长江下游洪水事件:基于南通河漫滩研究

Flood Events in the Lower Yangtze Reach: Inferred from the Floodplain at Nantong, Eastern China

沉积物中保留下来的信息载体可较准确地反映古环境变化,沉积物粒度与磁化率是沉积物中反映沉积环境变化的重要指标.研究了长江下游南通段河漫滩沉积剖面的粒度和磁化率特征,由此识别出长江下游南通段现代洪水事件.粒度分析表明,样品偏态系数为负偏,表明沉积物粒径总体相对较大,反映水流量大,搬运能力强.根据砂、粉砂、粘土百分含量变化,可将剖面分6层,呈粗—细—粗—细—粗—细变化.这与磁化率特征是一致的,磁化率呈高—低—高—低—高—低变化.将磁化率、中值粒径和近百年降水量变化对比分析发现,磁化率与沉积物中值粒径呈负相关关系,而沉积物粒度与降水量呈正相关关系.推断第1、3、5层的沉积特征指示该阶段水量较大(洪水期),第2、4、6层沉积特征指示水量较小,约130 a以来长江下游南通段至少发生3次较大的洪水,其中第5沉积层对应的洪水规模可能最大.

Information carder conserved in the sedimentary materials can accurately reflect the development of paleoenvironment. The characteristics of grain size and susceptibilities are major indexes for sedimentary environment. After analyzing the characteristics of grain size and the susceptibilities of the sedimentary samples, we identified several flood events in the lower reach of Yangtze River. Negative SK of the samples indicates the existing of coarse grain, which shows big water flow and strong transport ability. According to the percentage of different grain size sediment, we can cut the section into 6 layers of coarse-flne-coarse-fine-coarse-flne. The quality susceptibilities of these layers show the similar rhythm as high-low-high-low-high-low. Through comparing of susceptibilities, median particle size and changes of precipitation within nearly one hundred years, we found that particle size and precipitation show positive correlation, and susceptibility and grain size show negative correlation. Comprehensive analysis indicated that greater precipitation in the first, third, fifth stages led to larger floods, while smaller precipitation in the second, fourth, sixth stages led to smaller floods. Therefore, we speculated that there were at least 3 times of large floods at the lower reaches of the Yangtze River, and the flood scale at the fifth stage might be the largest one.