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Flood Investigation 60 Glenridge Drive, Cooroibah Mark Bain Constructions 21-0288-FS001-V5 5 May 2022
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Full text of sections 2 to 28 (of 77)
Section 2
COMMERCIAL IN CONFIDENCE
This document is commercial-in-confidence. The recipient of this document agrees to hold all
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disclosure of the said information to unauthorized parties, directly or indirectly, irrespective of the
acceptance or rejection of the presentation or at any time before, during or after an
agreement has been reached, without prior written consent.
PEAKURBAN Pty Ltd
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Brisbane
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Spring Hill Qld 4000
Sunshine Coast
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Birtinya Qld 4575
Melbourne
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Notting Hill Vic 3168
E [email protected]
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DOCUMENT CONTROL
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Version Date Details Prepared Checked Approved Signed RPEQ
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4 01/04/22 Issue B. Bolt B. Bolt B. Bolt 11523
5 05/05/22 Issue B. Bolt B. Bolt B. Bolt 11523
Section 3
EXECUTIVE SUMMARY
PEAKURBAN Pty Ltd have been engaged by Mark Bain to prepare a Flood Investigation in
support of a development application to Noosa Shire Council (NSC) over land located at 60
60 Glenridge Drive, Cooroibah (Lot 47 RP 860577). This development application seeks a
development permit for a Reconfiguration a Lot (ROL) to create 8 rural residential lots.
The site is influenced by two unnamed tributaries of the Noosa River. Both catchments extend
upstream across McKinnon Drive to the west and join at the eastern boundary of the site. The
total catchment of both tributaries is 130ha. The catchment is characterised by steep upper
catchment and flattens out along the lower part of the flood plain. Elevations on site range
from 10 to 44m AHD.
Suitable flood immunity is achievable for future dwellings on the site, with the recommended
minimum building envelope / finished floor level (FFL) for each lot has been taken as the
greater of the DFE + freeboard (0.3m) or the PMF (refer Table 4-5). Each lot will be have a
place to refuge above Probable Maximum Flood within the future dwelling.
The investigation has found that there is a no off-site impacts, reduction in peak flows and
increase in flood storage provided. As such, any further formal on-site detention is not
recommended as development has been designed to achieve the outcome of non-
worsening.
A flood risk assessment was completed which identified that the use of the building envelope
as a flood refuge should a flood event greater than the 1% AEP occur.
The new internal road will achieve flood immunity in the 10% AEP and remain trafficable in the
1% AEP event. A 9 x 1200 (w) x 600 (h) box culverts are proposed under the road crossing.
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah |i
Section 4
CONTENTS
EXECUTIVE SUMMARY ............................................................................................................................ I
1. INTRODUCTION .............................................................................................................................. 1
Site Location .................................................................................................................................. 1
Response to Further Advice ........................................................................................................ 3
Response to Information Request ............................................................................................... 4
2. AVAILABLE DATA ........................................................................................................................... 6
3. HYDROLOGY .................................................................................................................................. 7
Catchment .................................................................................................................................... 7
Rainfall Depths .............................................................................................................................. 8
Peak Flow Rates and Verification ............................................................................................... 8
3.3.1. TUFLOW Peak Flows .................................................................................................. 8
4. HYDRAULICS ................................................................................................................................. 10
Model Setup ................................................................................................................................ 10
4.1.1. Topography ............................................................................................................. 10
4.1.2. Inflow ........................................................................................................................ 10
4.1.3. Roughness ................................................................................................................ 10
4.1.4. Boundary .................................................................................................................. 10
4.1.5. Culverts..................................................................................................................... 10
Model Scenarios ......................................................................................................................... 11
4.2.1. Existing Scenario (E02) ............................................................................................ 11
3.2.3 Proposed Scenario (P03) ........................................................................................ 11
Results ........................................................................................................................................... 12
