Response to Information Request Letter · OPW25/0057

Response:

Amendments were undertaken to the 12dModel to ensure the culvert determining factor was inlet control and
corresponding upstream head loss coefficients used. Stormwater Calculation Results are contained on Covey Drawing
233467_C380_RevE.

ITEM – STORMWATER QUANTITY

Information Required:

3. Provide an update to the Flood Hazard Assessment report which specifically examines the impact resulting from
   changes in the site land use / imperviousness on stormwater characteristics within the downstream road
   network and downstream private land. Where unacceptable impacts are predicted to occur, these impacts are
   to be mitigated through the use of a detention basin or other means.

Response:

No hydraulic modelling has been undertaken to determine the detention requirements for each residential lot (if any).
The developer(s) of the individual lots must seek approval from Noosa Shire Council for any stormwater detention
infrastructure that may be required to facilitate the development of each lot. Stormwater detention infrastructure
must be contained within the lot that it serves.

The OPW application includes earthworks and civil infrastructure to suit a 1 into 3 development providing lots of similar
characteristics to those currently on site. The lots are to be stabilised with appropriate grass cover and retain the
‘Community Use’ zoning. Final development form of each lot is to be determined.

Flows generally move throughout the site in a similar manner to the existing regime, with Lot 1 and Lot 2 flows
converging towards the intersection of Lake Macdonald Drive and Dianella Court. Lot 3 continues to drain to the south
in line with the pre-developed case.

ITEM – CLIMATE CHANGE

Information Required:

4. Clarify what climate change assumptions have been adopted in the drainage design in order to meet PO2 of the
   Water Quality and Drainage Code. It is suggested that SPP2-4.5 and a planning horizon of 2100 be adopted
   consistent with the current methodology of ARR. This should be applied to both the drainage design and flood
   hazard assessment.

Response:

The stormwater design has been amended in consideration of the application of SPP2-4.5 and a planning horizon of
2100. To meet the flow requirements of the increased rainfall intensities, the following stormwater network
amendments have been applied:

-   4/A to 5/A increased from ø375 to ø450 for the increased stormwater flows
-   1/H to 2/G increased from ø450 to ø525 for the increased stormwater flows
-   Line K (Lot 3 outlet) increased from ø375 to ø450 for the increased stormwater flows
-   Q100 overland flows updated
-   Stormwater longitudinal sections updated for new pipe sizing and flows
-   HGL updated for increased stormwater flows
-   Swale calculations updated to suit increased flows – no change to swale sizing required
-   Bio-basin weir lowered by 100 mm to cater for increased 1 % AEP flows

ITEM – STORMWATER STRUCTURES 2/A & 4/A

Stormwater structures (2/A & 4/A) do not align with the 600 mm channel (900 mm x 900 mm structures).

Information Required:

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