A Comparative Study On Floods Of Main Rivers On Both Sides Of Qinling Mountains In Different Time Scale

With the development of global change science, people have realized that the research of regional change is becoming more and more important in this field. Therefore, an increasing numbers of people have focused on the comparative study of different regions. As the mid-latitude region is sensitive area respond to the global change, through the comparative study of different regional the research can result a more comprehensive understanding of the regional response to global change. Based on this background, this paper has mainly research on the different scales of flood on both sides of Qinling Mountains. By collecting and sorting the research results of palaeoflood, history flood, gauged flood of the upper reach of Hanjiang River and Weihe River, this paper analyzed the difference of time and discharge of different time scale flood on both sides of Qinling Mountains, so did to the relationship between the flood and global change. This paper’s conclusions mainly are as follows:(1)By comparing the gauged flood data in the upper reach of Hanjiang River and Weihe River on both sides of Qinling Mountains, we have found that floods of short time scale on both sides of Qinling Mountains show different characteristic. Although in some years, for example the year in 1981,2005,2011, et al., both the upper reach of Hanjiang River and Weihe River occur extreme flood, however, in most years of gauged flood series, there is no extreme flood occurred on both sides of Qinling Mountains. And further research showed this was caused by blocking function of Qinling Mountains, the influence of water vapor sources, influence of basin shape and watershed underlying surface conditions.(2) By analyzing the history flood data of the reach upper of Hanjiang River and Weihe River, the following conclusions were got:from the aspect of flood frequency, the flood in the upper reach of Hanjiang River was less than the Weihe River; from the aspect of flood disaster grade, the flood in the upper reach of Hanjiang River mainly were great flood or extreme flood, while the flood in Weihe River mainly were moderate flood or great flood, there was only a few extreme flood. Besides, the flood in both sides of Qinling Mountains had obvious periodic and cyclical, and the period had consistent and synchronicity with north China’s disaster, as well as to the macula cycle. These had showed that the flood events on both sides of Qinling Mountains were relative to the active of solar activity.(3)By comparing the long-time scale palaeoflood events in upper reach of Hanjiang River and Weihe River, we had found that palaeoflood events occurred in different period on both sides of Qinling Mountains, however, most of the palaeoflood records occurred in the period of 4 200～4 000 a B.P. and 3 200～3 000 a B.P.. Meanwhile, the research of monsoon climate variation showed that the period of 4 200～4 000 a B.P. and 3 200～3 000 a B.P was also the period of monsoon climate mutation and deterioration. Abnormal extreme weather events caused by monsoon climate mutation and deterioration resulted the extreme palaeoflood events in the upper reach of Hanjiang River and Weihe River on both sides of Qinling Mountains. However, confining by the OSL dating method, it was only proper to argue that palaeoflood events in the upper reach of Hanjiang River and Weihe River on both sides of Qinling Mountains occurred at same period in long-time scale aspects.(4) The palaeoflood in the upper reach of Hanjiang River and Weihe River recorded by typical profile were choose to reconstruct flood discharge by HEC-RAS model. In this paper TJZ profile in the upper reach of Hanjiang River and GCZ profile in Weihe River were choose as objects of study. TJZ profile recorded four palaeoflood events, which discharge calculated by HEC-RAS model were 54 920 m3/s,51300 m3/s,43 500 m/s,37 050 m/s. the discharge error between HEC-RAS model and slope-area method respectively was 5.41%,2.66%,3.57%,0.28%. GCZ profile recorded two palaeoflood events, which discharge calculated by HEC-RAS model were 26 000 m3/s and 21 300 m3/s, the discharge error between HEC-RAS model and slope-area method respectively was 0.16% and 5.58%. In order to guarantee the rationality of using HEC-RAS model, the discharge of gauged flood was calculate by HEC-RAS model to compare with its true value. Besides, the discharge in TJZ and GCZ profiles were also verified by global peak discharge versus drainage area for respectively global palaeoflood, and the result showed that it was rational to use HEC-RAS model to reconstruct the discharge of palaeoflood.(5) By combining the history flood and palaeoflood with gauged data respectively, different time scale of flood data were compared with its influence to flood frequency curve. The comparative study result showed that both history flood and palaeoflood would make some influence to flood frequency curve when they were join with gauged flood data. While history flood data make a difference only when the discharge were far more than the biggest gauged discharge. The discharge of palaeoflood in upper reach of Hanjiang River and Weihe River were much larger than the biggest gauged flood discharge, therefore, palaeoflood data would make a great difference to flood frequency curve. Therefore, it was great significant to research the palaeoflood in flood frequency analysis.