汉江与渭河大洪水滞流沉积物性质对比分析

通过广泛野外考察,在汉江上游和渭河干支流发现多次现代大洪水滞流沉积物(SWD),进行样品采集和粒度成分、磁化率特征对比分析。结果表明:汉江上游与渭河大洪水SWD的磁化率都很低,在(20～60)×10-8 m3/kg之间。这与渭河流域马兰黄土L1的磁化率相当。这也正是那些未受到风化成壤作用影响的新鲜沉积物的基本特征。汉江上游与渭河大洪水SWD粒度成分均以粉沙为主,沙级颗粒含量次之,粘土成分含量最少。它们主要是河流洪水悬移质和跃移质成分在高水位滞流环境当中的沉积物。与渭河相比,汉江上游大洪水SWD粒度成分更粗,沙级颗粒成分含量略多。汉江上游大洪水SWD性质主要是粉沙质亚沙土和沙质粉沙土。渭河大洪水SWD性质则主要是粉沙土、沙质粉沙土和粘土质粉沙土。这是因为汉江上游穿行于秦岭和巴山基岩山地之间,羽毛状分布的支流水系比降大,河流搬运动力强,在暴雨洪水期间,不断地向干流输入粗颗粒泥沙。而渭河流域为黄土高原,河流泥沙主要来源于粉沙质的黄土层和地表土壤。该研究成果对汉江上游和渭河流域防洪减灾和水土流失防治具有重要的科学意义。

Comparative Study of the Modern Flood Slackwater Deposits in the Hanjiang and Weihe River Valleys

Modern flood slackwater deposits(SWD) were investigated and sampled in the upper reaches of Hanjiang river and the Weihe river basin.Grain-size distribution and magnetic susceptibility were analyzed in the laboratory.The results show that magnetic susceptibility of the modern flood SWD are(20～60)×10-8 m3/kg,very close to that of the Malan Loess over the Weihe river basin.This means they were newly deposited and were not influenced by the weathering and pedogenesis.The grain size of the modern flood SWD in the upper reaches of Hanjiang river and the Weihe river is dominated by silt,and with a certain proportion of sand and clay.This indicates that they were deposited from the suspended sediment load of the floodwater at a high-stage stagnant environment.Compared with that in the Weihe river valley,The SWD in the Hanjiang river valley contains more sand-sized particles.The modern flood SWD in the Hanjiang river valley is defined as sandy silt and silty sand,while in the Weihe river valley,it is defined as silt,sandy silt and clayey silt.The upper reaches of Hanjiang river flows between the Qinling and the Bashan bedrock mountains,the torrential tributary rivers bring much coarse sediment into the main stream of the Hanjiang river.The Weihe river drains the central part of the Loess Plateau.The sediment load in the floodwater is mainly sourced from eolian loess and the related surface soil.These results are of great importance to mitigate flood disasters,and to soil and water conservation.