Mangrove and shoreline erosion in the Mekong River delta, Viet Nam

Study Number:

109

Author:

Manon Besset, Nicolas Gratiot, Edward J. Anthony, Frederic Bouchette, Marc Goichot, Patrick Marchesiello

Abstract:

The question of the rampant erosion of the shorelines rimming the Mekong River delta has assumed increasing importance over the last few years. Among issues pertinent to this question is how it is related to mangroves. Using high-resolution satellite images, we compared the width of the mangrove belt fringing the shoreline in 2012 to shoreline change (advance, retreat) between 2003 and 2012 for 3687 cross-shore transects, spaced 100 m apart, and thus covering nearly 370 km of delta shoreline bearing mangroves. The results show no significant relationships. We infer from this that, once erosion sets in following sustained deficient mud supply to the coast, the rate of shoreline change is independent of the width of the mangrove belt. Numerous studies have shown that: (1) mangroves promote coastal accretion where fine-grained sediment supply is adequate, (2) a large and healthy belt of fringing mangroves can efficiently protect a shoreline by inducing more efficient dissipation of wave energy than a narrower fringe, and (3) mangrove removal contributes to the aggravation of ongoing shoreline erosion. We fully concur, but draw attention to the fact that mangroves cannot accomplish their land-building and coastal protection roles under conditions of a failing sediment supply and prevailing erosion. Ignoring these overarching conditions implies that high expectations from mangroves in protecting and/or stabilizing the Mekong delta shoreline, and eroding shorelines elsewhere, will meet with disappointment. Among these false expectations are: (1) a large and healthy mangrove fringe is sufficient to stabilize the (eroding) shoreline, (2) a reduction in the width of a large mangrove fringe to the benefit of other activities, such as shrimp-farming, is not deleterious to the shoreline position, and (3) the effects of human-induced reductions in sediment supply to the coast can be offset by a large belt of fringing mangroves.

Main Results and Conclusions:

