黄河下游辫状、弯曲和顺直河段间沉积动力特征比较

Comparison of Depositional Dynamics among the Braided,Meandering and Straight Channel Reaches in the Lower Yellow River

黄河下游沿程出现了辫状、弯曲和顺直河型段,河道平面形态的差异势必受到不同的沉积动力特征的制约。本研究根据对收集到的有关实测资料的统计和计算,对可以表征河道沉积动力特征的相关指标进行了量化分析,发现不同河型段间的沉积动力特征存在明显差别。河道比降在辫状河段最大,介于0.2‰～0.14‰之间;弯曲河段的次之,介于0.14‰～0.10‰之间;顺直河段的小于0.10‰。平滩流量时的流速在辫状河段的较大,不同年份间的变幅较大;弯曲河段的次之,不同年份间的变幅较小;顺直河段的最小,不同年份间的变幅不大,并向下游显著减小。单位河长能耗率在辫状河段最大,弯曲河段次之,顺直河段的最小,其间的比值为2.31:1.35:1。单位面积能耗率在辫状河段最小,弯曲河段最大,顺直河段次之,其间的比值为0.52:1.18:1。河床沉积物粒度较细,并普遍缺失推移质组分,中值粒度在辫状河段、弯曲河段和顺直河段分别为3.0、3.2、3.67;河床沉积物的分选性也依次变好。受制于上述相关沉积动力特性,河床沉积速率在不同河型段有明显差别:中大流量时,辫状河段的最小,弯曲河段的最大,顺直河段的次之;在小流量时,三个河型段的河床沉积速率非常接近,但沿程略有减小。另外,辫状河段河床沉积速率明显依赖于流量的变化,且随流量的增大而明显减小。

There are braided,meandering and straight channel reaches along the lower Yellow River.The difference in channel planform certainly is influenced by different depositional dynamics.In this work,according to a series of data collected from the yearbook of the water and sediment of the Yellow River,the different indexes of representative channel depositional dynamics has analyzed.The results show that,there are evident differences in depositional dynamics among the braided,meandering and straight channel reaches in the lower Yellow River.The channel gradient is the maximum for the braided reach,which ranges 0.2-0.14,is moderate for the meandering reach,which ranges0.14-0.10,and the minimum for the straight reach,which less than 0.10%.The flow velocity of the bankfull discharge is also larger for the braided,moderate for the meandering and least for the straight reaches.Its variational extent decreases downwards along the three channel patterns.The gross power of stream flow is decreases from the braiding via meandering to straight reaches and its ratio is 2.31∶1.35∶1.The specific power of stream flow is minimum for the braided,maximum for the meandering and moderate for the straight reaches and its ratio is 0.52∶1.18∶1.The channel bed sediment is relatively fine and generally lacks the roll component.The median grain size is 3.0,3.2 and 3.67 for the braided,meandering and straight reaches,respectively,furthermore,the sediment sorting is also enhanced along the different channel reaches downwords.Be enslaved to the abovemationed depositional dynamics,the sedimentation rate on channel bed is different among the three channel pattern reaches.It is the minimum for the braided,maximum for the meandering and moderate for the straight reaches when discharge is moderate to large,and it is approximative among the three channel pattern reaches,but a decrease tendency downwards,when discharge is small.In addition,the sedimentation rate on braided channel bed evidently depends on the variation of discharge,and decreases with increasing discharge.