FLOOD SEDIMENT IN VALLEY FLAT AND THE CLIMATIC IMPLICATIONS OF LANZHOU SECTION OF THE YELLOW RIVER

According to grain size analyses of 60 samples, we studied flood variations indicated by deposits in valley flat of the Yellow River in Lanzhou section over recent 150 years. Results indicated that the profile section was obviously stratified in Lanzhou area during these years, which can clearly reflect the variations of flood and short-term precipitation with high resolution, and is a good index indicating the variations of flood and precipitation. The grain composition reflected changes in the strength of flood conveying power during sedimentation, thick grains showing stronger conveying power and thin grains suggesting weaker conveying power. The section was divided into 10 sediment stages,indicating 10 floods of different scales and 10 years with much precipitation. The stages between 10 flood stages show no flood occurrence or that the floods' scales were very small, indicating normal or less precipitation in this river basin. The forth layer formed in 1904, and the biggest runoff was 8 500 m³/s. Flood scales of the fifth, sixth, seventh, eighth, ninth and tenth stages were all smaller than that in 1946, and their biggest runoffs were all smaller than 5 900 m³/s, that is to say, the biggest runoff was smaller than 5 900 m³/s for recent 100-150 years. The grain composition varied obviously from up to bottom. According to the grain composition characteristics of each layer and the criterions determining the flood depth in the valley flat, we come to the conclusion that the order of depth and scale of floods of the section from big to small is 4th, 1st, 2nd, 8th, 3rd, 9th, 7th, 5th, 6th and 10th. According to characteristics of precipitation in Lanzhou City, the causes of floods and comparatively big sediment depth of all layers in the section, it was concluded that 10 flood stages were all caused by obvious increase of precipitation in Lanzhou section of the Yellow River or such branches as Taohe, Datong and Daxia Rivers of that year.