基于粒径和 137Cs的三峡水库消落带泥沙来源示踪研究

为分析三峡水库消落带泥沙输移与沉积规律,采用原位观测方法,沿库岸H=148 m高程带选择15个采样点,并甄选出3个典型断面采集沉积泥沙样品,利用激光粒度仪和γ能谱仪分别测试样品的粒径与 ¹³⁷Cs活度,分析二者在水平和高程2个维度的变化特征,探讨消落带沉积泥沙来源。结果表明①三峡水库消落带沉积物的中值粒径(D₅₀)沿水流方向逐渐降低,并在忠县任家镇及其下游河段基本保持稳定,而 ¹³⁷Cs活度沿水流方向逐渐增大,但在任家镇及其下游河段仍有波动。②消落带邻江区域沉积物的粒径最粗、 ¹³⁷Cs活度最低,除这一区域外,沉积物粒径沿高程的变化取决于消落带上方坡面的土壤侵蚀状况,若上方存在侵蚀,沉积物粒径随高程的增加而增大,若上方微弱侵蚀或无侵蚀,则沉积物粒径随高程的增加而降低,但 ¹³⁷Cs活度不受上方土壤侵蚀状况影响,均表现为随高度增加先降后升的变化趋势。③粗颗粒与细颗粒沉积物具有不同的来源特征,变动回水区的粗颗粒也主要源于金沙江和嘉陵江,乌江入库的粗颗粒主要沉积在涪陵至忠县任家镇河段,任家镇下游的粗颗粒主要源于库区内侵蚀产沙;变动回水区的细颗粒主要源于金沙江和嘉陵江,常年库区的细颗粒泥沙来源较多,金沙江、嘉陵江、乌江和库区内侵蚀产沙均存在明显贡献,而库水位涨落过程中的波浪侵蚀对沉积物存在再分配作用,它能使表层沉积物重新进入水体,并在较低的位置实现二次沉积。

Sediment Sources Tracing in Riparian Zone Based on Grain Size Distribution and 137 Cs Activity in the Three Gorges Reservoir

Sediment source is an important information that connects particles from hill slopes and small watersheds to river channels.In order to reveal the basic law of sediment transport and deposition in the riparian zone of the Three Gorges Reservoir,we investigated the horizontal and vertical variation of particle size distribution and 137 Cs activity among sediment samples collected in the study area,and analyzed the implication of such variations on sed-iment sources.Results showed that median particle size (D50 )of sediments gradually became finer along the flow direction and stabilized in general in Zhongxian County and its lower reach;while the 137 Cs activity of sediment samples rose along the flow direction but was not as stable as particle size in the downstream of Zhongxian County. Sediments collected around H=148 m had the largest particle size and the lowest 137 Cs activity compared to samples from other altitudes.Except for this river-adjacent area,particle size grew larger with the increase of altitude when the soil could be eroded easily in the upper hill slope;on the contrary,particle size became smaller with the in-crease of altitude when the upper hill slope was used for forest,indicating slight soil erosion.Therefore,vertical variation of particle size was greatly influenced by the land use of the hill slope above the riparian zone.In contrast, vertical variation of 137 Cs activity in three sampling transects were all falling at first and rising later with the increase of altitude despite the different land use of upper hill slope.On the basis of these variations,it could be judged that sediment deposited at the riparian zone in the fluctuant backwater area was sourced mostly from the Jinsha River and the Jialing River.In the permanent backwater area,coarse sediment with a grain diameter≥62μm in the ripa-rian zone had distinct sources compared to fine sediment whose grain diameter was <62 μm.Coarse sediments from the Wujiang River deposited at the reach from Fuling to Renjia.In the reach from Renjia to the Three Gorges Dam, coarse deposited sediment sourced dominantly from soil erosion in the tributaries within the Three Gorges Reservoir basin.All potential sources including the Jinsha River,the Jialing River,the Wujiang River and soil erosion within the basin all showed significant contributions to the fine sediments in the permanent backwater area.In addition, wave erosion redistributed deposited sediments.Surface particles could be eroded into the water and deposit on the locations with lower altitudes.