Edward J Anthony

Manon Besset

Philippe Dussouillez

Marc Goichot

Hubert Loisel

Overview of the Monsoon-influenced Ayeyarwady River delta, and delta shoreline mobility in response to changing fluvial sediment supply

Keywords: monsoon-influenced delta, delta erosion, river sediment supply, coastal suspended particulate matter, Ayeyarwady, Gulf of Martaban Highlights

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Many deltas are becoming economically and environmentally vulnerable and their physical and biological resilience declining as a result of human activities [START_REF] Brondizio | Catalyzing action towards the sustainability of deltas[END_REF][START_REF] Seijger | An analytical framework for strategic delta planning: negotiating consent for longterm sustainable delta development[END_REF][START_REF] Hagenlocher | Vulnerability and risk of deltaic social-ecological systems exposed to multiple hazards[END_REF]. Increased vulnerability and loss of resilience occur as a result of reduced sediment flux from rivers and various other modifications caused by human interventions (Ericson et al., 2007;[START_REF] Syvitski | Morphodynamics of deltas under the influence of humans[END_REF][START_REF] Syvitski | Sinking deltas due to human activities[END_REF][START_REF] Evans | Deltas: the fertile dustbins of the world[END_REF]Besset et al., 2019;[START_REF] Grill | Mapping the world's free-flowing rivers[END_REF]. These conditions are aggravated by climate change and sea-level rise. The Ayeyarwady River, in Myanmar (Fig. 1), is increasingly affected by problems related to water and sediment management (WWF, 2018) that are affecting the delta [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF]. The Ayeyarwady delta is an important component of the developing economy of Myanmar, the largest country (676,600 km 2 ) in mainland Southeast Asia. Myanmar is heavily reliant on the economic advantages provided by the river and its delta, and is also among the 15 nations that, together, account for 80% of the world's population exposed to river (including delta) flood risk world-wide [START_REF] Ward | Assessing flood risk at the global scale: model setup, results, and sensitivity[END_REF][START_REF] Winsemius | A framework for global river flood risk assessments[END_REF]. This situation is of particular concern to the disproportionately large river delta (Fig. 2A). As in most developing countries, population growth in Myanmar has been extremely rapid, exacerbating exposure to the risks of river flooding, cyclones, and coastal erosion. The Ayeyarwady delta is particularly populous with over 15 million people, and registered an increase of nearly 150% in about 30 years [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF]. The advantages provided by the delta as a home to millions of people and as Myanmar's food basket could be seriously offset by massive on-going inchannel aggregate extractions and, in the coming years, by the impacts of numerous projected dams [START_REF] Hennig | Damming the transnational Ayeyarwady basin. Hydropower and the waterenergy nexus[END_REF][START_REF] Taft | A review of current and possible future human-water dynamics in Myanmar's river basins[END_REF] that could further stifle sediment supply (WWF, 2018).

Compared to most Asian and Pacific mega-deltas, the knowledge acquired on the Ayeyarwady system is quite sparse, and the morpho-sedimentary dynamics of the delta are still relatively understudied, notwithstanding a few recent efforts geared at shoreline changes [START_REF] Hedley | Evolution of the Irrawaddy delta region since1850[END_REF][START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF], flood impact [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF], shoreline response to cyclone activity [START_REF] Besset | The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): towards degraded delta resilience[END_REF], and Holocene evolution [START_REF] Giosan | On the Holocene evolution of the Ayeyawady megadelta[END_REF]. Acquiring data on this large delta should be an important objective with high relevance for the future development of Myanmar. The delta, like the catchment of which it is a part, crystallizes factors related to environment and water management, climate, economic and social development, land-use changes [START_REF] Taft | A review of current and possible future human-water dynamics in Myanmar's river basins[END_REF], as well as the sediment issue, which is particularly pertinent to delta sustainability.

The aim of this paper is to characterize the 450 km-long delta shoreline and its coastal sediment transport pathways, define the recent to current shoreline status (stability, erosion, accretion), and identify potential causes of shoreline change and the future outcome of this status in terms of delta vulnerability. In particular, the link between changing fluvial sediment supply, increasingly influenced by a variety of drivers, notably anthropogenic, and shoreline mobility, forms the overarching discussion theme.

The Ayeyarwady River and delta: a general overview

The Ayeyarwady has its source in the eastern Himalaya and lies in a zone of active tectonics in the collision zone between the Indian and Eurasian Plates [START_REF] Wang | Active tectonics and earthquake potential of the Myanmar region[END_REF], a context favourable to large fluvial water and sediment discharges. The river's catchment covers 404,100 km 2 and comprises three large tributaries, the Chindwin, Shweli, and Myitnge rivers (Fig. 1), separated in the northern reaches by alluvial floodplains. The downstream end of this drainage system has formed a large delta (Fig. 2A) in the Andaman Sea that merges eastward with the mouth of the smaller catchment of the Sittaung River (48,100 km 2 ). The Ayeyarwady delta and the contiguous coastal deposits of the Sittaung have formed 20,570 km 2 of low, fertile plain [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF]. Although only 23 rd in world catchment size rankings, the Ayeyarwady delta, with its Sittaung extension, ranks 11 th in size among the deltas of the world [START_REF] Coleman | Major Deltas of the World: A Perspective from Space[END_REF].

Overviews on the hydrology, geomorphology, sedimentology, and flood risks of the Ayeyarwady have been provided recently by van der Velden (2015), [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF]WWF (2018). The Ayeyarwady catchment experiences a tropical seasonal monsoon climate with rainfall concentrated in the hot humid months of May to October (Indian monsoon) as southwest winds blow northeastwards across the Bay of Bengal in the wake of the northward movement of the Intertropical Convergence Zone (ITCZ). In contrast, dry northwest winds blow from the continent during the cool months of December to March (Asian monsoon). The mean annual rainfall across the delta ranges from 2000 to 3000 mm, but other parts of the river's catchment experience up to 6000 mm of rainfall [START_REF] Frenken | Irrigation in Southern and Eastern Asia in figures[END_REF]. Tropical cyclonic storms make landfall in Myanmar between May and October, generate heavy rainfall and severe flooding [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF], and their frequency seems to be affected by the Pacific Decadal Oscillation (PDO) [START_REF] Haggag | Indian Ocean Tropical Cyclones and Climate[END_REF].

