Prediction of stormwater particle loads from impervious urban surfaces based on a rainfall detachment index

Abstract

This paper makes use of Non-Coarse Particle (NCP) data collected from three different impervious surfaces in Toowoomba, Australia. NCP is defined as suspended solids less than 500μm in size. NCP loads (in mg/m2) were derived for 24 storms from a galvanized iron roof, a concrete carpark and a bitumen road pavement. A scatter plot analysis was used to identify potential correlations between NCP loads and basic rainfall parameters such as rainfall depth and intensity. An exponential-type trend, consistent with many washoff models, was evident between load and average rainfall intensity for all surfaces. However, load data for some storms did not fit this general trend. Various indices, comprising different combinations of basic rainfall parameters, were evaluated as an alternative to rainfall intensity. A composite index, referred to as the Rainfall Detachment Index, was found to be better than average rainfall intensity in explaining a relationship between NCP load and storm rainfall characteristics. The selected rainfall index utilizes 6-minute rainfall intensities and is a variant of the well known Rainfall Erosivity Index (EI30) used for soil erosion estimation.