Recent morphodynamics, evolution and vulnerability of deltaic shorelines : a global analysis

Les deltas fluviaux sont des écosystèmes riches et vulnérables. Les apports de sédiments fluviaux assurent leur développement face à la subsidence et l’érosion induite par les agents marins. Les deltas sont principalement exposés à des crues et des submersions marines. Pourtant, ils restent des lieux stratégiques de peuplement, de production économique et d’enjeux géopolitiques, les rendant d’autant plus vulnérables et exposés aux risques. Cette thèse a pour objectif de cerner le fonctionnement passé et actuel de ces systèmes à partir de l’analyse de soixante deltas et d’une approche holistique systémique, à l’aide de données spatiales, environnementales et sociétales. Après l’étude de l’évolution côtière récente montrant une tendance à la diminution de la progradation de nombreux deltas, une classification conceptuelle et qualitative de la morphologie des deltas axée sur les influences fluviomarines a été conduite. La mise à jour de cette classification et la proposition de nouvelles approches, en termes de morphologie, de dynamique et de vulnérabilité, ont nécessité une revue des acquis antérieurs, une quantification de l’influence du fleuve, de la houle et de la marée, et une approche quantitative qui a mis en évidence la complexité des interactions. Le travail débouche sur une nouvelle classification quantitative et objective. La thèse compare aussi les réponses deltaïques à des perturbations exceptionnelles, montrant les limites de la résilience deltaïque. Elle renseigne sur l'impact anthropique sur ces espaces fragiles fortement dépendants de l’apport sédimentaire. Cet équilibre est d’autant plus fragilisé aujourd’hui par les impacts du changement climatique.River deltas are rich and fragile ecosystems. Deltas depend on fluvial sediment supply to balance natural subsidence and erosion caused by waves and currents. Deltas are mainly affected by river flooding, marine submersion. However, deltas are strategic sites of human settlement, economic hotspots, and geopolitical issues. This attraction increases the pressure, rendering these deltas more and more exposed to risks and vulnerable.The main objective of this thesis is to analyze the past and present functional dynamics of delta shorelines based on 60 of the world’s deltas and a holistic and systemic approach with spatial, environmental, and societal data. After the study of recent coastal evolution showing a tendency to decreasing progradation of many deltas, a conceptual and qualitative classification of deltaic morphology based on fluviomarine influences was conducted. Updating of this classification and the proposal of new approaches, in terms of morphology, dynamics, and vulnerability, have necessitated revisiting these older schemes, and the adoption of a methodological and interpretative approach aimed at quantification of the weight of each of these three parameters showing the complexity of the interactions. The thesis proposes a new quantitative and objective classificatory framework, including the human dimension. Finally, the thesis highlights the responses of deltaic shorelines to exceptional perturbations, and highlights the limits of resilience. The thesis advises over the impact of humans on these fragile coastal environments, the equilibrium of which strongly depends on sediment supply. This fragility is further exacerbated by the impacts of climate change

Multi-decadal variations in delta shorelines and their relationship to river sediment supply: An assessment and review

International audienceThe inception, growth, and decline of numerous large and small river deltas on Earth have been strongly influenced by human population dynamics and interventions on catchments, notably deforestation and reforestation. Over the last half century, the effects of catchment conditions in determining fluvial sediment supply have been exacerbated or moderated by dams and reservoirs. The sediment balance of river deltas, crucial in terms of delta shoreline stability, advance or retreat, and subsidence, has, in turn, been affected by variations in fluvial sediment supply. The shoreline mobility and resulting subaerial coastal area changes of a selection of 54 of the world’s deltas was quantified over 30 years based on data culled from the literature and from satellite images. These changes were analyzed alongside fluvial sediment loads. Delta shoreline mobility in response to changing fluvial loads has been variable, reflecting the miscellaneous factors that influence the supply of sediment to deltas. 29 deltas are in overall erosion, 18 show shoreline advance, whereas seven do not show any significant change. The sediment loads received by 42 deltas diminished relative to values prior to 1970, by more than 50% for 28 of them. Ten deltas showed advance, some significantly, notwithstanding fluvial sediment load decreases exceeding 25%. Overall, with the exception of the Colorado (Tx) and the Indus, losses in subaerial coastal area have been rather low. It would appear that diminishing fluvial sediment supply, the driving force in deltaic equilibrium at a multi-decadal timescale, has not, thus far, had a significant negative impact on multi-decadal delta shoreline mobility. This is important in terms of gauging currently perceived delta vulnerability. Notwithstanding, a clear link exists between the mobility of delta shorelines and the reduction in fluvial sediment loads. Eroding deltas have been affected by a reduction that is twice as important as that of stable or advancing deltas since 1970. Dams currently in place will reduce, in the future, the sediment load to their deltas of 25 of the 54 rivers by more than 50% and 100% for 15 of them. It is important to envisage the supply of sediment to deltas less in terms of its direct role in generating accretion, and eventual delta shoreline advance, and more in terms of an agent of resilience. The reduction of fluvial sediment supply to deltas will negatively impact their resilience to other drivers in the future: anthropogenic, climate change, and sea-level rise. The variability of delta shoreline behavior in the face of changing fluvial sediment loads also calls for more in-depth studies of individual deltas in order to build up future management plans addressing vulnerability and loss of resilience to marine forcing, subsidence, and sea-level rise