The present Ayeyarwady deltaic complex (Fig. 2A) developed in a confined setting between north-south-aligned bedrock ridges (Western Hills and Eastern Hills) as a bay-head delta (Fig. 1), a common development setting for many river deltas worldwide [START_REF] Anthony | Wave influence in the construction, shaping and destruction of river deltas: A review[END_REF][START_REF] Anthony | River deltas[END_REF]. Embayed settings are particularly favourable to the trapping of fluvial (and marine) sediments, as wave reworking, redistribution, and dispersal are commonly hampered. The delta coast, including the Gulf of Martaban (Gulf of Moutama), is adjacent to a relatively shallow shelf, and with a maximum depth of only about 50 m, composed of sediments deposited by the Ayeyarwady, the Sittaung, and the Salween (324,000 km 2 ) [START_REF] Ramaswamy | Tidal influence on suspended sediment distribution and dispersal in the northern Andaman Sea and Gulf of Martaban[END_REF]. The shelf width is about 170 km off the Ayeyarwaddy delta and increases to more than 250 km in the centre of the Gulf of Martaban [START_REF] Damodararao | Lithology, monsoon and sea-surface current control on provenance, dispersal and deposition of sediments over the Andaman continental chelf[END_REF].

Coastal currents in the Andaman Sea off the delta are mainly governed by the monsoon winds, as well as by Rossby waves triggered by Ekman transport and by Equatorial Kelvin waves [START_REF] Suwannathatsa | Seasonal Currents in the Bay of Bengal and Andaman Sea Revealed by Reprocessed Observations[END_REF]. Current flows are dominantly alongshore from west to east, and this is important in the alongshore mobility of fine-grained sediment exiting from the multiple distributary mouths in the west, as discussed below. The wave regime is characterized by monsoon wind waves of moderate fetch (wave periods of 8-12 s with a mean of about 10 s) generated in the Andaman Sea by the southwesterly winds, and thus, almost exclusively from a southwest direction (Fig. 2B), but with a marked seasonal variability in height [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF]. Waves are highest in July (mean values of up to 2 m) at the peak of the rainy Indian monsoon season. Their heights diminish during the dry season when the influence of southwest monsoon winds becomes attenuated due to the southward migration of the ITCZ. Maximum wave heights can reach 5 m during cyclones.

Figure 2C shows a map of the dominant M2 tidal amplitude in the Andaman Sea and a 12month extract of the tidal record from an offshore tide gauge in the delta (see location in Fig. 2A) for 2018. According to [START_REF] Kravtsova | Hydrological regime,morphological features and natural territorial complexes of the Irrawaddy River Delta (Myanmar)[END_REF], the mean tidal range along the western shores of the delta is 3.0 m and about 3.2 m along the eastern shores. This data source further indicates that the tidal range during spring and neap tides along the western extremity of the delta are 2.2 m and 1.8 m, respectively, but up to 5.7 m and 4.0 m, respectively, along the eastern extremity of the delta off the mouth of the Sittaung. These values are similar to those of [START_REF] Volker | The deltaic area of the Irrawaddy river in Burma. In: Scientific problems of the humid tropical zone deltas and their implications[END_REF] who identified spring and neap tides at the eastern periphery of the Ayeyarwady delta amounting to 5.8 m and 4.0 m, respectively. These ranges attain 5.1 m and 3.5 m, respectively, in Yangon located 72 km from the sea.

Spring-tide influence extends almost 300 km inland to the apex of the delta [START_REF] Hedley | Evolution of the Irrawaddy delta region since1850[END_REF]. Tropical cyclones, thoroughly reviewed by [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF], may generate strong coastal surges, resulting in significant damage periodically [START_REF] Dube | Storm surge modeling and applications in coastal areas[END_REF].

Tropical Cyclone Nargis (2-4 May 2008), a category 5 event just prior to landfall in Myanmar, is the latest and strongest meteorological event to have affected the delta in historic times, causing the worst natural disaster in the recorded history of Myanmar [START_REF] Fritz | Cyclone Nargis storm surge in Myanmar[END_REF], and massive shoreline retreat [START_REF] Besset | The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): towards degraded delta resilience[END_REF].

According to data collated by [START_REF] Kravtsova | Hydrological regime,morphological features and natural territorial complexes of the Irrawaddy River Delta (Myanmar)[END_REF], the mean fluctuation of sea level in the Andaman Sea within a year is about 8 cm. Sea level is slightly higher during the rainy season than the dry season due to the impact of river runoff. Seasonal sea-level fluctuations do not therefore play a significant role in the coastal hydrodynamic regime, which is dominated by winds, wind waves and moderately large tides.

In terms of the dominant marine processes acting to shape the delta's shoreline, the Ayeyarwady comes out as an example of a mixed wave-and-tide-dominated system in the commonly used classification system of [START_REF] Galloway | Process framework for describing the morphologic and stratigraphic evolution of delta depositional systems[END_REF]. Five main distributary mouths have developed downstream of the main-stem Chindwin and Ayeyarwady channels to convey the high water and sediment flux to the prograding delta in its western sector, where four main distributaries debouch (the fifth main one corresponds to the Yangon distributary in the eastern sector, Fig. 2A). The western sector is more significantly wave-influenced than the less prograded embayed eastern sector which is more wave-sheltered and essentially composed of open-coast muddy-sandy tidal flats. Wave influence is reflected in the formation of beach-ridge sets in some of the inter-distributary plains, especially the large 'promontory' between sites 7 and 10 (Fig. 2A), indicating that this western sector has formed a depocentre for sand supplied by the rivers [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF][START_REF] Giosan | On the Holocene evolution of the Ayeyawady megadelta[END_REF][START_REF] Kuehl | Sediment dispersal and accumulation off the Ayeyarwady delta -tectonic and oceanographic controls[END_REF]. Wave influence in this sector is both a cause and consequence of a more prograded sector of coast (Fig. 1). Progradation has culminated in a more protruding delta lobe in this sector relative to the southwesterly monsoon waves, which enhances the efficacy of wave-reworking of the shoreline in the context of a moderate tidal range. As in other deltas, the progressive growth and seaward protrusion of the delta have been tantamount to increasing wave influence [START_REF] Anthony | Wave influence in the construction, shaping and destruction of river deltas: A review[END_REF]. However, this influence has been moderated by the presence of the multiple distributary mouths, as well as by tidal amplification in the embayed eastern sector of the delta. Beach-ridge development has been both more recent [START_REF] Giosan | On the Holocene evolution of the Ayeyawady megadelta[END_REF] and more limited in the eastern sector of the delta, probably sourced from the Yangon tributary.

