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

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.

X:\JOBSICT Projects in progress\1225 30 lots lake MacDonald Drive\Correspondence\stage 3 & 4\Response to Info_Request_Jan2014.doc