Callaghan & Toth 24-4-14 Response to Information Request

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6. Provide comment on the long stability of the road formation and drainage structures /
features in those sections potentially affected by flooding from the waterway.

Only a small percentage of the proposed Road 1 will have Q100 water level reaching it, see
drawing 1225-33 for developed flood line and depths below:
e CH50 at boundary of Stage 3: Q100 4mm deep at lowest edge of road (Q10
200mm below edge).
e CH121 at intersection of Road 2: Q100 below road level. Q100 is 100mm
below edge of road (Q10 300mm below road edge).
e CH187 near culdesac centre: Q100 50mm deep at lowest edge of road (Q10
150mm below road edge).

The road batters are relatively flat (refer to cross sections 1225-36A and 1225-37) and
batters towards the creek will be vegetated to stabilize the completed works. The road
alignment in front of lots 17-20, and 29 has been moved approximately 4m further from the
creek.

Flow velocity is very low in a Q100 event, approximately 0.6 to 0.7 metres per second in the
overbank area beside the road, and 1-1.3 metres per second in the main channel.

Erosion and long term stability should not be an issue for such low velocities in the main
channel, Bio-retention swale and edge of road area.

The road will have seepage drains on both sides to minimize ground water effects. The
attached ‘Geotechnical Investigation Proposed Subdivision, 55 Lake Macdonald Drive,
Cooroy, Revision 1’ by Tectonic Geotechnical and Environmental Engineers shows an
indicative CBR of 3-5, with one test at Bore Hole 12 with possible CBR less than 3. Overall
the soils are suitable for Road Construction in accordance with the Geotechnical Report.

Proposed Masonry Street formation is all above the Q100 flood level.

Flooding

7. Provide a flood study taking into account issues in information request.

Response:

Attached is BSM Flood study as an Appendix to SQWMP with plans and cross sections of
the existing and proposed improvements to ensure:

Hydraulic performance is the same for pre and post development

No loss of flood storage

New roads have QUDM flood immunity

Surface levels of all lots provided with flood immunity, refer to plan 1225-45.

8. Provide an amended plan in accordance with the recommendation of the flood study

Max Watterson & Associates will submit a new Allotment reconfiguration plan under separate
cover which complies with our plan 1225-45.

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Stormwater Quality

9. Submit a stormwater quality management plan prepared by an RPEQ for the ultimate
development of the site showing how stormwater from the development will be treated
to meet current best practice load based reduction targets.

Response:

A Stormwater Quality Management Plan has been prepared to demonstrate that load based
reduction targets are achievable for the ultimate development. The SWQMP assesses the
pollutant loads from all five stages of the development including the already approved and
completed Stage 1 and 2.

The attached SWQMP shows that the stormwater treatment train which was modelled using
MUSIC V6.0.3 achieves the following load based reductions:

 

 

 

 

 

 

: MUSIC REDUCTION | POLLUTANT LOAD
Ge RESULTS _|_ REDUCTION TARGETS
Flow 3%
Total Suspended Solids 81% 80%
Total Phosphorous 58% 60%
Total Nitrogen 51% 45%
Gross Pollutants 99% 90%

 

 

 

 

 

Development in Riparian Buffer

10. Provide an amended plan for the development indicating
e. Any new lots and infrastructure removed from within the riparian buffer
f. Disturbance for roadworks and infrastructure (only for the access road from
Blue Wren Place) minimised within the riparian buffer and a minimum setback
of 5m from the top of the most eastern bank of the waterway to any
infrastructure / road

Response:

Drawing Number 1225-33 shows the riparian zones for Stages 3 and 5. The plan shows
the 5m and 10m riparian buffers from top of bank along the waterways to correspond to
Council's Biodiversity Overlay map. The eastern tributary from Lake MacDonald Drive, is
only partly mapped as Riparian Buffer Area on the Biodiversity Overlay (Map OM3.1) and as
such a buffer of 5m from the invert of the creek is provided on this section of the waterway
which is not riparian buffer area.

Areas of disturbance to the riparian buffer areas are highlighted in orange and include:
e Lot 15 encroaching into the 10m buffer;

A pedestrian walkway bridge over the eastern tributary between Stages 3 and 5;

Stormwater drainage outlets into the western tributary;

Bioretention Swales encroaching into 10m buffer on the western tributary; and

Some road encroaching into the 10m buffer on the western tributary.

