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

COOPERATIVE RESEARCH CENTRE FOR CATCHMENT HYDROLOGY
The Coburg gross pollutant wet load data have
incorporated the effect of Moreland City Council’s
street sweeping practices which range from daily to
fortnightly, depending on land-use. How exactly any
alterations made to the street sweeping frequency
would affect the gross pollutant load in stormwater
(see Figure 8.2) is not known and cannot be
ascertained from the data collected. However, it is
possible for some inference of the effectiveness of
street sweeping in limiting the export of gross
pollutants from street surfaces to the stormwater
system to be made, and this will be discussed in
Section 9.2.
Despite rainfall wash-off being the dominant factor
transporting gross pollutants from street surfaces,
litter can also reach the stormwater system during dry
weather periods. The litter monitoring study,
conducted by Hall and Phillips (1997), in the
Carnegie commercial catchment indicated that during
dry days numerous gross pollutant items are
transported into the stormwater system by factors
other than stormwater runoff (eg. wind or direct
dumping). That study focused on measuring the
number of litter items as well as material composition
collected daily over seven days, from identified
catchment pollutant sources. SEPTs were placed in
drain entry pits located in the study area to determine
the number of litter items reaching the stormwater
system from the identified catchment pollutant
sources (including bins, footpaths and street surfaces).
The results showed that up to 78 items of litter in total
(per day) were collected in SEPTs during periods
without rainfall. A substantial amount of the material
trapped during recorded dry days were lighter items
(polystyrene) although numerous heavier items were
also found, indicating possible direct littering rather
than wind blown transportation of street surface
pollutants.
Gross Pollutant Load Generation:
Data collected in the Coburg catchment indicated washoff of gross pollutants becomes significant for
storm events greater than 3.7 mm of rainfall depth or 0.70 mm of runoff.
The limiting mechanism affecting the transport of gross pollutants in the majority of cases appears to be
re-mobilisation and transport processes (ie. stormwater runoff rates and velocities) and not the supply of
gross pollutants.
Approximately 35% of all recorded rainfall events in Melbourne 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.
27