Morphodynamics of an eroding beach and foredune in the Mekong River delta: Implications for deltaic shoreline change
Introduction
River delta shorelines exposed to significant wave influence may be characterized by complex patterns of accretion or erosion over various timescales. These patterns are generally the outgrowth of interactions involving sediment supply, river discharge, waves, tides, currents, grain size, and long-term shoreline orientations resulting from delta growth (Anthony, 2015). In a time of large-scale concern regarding the vulnerability, resilience and rehabilitation of river deltas (Foufoula-Georgiou, 2013, Giosan et al., 2015, Tessler et al., 2015, Brondizio et al., 2016), two important questions are: (1) determining to what extent delta shoreline erosion is prevalent and lasting, and (2) the causes of erosion. These aspects are examined with reference to the mouths of the Mekong River delta (Fig. 1), the world's third largest delta, backbone of the Vietnamese economy, and home to nearly 20 million people. Following concerns regarding the effects of dams (Kummu et al., 2010, Kondolf et al., 2014) and river-bed mining (Bravard et al., 2013; Brunier et al., 2014) on decreasing fluvial sediment supply to the Mekong delta, and how this will affect its stability, erosion of the delta's shoreline has become a particularly important issue (Tamura et al., 2010, Schmitt and Albers, 2014, Phan et al., 2015). In addition to erosion, subsidence, accelerated by massive groundwater extraction in this populous delta (Erban et al., 2014), is deemed to be rendering the Mekong delta increasingly vulnerable. This vulnerability constitutes a threat to the safety and livelihood of subsistence farmers and fishermen (Ghimire et al., 2015). Severe erosion has been affecting coastal settlements, leading to the relocation of hundreds of households (Viet Nam News, 2015). The army is commonly called upon to set up hasty coastal defences along eroding sectors of the delta (Fig. 2).
Large deltas such as the Mekong are, however, intrinsically complex, with a strong potential for self-organization, involving, among other things, changes in sediment transport pathways and sediment redistribution that are reflected in space- and time-varying shoreline patterns (Anthony, 2015). This paper describes the morphodynamics of an eroding sector of beach and foredune at the distributary mouths of the Mekong delta, based on field experiments and observations conducted in 2010, 2011, and 2012. The reasons for the prevalence of erosion are discussed in the light of the complexity of patterns of deltaic shoreline evolution.
Section snippets
The Mekong River delta
The Mekong delta prograded rapidly in a sheltered bay in the East Sea under the influence of high fluvial sediment supply, and developed from an estuary into a delta between 5300 and 3500 years BP (Nguyen et al., 2000; Ta et al., 2005). As the developing delta prograded, it became increasing exposed to ocean waves that led to the construction of numerous sets of beach ridges in the eastern sector (Tamura et al., 2012a, Tamura et al., 2012b), which is partitioned into 6–7 distributary mouths.
Methods
Three field missions along the shores of the Mekong delta were carried out:
- 1.
21/01 to 02/02/2011 (preliminary reconnaissance mission - PRM),
- 2.
26/05 to 6/6/2011 (Ba Dông field experiment FE1, from 31/05 to 04/06),
- 3.
30/01 to 16/02/2012 (Ba Dông field experiment FE2 from 11/02 to 13/02).
The PRM consisted solely of field observations of shoreline morphology and sedimentary facies along ∼500 km of the delta shoreline from the eastern confines of the delta mouths to Cà Mau Point and parts of the Gulf of
Results
Ba Dông beach is up to 200 m wide at low tide and is a typical bar-trough system (Masselink et al., 2006) composed of two to three couplets of alternating bars and troughs and an upper beach bar that is welded to the foredune (Fig. 4d). The beach and dune are composed of homogeneous well-sorted fine to medium quartz sand (mean = 200–250 mm for beach sand, and mean of 150 mm for dune sand). In addition, each sample comprised ∼5 to 30% of comminuted shelly debris.
The data from the topographic
Discussion
The discussion will focus on two related aspects: (1) the short-term beach-foredune morphodynamic regime highlighted for Ba Dông beach, and deemed to be representative of the sandy, river-wave-tide-influenced mouth sector of the Mekong delta, and (2) the relationship between the currently observed beach morphodynamics and erosion, and the longer-term shoreline evolution in this sector.
Conclusions
The following conclusions can be drawn from this study.
- (1)
The Mekong delta shows alongshore morphological, grain-size, and wave and tidal-range characteristics that have been interpreted as reflecting a change from a mixed ‘river-wave-tide-influenced’ delta in the sand-rich inter-distributary mouth sector, to a ‘wave-tide-influenced’ muddy delta between the mouths of the Hâu Tiang and Cà Mau Point, and finally a ‘wave-influenced’ but low-energy muddy delta along the Gulf of Thailand.
- (2)
The seasonal
Acknowledgments
We acknowledge funding from the Fonds Français pour l′Environnement, WWF Asia project “Decision Support for Generating Sustainable Hydropower in the Mekong Basin - Knowledge of sediment transport and discharges in relation to fluvial geomorphology for detecting the impact of large-scale hydropower projects”, and from the Belmont Forum Project “BF-Deltas: Catalyzing Action Towards Sustainability of Deltaic Systems with an Integrated Modeling Framework for Risk Assessment”. Further support was
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