The minimum setback of 5m from the top of the most eastern bank of the western tributary to
any infrastructure / road is generally achieved, apart from stormwater infrastructure
encroaching slightly into this 5m buffer.

It should be noted that the western tributary will be reshaped and the following works
undertaken to improve flood storage for the site, reduce maintenance requirements, remove
mosquito breeding habitat, and improve drainage. See Drawing 1225-33 which shows the

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extent of earthworks and western tributary plan, long section, and cross-sections in Figures
9, 10, and 11 of the flood study showing the minor reshaping of the channel.

Natural channel design principles will be incorporated in the reshaping of the tributary and
features will include:
e Arock lined invert approximately 1.5m wide
e Removal of vegetation acting as weeds and currently clogging the watercourse
e Plant out the banks that have been reshaped with groundcover and stabilising plants
e.g. Lomandra spp to a density of one plant per 2m”
e Retain existing trees in the riparian buffer areas
e Infill planting in riparian areas to achieve a density of 1 plant per 1.5m” focusing on
canopy and pioneer species to achieve shade cover of the watercourse.

17. Justification for any infrastructure / road within the riparian buffer area.

Response:

Even though there is some encroachment it is considered that the overall intent of the
Biodiversity Overlay and mapped Riparian Buffer Areas is achieved on site by:

e Maintaining a riparian corridor along both waterways, generally a minimum of 10m
wide. In some areas the riparian buffer is much wider.

e Existing native vegetation is to be protected.

e Revegetating the riparian buffer areas on both waterways to maintain wildlife
corridors.

e Minimising encroachment into the riparian buffer areas. In some locations a 10m wide
buffer is not achieved as shown in orange on plan 1225-33.

o Lot 15: The defined bank is heavily vegetated with shrubs and mature trees at
present, and Q100 flood level in contained in the bank. We have moved the
rear boundary back as far as practical to allow the minimum 5m vegetated
buffer from top of bank.

o Proposed pedestrian footbridge on Eastern Tributary has been removed from
riparian area.

o Lot 26: We have allowed for a 3m wide grassed area behind the lot so Park
maintenance access is provided.

o Road 1 Road Reserve: Some minor areas have been removed where
necessary for turfed sections beside the road. On plan 1225-45 the Typical
Bio-retention Swale Detail shows that base of swale and batter on western
tributary side are vegetated to form part of the riparian planting.

The area of riparian buffer that could not be achieved as noted above has been offset
by planting an area of the Eastern Tributary (see yellow areas on plan 1225-33). An
offset planting area of 580sqm area has been used to offset the 340sqm shortfall.

Road 1 centreline, Road Reserve and lots 17-20, 29 front boundaries have been
moved approximately four metres further back from the Western Tributary to allow
more riparian planting and move the road away from the banks. The road centreline
and swale have also been curved to minimize tree removal.

e Maintaining water quality by providing a stormwater treatment system that achieves
best practice pollutant load reductions.

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e Removing weed species within existing riparian vegetation during the revegetation of
these areas.

As detailed above the western tributary invert will be reshaped, lined with rock in the invert
and revegetated. The justifications for undertaking earthworks in the watercourse are as
follows:

e improve flood storage for the site,

e reduce maintenance requirements,

e remove mosquito breeding habitat, and

e improve drainage from Blue Wren Place which currently ponds.

See Drawing 1225-33 which shows the extent of earthworks in the western tributary plan,
long section, and cross-sections in Figures 9, 10, and 11 of the flood study showing the
minor reshaping of the channel.

Land Dedication

12. Provide a plan of areas to be dedicated unencumbered by services and service
easements and the like and showing:
g. Restoration of the mapped riparian buffer and areas below the Q100
h. Revegetation of the mapped riparian buffer and areas below the Q100.

Response:

Areas to be dedicated unencumbered by services and service easement includes:

e Total Passive Parkland without services or easements = 2.5Ha

e Total Active Parkland without services or easements = 622sqm (63 sqm sewer
easement)

e Total area of restoration (including weed removal and revegetation of riparian buffer
areas) = 12,500sqm

See Drawing Number 1255-33 which shows the dedicated areas to riparian buffer, in
accordance with Council’s Biodiversity Overlay.

Yours faithfully,
Callaghan & Toth

Vi

A D Toth BE Environmental RPEQ (Env)(civil) MIEAust CPEng NPER CPESC

Director
Please view www.callaghanandtoth.com.au for all information on our company profile and terms and conditions for services rendered

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