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The combined system hydraulic model estimates the sewage flows generated by residential, commercial and
industrial premises and the stormwater flows generated by rainfall. The model also routes these modelled flows
via a one dimensional network to the point of discharge (i.e. either the sewage treatment plant or the Estuary).
The model is then able to estimate the content of sewage within the combined flow discharged to the Estuary.
The model showed that while there are 15 possible combined stormwater and sewage overflow points, three
distinct sites were identified that contribute around 95 per cent of combined overflows to the Estuary:
New + Old Margaret Street Pump Station located in Royal Park off Paterson Street;
The Esplanade (including the Shields Street, Tamar Street and Willis Street Pump Stations); and
Forster Street Pump Station.
During periods of dry weather, pump stations associated with these locations transfer sewage to the Ti Tree
Bend Sewage Treatment Plant. During wet weather events when combined flows exceed the sewage pumping
capacity of the stations, excess combined flows are discharged to the Estuary to mitigate flooding behind the
levee, either by gravity weir or rising mains attached to pumps. These sites became the focus of the risk
management scenarios, or treatment options.
Treatment option
High level description
1.
Legislation, regulation
and policy improvement
Changes to the legislative and regulatory environment to
incentivise continuous improvement of the combined system
2.
Community information
and education
Ongoing monitoring of river health to facilitate continuous
system
improvement, education streams and warnings in the event of
an overflow
3.
Operational
improvements and
system optimisation
Review existing operational environment of the combined
system to ensure existing infrastructure is operating efficiently
and effectively (i.e. Margaret Street Detention Basin and weir
levels at CSO locations)
4.
Green infrastructure
(primarily WSUD
treatments)
Develop the framework required to transition from
"traditional" drainage systems to WSUD drainage systems
including detention, wetlands, ponds, bio-filtration systems and
infiltration systems to
decrease runoff frequency, volume and peak flow. Green
infrastructure would also be considered for the immediate
mitigation
options
5.
Screening, preliminary
treatment and/or
disinfection at CSO
locations
Installation of screening and chemical treatment facilities at the
3 key CSO locations
6.
Offline storage
Underground storage tanks located at the key CSO locations
7.
Live storage
Storage within the existing system, requiring baffles, weirs,
actuators at the 3 key CSO locations
8.
Separation
Full separation of the combined system and construction of a
separated sewer and stormwater network
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9.
Diversion of separated
sewage catchments
Diversion of the West Launceston and South Launceston trunk
sewerage mains directly to the Ti Tree Bend STP
10.
Diversion of separated
stormwater catchments
Construction of required stormwater drainage components to
enable direct discharge to the Estuary at Margaret Street
11.
System upgrade i.e.
additional combined
rising main to Ti Tree
Bend and reconfiguration
of network components
Increase the pump rate to Ti Tree Bend for the key CSO
locations
12.
Consolidation and
movement of discharges
further downstream.
Pump combined discharge further downstream to where the
Estuary widens and dilution is increased
Table 2. Combined system overflow – grouped treatment options
The risk management scenarios were initially grouped into the twelve categories listed in Table 2 and were then
shortlisted using multi-criteria analysis and preliminary investigation.
The Working Group’s Investment Plan can be found at Appendix 3, which provides detailed information on the
processes of model validation and calibration, the options considered and the process of shortlisting.
5.4
Water quality
Water quality parameters have been monitored in the Tamar Estuary and the North and South Esk rivers since
the 1970s, with historical data predating the Ti Tree Bend and Hoblers Bridge sewerage treatment plants.
Thermotolerant coliforms in the North Esk River at Hoblers Bridge and in the Estuary at the yacht basin were
observed to be present in the millions of cells/100mL in the 1970s, with the highest count peaking at 8.8 million
cells/100mL at Hoblers Bridge in June 1991.
Mirroring the trend observed globally, there is a strong trend of significantly improved water quality following the
construction of wastewater treatment plants. While pathogens in the Estuary are demonstrably much lower than
in previous decades, they are still observed to peak, particularly during times of rainfall, rendering the water in
Zone 1 unsuitable for primary recreational contact some of the time.
A monitoring program implemented by the Launceston City Council in 2016 collected water quality data from a
number of waterways upstream of inputs from Launceston, stormwater sites and downstream sites in the lower
North Esk River and within Zone 1 of the Estuary. The data show a strong relationship between rain events and
elevated Enterococci levels in the waterways. Rainfall causes a statistically significant increase in pathogens at sites
in the lower North Esk River and upper Tamar Estuary. This relationship is evident when rainfall in the catchment
exceeds 1mm in a 24-hour period. On average, Launceston experiences 89 days per year where rainfall exceeds 1
mm. At sites upstream of Launceston’s urban discharges (e.g. the North Esk River at St Leonards), the water
quality meets the recreational guidelines most of the time, but fails to meet the guidelines after rain.
Samples collected on 5 consecutive days in September 2017 captured data from 11 sites in waterways in
Launceston, including four sites in Zone 1 in the Estuary. A total of 11 mm of rain fell during the second day of
sampling, causing the New Margaret Street pump station to discharge untreated effluent to the estuary. The
rainfall event (and associated combined system overflow) resulted in elevated turbidity and Enterococci, with