咸阳渭河高漫滩沉积洪水变化研究

根据咸阳高漫滩XY剖面中116个样品的粒度分析，结合历史文献记载的年代资料得出，渭河约120年来形成的高漫滩洪水沉积分层明显，对洪水的变化反映清楚，分辨率高，能够作为很好地指示洪水变化的指标。XY剖面最上层为2003年大洪水沉积，最上层之下的其它15层为古代洪水发生时期形成的沉积层，整个剖面记录了16次规模较大的洪水。XY剖面随深度变化具有明显的粗细变化节律，其中第1、3、5、7、9、11、13、15层粒度组成较第2、4、6、8、10、12、14、16层细。在充分考虑了因洪水沉积逐步叠加引起的地形增高对粒度成分的影响之后，可以确定这16个阶段洪水深度和规模由大到小的变化顺序为:第12阶段>第15阶段>第6阶段>第9阶段>第16阶段>第1阶段>第5阶段>第14阶段>第7阶段>第11阶段>第3阶段>第13阶段>第8阶段>第10阶段>第4阶段>第2阶段。其中第12、15、6、9、16、1阶段的洪水规模较第5、14、7、11、3、13、8、10、4、2阶段洪水规模更大。根据2003年河漫滩上的洪水沉积层粒度成分和洪水深度为2.2 m左右确定，所研究的剖面中第6、9、12、15、16、1层形成时的河漫滩上的洪水深度大于2.2 m，第5、14、7、11、3、13、8、10、4、2层形成时的洪水深度小于2.2 m。通过与现代大洪水发生时期年降水量的对比可知，这些大洪水发生的主要是渭河流域一些地区当年降水量显著增加造成的。在这16个洪水阶段之间没有洪水发生的阶段是降水量正常或偏少的阶段。

Research on Flood Change Indicated by Deposit in High Valley Flat Profile of Weihe River in Xianyang

On the basis of analysis of 116 samples in grain size from high valley flat section in Xianyang and historical book material of Weihe migration time,we knew that the delamination of 120 years' flood deposition of Weihe River is obvious and can be regarded as an indicator of flood and change.The XY profile indicated that there are 16 flood deposition stages,including the top major flood deposition layer formed in 2003 and the other 15 flood deposition layers formed in the ancient flood stages.There is a obvious variety rhythm of granularity along with the depth in the XY.The granularity in the 1st,3rd,5th,7th,9th,11th,13th and 15th layer was thinner than the 2nd,4th,6th,8th,10th,12th,14th and 16th layer in the XY profiles.After thinking over the effect of the heightening landform caused by the accumulating deposition thickness on grain size composition,the order of flood depth and the flood scale of these 16 flood stages was:12th >15th >6th >9th >16th >1st >5th >14th >7th >11th >3rd >13th >8th >10th >4th >2nd>.The 12th,15th,6th,9th,16th and 1st were the catastrophic flood stages and the 5th,14th,7th,11th,3rd,13th,8th,10th,4th and 2nd were the big flood stages.The origin of the flood was obviously increased of annual rainfall at the year in some areas in the Weihe basin.The stages in which no flood occurred in high valley flat were of normal rainfall or less rainfall.