4.3.1. Existing Scenario ...................................................................................................... 12
4.3.2. Proposed Scenario ................................................................................................. 12
4.3.3. Flood Hazard Overlay Code Response ................................................................ 17
5. CONCLUSION .............................................................................................................................. 18
6. REFERENCES ................................................................................................................................. 19
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Section 5
LIST OF TABLES
Table 2-1: Summary of Study Data ..................................................................................................... 6
Table 3-1: Catchment Details .............................................................................................................. 7
Table 3-2: Rainfall Depths ..................................................................................................................... 8
Table 3-3: 1% AEP Peak Flow Verification .......................................................................................... 8
Table 3-4: 1% AEP Peak Flow Comparison ......................................................................................... 9
Table 4-1: Model Roughness Values ................................................................................................. 10
Table 4-2: Model Culvert Information ............................................................................................... 11
Table 4-3: Existing and Proposed Peak discharge at the Site Boundary. ..................................... 12
Table 4-4: 1% (2100) AEP Flood Storage Comparison. ................................................................... 13
Table 4-5: Recommended Minimum Building Envelope and Finished Floor Level ..................... 14
Table 4-6: PMF Time of Inundation and Rate of Rise. ..................................................................... 17
LIST OF FIGURES
Figure 1-1: Site Location (Source: QGIS Open Street Map 2021) ................................................... 1
Figure 1-2: NSC Flood Hazard Overlay Mapping (Source: NSC Interactive Mapping) ............... 2
Figure 3-1: NSC Flood Search Levels (orange box) versus Site Model Results (white box) ......... 9
Figure 4-1: Section Across Internal Road Extension ........................................................................ 14
Figure 4-2: DFE (1% (2100) AEP) Peak Water Surface Elevation Reporting Locations ................ 15
APPENDICES
Development Plans
Catchment Plan
Rainfall Depths
TUFLOW Model Layout Plan
ARR Blockage Assessment
Existing Flood Maps (E02)
Proposed Flood Maps (P03)
Flood Afflux Maps (P03-E02)
Severe Storm Assessment (P03a)
GCCC Flood Refuge Guideline
Flood Hazard Overlay Code Response Flood Hazard Overlay Code Response
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | iii
Section 6
1. INTRODUCTION
PEAKURBAN Pty Ltd have been engaged by Mark Bain to prepare a Flood Investigation in
support of a development application to Noosa Shire Council (NSC) over land located at 60
60 Glenridge Drive, Cooroibah (Lot 47 RP 860577). This development application seeks a
development permit for a Reconfiguration a Lot (ROL) to create 8 rural residential lots.
The proposed outcome of this investigation is to inform the layout design with respect to
managing flood impacts, demonstrate non-worsening and sizing of cross-drainage culverts in
order to achieve the requirements and targets of NSC Planning Scheme “Flood Hazard Overly
Code”.
The proposed Plan of Subdivision and concept engineering plans can be found in Appendix
A.
Site Location
The location of the proposed development site is shown in Figure 1-1.
The site is influenced by two unnamed tributaries of the Noosa River. Both catchments
extend upstream across McKinnon Drive to the west and join at the eastern boundary of the
site. The total catchment of both tributaries is 130ha. The catchment is characterised by
steep upper catchment and flattens out along the lower part of the flood plain. Elevations
on site range from 10 to 44m AHD.
The site is located within the mapped NSC Flood Hazard Overlay (refer Figure 1-2).
SITE
Figure 1-1: Site Location (Source: QGIS Open Street Map 2021)
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Section 7
SITE
Figure 1-2: NSC Flood Hazard Overlay Mapping (Source: NSC Interactive Mapping)
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Section 8
Response to Further Advice
A Further Advice Request was received on 14 March, 2022 as part of Reconfiguration of Lot
Application No: RAL21/0033. Items 1 to 3 relate to Flooding, with response to each item
detailed below:
Issue
Blockage of hydraulic structures needs to be accounted for in the design of the new internal
roadway and assessment of impacts of the development.
Information Request:
1. Provide an updated Flood Investigation report which documents the blockage
assumptions for each of the culverts represented in the ‘existing’ and ‘proposed’ model
scenarios and demonstrate the assumptions are consistent with the guidance of ARR2019
Book 6 Chapter 6.
Response:
A blockage assessment was completed and identified a 20% blockage should be applied to
the culverts within the model. With the inclusion of 20% blockage on the new culvert
crossing, an additional cell is required (9 x 1200 (w) x 600 (h)).
Issue
Building envelopes need to be entirely flood free.
Information Request:
2. Update the TOD subdivision plan such that the entirety of the building envelopes
achieves the minimum levels specified in Table 4-5 of the updated ‘Flood Investigation’
report.
Response:
Site plan has been updated to show building envelope extents being filled.
Issue
Lot 1 building envelope flood immunity for the 1% (2100) AEP is required to be achieved by
filling. The access of the building envelope to Glenridge Drive is inundated in the 39%AEP
event. These outcomes are unacceptable and contrary to the Flood Hazard Overlay Code.
