Study On The Characteristics Of Rainfall Energy And The First Flush Effect Of Urban Runoff Pollution

With the development of society and economy, urban water pollution situation is more and more serious, when agricultural and industrial point source pollution was gradually controlled, urban water environment quality has not been completely changed, which makes the focus of people's attention shifted to the non-point source pollution, and contamination of urban surface runoff is the main component of non-point source pollution, the migration regularity of the study on urban surface runoff pollution become the problems to be solved urgently.In this paper, the natural rainfall in Tianjin area was observed and correlated with the sampling, using filter paper stain method to get the rainfall energy features, including Raindrop spectrum function, the corresponding raindrop velocity and the rainfall energy. And in the light of the characteristics of natural rainfall, to design indoor artificial rainfall simulation device independently. Through lots of experiments on the device of performance test, rainfall characteristics of the device are analyzed and summarized, in contrast to the natural and simulated rainfall similarities and differences. At the same time, using indoor needle tube type artificial rainfall system, to use sodium chloride as simulate pollutant in rainfall flush experiment, to get the scouring law under different initial conditions, and analyze the first flush effect in different underlying surfaces. Main research results are as follows:1. Tianjin area can be divided into ordinary type and shower type, the raindrop size distribution meets the Best distribution formula, its raindrops spectrum function is as follows:Normal rainfall :Shower rainfall :2. Rainfall intensity have close link with raindrop median diameter and rainfall kinetic energy, with the rainfall intensity increases, the raindrop median diameter and rainfall kinetic energy are increased, the corresponding relationship is as follows:3. Indoor artificial rainfall simulator can simulate the function of the three major characteristics of the natural rainfall, the rainfall device was tested and analysised, including rainfall intensity and stability test, uniform rainfall and raindrop terminal landing speed. The experimental results show that the indoor artificial simulation rainfall device can obtain the ideal effect of rainfall and its performance is stable and reliable, the acquisition of rainfall intensity and rainfall uniformity of rainfall characteristics meet the design requirements.4. Rainfall intensity, slope gradient, rainfall kinetic energy and initial pollutant strength of pollutants have certain influences in the scouring process. Among them, rain intensity and slope of pollutants in the scour process influence is big, it is mainly due to the increase of rainfall intensity, runoff can accumulate rapidly, and increase the stability of runoff water. The scour process migration of rainfall energy and pollutants initial intensity of pollutants is relatively limited.5. According to the definition of first flush effect, cement, road brick and porous brick can appear the first flush effect, with rainfall intensity and slope increased, the MFFn value increased by a big margin, rainfall kinetic energy and initial pollutant intensity can also increase the value of MFFn, but the growth is limited, indicating that the kinetic energy of rainfall can accelerate the migration rate of pollutants, but its influence is weak. The influence of rainfall intensity.6. To make the order of flushing efficiency of pollutants in cement, road brick,permeable brick and grassland, cement > road brick > water permeable brick > grassland,with the increase of permeability, the size of the flushing efficiency of pollutants gradually weakened, cement and road brick, this two underlying surfaces have a higher flushing efficiency, they are more easily to appear serious first flush effect during the first stage of runoff. In cement and brick road, the top 10% runoff can control more than 90% of the pollution load, in brick, the top 20% runoff can control 80% of the pollution load, in grassland,the top 30% runoff can only control more than 60% of the pollution load. Therefore, cement and brick road is more suitable to use intercepting measures to control the pollution load.7. Cement and road brick is not quite suitable for the flush model, and permeable brick and grassland is more suitable relatively, indicating that the exponential model is more suitable for in impervious underlying surface, not suitable for the opposite. This is mainly because waterproof underlying surface condition is more complex, the scouring of pollutants is more complicated.