Flood Change Research Of Weihe River In Wei Nan In Recent 120 Years

Many researches have been conducted on riverway shift of Weihe River in historical period in China, and some progress has been made in research of water level changes of Weihe River. However, there are less researches about flood intensity variability and evolvement of water environment in historical period at home and abroad so far. So it is very important in science and reality to forecast the trend of water environmental development and govern pollution of Weihe River for revealing the rules of water environment evolvement of Weihe River, the climate changes in Weihe basin and water quality deteriorating from flood convey power and flood sediments. Based on the detailed investigation, 376 samples of flood sediments of WN1 and WN2 profiles in high floodplain of Wei Nan and those in 2005 have been collected, and the grain composition,magnetic susceptibility,calcium carbonate and TOC of samples have been determined. Based on analyzing experimental datum and combining historic literature from 1700 A. D to 1949 A. D, some conclusions have been summarized as follows:1. The flood sediment layers are obvious of Weihe river in Wei Nan in recent about 120 years, which can reflect clearly flood occurrence and short-term precipitation, and have high differentiation rate, and is a good target indicating the change of flood and precipitation. The grain composition reflects the change of flood convey power in sediment period, and thicker grains indicate stronger convey power and thinner grains indicate weaker convey power.2. WN1 and WN2 profiles are respectively 5.28m and 5.4m wide, both are modern sediment layers, divided into 19 sediment phases and indicate 19 different flood sediments. The grain composition of flood sediments in high floodplain of Weihe River in 2005 has finely instructed the flood level on high floodplain in the same year.3. The grain composition of WN1,WN2 profiles has obvious change rule from above to bottom. Compared the 2nd,1st,3rd,4th,6th,8th,10th,12th,14th and 15th layer of WN1 profile, the grain size of the 5th,7th,9th,11th,16th, 17th,18th and 19th layer is smaller, size classification is comparatively worse, slanting degree is smaller, and peak state value is lower, which indicates the flood convey power of the 5th,7th,9th,11th,16th,17th,18th and 19th layer is weaker than that of the 2nd,1st,3rd,4th,6th,8th,10th,12th,14th and 15th layer. Sufficiently eliminating the effect of landform heightened by sediment on grain composition, the order of depth and scale of floods of WN1 profile from big to small is 15th, 14th, 10th, 3rd, 12th, 4th, 13th, 6th, 8th, 2nd, 1st, 7th, 11th, 5th, 16th, 9th, 18th, 17th and 19th. Among all flood stages, the floods of the 15th,14th,10th,13th,3rd,6th and 8th are extraordinary, and flood levels on high floodplain are higher than 1.56 m. The floods of the 11th,7th,2nd,5th are big, and flood levels on high floodplain are lower than that of Weihe River in 2005. The scales of 19 floods of WN2 section also have alternative change.4. CaCO₃ in sections mainly comes from chipping of river transportation. The high content of CaCO₃ reflects that the granularity of flood sediments is rude, the hydrodynamic force is strong, flood scale is big and precipitation is much. While the low content of CaCO₃ reflects that the granularity of the flood sediments is thin, the hydrodynamic force is weak, flood scale is small and precipitation is little. The experimental result of CaCO₃ content of WN1 section is shown that the content of CaCO₃ in the 15th,19th,7th,9th,14th,18th,13th,17th,10th,8th and 6th layers is smaller than that in the 1st,5th,2nd,12th,4th,16th,3rd and 11th layers. Eliminating the effect of landform heightened by sediment on CaCO₃ content, the hydrodynamic force of the 1st and 4th floods are strong and they are catastrophic flood, while the hydrodynamic force of the 7th and 19th floods are weak and they are big flood.5. Magnetic susceptibility of samples in the floodplain of Weihe River indicates change of precipitation and water power sensitively. The deposit layer of thick granularity and high magnetic susceptibility, formed in condition of more precipitation and stronger water power indicates higher flood level. Otherwise, the deposit layer of thin granularity and low magnetic susceptibility formed in condition of less precipitation and weaker water power indicates lower flood level. The change of magnetic susceptibility of WN1 and WN2 profiles is obvious and rhythmical, which reflects the different flood levels and climate change.6. Low organic carbon content of flood sediments of Weihe River is often relevant to change of flood power caused by the increasing and concentrating precipitation, and high organic carbon content is often relevant to the decreasing and even precipitation. The order of organic carbon content of WN2 profile from big to small is 18th, 2nd, 19th, 13th, 5th, 9th, 12th, 4th, 6th, 3rd, 7th, 1st, 8th, 10th, 16th, 17th, 14th, 11th and 15th layer. The low organic carbon contents of the 15th, 14th, 17th, 16th, 10th, 8th and 6th layer indicate the big flood scales, which is basically accordant to the result indicated by granularity component.7. In Wei Nan area there was no flood disaster in 1710～1735 and 1790～1799, which indicates the precipitation was less in these stages. In 1736～1789 and 1839～1933 there were much floods and there was a full section flood in 1931 in entire Wei Nan, which indicates the precipitation was more in these stages.8. In Wei Nan. area in 1700～1949 the flood periods of 12.8 years, 8.55 years and 5.85 years have passed the test of 95%, and that of 18.25 years and 14.35 years have passed the test of 85%. The period of 5.85 years is accordant to 2-7 years period of ENSO, which indicates the floods in Wei Nan area are relate to atmosphere activity above ocean in low latitude.