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本文研究了密西西比河下游河道裁弯工程前(19世纪80年代至20世纪30年代)、后(1943年至1992年)河道比降与水流功率的变化;研究河段上自新马德里(密苏里州)下至纳齐兹(密西西比州),全长约900km;具体分析了裁弯工程前、后6个主河段及13个次河段的河道演变。(1)研究表明:研究河段目前的河道比降与水流功率均较裁弯前有较大幅度的增加,其中增幅最大的是富尔顿(田纳西州)至莱克普罗维登斯 (路易斯安娜州)河段,其河道比降与水流功率分别增加了27-36%和20-38%;裁弯工程上、下游河段的河道比降与水流功率也有所增加,但其增幅相对较小。(2)过去研究表明:1932年之后,研究河段的床沙非但没有粗化,反而总体发生了某种程度上的细化;由于密西西比河下游河道不是缺沙河道,因此,水流功率的增加和泥沙中值粒径D50 的恒定不变只能通过床沙质输沙量的增加来予以补偿,以达到河道新的输沙平衡。(3) 过去研究还表明:20世纪密西西比河的河道输沙量是减少的,但由于这一结论主要是依据于实测总输沙量的分析结果,因此并不能据此认为下游河道床沙质输沙量也相应减少了;本文研究表明:裁弯后的床沙质输沙量大于裁弯前,这是因为:裁弯工程直接导致的次河段水流功率的增加,造成了下游河道的冲刷,从而增加了下游河道的床沙质输沙量;事实上,裁弯工程实施后,在通过河道演变调整达到目前新的动态平衡过程中,这部分新增的床沙质输沙量起着十分重要的作用。(4)研究河段目前的平衡状态可概括为: 上游河段保持动态平衡,中游河段通过极度冲刷、下游河段通过淤积达到新的动态平衡; 仅考虑河道比降与水流功率的变化并不能完全解释裁弯后研究河段的河道演变趋势,还必须同时考虑裁弯工程上游河段河道演变引起的上游河道来沙量的变化。
In this paper, the changes of river channel slope and current power before the river cutoff project in the lower Mississippi (from the 1880s to the 1930s) and after (1943 to 1992) were studied. From the New Madrid ) To Natchez (Mississippi), with a total length of about 900km. The evolution of the river channels in the six main and 13 sub-river sections before and after the tail-bending project was analyzed in detail. (1) The study shows that the current river channel current and river current power have a more substantial increase than the before cut, the biggest increase is Fulton (Tennessee) to Lake Providence (Louisiana) State river), the river channel drop and current power increased by 27-36% and 20-38%, respectively. The river channel drop and current power in the upstream and downstream sections of cut-bending project also increased, but the increase was relatively small . (2) Past studies show that, after 1932, not only the bed of sediment in the study section has not been roughened, but an overall refinement has been made to some extent. As the lower Mississippi River is not a river with a lack of sand, the increase of water flow power And the constant sediment median diameter D50 can only be compensated by the increase of bed sediment transport, in order to reach the new balance of sediment transport. (3) Past research also shows that the sediment discharge of the Mississippi River decreased in the 20th century. However, since this conclusion is mainly based on the analysis of the measured total sediment load, it can not be considered that the sediment of the downstream channel bed The results of this paper show that the sediment discharge of sandy bed after cutting is larger than that before cutting, because the increase of water flow power in the sub-river directly resulted from the cut-bending project, In fact, after the cut-off project is implemented, during the course of the adjustment of the river course to reach the new dynamic balance, the newly added amount of sandy sediment transport A very important role. (4) The current equilibrium state of the study reach can be summarized as follows: the upstream reach maintains a dynamic equilibrium, the mid-reach reach the new dynamic balance through sedimentation, and the lower reaches only reach the new dynamic balance; We can not fully explain the evolution trend of the river course after cutting the bend, and we must also consider the change of the sediment yield of the upper reaches caused by the evolution of the upper reaches of the tail bend.