Linking rapid erosion of the Mekong River Delta to human activities

Study Number:

115

Author:

Edward J Anthony, Guillaume Brunier, Manon Besset, Marc Goichot, Philippe Dussouillez, & Van Lap Nguyen

Abstract:

As international concern for the survival of deltas grows, the Mekong River delta, the world’s third largest delta, densely populated, considered as Southeast Asia’s most important food basket, and rich in biodiversity at the world scale, is also increasingly affected by human activities and exposed to subsidence and coastal erosion. Several dams have been constructed upstream of the delta and many more are now planned. We quantify from high-resolution SPOT 5 satellite images large scale shoreline erosion and land loss between 2003 and 2012 that now affect over 50% of the once strongly advancing >600km-long delta shoreline. Erosion, with no identified change in the river’s discharge and in wave and wind conditions over this recent period, is consistent with: (1) a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment, (2) large-scale commercial sand mining in the river and delta channels, and (3) subsidence due to groundwater extraction. Shoreline erosion is already responsible for displacement of coastal populations. It is an additional hazard to the integrity of this Asian mega delta now considered particularly vulnerable to accelerated subsidence and sea-level rise, and will be exacerbated by future hydropower dams.

Main Results and Conclusions:

• Anthropogenic activities have caused a large decline in sediment supplies, which are critical to the health and stability of river deltas.
  • “River deltas crucially depend on sustained sediment supplies in order to maintain delta shoreline position and to balance subsidence. Because they are increasingly starved of sediment trapped behind dam reservoirs, many of the world’s river deltas are becoming vulnerable to accelerated subsidence and erosion, losing large tracts of land and becoming more exposed to flooding and sea-level rise (1,2).”(1)
  • “Erosion, with no identified change in the river’s discharge and in wave and wind conditions over this recent period, is consistent with…a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment…”(1)

• The Mekong River Basin is where an extremely large amount of  people in Southeast Asia get their food and is starting to deteriorate.
  •  “The Mekong River basin (Fig. 1) is 12th in size in world rankings and drains six countries. It also has the world’s third largest delta(6 ). The Mekong delta hosts a population of nearly 20 million (7).”(1)
  • “Crucial to the food security of Southeast Asia, it provides 50% of Vietnam’s food (8). Significantly, it accounts for 90% of Vietnam’s rice production making this country the world’s second most important rice exporter, and 60% of its seafood, both with export values of several billion US$.”(1)
  • “These important advantages are increasingly threatened by a number of rapid drivers of development, notably planned large-capacity dams (10) (Fig. 1b) that are rendering the Mekong delta an iconic example of an economic, social, political and environmental hotspot.” (2)  
• There has been a large amount of erosion along the Mekong Delta, a lot of this can be attributed to the fact that there has been a lot of deforestation to keep up with the growing population.
  •  “Erosion is essentially affecting the muddy sectors with shoreline retreat rates commonly exceeding 50m/yr in places, especially along the 180 km-long SCS coast nearly 90% of which is in retreat (Fig.  3). Over 50% of the >600 km-long Mekong delta coast has been in erosion between 2003 and 2012 but with noteworthy variations (Fig. 4).” (3)
  • “This rampant erosion contrasts with the massive growth of the delta towards the southwest over the last three millennia (Fig. 2a). The net loss rate is mitigated by the sandy DDM sector, which shows mild net accretion, notwithstanding an irregular alongshore pattern of erosion and advance (Fig. 3).” (5)
  • “High-resolution satellite images show that the Mekong delta is now largely prone to erosion, with shoreline retreat over the period 2003–2012 having affected over 50% of the >600 km-long coast, and even up to 90% of the muddy South China Sea coast. A decreasing river sediment supply to the coast is deemed to be the prime cause of this erosion, and most likely due to existing dam retention of sediment and to massive channel-bed sand mining in the delta, an activity on the increase over the last decade.” (9)
  •  The coastal mangrove system along the muddy SCS and GT coasts has been classified as ‘fringe mangrove’ occupying a narrow coastal band(30). The vicissitudes of war and timber overexploitation have had a heavy toll on mangroves in the delta, especially heavy downcutting in the 1980 s and 1990 s to provide timber for the construction industry and charcoal, and for conversion into shrimp farms(53,54). (9)
• Currently there is a lot of uncertainty on how the disintegrating Mekong River Delta can sustain so many people.
  • “The uncertainty surrounding the impact of existing dams on the sediment supply to the delta is not shared by any of the future impact scenario studies. There is agreement that the planned set of future hydropower dams will definitely impact the sediment budget of the Mekong delta(18,20–22).” (10)
  • “This depletion stage may be attained well before 2020 if sand mining in the delta and in the river reaches upstream is to continue at its present rate. Given the already high vulnerability of the Mekong delta, the sediment supply necessary to mitigate wave and current induced shoreline erosion, and balance subsidence and rising sea level, will decrease more drastically.” (10) 
  • “Understanding the links between erosion of the Mekong delta and sediment supply reduction by dams, channel sand mining, subsidence, and the additional effects of competition for a decreasing sediment load between the delta plain and the shoreline, is imperative for a better apprehension of the increasing vulnerability of this mega delta. This understanding, underpinned by more reliable measurements of sediment flux, is also necessary in the search for solutions to mitigate such vulnerability.” (10)

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(10) Mekong River Commission. Basin Development Plan Programme, Phase 2: Assessment of
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(7) Mekong River Commission. 2010. State of the Basin Report. Vientiane, Lao PDR. 232.
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(30)Phan, S. K., van Thiel de Vries, J. S. M. & Stive, M. J. F. 2015. Coastal mangrove squeeze in
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