昭和58年7月豪雨による益田開拓農地造成域の降雨・流出特性

The western parts of Sanin District were suffered severe disasters from flood caused by the record heavy rainfall on 20-23 July, 1983.
This paper discussed the rainfall and runoff characteristics caused by the heavy rain fall in the Otonaga River basin of Masuda reclamation land located in the hilly area of Masuda City.
Based on the observed rainfall data in the Otonaga River basin, it was found that the records of total and maximum daily rainfalls were both renewed and the daily rainfall exceeded the expected value of 200 years retern period in Masuda City.
The observed rainfall intensity of short time during 1-12 hours also exceeded the expected value of the planning scale of drainage channel systems. Consquently, it became clear that the severe disasters originated mainly in the unheard heavy rainfall in this district.
Nextly, using the hydrographs of flood water level and discharge on 20-23 July at the dam site of Otonaga River, the parameters of runoff characteristics were evaluated 0.96 in the peak runoff coefficient and 160 in the coefficient of formula for concentration time of flood. While, the concentration time of flood was 25 minutes and the mean effective rainfall intensity during the that time was 99.9 mm/h.
Moreover, the specific peak discharge of flood was 27.8m^3/s/<km>^^^2. And its value was nearly plotted on the enveloping curve for regional flood peaks in Sanin District. Therefore, it was concluded that the magnitude of peak discharge recorded in the experimental basin was the class of the regional maxima of flood peak in Sanin District.