| The impervious area in urban areas,represented by asphalt pavements,has increased significantly with urbanization.As asphalt pavement absorbs solar radiation in summer,it can lead to substantial warming,which in turn significantly raises the surface runoff temperature.When high-temperature runoff directly feeds into urban receiving water bodies,it may cause thermal pollution,which may have serious impacts on the inherent ecological communities of the water bodies.Some source control practices such as vegetative-filter strips,rainwater gabions,grassed swales,underground infiltration ponds,permeable pavement,bioretention,and other green infrastructure with the function of promoting infiltration effectively control thermal pollution while reducing runoff volume and improving runoff quality,among which bioretention are most widely used.In this study,laboratory simulation experiments were conducted to simulate actual bioretention,and experimental soil columns were set up using the principle of similarity.The construction and rainfall characteristics were changed separately to quantify the effectiveness of bioretention in reducing the thermal pollution of stormwater runoff under different design parameters.The rainfall was simulated by an artificial rainfall system,and the discharge temperature,as well as runoff temperature of different layer of bioretention,were recorded using sensors,and the inlet and outlet flow rates were measured using flow meters.Field experiments and monitoring were conducted based on laboratory experiments,and three typical sites of the built bioretention in the Beijing Yizhuang Economic Development Zone were selected for field monitoring from June to September 2021.The average runoff temperature(EMT)and heat load of different rainfall events were calculated,and the average temperature of inflow as well as outflow during the whole process of each rainfall event was analyzed.Based on the continuous field monitoring of rainfall flow and temperature data during the rainfall process,the relationship between runoff volume,stormwater runoff temperature,and the thermal load was studied.Based on the field monitoring data and experimental data,combined with the rainwater runoff control mechanism of bioretention and the heat exchange mechanism in the process of runoff transfer,study the mechanism of runoff thermal pollution control.The design scheme of bioretention based on rainfall characteristics,main parameters of bioretention,and catchment characteristics is constructed by integrating the production,discharge,and heat exchange process.The relationship between runoff volume control rate and thermal load reduction rate of bioretention is obtained by calculating the inlet and outlet water temperatures and the thermal load reduction under the set scenarios.The main findings of the study are as follows.(1)Bioretention has a good control effect on the thermal pollution of stormwater runoff.The monitoring results show that under the condition of 1-year rainfall of 22.34～41.53 mm and average rain intensity of 0.45mm/min,the bioretention effects on runoff thermal pollution control are as follows: temperature reduction rate was9.37～16.60%,the volume reduction rate was 14.46%～81.86%,and thermal pollution reduction rate was 23.19～84.12%.(2)The temperature change pattern of each structural layer during rainfall varies greatly,the mulch layer and the planting soil layer are easily affected by the runoff temperature,temperature was raised fast and changed the most;the soil temperature change of the filler layer was the less.We found that the deeper the layer the more stable the soil temperature is,and the filler thickness is at least 90 cm to ensure the thermal pollution reduction effect.Soil temperature can be decreased by 1℃ on average for every 30.56 cm increase in depth of the bioretention soil column.In addition,the facility emptying time,as a key design parameter,has a certain influence on the runoff thermal pollution control effect.(3)Rainfall duration,facility service area ratio,and whether to add internal water storage layers of different heights have certain effects on the runoff thermal pollution reduction effect of bioretention facilities.The monitoring results show that under the same rainfall recurrence period,the temperature reduction rate and the volume reduction rate of bioretention facilities have opposite relationships due to the rainfall duration,and the temperature reduction rate decreases with the increase of rainfall duration,while the runoff volume reduction rate increases with the increase of rainfall duration.The monitoring results showed that the rainfall duration increased from 30 min to 120 min,the temperature reduction of runoff through bioretention facilities decreased from 4.98℃ to 3.08℃,the volume reduction rate increased from 19.25% to22.63%,and the thermal pollution reduction rate reached 32.75% from 32.66% at 30-60 min and decreased to 30.57% at 120 min.30.57% at 120 min.(4)The ratio of bioretention area to service catchment area is an important factor affecting the effect of thermal pollution control.The experimental results show that if the ratio is increased by a factor of 1,the volume reduction rate of facilities can be increased by 7.63% on average,and the thermal pollution reduction rate can be increased by 7.42% on average.The response of the effluent temperature to the ratio of bioretention area to service catchment area was not significant,but it showed a pattern that the effluent temperature becomes more stable with the service area ratio increasing.Therefore,increasing the service area ratio can improve the volume reduction rate of rainwater runoff and thus improve the total thermal pollution control rate.(5)Internal water storage structure can significantly improve the effect of bioretention thermal pollution control effect.The height of the IWS layer increased from 0cm to 80 cm,and the thermal pollution reduction rate increased from 32.75% to84.12%,on average,each additional 10 cm height of IWS can increase the thermal pollution reduction rate by 5.18%～7.17%.(6)Thermal pollution of summer rainfall runoff from the parking lot is serious, and only one rainfall without thermal pollution exists in 15 rainfalls.Field rainfall average temperature range between 19.56 ℃ ～ 28.96 ℃,the peak temperature of up to31 ℃,and rainwater temperature roes exceeds the national standard 2 times.The correlation analysis method shows that meteorological characteristics and rainfall characteristics jointly affect the level of rainfall-runoff thermal pollution.EMT shows a strong positive correlation with maximum temperature,maximum solar radiation,and maximum 5mins rain intensity;unit area runoff thermal load shows a strong positive correlation with rainfall amount,rainfall calendar time,and dry days before rain.Meanwhile,based on a large amount of monitoring data,a multiple linear regression model was established to predict the EMT and heat load of rainwater runoff.(7)The trend of stormwater runoff temperature over stormwater events shows a pattern that higher runoff temperature happening at the earlier stage in the whole event.The implementation of interception measures at the early stage of rainfall can effectively control 80% of the thermal load.Bioretention has a good control effect on EMT and thermal load.Through analysis of 15 typical rainfall events,it was found that the EMT of stormwater runoff from ecological parking lots can be reduced by 2.51℃on average in summer,and the control rate of thermal load can reach 90.86% on average.The control effect of single bioretention is lower than that of the whole area,the average EMT of stormwater runoff can be reduced by 2.10℃,and the average heat load control rate is 70.43%,for this parking lot,consider that the ratio of bioretention to its service area to be 1:4～1:9 is more suitable. |
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