Original source · versioned page text
SWMP Technical Memo Climate Change as submitted with Further Advice response
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
TECHNICAL MEMORANDUM
Response to RFI Item 4 - Climate Change Allowance
To Development Assessment, Noosa Shire Council
From Eoin Whitmore, RPEQ 31423 - RECOR Pty Ltd
Date 12 August 2026
Project Cooroy Social Housing - 45 Maple Street and 9 Myall Street, Cooroy
RECOR reference P0478-26-001
Subject Supplement to the Stormwater Management Plan dated 24 June 2026
1. Purpose and response summary
This technical memorandum supplements the Stormwater Management Plan (SWMP) dated 24 June 2026
and responds to Council's RFI Item 4 concerning the climate-change allowance. The DRAINS model has
been revised and rerun for the 1% AEP event with rainfall depths increased by 20%. The updated model
confirms that the current civil design achieves a non-worsening discharge outcome without increasing the
nominated 75 kL main detention tank.
2. Council requirement and applicable guideline
Council's RFI refers specifically to section SC6.7.6(1)(d)(i) of Planning Scheme Policy 6 - Engineering Design
Standards. Under the 'Climate Change' heading, the Noosa Plan 2020 requires drainage design and flood
impact assessments to use the latest Bureau of Meteorology IFD data for the study location. It identifies the
major drainage design event and defined flood event as the BoM 1% AEP IFD with a 20% increase in rainfall
intensity for future climate change.
Accordingly, the climate-change verification undertaken for this response applies the 20% uplift to the
complete 1% AEP rainfall-depth series used by DRAINS. The other AEP events remain as assessed in the
submitted SWMP. The adjoining 800 mm tidal-tailwater allowance is not applicable because the site
discharges to Council's inland stormwater network in Myall Street rather than directly to a tidal waterway.
Guideline source: Noosa Plan 2020 - Schedule 6 Planning Scheme Policies, PSP6, SC6.7.6(1)(d)(i), PDF page 54
P0478-26-001 | Cooroy Social Housing | RFI Item 4 Page 1 of 6
See the original
The page text is free to read. Viewing or downloading an original file needs an account.
Log in to see the originalSearchable page text hides email addresses. Original files are unchanged and may show email addresses.
Full text of the other 5 pages
Page 2
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
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
Page 3
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
landscaped areas. The revised classification was applied consistently to the like-for-like pre-development
and post-development climate scenarios.
4.3 External stormwater pipe
The downstream external DN375 RCP link was refined in the DRAINS model using the lesser proposed grade
shown on the current operational works longitudinal section. Drawing C052, Revision C, identifies the final
external connection grade as approximately 4.77%. This amendment more accurately represents the
proposed constructed system and its hydraulic routing and conveyance characteristics.
4.4 Online 15 kL rainwater harvesting tank
The current Revision C drawings include a 15 kL online rainwater harvesting tank upstream of the 75 kL
main OSD tank. Drawing C052 notes that the tank is to detain 15 kL before overflow continues to the
downstream detention tank, and Drawing C054 shows both tanks in the coordinated layout. The online
tank provides rainwater harvesting and reuse, regular operational drawdown and additional upstream
attenuation before overflow reaches the main tank.
The two tanks provide 90 kL of nominal gross tank volume across the site. Importantly, the climate-change
compliance result reported below does not rely on the full 15 kL harvesting volume being empty at the start
of the design storm: the modelled 75 kL main OSD tank remains within capacity on its own. The harvesting
tank therefore provides additional resilience while also supporting Council's rainwater harvesting and reuse
objectives.
5. Climate-adjusted modelling results
DRAINS was run using the Full Unsteady hydraulic model. The ensemble analysis identified the 25-minute
burst, Storm 4, as the critical storage case for the climate-adjusted 1% AEP event. The updated results are
summarised below.
