Evaluation of First Flush Phenomenon from Bridge and Parking Lot Sites in the Gyeongan Watershed in Korea

Urban land-use areas with high imperviousness, such as bridges, highways, and parking lots, are known to be responsible for relatively large amounts of stormwater runoff and high concentration of pollutants particularly in the "first flush". This study was conducted to determine the effect of first flush from urban land-use sites located in Yongin City, Korea, on runoff amounts and pollutant concentration. It was observed that, on most pollutant parameters, the effect was apparent when the data were analyzed qualitatively but it was relatively weak when analyzed quantitatively. A stronger effect was observed from a bridge site than from a parking lot site. Pollutant loads, washed off at 40 and 60% runoff volume, respectively, were also determined for further analysis of the first flush effect to find an improved strategy for best management practices. In this study, no correlation was observed between rainfall intensity and runoff duration with first flush effect.