Effect Of Big Dams On Sediment In Large Asian Rivers

Sedimentation changes resulted from human activities has been a main issue of global reservoirs and rivers.It is of great significance to investigate dam construction effects on sedimentation.This study attempts to carry out a thorough investigation of big dam impacts on sedimentation in large Asian rivers,e.g.,Yellow River,Yangtze River,Pearl River and Mekong River,based on multi-source datasets,including gauged water discharge,sediment flux,and extracted information of dams/reservoirs from DEM.A framework was built and practiced to investigate dynamics of soil erosion from big dams of large Asia Rivers during the past decades,followed by discussions on potential environmental risks.The key findings of this investigation are described as followings.Firstly,it is clear that the specific quantity of sedimentation has been transferred in the stream,yet a portion is less as compare with the total amount of sedimentation.The information related with diminutions in sedimentation load because of trapping sedimentation behind the dams/reservoirs has been demonstrated to be clearly evidently for the large Asian rivers,e.g.,in the Yellow River,;the operation of the Xiaolangdi reservoir,which began in 1999,had a particularly large impact.The river channel in the lower river reaches has experienced 13.79×10~8 m~3 of erosion in the 2000 s,and the Yellow River delta has exhibited signs of erosion since 2000.The operation of the Three Gorge Dam which began in 2003 played a dominant role in sediment reduction.In the Yangtze River,the sediment flux from upstream(Yichang station)and into the sea(Datong station)have decreased to 0.49×10~8 t/yr(9.9%)and 1.43×10~8 t/yr(33.5%),respectively,in the 2003–2015 period compared to the period between the 1950 s and 2002.The dramatic decrease of sediment input resulted in the erosion of the river channel in its mid-lower reaches,with an erosion rate of 61 106 t/yr from 2003 to 2015.In the Pearl River,the geomorphologic evolution of the river channel and the river delta changed after the 1990s;for example,the river beds in the West River,the North River and the East River were lowered by 0.59–1.73 m,0.34–4.43 m and 1.77–6.48 m.Secondly,the reduction in sedimentation load from the large rivers has induced environmental changing in the coastal zone such as ecological changing and morphological changing.The deltas of the rivers of these study are in dangerous due to diminutions in the sedimentation load from these basin,which small aggradation rates or raise coast erode,thus resulting in comparative sea–level go up and improved chance of coast flooding.Delta loss as a result of the decline continued subsidence and sedimentation supply has raised the flood risk on the delta area.The Yangtze delta provides prograded greater than 200 km into the East China Sea over the past 6000 yr;nevertheless,over the past 60 years anthropogenic intervention as an alternative to monsoonal weather has changed into a principal factor to the reducing sedimentation load to the ocean.Therefore,the Yangtze delta has dealt with coastal and erosion wetlands in the intertidal region have decrease.The largely populated in Pearl River delta has become mainly protected from storm by channel barriers and coast,but delta subsidence and shrinkage with the decreased sedimentation load have relative sea-level go up that is often faster than the global sea–level growth.The Mekong River delta Started on 8000 yr BP and has created a large delta plain in Cambodia to Vietnam during the Holocene(area between 62,520 and 93,781 km~2).The delta has been progdading at a predicted mean of 10–20 m/yr for the last 3000 yr BP.However,the sedimentation load of Mekong River basin is steady at millennial scale,coast loss has developed into a major issue at present.A majority of deltaic coasts among distributary of the Mekong River have familiar characteristic,that are erosional at the northeastern zone and depositional at the southwest zone of each section,managed by formidable wave event in winter Thirdly,the trapped sediments by dams/reservoirs display that deposits have been affectedly transferred in streams or valleys,exposing tremendous benefits of human-made reservoirs on minimizing sedimentation load.Using the sedimentation TE higher than 80% for those tributaries,reservoirs/dams in the channel have the ability to trapping almost sedimentation getting to the river basins.With the Yellow River basin,the sedimentation TE of 95.2% resembles another river basin that implies the reservoirs/dams may cooperatively generate a substantial human signature on wide basin-sedimentation deposit.Sedimentation load interpolation summarize results the sedimentation load annually getting to stream for 1950–1970 was around 2.483 Gt.Considering,the reservoir/dam construction and the storage changing,the essentially trapping sedimentation load was fixed.For present year,the reservoir/dams sediment loads were predictable at 0.59 Gt/yr,which about 47.6% of sedimentation load annually reduction.Overall,19.32 Gt of sedimentations have been trapping by 2010,5.9 Gt was impounded in the last century as well as the excess 13.42 Gt were intercepted before 2000 by large dam/reservoir.The trapping sedimentation by check-dam demonstrates around 40.32 Gt of sedimentation have been resolved in river stream or valleys,showing the tremendous consequences of human-made dam/reservoir on minimizing amount of sedimentation.It is concluded that sediment amount from the large Asian rivers was reduced by accelerating anthropogenic in relation to the operation of whole basin and delta area in recent years.Soil and water conservation projects and engineering projects also were among the key contributors of sedimentation decreased.Thus,if the plan of dams construction to be continued,the section of the river basin sediment will be trapped,and leads to numerous environment effects in lower reaches.It is also really essential to identify the effects well and therefore,the problems need to be well in consideration.Principally the influences on the delta area and floodplain are likely to be serious and need to be evaluated in specifics in further research.