Influence of rainfall characteristics towards first flush behaviour

Abstract

Urban stormwater runoff has been recognised as one of the main contributor in degradation of water quality as it wash-off varies pollutants on impervious surface to receiving water. One important characteristic of urban stormwater runoff is that pollutant discharge in the early phase of runoff is significantly higher than that in later phase of the runoff and this process is identified as first flush phenomenon. The understanding of first flush is important to provide information on pollutant control and water quality management. This research investigates the influence of rainfall characteristics toward first flush behaviour. This study involved field work, sampling of stormwater runoff, laboratory testing and data analysis. A total of 115 stormwater runoff samples were collected from 11 rainfall events in the study area, located at School of Civil Engineering, Universiti Teknologi Malaysia, Johor. Rainfall characteristics and runoff data were collected from May 2017 to April 2018. Stormwater quality parameters analysed were total suspended solid (TSS), total dissolved solids (TDS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and zinc (Zn). In order to analyse first flush occurrence, two methods were employed; concentration-based First Flush (CBFF) method (using data from pollutant’s concentration and hydrograph) and mass-based First Flush (MBFF) method (distribution of M(V) curves). CBFF confirmed that rainfall events with peak pollutant concentration which occurred before peak flow-discharge indicated the occurrence of first flush phenomenon. In contrast, first flush was absent when peak concentration occurred after peak flow-discharge. It was found from the distribution of M(V) curves that the percentage of first flush occurrence is different for each pollutant. Based on the mean value of M(V) curve for each pollutant, the relative strength of the first flush was COD>TSS>Zn>BOD>TDS. Detail analysis on the influence of rainfall characteristics on first flush behaviour was carried out by PROMETHEE and GAIA software. Result of rainfall characteristics showed that rainfall duration and intensity were significant in controlling the wash-off of pollutants while antecedent dry period (ADP) is important for understanding the build-up process of pollutants. In addition, high rainfall depth and intensity have influenced the behaviour of first flush for TSS, TDS and BOD. For COD, longer rainfall duration and ADP play a significant role in influencing the first flush behaviour. Meanwhile, Zn was influenced by longer duration of rainfall. Therefore, it is concluded that the behaviour of first flush varied due to variations in the characteristics of rainfall events.