大野川上流域の水文特性に対する一考察
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概要
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Reconaissance survey was carried out to investigate hydrologic characteristics of flood flow events occur-ring in the upper basin of Ohno river, Oita prefecture. The studied area had suffered devastating floods on three different occasions since 1983,in July, 1983,July 2,1990 and September 3,1993. Until then, the studied area had not experienced any serious damage by floods for nearly 90 years. Before 1983,the oldest record of flood damage dates back as far as 1870. These three flood events were caused by storm rainfall due to Bai-u or typhoon. Heavy rainfall attaining more than 50 millimeters per hour caused flood flows in the upper basin of Ohno river. As a result, in the urban area of Taketa city, many houses were innundated and railroad bridges and road bridges were destroyed or washed away. Among these three floods, the first and third ones were due to Bai-u or frontal rainfall and second one was due to typhoon #9313. Unexpectedly, the flood events did not always result in disaster. For example, there was heavier rainfall on August 26th, 1982,(430.0 millimeters per day), the studied area had no serious damage from this heavy rainfall. This might be explained by the distribution of hourly precipitation and amounts of effective rainfall. The details of hydrogeomorphic characteristics of the drainage basin were already described in the previous paper, which readers may refer to concerning these data. The annual change of precipitation and runoff at Miyado and Namino stations are shown in Fig. 2. The maximum ratio of runoff rate was 75.55% of 1980 and minimum rate was 47.4% of 1985. Judging From Fig. 2 there is a increasing tendency for annual precipitation. As has already been pointed out by Kayane and Takeuchi (1971) that the runoff ratio exceeding 80% is doubtful and annual evapotranspiration is fairly constant, say, 800millimeters in the upper basin of Ohno river. Fig. 3 shows the monthly change of precipitation and runoff height. Although seasonal change can clearly be recognized, there is a slight lag between change of precipitation and that of runoff height. The streamflow discharge of Ohno river decreases downstream from Inukai to Shiratakibashi. This phenomenon can be explained by influent seepage to the alluvium of the downstream area. If we denote probability density function as f(x), then exceeding probability of W(x) is expressed as follows ; [numerical formula]Return period T is defined as [numerical formula]where n is the number of years, in this case 24 years. Twenty-four consecutive years (1974-1997) of daily rainfall and runoff data lead to a total of some 17520 input-output pairs for the drainage basin. A Hazen plot of daily maximum rainfall of each year on probability paper is as shown in Fig. 5. From the regression line of this figure the maximum daily precipitation of 400 millimeters will occur only once in 100 years, that is, a return period of 400mm per day could be 100 years and 99% of daily precipitation will not exceed this value.
- 敬愛大学・千葉敬愛短期大学の論文