Effect Of Permeable Sidewalk On The Rainwater Runoff Control And Pollutant Reduction

In recent years,unhealthy urban construction methods have led to a substantial increase in hardened and impervious surfaces in urban areas,which has significantly changed the characteristics of rainwater runoff.Urban rainwater runoff and flood peak flows have increased significantly.Urban waterlogging has become serious,and non-point source pollution has increased."Seeing the sea"is not uncommon,and urban water security has become one of the main factors that disturb urban healthy development.Therefore,transforming urban construction methods,improving urban stormwater management and strengthening runoff pollution control have become urgent tasks.Permeable pavement is one of the specific measures for decentralized source control under the"sponge city"concept.It can not only delay the flow time,peak time,and reduce the peak flow,but can also effectively intercept and filter rainwater runoff pollutants and has a certain reduction effect on urban non-point source pollution.In this study,eleven different structures and types of sidewalk permeable paving were designed and constructed.The surface layer includes ceramic tiles,concrete bricks,and cast-in-place concrete with different porosity.Pervious concrete or graded crushed stone base is disposed above the ash soil subgrade.By changing the structure and thickness of the water-permeable pavement surface and base course,and setting an impervious concrete pavement as a control,the effect of water-permeable pavement on rainwater runoff control and pollutant reduction was compared and studied,and its pollutants were intermittently adsorbed.The desorption characteristics were analyzed.The main conclusions are as follows:（1）Ceramic permeable bricks and permeable concrete paving can absorb the rainfall corresponding to the annual runoff control rate of 80%in the construction of sponge cities after one year of construction,and the cast-in-place concrete with a thickness of more than 30cm can completely absorb the return period within 20 years Rainfall.Runoff time of periodic cleaning of non-gap permeable ceramic tiles and gap-type permeable ceramic tiles is delayed by 1 to 13 minutes and 4 to 22 minutes,and peak time is delayed by 0 to 7 minutes and 2 to 16 minutes,respectively,compared to ordinary permeable ceramic tiles.The delay effect of the permeable ceramic tile is more obvious.When the rainfall interval was 1 day,the peak reduction rate of permeable ceramic tiles and permeable bricks increased by 6.4%～47.7%compared with the interval of 1 hour,and the peak growth rate of concrete tiles was higher than that of ceramic tiles.The permeation rate of permeable concrete bricks decreases rapidly with the running time,and the permeation effect is not significantly improved after periodic cleaning treatment.It is recommended to use the gap-type paving method when using this brick,and use coarse aggregate to fill the joints or do not fill the joints.In addition,the permeable bricks have almost completely lost their permeability due to surface pore blockage after one year of operation.Based on this feature,the existing permeable brick structure can be changed to make the surface layer a replaceable structure to extend the service life of the permeable sidewalk and reduce the cost of reconstruction.（2）Permeable concrete pavement has an effect of removing SS from road surface runoff between 59%and 100%.After one year of operation,permeable concrete pavement can still maintain good removal and penetration effects.But when the rainfall interval is greater than 3 days,the concentration of outflow SS in the early stage of rainfall is relatively high,and it is recommended that the outflow rainwater be collected for reprocessing at this stage.The effect of reducing permeate COD in pollutants of rainwater runoff is unstable due to permeable cast-in-place concrete pavement;the removal rate of NH₄⁺-N is between-17%and 60%under different rainfall recurrence periods;The purification effect of NO₃^--N does not change much,and eventually stabilizes,the removal rate is between 4%and 51%;the removal rate of TP is more than60%.（3）At 7 days and 3 days during the rain intermittent period,the removal rate of pollutants by the pervious concrete pavement decreased by 23%～46%and 30%～-51%respectively compared with 0.5 days during the rain intermittent period.When the rainfall was intermittent for 3 days,the nitrate nitrogen in the outflow rainwater reached the Class III standard in the Groundwater Quality Standard（GBT 14848-2017）,and the ammonia nitrogen exceeded the threshold value of the Class V standard in the Groundwater Quality Standard（GBT 14848-2017）2.0 to 6.6 times,there is a risk of contaminating groundwater.Therefore,it is recommended that drainage pipes should be laid at the bottom of the permeable pavement in areas with large rainfall periods to drain the rainwater in time to prevent groundwater pollution.（4）The removal effect of dissolved COD by the permeable concrete structure layer is unstable;the ammonia nitrogen adsorbed inside it is easily desorbed,and the removal rate reaches 100%at the time of outflow for 10 minutes.The removal of ammonia nitrogen is in the adsorption and desorption amount equivalent dynamic equilibrium state;relatively stable adsorption of nitrate nitrogen,slow desorption process and can not be completely desorbed in a short period of time,the removal rate of pure water within one hour is between 65%～83%.Total phosphorus is available during the flushing time desorption phenomenon,and the total phosphorus was not completely desorbed within 15 minutes of scouring,and the removal rate was between 84%and95%.In the dry state,the water-retaining process of pervious radon can greatly improve the apparent removal effect in the early stage,but it is very easy to desorb and re-enter the rainwater,causing the concentration of pollutants to increase.The removal of nitrate nitrogen is mainly physical adsorption,which can stably maintain a good reduction effect on total phosphorus,mainly due to chemical adsorption.