A study of the movements of bed-sediment along azusa river, japan
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概要
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1. Field work was first carried out on the Upper River as the geological materials in the bed-sediment are more easily identified than in the Lower River. However, the Debris Proportions for the confluences in the middle section could not be found due to the width of the active flood plain there and the relatively small size of the tributaires which results in poor mixing of the debris below the confluence. The most important results here are the Sampling Accuracy Tests carried out at Kamikochi. Samples were taken from along a 〓/4 Km stretch of the river which is free from addition of debris for some kilometers upstream. Sampling was conducted on three size ranges, referred to as 10mm. (9.52-15.8mm), 5mm. (4.6-9.52mm.), and 2.5mm. (2.5-4.6mm.) size ranges respectively. At this Station a few samples were taken above and below these sizes, but above this size samples become too bulky to handle by a small field party, and below this size the individual particles become partly mineral rather than rock fragments and it is difficult or impossible to identify. It was established from these tests that a sample size of 2,250 particles gave the required accuracy, and was used as a standard throughout this project. Using this sample size it was concluded that: i. The geological composition can be measured from relatively small samples and that the composition is independant of the grain size distribution. The accuracy of measurement depends upon: (a) ease of identification and skill of operators, (b) random variations in the composition of the bed material which depend upon the river's characteristics (eg. wide flood plains show greater variations than rivers confined between narrow banks), and (c) sample size. How-ever increasing the latter will not overcome errors due to (a) and (b), but too small a sample introduces unacceptably large variations. ii. The grain size distribution varies widely, and a reliable average can-not be found even from large samples. The surface layer of the bed is almost always found to contain a large proportion of larger particles, which form an "insulating layer" against removal of material from below them. These pebbles are too large to be moved by the existing flow and so remain covering the bed, the smaller sizes have for the most part been moved away. This phenomena occurs during floods and prevents the river from carrying its capacity load. iii. A practical difficulty is to allow a long enough stretch of river below confluences to allow for complete mixing of the bed-sediment, without having extra material added to the river below the confluence. Large scale aerial photographs are indispensable in this connection in the field. At Kamikochi the standard deviation of the measured geological compositions of five samples was less than 2.2% in all cases (expressed as% of the whole sample) for the three main geological components, and of this it is estimated that less than 1% was due to errors in identification, the rest being due to random variations in the composition Geological grain size distribution curves are shown on Fig. 4, where the heading inferred refers to sizes below 2.5mm: larger sizes were measured directly. "Ss" corresponds to symbol "P" on Fig. 2; other symbols correspond directly." One other result for the Upper River must be mentioned: Between Station "M" and Kamikochi, no Paleozoic Rocks enter the river, but the concentration of Paleozoic Rocks at Kamikochi is nearly 6% higher, which is greater than the known variations in measurement. This trend has been also found on the Lower River, and is thought to be due to non-uniform transport mechanism during floods owing to the differences in gradient and discharge between the main stream and the tributaries at different stages of the flood and hence their transporting capacities. 2. In order to calculate the Debris Proportions for the Lower River, it was necessary to separate the sandstones, shales and cherts of the Paleozoic sediments; however, results were obtained for all the main confluences on the three size ranges (Step 1), and considerable differences exist between the movement on the different sizes. The accuracy with which this can be done depends upon the differences in geological composition of the bed-sediment between the tributaries in addition to the accuracy of sampling. If this difference is greater than 15%, the error in the results is less than 10%, and in practice better than this as several results can be averaged (for different geological materials). This accuracy is also affected by the relative volume of debris carried by the tributaries and their gradients, both of which should preferably be of the same order, but limits cannot be easily given. 3. The movement of debris along reaches of a river between confluences (Step 2) can be measured if: i. The Debris-delivery Ratio is 100% (i.e. no aggradation or degradation). ii. and one of the geological groups in the bed-sediment does not enter the river along the reach, the percentage reduction of which indicates the volume of material entering the river along the reach. The first requirement is almost never found in nature, in which case it is necessary to calculate or estimate the Debris-delivery Ratio indepen-dently (eg. from bed level surveys). Alternatively, one may assume a Debris delivery Ratio of 100%, and work out the results in terms of the downstream point on the system, in which case the results indicate the source of material arriving at that point. In order to obtain the results in volume per annum, the results in percentage must be correlated with measured Debris Yields somewhere on the river, usually in the form of measured accumulations behind debris control dams. However, such data gives the yield behind the dam but gives no indication of the yield below the dam, and cannot be used directly unless the river was also sampled before the construction of the dam, except in the case of a dam situated at the downstream end of the river system under consideration. There is also the difficulty that the data may not be for the same period as the river analysis, and there are large fluctuations in the yield from year to year. Practical results for the case of the reservoir to be formed behind Nagawa Dam are summarised in Fig. 3. The author has suggested a long-term study of a small catchment area chosen with suitable geological and other conditions in order to clarify some of the problems encountered during a study of this nature.
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