Flush Characteristics of Run off Pollution in a Rapidly Urbanizing Catchment

Understanding of the flush characteristics of runoff pollution can help to develop efficient measures for pollution control. Although the runoff pollution in the urban catchments has been extensively investigated, few studies have attempted to reveal the pollution characteristics in the urbanizing catchments with mixed urban and agricultural land use. In this paper, the Shiyan River catchment, a typical rapidly urbanizing catchment in China, is chosen as a study area, and a temporary monitoring site was set at the downstream of the catchment to synchronously measure rainfall, runoff and water quality during 26 storm events from 2009 to 2012. The flush characteristics for each storm event were assessed using a dimensionless representation of cumulative loading ratio against cumulative runoff ratio. In order to analyze pollutant flushing characterizations, rainfall-runoff process is divided into three stages according to the percentage of total runoff volume, including prophase (0 similar to 33%), metaphase (33%similar to 67%) and anaphase (67%similar to 100%). The results indicated that only 34% of the storm events have "first-flush" phenomenon, in which more than 1/3 of the pollutant loads were washed off during prophase; while more than 96% of the storm events have "middle flush" phenomenon, in which pollutant loads were mostly washed off during the metaphase stages. The relationship analysis between flush characteristics and rainfall characteristics of the storm event indicated that "middle flush" phenomenon usually occurred in the storm events with a total rainfall below a threshold value. The reason for middle flush can be explained by the mixed urban and agricultural land use in the catchment. In addition, a novel pollutant wash off model was developed to simulate the flush process of runoff pollution in the catchment. The result indicated the model can fit well with the middle flush phenomenon. The findings in this study provide support in decision making of runoff pollution control in the rapidly urbanizing areas.