.  Photo: Supplied

Vietnamese researchers have better data sources, hence a more accurate estimate of the elevation level of the Mekong Delta compared with numbers suggested by their international peers who rely mostly on satellite sources, a Dutch physical geographer has argued.

In a report published by the journal Nature Communications, Dr. Philip Minderhoud, from the Utrecht University and Deltares Research Institute, confirmed that Vietnam’s Mekong Delta region is an average of only 80 centimeters above the local sea level, approximately two meters lower than international researchers previously thought.

The sinking situation in the Mekong Delta, according to Minderhoud’s estimates, makes the salinization and flooding plaguing the region “even more urgent for the 18 million inhabitants”.

The incorrect estimate given by international researchers is due to a lack of consideration for their reference point - satellite data – as well as the dearth of information available in public domain data and minimal collaboration between scientists, according to Minderhoud.

Unfortunately, this probably is most likely not relegated to just the Mekong Delta and possibly exists in other parts of the word, he added.

The researcher said his newest publication is primarily for the international researcher community as pundits in Vietnam have better data at their disposal.

From Minderhoud’s point of view, development in the Mekong Delta will need to follow sustainable policies and practices to tackle the challenge of sea level rise and the sinking of the region. 

Tuoi Tre News has had a phone interview with Minderhoud, currently based in the Mekong Delta hub of Can Tho, to discuss the findings of his study.                                         

What can you tell us about your newest publication on the actual elevation of the Mekong Delta?

There are two issues with satellite DEM data (Digital elevation models) which explain why the elevation was incorrectly assumed before:

(1) The accuracy of the measurements is in the order of meters, which is an error in each individual measurement.

(2) Global satellite elevation is referenced (zero elevation) to a global model of earth gravity (geoid), which is roughly equal to sea water level. However, the global geoid model is not very accurate for Vietnam as it doesn’t contain local Vietnamese data. What we found in our study is that there is a difference of nearly two meters between the 0-level in the global geoid and actual sea level height in the Mekong Delta (dependent on local earth gravitation, ocean currents, and tides).

Many previous scientific studies did not correct for this difference because they didn’t believe this difference could be so large and there is no data available to do a correction. We discovered this large offset, which is fundamental to the incorrect estimation when we compared it local Vietnamese topographical elevation data.

One very important remark - we show in our study that the Mekong Delta actually sits at a much lower elevation than is assumed by the international scientific community based on comparison to local Vietnamese topographical data.

The mistake in elevation was made because international researchers used less accurate, freely available data. This is an example of why it would be beneficial that data, such as accurate elevation data, is so important for scientific analyses and should be freely accessible for national and international researchers.

What is the main challenge that needs to be urgently addressed in the Mekong Delta?

New sediments can counterbalance subsidence.  You can have subsidence, but if you have enough sedimentation, your land level can still go up. This is the case for some mangrove forests at the coast of the delta. There are subsidence rates of two to four centimeters per year, but as sedimentation is five to six centimeters per year the land level still rises.

The general reduction of sediment coming down the river system due to damming and changing monsoon rains decreases the ability to elevate the land by flooding and new sedimentation.

In the Mekong Delta, we see the subsidence rates going up during the past decades, while the sediment in the river goes down. There is simply not enough sediment to compensate for the present day rates of land subsidence in the delta.

Dr.Philip Minderhoud near a ground water pump in the Mekong Delta Vietnam where subsidence has caused the ground to sink by 20-30 cm.  Photo: Supplied
Dr.Philip Minderhoud near a ground water pump in the Mekong Delta Vietnam where subsidence has caused the ground to sink by 20-30 cm.  Photo: Supplied

What can we do for the sinking problem of the Mekong Delta?

A first priority should be reducing subsidence rates as much as possible. This requires determining what drivers are causing subsidence and where they are located. Natural subsidence cannot be slowed down, but human induced subsidence, such as groundwater extraction and heavy infrastructure, can.

Reducing or stopping groundwater extraction will also reduce land subsidence rates, even though this will not stop them completely, as land subsidence is a slow process that once triggered continue for many years, although at reducing rates.

The amount of subsidence caries from place to place, depending on geology and groundwater recharge rates, but even without knowing exact numbers, any reduction in groundwater use will have a positive effect on subsidence rates. This means local governments don’t need to wait for more research and can begin taking action straight away. The biggest challenge will be to find an alternative fresh water source to meet the freshwater demand currently met by groundwater.

In my view groundwater in the Mekong Delta should be regarded as a strategic reserve, to be used, for example, in extreme dry conditions caused by future climate change, not as a primary water source. The precious water stored in the subsurface should be preserved for the future when locals will need it the most.

Should Vietnam follow the Netherlands’s example in the Dutch Delta and build a strong dyke system to protect the Mekong Delta?

The Dutch delta should not be taken as example of how the Mekong Delta should be managed in the future, but what can be learned from the Dutch is which mistakes Vietnam should not make; and the country should only copy the successful aspects of our delta management. 

I think it is unwise to try and construct a large sea dyke around the entire Mekong Delta for two reasons.  One: it is much too expensive to construct in the first place due to the size of the delta and as the sea dyke will sink as well.  The maintenance costs will be astronomical.  Two: such a dike system would cut off the remaining sediment supply which could otherwise counterbalance subsidence and sea level rise.

At the coastline and in large rural areas, soft, nature based solutions, such as mangrove forests are much more preferable, with the focus on living dynamically with floods and allowing for natural sedimentation.

For high economical areas, for example larger delta cities, the construction of a dyke system to protect the city from flooding may be an economically feasible solution to protect these areas from the effects of subsidence and sea level rise. Photo:

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Hong Van / Tuoi Tre News


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