三峡水库干支流悬移质泥沙及支流泥沙沉积特征——以忠县段为例

摘要

Construction of cascade dams along the main stream and tributaries of the Upper Yangtze River is leading to a considerable reduction in the amount of suspending sediments,as discharged into the upper reaches of the Three Gorges Revoir.However,with the increase of sediment yield caused by sloping surface,the Three Gorges reservoir is constantly fed with reservoir sedimentation and alter its sediment origins.The impounding of Three Gorges Dam pro-duces interaction of biological,physical and chemical matter between the main stream and tributaries and lasting sedimentation in riparian zone.Decreasing flow velocity caused by impoundment in tributaries has caused noticea-ble eutrophication.Therefore,there is an urgent need to investigate composition,characteristic of suspending sedi-ment and deposition processes in riparian zone of the Three Gorges Reservoir.The Ruxi River,a tributary of the Yangtze River located at Zhong County,as well as an adjacent section of the mainstream near its confluence,were chosen as the study area.Monthly suspending sediments in the mainstream and tributary were trapped for testing from July to September,2015.Stratified sediment samples in riparian zone of the Ruxi River were collected in July, 2015 and the Grain-size composition and137Cs activity were analyzed.Results indicated that the median particle size of the suspending sediments collected in the mainstream and tributary were 12.81 μm and 18.87 μm,respectively. The sand percentage of tributary sediment samples was higher than those of the mainstream samples in July,impl-ying that the particle size of tributary sediment was coarser than those of the main stream sediment.The grain-size composition of sediments in the mainstream during August and September was identical with those in July.As to suspending sediments in the tributary,it had a finer size than those in July.137Cs activities of suspended sediment collected in July were 0.87 and 0.65Bq/kg in the mainstream and tributary,respectively.They were both lower than those of sediments collected in August,especially ones collected in the tributary with an average 137Cs activity of 2.16Bq/kg.The coarser particles and lower 137Cs activity of suspending sediment in the tributary in July could be ascribed to subsurface erosion from heavy rainstorm events,whereas finer sediment composition and higher 137Cs ac-tivity in August could be attributed to surface erosion caused by frequent light rain.The stratified sediment instru-ment was installed in 2008 in the riparian zone of the Ruxi River,which trapped a sediment layer with a thickness of 7 cm till the sampling date.The median size and 137cs activity of cored stratified sediment ranged between 4.6-13.7 μm and 1.39-1.97 Bq/kg,respectively.According to the distribution of 137Cs activities combining with sedi-ment grain-size composition,the sediment deposition processes in riparian zone of the Three Gorges Reservoir during rainy and dry seasons were roughly distinguished.%长江上游水库群的开发和利用,导致上游入库泥沙的减少,坡面侵蚀产沙贡献增加,造成三峡水库入库泥沙沉积及其来源的变化;同时,大坝蓄水使干支流之间存在水流和物质能量的交换作用,缓慢的流速导致三峡水库支流富营养化现象较突出.所以,三峡水库入库泥沙的构成,库区干支流悬移质泥沙特征以及消落带泥沙沉积过程是迫切需要研究的问题.本文选择三峡库区中游忠县境内长江干流和一级支流汝溪河作为研究对象,采集雨季7—9月长江干流和支流汝溪河悬移质泥沙和汝溪河库湾消落带沉积泥沙剖面分层样品,并测试样品的颗粒组成和Cs-137活度.结果表明:7月份低水位时期,干流和支流悬移泥沙的中值粒径分别为12.81 μm和18.87 μm,且支流砂粒体积百分比较干流高,说明支流悬移泥沙比干流粗;137Cs比活度分别为0.87和0.65 Bq/kg,均较低.而8月份和9月份,干流悬移泥沙颗粒粗细和7月份相当,而支流悬移泥沙的平均中值粒径仅为6.05 μm,与干流悬移质泥沙相比偏细;137Cs比活度分别为1.23和2.16 Bq/kg.可以发现无论是干流还是支流,137Cs比活度变化均表现为8月份比7月份偏高,且8月份支流137Cs比活度比干流偏高.这是因为7月份是暴雨导致的表下层侵蚀泥沙,泥沙颗粒较粗且137Cs比活度较低;而8月份的悬移泥沙主要归结于频繁的中小雨引起的坡面表层侵蚀产沙,泥沙颗粒较细且富含137Cs.对于支流典型泥沙沉积剖面的研究表明,从2008至今,汝溪河库湾消落带泥沙总淤积厚度在7 cm左右,中值粒径介于4.6~13.7 μm之间,137Cs比活度的范围为1.39~1.97 Bq/kg.根据137Cs比活度结合河流输沙颗粒组成的旋回分层特征,可以大致区分出雨季和旱季的沉积泥沙,初步弄清支流消落带泥沙沉积过程.