• Mangroves provide coastal protection, which is important for both humans and animals alike.
  • “A wide and healthy belt of mangroves fringing the shoreline also plays a significant role in contributing to coastal protection by dissipating waves under normal energetic ocean forcing conditions. This protective role has been demonstrated in several studies conducted theoretically (Massel et al. 1999), in the laboratory (Hashim and Catherine 2013), and from field monitoring (Mazda et al. 1997; Quartel et al. 2007; Barbier et al. 2008; Horstman et al. 2014), but also from geomorphological and coastal management-oriented approaches.” (Anthony and Gratiot, 2012; Winterwerp et al. 2013; Phan et al. 2015).” (1)    
  • “The protective role of mangroves during the course of extreme climatic and tsunami events and disasters has been underlined (e.g., Alongi, 2008; Gedan et al. 2011; Marois and Mitsch 2015). Mangroves are closely linked with their physical environment and contribute to land-building by trapping sediment through their complex aerial root structure (e.g., Carlton 1974; Kathiresan 2003; Anthony 2004; Corenblit et al. 2007; Kumara et al. 2010).”  (1)            
  • “Healthy mangroves can trap more than 80% of incoming fine- grained sediment (Furukawa et al. 1997) and contribute to sedi- mentation rates of the order of 1–8 mm/year, generally higher than local rates of mean sea-level rise (Gilman et al. 2006; Gupta 2009; Horstman et al. 2014).”  (2)
• Erosion at the edge of a mangrove forest can reach a “tipping point” from which there is no return to the ecosystem’s original state and the mangrove forest cannot survive.
  • “On coasts characterized by mangroves, resilience to high-energy events such as tsunami or repeated storms can be impaired where mangrove loss has been generated and sustained by human activities. This can be envisaged through consideration of the concept of the tipping point, which corresponds to a threshold value beyond which a system cannot return to its original dynamic equilibrium (Kéfi et al. 2016).” (2)        
  • “Tipping points occur where one or more of the driving processes go beyond a threshold, resulting in destabilized dynamic feedback loops that link all processes together. This can be expected where the sediment supply is drastically reduced (sediment trapping by dams, sand mining, etc.), or where oceanic forcing is modified over a long period of time (18.6-year tidal cycles, ocean oscillations, etc.). This is also the case where a mangrove fringe is reduced in width by coastal ‘squeeze’ or by deforestation (Lewis 2005; Anthony and Gratiot 2012).” (2)
• The delta is a key player in food security for Southeast Asia    .
  • “The Mekong delta in Viet Nam (Fig. 1), the third largest delta in the world (Coleman and Huh 2004), has a particularly well-developed mangrove environment (Veettil et al. 2019). The delta makes up for 12% of the country’s natural land and 19% of its national population, and hosts a population of 20 million inhabitants (Mekong River Commission 2010).” (2)        
  • “The delta is crucial to the food security of South- east Asia, and provides 50% of Viet Nam’s food (General Statistics Office of Viet Nam) and is part of a river with the most concentrated fish biodiversity per unit area of any large river basin in the world, with 454 fish species in the delta alone (Vidthayanon 2008), and ranking second only to the Amazon in overall biodiversity (WWF 2012).” (2)
• Erosion on the Mekong River Delta is becoming a serious problem.
  • “Sea dykes are being increasingly built along parts of the muddy East Sea and Gulf of Thailand coasts for protection from marine flooding and for shrimp farms, generating a process of ‘mangrove squeeze’ (Phan et al. 2015).” (2)
  • “The erosion of the Mekong delta has been attributed to sediment depletion associated with three main factors (Anthony et al. 2015): (1) potential trapping of sediment by the increasing number of dams con- structed in the Mekong catchment, (2) large-scale commercial sand mining in the river and delta channels, and (3) accelerated subsidence due to groundwater pumping.” (2)
• There is less shoreline erosion when mangroves are present.
  • “Mangrove loss thus comes out as an additional factor in modulating erosion of the Mekong delta. Phan et al. (2015) showed that dissipation of waves incident on the delta shoreline was not effective where mangroves had been removed, especially in the case of infragravity waves which require a large mangrove cover several hundred metres wide to be significantly attenuated, such that mangrove removal indeed contributed to shoreline erosion.” (2)        
  • “Notwithstanding their limited number of data points and the large error bars of these points, Phan et al. (2015) identified a minimum critical width of 140 m for a stable mangrove fringe, and, above this minimum width, a capacity to promote sedimentation. The authors considered that the larger the width of the mangrove fringe the more efficient the attenuation of waves and currents will be, offering a successful environment for both seedling establishment and sedimentation.” (2)
• The relationship between mangrove fringe width and erosion protection levels is not linear; this means that though a healthy and full mangrove ecosystem at the water’s edge is always helpful it can not stop the erosion that is already happening or work retroactively.
  • “At the overall regional scale, our results reveal a pattern that is more complex than the simple linear relationship proposed by Phan et al. (2015) between mangrove width and the status of the shoreline in the Mekong delta. The results obtained in the present study, and based on a comprehensive analysis of 3687 pairs of shoreline change and man- grove width spaced 100 m (i.e., covering a total shoreline length of 369 out of ca. 500 km of delta shoreline), show no statistically significant relationships, whatever the scale considered (Figs. 3–5).” (5)
  • “There is no doubt that mangroves, by dissipating waves and currents, can contribute actively to protection of a variably wide coastal fringe (which is not quite the same thing as protection of the shoreline on which waves impinge), and can, especially, promote rapid coastal accretion where fine-grained sediment supply is adequate, or delay, but not halt, coastal retreat, where the sediment supply is inadequate.” (6)
  • “Ignoring these basic aspects may imply that high expectations from mangroves could be met with disappointment. This can have important shoreline management implications because of the following wrong deductions: (1) a large mangrove fringe is enough to stabilize a (eroding) shoreline, (2) some reduction of the mangrove width to the benefit of other activities such as shrimp-farming is not delirious, and (3) the effects of human-in-duced reductions in sediment supply to the coast can be offset by mangroves.” (6-7)

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