Stormwater particle characteristics of five different urban surfaces
A network of five stormwater monitoring sites was established in 2004 within the inner city area of Toowoomba, Australia. Each site has a passive sampling device to capture flow-weighted samples to determine the event mean concentrations (EMCs) of stormwater particles. The monitoring sites were small catchments (<500m2) each covered by a single type of urban surface including impervious areas (galvanized iron roof, concrete carpark, asphalt road pavement) and pervious areas (grassed and exposed bare soil).
This paper will discuss the outcomes of monitoring during the wet summer period from December 2004 to end February 2005. Stormwater samples were collected for 15 storms with rainfall totals ranging from 2.5 to 48.5mm. Runoff occurred from the pervious surfaces only when rainfall depth exceeded 20mm or during high intensity rainfalls (~ 40mm/hr). Impervious surface runoff occurred for all storms greater than 1mm.
Stormwater particles were graded into size ranges in order to describe their characteristics. A simple classification system was adopted that divides solids into four size ranges; Very Fine (VFPs, <8μm), Fine (FPs, 8-63μm), Medium (MPs, 63-500μm) and Coarse Particles (CPs, >500μm). This paper investigates the EMCs and loads of non-Coarse Particles (<500μm ) in addition to organic and inorganic contents.
Comparisons were made of the EMC of each particle class and inorganic content from each type of surface. EMCs exhibited a high variability with typically a 10 to 100-fold range of measured values. Road EMCs had a narrower range with approximately a 5-fold difference between maximum and minimum values. Overall, the inorganic content for non-Coarse particles fell within a range from 55% to 85%. Inorganic content for the roof, carpark and grass were of similar magnitude with the road and bare soil values slightly higher. A variation in the distribution of particle sizes and inorganic content due to rainfall intensity was apparent.
Hydrological modelling was undertaken to predict runoff volumes and derive estimates of particle loads for individual storms and the cumulative total over the three month monitoring period. On a per m2 basis, the road surface generated the highest cumulative load of non-Coarse particles; an order of magnitude greater than the roof surface. The grass surface contributed the lowest cumulative load.