The distributary mouths have remained relatively fixed in the western sector of the delta, as suggested by radiometric ages from beach-ridge deposits [START_REF] Giosan | On the Holocene evolution of the Ayeyawady megadelta[END_REF]. The inception and development of these distributaries appear to stem from channel bifurcations [START_REF] Jerolmack | Conceptual framework for assessing the response of delta channel networks to Holocene sea level rise[END_REF] around distributary mouth bars, as has been proposed for the multiple mouths of the Mekong River delta [START_REF] Tamura | Origin and evolution of interdistributary delta plains; insights from Mekong River delta[END_REF]. The increase in tidal range eastward is the outcome of amplification of the tidal wave propagating over the shallow Andaman shelf and Gulf of Martaban embayment. The shallow shelf itself is a product of pronounced river, coastal and marine sedimentation through geological times that has produced the thick sedimentary deposits of the Central Burma Basin down to the Andaman shelf (Rodolfo, 1969b).

Fluvial water and sediment discharge to the coast and alongshore transport

The disproportionately large size of the Ayearwady delta relative to the size of its river catchment reflects a favourable combination of a high river sediment supply related to an active tectonic context, a steep upper catchment topography, high monsoon rainfall, the confined bay-head morphological setting of delta growth between bedrock promontories, and the moderate hydrodynamic conditions of the Andaman Sea [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF]. The Ayeyarwady river catchment collects nearly 70% of the surface water volume of Myanmar [START_REF] Kravtsova | Hydrological regime,morphological features and natural territorial complexes of the Irrawaddy River Delta (Myanmar)[END_REF]. The combined discharge of the Ayeyarwady and Chindwin Rivers at Chauk (Fig. 1) ranges from about 1500 m 3 /s to about 30,000 m 3 /s (van der Velden, 2015), and shows a strongly seasonal monsoon pattern [START_REF] Stamp | The Irrawaddy River[END_REF]. [START_REF] Robinson | The Irrawaddy River sediment flux to the Indian Ocean: the original nineteenth-century data revisited[END_REF] calculated, from data collected between 1969 and 1996 at Pyay (Fig. 1), an annual water discharge of 44,241 km 3 and a sediment discharge to the ocean of 226-364 Mt, concentrated in the rainy season, and among the highest in the world. Using these data, [START_REF] Furuichi | Discharge and suspended sediment transport in the Ayeyarwady River, Myanmar: centennial and decadal changes[END_REF] estimated the water discharge (379 ±47.10 9 Mt/year) and suspended sediment load (325 ±57. Mt/year) for the river upstream of the delta head. These values are similar to those derived by WWF ( 2018) who also estimated a widely-ranging bedload volume of 7.2 to 34 Mt/year (i.e., about 3 to 15% of the total load). A statistical comparison carried out by [START_REF] Furuichi | Discharge and suspended sediment transport in the Ayeyarwady River, Myanmar: centennial and decadal changes[END_REF] with earlier, 19 th century data (1871 to 1879), shows a significant decrease in discharge in the last 100 years. Sediment discharge over the last few decades has been strongly affected by anthropogenic pressures in the catchment (WWF, 2018), as discussed later. Shorter-term fluctuations would also be expected from climatic variability.

Precipitation in Myanmar is known to be affected by the inter-annual El Niño/Southern Oscillation (ENSO) [START_REF] Krishnamurthy | Indian monsoon-ENSO relationship on interdecadal timescale[END_REF][START_REF] Furuichi | Discharge and suspended sediment transport in the Ayeyarwady River, Myanmar: centennial and decadal changes[END_REF], by the ENSO counterpart of El Niño (La Niña; equatorial Pacific cold water anomalies) [START_REF] Rojas | Understanding the Drought Impact of El Niño on the Global Agricultural Areas: An Assessment Using FAO's Agricultural Stress Index (ASI)[END_REF][START_REF] Sein | Inter-annual variability of summer monsoon rainfall overMyanmar in relation to IOD and ENSO[END_REF], by the intra-seasonal Madden-Julian Oscillation [START_REF] Hendon | The life cycle of the Madden-Julian Oscillation[END_REF], which is a tropical to near-global alternation in atmospheric and oceanic circulation patterns and rainfall, and finally by the Indian Ocean Dipole with which rainfall in Myanmar is negatively correlated, and which notably modulates monsoon rainfall [START_REF] Ashok | Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO[END_REF].

Finally, rates of meltwater, and the frequency of the tropical cyclonic storms that make landfall in Myanmar between May and October, and which are accompanied by exceptionally heavy rainfall [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF], are deemed to be affected by the Pacific Decadal Oscillation [START_REF] Haggag | Indian Ocean Tropical Cyclones and Climate[END_REF]. Based on a sediment discharge at Chauk ranging from 165 kg/s in the dry season to 18,000 kg/s at the peak of the Indian monsoon, [START_REF] Kravtsova | Hydrological regime,morphological features and natural territorial complexes of the Irrawaddy River Delta (Myanmar)[END_REF] calculated the mean water turbidity at the mouths of the Ayeyarwady at approximately 600 g/m 3 , which corresponds to relatively high values. 60 ±10% of the total sediment flux downstream is supplied by the Chindwin, the subcatchment of which is characterized by more readily erodible rocks [START_REF] Garzanti | Provenance and sediment fluxes in the Irrawaddy (Ayeyarwadi) River[END_REF]. It is not clear, however, how water and sediment are apportioned among the five major and various smaller distributaries of the Ayeyarwady. Sand constitutes much of the shoreline in the western sector of the delta (Fig. 2A). There are no data on shoreline sand budgets for the individual beaches bounding the inter-distributary plains. It is clear, from the more advanced progradation of this western sector (Fig. 2A), that the sand supplied by the river to the deltaic shorelines has been sequestered in this sector, possibly throughout the development of the delta.

Figure 3A shows 10-year (2002-2012) averaged suspended particulate matter (SPM) concentrations offshore of the delta, compiled from the GlobCoast MERIS satellite (http://sextant.ifremer.fr/en/geoportail/sextant). The data were generated through a combination of the Polymer algorithm [START_REF] Steinmetz | Atmospheric correction in presence of sun glint: application to MERIS[END_REF] adapted for coastal waters [START_REF] Loisel | Suspended particulate matter variability of the global coastal waters over the MERIS time period[END_REF] and the SPM algorithm of [START_REF] Han | Development of a semi-analytical algorithm for the retrieval of suspended particulate matter from remote sensing over clear to very turbid waters[END_REF]. We assume that these concentrations dominantly reflect fine-grained terrestrial inputs to the coastal sea, an assumption supported by analysis of SPM concentrations for 2008 prior to, in the course of, and following Cyclone Nargis in May 2008 [START_REF] Besset | The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): towards degraded delta resilience[END_REF].

