The Study On The Deposit Of High Valley Flat And The Change Of Flood In Zhouzhi Of Weihe River

Many studies have been conducted on river channel's change of Weihe River in China in history, and some progress has been made in understanding of water level change of Weihe River. However, there has been few studies about flood intensity variability, flood scale variability and height of flood variability in historical based on River Sediment of the Weihe River so far. Therefore, it is important and practical to prediction the trend of Water Environment in Weihe River and flooding prevention through studying the evolution of the water environment law and the historical period of the Climate in Weihe River Basin, from studying on Hydrodynamic and Water Environment. Based on detailed observations of the field on the basis of the Weihe River riverbed to the South 500 m, 1000m high alluvial flat of the central and trailing edge of flood plain has been selected ZZ₂, ZZ₃ flood sediment profiles, the two sections were collected 130 samples. Based on analyzing experimental datum such as the grain composition,magnetic susceptibility of samples, then combining historic literature to analysis the scale and the reasons from 1644 A.D to 2005 A.D, some conclusions have been summarized as follows:(1) The flood sediment layers are obvious of ZZ₂ and ZZ₃ flood sediment profiles in Zhouzhi of Weihe River, 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) Both ZZ₂ and ZZ₃ profiles are respectively 1.95m wide, ZZ₂ is divided into 18 sediment phases and indicate 18 different flood sediments.ZZ3 is divided into 20 sediment phases and indicate 20 different flood sediments. According to comparing flood sediment characteristics, we know that the layer of ZZ₃ profile is two tiers more than ZZ₂ profile's, and the remaining layer come into being at the same period with ZZ₂ profile.(3) The grain composition of ZZ₂ and ZZ₃ profiles has obvious change rule from above to bottom. Sufficiently eliminating the effect of landform heightened by sediment on grain composition, the order of depth and scale of floods of ZZ₂ profile from big to small is 7th, 8th, 2th, 4th, 5th, 6th, 13th,3th,lth, 9th, 11th, 10th, 14th, 12th, 16th, 18th, 15th and 17th; the order of depth and scale of floods of ZZ₃ profile from big to small is 1th, 4th, 2th, 5th, 3th, 11th, 6th, 8th, 12th, 7th, 13th, 10th, 9th, 14th, 19th, 15th, 17th, 16th, 18th and 20th.(4) Magnetic susceptibility of samples in the floodplain of Weihe River indicates change of environmental change sensitively. The deposit layer of thick granularity and low magnetic susceptibility formed in condition of stronger water power indicates higher flood level. Otherwise, the deposit layer of thin granularity and high magnetic susceptibility formed in condition of weaker water power indicates lower flood level. The change of magnetic susceptibility of ZZ₂ and ZZ₃ profiles is obvious and rhythmical, reflecting water power frequent changes during the formation period of Weihe River Floods.(5) In Zhouzhi area there was no flood disaster in 1760-1779s 1800-1809 and 1970-1979,which indicates the precipitation was less in these stages. In 1650-1670,1690-1750,1810-1830, 1850- 1960, 1980- 1990 there were much floods, which indicates the precipitation was more in these stages.