Investigation of stormwater particles generated from common urban surfaces

Investigation of stormwater particles generated from common urban surfaces

[Abstract]:

Pollution due to urban stormwater runoff is a significant environmental issue. Large regional devices including sediment ponds and constructed wetlands are common features in the urban landscape to treat runoff. In keeping with this approach, data requirements to evaluate stormwater impacts have mainly been met by the monitoring of sizeable urban catchments, typically greater than 10ha in area. Urban runoff characteristics have thus been conventionally linked with broadly defined catchment attributes. Land use, as defined by zonings such as Residential, Commercial and Industrial, is an attribute often used to evaluate stormwater runoff from urban catchments.

The emergence of Water Sensitive Urban Design (WSUD) in Australia is changing the management focus from the reliance on a small number of large-scale devices to many smaller-scale source controls distributed throughout the catchment. This paradigm shift in stormwater management places greater emphasis on small-scale processes within urban areas. Subsequently there is a need for more knowledge about stormwater generated from specific urban surfaces (roads, roofs, grassed areas etc).

The objective of this study was to demonstrate how urban stormwater quality can be managed on the basis of urban surfaces. The study involved the collection of data for typical urban surfaces and the development of predictive models to estimate stormwater quality. A series of case studies is provided to illustrate the use of surface-related data and modelling tools in stormwater management, particularly in the context of WSUD.

Non-Coarse Particles (NCP), defined as suspended solids less than 500μm in size, was selected as the stormwater pollutant under consideration. NCP is divided into the following particle size classes; Very Fine Particles (VFP, <8μm), Fine Particles (FP, 8-63μm) and Medium Particles (MP, 63-500μm). Laboratory methods to determine the concentration of these particle classes within stormwater runoff were adapted and refined from current standard methods. Organic content of each stormwater particle class was also determined.

An innovative device, the flow splitter, was developed to collect runoff samples from urban surfaces. The flow splitter was designed to obtain a composite flow-proportional sample, necessary to derive the Event Mean Concentration of stormwater particles. Hydraulic and sediment testing of a prototype flow splitter confirmed that the device is an accurate and unbiased sampling method.

Flow splitters were installed at five monitoring sites within inner city Toowoomba, Australia. The sites have small catchments (50 to 450m2 area) representative of urban impervious areas (galvanized iron roof, concrete carpark and bitumen road pavement) and pervious areas (grassed and exposed bare soil). Overall, runoff from 40 storms with rainfalls from 2.5mm to 64.3mm was sampled during the period December 2004 to January 2006.

A scatter plot analysis identified potential correlations between measured NCP loads and basic rainfall parameters such as rainfall depth and intensity. An exponential-type trend, consistent with many washoff models, is evident between load and average rainfall intensity for all surfaces. A composite index, referred to as the Rainfall Detachment Index (RDI), was found to be better than average rainfall intensity in explaining a relationship between NCP load and storm rainfall characteristics.

The insight gained from the RDI led to the development of a particle Mass Balance Model for impervious surfaces. Depending on the surface type, the model was able for provide reasonable estimates (R2 = 0.74 to 0.97) against the measured NCP loads. Simpler analytical methods for particle load estimation were also developed in the study. A total of five methods were produced. An error analysis was conducted to compare the performance of each method to accurately reproduce the measured NCP loads. The analysis also included three methods used in current practice, which performed poorly compared to the new modelling techniques.

The analytical methods provide useful tools in urban stormwater planning. The Mass Balance Model and measured surface-specific data were used in a number of case study examples to demonstrate possible applications. The applications included assessments of 1) the relative contribution that different urban surfaces make to the particle load in runoff; 2) how surface-specific data can be directly transferred to represent a large-scale urban catchment located in a different climate; 3) the particle loads generated from Residential and Commercial land uses; 4) the effect of exposed areas of bare soil on the particle loads from a Residential catchment; 5) the effect that widespread adoption of rainwater tanks may have on particle concentration in Residential urban runoff and 6) the particle load reductions by the use of a grass swale to treat road runoff.