Study on Forest Damage at Sado Island by Strong Wind

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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.