The development of a novel approach for assessment of the first flush in urban stormwater discharges

摘要

The management of stormwater pollution has placed particular emphasis on the first flushnphenomenon. However, definition and current methods of analyses of the phenomena containnserious limitations, the most important being their inability to capture a possible impact of thenevent size (total event volume) on the first flush. This paper presents the development of a novelnapproach in defining and assessing the first flush that should overcome these problems.nThe phenomenon is present in a catchment if the decrease in pollution concentration with thenabsolute cumulative volume of runoff from the catchment is statistically significant. Using datanfrom seven diverse catchments around Melbourne, Australia, changes in pollutant concentrationsnfor Total Suspended Solids (TSS) and Total Nitrogen (TN) were calculated over the absolutencumulative runoff and aggregated from a collection of different storm events. Due to the discretennature of the water quality data, each concentration was calculated as a flow-weighted averagenat 2mm runoff volume increments. The aggregated concentrations recorded in each incrementn(termed as a ‘slice’ of runoff) were statistically compared to each other across the absolutencumulative runoff volume. A first flush is then defined as the volume at which concentrationsnreach the ‘background concentration’ (i.e. the statistically significant minimum). Initial resultsnclearly highlight first flush and background concentrations in all but one catchment supportingnthe validity of this new approach. Future work will need to address factors, which will helpnassess the first flush’s magnitude and volume. Sensitivity testing and correlation with catchmentncharacteristics should also be undertaken.