湘江衡阳段河漫滩沉积物粒度特征与历史洪水事件

Particle Size Characteristics of Flood Sediments in Floodplain and Historical Flood Events of Hengyang Reach of Xiangjiang River

[目的]分析河漫滩沉积记录的历史时期洪水信息,揭示极端洪水事件和洪水发生规律,为防洪减灾提供指导。[方法]根据湘江河漫滩沉积剖面91个样品的粒度分析,结合AMS ~(14)C精确测年,分析湘江衡阳段河漫滩剖面粒度特征与洪水事件。[结果](1)河漫滩沉积物粒度组成以粗粉砂、粗砂、极细砂为主。粒度参数显示,剖面洪水沉积动力较强,分选系数极好,宽型峰态居多。(2)吉祥剖面5个阶段洪水规模大小顺序依次为:B阶段>D阶段>A阶段>C阶段>E阶段。此外,剖面B阶段34~40,44~46 cm区间和D阶段中130~134 cm区间粗砂的含量异常的增高,粗粉砂和细粉砂含量极少,中值粒径值为最低值范围,洪水沉积动力异常强大,为衡阳特大洪水事件发生时期。(3)沉积剖面中值粒径值与平均粒径值、粗粉砂+细粉砂的含量成显著正相关;粗砂含量与平均粒径值、中值粒径值、粗粉砂+细粉砂含量成显著负相关性。[结论] 6 390 aBP以来,衡阳洪水规模整体上呈现增大增多的趋势,沉积阶段A和阶段B均为洪水多发时期;粗砂含量、平均粒径值、中值粒径值、粗粉砂+细粉砂含量4个指标能很好地用来识别洪水事件。

[Objective]The flood information recorded in floodplain sediments during the historical period was analyzed in order to reveal the extreme flood events and the regularity of flood occurrence,and to provide guidance for flood control and disaster reduction.[Methods]Particle size characteristics and flood events for the floodplain profile of Hengyang Reach of the Xiangjiang River in Hu’nan Province were studied based on particle size analysis of 91 samples from the flood plain combined with AMS 14 C dating.[Results]①The composition of floodplain sediments mainly included coarse silt,coarse sand,and fine sand.The grain size parameters showed that the profile flood sedimentary power was strong,the sorting coefficient was excellent,and the wide peak was in majority;②The flood scale of the five flood stages of the Jixiang profile followed the order of stage B>stage D>stage A>stage C>stage E.In addition,between 34—40,44—46 cm in stage B,and between 130 and 134 cm in stage D,the content of coarse sand increased abnormally,the content of coarse silt and fine silt was very small,and the median partilce size value was in the lowest range.Flood sedimentary power was extremely strong.The flood events were all in periods when Hengyang catastrophic flood events occurred;③The median particle size values were positively correlated with the average particle size values and the content of coarse silt+fine silt.There was a significant negative correlation between the content of coarse sand and average particle size values,the median particle size values,and the content of coarse silt+fine silt.[Conclusion]The flood scale of the Hengyang reach has,on the whole,shown an increasing trend since 6390 B P Both sedimentary stage A and stage B are flood prone periods.The four indicators(content of coarse sand,average particle size values,median particle size values,and content of coarse silt+fine silt)could well identify flood events.