Abstract:
According to the field investigation and grain composition analysis of 81 samples of the section on high floodplain of the Yellow River in Lanzhou city, the sediment characteristic of the section and the paleo-flood changes were studied. Results indicated that the flood sediment layers of the Yellow River are obvious in Lanzhou area, which reflects clearly the variation of the flood and short-term precipitation with a high resolution, and is a good index indicating the changes of the flood and precipitation. The crude grain of the sediment indicates the bigger scale flood and the more precipitation at that time, and the thin grain indicates the smaller scale flood and the less precipitation. The LC section is divided into 10 sediment layers, which indicating 10 bigger-scale flood stages, and the sand grain are composed mainly of the coarse silt, the very fine sand and the fine sand, the colloidal particle, the cosmid, the fine silt, the medium sand and the coarse sand are all less. The 10 sediment layers indicate 10 floods with different scales. The grain composition of the whole section has obvious change rule from above to bottom. Sufficiently considering the effect of landform heightened by the aggradation on grain composition, the order of the depths and scales of floods on the high floodplain of LC section from big to small is the 10th, 8th, 9th, 1st, 2nd, 7th, 3rd, 5th, 4th and 6th stage. The flood depths on the high floodplain are above 2.2 m when the 1st, 8th, 9th and 10th layers formed, and are lower than 2.2 m when the 2nd, 3rd, 4th, 5th, 6th and 7th layers formed. The flood sediment is not continuous, each of 10 floods generally indicates transitory rainy season of one year, and in the interval of every two adjacent floods there are no floods or small scales floods, indicating there are normal or less precipitation. According to a little precipitation in Lanzhou city and its upper area in the Yellow River drainage basin, it is concluded that the 10 flood stages are caused by obvious increasing of the annual precipitation.
