Development of Estimation Method for Wind Load on Forest Trees

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Meteorological observation around the Japanese cypress canopy was carried out in the experimental forest of Shinshu University, Minami-Minowa village in Nagano prefecture, Japan. We measured wind load on trees from wind speed profile and momentum flux observation. The results are :1)The momentum flux is constant for height over the forest canopy and is absorbed in the upper part of the canopy.2)Peak of wind speed was observed in the trunk space of the forest. Therefore, we cannot neglect the momentum flux beneath the canopy.3)The momentum flux is constant at the same height for the forest.Using these results and wind speed profiles, the relationship between drag coefficient and LAI of the forest canopy was investigated. Larger wind speed makes the coefficient smaller and the critical value is 0.2 for the canopy. The value is smaller than that for Picea abis and Tsuga heterophylla because we underestimated momentum flux under the canopy. Considering the result above, we proposed the equation to estimate the drag coefficient from roughness, displacement height, tree height and LAI.We tested this equation for three types of forests and the results were agreeing in the practical levels. Next, we simplified this equation with tree height and LAI.It is almost sufficient to estimate the coefficient.