Résumé
A morpho-sedimentary analysis of the Ayeyarwady delta shoreline was conducted based on a field mission in Myanmar in November 2016 and interpretation of satellite images spanning the period 1974-2019. These analyses were complemented by data on land-to-water conversion and vice versa within a 2 km-wide coastal fringe, and on MERIS-derived seasonal and decadal-scale evolution of suspended particulate matter (SPM) off the delta. The objectives were to: (1) characterize the 450 km-long delta shoreline and coastal sediment transport pathways, (2) define the shoreline status (stability, erosion, accretion), and (3) identify potential causes of shoreline change and the future outcome of this status in terms of delta vulnerability. The delta shoreline was characterized on the basis of qualitative alongshore tidal, wave-energy, and sediment grain-size patterns (muddy, sandy or a mixture of both), and morphology (sandy beaches and mudflats). The deltaic coast exhibits a mixed wave-and-tide-dominated morphology and comprises a western sector characterized by four of the five main distributary mouths separating inter-distributary plains bounded by low overwash-influenced beaches devoid of aeolian dunes. The eastern sector, in the Gulf of Martaban, is embayed, much less prograded and bounded by dominantly muddy shores. This simple shoreline dichotomy reflects the overarching alongshore sediment redistribution and storage patterns that have accompanied the growth of the delta, resulting in the two facies: sand dominantly retained in the western sector where the multiple distributary mouths have constrained potential alongshore sand transport by low- to moderate-energy monsoon-generated southwesterly waves, and mud transported by the regional monsoon-influenced coastal shelf circulation towards the eastern sector. The recent multi-decadal shoreline mobility in the Ayeyarwady delta points to the influence of fluvial sediment supply on these two fades. Between 1974 and 2019, 49% of the delta's shoreline underwent erosion, mainly affecting the sandy beaches in the western sector, whereas shoreline accretion is still prevalent along the large inter-distributary plain east of Yangon, where coastal mud preferentially accumulates in the Gulf of Martaban. We attribute erosion to reduced river sand supply generated by dams and by massive in-channel sand mining upstream of the delta, exacerbated by important channel dredging. Large-scale deforestation resulting from land-use changes (agriculture and mining) in the Ayeyarwady catchment are probably contributing to enhanced fine-grained sediment supply to the delta plain and the coast, as reflected by the relative stability of coastal SPM over a decade (2002-2012) and continued deposition in the accreting eastern sheltered part of the delta. The coastal sediment balance will be further impacted in the future if planned dams are constructed. Without proper sediment management, notably a significant reduction or prohibition of in-channel sand mining, the sandy beaches that armour the wave-exposed western sector of the delta will continue to erode, resulting in increased potential exposure of the delta to cyclones and sea-level rise.