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TEMPORAL AND SPATIAL CHANGES IN BRAIDING INDEX OF ALLUVIAL RIVERS IN JAPAN AND THEIR REASONS

日本冲积河流辫状指数时空变化及其原因

基本信息

批准号：
09680181
负责人：
SUZUKI Takasuke
金额：
$ 1.86万
依托单位：
Chuo University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 1999
项目状态：
已结题

项目摘要

Channel forms of alluvial rivers in the Meiji Era of Japan, I.e. 1870〜1910's, is assumed to be under the nearly natural condition, because no large dams were constructed before and during the Meiji Era. The temporal and spatial changes in channel braiding and sinuosity of 34 alluvial river during the past about 100 years were examined with morphometry using 1/50,000 topographic maps published repeatedly four or nine times in the different years since 1870's. The results are summarized as follows. 1) in the Meiji Era, the maximum value of braided index of a river tends to appear in the channed span of 1×10ィイD1-3ィエD1 in channel gradient, I.e. at the fan margin where the mean grain size of river sediment decreases from gravel to sand. 2) In the present, as compared with the data for the Meiji Era, the maximum value of braided index has increased markedly in the fan and appears at the mid-fan of 3×10ィイD1-3ィエD1 in channel gradient. Whereas the maximum value has decreased markedly in the large plains composed of fan, meander plains and delta, but it appears at the same area as that in the Meiji Era. 3) After the construction of weirs, the braided index increase in the upstream channel of the weirs in fan, but a little effect of weirs on the braiding appears in the meander plains. 4) The maximum value of sinuosity tends to appear in the channel span of 1×10ィイD1-3ィエD1〜1×10ィイD1-4ィエD1 in channel gradient. This threshold gradient is larger than those in the Mississippi River. These temporal and spatial changes in channel braiding and sinuosity is considered to be resulted from the river improvement and dam construction since the 1960's in particular.

日本明治时代冲积河流的河道形态，即1870〜1910年代，被假定为接近自然条件，因为明治时代之前和期间没有建造大型水坝。利用1870年代以来不同年份重复出版四、九次的1/50,000地形图，用形态测量法研究了34条冲积河近100年来河道辫状和蜿蜒度的时空变化。结果总结如下。 1）明治时代河流辫状指数最大值往往出现在河道坡度1×10ィイD1-3ィエD1的河道跨度处，即在扇缘，河流沉积物的平均粒度从砾石减小到沙子。 2）目前，与明治时期的数据相比，辫状指数最大值在扇内明显增大，并出现在通道梯度3×10ィイD1-3ィD1的扇中部。而在由扇形平原、曲流平原和三角洲组成的大平原中，最大值明显下降，但出现的区域与明治时代相同。 3)堰坝施工后,扇形堰上游河道的辫状指数有所增加,而河曲平原地区堰对辫状指标的影响较小。 4) 弯度最大值往往出现在河道梯度中1×10ィイD1-3ィエD1〜1×10ィイD1-4ィエD1的河道跨度内。该阈值梯度比密西西比河的阈值梯度大。河道辫状和蜿蜒度的这些时空变化被认为是特别是自 20 世纪 60 年代以来河流改善和大坝建设的结果。