Information Request:
3. Update the TOD subdivision plan to Delete Lot 1 and amalgamate with proposed Lot
Response:
Layout has been updated to address this item.
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Section 9
Response to Information Request
An Information Request was received on 21 January, 2022 as part of Reconfiguration of Lot
Application No: RAL21/0033. Items 22 to 25 relate to Flooding, with response to each item
detailed below:
Issue: Flooding
The Flood investigation:
i. includes comments regarding the impacts of “blockage of stormwater culverts” but
does not indicate the impacts (depth, velocity) of “blockage” on the proposed new
road, and
ii. does not include any comment regarding the probable maximum flood (PMF)
iii. does not include sufficient detail to demonstrate that the development (access and
all lots) achieve compliance with The Flood Hazard Overlay Code – Performance
Outcome PO4
iv. the investigation recommends that the floor levels be a minimum of 500mm above
the flood level while the planning scheme requires 300mm.
Information Request:
22. Provide additional detail on the impacts (depth, velocity, safety) of 100% blockage of
the stormwater culverts on the proposed new road
Response: Whilst it is not a design requirement for the 100% blocked scenario to achieve
QUDM design requirements, a comparison against the QUDM road drainage criteria has
been made for some context. In the 100% blocked scenario, the depth of flow exceeds the
0.2m QUDM depth criteria in the upstream lane (with depths up to 0.25m noted), however, is
less than 0.2m on the downstream lane. Flow velocity is up to 1.75m/s. Approximately 80m
of the road has a hazard greater than the 0.3m2/s QUDM hazard criteria (with hazard up to
0.5m2/s noted).
Information Request:
23. Include additional comments regarding the probable maximum flood (PMF) and the
impacts on the safety of residents (within the site and on adjoining properties) and road
users.
Response: An assessment of the PMF event found that flood levels in the PMF event are
approximately 0.5m higher that the 1% (2100) AEP flood levels. It is recommended that floor
levels be constructed to a minimum PMF level so as to provide a refuge for residents in this
event. It also noted that the cul-de-sac end of the internal road is also above the PMF level
and could act as a refuge.
Information Request:
24. Provide additional comment in regard to the recommended minimum floor levels
and the potential impacts of the PMF.
Response: The Planning Scheme requires a minimum floor level of dwellings to be 0.3m
above the 1% (2100) AEP event. An assessment of the PMF event found that flood levels in
the PMF event are approximately 0.4m to 1.0m higher that the 1% (2100) AEP flood levels.
The recommended minimum building envelope / finished floor level (FFL) for each lot has
been taken as the greater of the DFE + freeboard (0.3m) or the PMF, as summarised in Table
4-5, so as to provide a refuge for residents in this event.
Information Request:
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Section 10
25. Provide additional detail to demonstrate that the development (access and all lots)
achieves compliance with The Flood Hazard Overlay Code – Performance Outcome PO4.
Response: The Planning Scheme requires a minimum floor level of dwellings to be 0.3m
above the 1% (2100) AEP event. An assessment of the PMF event found that flood levels in
the PMF event are approximately 0.4m to 1.0m higher that the 1% (2100) AEP flood levels.
The recommended minimum building envelope / finished floor level (FFL) for each lot has
been taken as the greater of the DFE + freeboard (0.3m) or the PMF, as summarised in Table
4-5, so as to provide a refuge for residents in this event.
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Section 11
2. AVAILABLE DATA
The data detailed in Table 2-1 was reviewed and used in this assessment.
Table 2-1: Summary of Study Data
ID Data Description
1 Digital Elevation 1 metre resolution digital elevation model (DEM) based on
Model Aerial LiDAR survey captured by DERM in 2015 (vertical
accuracy of +/- 200mm [2 sigma error rate])
2 Aerial Photograph Aerial photography captured by NearMap (10 September
2021).
3 GIS Information Stormwater pipe, pit and infrastructure GIS information
supplied by NSC.
4 Layout AutoCAD layout of proposed development (29 April 2022 by
TOD Consulting).
5 ARR Data Hub North East Coast, Mary Rivers (accessed 26 October 2021 for
Latitude: -26.37, Longitude: 152.98)
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Section 12
3. HYDROLOGY
Hydrologic modelling of the catchment has been carried out using WBNM software (2019
version) developed jointly by the University of Wollongong, Rienco Consulting and Balance
R&D. Australian Rainfall and Runoff 2019 (ARR2019) methodology was used for this
assessment.
