Building reservoirs affects the process of water and sediment transport in the basin of the dam.The impact of the Three Gorges Reservoir (TGR) on sediment transport in the downstream has appeared.After the TGR impoundment,the total sediments in the downstream have been reduced.Sediments grouped with d ≤0.125 mm (fine particle) were increasing along the path,but the average amount was still less than the amount before the reservoir impoundment.Sediments grouped with d＞0.125 mm (coarse particle) have been recharged in the Yichang to Jianli section,but downstream of Jianli has a trend of deposition.In specific,the coarse sediment in the Yichang to Jianli section in 2003-2007 was above the value before the TGR impoundment,but increasing of the sediments concerning both coarse and fine sediments in 2008-2014 less than that in 2003 to 2007.Sediment retaining by the Dam is the major reason for the reduction of sediment in the downstream.However,the retaining in different reaches of the River is impacted by coarsening of the riverbed,the discharge,flow process,conditions of lake functioning and recharging from the tributaries.The main conclusions from our study are:(1) In the Yichang to Shashi section,the riverbed is relatively coarse that limits the recharge of coarse sediment,where the supplied fine sediment is controlled by the discharge by the duration of flood and its magnitude.(2) In the Shashi to Jianli section,recharging of both coarse and fine sediments is controlled by amount of total discharge;(3) In the Jianli to Luoshan section,the flow recharge from the Dongting Lake plays the most important roles on sediment recharging.Sediments supply from riverbed is higher in flood years than that from the dry years.The coarse sediment tends to deposit and the deposit amount in dry years is larger than that in flood years.(4) In the Luoshan to Hankou section,the recharge of fine sediment is controlled by supply from the riverbed.The recharging in 2008-2014 is more than that in 2003-2007.In about 2010,the coarse sediments were transited from depositing to scouring that is probably caused by the increasing of continuous large flow days.(5) In the Hankou to Jiujiang section,more fine sediments appear to deposit.The coarse sediment is recharged by the scouring of the riverbed and there were more coarse sediments recharged from the riverbed in the flood years than that in the dry years.(6) In the Jiujiang to Datong section,the rate of the fine sediment recharge from Poyang Lake to that from the riverbed was 1∶2.82.It is obvious that the sediment from scouring of riverbed contributed more to coarse sediment transportation and the contribution is mainly affected by magnitude and continuous days of large flow.%以三峡下游河段为研究对象,利用1987-2014年原型观测数据,分析粗细颗粒在时间和空间的输移过程,并以水文站作为划分单元探讨其成因.三峡水库蓄水后坝下游d≤0.125 mm(细颗粒)各粒径组沙量沿程递增,并小于蓄水前均值;d＞0.125 mm(粗颗粒)沙量在宜昌—监利河段得到补给,其下游为淤积趋势,其中2003-2007年至监利站恢复程度最大.在成因上:①宜昌—沙市河段,河床粗化限制了粗细颗粒泥沙补给,其中细颗粒补给受制于总水量,粗颗粒受制于大流量持续天数和量值;②沙市—监利河段,粗细颗粒补给均受制于总水量;③监利—螺山河段,洞庭湖入汇对细颗粒补给起决定作用,河床补给量中洪水年大于中枯水年,粗颗粒为淤积趋势,且中洪水年淤积量小于中枯水年;④螺山—汉口河段,细颗粒增加受河床补给决定,2008-2014年补给强度弱于2003-2007年,2010年前后粗颗粒泥沙由淤转冲,与大流量持续天数和量值增加有关;⑤汉口—九江河段,细颗粒泥沙淤积,粗颗粒因河床冲刷得到补给,大水年补给量高于枯水;⑥九江—大通河段,鄱阳湖入汇和河床补给对细颗粒增加的贡献比例为1∶2.82,河床冲刷对粗颗粒泥沙起补给作用,受大流量持续天数和量值影响.
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