The pattern depicted by the GlobCoast data is very similar to that derived by [START_REF] Matamin | Remote sensing of suspended sediment over Gulf of Martaban[END_REF]. The seasonal SPM pattern does not strictly adhere to the seasonal rainfall regime, as depicted by water levels in the river close to the delta at Pyay and Hinthada (Fig. 3B), as one would expect a significant increase during the rainy Indian monsoon months (May-October) and a decrease in the dry-season Asian monsoon (December-March). The pattern is strongly influenced by lag effects, given the size of the Ayeyarwady catchment, and, in the offshore zone, by marked cross-shore and alongshore spatial variations. The Indian monsoon is characterized by offshore displacement of the SPM plume starting in May, with a clear banding from the coast to the offshore (Fig. 3A). This stronger offshore displacement, which largely reflects spread of mud under the strong outgoing fluvial jets at the distributary mouths, prevails until January, as the water discharge diminishes during the dry season. However, the strong offshore concentration zone appears to attain a peak in June before waning significantly in July and the remaining rainy season months. This could suggest that offshore SPM concentrations increase rapidly at the start of the rainy season under favorable conditions of coastal supply as river liquid discharge increases in the distributary channels, prior to overbank flooding of the river flood plain and the delta plain. The concentrations then diminish as these terrestrial units sequester a significant part of the suspension-load supply with the rise in river level and widespread rainy-season flooding.

The alongshore spatial pattern shows that a significant fraction of the mud discharged by the Ayeyarwady into the sea in the western distributary-mouths sector during the Indian monsoon is transported by the regional coastal currents towards the Gulf of Martaban, where the SPM concentrations are highest (Fig. 3A). Here, further trapping of mud debouching from the Sittaung (Fig. 1) occurs. This eastward regional SPM transport pattern is in agreement with the finding of various authors who identified high mud concentrations in the Gulf of Martaban (Rodolfo, 1969a;[START_REF] Ramaswamy | Tidal influence on suspended sediment distribution and dispersal in the northern Andaman Sea and Gulf of Martaban[END_REF][START_REF] Rao | Sediment distribution and transport on the Ayeyarwady continental shelf, Andaman Sea[END_REF][START_REF] Matamin | Remote sensing of suspended sediment over Gulf of Martaban[END_REF][START_REF] Damodararao | Lithology, monsoon and sea-surface current control on provenance, dispersal and deposition of sediments over the Andaman continental chelf[END_REF][START_REF] Kuehl | Sediment dispersal and accumulation off the Ayeyarwady delta -tectonic and oceanographic controls[END_REF]. [START_REF] Matamin | Remote sensing of suspended sediment over Gulf of Martaban[END_REF] and the GlobCoast data also show high dry-season concentrations in this eastern part of the delta (Fig. 3A), in agreement with the observations of these authors. These high dryseason concentrations may also reflect some degree of mud resuspension by the strong tidal currents in this macrotidal part of the gulf. The Andaman Sea has been described as highly turbid, and clastic mud is burying carbonate deposits consisting of molluscan fragments and foraminifera tests on the delta shelf, leading to a vertical depositional rate in that area of about 200 cm in 1000 years (Rodolfo, 1969a(Rodolfo, , 1969b)). According to Rodolfo (1969a), and more recently [START_REF] Rao | Sediment distribution and transport on the Ayeyarwady continental shelf, Andaman Sea[END_REF], a small amount of mud reaches the Andaman Basin through the Martaban Canyon, and river-derived mud is not accumulating in the western part of the shelf fronting the delta.

Methodology

Field reconnaissance and alongshore sediment sampling

A field mission aimed at ground-truthing shoreline characteristics (morphology, sedimentology, presence and type of vegetation, qualitative observations of bedforms, and shoreline dynamics), and collection of sand-rich samples for grain-size analysis was conducted in November 17-22, 2016. This mission consisted essentially of reconnaissance work and visits to 17 sites from east to west along the delta shoreline (Fig. 2A). The choice of the sites was based on access permission from the Myanmar administrative authorities and on accessibility. A total of 54 sediment samples were collected from 11 sites characterized by sandy beaches (sites 8 to 17 in the western sector of the delta, and sites 6 and 7 transitional to the eastern sector). Four to five samples were collected on a cross-shore transect from the lower to the upper beach, using observed bedforms or facies changes, where apparent, to delimit sampling zones. No samples were collected between sites 1 and 5 where the foreshore was characterized by mud. All samples were initially homogeneously separated in the laboratory with a sediment inox splitter and dispersed using 0.3% sodium hexametaphosphate as a dispersing agent. The grain-size distribution was measured using a Beckman Coulter LS 13320 laser grain-sizer with a range of 0.04 to 2000 μm. The calculation model uses the Fraunhöfer and Mie theory. For the calculation model, water was used as the medium (RI = 1.33 at 20 °C), a refractive index in the range of that of kaolinite for the solid phase (RI = 1.56), and absorption coefficients of 0.15 for the 780-nm laser wavelength and 0.2 for the polarized wavelengths (Buurman et al., 1996).

Multi-decadal shoreline changes

Shoreline changes were analyzed using two metrics: (1) shoreline mobility and induced coastal area change over time, and (2) conversion of coastal land into water, or conversion of adjacent coastal water into land. Data on shoreline mobility and area change were obtained from 47 digitized LANDSAT satellite images (60-30 m pixel size) from 1974 to 2019. Following work on similar delta shorelines, such as the Mekong [START_REF] Anthony | Linking rapid erosion of the Mekong River delta to human activities[END_REF]Besset et al., 2019b), we used the mangrove fringe in the muddy mangrove-dominated sectors, and brush or plantation fringe in sandy, beach-dominated sectors as shoreline markers. These shoreline markers were confirmed in the course of the field observations at the 17 site stops covering over 300 km of the delta's shoreline in November 2016 (Fig. 2A). A specific focus was also set on the impact of Tropical Cyclone Nargis in May, 2008 (Besset et al., 2017), which will only be briefly evoked here insofar as it illustrates the increasing vulnerability of the delta. Shoreline variations were calculated using the ArcMap extension module Digital Shoreline Analysis System (DSAS), version 5 [START_REF] Himmelstoss | Digital Shoreline Analysis System (DSAS) version 5.0 user guide[END_REF], coupled with ArcGIS®10.2.2. Using a relevant cartographic frame (Projection UTM 48N), a baseline was set about 1 km offshore of the delta shoreline. This baseline was regular enough to: (i) smooth any small-scale alongshore instabilities, and (ii) delineate large-scale geomorphic features such as distributary mouths, capes or bays. We then set up regularly spaced transects normal to the baseline and extending from offshore to 3 km inland. We set up a transect every 100 m alongshore from which shoreline variations between the changing vegetation line and the inland base line were calculated between the successive sets of LANDSAT images. This distance, chosen as a compromise between quality of the interpretation and the important length of the Ayeyarwady delta shoreline (~ 450 km) was divided by the time in years between the two image dates to generate statistics of shoreline change in DSAS 5. Using ArcGIS®10.2.2, we further determined surface area differentials between shorelines of different dates to obtain annual area gains and losses. We retained a relatively large uncertainty shoreline change band of ±20 m, which is much more than commonly used in the literature. The reader is referred to Besset et al. (2019aBesset et al. ( , 2019b) ) for the complete error computation methodology.