River delta shoreline reworking and erosion in the Mediterranean and Black Seas: the potential roles of fluvial sediment starvation and other factors

The Mediterranean basin (including the Black Sea) is characterized by a plethora of deltas that have developed in a wave-influenced setting. Many of these deltas are sourced in sediments by river catchments that have been variably dammed. The vulnerability status of a selection of ten deltas subject to different levels of reduction in fluvial sediment supply following damming was analysed by quantifying changes in delta protrusion area and protrusion angle over the last 30 years. The rationale for choosing these two metrics, which do not require tricky calculations of longshore bedload transport volumes and river ‘influence’, is that as sediment supply wanes, increasing relative efficiency of waves leads to longshore redistribution of reworked sediments and progressive ‘flattening’ of the delta protrusion. The results show that eight of the ten deltas (Nile, Rhône, Ebro, Ceyhan, Arno, Ombrone, Moulouya, Medjerda) are in erosion, whereas two (Danube, Po) show stability, but the statistical relationship between change in delta protrusion area and sediment flux reduction is poor, thus suggesting that the role of dams in causing delta shoreline erosion may have been over-estimated. But this poor relationship could also be due to a long temporal lag between dam construction and bedload removal and transport to the coast downstream of dams, and, where the delta protrusion is being eroded, to bedload trapping by shoreline engineering structures and by elongating delta-flank spits. Other potential influential factors in shoreline change include subsidence, sea-level rise, storminess, exceptional river floods, and managed sediment releases downstream of dams. A longer observation period and high-resolution sediment-budget studies will be necessary to determine more definitively to which extent continued trapping of sediment behind dams will impact overall delta stability in the Mediterranean and Black Seas. Mitigation of delta erosion is likely to become costlier under continued sediment starvation and sea-level rise

Geomorphology of a tropical river delta under pressure: the Rufiji delta, Tanzania—context, channel connectivity and alongshore morpho-sedimentary and hydrodynamic variability

International audienceThe geomorphology of the Rufiji River delta was investigated with emphasis on the influence of the delta's context, including the river basin, and remote-sensed delta-plain channel connectivity and shoreline morphological, sedimentary and hydrodynamic variations. The Rufiji is influenced by its East African Rift Valley tectonic context, high inter-annual water discharge variability and large mud-dominated sediment supply. The delta exhibits 14 distributary channels, only five of which are currently functional and all debouching presently in a low-energy muddy north sector, probably in response to NE tectonic tilting of the coastal basin of Tanzania. The south sector is characterized by a clear loss of distributary connectivity with the main stem but this appears to have occurred gradually over time, and is not a product of sudden avulsion. This loss is manifested by stronger meandering and numerous meander cut-offs indicative of a weaker overall delta-plain gradient, and may reflect the influence of the aforementioned tilting. This sector also has more abundant beach-ridge deposits than the north sector. The difference in shoreline facies between the dominantly muddy north sector and the dominantly sandy south sector appears to reflect primarily exposure to wave energy determined by an archipelago fronting the delta, and this difference is presently reinforced by preferential channel sediment routing to the north and sectorial sediment sequestration. The Rufiji River basin and its delta represent important stakes in the development of Tanzania under the country's rapid demographic growth, hydropower dam development and climate change. Eventual expected delta retreat caused by fluvial sediment shortage from the basin due to current and projected hydropower dams could be temporarily mitigated by sediment release by deforestation and by the relatively sheltered hydrodynamic setting of this delta

The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): Towards degraded delta resilience?

