台風9117,9119号による森林被害の要因解析

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平成3年9月に九州北西部をかすめ,東北へと進路をとった2つの台風,17号と19号によって,とくに九州中北部に甚大な森林被害が発生した.この台風の強風による森林被害について,その発生に関連が大きいと考えられる要因群,強風,マクロ,ミクロな地形と樹木などの条件について分析を加えた.平均風速と瞬間最大風速の関係を明らかにし,最大風速発生時近傍の風速について,瞬間風速の推定を行った.被害の発生に対して強風の持続時間,斜面方位,斜面内位置,標高,樹齢と樹形などが強い関係があることが分かった.瞬間最大風速の大きさより,その時間帯近くの強風の平均風速と被害発生率の上限値との間に強い関係が認められた.一定方向の斜面で高い発生率を示し,この斜面方位は強風時の風向とほぼ一致した.同一山腹斜面内では比較的下部で,標高についてはその地域の中,下部で高い発生率を示すなど風害発生の恐れの高い斜面位置の推定が可能であることが示唆された.幼齢の林分で被客の発生は少なく,樹齢が20～40年生位以上の森林に高い被害率が見られた.樹冠長,胸高直径,樹高等によって,生じた被害の類型が異なることが分かった.被害類型の中,折損,倒伏について力学的解析を加えた.理論的折損高と実測折損高との差異について考察を加え,受風時の樹体の変形による影響を考慮する必要があることが示唆された.倒伏のメカニズムについては,根系部の倒伏モ-メントに対する抵抗モーメントを,土塊重量によるものと根鉢周辺部のせん断抵抗によるものについて検討し,両機構が成立する可能性が示唆された.Forests in northern and central Kyushu were severely damaged by typhoons 9117 and 9119 in September 1991. The two typhoons both traveled northeastward after skirting the northwestern part of Fukuoka Prefecture. Fukuoka, Oita and Kumamoto prefectures were chosen as target areas for study. Factors affecting forest damage were analyzed, including wind velocity and direction, topography and types of tree damage. (1) In order to demonstrate the relation between average wind velocity and momentary maximum velocity, wind velocity in the target areas was estimated during periods of the highest wind velocity. (2) It became clear that the occurrence of the damage was considerably influenced by such factors as the duration of high velocity wind, aspect of slope, position on the slope, altitude, tree age and form. (3) The upper limit of the occurrence rate of forest damage was more significantly affected by average wind velocity during periods of high velocity than by momentary maximum velocity. (4) Severe damage occurred on slopes with certain aspects, the direction of which coincided with the direction of high velocity wind. (5) It became clear that the damage occurrence rate was higher on slopes at lower positions and in areas of lower altitude. It is therefore suggested that it may be possible to predict the likelihood of damage at various positions. (6) The occurrence rate in young stands was low, and high in stands over 20 years old. (7) It was observed that the types of the damage were affected by tree height, diameter breast high and length of crown. (8) Mechanisms of stem breakage and overturn were analyzed statically. Significant differences between calculated and observed breakage heights were detected. It is suggested that the deformation of trees as a result of high wind velocity must be considered. In the overturn phenomenon, a balance must be considered between the moment of power induced by wind blowing on a crown and the resisting moment of the root system. With regard to the resisting moment, two cases were investigated. One moment was the product of the total weight of the root system and the horizontal radius of the root system. The other was the product of the shearing stress and the average radius of root system calculated three dimensionally from the length of the root system. Therefore, the possibility of two mechanisms was suggested.

台風9117,9119号による森林被害の要因解析

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