西安北郊渭河河漫滩沉积与洪水事件

The research on the sediment of floodplain and flood events of Weihe River at Xiangjiaxiang in the northern suburbs of Xi'an

根据西安北郊相家巷河漫滩沉积剖面中180个样品的颗粒分析和历史文献资料记载,研究了西安北郊相家巷漫滩沉积代表的洪水事件。研究表明:西安北郊相家巷剖面洪水沉积分层明显,对洪水的发生和短期降水量变化反映清楚。相家巷剖面洪水沉积厚度约为5.40m,可划分为28个层,指示发生了28次大小不同的洪水事件。剖面中的洪水沉积以中、粗砂和细砂为主,指示当时的洪水规模比近120年来关中渭河洪水明显大。这28次洪水事件发生时高漫滩上的洪水深度均大于2.2m,剖面中的第12、14、19～28层代表特大洪水事件。西安北郊相家巷洪水沉积代表的大规模洪水主要是由当年降水量的明显增加引起的。

According to the analysis of grain size for 180 samples from the floodplain deposition of Xiangjiaxiang in the northern suburbs of Xi＇an and the record of historical document materials,this paper has studied flood events represented by the floodplain deposition of Xiangjiaxiang.The materials indicate that the deposited layers of flood in Xiangjiaxian are obvious,which can clearly reflect the occurrence of flood and short-term changes of precipitation.High resolution can be a good indicator of flood events and precipitation change.The particle ingredients of sediments reflect the changes of hydrodynamic force in the depositional period： Thicker grain composition indicate the hydrodynamic force of floodplains in the depositional period is strong,however finer grain composition indicates the hydrodynamic force of floodplains in the depositional period is weak.The thickness of flood deposition of Xiangjixiang profile is 5.40 m,which can be divided into 28 layers,indicating 28 different scales of flood events.The changes of particle ingredients of Xiangjiaxiang profile have obvious changing laws in terms of thickness.Considering the fact that terrain increased have influences on particle ingredients caused by deposition thickness,we can see that when these 28 flood events happened,the flood depth and scale were changing from large to small.The flood deposition of the profile mainly includes medium sand,coarse sand and fine sand,indicating that the flood scales were greater at that time than Weihe in Guanzhong floods in recent 120 years.The size and thickness of Weihe floodplain deposition indicate that when these 28 flood events of Xiangjiaxiang profile happened,the depth of flood above the top surface of floodplain is averagely higher than 2.2 m,and the 12th,14th,and 19th to 28th layers of the profile represent extraordinary flood events.The huge scale of flood represented by the floodplains deposition of Xiangjiaxiang in the northern suburbs of Xi＇an is mainly resulted from increasing annual precipitation at that time,which was more than 800 mm.