Study on Flood Changes of Weihe River at Mafangtan in Gaoling
ZHAO Jing-bo 1,2 LI Sheng-li1 (1.College of Toruism and Environment Science, Shannxi Normal Unibersity; Xi'an 710062; 2. Center of Historical Environment and Socio-Econmic Development in Northwest China of Shaanxi Normal University, Xi'an 71006)
On the basis of the field investigation, grain size analyses for 147 samples from Mafang sections in valley flat in Gaoling and the historical record age material, flood and precipitation change between 1400 and 1100 years were studied. Ancient flood deposit layers of Weihe River in Gaoling is clear and can sensitively reflect the flood change and the short-term rainfall change, and also can make up the insufficiency of the historical literature record. The grain composition and the grain size parameter show that there are at least 9 sedimentary layers of different grain composition in the section with 3.5 m thick in valley flat at Mafangtan in Gaoling, which indicates 9 great flood stages. Among them, 2nd, 4th, 6th and 8th layer is bigger in thick and grain diameter than 1st, 3rd, 5th, 7th and 9th, and the flood scale was greater when the former 4 layers deposited than when the later 5 layers deposited. The scale of the 9 flood stages is greater than that in 2003, and 2nd, 4th, 6th, and 8th flood stage are catastrophic flood stages. According to the sedimentary principle and difference of grain diameter of the 9 flood stages, the magnitude order of the 9 flood stages is 2nd6th4th8th3rd1st5th9th7th. The water depth on the valley flat during the 9 flood stages was above 2m, the flood depth, flood energy and flood magnitude are greater at that time than at present, and the origin of the 9 flood stages is the precipitation increased obviously at that time.