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A basic understanding of the relationships between rainfall intensity, duration of rainfall and the amount of suspended particles in stormwater runoff generated from road surfaces has been gained mainly from past washoff experiments using rainfall simulators. Simulated rainfall was generally applied at constant intensities, whereas rainfall temporal patterns during actual storms are typically highly variable. This paper discusses a rationale for the application of the constant-intensity washoff concepts to actual storm event runoff. The rationale is tested using suspended particle load data collected at a road site located in Toowoomba, Australia. Agreement between the washoff concepts and measured data is most consistent for intermediate-duration storms (duration <5 h and >1 h). Particle loads resulting from these storm events increase linearly with average rainfall intensity. Above a threshold intensity, there is evidence to suggest a constant or plateau particle load is reached. The inclusion of a peak discharge factor (maximum 6 min rainfall intensity) enhances the ability to predict particle loads.

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Total Suspended Solids as a measure of suspended particles in urban stormwater has limitations and the alternative Suspended Sediment Concentration method was adapted to determine Non-Coarse Particle (NCP) concentration, defined as particles smaller than 500 μm. NCP was partitioned into the following classes: Very Fine Particles (<8 µm, VFP), Fine Particles (8-63 µm, FP) and Medium Particles (63-500 µm, MP). A Site Mean Concentration approach was adopted to differentiate the suspended particle characteristics between three impervious surfaces (roof, road and carpark) using runoff data collected for 35 storms.Runoff particle size distribution (PSD) of all surfaces was dominated by particles less than 63 μm. A weak trend of relatively constant VFP concentration was present in the road runoff data. Roof runoff PSD became finer as NCP concentration increased and, overall, the PSD of carpark runoff was coarser compared to road and roof runoff. These findings have runoff treatment implications as settling processes are influenced by particle size. IntroductionPollution due to urban stormwater is a significant environmental issue. A management philosophy to reduce, detain, infiltrate, treat or use stormwater at its source has emerged to reduce the adverse impacts of urban stormwater. This approach is variously termed 'low impact development' (LID), 'water sensitive urban design' or 'sustainable urban drainage systems'. A common LID feature is the strategic use of small-scale and on-site controls on urban stormwater, including non-structural measures such as alternative road layouts to minimize imperviousness. Several LID-type design guidelines are available (e.g. CIRIA, 2000 used in Scotland). The introduction of LID has highlighted a need for more information about the characteristics of stormwater generated from specific urban surfaces such as roads and roofs.3 Suspended particle load in urban runoff is a key pollutant leading to reduced water clarity in receiving waters. Due to the affinity of heavy metals and other contaminants to particles, stormwater treatment systems generally involve some form of particle separation process. On this basis, Total Suspended Solids (TSS) is a common performance indicator used in urban stormwater planning and analysis. Determination of TSS concentration is accepted practice to characterise suspended particles in runoff, but research has disputed the validity of using TSS as a reliable measure in stormwater (Knott, et al. 1992;Gray, et al. 2000). An alternative measure, referred to as Non-Coarse Particle (NCP) concentration is described in this paper.This paper is based on stormwater monitoring data collected from three impervious surfaces; a road, a carpark and a roof, located in Toowoomba, Australia (Brodie, 2007). The NCP test methods, the monitored impervious surfaces and a passive stormwater sampling device developed as part of the study are described.Suspended particle characteristics in runoff from each surface are also described based on statistical analysis of the m...