Data downloaded from the global database of [START_REF] Pekel | High-resolution mapping of global surface water and its long-term changes[END_REF] were used to identify coastal areas converted from land into water and vice versa in the Ayeyarwady delta. The rationale for examining these transformations is that they are a complementary input on multi-decadal shoreline mobility. Land-to-water conversion generally denotes erosion (but also back-shore land-use changes such as the installation of shrimp farms and rice cultivation), whereas water-to-land conversion implies land gain resulting from shoreline accretion (and less commonly from land reclamation, which is non-existent in the coastal fringe of the Ayeyarwady delta). The data from [START_REF] Pekel | High-resolution mapping of global surface water and its long-term changes[END_REF] are derived for each pixel (30 x 30 m) of LANDSAT satellite images covering a long-trend of 32 years that smooths out the impact, on land or water extent, of exceptional events such as droughts or floods. In addition to producing a map covering the delta, we calculated the land-to-water and water-to-land changes in a 2 km-wide coastal fringe relative to the 1984 shoreline. This 2-km fringe is well within the bandwidth of change liable to be caused by shoreline erosion (delta land conversion into coastal water) or accretion (coastal water conversion into delta land). The data also show seasonal patterns of conversion for each pixel. Overall, we retained four conversion categories over the 32-year period of observation, following Besset et al. (2019a): (1) new permanent water surfaces (land into permanent water), (2) loss of permanent water (permanent water into land), (3) permanent water into seasonal water, and (4) seasonal water into permanent water. A single category suffices to validate the presence of water. The category representing the year a change is identified is then considered as the first year of change. The category of the last year is always attributed to the last year of observation (October 2014 to October 2015).

Results

Shoreline morphological and grain-size variations

The November 2016 field reconnaissance enabled the identification of a clear westto-east change in shoreline morphology and sedimentology (summarized in Fig. 2A in terms of two sectors), and embedded in a transition from a wave-and-tide-dominated system in the western, to a more tide-dominated eastern sector. This difference is reflected in both the pattern of long-term delta development and the coastal landforms, briefly summarized in section 2, and grain-size gradients.

The multiple distributary channels in the western sector (sites 7 to 17, Fig. 4) alternate alongshore with inter-distributary plains and their beach-ridge sets, bounded seaward by low sandy beaches (Fig. 5A) that exhibit, in places, a morphology of multiple bars and troughs. The upper beach is commonly characterized by washovers, and the backshore devoid of foredunes. The beaches front wide back-barrier depressions that are bounded landwards by the most recent sets of beach ridges. Generally, in this sand-rich western sector, the low-tide beach exhibits a grain-size fining trend (Fig. 5A1) and is characterized by an abrupt transition to a muddy foreshore (Fig. 5A2) that commonly exhibits mangrove root remains, indicating erosion associated with barrier translation landwards. Between sites 8 and 6, the D50 grain-size values decrease significantly, and the percentage of mud increases, indicating the transition towards the mud-dominant foreshores of the eastern sector (sites 1 to 5). East of the mouth of the Yangon distributary, foreshore mud dominates throughout (Fig. 5B1), colonized by mangroves and, in places, tropical salt marshes (Fig. 5B2).

Multi-decadal shoreline changes

The 450 km of delta shoreline showed a mean overall advance of 9.5 m/yr ±3.9 m between 1974 and 2019, up from 7.5 m/yr for the period 1974-2015 [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF], thus suggesting a mild gain in accretion. This mean rate masks, however, a strong alongshore variability (Fig. 6). Areas of shoreline advance indeed show high rates: a mean of +26 m/year overall, and exceeding 50 m/yr along large stretches and even 100 m/year locally, in the muddy northeast corner of the delta on either side of the mouth of the Yangon tributary where patches of bare low-tide foreshore mud are also visible (Fig. 6). The analysis has not been carried out beyond this accretion zone. Observations of satellite images show that the shoreline in the approaches to the large macrotidal mouth of the Sittaung is characterized by rapidly changing muddy foreshores that probably correspond to the mud pool accumulating in this corner of the Gulf of Martaban. [START_REF] Shimozono | Large-scale channel migration in the Sittang River estuary[END_REF] have highlighted significant morphological changes in the mouth of the Sittaung associated with rapid bank erosion under tidal and fluvial forces.

The overall net retreat rate has been -8 m/yr. Although this rate is well below that of the net advance rate, erosion over the observation period affected, nevertheless, a significant stretch of the delta's shoreline, notably in the western sector of multiple distributary mouths, which are, paradoxically, the delta's main arteries for sediment supply to the coast. Net erosion has prevailed, over the 45-year period of analysis, along 220 km of delta shoreline, i.e., nearly 49%. Erosion has particularly affected the western, distributarymouth sector (Fig. 6). In order to highlight the temporal trends, an analysis was carried out of shoreline gains and losses in km 2 /yr on a time-slice basis (Fig. 7). The western sector has been largely in erosion since 1988, albeit with a progressively attenuated trend. Although this sector has registered very mild gains since 2010, much of this is accounted for by the sandy-muddy transitional zone (between sites 6 and 8, Fig. 2A) to the eastern sector, where one of the two branches of the Yangon distributary debouches (Fig. 6). In this critical western sector, where the delta has prograded most, the current status of the beaches over the latest 2015-2019 period is one of rampant erosion. In contrast to the western sector, the presently accreting northeast sector showed retreat prior to 1988, losing up to 3.35 km²/year, followed by fluctuations and important advance between 1992 and 2000 related to the formation of a large mud bank that was partially colonized by mangroves and, locally, tropical salt marshes. Shoreline changes since 2000 in the extreme northeast corner of the delta have not been included in the analysis as they show an anomalous erosion hotspot that, in fact, corresponds to in-situ reworking of this mud bank close to the mouth of the Sittaung (Fig. 8). It is important to stress the net coastal land gain of > 25 km²/yr in this eastern sector since 2015 (Fig. 7).

The land-water (water-land) conversion of the delta is depicted in Fig. 9, together with changes in the 2 km-wide coastal fringe. Conversion of land into water has occurred in large areas of the delta plain, and this pattern also prevails along the 2 km-wide coastal fringe where land-to-water (all categories considered) dominated between 1984 and 2015 (> 70%). This figure largely exceeds that of the shoreline stretch in erosion, but this apparent discrepancy may be due to the choice of a 2 km-wide band. Large stretches of the eroding western sector of the delta comprise narrow beaches behind which wetlands have appeared that correspond to areas of rice cultivation, but several small areas of water-to-land conversion also appear, and correspond to accreting spits, further discussed below.

