First flush effect from vegetated roofs during simulated rain events

Abstract

Vegetated roofs are becoming increasingly popular in urban environments. Still, several aspects of their influence on various urban infrastructures are not sufficiently investigated, including the roofs' influence on runoff water quality. In this study a first flush effect in runoff water from vegetated roofs during simulated artificial rain events is investigated. Example extensive (Sweden) and intensive (Japan) vegetated roofs are studied. The first flush effect is typically occurring in urban runoff from hard surfaces and is not observed when the source of pollutants is unlimited (e.g. soil particles). Vegetated roofs would thus not be expected to exhibit a first flush effect. However, the results show that concentrations of the studied chemical compounds and elements were higher in first runoff samples than in samples taken at higher runoff depths. Analysis of the plots of normalized cumulative mass of studied runoff constituents as a function of normalized cumulative flow showed that, generally, the values are above the diagonal line with the exception of potassium (K) and dissolved organic carbon (DOC). This indicates that, with the exception of K and DOC, proportionally more mass is washed off in the earlier runoff than in the later, which can be interpreted as the occurrence of a first flush effect.