Analysis of first flush in rainfall runoff in Shenyang urban city

As urbanization progresses,urban land use changes,leading to an increase in the area of impervious surfaces and,as a consequence,increases in runoff volume and peak flow rates. Studies have shown that urban stormwater runoff contains a variety of pollutants,such as sediment,organic material,micro-organisms,nutrients and heavy metals,which can seriously affect public health and threat the quality of environment. The distribution and concentration of these pollutants depends on the characteristics of the urban surface,and dry and wet atmospheric depositions. Urban runoff pollution problems are more difficult to control than steady-state point discharges because of the intermittent rainfall and runoff,the large variety of pollutants,and the complex natural conditions. In a storm event,a first flush phenomenon occurs when the main proportion of the pollution load is transported in the first part of the runoff volume. However,the peak concentration may vary for different pollutants during the same storm event,or in the same watershed during different storm events. Many factors influence the first flush intensity. Watershed area, rainfall intensity, impervious percentage,antecedent dry weather period and sampling methods are the common influences.As an established industrial base in China,Shenyang has experienced rapid urbanization. The impervious area is increasing rapidly due to the revitalization of the northeast old industrial base and other policies in recent years. As a result, non-point source pollution from urban runoff is becoming more and more serious. Various experimental catchments were equipped in order to assess the quality characteristics of urban runoff in the past. And it is very important to examine runoff from different surfaces. So in the present study,roof and road runoff quality in Shenyang was monitored and analyzed. Samples of roof runoff were collected at the pipe outlets and road runoff was collected at rain grate. The rainfall depth through events was recorded by automatic gauge. Once runoff flow was observed,we collected samples every 10 min for the first 60 min,every 30 min between 60 and 180 min and then every 60 min beyond 180 min. Flow rates for the three surfaces were simulated by SWMM( Storm Water Management Model). The samples were collected,treated and analyzed in the laboratory within 24 h. Total suspended solids( TSS),chemical oxygen demand( COD),total nitrogen( TN),total phosphorus( TP),and some typical heavy metals( Cd,Cr,Cu,Ni,Zn) using standard methods. M( V) curve and MFF 30 was used to analyze the first flush intensity and occurrence probability of different rainfallrunoff events. The study demonstrated that first flush occurred for between 50% and 90% of the monitored rainfall events. The intensity of the first flush intensity varied by surface and was most intense for the flat roof,followed by the slope roof and then the road. Rainfall duration and antecedent dry weather period were positively correlated with the first flush. Rainfall intensity was positively correlated with the first flush at first,but when dilution effect is greater than the flushing effect,the correlation changed to negative. Heavy metal pollutants frequently occurred in the first flush,while TN prone to occur in strong flushes.