Conversion of land to water is particularly prominent in the eastern sector (Fig. 9), highlighting some apparent disagreement with the dominant shoreline accretion in this sector (Fig. 6). This inconsistency may, in fact be explained by the tendency for the conversion to include areas that correspond to intertidal shore-detached mud, which may fluctuate considerably in shape and size over time, and with the macrotidal frame. The shoreline is, in contrast, dominated by water-to-land conversion in this sector, reflecting the multi-decadal accretion trend (Fig. 6).

Discussion

Delta shorelines are dependent on fluvial sediment supply to balance marine forces of sediment dispersal, subsidence and sea-level rise. There is no simple straightforward relationship between fluvial sediment supply and shoreline mobility, since the latter can also be influenced by various other drivers acting at a variety of timescales (Besset et al., 2019a). However, fluvial sediment supply is the overarching driver of the shoreline mobility of the Ayeyarwady delta, complemented by sediment redistribution processes by monsoongenerated waves and currents along the coast, including regional trapping in the Gulf of Martaban. The simple shoreline dichotomy adopted in this study reflects the overarching alongshore sediment redistribution and storage patterns that have accompanied the growth of the delta and the two resultant facies. Sand is 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 facies. According to Rodolfo (1969a), the Ayeyarwady delta prograded at an average rate of 2.5 km in 100 years into the Andaman Sea up to the start of the last quarter of the 20 th century. The shoreline erosion pattern drawn out of the satellite data thus shows a significant slow-down in progradation (Figs. 6,7). These results are in agreement with those of [START_REF] Hedley | Evolution of the Irrawaddy delta region since1850[END_REF] who identified a turnaround to erosion starting from 1989.

Although the Ayeyarwady is a high-sediment load river relative to the size of its basin, the erosion that affects the western sector (Fig. 6) is striking, inasmuch as the main sediment sourcing of the delta occurs in this sector of active distributary mouths. Attempting to explain this situation needs taking into account complex mechanisms of catchment-scale sediment release, storage and redistribution strongly mediated by human activities in the last three to four decades, and the propensity for transport of sand and suspension load to the delta's shoreline. To summarize, the western sandy shoreline undergoes erosion whereas the muddy eastern shoreline is accreting more or less strongly, thus calling for an explanation that takes into account the impact, on shoreline mobility, of: (1) the segregation of fine-grained sediment and bedload supplied by the river, (2) the preponderance of sand in the western sector of the delta where the multiple distributary mouths refract waves, leading to immobilization of this fraction in this sector, and (3) the regional-scale transfer, by the hydrodynamic regime, of mud exiting from these distributary mouths towards the Gulf of Martaban.

Fluvial sediment supply in the Ayeyarwady River is influenced by dams, aggregate mining in channels, channel dredging, deforestation, and climate change. Gauging the magnitude of each of these factors at this stage is an impossible task, and the issue is further complicated by differential impacts on bedload and suspended load that can only be grossly outlined, and by the fact that activities that lead to deforestation, such as agriculture and mining on catchment slopes, but also climate change, could actually enhance catchment sediment supply. A 30% drop in sediment supply generated by trapping of the fluvial load behind reservoirs in the Ayeyarwady catchment was calculated by [START_REF] Syvitski | Morphodynamics of deltas under the influence of humans[END_REF]. This is a significant amount. Between 1988 and 2004, for instance the storage capacity of reservoirs on the small tributaries of the Ayeyarwady and Chindwin grew from 2.34 to over 18 km 3 (Myanmar Irrigation Works Department, 2004). Many dams have recently been constructed along the smaller rivers and tributaries, and more are planned for the larger ones [START_REF] Kattelus | Myanmar under reform: Emerging pressures on water, energy and food security[END_REF][START_REF] Hennig | Damming the transnational Ayeyarwady basin. Hydropower and the waterenergy nexus[END_REF][START_REF] Taft | A review of current and possible future human-water dynamics in Myanmar's river basins[END_REF][START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF].

Assessment of the energy potential of Myanmar identified 92 new large hydropower projects (ADB, 2012), including the controversial Myitsone dam (with a projected potential of 3,600 kW). The project has been suspended by the Government, a decision taken in the broader interests of Myanmar. Data collected by [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF] show that there are currently almost 200 large dams in Myanmar. The impacts of trapping of bedload (sand and gravel) by dams are further aggravated by channel dredging and in-channel mining. The massive scale at which aggregate mining in the Ayeyarwady river channels is practiced has been considered as being more pernicious on the river's bedload supply to the delta than river dams (WWF, 2018). Current volumes of aggregate mining in the river, to the tune of 9 Mt (5-6 Mm 3 of sand) and nearly 0.5 Mt (about 0.3 Mm 3 of gravel a year (Fig. 10A), and considered as conservative estimates by WWF (2018), must constitute a significant proportion of the annual bedload supply by the fluvial system, and the true values, are, in fact, likely to be much higher. In addition to aggregate mining, dredging has also been carried out on a large scale in the river channels. Data collated by WWF (2018) show an estimated 24 Mm 3 of sediment dredged in the Ayeyarwady between 1989 and 2017, the total volume of dredged material between 2012 and 2017 being 7.72 Mm 3 (about 12.4 Mt), i.e., about one-third of the total volume dredged since 1989. Most of the dredging has occurred since 2015 (78%), a year when 2.5 Mm 3 of material were removed from the river.

The dredging volumes are considerably lower than the estimated volumes of material removed through sand and gravel mining, which are at least ten-fold higher, but they also contribute to the aggravation of the sediment balance of the river. Although quantified changes in the fluvial sediment load other than estimations of potential reservoir trapping, aggregate extractions and dredging are still lacking (WWF, 2018), several activities in the river catchment, notably mining (Fig. 10B) and agriculture, are currently generating massive deforestation (Fig. 10C), which has been important in Myanmar in the recent decades [START_REF] Leimgruber | Forest cover change patterns in Myanmar (Burma) 1990-2000[END_REF][START_REF] Hansen | High-Resolution Global Maps of 21st-Century Forest Cover Change[END_REF]Webb, 2013). Erosion of the bare slopes generated by mining and deforestation is contributing to an increase in sediment supply to the river system (WWF, 2018), thus counter-balancing the sediment reduction generated by dams, in-channel mining and dredging.

