Research On The Size Optimization Of The First Flush Remission In Sponge City Stormwater Runoff

In recent years,as the process of urbanization accelerates,the destruction of urban aquatic ecological environment has become more serious.Urban surface runoff pollution,especially initial runoff pollution which is one of the sources of pollution that can't be ignored,has also been gradually attracting the attention from our country and social.Many studies show that sponge city can effectively improve the status of runoff pollution as a promising new strategy.However,most of the LIDs are planned and designed based on the entire runoff process.In order to improve runoff water quality,we can try to optimize the LID size based on the runoff with great concentration of pollutants which can increase the initial pollutant removal efficiency and reduce the pollution of the entire runoff process.In this study,a district of Xuancheng in Anhui Province was selected as the research object.The SWMM model was built and three kinds of LID measures were considered including green roof,permeable pavement and biological retention pool.SS and COD,as two pollutants,were selected as the presentation of the Hydrology and water quality.The influence to it through the three kinds of LID measures with different LID scales were simulated and analyzed under four rainfall reappearance period including 1a,3a,5a,10a.What's more,through the qualitative identification of dimensionless cumulative M?V?curves and quantitative identification of MFF₃₀values,the optimal LID size to avoid the initial flushing effect of rainfall runoff is determined to effectively reduce the urban surface runoff pollution.The results show:?1?It shows that under each recurrence period,the larger the three LID measures,the better the runoff control and pollutant reduction.In the process of 1a to 10a,when LID size of the measures of green roof,permeable pavement and biological retention pool each reaches the maximum design size of 23.82%,29.33%and 14.13%,the runoff control rates are respectively decreases from 68.09%,58.48%and 62.53%to63.19%,49.77%,55.49%.The removal rates of SS decreased from 56.59%,50.02%and 55.89%to 50.32%,40.46%and 48.72%.The COD removal rates decreased from55.43%,47.92%and 52.46%to 47.99%,38.52%and 45.29%.Under the same LID size,the runoff control rate and pollutant reduction rate both decrease with the increase of recurrence period,and gradually become gentle.?2?During different recurrence periods,the change processes of runoff flow and pollutant concentration under three LID measures at different sizes generally show similar trends.They both increase with time and decrease after reaching the peak of flow rate or pollutant concentration.As the size of LID increases,the peak flow rate and the peak pollutant concentration both decrease and the runoff duration becomes short gradually.Under the same LID size,the peak flow and peak pollutant concentration both increase with the increase of the recurrence period,and the runoff duration becomes longer.?3?Under different recurrence periods,the early SS and COD flushing intensity decreases firstly and then increases and then decreases with the increasing size of the present green roof measure.With the increase of the size of the pavement permeability,it firstly decreases and then unchanged,afterwards decrease.With the increase of the size of the biological retention pool,it shows the decreasing trend.When the scale of the three LID measures is constant,the initial flushing intensity of SS and COD shows an increasing trend with the increase of recurrence period.?4?When the recurrence period is less than 3a,the three measures of green roof,permeable pavement and biological retention pool all can avoid the occurrence of initial rainfall runoff flashing.The optimal size of 1a recurrence period are 23.82%,27.13%,12.62%,respectively.When the recurrence period is 3a,only the biological retention pool measures can avoid the initial flushing effect,and the optimal size is14.13%.When the recurrence period is greater than 3a,three kinds of measures all can't avoid the initial scouring effects.When it each reaches the optimal size of23.82%,29.33%and 14.13%,it can well relieve the initial flushing effect,make the flushing weak.Under different recurrence periods,when the size of the three LID measures are the same,the priority of the effect of alleviating the initial flushing effect to pollutants are biological retention tank>green roof>infiltration pavement.