Large-scale suspended sediment transport and sediment deposition in the Mekong Delta

Sediment dynamics play a major role in the agricultural and fishery productivity of the Mekong Delta. However, the understanding of sediment dynamics in the delta, one of the most complex river deltas in the world, is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains, and the scarcity of observations. This study quantifies, for the first time, the suspended sediment transport and sediment deposition in the whole Mekong Delta. To this end, a quasi-2D hydrodynamic model is combined with a cohesive sediment transport model. The combined model is calibrated using six objective functions to represent the different aspects of the hydraulic and sediment transport components. The model is calibrated for the extreme flood season in 2011 and shows good performance for 2 validation years with very different flood characteristics. It is shown how sediment transport and sediment deposition is differentiated from Kratie at the entrance of the delta on its way to the coast. The main factors influencing the spatial sediment dynamics are the river and channel system, dike rings, sluice gate operations, the magnitude of the floods, and tidal influences. The superposition of these factors leads to high spatial variability of sediment transport, in particular in the Vietnamese floodplains. Depending on the flood magnitude, annual sediment loads reaching the coast vary from 48 to 60% of the sediment load at Kratie. Deposited sediment varies from 19 to 23% of the annual load at Kratie in Cambodian floodplains, and from 1 to 6% in the compartmented and diked floodplains in Vietnam. Annual deposited nutrients (N, P, K), which are associated with the sediment deposition, provide on average more than 50% of mineral fertilizers typically applied for rice crops in non-flooded ring dike floodplains in Vietnam. Through the quantification of sediment and related nutrient input, the presented study provides a quantitative basis for estimating the benefits of annual Mekong floods for agriculture and fishery, and is an important piece of information with regard to the assessment of the impacts of deltaic subsidence and climate-change-related sea level rise on delta morphology.

[1]  Xinwu Li,et al.  Flood Mapping and Flood Dynamics of the Mekong Delta: ENVISAT-ASAR-WSM Based Time Series Analyses , 2013, Remote. Sens..

[2]  S. Lawrence Dingman,et al.  Effective sea-level rise and deltas: Causes of change and human dimension implications , 2006 .

[3]  C. Paola.  BOOK REVIEW | River Discharge to the Coastal Ocean: A Global Synthesis , 2011 .

[4]  G. M. Kondolf,et al.  Dams on the Mekong: Cumulative sediment starvation , 2014 .

[5]  Matti Kummu,et al.  Sediment: Curse or Blessing for Tonle Sap Lake? , 2008, Ambio.

[6]  M. Kummu,et al.  Water balance analysis for the Tonle Sap Lake–floodplain system , 2014 .

[7]  D. DeMaster,et al.  Late Holocene Evolution of the Mekong Subaqueous Delta, Southern Vietnam , 2010 .

[8]  A. Bárdossy,et al.  Multi-objective automatic calibration of hydrodynamic models utilizing inundation maps and gauge data , 2011 .

[9]  N. Kabeya,et al.  Isotopic investigation of river water mixing around the confluence of the Tonle Sap and Mekong rivers , 2008 .

[10]  M. Abbott,et al.  On The Numerical Computation Of Nearly Horizontal Flows , 1967 .

[11]  Xixi Lu,et al.  Basin-wide sediment trapping efficiency of emerging reservoirs along the Mekong , 2010 .

[12]  D. Walling.  The Changing Sediment Load of the Mekong River , 2008, Ambio.

[13]  I. Overeem,et al.  Sinking deltas due to human activities , 2009 .

[14]  Xixi Lu,et al.  Sedimentation in the Manwan reservoir in the Upper Mekong and its downstream impacts , 2008 .

[15]  Bruno Merz,et al.  Sedimentation in the floodplains of the Mekong Delta, Vietnam Part II: deposition and erosion , 2014 .

[16]  E. Wolanski,et al.  Fine-sediment Dynamics in the Mekong River Estuary, Vietnam , 1996 .

[17]  Harish Gupta,et al.  The role of mega dams in reducing sediment fluxes: A case study of large Asian rivers , 2012 .

[18]  B. Merz,et al.  Sedimentation monitoring including uncertainty analysis in complex floodplains: a case study in the Mekong Delta , 2013 .

[19]  K. Fu,et al.  Analysis and prediction of sediment trapping efficiencies of the reservoirs in the mainstream of the Lancang River , 2007 .

[20]  Xixi Lu,et al.  Water discharge and sediment flux changes over the past decades in the Lower Mekong River: possible impacts of the Chinese dams , 2006 .

[21]  Katherine L. Farnsworth,et al.  River Discharge to the Coastal Ocean: A Global Synthesis , 2011 .

[22]  Olli Varis,et al.  Sediment-related impacts due to upstream reservoir trapping, the Lower Mekong River , 2007 .

[23]  Avijit Gupta.  Large rivers : geomorphology and management , 2007 .

[24]  J. Syvitski,et al.  Going under: The world's sinking deltas , 2012 .

[25]  Stefan Dech,et al.  Characterisation of land surface phenology and land cover based on moderate resolution satellite data in cloud prone areas — A novel product for the Mekong Basin , 2013 .

[26]  T. Heege,et al.  Operational multi-sensor monitoring of turbidity for the entire Mekong Delta , 2014, Remote Sensing the Mekong.

[27]  Roger A Falconer,et al.  Longitudinal dispersion coefficients in natural channels. , 2002, Water research.

[28]  M. Kummu,et al.  Reappraisal of sediment dynamics in the Lower Mekong River, Cambodia , 2014 .

[29]  Y. Saito,et al.  Monsoon-influenced variations in morphology and sediment of a mesotidal beach on the Mekong River delta coast. , 2010 .

[30]  Sedimentation in the floodplains of the Mekong Delta, Vietnam. Part I: suspended sediment dynamics , 2014 .

[31]  J. Syvitski,et al.  Morphodynamics of deltas under the influence of humans , 2007 .

[32]  T. Sturm,et al.  Open Channel Hydraulics , 2001 .

[33]  C. T. Haan.  4 – Open Channel Hydraulics , 1994 .

[34]  D. He,et al.  A new assessment method for comprehensive impact of hydropower development on runoff and sediment changes , 2012, Journal of Geographical Sciences.

[35]  C. Liu,et al.  Changes in the sediment load of the Lancang-Mekong River over the period 1965–2003 , 2013 .

[36]  Bruno Merz,et al.  Floodplain hydrology of the Mekong Delta, Vietnam , 2012 .