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REPS - Technical Memorandum
Technical Memorandum 51 Elanda Street Stormwater Management Plan Addendum Client: Archend Council: Noosa Shire Council Document reference: 51 Elanda Street- Response to Council information request Prepared By: Adam Young Signed: Eoin Whitmore RPEQ: 31423 Date: 26/09/2025 Purpose This technical memorandum has been prepared in response to Council’s Further Advice Request for application MCU24/0153, OPW24/0202, and PLM24/0024 at 51 Elanda Street, Sunshine Beach. The purpose of this memo is to address the issues raised by Council and provide a detailed response to ensure the resolution of outstanding matters related to the civil scope of works. As part of this submission, amended Operational Works plans have been attached. These plans have been updated to reflect the responses outlined in this memorandum and demonstrate compliance with the relevant requirements. Issue – Updated Stormwater Engineering Drawings and Report Amended plans and associated reports must be submitted to Council prior to any further development applications being made and must include the following to the reasonable satisfaction of the Manager, Development Assessment: 1.1. Provide updated stormwater and engineering drawings including: a. Additional detail demonstrating that the post-development peak flow rate discharging to structure “7/1” does not exceed the pre-development peak flow rate. The detail must clearly demonstrate that the development and calculations incorporate appropriate climate change allowances in accordance with AO2.4 of the Water Quality and Drainage Code. b. Additional detail confirming an appropriate climate change allowance has been T 1300 469 707 U WWW.RECOR.AU PAGE 1 OF 7
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applied to the sizing of all detention tanks.
c. A stormwater design calculation table.
d. Details of detention outlet orifices / controls.
f. Prepared by a suitably qualified person (RPEQ)
RECOR RESPONSE:
A 12d Model drainage analysis was conducted for the entire site. The 1% AEP (Q100) and 10% AEP
(Q10) hydraulic grade lines (HGLs) are presented in the stormwater long section plans for reference.
The site drainage and detention/ rainwater harvesting tanks have been modelled in 12d using the
ILSAX method of dynamic drainage analysis. All rainfall data was obtained from the Bureau of
Meteorology (BOM) Intensity-Frequency-Duration (IFD) data website and is based on the latest 2019
data. The rainfall data obtained was then multiplied by a factor of 1.2 to simulate an increase of 20%.
Refer tables 1 and 2 below for the original and climate change adjusted rainfall data.
Table 1: IFD Table
Annual Exceedance Probability (AEP)
Duration 63.2% 50%# 20%* 10% 5% 2% 1%
1 min 173 195 263 308 351 405 446
2 min 147 167 230 273 316 375 420
3 min 138 157 215 254 293 346 386
4 min 131 149 203 239 275 321 357
5 min 125 142 193 227 259 301 333
10 min 103 116 155 181 205 236 259
15 min 87.0 98.1 132 153 174 199 218
20 min 76.0 85.7 115 134 152 175 192
25 min 67.8 76.5 103 120 137 158 173
30 min 61.4 69.3 93.5 109 125 144 159
45 min 48.6 55.0 74.9 88.2 101 118 131
1 hour 40.8 46.3 63.5 75.2 86.5 102 113
1.5 hour 31.7 36.1 50.2 59.8 69.4 82.2 92.2
2 hour 26.5 30.3 42.4 50.8 59.2 70.6 79.5
3 hour 20.5 23.6 33.5 40.4 47.4 56.9 64.4
4.5 hour 16.0 18.4 26.5 32.2 37.9 45.8 52.0
6 hour 13.4 15.5 22.5 27.4 32.4 39.2 44.6
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9 hour 10.5 12.2 17.8 21.8 25.9 31.5 35.9 12 hour 8.78 10.3 15.1 18.6 22.1 26.9 30.7 18 hour 6.85 8.02 11.9 14.7 17.5 21.4 24.6 24 hour 5.71 6.70 9.96 12.3 14.8 18.2 20.9 30 hour 4.95 5.80 8.65 10.7 12.9 15.9 18.3 36 hour 4.38 5.14 7.68 9.54 11.5 14.2 16.4 48 hour 3.60 4.22 6.31 7.87 9.50 11.8 13.7 72 hour 2.68 3.14 471 5.89 7.14 8.98 10.5 96 hour 2.16 2.52 3.78 4.73 5.76 7.26 8.52 120 hour 1.81 2.12 3.17 3.97 4.83 6.10 7.16 144 hour 1.57 1.83 ae) 3.42 4.16 5.24 6.15 168 hour 1.39 1.62 2.41 3.01 3.65 4.59 5.37 Table 2: IFD Table - Rainfall intensities with 20% increase Annual Exceedance Probability (AEP) Duration 63.2% 50%# 20%* 10% 5% 2% 1% 1min 207.6 234 315.6 369.6 421.2 486 535.2 2 min 