Study on Forest Damage at Sado Island by Strong Wind

本文对1991年第19号台风在佐渡岛造成的森林破坏进行了研究，主要研究了当时大风的特征和地形对阵风的影响。在本研究的早期阶段，从森林受损的第一印象就很难确定主风的方向，因为拔除的树木和折断的树干在高地地区以不同的方向躺着。为进行后续研究，从多个气象观测站的记录和对低地大风吹来的盐雾对日本冷杉(Crypromeria Japan Onica)枯叶的实地调查中，考察了台风过境该岛附近的风特征。通过对样点的详细实地调查、地形分析和力学分析，可以估计佐渡岛大仓河流域台风成熟的时间和地点。强风对森林造成较大破坏的地理特征是：(1)沿着深谷形成的风道交界处，气流的聚集形成了强风；(2)地形裸露区域，高大的树木，风从任何方向吹来；(3)当大风连续吹时，形成垂直涡流的陡峭的斜坡特征。垂直涡旋使风的下流比上流更容易使树木顺着坡面向下移动。

The forest damage which had been caused by Typhoon 19th in 1991 at Sado Island was researched in this study, the main results of which were the general aspect of the feature of strong wind at that time and the topographical influence on the gusts that injured the trees. At the early step of this study, it was not easy to decide the direction of main strong wind from the first impression of the aspect of damaged forest, where the uprooting trees and snapping stems lay down in various directions at upper land area. For the purpose of subsequent study, the feature of the wind of typhoon passing through near the island was investigated from the records at several meteorological observation stations and the field investigation of withered leaves of Japanese ceder (Crypromeria japonica) by the salty spray which was blown by the strong wind at low land area. After the detailfield surveying at sampling spots, topographical analylsis, and mechanical consideration, it could be estimated when and where were the traces of gales in mature of the typhoon at the basin of Ohkura River at Sado Island.The geographical features that the forest was much damaged by strong wind were as follows ; (i)the junction of the wind channels which were made along with the deep valleys, where covergence of air flow made the strong wind force.(ii)the topographical exposure area with tall trees where wind blew from any direction, and(iii)the feature with steep slpoe which generated the vertical vortex of air flow when the strong wind blew continuously. The vertical vortex makes the down-flow of wind which logdes down the trees at slope more easily than the up-flow.