First flush flows management practice in the urban environment of Genoa

In the framework of an integrated approach to urban drainage systems, first flush flows management is going to become one of the primary targets for the protection of receiving water bodies and the related pollution control. It has been demonstrated that, during storm events, the pollutant load associated with first flush flows is sometimes higher than the waste water one in ordinary conditions. However the evaluation of the first flush runoff volume and the design and optimisation of possible detention tanks and treatment facilities require a deeper understanding of the first flush phenomenon. In order to investigate runoff pollution and the relationship between pollutant load and runoff, two monitoring systems have been installed in the experimental catchment of Villa Cambiaso, University of Genoa, for sampling separately roof and road runoff. Since January 2002, both water quality and flow measurement data have been collected by analysing the following parameters: total suspended solids, COD, NH4+, pH and heavy metals in dissolved form (Zn, Pb, Cu, Cd, Cr, Ni). Cumulative load curves were produced for all monitored water quality parameters. As for the road runoff, the magnitude of the first flush in the monitored events strongly depends on the duration of the antecedent dry weather period, while the influence of the initial rainfall intensity was observed on the occurrence of the phenomenon. Regarding roof runoff, the entity of first flush of heavy metals in dissolved form is affected by both rainfall event characteristics (intensity and total depth) and environmental conditions prior to the rain event; the occurrence of first flush of suspended solids is mainly affected by the duration of the antecedent dry weather period, which regulates pollutant, building up