176.4 200.4 276 327.6 379.2 450 504 3 min 165.6 188.4 258 304.8 351.6 415.2 463.2 4min aia 178.8 243.6 286.8 330 385.2 428.4 Smin 150 170.4 231.6 272.4 310.8 361.2 399.6 10 min 123.6 139.2 186 217.2 246 283.2 310.8 15 min 104.4 117.72 158.4 183.6 208.8 238.8 261.6 20 min 91.2 102.84 138 160.8 182.4 210 230.4 25 min 81.36 91.8 123.6 144 164.4 189.6 207.6 30 min 73.68 83.16 112.2 130.8 150 172.8 190.8 45 min 58.32 66 89.88 105.84 121.2 141.6 157.2 1 hour 48.96 55.56 76.2 90.24 103.8 122.4 135.6 1.5 hour 38.04 43.32 60.24 71.76 83.28 98.64 110.64 2 hour 31.8 36.36 50.88 60.96 71.04 84.72 95.4 3 hour 24.6 28.32 40.2 48.48 56.88 68.28 77.28 4.5 hour 19.2 22.08 31.8 38.64 45.48 54.96 62.4 6 hour 16.08 18.6 27 32.88 38.88 47.04 53.52
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9 hour 12.6 14.64 21.36 26.16 31.08 37.8 43.08
12 hour 10.536 12.36 18.12 22.32 26.52 32.28 36.84
18 hour 8.22 9.624 14.28 17.64 21 25.68 29.52
24 hour 6.852 8.04 11.952 14.76 17.76 21.84 25.08
30 hour 5.94 6.96 10.38 12.84 15.48 19.08 21.96
36 hour 5.256 6.168 9.216 11.448 13.8 17.04 19.68
48 hour 4.32 5.064 7.572 9.444 11.4 14.16 16.44
72 hour 3.216 3.768 5.652 7.068 8.568 10.776 12.6
96 hour 2.592 3.024 4.536 5.676 6.912 8.712 10.224
120 hour 2.172 2.544 3.804 4.764 5.796 7.32 8.592
144 hour 1.884 2.196 3.276 4.104 4.992 6.288 7.38
168 hour 1.668 1.944 2.892 3.612 4.38 5.508 6.444
The below table shows the climate change adjusted Pre-Development Catchment Runoff estimations
provided in the original SCCE SMP prepared in November 2024.
Table 3: Predevelopment site discharge (m3/s)
Flow Event Existing Scenario(m³/s)
Q10 (10% AEP) 0.040
Q1 (1% AEP) 0.062
The following unmitigated post-development flow results were derived from the 12d model which
was run with the climate change adjusted rainfall data in Table 2 Above.
Table 4: Unmitigated post-development site discharge – adjusted for climate change (m3/s)
Flow Event Climate Change Incl. (m³/s)
Q10 (10% AEP) 0.050
Q1 (1% AEP) 0.084
The following mitigated post-development flow results were achieved with the use of an onsite
detention tank
Table 5: Mitigated post-development site discharge -adjusted for climate change (m3/s)
Flow Event Climate Change Incl. (m³/s)
Q10 (10% AEP) 0.035
Q1 (1% AEP) 0.057
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Table 6: Detention Tank Parameters
Detention Tank Parameters
Tank Volume 5.0m³
Storage Volume 4.8m³
Area 5.0m²
Base RL 42.849m
AEP1% max. water level RL 43.807m
AEP1% max. water level depth 0.958m
Orifice plate outlet diameter 150mm
Outlet pipe diameter 225mm
Orifice plate outlet control RL (middle of pipe) 42.924m
The 12d dynamic inflow and outflow outputs for the rainwater harvesting tank (structure 213) and
the detention tank (structure 6-1) are below:
AEP10%
Table 7: Structure 2-3 AEP10% inflows and outflows
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Table 8: Structure 6-1 AEP10% inflows and outflows AEP1% Table 9: Structure 2-3 AEP1% inflows and outflows 6
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Table 10: Structure 6-1 AEP1% inflows and outflows
In both the major and minor events, the climate change adjusted post development flows have been
effectively mitigated such that the outflows to the existing stormwater network are lower than the
predevelopment flow. It is therefore confirmed that Acceptable Outcome 2.4 of the water quality
and drainage code has been achieved.
The model also confirms that there is sufficient capacity in the external pit to take all flows from the
development. We trust that this Technical Memorandum, in conjunction with the amended civil
drawings, is sufficient to evidence to council that the stormwater quantity measures have been
designed to mitigate the 20% increase in rainfall allowing for climate change.
All other engineering items within the Decision Notice for application OPW24-0202 dated the 11th of
August 2025, have been addressed within the engineering drawings.
Please contact the undersigned for any further clarifications:
➢ Email- [email protected]
➢ Phone- 0451453640.
Eoin Whitmore, Associate Engineer
RPEQ Number: 31423
Date: 26/09/2025
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