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MEKONG

Par ECOLA Dernière modification 07/11/2018 15:24

LMDCZ-AFD project: study of erosion processes and protection measures for the Lower Mekong Delta Coastal Zone

Collaborators: I am team leader of this French-Vietnamese project, coordinating with Dinh Cong San (SIWRR), Nguyen Thuy Anh and Sandra Rulliere (AFD). Other close collaborators are Nguyen Thong (HCMUT/CARE), Nguyen Lan Anh (VNU), Nguyen Nguyet Minh (USTH/CARE),  Nicolas Gratiot (IRD-LTHE/CARE), and Rafael Almar, Rachid Benshila, Elodie Kestenare (LEGOS) ...

Executive summary and WP summary reports
IRD MOVIE (J.M. Boré)
Project's web page

 

Because of land subsidence due to groundwater extraction and diminishing sediment supplies due to river damming, coastlines of many of the world’s river deltas are retreating with significant political, economic and environmental consequences. A typical example of such delta systems is given by the Mekong. Here as in other delta systems, coordinated international efforts are needed for appropriate research studies leading to sustainable protection measures. The present project aims in that direction. It is funded by the European Union (EU-DEVCO) and the French Development Agency (AFD) and coordinated by Vietnamese and French institutes (notably the IRD) and under the guidance of about 10 international experts from France, Germany and the Netherlands.

 Mekong3

EROSION OF THE COASTAL MEKONG DELTA

The Mekong River basin drains six countries, ending in South Vietnam where the coastal Mekong delta covers 13 provinces and cities. The total area accounts for 12% of the country, with a population of 19 millions (21% of the national population). It provides 50%, 90%, and 60% of Vietnam’s food, rice production (the world’s second most important rice exporter), and seafood, respectively. Besides strong activity in agriculture and animal husbandry, the delta has the most concentrated fish biodiversity per unit area of any large river basin in the world and is ranked second only to the Amazon in overall biodiversity. The coastal mekong delta  is influenced by winds, waves, tides, land subsidence (larger than sea level rise) and changing sediment loads from the Mekong and Saigon-Dong Nai river system. In addition, human activity has an impact on erosion and deposition processes through dyke construction and drainage, agriculture, aquaculture, and fishery exploitation along the coastal areas. But how much of the human effects are accountable for the observed shoreline erosion is an open question.

Mekong2 

APPROACH

In this project, we provide arguments to separate local from regional phenomena and natural redistribution by currents from actual effect of river sediment supply deficit. This investigation is first conducted at regional scale to assess system-wide sediment budgets using in-situ (WP1) and remote observations (WP3), combined with regional numerical modeling (WP2). We then assess the local sediment budgets in two selected sites: Go Cong (Tien Giang Province) and Phu Tan (West coast of Ca Mau Province). Based on regional and local sediment budget understanding (WP4 and WP5), we test soft and hard measures of protection using physical models (SIWRR flume facility) and local numerical models (WP6). Model assessments are also examined in the face of on-site observations and monitoring of existing protection measures along the LMDCZ (particularly at U-Minh and Go Cong). All these elements are finally assembled to provide solid arguments and propose a selection of protection measures for the local study sites (WP6). 

OBSJECTIVES

  1. Understanding the natural and anthropogenic dynamical mechanisms of coastal erosion and assessing their relative contribution to the erosion problem at regional LMD scale and at local scale for the two selected study sites Go- Cong and Phu-Tan, based on the review/analysis of the literature and other unpublished reports, analysis of satellite images, in-situ measurements and numerical modeling (hydro- morphodynamics); 
  2. Identifying/evaluating appropriate measures against coastal erosion for Go-Cong and Phu-Tan with a focus on hard structure, nature-based and hybrid solutions, based on the lessons drawn from previous implemented projects and from physical and numerical modeling done within this project; 
  3. Building capacity for the continuation of the initiated studies and for conducting similar future projects. 

RESULTS ON EROSION PROCESSES

In agreement with satellite observation of shoreline evolution, we find that the coastal Mekong delta is locally in disequilibrium. The observed erosion patterns can largely be explained by natural redistribution of sediments by coastal currents that is not limited to the nearshore zone. Our results also suggest that a decrease in river sediment supply would affect the estuarine sub-region, much before any other area in the coastal zone. Far from the estuarine zones, land subsidence and mangrove squeeze are additional candidates to explain erosion in areas that should naturally be accreting. The fact than Go Cong, in the estuarine region, has seen strong erosion in recent decades suggests that processes other than coastal dynamics are acting there. The most obvious candidate is a large reduction of supply from the Saigon-Dong Nai river (Soai Rap mouth) due to upstream dams. Dau Tieng dam, constructed in the 1980s, is just 70 km from Ho Chi Minh City on the Saigon River. It is therefore much closer to the coastal ocean than dams built along the Mekong. If confirmed, Go Cong may offer an early warning of what could happen to the LMDCZ when Mekong dams will have largely affected coastal discharge. On the other hand, our results show some recent increased erosion along the Ca Mau peninsula must be explained by other mechanisms that river deficit: redistribution by coastal dynamics and land subsidence for sure, but also weakening of mangrove belt protection and possibly others. 

CONCLUSIONS ON COUNTER-MEASURE EFFICIENCY 

PHU TAN: from existing experiments and model results, we can expect high accretion of order 5-40 cm/y inside any structure that is sufficiently porous for tides to get through. It is important to note that accretion here does not result primarily from longshore drift due to nearshore breaking of oblique waves (littoral drift paradigm) but from a combination of alongshore transport over the entire subaqueous delta front and onshore transport associated with tides and winds. This means that the protection structure may have limited impact on the surrounding area, as opposed to what is common in sandy beach environments. The paradigms of sandy littoral protection, which has been the focus of most engineering work does not seem applicable to muddy environments such as these.
GO CONG: the conclusions above must be moderated for Go Cong. Our modeling experiments show that Go Cong being directly under the influence of the Soai Rap river, SSC and nearshore accretion efficiency also largely depends on river sediment supply. Protection structures can turn annual net erosion to net deposition if sediment supply is available. But with a large decrease of river flux, the efficiency of protection measures is greatly reduced in the models and cannot overcome erosion. The exact numbers will thus be dependent on an exact knowledge of river fluxes and erosion parameters.

 

Institutes

  • VIETNAM: SIWRR, HCMUT (CARE lab), WRU, ...
  • FRANCE: LEGOS, IGE, ...

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