昭和46年度台風による一ツ瀬川水源地域の被災状況

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タイトル別名
  • Damages at the Source Head of Hitotsuse-River by the Typhoons of 1971
  • ショウワ 46ネンド タイフウ ニ ヨル ヒトツ セガワ スイゲン チイキ ノ ヒサイ ジョウキョウ

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Serious damages occurred in the area of headwaters of Hitotsuse River in Miyazaki prefecture extending 10,000 ha., which areas are those including the Miyazaki District University Forest of Kyushu University caused by the typhoons No. 19 and No. 23 in 1971. Land slide damages as much as 400 places were recognized by means of aerial photographic interpretation. Total amount of the landslides was estimated about 200,000 m^3, countless forest roads were cut, and two check dams were destroyed. It is thought that most of these damages are due to the unprecedented storm rainfalls amounting to 1,300 mm and 700 mm in No. 19 and No 23 typhoons respectively, but it is recognized remarkable difference of damaging degrees in various districts and it is construed from the difference of natures in various soils. Damages are developed remarkably in the area of topographic features where gradients show the angle of 30～40 degrees. From our investigations on land slides occurred in cutting slope of forest road between Okochi and Ozaki, we recognized most of land slides tend to arize in newly earthworked zones and within 50 m from line of valley center. Considering roadway diagraph problem, failure percentage has been increased at the places such as the length of cutting slope of 4.77m, gradient of ground surface of 31.5 degree and cutting area of 13.54 m^2. Highest failure percentage was recognized in the conditions as follows: length of cutting slope 8.50 m, gradient of ground surface 42.5 degree, cutting area 32.5 m^2. In the Iwaya valley consisted of granite bed mostly, a land slide occurred in the scale such as inclines of 300 m, width of 50 m and soil amount of 50,000 m^3. Land slide flew down as debris flow having velocity of 4～11 m/sec and it caused considerable erosion at the place of steep slope of valley floor, but at gentle slopes contrary made the debris depositin. One check dam seemed to have been damaged gradually for several years, another was thought damaged by single attack of debris flow, and our two dimension calculation of stability indicated the area of cross section was insufficient for debris flow.

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