CRC Street Sweep Eval 1999.pdf( 489.9 KB ) - Clearwater

COOPERATIVE RESEARCH CENTRE FOR CATCHMENT HYDROLOGY
8 Gross Pollutant Wash-Off
Characteristics
8.1 Gross Pollutant Load Generation
The study by Allison et al. (1998) showed that
stormwater runoff is the principal means by which
gross pollutants are transported to the stormwater
system. Ten storm events (larger than 3 mm of
rainfall) and their transported gross pollutant loads in
the Melbourne suburb of Coburg were monitored
using the CDS unit from May to August 1996
(Allison et al., 1998). Monitoring was carried out in a
50 hectare catchment and the amount of gross
pollutants transported during each of the 10 events
was found to be correlated with the event rainfall
depth as shown in Figure 8.1. A similarly high
correlation between the gross pollutant load retained
in the CDS unit and event runoff was also obtained as
shown in Figure 8.2.
According to the fitted relationship between the wet
gross pollutant load generated and the depth of
rainfall (see Figure 8.1), events of less than 3.7 mm
may be considered to be insufficient for remobilisation
and transport of deposited street surface
loads. The corresponding threshold for runoff (see
Figure 8.2) is 0.70 mm. The fitted relationships
between gross pollutant wet load and event rainfall
depth or runoff show a trend of increasing gross
pollutant load with increasing rainfall or runoff.
Although the curves are monotonically increasing, the
rate of increase in gross pollutant loads decreases
with rainfall and runoff indicating a possible upper
limit of gross pollutant load transported into the
stormwater system during large rainfall or runoff
events. The fitted curves in Figure 8.1 and 8.2 may
be interpreted as indicating that the limiting
mechanism for stormwater gross pollutant transport,
in the majority of cases, is not the supply of gross
pollutants but rather the processes (ie. the stormwater
runoff rates and velocities) influencing the
mobilisation and transport of these pollutants.
If the mobilisation and transportation of gross
pollutants from the street surface depends on a
rainfall depth greater than 3.7 mm, it is likely that the
inter-event dry period for gross pollutant transporting
storm events, in Melbourne will be longer than the
calculated 2.6 days for all recorded storm events.
Analysis of the cumulative frequency distribution of
event rainfall depth for Melbourne over a 105 year
record is presented in Figure 8.3. The analysis shows
that approximately 35% of all recorded rainfall events
are greater than 3.7 mm giving an average inter-event
dry period of 178 hours (7.4 days) for gross pollutant
transporting storm events.
Figure 8.1 Gross Pollutant Wet Loads v’s Rainfall (after Allison et al., 1998)
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