Study On Wind Roads Of Seven Street Trees In Shenzhen Based On Wind Tunnel Experiment

China’s coastal cities have been disturbed by typhoons for a long time.Strong winds have brought losses to urban street trees and posed a threat to urban infrastructure and citizen safety.Therefore,reducing the wind loads of urban street trees and improving their wind resistance have objective research prospects and significance.Taking 7 kinds of trees Bischofia javanica Blume,Schefflera macrostachya(Benth.)Harms,Acacia confuse Merr.,Ormosia pinnata(Lour.)Merr.,Ficus concinna Miq.,Khaya senegalensis(Desr.)A.Juss.and Dracontomelon duperreanum Pierre as experimental materials,this paper study the crown shape and physical structure parameters of different types of trees through crown scanning experiment and wind tunnel experiment,and build the wind resistance mathematical model between wind loads(Y)and five independent variable:the wind speed(X1),the overall projected area of windward face(X2),the leaf area of windward face(X3),the cavity area of windward face(X4)and the windward void rate(X5),predicted and analyzed the model,and summarized the appropriate pruning measures of the seven experimental tree species.The main results are as follows:(1)The canopy types in this experiment can be roughly divided into three categories:(1)Leaf large and crown sparse(Bischofia javanica,Schefflera macrostachya);(2)Leaf small and crown sparse(Ficus concinna);(3)Leaf small and crown dense(Acacia confusa,Ormosia pinnata,Khaya senegalensis,Dracontomelon duperreanum).The results of crown scanning data show that Ficus concinna,Bischofia javanica and Schefflera macrostachya have good cavity permeability.When not trimmed,the crown cavity rate can reach 30%～45%.The single leaf area of Bischofia javanica and Schefflera macrostachya is large,so their total leaf area is also large.Khaya senegalensis.Acacia confusa,Ormosia pinnata and Dracontomelon duperreanum belong to the type of lush crown,with dense branches and leaves.When not trimmed,the crown cavity rate is only 25%～30%.(2)Adjusting the mathematical model of each tree species,the result show that the overall significance of R~2 and the model is good.For each wind resistance mathematical model of each tree species,basically,the wind speed,the overall projected area of the windward side and the leaf area of the windward side are positively correlated with the wind load,and the cavity area and cavity rate of the windward side are negatively correlated with the wind load.From the point of view of model fitting degree,the good model fitting degrees are Dracontomelon duperreanum,Ficus concinna and Khaya senegalensis,and the general model fitting degrees are Ormosia pinnata and Schefflera macrostachya.From the overall situation of test error,the order from small to large is:Dracontomelon duperreanum(7.38%)<Ormosia pinnata(10.27%)<Ficus concinna(10.72%)<Khaya senegalensis(12.14%)<Schefflera macrostachya(13.46%)<Bischofia javanica(20.07%)<Acacia confusa(29.87%).(3)Through the horizontal comparison of the force data of wind tunnel experiment,it is found that the overall trend of the wind roads from large to small is:Acacia confusa>Khaya senegalensis>Bischofia javanica>Dracontomelon duperreanum>Ormosia pinnata>Schefflera macrostachya>Ficus concinna.The tree crown of layered tower type and inverted egg multi cavity type has good air permeability.Under the same wind speed,it can withstand less than 50%or even less wind roads than the tree crown of round egg dense type.Pruning the tree crown can significantly reduce the wind roads on the tree crown.The first pruning can reduce the wind roads on the tree crown by 12%～23%,the second pruning can reduce the wind roads on the tree crown by 22%～37%,and the third pruning can reduce the wind roads on the tree crown by 37%～50%,which proves that the wind resistance benefit of pruning the tree crown is significant.(4)Based on the mathematical model 1,set the wind speed as 12m/s;set the overall projected area of the tree crown as 30m~2 and 40m~2,and calculate the predicted wind load.The results show that under the same wind speed and the same crown area specification,pruning to increase the crown cavity can effectively reduce the wind load on trees in theory:assuming that the initial cavity rate of urban street trees is 10%,when the pruning amount is 10%of the crown area,the wind load can be reduced by 13%～18%;when the pruning amount is 20%of the crown area,the wind load can be reduced by 26%～38%;when the pruning amount is 30%of the crown area,the wind load can be reduced by 35%～47%.From the prediction results,when the crown void rate is 10%,the overall wind load is sorted from small to large as Schefflera macrostachya<Ormosia pinnata<Dracontomelon duperreanum<Bischofia javanica<Ficus concinna<Acacia confusa<Khaya senegalensis,and when the crown void rate is 40%,the overall wind load is sorted from small to large as Ormosia pinnata<Schefflera macrostachya<Acacia confusa<Dracontomelon duperreanum<Bischofia javanica<Ficus concinna<Khaya senegalensis.(5)Based on the above experimental results,specific pruning measures were formulated for the seven tree species.There are two pruning strategies in the action summary:(1)Reduce the overall area of the tree crown and reduce the outer contour size of the tree crown.This kind of tree crown itself has a certain degree of cavity and permeability,so it is mainly to trim the branches and leaves outside the crown,and the crown cavity can be increased appropriately.The applicable tree species of this strategy are Bischofia javanica,Schefflera macrostachya,Acacia confusa,Ficus concinna and Dracontomelon duperreanum.(2)Increase the area of canopy cavity.The internal branches and leaves of this kind of tree crown are dense.The primary task is to trim the internal branches and leaves of the tree crown and increase the permeability of the tree crown.The applicable tree species of this strategy are Ormosia pinnata and Khaya senegalensis.