Deltas need to keep up with sea-level rise and marine forcing. The chronic erosion that affects the western sector of the delta (Fig. 6,7) indicates, thus, that the sand supply to the Ayeyarwady delta's shoreline is no longer sufficient to balance forcing. The shoreline is deprived of sand needed to maintain its beaches by massive aggregate mining in the river reaches (Fig. 10A) as well as by dams. The ensuing beach erosion occurs through both the well acknowledged mechanism of diminished sand supply to inter-distributary beaches, and the less well acknowledged effect of reversed (towards upstream) bedload transport in the distributary mouths caused by salt intrusion resulting from artificial channel deepening generated by aggregate extraction, as has been postulated for the Mekong delta [START_REF] Anthony | Wave influence in the construction, shaping and destruction of river deltas: A review[END_REF]. In addition, the considerable amount of sediment stored in the river bed and fluvial floodplain as a result of rampant deforestation (Fig. 10C), could also be affected by the buffering of peak flows by reservoirs, causing reduction of the capacity for bedload mobilization towards the delta (WWF, 2018).

The narrowing beaches in the Ayeyarwady delta are being reworked, with landward migration through abundant washovers along the central sectors of beaches bounding the inter-distributary plains, accompanied by bi-directional drift that results in sand being stored in peripheral accreting spits [START_REF] Anthony | Recent shoreline changes and morphosedimentary dynamics of the Ayeyarwady River delta: assessing the impact of anthropogenic activities on delta shoreline stability[END_REF]. This weakening of the delta's sandy 'armour' and the absence of aeolian dunes potentially favour massive landward translation of the high-water line during cyclones, as happened under the strong surge conditions generated by Cyclone Nargis in May 2008. The shoreline was translated landwards by hundreds of metres in places, resulting in weakening of, and lengthening of the time of, delta shoreline resilience as beach erosion became aggravated in the months following this event [START_REF] Besset | The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): towards degraded delta resilience[END_REF]. This event was followed by slow recovery that has never been totally achieved in the dominantly erosional multi-decadal context in which these beaches now evolve.

An additional contributor to the erosion that affects the western sector of the delta could be sediment redistribution modulated by the numerous embankments built upstream in the fluvial reaches. The transport of sediment to the sea should, theoretically, be aided by river embankments, as such structures confine sediment within the channels and, thus, lower the spill-over of fluvial load onto the delta plain during the flood season [START_REF] Woodroffe | Deltaic and estuarine environments and their late Quaternary dynamics on the Sunda and Sahul shelves[END_REF]. Recognition of the loss of sediment from flooding for land in the delta led to a call for non-rebuilding of such embankments as early as 1939 [START_REF] Kravtsova | Hydrological regime,morphological features and natural territorial complexes of the Irrawaddy River Delta (Myanmar)[END_REF]. However, while inhibiting lateral channel migration and deposition in the protected floodplain areas, these structures also enhance channel-bed aggradation, which exacerbates local flood hazard [START_REF] Brakenridge | Design with nature: Causation and avoidance of catastrophic flooding, Myanmar[END_REF] and reduces delivery of sediment to the coast. Over time, channel-bed sedimentation induces higher river levels for the same discharge [START_REF] Pinter | One step forward, two steps back on U.S. floodplains[END_REF], with overbank flooding resulting in spillover of sediment onto the delta plain rather than sediment transport to the coast. The flood hazard and potential sequestering of bedload in the channel reaches are also exacerbated by the marked difference in slope between the main-stem Ayeyarwady and its various distributaries, all of which exhibit much steeper slopes (WWF, 2018). This difference is deemed to be contributing to a glut in sediment in the middle reaches well upstream of the delta, and the ensuing accumulation of sediment and the river-bed aggradation exacerbate rainy season flooding (WWF, 2018).

The more sheltered eastern sector is, in contrast, the current main depocentre for mud transported by coastal currents from the multiple mouths towards the Gulf of Martaban (Fig. 3A). The overall trend indicated by the 10-year coastal SPM data (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) from GlobCoast suggests that the fluvial fine-grained sediment supply exiting at the mouths of the Ayeyarwady has been relatively stable overall (max ± 5%) (Fig. 11), assuming, of course that these concentrations are a faithful reflection of fine-grained fluvial sediment supply, which, as indicated earlier, seems to be the case. By balancing losses generated by fine-grained sediment retention behind reservoirs, sediment released by deforestation could explain the relative stability of the coastal SPM concentrations between 2002 and 2012 (Fig. 10), given the propensity for easier mobilization and transport of suspension load from the fluvial reaches to the sea. This load largely dominates the sediment supply of the Ayeyarwady (WWF, 2018), and has been dominant in the construction of the delta [START_REF] Rodolfo | The Irrawaddy Delta: Tertiary setting and modern offshore sedimentation[END_REF]. Another issue in the Ayeyarwady delta is that of aggravated subsidence (Syvitiski et al., 2009), but the areal pattern of this process is not known. van der Horst et al. ( 2018) have monitored very high local subsidence rates in Yangon (Fig. 1), Myanmar's capital city (5 million inhabitants), aggravated by urban development needs in terms of buildings, infrastructure and ground-water abstraction. Tidal gauge data for the delta indicate a 3.4-6 mm/y relative sea-level rise [START_REF] Syvitski | Sinking deltas due to human activities[END_REF]. Aggravated subsidence and sea-level rise can only be balanced by the trapping of more fine-grained sediment by the deltaic plain. An increased supply of suspension load resulting from deforestation may essentially be compensating for delta-plain subsidence, while also contributing to the current progradation of the relatively sheltered eastern sector of the delta where sediment is trapped.

Significant losses of mangrove forests in the Ayeyarwady delta to shrimp farms and rice fields, notably in the decade prior to Cyclone Nargis, have been identified [START_REF] Hedley | Evolution of the Irrawaddy delta region since1850[END_REF][START_REF] Frenken | Irrigation in Southern and Eastern Asia in figures[END_REF][START_REF] Veettil | Rapidly diminishing mangrove forests in Myanmar (Burma): a review[END_REF][START_REF] Chatagno | Evolution des forêts de mangroves dans les deltas tropicaux : causes et conséquences[END_REF]. According to [START_REF] Hedley | Evolution of the Irrawaddy delta region since1850[END_REF], the total mangrove forest area decreased from 2345 km 2 to 1786 km 2 between 1924 and 1995 from clearance for agriculture and aquaculture. It is projected that unprotected Ayeyarwady delta mangrove forests could be completely deforested by 2026(Webb, 2013)). [START_REF] Chatagno | Evolution des forêts de mangroves dans les deltas tropicaux : causes et conséquences[END_REF] mapped a loss of 636 km 2 of mangroves but also a much lower gain of 279 km 2 between 1998 and 2018, yielding a net loss of 29%. The contribution of mangrove loss to shoreline change is hard to identify, since much of the erosion of the delta's shoreline concerns the sandy beach sector in the west, where mangroves essentially occur in backbarrier plains and fringe the delta distributary channels. If not controlled, mangrove removal in the mud-dominated eastern sector of the delta could contribute to the local exacerbation of erosion.

