Simulation Study On Regulation Of Road Rainwater Runoff And Water Purification By Biological Retention Zone Based On HYDRUS Model

With the development of urbanization in China,the nature of urban underlying surface is constantly changing,and the phenomenon of“city looking at the sea”caused by rainfall occasionally occurs.In recent years,the construction of sponge city has not only greatly reduced the amount of rainwater runoff,effectively slowed down the peak flow of rainwater,purified the quality of runoff,but also effectively improved the urban living environment.In China,the proportion of roads in urban underlying surface is as high as 10%～15%,some even more than 20%,therefore,the construction and reconstruction of the roadside bioretention zone in sponge city is particularly important.In this paper,through one-dimensional vertical and two-dimensional vertical migration experiments in laboratory scale,the precision of the effect of biological retention zone on road rainwater runoff control and water purification was studied;the HYDRUS-1D model of one-dimensional vertical migration was established to simulate and study,and the parameter rate was determined;on this basis,the lateral dispersion is introduced to establish the HYDRUS-2D model of vertical two-dimensional migration,and the reliability of the selected parameters is verified;finally,the simulation results of HYDRUS-1D and HYDRUS-1D models with optimized parameters were fitted with the experimental results of vertical two-dimensional migration,it is found that the simulation accuracy of HYDRUS-2D model is high,which can provide the optimization basis for the area,depth and other structural parameters of the bioretention zone,and provide technical support for the construction and reconstruction of urban road sponge,and provide the basis for saving the construction investment.The main research contents and conclusions are as follows:（1）Based on the analysis of runoff pollution characteristics of six times of rainfall（including light rain,moderate rain and heavy rain）in a year in Xi’an City,the characteristics of rainfall runoff pollution in different grades of road sections were mastered;the results show that:there is a significant correlation between the pollution load of road runoff and road grade,and the pollution concentrations of COD,NH₄⁺-N,TN and TP in the runoff rainwater conform to the rule of main road>secondary road>branch boad,and the average pollution concentrations of several pollution indexes of each rainfall are higher than the surface water quality standard of class v;（2）The one-dimensional vertical migration experiment of biological retention zone can regulate the road runoff of Xi’an with return periods of 2,3 and 5 years,and the change trend is relatively stable.When the return period P=2 a,3 a and 5 a,the average delay time of one-dimensional vertical migration experiment is 182 s,155 s and 135 s,respectively;The runoff reduction rates were 58%～74.6%,54.4%～73.3%and 50.7%～65.2%respectively.It has a certain purification effect on rainfall runoff with different pollution loads;The pollutant removal rate decreased with the extension of rainfall duration and the increase of rainfall pollution load;（3）Compared with the one-dimensional vertical migration,the two-dimensional vertical migration has a certain improvement effect in rain peak delay,water reduction and water purification accuracy.When the return period P=2 a,3 a and 5 a,the accuracy of rain peak delay of two-dimensional vertical migration is 16.9%,17.4%and 16.3%higher than that of one-dimensional vertical migration,and the accuracy of water reduction is 7.0%～7.4%,5.1%～5.7%and 7.3%～9.4%higher than that of one-dimensional vertical migration;when the return period P=3 a,the removal accuracy of COD,NH₄⁺-N,TN and TP by vertical two-dimensional migration was 4.46%～5.75%,3.54%～5.85%,5.34%～7.17%and 5.74%～6.82%higher than that by vertical one-dimensional migration,respectively;（4）Based on the bioretention zone test designed in this paper,the parameters of HYDRUS-1D and HYDRUS-2D models were calibrated and verified.Compared with the HYDRUS-1D model,the HYDRUS-2D model considering the lateral migration has higher simulation accuracy for the water reduction and water purification effect of road rainwater runoff in the biological retention zone.