Catchment
The objective of this component of the study was to investigate the peak flows contributing
to the site. The catchment has been delineated based on 2015 LiDAR survey data. The
catchment layout is shown in Appendix B, with a total catchment area of 434 ha being
assessed. The proposed site is located within sub-catchment 1H and 1L, with a contributing
catchment area of 130 ha. A summary of the catchment details is presented below in Table
3-1. The impervious values applied are representative the rural, rural residential and
bushland land-use of the catchment. The existing and proposed impervious values are
noted for catchments 1H and 1L to represent the development.
The default catchment lag parameter within WBNM of 1.6, was applied to the model setup.
Rainfall losses have been applied as per the ARR Data Hub, with an initial loss of 35mm, and
continuing loss of 3.0mm/hr for pervious areas. 0mm was applied for impervious. The median
pre-burst rainfall depth was applied to the model setup.
Table 3-1: Catchment Details
Catchment Downstream Area (ha) Impervious
Name Catchment (%)
1A 1C 29.842 5
1B 1C 14.787 10
1C 1F 18.826 10
1D 1H 19.329 0
1E 1G 24.075 0
1F 1K 4.325 10
1G 1H 13.695 10
1H 1M 6.056 0 / 10
1I 1K 4.445 10
1J 1L 3.637 20
1K 1L 1.461 20
1L 1M 11.036 5 / 20
1M 2F 5.527 10
2A 2C 64.299 0
2B 2C 62.751 2
2C 2D 28.965 2
2D 2E 38.306 5
2E 2F 43.203 10
2F SINK 39.526 10
TOTAL 434.091
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Section 13
Rainfall Depths
Rainfall depths for events up to the 1% AEP were obtained from the BoM website and the
Probably Maximum Precipitation (PMP) event was determined using the Generalised Short-
Duration Method (GSDM) and the Generalised Tropical Storm Method Revised (GSTMR).
Calculation sheet for the GSDM and GSTMR is provided in Appendix C. The rainfall depth for
the 39%, 10%, 1%, 1% (2100) AEP and PMP events are summarised below in Table 3-2.
Table 3-2: Rainfall Depths
Duration 39% AEP 10% AEP 1% AEP 1% (2100) AEP PMP
(hours)
0.25 26.2 37 52.9 63.5 190
0.50 36.9 52.7 76.5 91.8 290
0.75 43.8 63.3 93.5 112.2 360
1.00 49.1 71.7 107 128.4 420
1.50 57.4 85.1 130 156 540
2 64.1 96 149 178.8 630
2.50 69.8 106 165 198 700
3 74.8 114 180 216 760
4 83.7 129 206 247.2 880
5 87.7 136 218 261.6 960
6 91.4 143 229 274.8 1020
12 98.3 154 249 298.8 1250
Peak Flow Rates and Verification
The peak flow rates estimated from the WBNM model have been verified against the rational
method for the 1% AEP event, with the Bransby-Williams equation adopted to determine the
time of concentration as recommended by the Department of Transport and Main Roads
“Road Drainage Manual 2015”. The flow verification was completed in relation to two sub-
catchments (1A and 2C). Details of the flow verification are presented in Table 3-3. Based
on this assessment it can be seen a suitable verification has been achieved.
Table 3-3: 1% AEP Peak Flow Verification
Parameter Value Value
Sub-Catchment 1A 2C
Contributing Area 29.8 ha 156.0
Time of Concentration 38 min 92 min
Runoff Coefficient 0.7 0.7
Rational Peak Flow 9.1 m3/s 31.2
WBNM Peak Flow 8.1 m3/s 33.0
Critical Duration 720 min 720 min
3.3.1. TUFLOW Peak Flows
A comparison of peak flows from the WBNM hydrology model to that estimated within the
TUFLOW hydraulic model is presented below Table 3-4 at catchment 1A and 2C for the 1%
AEP event. It can be seen that the TUFLOW hydraulic model estimates for these two
catchments correlate well to the WBNM output, with peak flows from Catchment 1A slightly
less in the hydraulic model and Catchment 2C slightly more. The critical duration also
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Section 14
changed from 720min to the 120min, with the TUFLOW hydraulic model considered a better
representation of flow routing within the catchment.