International audienceThe Ayeyarwady River delta (Myanmar) is exposed to tropical cyclones, of which the most devastating has been cyclone Nargis (2-4 May 2008). We analysed waves, flooded area, nearshore suspended sediments, and shoreline change from satellite images. Suspended sediment concentrations up to 40% above average during the cyclone may reflect fluvial mud supply following heavy rainfall and wave reworking of shoreface mud. Massive recession of the high-water line resulted from backshore flooding by cyclone surge. The shoreline showed a mean retreat of 47 m following Nargis. Erosion was stronger afterwards (-148 m between August 2008 and April 2010), largely exceeding rates prior to Nargis (2000-2005: -2.14 m/year) and over 41 years (1974-2015: -0.62 m/year). This implies that resilience was weak following cyclone impact. Consequently, the increasingly more populous Ayeyarwady delta, rendered more and more vulnerable by decreasing fluvial sediment supply, could, potentially, become more severely impacted by future high-energy events

Multi-Decadal Deltaic Land-Surface Changes: Gauging the Vulnerability of a Selection of Mediterranean and Black Sea River Deltas

International audienceAreal changes over delta surfaces determined by land and water ratios are a promising tool for identifying spatial and temporal changes in deltas that may reveal subsidence and shoreline erosion. Such changes can also provide the basis for more detailed studies on variations in land-cover and vegetation. Changes in land and water areas over a 35-year period (1984–2019) were determined for a selection of ten river deltas in the Mediterranean (Nile, Rhône, Po, Ebro, Moulouya, Ceyhan-Seyhan, Medjerdja, Ombrone, Arno) and the Black Sea (Danube), with a particular focus on aspects of subsidence and shoreline erosion. With the exception of the Ombrone, Arno, and Moulouya, and to lesser extent the Medjerdja, where notable changes dominate in the coastal zone and are tantamount to net erosion, the spatial pattern is largely dominated by delta-plain changes characterized by increasing areas of water. The pattern reflects a mix of shoreline erosion, land-use and land-cover changes, such as the ecological restoration of wetlands, but also increasing subsidence in these deltas, all of which have been exposed to a declining fluvial sediment supply due to human influence. The use of data on land-water ratios needs to be complemented by more detailed studies devoted to each delta in order to clearly disentangle changes related to land-use, vegetation, and subsidence. It is also important to determine how wetlands are interpreted in such ratios, as these important ecological elements are sensitive to ratio variations. It would also be interesting in future studies to examine how these variations play out over time, notably in deltas where changes have been significant over the period 1984–2019

Shoreline change of the Mekong River delta along the southern part of the South China Sea coast using satellite image analysis (1973-2014)

16th Young Geomorphologists Days, Nantes, FRANCE, JAN 29-30, 2015International audienceThe Mekong River delta is the world's third largest delta, consequence of a favourable morphosedimentary setting, high sediment supply and rapid growth during the Holocene. Analysis of Landsat satellite images from 1973 to 2014 shows that nearly 70% of the 160km - long South China Sea shoreline of the delta has strongly eroded. This trend represents a reversal of the massive, long-term Holocene progradation that characterized this part of the delta. Erosion and land loss along the South China Sea coast are not related to lobe switching, but more likely to decreasing river sediment supply and variations in patterns of sediment storage in the delta that appear to be due to human-induced modifications. These include fluvial sediment trapping by dams, enhanced subsidence due to massive groundwater abstraction, and riverbed aggregate extractions. Shoreline erosion is further exacerbated by the replacement of protective mangroves by shrimp farms. This erosion constitutes an additional hazard to the future integrity of a mega-delta already considered particularly vulnerable to subsidence, and to future large-capacity dams

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

International audienceA 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

Concerns about data linking delta land gain to human action

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

International audienceThe 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