黄河银川平原段河床沉积速率变化特征
摘要: 黄河银川平原河段近几十年来面临较大的洪灾威胁，几次大洪水造成了很大的经济损失，人们常常将这归结为黄河上游修建大坝导致该河段河床沉积增强所引起的结果。为了分析该河段河床的沉积速率的历史变化特征，用地质尺度时段平均方法、90万年以来不同时段的相关性分析方法以及当前河道断面实测方法分别对其沉积速率进行了估算。结果表明，相关性分析方法中的脉冲函数关系可以很好地估算其历史沉积速率的变化趋势，并且自距今90万年以来其沉积速率大致具有增大的趋势，在2万年左右达到峰值3.43 cm/a；自距今2万年以来其沉积速率有减小的趋势；根据该关系式估算的目前河床沉积速率数值为0.51 cm/a，这与河道断面实测方法估算的该河段河床的平均沉积速率为0.3 cm/a比较接近。显然，该河段的现代河床沉积速率远小于历史上最高的沉积速率，近年来水土保持、大坝建设等人类活动使得进入银川平原河段的粗泥沙明显减少，从而降低了该河段的河床沉积速率。
Variations of the Channel Sedimentation Rate in the Yinchuan Plain Reach of the Yellow River
Abstract: In the latest decades it has been confronted with the enhanced flood risk in the Yinchuan plain reach of the Yellow River and occurred several large floods have induced much economic losing. Many researches concluded that these flood disasters were resulted from the three large reservoir (Qingtongxia, Liujiaxia and Longyangxia) construction in the upper Yellow River because of the operation of the reservoirs and the water derivation from the reservoirs and the main stream in the upper Yellow River induced the enhanced channel sedimentation rate in the studied river reach. However, many previous studies in the world have showen that large dam or reservoir construction could induce the enhanced channel erosion or decreasing sedimentation rate in the dwonstream reach below the dam or reservoir. The change trends of channel sedimentation rates in longtime scale which includes anthropogenic dominant period (e.g. last 50 years) and natural dominant period (e.g. 1 000 years ago) could be used to reveal the influences of human activities on the channel change in the last decades. The aims of this study are to reveal the variation trend of the channel sedimentation rates in the Yinchuan plain reach of the Yellow River since 0.9 Ma B.P. and to evaluate whether the human activities have led to increase the channel sedimentation rates during the last decades. The methods used in this study are mainly the caculation of the average rate of channel sedimentation in geomophological scale according to the chronological analysis of a bore core with depth about 300 m, correlation analysis in different time periods since 0.9 Ma B.P., and the comparison of channel bed variation according to the 9 channel cross-sections in the river reach measured in the two different years of 1993 and 2001 that could be uesed to assess the present channel sedimentation rate. The results indicate that the pulse function relationship between years and channel sedimentation rates obtained from the correlation analysis method can be used to estimate commendably the historical variation trend of the channel sedimentation rate in the study river reach in geomophological scale. The sedimentation rate has had an increase trend since 0.9 Ma B.P. as a whole, however, a decrease trend of channel sedimentation rates has occurred since 20 ka B.P. The peak value of 3.43 cm/a appeared in 20 ka B.P., it indictes that the maximum channel sedimentation rate was in 20 ka B.P. The average channel sedimentation rate since 0.04 Ma B.P. calculated from the pulse function is 3.16 cm/a. The present sedimentation rate estimated from this function is 0.51 cm/a which is close to the result of 0.3 cm/a caculated from the actual measurement of the channel cross-section variation. Obviously, the present channel sedimentation rate is far less the maximum sedimentation rate in the geomophological scale as well as the average channel sedimentation rate since 0.04 Ma B.P. The human activities, such as the water and soil conservation, large reservoir or dem constructions, and water derivation in the main stream of the upper Yellow River drainage basin, have decreased a lot of coarse sediment feeding to the Yinchuan plain reach of the upper Yellow river and decreased the channel sedimentation rate in the studied river reach.
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