Table 3-4: 1% AEP Peak Flow Comparison
Parameter Value Value
Sub-Catchment 1A 2C
WBNM (Hydrology) 8.1 (720min) 33.0 (720 min)
TUFLOW (Hydraulics) 7.5 (120 min) 33.5 (120 min)
Difference -0.6 +0.5
A comparison of the peak 1% (2100) AEP flood levels from the NSC flood search and TUFLOW
model results from this investigation is presented below in Figure 3-1. This highlights that the
estimated levels are a close match across the site with a levels at the downstream boundary
of the site slightly higher in this current investigation (which is considered to create a
conservative outcome). As such it is considered that a suitable verification between the NSC
regional flood model and the current investigation has been achieved.
12.56
.36
13.36
12.37
.36
12.37
13.00
Figure 3-1: NSC Flood Search Levels (orange box) versus Site Model Results (white box)
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Section 15
4. HYDRAULICS
Hydraulic modelling has been carried out using TUFLOW software developed by BMT WBM.
TUFLOW is a computational engine that provides one-dimensional (1D) and two-dimensional
(2D) solutions for the free-surface flow equations to simulate flood and tidal wave
propagation. The Heavily Parallelised Compute (HPC) computational method has been
applied using 2020-10-AA version of TUFLOW. Modelling assumptions and outcomes are
described below.
Model Setup
4.1.1. Topography
TUFLOW model topography is based upon the DEM that was created from 2015 LiDAR and
detailed site survey. The catchment has been represented using a 5m hydraulic grid size.
The internal road was represented the proposed road profile using a 2d_zsh, with the crown
having a minimum level of 12.81m AHD above the culvert.
The building envelopes were also raised to ensure flood immunity using a 2d_zsh, with minor
regrading of lot beside building envelope of Lot 1.
4.1.2. Inflow
Inflows have been incorporated into the model setup using inflow polygons (2d_sa) to read
in the inflow hydrographs from the WBNM hydrology model.
4.1.3. Roughness
The roughness of the flow path has been incorporated into the model using Manning’s ‘n’
values to represent the differing land-uses. Table 4-1 outlines the applied roughness values.
Table 4-1: Model Roughness Values
Description Manning’s ‘n’
Road 0.015
Road Reserve 0.03
Cleared / Sporadic Vegetation 0.04
Low Dense Vegetation 0.06
Highly Dense Vegetation 0.10
4.1.4. Boundary
The TUFLOW model has been extended 1.5 km downstream of the study area such that tail
water assumptions do not influence flooding at the site. A normal depth rating curve has
been adopted for the downstream.
4.1.5. Culverts
A total of eighteen (18) existing culverts were represented within the model extent, with
culvert information obtained from Noosa Shire Council GIS information. The new ‘internal’
culvert has been sized to ensure the internal road servicing the development can achieve
immunity in the 10% AEP and remain trafficable in the 1% (2100) AEP event. A summary of
the culverts represented in the model are detailed in Table 4-2.
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Section 16
A blockage factor of 20% has been applied as per the ARR blockage assessment completed
in Appendix E.
Table 4-2: Model Culvert Information
ID Size Length USIL (m AHD) DSIL (m AHD)
1 1 x 1800 x 900mm RCBC 7.00 14.66 14.56
2 1 x 900mm RCP 11.15 14.08 14.01
3 1 x 300mm RCP 11.17 14.04 13.95
4 1 x 300mm RCP 10.74 13.63 13.60
5 1 x 450mm RCP 10.78 13.13 13.06
6 1 x 375mm RCP 15.00 12.89 12.78
7 4 x 750mm RCP 15.00 12.53 12.49
8 1 x 300 x 450mm RCBC 11.83 12.77 12.74
9 1 x 380mm RCP 13.32 13.40 13.30
10 1 x 380mm RCP 11.55 14.60 14.50
11 10 x 750mm RCP 11.2 13.95 13.80
12 3 x 1200 x 900mm RCBC 11.2 11.94 11.83
13 1 x 1500mm RCP 9.79 11.30 11.10
14 5 x 900mm RCP 11.2 15.68 15.62
15 1 x 380mm RCP 10.99 15.95 15.8
16 10 x 750mm RCP 11.2 14.05 13.95
17 5 x 2400 x 1500 9.24 6.50 6.40
18 1 x 450mm RCP 6.2 8.72 8.68
Internal 9 x 1200mm x 600mm RCBC 10.00 11.80 11.75
Driveway 1 x 300mm RCP 5 12.94 12.92
Model Scenarios
The Existing (E01) and Proposed (P01) scenarios are described below and depicted in
Appendix D.