Conclusion and perspectives

Like many deltas worldwide, the Ayeyarwady deltas is faced with increasing urbanization and investments in agriculture, fisheries and aquaculture, transport and industry. But the river is also part of a large catchment in which issues of environment, water management, climate change, economic and social development, and land use are driving significant change [START_REF] Taft | A review of current and possible future human-water dynamics in Myanmar's river basins[END_REF], with strong implications for the sediment balance of the delta. Irrigation and hydropower are going to be key elements of this development [START_REF] Ifc | Strategic Environmental Assessment (SEA) of the Hydropower Sector: Baseline Assessment Reports[END_REF], but could also pose a threat to the stability of the Ayeyarwady delta. A considerable amount of sediment is reported to be stored in the river bed and fluvial floodplain as a result of deforestation, such that the combined impact of hydropower and irrigation dams will be more important, in the short term, in buffering peak flows and thus reducing the capacity to move sediment to the delta, notably bedload, while exacerbating the flood hazard (WWF, 2018). Future dams could only compound the deleterious effects of existing reservoirs and of the current massive fluvial aggregate mining, which, by curtailing sand supply to the coast, are the most important threat to the stability of the delta's shoreline. In the future, the distributary mouths could be increasingly affected by upstream bedload transport, and salt intrusion if aggregate extractions (and attendant channel deepening) in the tidally-influenced reaches of the Ayeyarwady are maintained at current levels. Unless such mining is regulated towards much more reasonable and less damaging levels, and alternative solutions found to compensate for such regulation, the sandy beaches of the delta will continue to recede, leading to impairing of their energybuffering capacity and their ability to protect large fronts of the delta. This could, in turn, exacerbate the lowering of resilience to cyclones, thus increasing risks to humans and to the delta ecosystem. Wetlands behind narrow beaches in this 2 km-wide band probably correspond to areas converted to rice cultivation, whereas small areas of water-to-land conversion correspond to accreting spits. -year (2002-2012) overall evolution pattern of SPM concentrations (A), and monthly seasonal evolution trends (B) over the same period in the nearshore zone and over the continental shelf of the Ayeyarwady delta (data from GlobCoast, http://sextant.ifremer.fr/en/geoportail/sextant). The data show relatively stable concentrations over the ten-year period.

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS Figure 1. Shaded-relief map showing the setting and main tributaries of the Ayeyarwardy River delta. Figure 2. 2016 Google earth image of the Ayeyarwady delta (A), and wave (B) and tidal (C) characteristics. The delta has been divided into two main sectors, eastern and western. Red dots along the shoreline show field sites visited during a reconnaissance survey in November, 2016. The wave climate was culled from the ERA-Interim hindcast wave database (1978-2016) generated by the ECMWF (European Centre for Medium-Range Weather Forecasting) Wave Atmospheric Model, the M2 tidal amplitude map derived from the tide and current prediction programme WXTide32© (Version 4.6), and the one-year tidal record from the nearshore tide gauge the location of which is shown in Fig. 2A.

Figure 3 .

 3 Figure 3. Monthly mean values of suspended particulate matter (2002-2012) off the mouths of the Ayeyarwady delta and along the Andaman coast culled from the GlobCoast database and obtained from MERIS sensor with POLYMER atmospheric corrections (Han et al., 2016) (A), and monsoon-influenced water levels in the river at Pyay and Hinthada (Fig. 1) at the apex of the delta (B). The SPM show high concentrations in June (start of the rainy season) and December (dry season, reflecting, probably, a lag in terrestrial fine-grained sediment supply to the coast following overbank sedimentation in the river flood plain and delta plain during the rainy season which lasts till October).

Figure 4 .

 4 Figure 4. Examples of mid-beach sand samples depicting alongshore variations in shoreline sedimentology in the sandy, western sector of the Ayeyarwady delta (sites 8-17) and transitional sandy-muddy shoreline (sites 6, 7) towards the mud-dominated eastern sector (not shown here).

Figure 5 .Figure 6 .

 56 Figure 5. Ground photographs (November, 2016) of the two dominant shoreline types in the Ayeyarwady: sandy beaches with muddy low-tide foreshores (A), and dominantly muddy mangrove shorelines with local concentrations of sand (B), and sometimes colonized by shore-front saltmarshes (foreground in B1).

Figure 7 .

 7 Figure 7. Coastal area change at different time frames between April 1974 and April 2019, highlighting both the accretion in the eastern sector that has gained momentum in 2015-2019, and the decreasing erosion trend in the western sector since 1988 and turnaround to very mild accretion since 2010, much of it explained by the area gains in the northeast portion of this sector between sites 7 and 10.

Figure 8 .

 8 Figure 8. Massive local reworking of muddy foreshore deposits in the eastern sector of the delta close to the mouth of the Sittaung, where several mud banks have accumulated, reflecting the important mud storage and mud remobilisation in this part of the delta.

Figure 9 .

 9 Figure 9. Map of land-water and water-land conversion of the coastal Ayeyarwady delta between 1984 and 2015 generated from data downloaded from Pekel et al. (2016), and percentages of various categories of conversion within a 2 km-wide coastal band. The coastal band shows the dominance (78%) of conversion of land into permanent or seasonal water, and of seasonal water into permanent water, largely indicative of shoreline erosion.

Figure 10 .

 10 Figure10. Summary of river-use and land-use pressures in the Ayeyarwady catchment that are affecting the sediment discharge (adapted from WWF, 2018), with impacts on sediment supply to the delta shoreline and its mobility. Massive aggregate extraction in the lower fluvial reaches of the Ayeyarwady and upper delta distributary channels is deemed to be leading to chronic erosion of the delta's sandy beaches in the western sector, whereas new sediment releases from deforestation caused by agriculture and mining may be contributing to the maintenance of sustained levels of suspended sediments that are mainly contributing to progradation of the muddy eastern sector of the delta.

Figure 11 .

 11 Figure 11. Map of 10-year (2002-2012) overall evolution pattern of SPM concentrations (A),

Acknowledgements

We thank WWF Asia for funding this project. Sami Tornikoski, Salai Thura Zaw, Hten Hten Thazen, and Htayla Aung of WWF Myanmar are thanked for their valuable support.

Colin Woodroffe and another reviewer provided constructive suggestions for improvement.