著者

斉藤 享治

出版者

埼玉大学教育学部

雑誌

埼玉大学紀要. 教育学部 (ISSN:18815146)

巻号頁・発行日

vol.63, no.1, pp.185-192, 2014

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There is a discussion whether megafans are alluvial fans or not. It is difficult to define whether the formation processes of megafans are the same as those of alluvial fans or not, because the surface deposits are sometimes different from the fan deposits. However, strong relationships between drainage basin areas and fan surface areas exist in alluvial fans with the same processes. Kesel (1985a, b) stated that alluvial fans in the General Valley of Costa Rica are larger than many arid fans with comparable drainage basin areas and that the position of the Kosi megafan closely coincides with the trend line of the Costa Rican fans. The statement suggests that the megafan is a member of the alluvial fans. Hence, the relationships between drainage basin areas and fan areas, including the Costa Rican fans are examined in this study. Alluvial fans over 2 km2 in area and steeper than 0.002 in mean fan slope were collected, and the drainage basin and fan areas were measured by use of 1: 50,000 topographic maps. In Costa Rica there are 18 alluvial fans, including four fans in the General Valley. The regression line between drainage basin areas and fan areas is shown by Af = 0.59 Ad0.71, where Af and Ad mean fan area and drainage basin area respectively. The regression line in Costa Rica is similar to those in Japan, Taiwan, the Philippines and New Zealand. For the comparison between alluvial fans and megafans, the Kosi megafan, Okavango fan, and five megafans in the Gran Chaco to the east of the Andes are chosen. The positions of seven megafans are beyond regression lines of five regions, including Costa Rica. Seven points are around regression lines in the San Joaquin Valley by Bull (1964) and in the General Valley by Kesel (1985a). The fan areas by Kesel are overestimated in the General Valley. Therefore, the comparison between drainage basin areas and fan areas in the San Joaquin Valley will be required in future.