4.2.1. Existing Scenario (E02)
In representing the Existing Scenario, the model was setup to represent the overland flow
characteristics that existed prior to the site being developed. A combination of LiDAR and
detailed survey was used to represent the existing model terrain. The model was run for the
63.2%, 39%, 10% and 1% AEP for durations ranging from 30min to 12hrs.
3.2.3 Proposed Scenario (P03)
The Proposed Scenario has utilised the Existing Scenario model to represent the proposed
development. The following changes were made:
• Represent internal road at RL12.7m AHD (2d_zsh).
• Represent new 9 x 1200 x 600mm RCBC under internal road (1d_nwk).
• Represent filling of building envelopes of Lot 1 and 4 to achieve flood immunity
(2d_zsh).
• Represent minor regrading beside building envelope of Lot 1 (2d_zsh).
• Adjustments to roughness to account for new road (2d_mat).
The model was run for the 63.2%, 39%, 10% and 1% AEP for durations ranging from 60min to
24hrs.
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Section 17
Results
The TUFLOW hydraulic model been assessed for a range of flood events including the 63.2%,
39%, 10%, 1%, 1% (2100) AEP storms and the Probable Maximum Flood (PMF) event. The
assessment has considered a range of storm durations to ensure that the critical storm is
defined. The results of the hydraulic assessment are presented as a series of flood maps to
illustrate the flooding characteristics of the overland flow path and the resulting flood
behaviours including flooded depth, height, velocities, hazard and afflux.
The flood maps are presented in Appendix F and Appendix I.
4.3.1. Existing Scenario
The existing scenario flood maps are shown in Appendix F. This details the existing flood
extent and behaviour in relation to the development site. The results of this have been used
as the basis for assessing the potential impacts that may result from the development.
4.3.2. Proposed Scenario
The proposed scenario flood maps are shown in Appendix G. This details the proposed flood
extent and behaviour in relation to the development site. An indicative lot plan is overlaid on
this map set to highlight how the development relates to the flood plain.
The PMF event was run for the proposed scenario to identify flood behaviour in this extreme
event and how residents would be able to safely manage this event from their property. The
PMF level across the site ranges from 13.48 to 13.51m AHD, which is approx. 0.4m and 1.0m
higher at the upstream and downstream boundary of the site, respectively.
Each building envelope will require fill to achieve appropriate freeboard, however, all filling is
generally above the 1% (2100) AEP flood level. Minor filling of Lot 1 is required and some
minor shaping of the land beside the Lot 1 building envelope to manage afflux cause by
flooding backing up behind the building envelope.
4.3.2.1. Peak Discharge
The assessment found the peak discharge at the site boundary are predicted to decrease
across all modelled events. This is shown in Table 4-3 which compares the Existing and
Proposed peak discharge at the site boundary for each event. As such it is considered that
a formal detention is not required to service this development as the design has achieved
the required non-worsening outcome.
Table 4-3: Existing and Proposed Peak discharge at the Site Boundary.
Design Critical Duration Existing Peak Proposed Peak Difference (m3/s)
Event (AEP) (min) / Temporal Discharge (m3/s) Discharge (m3/s)
Pattern
1% (2100) 120 (TP3) 49.00 48.81 -0.19
1% 120 (TP6) 38.83 38.70 -0.13
10% 180 (TP8) 22.93 22.45 -0.48
39% 540 (TP5) 8.94 8.89 -0.05
63.2% 540 (TP5) 6.19 6.16 -0.02
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Section 18
4.3.2.2. Afflux
Afflux mapping has been undertaken for the 63.2%, 39%, 10% and 1% AEP to identify whether
there are any impacts resulting, and if so, whether an alternative development arrangement
needs to be investigated. Afflux maps have been provided in Appendix H.
The afflux mapping shows that there are no external impacts resulting to any adjoining
properties as a result of the development. An increase in flood levels of up to 330mm is
estimated upstream of the proposed Internal Road extension in the 1% (2100) AEP, however,
this is contained within the site boundary.
4.3.2.3. Flood Storage
A flood storage assessment has been completed over the site to ensure there is no loss of
flood plain storage as a result of the development. For the existing flood storage calculation,
the existing peak 1% (2100) AEP flood surface was compared to the existing model terrain.
Similarly with the proposed flood storage calculation, the proposed peak 1% (2100) AEP flood
surface was compared to the proposed model terrain. Details of the flood plain storage in
the existing and proposed cases is summarised below in Table 4-4, where it is shown there is
an increase in the amount of flood storage as a result of the development, which is
considered positive outcome.
