Response to Information Request Letter 10-6-2025 · OPW25/0042

Page 2

   Provide a table identifying each property fronting Serenity Close for the section of Serenity
   Close which grades to the sag and provide an explanation of how the direction of drainage for
   each property has been determined (in particular, where it is assumed to not fully drain to
   Serenity Close).

Response
ALTA advise that upon further detailed review it was found that ‘Elandra’ discharges runoff to
internal Rapid Infiltration Pits (RIPs) sized for Q100 events and surcharging northward away from
Serenity Close in the event of failure.

Refer attached amended catchment drawing CA01 with tabulation of contributing external
catchments and explanatory comments now added. Refer also collated set of catchment drawings
for all developments along the northern side of Serenity Close serviced by RIPs. Note also that
RIP infiltration capabilities have been conservatively ignored.

2. Clarify what design blockage values have been adopted for the stormwater inlets in the design
   case in the overland flow assessment.

Response
Engeny advise that blockage applied as per QUDM (field inlets 50%, which is applied to the rating
curve for each pit).

3. Clarify what climate change assumptions have bene adopted in the overland flow assessment.

   It is suggested that SPP2-4.5 and a planning horizon of 2100 be adopted consistent with the
   current methodology of ARR.

Response
Engeny advise that the SPP2-4.5 and a planning horizon of 2100 have been adopted. Current
climate has applied an uplift to the IFD’s as recommended recently by ARR.

4. It is noted that 1% AEP (2100) immunity for the staff facilities building is achieved by the use of
   a levee (block wall).
   It would be preferable if the levee was only used to achieve freeboard rather than 1% AEP
   (2100) immunity.

Response
ALTA advises that immunity to staff facilities will be provided by a concrete pathway adjacent to
the ephemeral stream’ which maintains a minimum height of 1m above the adjacent stream base.
The staff amenities area will have no openings (doors, windows etc) addressing the pathway or
‘ephemeral stream’. By way of better explanation see Section D1 now provided on amended
drawing D03.

5. Provide further contours or sections in the location of RIP 3B and RIP 5 to demonstrate that
   surface levels or an overland flow path (which may be informal) will allow and surcharge flows
   from these devices to flow to a lawful point of discharge without impacting any of the
   development buildings or facilities.

Page 3

Response
ALTA advises that all discrete RIPs have been conservatively sized to contain up to 100 year ARI
events. Should RIPs fail or more intense storms be experienced overflow would be to adjacent
roadway (legal point of discharge).

In the absence of final landscape design ALTA is yet to finalise the exact levels and contours, and
suggest this could this be dealt with via a condition.

6. It is noted that a gabion rock wall is specified as the method of retaining along the edge of the
   overland flow swale in places. Given the ability of water to seep into this type of retaining (large
   voids), it is unclear if this method of retaining is suitable for the edge of a drainage channel and
   if a more suitable method is available that provides better long term performance of the
   channel and adjoining areas.

Response
ALTA advise that the proposed Gabion Walls would be constructed with a heavy geofabric filter
(Bidim A24 or equivalent) along their base and rear faces such that any water entering the gabions
would not export sand from behind or beneath the walls.

Gabions are frequently used in waterways for batter retention (see below link)
https://www.gabionbarriers.com/application/gabion-channel.html

Issue – Overland Flow Channel

The design of the Overland Flow Channel includes roughness assumptions consistent with
mown grass or rock at a maximum batter of 1:2. Council has concerns with the ability to
provide long-term stabilisation for a grass or rock 1:2 batter.

Information Required
7.    Provide additional detail demonstrating how long-term stabilisation will be provided to the
      batters of the Overland Flow Channel. The additional detail must be prepared by a suitably
      qualified person (RPEQ) and include treatment options consistent with the current
      hydraulic design assumptions of the Overland Flow Channel (i.e., turf or rock).
      Where long-term stabilisation of the proposed 1:2 turfed/rocked batter isn’t achievable,
      provide a revised set of drawings amending the Overland Flow Channel to include a
      reduced batter (i.e., a 1:3 vegetated batter).

Response
ALTA advises that the modelled roughness co-efficients were consistent with mown grass (n=.035)
along the channel base with vegetated batters (n=.06) on one side and gabion walls on the other.
An additional run has now been completed with the base roughness coefficient increased to reflect
a vegetated treatment lest that landscape outcome be pursued. An additional channel flow depth
of less than 100mm resulted which is insignificant to the performance or freeboard of the channel
(see graph in attached Channel base roughness comparison).

Inspection of Figures C11 and C12 of the Engeny report suggests channel flow velocities less than
1m/second for 1% AEP flows (with 2100 climate change) where jute mat and landscaped batters
with grassed base are proposed.

Page 4

Such velocities are less than maximums suggested in QUDM table 9.05.3 for easily eroded soils
with 50% coverage and less than maximums recommended for vegetated channels in QUDM
table 9.05.1

The Ephemeral Stream area will enjoy interaction with hotel guests and will be regularly
maintained by hotel groundkeepers such that any damage after an overland flow event would be
promptly rectified.

ALTA notes that the stream analysis is conservative and completely ignores infiltration afforded by
RIPs located on existing external catchment sites.

Issue – Extent of Vegetation in Overland Flow Channel
The higher order development plans suggest that the trapezoidal channel will include
landscaping (beyond turf) in the Overland Flow Channel. This is Council’s preferred outcome.
However, the inclusion of vegetation within this channel is inconsistent with the current
hydraulic design assumptions and could lead to flow breaking out of the channel and cause
flooding.

Information Required
8.    Confirm the Overland Flow Channel will be vegetated to achieve a natural channel
design and meet the required hydraulic design assumptions.

Response
ALTA has relied on the above response regarding hydraulic performance and suggest it
demonstrates that a grassed or vegetated channel with 1:2 batters would provide adequate
performance and stability.

Which channel landscape treatment is adopted is subject to a future landscape OPW application to
determine.

Item – Construction Traffic
A Traffic Impact Assessment has not been provided as part of the application material.
Council recognise that the haulage operations are likely to cause impacts to the local road
network and associated road infrastructure.

Information Required
9.    Provide a Traffic Impact Assessment (TIA) to assess the impacts of construction traffic
      associated with the prescribed works. The TIA must be prepared by a suitably qualified
      person (RPEQ) with Traffic Management Design qualification, in accordance with the
      Department of Transport and Main Roads Guidelines, and must include, but not be
      limited to:
        a. a Road Safety Audit;
        b. Pavement Impact Assessment;
        c. a Traffic Management Plan – the Traffic Management Plan must be developed in
            accordance with the Queensland Manual of Uniform Traffic Control Devices
            (MUTCD) and the Queensland Guide to Temporary Traffic Management (GTTM);

Page 5

       d. proposed haulage route – including the scheduled duration of excavation/haulage
          works, number of haulage vehicles exiting the site per hour, and the type of
          haulage vehicle (i.e., truck or truck and dog); and
       e. the receiving location(s) of the exported spoil.

Response
Refer attached Traffic Impact Assessment, Traffic Management Plan and Traffic guidance scheme.


                                END INFORMATION RESPONSE


Part 3: Information Request has now concluded, and the application is now in Part 5: Decision.

Should you have any questions with respect to the above, please do not hesitate to contact me
direct.

Yours faithfully
PROJECT URBAN




Scott Sobey
DIRECTOR
Email: [email protected]