沈阳市降雨径流初期冲刷效应

研究探讨城市降雨径流的初期冲刷效应对于城市水环境治理和水污染防治具有重要意义。以沈阳市典型区域为例,对坡顶屋面、平顶屋面和路面3种典型下垫面的降雨径流进行了监测,分析的污染物包括总悬浮颗粒物(TSS)、总氮(TN)、总磷(TP)、化学需氧量(COD)和主要重金属(Cd、Cr、Cu、Ni、Zn)。利用M(V)曲线和污染物初期冲刷率(MFF₃₀),分析了不同降雨径流事件的初期冲刷强度和发生强冲刷、弱冲刷的概率。结果显示:研究区较易产生初期冲刷现象,约有50%-90%的降雨事件能产生初期冲刷现象。3种下垫面发生初期冲刷的强度依次是:平顶屋面>坡顶屋面>路面。降雨历时和干期长度与初期冲刷效应具有正相关关系,降雨量和降雨强度与初期冲刷先是正相关关系,但当稀释效应大于冲刷效应时,呈现负相关关系。重金属污染物较容易发生初期冲刷现象,而TN易发生强初期冲刷。

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 MFF₃₀ was used to analyze the first flush intensity and occurrence probability of different rainfall-runoff 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.