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Science Why it's so hard to treat pain in infants. Science The controversial sale of 'Big John,' the world's largest Triceratops. Science Coronavirus Coverage How antivirals may change the course of the pandemic. Darby, S. Hackney, J. Leyland, M. Kummu, H. Lauri, D. Parsons, J. Best, A. Nicholas, and R. Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity. DeMaster, D. Liu, E. Eidam, C. Nittouer, and T. Determining rates of sediment accumulation on the Mekong shelf: Timescales, steady-state assumptions, and radiochemical tracers.
Eidam, E. Ogston, D. DeMaster, J. Liu, T. Dynamic controls on shallow clinoform geometry: Mekong Delta, Vietnam. Erban, L. Gorelick, and H. Groundwater extraction, land subsidence, and sea-level rise in the Mekong Delta, Vietnam.
Fagherazzi, S. Bryan, and W. Buried alive or washed away: The challenging life of mangroves in the Mekong Delta. Ge, Q. Liu, Z. Xue, and F. Gupta, A. Geology and landforms of the Mekong Basin. Campbell, ed. Hanebuth, T. Proske, Y. Nguyen, and K. Early growth stage of a large delta: Transformation from estuarine-platform to deltaic-progradational conditions the northeastern Mekong River Delta, Vietnam.
Koehnken, L. Li, X. Liu, Y. Saito, and V. Recent evolution of the Mekong Delta and the impact of dams. Earth Science Review. Liu, C. He, D. Walling, and J. Changes in the sediment load of the Lancang-Mekong River over the period — DeMaster, C. Nittrouer, E. Eidam, and T. A seismic study of the Mekong subaqueous delta: Proximal versus distal accumulation. Li, K. Xu, D. Veiozzi, Z. Yang, J.
Milliman, and D. Sedimen-tary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea. Xu, A.
Li, J. Milliman, D. Velozzi, S. Xiao, and Z. Xue, K. Ross, H. Wang, Z. Yang, A. Mud and sand sequestering behind dams and large-scale riverbed sand mining are deemed to be the overarching causes in the decrease in sediment supply to the coast responsible for delta erosion. Channel mining creates pools and pits, generating deepened channels that trap sand coming from upstream in order to restore channel geometry.
These extractions and pit and pool infill, are deemed to lower the amount of sand attaining the mouths and to be responsible for the significant slow-down in progradation of the sand-dominated mouth sector of the delta. Enhanced delta-plain deposition to fill the accommodation space created by accelerated subsidence may be having a similar effect on the balance of mud routed to the coast, potentially depriving the coastal zone of mud and favouring accelerated muddy shoreline erosion in the GT and especially, southern sector of the SCS.
Possible seasonal but stronger tidal pumping of mud from the mud reservoir at the mouths into the artificially deepened deltaic distributary channels may also further deprive the coastal zone of mud during the high wave-energy, low-flow season. The temporal trend in SPM concentrations at the mouths of the Mekong provide a reasonable proxy highlighting a decrease in Mekong river sediment supply Fig.
This annual fall in SPM was attributed to a persistent decrease in Mekong river sediment output during the critical high-flow season when the river supplies sediment to the sea The alongshore-uniform January pattern Fig. The significant role of infragravity wave energy impinging on the muddy SCS coast following gravity wave dissipation by the shoreface and mangroves has been identified This highlights the overarching role of the more energetic and longer-period northeast Monsoon waves with their larger infragravity component.
The recent persistent decrease in suspended sediment concentrations off the delta is attributed essentially to dam impoundment of sediment 46 and corroborates the conclusions of a study that has quantified significant sediment retention by dams at the scale of the Mekong basin Although there is a consensus, however, on the negative impacts of existing and planned dams on the sediment supply of the Mekong to its delta 5 , 18 , 20 , 21 , 22 , the poorly estimated Mekong river load and, therefore, the uncertainty regarding what fraction of this load may be trapped behind dams, precludes linking without doubt the present delta erosion to existing dams.
Dams are, not, however, the only source of a potential decrease in sediment supply to the coast. The massive channel bed mining in the Mekong Fig. This rate represents nearly 20 times the annual Mekong sand flux estimated at Kratie These losses occurred along much of the reaches of the two channels Fig. The numerous pits and pools created by large-scale sand mining actively trap bedload transported downstream during the high-discharge season This should be resulting in a net decrease in sand supply to the DDM sector Fig.
We interpret the present irregular pattern of change in this sector Fig. This activity will increasingly impact on rates of progradation in this sector, as suggested by the decrease in shoreline advance between and Table 1. Another mechanism likely to be activated by sand mining is that of enhanced saltwedge intrusion in the delta channels in the dry season, a process that leads to up-channel tidal pumping of mud Fig.
Deeper channels favour stronger upstream intrusion of saltwater and more mud-trapping at the upstream edge of the intrusion in estuaries In the Mekong, this occurs at a time of the year when mud needs to be stored along the coast to dissipate wave energy and mitigate shoreline erosion downdrift of the DDM sector.
The hypothesis of enhanced up-channel mud pumping from the coastal zone as the distributary channels in the lower Mekong delta become deeper as a result of sand mining is supported by increasing dry-season inland saltwater intrusion into the delta 47 , 51 , which has also been orally confirmed to us, especially for the lower reaches of the Bassac channel which now require almost continuous dredging of mud to maintain navigation for large vessels.
The enhanced salt-wedge intrusion further poses the problem of increased salinization of cultivated land in the Mekong, especially given the accelerated subsidence caused by groundwater exploitation Subsidence rates are highest in the southwestern sector of the delta Fig.