Table 4-4: 1% (2100) AEP Flood Storage Comparison.
Scenario Flood Plain Storage (m3)
Existing 36,831
Proposed 45,932
Difference +9,102
4.3.2.4. Internal Road
The Internal Road extension has been designed achieve flood immunity in the 10% AEP
event, whilst allowing some overtopping in the 1% AEP (major event). The high point of the
road (which acts as a weir) is at a minimum 12.81m AHD (refer Figure 4-1). This results in a
flow depth of less 0.2m occurring over the road pavement in the 1% AEP, with flow velocities
less than 1.6 m/s and flow hazard less than 0.3m2/s. These parameters are below the
trafficability requirements of QUDM, which require a maximum depth of 0.2m and hazard less
than 0.3m2/s in the 1% AEP event.
The new culverts have a flow velocity of 2.1m/s. Outlet scour protection will be required to
be provided with D50=200mm rock provided and extend 3.0m past apron.
It is recommended that appropriate depth markers and flood warning signs be placed on
either side of the internal road flood extents to warn residents of flood risk and depth of water
over road should it overtop.
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 13
Section 19
Flow depth < 0.2m
1% AEP WSL
Internal Road
Figure 4-1: Section Across Internal Road Extension
4.3.2.5. Flood Levels and Immunity
Flood levels around the site for the Defined Flood Event (DFE) (i.e. 1% (2100) AEP) are detailed
for various reporting locations as shown in Figure 4-2. This shows the proposed new building
envelopes are above the DFE level. The recommended minimum building envelope /
finished floor level (FFL) for each lot has been taken as the greater of the DFE + freeboard
(0.3m) or the PMF, as summarised in Table 4-5.
Table 4-5: Recommended Minimum Building Envelope and Finished Floor Level
Lot 1% (2100) Minimum 1% (2100) PMF Peak Min Building
AEP Peak Required AEP + 0.3m Flood Level Envelope / FFL
Flood Level Freeboard freeboard (m AHD) (m AHD)
(m AHD) (m) (m AHD)
1 13.24 0.3 13.54 13.52 13.54
2 12.99 0.3 13.29 13.51 13.51
3 12.99 0.3 13.29 13.52 13.52
4 12.99 0.3 13.29 13.53 13.53
5 12.99 0.3 13.29 13.52 13.52
6 12.58 0.3 12.88 13.50 13.50
7 12.58 0.3 12.88 13.48 13.48
8 12.48 0.3 12.78 13.46 13.46
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 14
Section 20
Figure 4-2: DFE (1% (2100) AEP) Peak Water Surface Elevation Reporting Locations
4.3.2.6. Probable Maximum Flood
The PMF event was run for the proposed scenario to identify flood behaviour in this extreme
event and how residents would be able to safely manage this event from their property. The
PMF level across the site ranges from 13.48 to 13.51m AHD, which is approx. 0.4m and 1.0m
higher at the upstream and downstream boundary of the site, respectively. Flood depth
map is provided in Appendix G, noting that an indicative fill envelop has been represented
within the model (with future allotment owners to install required minimum building envelop).
It is proposed that the building envelope and finished floor level of Lots 1 to 8 will be located
above the PMF flood level. From which, if this event was to occur, residents would be able to
seek refuge within their homes until flood waters recede.
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 15
Section 21
4.3.2.7.
4.3.2.7. Severe Storm Assessment
A severe storm assessment was completed for the 1% (2100) AEP event assuming all of the
stormwater culverts are 100% blocked to ensure the future dwellings within the development
site do not become inundated and no new flow paths are formed that may impact
adjoining properties.
The assessment found that the building envelopes are not impacted by the 100% blocked
scenario with a flood level increase of up to 0.08m noted, which is less than the minimum
0.3m freeboard required. It is also noted that no new flow paths form as a result of the 100%
blocked scenario and adjoining properties are not impacts.
Whilst it is not a design requirement for the 100% blocked scenario to achieve QUDM design
requirements, a comparison against the QUDM road drainage criteria has been made for
some context. In the 100% blocked scenario, the depth of flow exceeds the 0.2m QUDM
depth criteria in the upstream lane (with depths up to 0.25m noted), however, is less than
0.2m on the downstream lane. Flow velocity is up to 1.75m/s. Approximately 80m of the
road has a hazard greater than the 0.3m2/s QUDM hazard criteria (with hazard up to 0.5m2/s
noted).
