Study On Water Environment Of Yellow River In Lanzhou In Recent About 300 Years

Many study have been done on space variation of sand amount, function of sand convey, sediment speed, frequency of water and sand and modern flood in middle and lower reaches of yellow river. By far, study on change of strength of river flood and evolution of water environment in historical period is less at home and abroad, and there is no study on flood in upper reach of yellow river in historical period. Through analyzing sediment of yellow river, we try to research the regulation of water environmental evolution of yellow river and disclose climate change in drainage area of yellow river in historical period from the aspect of water power and water environment. This has the important point of science and reality in predicting the development diction of water environment and river management. On base of elaborate field investigation, we selected such three representative sections as LS, LC, LM on the beach of shajingyi, cuijia and majia, and collected 220 samples according to distance of 2-4cm. Then granularity and magnetization rate of samples has been made determination, and experiment result have been analyzed, and combining historical literature datum, flood change cycle has been calculated. Conclusions are as follows:(1) The flood sediment layers are obvious of yellow river in Lanzhou area in recent about 300 years, which reflect clearly to flood and short-term precipitation and have high differentiation rate, and is good target indicating the change of flood and precipitation. The grain composition reflects the change of flood convey power in sediment period, and thick grains indicate stronger convey power and thin grains indicate weaker convey power.(2) The widths of sediment of LS, LC and LM are separately 1.2m, 1.56m and 1.62m and divided into 10, 11, 15 moments and indicate 10, 11, 15 different flood sediments.(3) The grain composition has obvious change rule from above to bottom. Sufficiently considering the effect of landform heightened by sediment on grain composition, the order of depth and scale of floods of LS section from big to small is 4th, 1st, 2nd, 9th, 8th, 7th, 3rd, 5th, 6th, 10th, LC section is 11th, 9th, 10th, 8th, 1st, 6th, 2nd, 3rd, 5th, 7th, 4th, and LM section is 10th, 12th, 11th, 13th, 15th, 14th, 3rd, 5th, 7th, 8th, 6th, 1st, 4th, 9th, 2nd.(4) LS and LM sections come into being before about 200 years, and LC section is in about 200-300 years to now. Through comparing grain composition and sediment width, it can be conferred the climate in recent 200-300 years is relatively wet and in recent 200 years is relatively dry.(5) There is no flood in 1655-1666 and 1668-1727, which indicates the precipitation is little and the climate is dry in both periods. There are many floods recorded in1752-1780, 1801-1868, 1884-1909 and 1921-1948, which indicates the precipitations of four periods are much.(6) The floods in 1644-1949 in Lanzhou have periodicity. The periods of 102 years, 76 years, 25.5 years, 30.6 years and 3.3 years have passed the test of 95%, and the periods of 152.9 years, 61 years, 11 years, 8.8 years, 6.3 years, 2.8 years and 2 years have passed the test of 85%. The periods of 6.3 years, 3.5 years, 2.8 years and 2 years are corresponding to the period of 2-7 years of ENSO and the period of 11 years is corresponding to the period of macula activity, which indicates the flood occurrence is possibly relate to atmosphere activity above ocean in low latitude and macula activity.