Analyses of the Wind Damage Caused by Typhoon 15 in 1981 at Tokyo University Forest in Hokkaido

Abstract

東京大学北海道演習林において，1981年夏の台風第15号により大規模な風害が発生した。そこで筆者らは，数量化による多変量解析の手法を用いて，被害率と地形および林況要因との関係を検討した。地形要因には，標高，露出度，斜面方位，斜面上の相対位置，斜面形状，傾斜角，土壌深をとり上げ，地形図および土壌図から読みとった。林況要因には，森林率，林冠疎密度，高木層のみの林冠疎密度，針葉樹混交率をとり上げ，空中写真から読みとった。多変量解析に先立って，被害率と各要因とを個別に比較した結果，露出度と斜面方位が被害発生に大きく関与することがわかった。数量化II類による判別分析では，無被害と小，中，激害の正判別率がいずれも70パーセント以上となり，かなりよく判別された。数量化I類による重回帰分析では，重相関係数は0.60であった。これらの多変量解析の結果から以下のことがわかった。i）周囲の3/4以上が露出していてかつ，斜面方位が暴風の主風方向を中心に左右約70°の範囲にある林分が，とくに大きな被害を受けた。ii）傾斜がゆるやかな林分および針葉樹混交率が高い林分も，概して被害を受けやすい傾向にあった。また，重回帰分析の予測値を用いて，調査地内の風害発生予測図を描くと，一部の地域を除いて実測値の分布図とよく一致した。The forests in Tokyo University Forest in Hokkaido were severely damaged by typhoon 15 in 1981. We tried analyses of the relation between wind damage and some topographical and certain stand condition factors by consulting topographical map, soil map and air photographs. Seven items for topographical factors―altitude, exposure, facing of the slope, relative position on the slope, land type on the slope (relief), inclination, soil depth―and 4 items for stand condition factors―forest area percentage, canopy density of trees higher than 4m, canopy density of trees higher than 14m and mixture percentage of conifer trees―were selected for quantification analyses. On the quantification analysis Hayashi-2, correct discrimination percentage between no damaged areas and damaged areas was more than 70 per cent. On the quantification analysis of Hayashi-1, multiple correlation coefficient was 0.60. Among the 11 items, exposure and facing of the slope were significantly related to the occurrence of wind damage. The following stands were severely damaged: those which face to the main direction of the stormy wind in the range of approximately 70 degree from the center. Generally, it is recognized that stands which are on gentle slopes or have high mixture percentage of conifer trees were easily damaged. As the result of drawing a predictive map of wind damage with the predictive scores of Hayashi-1, the map is generally fitted to the distribution of wind damage percentage of actual scores with some exceptions