Table 3 - Updated 1% AEP + 20% climate-change results
Result Model output Assessment
Pre-development discharge 0.151 m3/s Like-for-like climate scenario
Post-development unmitigated discharge 0.197 m3/s Before detention
Post-development mitigated discharge 0.148 m3/s After the 75 kL OSD controls
Change from pre-development -0.003 m3/s (-2.0%) Non-worsening achieved
Critical storage case 25 min, Storm 4 1% AEP ensemble
Peak main-tank storage Approx. 74.0 m3 Approx. 98.7% of 75 kL
Peak main-tank level RL 117.98 m Storage curve upper level RL 118.00 m
Controlled outlet split 0.088 + 0.060 m3/s Total 0.148 m3/s
Modelled overflow 0.000 m3/s No overflow reported
Assessment. The controlled climate-adjusted discharge of 0.148 m3/s is below the corresponding
pre-development discharge of 0.151 m3/s. Peak storage remains below the 75 kL modelled capacity
and no overflow occurs. The current main tank sizing and outlet controls therefore remain suitable
for the required climate-change design case.
P0478-26-001 | Cooroy Social Housing | RFI Item 4 Page 3 of 6
Page 4
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
Figure 1 - DRAINS climate-adjusted 1% AEP model results (Full Unsteady hydraulic model)
The upper model branch reports the pre-development discharge of 0.151 m3/s. The middle branch reports
0.197 m3/s unmitigated. The lower detained branch reports two controlled outlet components of 0.088
and 0.060 m3/s, combining to 0.148 m3/s, with zero overflow.
P0478-26-001 | Cooroy Social Housing | RFI Item 4 Page 4 of 6
Page 5
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
6. Storage verification
Figure 2 - Main OSD tank storage-elevation relationship
Figure 3 - Critical 1% AEP + 20% storage hydrograph: 25-minute burst, Storm 4
The storage-elevation curve provides approximately 75 m3 at RL 118.00 m. The critical hydrograph peaks at
approximately 74 m3, corresponding with the modelled peak level of RL 117.98 m. The storage
subsequently draws down through the adopted dual outlet controls.
P0478-26-001 | Cooroy Social Housing | RFI Item 4 Page 5 of 6
Page 6
ENGINEERING & PROJECT SERVICES
Sunshine Coast | Brisbane
www.recor.au | ABN 89 647 469 707
7. SWMP implications and conclusion
This memorandum is to be read as an addendum to the SWMP dated 24 June 2026. The lawful point of
discharge, catchment strategy, 75 kL main OSD tank, 170 mm and 245 mm discharge controls and DN375
outlet remain as described in SWMP sections 4.5, 4.6 and 7.2. The updated climate scenario supplements
the original Table 4-3 by demonstrating non-worsening under the specific 1% AEP + 20% Council
requirement.
• The 1% AEP rainfall-depth series has been increased by 20% in accordance with Noosa PSP6 section
SC6.7.6(1)(d)(i).
• The Type 2 ILSAX soil classification reflects sandy soils while conservatively allowing for fines and
development compaction.
• The external DN375 RCP model grade has been refined to match the lesser proposed grade on
Drawing C052, Revision C.
• The 15 kL online harvesting tank overflows to the 75 kL OSD tank and provides additional upstream
attenuation and rainwater reuse functionality.
• The mitigated climate-adjusted discharge is 0.148 m3/s, which is below the like-for-like pre-
development discharge of 0.151 m3/s.
• The main OSD tank peaks at approximately 74 m3 with no modelled overflow; no increase to the
nominated 75 kL main tank is required.
8. Reference documents
• RECOR, Stormwater Management Plan - 45 Maple Street and 9 Myall Street, Cooroy, P0478-26-001,
dated 24 June 2026.
• RECOR, Cooroy Social Housing Operational Works Drawings, P0478-26-001, Revision C, dated 6
August 2026, including Drawings C050, C052 and C054.
• Bureau of Meteorology, All Design Rainfall Depths for latitude -26.41952122, longitude
152.9095387, issued 11 June 2026; dedicated climate-adjusted 1% AEP file prepared with a 1.20
multiplier.
• Noosa Plan 2020, Schedule 6 - Planning Scheme Policies, PSP6 Engineering Design Standards,
SC6.7.6(1)(d)(i), Climate Change.
• Watercom DRAINS User Manual, ILSAX hydrological model soil classifications.
Prepared by
Eoin Whitmore | RPEQ 31423 | RECOR Pty Ltd
P0478-26-001 | Cooroy Social Housing | RFI Item 4 Page 6 of 6