Relatively high subsidence rates, exceeding 1. Interestingly, this is also the only area of the delta where shoreline erosion has been reportedly persistent since The secular erosion affecting this muddy sector largely antedates dams and the expected effects of channel-bed deepening on mud storage. This erosion may be due to a persistently weak supply of mud released from the DDM mud-storage sector as a result of delta-front sediment dynamics, but also possibly to higher incident wave energy due to a more normal shoreline orientation relative to the northeast Monsoon waves.
This finding further reinforces the argument that the January SPM along this sector of coast, shown in Fig.
Sediment partitioning and storage between the Bassac and the various Mekong distributaries to the northeast of the Bassac, more distant from this erosion hotspot, may play a role in this deficit. Aspects of mud partitioning and routing in the Mekong delta between its multiple mouths, where mud is stored during the high-season discharge before being transported alongshore and its subaqueous front are, however, poorly known.
If less mud is being supplied from the mouths to the rest of the delta, then it may be inferred that the lower rates of erosion of the even more distant GT coast, compared to the SCS coast Table 1 , may be due to its less energetic wave regime Fig.
Aspects of sand partitioning are better known. Much of the sand supplied by the river is sequestered in the DDM sector where delta progradation has been dominated by successive sets of beach ridges Two final but unrelated points regarding delta erosion and human activities in the populous Mekong delta are the impacts of large-scale removal of mangroves and the joint effects of accelerated subsidence and of the numerous canals on mud storage and supply to the coast Fig.
The marked alongshore variability in erosion rates of the SCS sector Fig. Field visits along much of the muddy SCS and GT coast during the high-energy season in confirmed active wave erosion of muddy mangrove-bearing bluffs.
Accelerated subsidence creates additional accommodation space for sediment. A supplementary effect of accelerated subsidence, therefore, besides that of contributing to exacerbated muddy shoreline erosion, may be that of potential lowering of mud supply to the sea as enhanced delta-plain deposition occurs to balance this subsidence.
The numerous artificial canals in the delta plain are also likely to have an additional effect on mud supply to the coast by trapping more mud. The relationship between canals, many of which are diked and delta-plain sedimentation is, however, far from being straightforward, especially given the large variability in such sedimentation as a function of flow volume A decreasing river sediment supply to the coast is deemed to be the prime cause of this erosion and most likely due to existing dam retention of sediment and to massive channel-bed sand mining in the delta, an activity on the increase over the last decade.
An important recent decrease in mud supply to the coast during the high river-discharge season has been highlighted from MERIS staellite images 46 , whereas decreasing rates of sandy shoreline progradation in the mouths sector of the delta are in agreement with large-scale sand mining in the delta channels, including in reaches very close to the sea. Annual sand mining rates 24 exceed by more than an order of magnitude the annual estimated bedload in transit at Kratie Sand trapping in the numerous channel bed pools and pits created by large-scale mining is expected to lower the sand supply to the beaches lining the mouths of the Mekong delta.
Subsidence accelerated by groundwater extraction is highest along parts of the muddy South China Sea coast most severely affected by erosion. This reduction in the quantity of coastal mud results in lesser wave energy dissipation, and, consequently, in shoreline erosion. A finer clarification of the mud partitioning processes and sediment budgets involved will require, however, robust data on various aspects of sedimentation in the delta.
The uncertainty surrounding the impact of existing dams on the sediment supply to the delta is not shared by any of the future impact scenario studies. There is agreement that the planned set of future hydropower dams will definitely impact the sediment budget of the Mekong delta 18 , 20 , 21 , These dams, together with uncontrolled sand mining, will thus aggravate the on-going erosion of the delta.
A recent modelling effort aimed at assessing the response of the floodplain hydrology and sediment dynamics in the delta to anthropogenic and environmental changes concluded on the overarching role of hydropower development, compared to climate change and the combined effects of sea-level rise and deltaic subsidence These are substantial reductions, whatever the true sediment load of the Mekong.
This depletion stage may be attained well before if sand mining in the delta and in the river reaches upstream is to continue at its present rate.
Given the already high vulnerability of the Mekong delta, the sediment supply necessary to mitigate wave- and current-induced shoreline erosion and balance subsidence and rising sea level, will decrease more drastically. Understanding the links between erosion of the Mekong delta and sediment supply reduction by dams, channel sand mining, subsidence and the additional effects of competition for a decreasing sediment load between the delta plain and the shoreline, is imperative for a better apprehension of the increasing vulnerability of this mega delta.
This understanding, underpinned by more reliable measurements of sediment flux, is also necessary in the search for solutions to mitigate such vulnerability. Although SPOT 5 images also exist for and , the coverage is incomplete and we therefore chose to limit our study to the complete — sets.
The images have a high Super-Mode 2. A total of change rates, each corresponding to a DSAS transect, were determined for each set of dates. We then defined the annual error E of shoreline change rate from the following equation:. The hydrography, relief and bathymetry on all maps are derived from How to cite this article : Anthony, E.
Linking rapid erosion of the Mekong River delta to human activities. Ericson, J. Effective sea-level rise in deltas: Causes of change and human dimension implications. Change 50, 63—82 Syvitski, J. Sinking deltas due to human activities.
Foufoula-Georgiou, E. Google Scholar. Giosan, L. Nature , 31—33 Wang, H. Recent changes of sediment flux to the western Pacific Ocean from major rivers in East and Southeast Asia. Coleman, M. Available at: www. Accessed: 12 April Mekong River Commission. The region produces considerable quantities of rice and fish products. Text Source: Prof. Home About Delta Alliance What and why?
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