AGENDA - Sunshine Coast Council

Strategy and Planning Committee Agenda 16 February 2011
A fundamental problem with the proposed stormwater quality treatment strategy proposed is
that all stormwater which bypasses a rainwater tank is required to pass through a Gross
Pollutant Trap before it enters a bioretention device. For this strategy to be implemented it
means that stormwater needs to be treated in “at source” Gross Pollutant Trap or in “inline”
Gross Pollutant Trap. Either way stormwater needs to enter the piped drainage system
before being treated in a bioretention device. This approach works for sites with some grade
but does not work for sites which are essentially flat and discharge underwater to a lake
system. On flat sites it is not possible to treat stormwater in bioretention devices once the
stormwater is in the piped system. On flat sites stormwater must be treated in “at source”
bioretention devices.
There are a number of errors which have been made when modelling the proposed
stormwater quality treatment train in MUSIC.
The saturated hydraulic conductivity of the filter media has been modelled at 90mm/hr. The
saturated hydraulic conductivity of the filter media which meets the FAWB specification as
required by Council is 200mm/hr. The result of using the lower hydraulic conductivity is that
the device treats less stormwater but the stormwater it does treat is treated to a higher
standard. Using the correct filter media saturated hydraulic conductivity the performance of
the bioretention systems is improved and compliance with best practice load based reduction
targets is still achieved.
The second error is that the width of the overflow weir has not been increased to reflect the
lumped approach taken in the modelling. The overflow weir width for a basin with a surface
area of 8000m2 has been modelled with a weir width of 2m. Following the Water by Design
MUSIC Modelling Guidelines the weir width for a basin with a filter media surface area of
8,000m 2 should be 800m.
The third error is that Gross Pollutant Traps are included in the treatment train in locations
where, due to the flat nature of the site, they cannot be incorporated.
The fourth error is that the bioretention devices have been modelled with an extended
detention depth of 0.3 metres. This extended detention depth is suitable for “end of line”
bioretention devices, but is unsuitable for at source devices. Because “at source” devices are
located within the road reserve, the level difference between the top of the filter media and
the top of extended detention needs to be minimised. Previously an extended depth of
150mm has been accepted for at source bioretention devices.
The fifth error is that rainwater reuse has been calculated at 700L per dwelling per day. In
accordance with the Water by Design MUSIC Modelling Guidelines the daily reuse should be
calculated at 205 litres per dwelling per day.
A sixth error which makes the design conservative is that the storage surface area has been
set as the same as the filter media surface area. This is the case where there are vertical
walls associated with the bioretention devices. Generally, at source bioretention devices
have 1:2 batters which means for a typical device that the storage area is 1.4 times greater in
area than the filter media surface area.
With all of these changes the proposed treatment train does not meet current best practice
load based reduction targets for total suspended solids or nitrogen. This is expected as the
bioretention filter media surface are represents only 1.1% of the total catchment area which it
is treating and generally around 1.4% of the catchment area is required as bioretention filter
media. Increasing the bioretention filter media surface area to the equivalent of 1.4% of the
catchment area is required to meet best practice load based reduction targets.
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