Stormwater first flush analysis in the Auckland region

Asaad Y Shamseldin
Auckland Council Research, Investigations and Monitoring Unit, RIMU
Publication date:

In broad terms, this study is concerned with improving the scientific understanding of the water quality characteristics of stormwater runoff in the Auckland region. The main objective of this study is to undertake an assessment of existing water quality data in the Auckland region for evidence of contaminant first flush and its characteristics.

A literature review with regard to the first flush, the methods used to identify the first flush from field data and the management of first flush is given in this study. The literature review shows that designing Best Management Practices (BMPs) to focus on treating the first flush/early stormwater runoff which constitutes a small portion of the event runoff volume is considered as being a more economical approach for reducing pollutants from stormwater (Barco, 2008). As the pollutant concentration during an event generally decreases with time (Sansalone and Buchberger 1997; Larsen et al. 1998; Krebs et al.1999; Li et al. 2005), the enhancement of the treatment of the first flush runoff can lead improvements in the overall performance of the stormwater management facilities (Li et al., 2008).

The literature review reveals that the first flush has been the focus of hot debate between the believers and the disbelievers of its existence. The literature review has also discussed the concentration, the mass-based and the empirical frameworks used in identifying the existence/non-existence of the first flush from the observed data. These frameworks have different levels of subjectivity. However, the mass-based framework appears to be the most widely used and more objective as some of the methods which belong to this framework use an objective numerical criteria for identifying the first flush with less conceptual flaws. Furthermore, the identification of the first flush using these methods is not site-specific unlike the concentration based methods where the definition of the first flush can be site specific.

In this study, the first flush analysis is conducted using a comprehensive data set from 16 sites in the Auckland region. The analysis presented in this report focuses on the first flush analysis for 22 water quality parameters. The results of the first flush analysis confirm the existence of the first flush phenomena in the Auckland region. However, the strength of the first flush varies from site to site and also depends on other factors such as the water quality parameters being analysed.

Prepared by Auckland UniServices Ltd

Auckland Council technical report TR2011/007

Last updated: 2018-07-09