暑熱適応策とヒートアイランド緩和策からみたオープンスペースにおける樹木配置の最適化と影響評価

抄録

 Urban trees as a heat adaptation measure by solar radiation shielding and as a heat island mitigation measure by transpiration are expected to improve the summer thermal environment. An increase in the amount of trees leads to an increase in the initial cost for tree planting and in maintenance costs such as pruning and irrigation. It is thus important when planning to green urban open spaces to consider the optimal tree arrangement to maximize the effect with the minimum amount of trees. However, very few studies have discussed the effect of the tree arrangement on the thermal environment. Therefore, we carried out tree arrangement optimization to both minimize the summer mean radiant temperature (MRT) as an urban heat adaptation measure and maximize the transpiration rate as a heat island mitigation measure. Based on optimization, we showed to what extent the MRT and the transpiration rate change by the arrangement and what kind of features of the arrangement contribute to improving the effect of the trees.

 The target of optimization was an open space with no buildings around it with a 36 m × 36 m tree arrangement area and a 12 m × 12 m evaluation area in the center. The design variable of optimization was the two-dimensional tree arrangement in the 36 m × 36 m area. To derive tree arrangements with the maximum effect by the minimum number of trees, we conducted optimization to minimize both MRT and vegetation coverage ratio (number of trees) in the evaluation area as the objective functions, focusing on urban heat adaptation. We also conducted optimization to maximize the transpiration rate and minimize the vegetation coverage ratio in the evaluation area, focusing on heat island mitigation. We calculated the MRT and the transpiration rate using typical sunny summer weather in Tokyo. We conducted numerical optimization and derived Pareto solutions.

 As a result, we obtained the following findings for typical sunny summer sunny weather in Tokyo at a location with no building around it. Comparisons between the effective and ineffective tree arrangements obtained by optimization showed the daily average MRT differed up to 7.5°C and the amount of transpiration differed up to about 20%, even when using the same amount of trees. This result shows that tree arrangement could have a non-negligible impact on the effects of urban heat adaptation and heat island mitigation. The arrangements of the optimal solutions for the MRT and the transpiration rate showed the same tendency, suggesting that similar tree arrangements could realize the maximization of effects for both urban heat adaptation and heat island mitigation. The tree arrangement of the optimal solutions showed that distributed arrangements with separation between trees are effective for reducing the MRT and increasing the transpiration rate, and that separation between trees in the east-west direction is more effective than separation between trees in the north-south direction. The difference of the MRT and the transpiration rate between effective and ineffective arrangements was large when the vegetation cover ratio was around 50%, suggesting that consideration to tree arrangement at the time of green planning is particularly important. The gradient of the Pareto solutions in the evaluation space was smaller when the vegetation coverage ratio was larger than 50%, suggesting that the cost performance of urban heat adaptation and heat island mitigation by trees decreased above a 50% vegetation coverage ratio.