SWMP Technical Memo Climate Change as submitted with Further Advice response

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3. Relationship to the submitted SWMP
The SWMP established the lawful point of discharge to Council's network in Myall Street, adopted DRAINS
for final hydrologic and hydraulic assessment, and proposed a 75 kL OSD tank with dual discharge controls.
The values below provide the relevant basis from SWMP sections 3.3, 4.5, 4.6 and 7.2.
Table 1 - Submitted SWMP design basis

   Item                                               Value                          SWMP reference

   Main detention tank                                           75.00 kL            SWMP section 7.2 / Drawing C054

   Submitted 1% AEP storage                                     74.53 m3             SWMP Table 4-2

   Submitted 1% AEP water depth                                  1.49 m              SWMP Table 4-2

   Lower discharge control                              170 mm at RL 116.84          SWMP Table 4-2

   Secondary discharge control                          245 mm at RL 117.84          SWMP Table 4-2

   Outlet pipe                                                 DN375 RCP             SWMP Table 4-2

                                                                                     Pre-development / mitigated; SWMP Table 4-
   Submitted 1% AEP flow                                    0.179 / 0.161 m3/s
                                                                                     3




4. Model amendments
4.1 Climate-adjusted 1% AEP rainfall
The BoM IFD file issued 11 June 2026 for latitude -26.41952122 and longitude 152.9095387 was duplicated
as a dedicated climate scenario. Every rainfall depth in the 1% AEP column was multiplied by 1.20; all other
AEP columns were retained unchanged. The revised file was imported into DRAINS and the 1% AEP
ensemble was analysed across the relevant durations and temporal patterns.
Table 2 - Representative climate-adjusted 1% AEP rainfall depths

   Duration                    BoM 1% AEP depth (mm)                             Climate-adjusted depth (mm)

            5 min                                   26.30                                             31.56

            10 min                                  41.20                                             49.44

            15 min                                  52.00                                             62.40

            20 min                                  60.80                                             72.96

            25 min                                  68.30                                             81.96

            30 min                                  74.90                                             89.88

            45 min                                  91.50                                             109.80




4.2 ILSAX soil classification
The ILSAX soil parameter was revised from Soil Type 3 to Soil Type 2. Soil Type 2 corresponds generally to
Hydrologic Soil Group B and represents sandy or sandy-loam soils with moderate infiltration. This better
reflects the sandy character of the site while remaining conservative relative to a Soil Type 1 clean, deep
sand assumption by allowing for fines, construction disturbance and compaction within developed


P0478-26-001 | Cooroy Social Housing | RFI Item 4              Page 2 of 6