A Study On Interception Of The Initial Runoff In Futian Watershed Of Shenzhen

As point source pollution was controlled step by step, the proportion of rainfall-runoff pollution in the deterioration of urban water environment is increasing.Interception and treatment are the main means of controlling the pollution caused by urban rainfall-runoff。The scale of system which based on the proposed experience value(Shenzhen Planning Bureau) of n0 is insufficient to achieve effective effect on the interception of the initial runoff of small-scale patches under separate system area,and its value is too small. In fact, n0 is suitable for Combined system.we can not get a reasonable scale of small-scale patches under separate system.Rainfall-runoff from the stormwater outfalls of a small area in Futian watershed of Shenzhen was studied for one year . COD, SS, BOD, TN, TP and Dynamic Flow of 17 rainfall was Monitored to determine the intercrption parameters of rainfall runoff.The relationship between Pollutant concentration and the rainfall duration and the first flush effect were analyzed. Three methods were used to identify the the first flush effect.The The results are as follows. The concentration of pollutants in initial runoff is high in most cases. However, its latter concentration decreases and tends to smooth.The-average pollution concentration in the former rainfall-runoff is more than 1.8 times of the latter.In most cases, COD,SS,BOD,TN exist first flush effect in the initial rainfall-runoff of study area. The first flush effect of TP is not obvious .The frequencies of COD, SS, BOD, TN, TP existening first flush effect is 86%,59%,80%,52%,42%.The fact of existence of the first flush effect illustrates that such an approach of intercepting the initial rainfall-runoff is feasible.By analyzing the relations among interception ratio of runoff volume and the quality of each pollutant and rainfall runoff and rainfall duration , We can determine the reasonable interception rate of Runoff volume.From the study we can draw the following conclusions:(1)The analysis of MFFn curve shows,the effect of Increasing interception rate is Significant on the removal of pollutants when the interception ratio is less than 0.3-0.4. Increasing interception rate is insensitive for improving the water quality of rivers.(2) non-dimensional fitting curve of Rainfall-runoff shows : The fractions of pollution load transported by the first 30%-40% of runoff volume were 49.3% -61.7% for SS, 47.6% -60.2% for COD, 41.8% -53.5 for BOD, respectively. The interception rate of previous 30%-40% runoff volume is appropriate .(3) In order to achieve 30%-40% interception rate, the interception time of the initial rainfall can be set to former 30-40 minutes .Themaximum design flow rate and storage capacity of interception facilities can be obtained from calculating the time distribution (5 minutes Step length, Specific rainfall duration and return period )of rainfall based on the interception time(35-45minutes).