The influence of particle size on the first flush strength of urban stormwater runoff

Abstract

The presence of a first flush (FF) of suspended solids (SS) in stormwater runoff has important implications for the design of treatment facilities, as does the particle size of solids. Whilst numerous studies have examined the FF behaviour of SS, few have disaggregated FF trends by particle size. In this study, the FF behaviour of SS was investigated in five size ranges, sampled from an urban stormwater drainage system located in Dublin, Ireland. A weak FF was exhibited in the gross fraction of SS, with just two events from 14 transporting more than 50% of the SS mass in the first 25% of runoff, implying that treatment structures should be capable of removing SS throughout the storm event. In the majority of rain events, the FF strength increased with decreasing particle size, probably related to the lower intensities required to dislodge solids at the onset of rainfall. Although FF strength was correlated with rain event characteristics, prediction intervals were too broad to confirm FF presence based on rainfall data alone. Therefore, the design of smaller treatment volumes based on an assumption of FF must be justified by local monitoring data.

Study on pollution characteristics of urban pavement runoff

“…Therefore, the determination of the starting point of flush and first flush effect (FFE) is a key factor in the design of pollution treatment facilities. The M(V) curves discrimination is a widely adopted method to judge the FFE (Morgan et al 2017;Perera et al 2019).…”

Urban Stormwater Characterization, Control and Treatment

“…Transport behavior of suspended sediments from urban watersheds is often assumed to exhibit a first flush effect and, as demonstrated by Memon et al (2017), total suspended solids (TSS) may follow a first flush pattern. However, Morgan et al (2017) highlighted the importance of particle size to washoff behavior, as they found partic most likely to exhibit first flush effects while gross suspended sediments rarely followed a first flush pattern.…”