三峡水库消落带泥沙沉积动态变化及其对水文节律的响应

Impoundment of the Three Gorges Reservoir reduces flow velocity and undermines its sediment carrying capacity, leading to significant sedimentation. Annual sediment trapping rate and sedimentation amount range between 64%-95% and 0.125Gt-0.305Gt, respectively. The reservoir water-level fluctuation zone represents a unique geomorphological unit that undergoes periodic flooding and exposure, which subsequently provides an important space for sediment storage. Sedimentation in the water-level fluctuation zone of the Three Gorges Reservoir differs from that in river natural riparian zone, which is mediated both by natural river hydrology and reservoir flow regulation. It is of great necessity to investigate changes of sedimentary dynamics in the water-level fluctuation zone of Three Gorges Reservoir and its response to the hydrological regime, reveal the sedimentary processes and its source composition, which provides scientific basis for sediment mitigation of the Three Gorges Reservoir. Based on our previous studies, by combining field sampling, laboratory analysis, model calculation and numerical statistics, this project attempts to depict the spatial and temporal pattern of Cs-137 in suspended sediment, establish a method using Cs-137 to differentiate sedimentary stratigraphy in the water-level fluctuation zone, reconstruct temporal sedimentary processes, quantify the relative source contributions of proximal upstream rivers, local tributaries and bank erosion to sink sediment, analyze its response to the specific hydrological regime, and explore the coupled effect of fluvial seasonal sediment transport and flow regulation.

三峡水库蓄水运行导致水流流速变缓，水体挟沙能力下降，泥沙落淤严重，年均拦沙率64%-95%，年淤积量达1.25-3.05亿吨。消落带是三峡水库特殊的地貌单元，经历周期性的淹没和出露，是泥沙淤积的重要场所。不同于河流自然消落带，三峡水库消落带泥沙沉积受河流自然水文特征和水库水位调节过程叠加影响。有必要开展三峡水库消落带泥沙沉积动态变化及其对水文节律的响应研究，揭示水库消落带泥沙沉积过程及来源构成，为水库泥沙调节提供科学依据。基于前期研究进展，本项目拟通过野外采样、实验室分析、模型计算和数理统计，表征悬移质泥沙Cs-137的时空分异特性，建立消落带季节性沉积旋回的Cs-137辨识手段，重构消落带泥沙沉积动态过程，定量估算上游河流输沙、库区流域产沙和消落带侵蚀对沉积泥沙的物源贡献，分析消落带泥沙沉积动态与水库特定水文节律的响应关系，探讨河流季节性输沙和水库水位调节的协同影响机制。

消落带是三峡水库重要的结构和功能单元，经历周期性的淹没和出露过程，成为水库拦沙重要的“汇”。不同于自然河流消落带，三峡水库消落带泥沙沉积受河流自然水文过程和水库水位调节过程的叠加影响。本项目通过野外采样、实验室分析、模型计算和数理统计，联合沉积物多因子定年和泥沙来源指纹示踪技术，表征水库悬移质泥沙理化性质的季节性分异特征，揭示消落带横向断面高程梯度泥沙沉积分异规律，建立消落带季节性沉积旋回辨识手段，重构消落带泥沙沉积时间序列动态过程，定量估算上游河流输沙、库区地表产沙和消落带侵蚀对消落带泥沙沉积的物源贡献，阐明消落带泥沙沉积动态与水库特定水文节律的响应关系，探讨河流季节性输沙和水库水位调节的协同影响机制。结果表明：（1）三峡水库干流悬移质泥沙粒度存在明显的季节性分异特征，悬移质泥沙粒度在汛期粗于非汛期；汛期粒径分布特征更复杂，表现为粒径分布范围较宽、集中度较低、在测量间隔上分布较均匀。（2）消落带横向断面高程梯度上，泥沙沉积速率随高程增加显著降低。（3）消落带沉积剖面210Pbex活度具有明确的季节性指示意义，即汛期沉积泥沙的210Pbex活度较低，而非汛期沉积泥沙210Pbex活度较高；多因子定年结果表明消落带泥沙沉积速率具有阶段性特征，2006-2007年为慢速沉积期，总沉积厚度为25 cm；2008-2013年为快速沉积期，总沉积厚度为270cm；2014-2017年为慢速沉积期，总沉积厚度为80cm；2018-2019年为快速沉积期，总沉积厚度为65cm。汛期和非汛期泥沙沉积厚度多年平均值分别为20.77cm和13.08cm。（4）三峡水库纵向沿程，水体悬移质泥沙和消落带表层沉积物粒度呈现细化趋势。（5）运用混合模型估算得到消落带沉积泥沙的物源贡献范围为长江干流55.9%-57.6%，嘉陵江0.3%-6.2%，乌江0.6%-2.4%，三峡库周坡面侵蚀31.4%-38.3%，消落带岸坡侵蚀2%-8.3%。（6）水库特定水文节律是主导消落带泥沙再分布过程的首要因素，淹水特性在消落带高程梯度上形成的差异（淹水时长、频率、时令）与水体悬移质泥沙浓度的耦合，导致消落带沉积速率、泥沙来源和沉积过程的筛选分异。