长江中游河漫滩沉积序列对洪水事件的指示——以荆州扬子江剖面为例

Indication of flood events based on floodplain sedimentary sequence in middle reaches of Changjiang River since 19th century:case of Yangzijiang profile in Jingzhou City

作为全球变化研究的热点之一,古洪水研究对区域防洪减灾有着重要的意义。基于长江中游荆江河段8.30 m高时间分辨率的河漫滩沉积记录,通过137Cs比活度和植物残体14C建立剖面年代框架,分析沉积物的粒度特征指示的沉积环境,并结合历史资料和水文数据,探讨了各个指标对洪水事件的指示。结果表明:(1)河漫滩沉积中存在4类粒度频数曲线,表示了弱而稳定的平流沉积到强且不稳定的(似)河床相沉积环境,分别指示了一般洪水、大洪水和特大洪水事件;(2)平均粒径、分选、粒度频数分维值等13个粒度指标特征表明,平均粒径、砂、(粗粉砂+砂)/(细粉砂+黏土)、中值粒径和粒度频数分维值这5个指标对环境响应敏感,较好地区别了各类洪水事件,它们检出了扬子江剖面记录的1788年来45个洪水年份,与历史文献和水文数据中的洪水年份吻合度在90%以上;(3)这5个指标对洪水年份的检出率分别是82.61%、89.12%、80.43%、71.74%和76.09%。研究结果可为河漫滩沉积序列在高分辨率洪水事件重建中的研究提供参考,也将进一步拓展古洪水的研究内容和内涵。

As one of the hotspots in global change research,the study of ancient floods is of great significance for regional flood prevention and mitigation.In this paper,we established a profile dating framework through 137Cs specific activity and 14C of plant residues,and analyzed the sedimentary environment indicated by the particle size characteristics of the sediments.Based on this,we discussed the indications of flood events by historical and hydrological data based on the 8.3 m high temporal resolution of floodplain sedimentary sequence in the Jingjiang River section in middle reaches of the Changjiang River.The results are as follows:(1) there are four types of particle size frequency curves in the floodplain sediments,which indicate the deposition environment from weak and stable stratospheric sediments to strong and unstable(like)bed-phase sediments,indicating the normal flood,large flood and extreme flood events respectively.(2) 13 particle size index characteristics indicate that the mean particle size,sand,(coarse silt + sand)/(fine silt + clay),median particle diameter and particle size frequency fractal dimension D are sensitive to environmental response and can well distinguish the sedimentary environment under various flood events.They detected 45 flood years recorded in the Yangzijiang profile since 1788,and the consistency with the flood years in historical and hydrological data were over 90%.The detection rates of the five indicators for flood years were 82.61%,89.12%,80.43%,71.74% and 76.09%,respectively.The results can provide a reference for floodplain sedimentary sequence in the reconstruction of high resolution flood events,and will broaden the content and connotation of the ancient flood research.