黄河永和关段全新世古洪水研究

通过对黄河中游晋陕峡谷段进行系统地考察, 在多个地点发现全新世古洪水滞流沉积物。选择永和关段YHG剖面进行了沉积学和水文学研究。根据野外宏观沉积学特征, 确定YHG 剖面含有一组五层古洪水滞流沉积层。采样分析测试表明其磁化率低、烧失量小、CaCO₃含量低, 粒度成分分类为粉沙质细沙和细沙质粉沙。它们是黄河洪水悬移质泥沙在高水位滞流情况下的沉积物, 记录了黄河晋陕峡谷段一期五次特大古洪水事件。通过地层学对比分析和OSL 测年断代, 确定这五次洪水发生在全新世中期-晚期转折阶段, 即3200-3000 aBP。这也是我国历史上商末-西周初年的气候恶化转折阶段。利用“古洪水SWD厚度与含沙量关系法”恢复古洪水洪峰水位, 采用面积-比降法计算出5 次古洪水事件洪峰流量介于28380~48590 m³/s 之间。该结果有效地延长了黄河中游洪水数据序列, 为万年尺度洪水流量-频率关系的计算提供了基础数据。这对于黄河中下游水利水电工程洪水设计和洪水资源化开发管理具有重要的现实意义, 对于揭示黄河中游水文过程对于全球变化的响应规律具有重要的科学意义。

Palaeoflood Sedimentological and Hydrological Studies on the Yongheguan Reach in the Middle Yellow River

Typical palaeoflood slackwater deposits of the Holocene age were found at many sites on both sides of the Shanxi-Shaanxi Gorges. A set of 5 beds palaeoflood SWD at the Yongheguang (YHG) site was sampled and investigated in detail. Analyses of the grain-size distribution, loss-on-ignition and CaCO₃ content indicate that these SWD consist of sandy silt and silt fine sand. They were sourced from the suspended sediment load of the Yellow River floodwater, which was brought in by storm rains and floods from the triangle zone between Shanxi, Shaanxi and Inner Mongolia. They have recorded 5 extraordinary palaeoflood events on the Yellow River. The age of the palaeoflood events was determined by OSL dating of the sediment and stratigraphic correlation with the Holocene chronological framework established in the drainage basin of the middle reaches of the Yellow River. This shows that these extreme floods occurred during 3200-3000 a BP at the turn from the middle Holocene climatic optimum to the late Holocene dry period, which is the time of climatic decline from the Shang Dynasty to Western Zhou Dynasty, with occurrence of serious drought and flood disasters, harvest failure, famine and major social changes. The reconstructed peak discharges of the palaeoflood range from 28380 to 48590 m³/s, which are much larger than the gauged largest floods in the Shanxi-Shaanxi Gorges in the middle reaches of the Yellow River, thus the flood data series of the river extended to a 10000-year time-scale.