Research On The Application Of Urban Trees In Sponge City Construction Of Old City

Trees affect the production of stormwater runoff through interception,transpiration and infiltration.At present,the sponge city construction of some old urban areas is not only facing the difficulties of insufficient greening space,complex underground pipelines and high cost,but also due to the special historical significance of the old urban areas,taking into account the strict protection of the heritage and environment of the old urban areas.These problems have become bottlenecks affecting the construction of sponge city in the old urban areas.If we can make full use of the function of trees to reduce stormwater runoff,we can not only alleviate the urban water environment problems facing the old city to a certain extent,but also help to create a green,ecological and livable living environment,so as to protect the heritage environment better.Firstly,aiming at the problem that the function of urban trees in reducing stormwater runoff is not paid enough attention in the construction of sponge cities in China,the experience of urban tree management in basic research,engineering practice and policy guarantee in the United States are analyzed from the perspective of stormwater management through comparative study,and the phases of stormwater management in tree management regulations,planning and standards are analyzed in depth.It will provide reference for promoting the application of trees in the construction of sponge cities in China.Secondly,on the basis of previous research on urban tree species in China,taking the old urban area in the Second Ring Road of Beijing as a representative,field investigation is carried out on different types of green space to comprehensively understand tree species selection,crown width,underlying surface status of tree canopy cover and tree planting location.The influence of different tree species and planting location on tree canopy cover is analyzed.Photoshop pixel method is used to calculate the existing tree canopy cover of representative old urban areas in China.Taking 19 historical and cultural areas in Beijing as samples,potential tree canopy cover is analyzed to understand the potential of tree canopy improvement in old urban areas,and estimation methods of tree planting under different tree canopy goals are proposed.The results show that although the tree canopy cover are generally low in the old urban areas,there are still room for the growth of tree canopy in small and medium-scale sites if tree species and planting spacing can be reasonably selected.Thirdly,based on the existing research results of tree canopy interception capacity at home and abroad,the estimation methods of stormwater runoff reduction effect under different tree canopy and underlying conditions of tree canopy are proposed.With the existing tree canopy in the old urban area and the design rainfall corresponding to the 70% annual total runoff control rate,the stormwater interception rate of tree canopy in different areas is between 2%-8%.However,in the small and medium-scale sites such as municipal roads,residential areas,historical and cultural blocks,which may reach a higher canopy coverage rate,the stormwater runoff reduction capacity will be greatly enhanced to 11%-27%.In addition,the ability of trees in different planting sites to intercept stormwater is significantly different.Under the condition of certain rainfall,the more impervious the crown covers,the stronger the interception ability.This also provides a reference for the selection of trees and the optimization of planting location in the future.Finally,in order to give full play to the role of trees,the shortcomings of existing greening laws,special planning and standards in China are analyzed.It is pointed out that there is a lack of detailed analysis of urban underlying surface and tree canopy,insufficient attention to the ecological functions and related indicators of urban tree canopy,and a lack of consideration of stormwater runoff control in the selection of tree species and planting location.Some problems and corresponding suggestions are put forward.