A flood depth map for this scenario is provided in Appendix I.
4.3.2.8. Flood Risk
This development has been designed to provide flood free access to all lots via the new
internal road in events up to the 10% AEP and will remain trafficable in events up to the 1%
AEP event in accordance with the Queensland Urban Drainage Manual (QUDM).
Further consideration of flood risk has been undertaken in events greater than the 1% AEP
event, with assessment of the PMF event taking place. The assessment found that the lots
within the development would become isolated in extreme events such as the PMF, with the
internal access road having flow conditions that would not be trafficable. An assessment was
completed as to the length of time the internal access would be considered not trafficable
(flow depths > 0.2m) along the warning time between the start of the rain event to when the
internal crossing becomes not trafficable. The assessment was completed for the PMF event
for a number of durations and is highlighted below in . From this, it is considered that a site
flood warning system would not be appropriate in this situation due to the short response
times for flood waters to rise. It would be safer outcome for residents to seek refuge within
their houses and communication about a extreme weather system come from Council or
State Disaster Management Services.
This assessment highlights that there would be a limited amount of time to warn / evacuate
residents due to the relatively short amount of time it takes for the internal road to become
inundated in an extreme (i.e PMF) event. This would be typical of all catchments where the
upstream catchment is relatively small. From this, the time of submergence is also relatively
small with times ranging from 2.0 to 8.0 hours depending on the event duration.
From this, it is considered that the provision of suitable flood refuge or evacuation to higher
ground (i.e. to Glenridge Drive) would be the preferred risk management measure as
opposed to evacuating the area to a formal evacuation centre. Each lot is provided with
an 1,100m2 building envelope above the PMF level that would be suitable to act as a flood
refuge until flood waters recede.
The application of a flood refuge is recommended in similar circumstances as means to
address flood risk and potential burden by Gold Coast City Council (GCCC), who have large
amount of flood plain on which developments are managed. The Guideline developed by
GCCC is presented in Appendix J, and in summary states “a shelter-in-place approach
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 16
Section 22
provides an opportunity to address the risks of isolation and subsequent risk to life and
potential burden to emergency services and has been commonly used in development
assessment for larger developments”.
Further to this, that subsequent that Council Disaster Management Plan includes
communications to these future residents that their building envelopes serve as a flood
refuge should an event greater than the 1% AEP event occur. This information could also be
conveyed via property note.
Table 4-6: PMF Time of Inundation and Rate of Rise.
Duration (hours) Time to become not Time of Submergence
trafficable (hours) (hours)
2 0.5 2.0
3 0.6 2.8
6 0.8 4.9
12 1.3 8.0
24 6.9 5.75
48 22.8 4.8
4.3.3. Flood Hazard Overlay Code Response
Code responses to Flood Hazard Overlay Code is presented in Appendix K.
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 17
Section 23
5. CONCLUSION
PEAKURBAN have completed a Flood Investigation to support the Reconfiguration of Lot
application for the development at 60 Glenridge Drive, Cooroibah. The objective of this
investigation was to design an appropriate road crossing to service the development,
demonstrate that no impacts and no loss of flood plain storage will result from the proposed
development (and associated earthworks).
This investigation demonstrates that a suitable development strategy has be developed
without creating any impacts to any adjoining property owners. Afflux maps for the 63.2, 39%,
10%, 1% and 1% (2100) AEP flood events and PMF provided in Appendix H demonstrate this.
The investigation has found that there is a reduction in peak flows and increase in flood storage
provided. As such, any further formal on-site detention is not recommended as development
has been designed to achieve the outcome of non-worsening.
The proposed development lot building envelopes are located above the 1% (2100) AEP, and
it is recommended that finished floor levels of future dwellings achieve a minimum 0.3m
freeboard above the DFE or the PMF level as detailed in Table 4-5.
This flood investigation plan successfully demonstrates that:
• The site is developed to create building envelopes that are flood free and enable
future finished floor levels to be a maximum of 0.3m above the DFE or the PMF
event.
• There is no impact to flood levels to adjoining properties as a result of this
development.
• Non-worsening of peak flows downstream of the development is achieved.
• Internal Road has been designed to achieve flood immunity in the 10% AEP event
and trafficable in the 1% AEP event.
• There is no loss in flood plain storage, with the proposed earthworks actually
increasing the available storage.
• Severe storm assessment found that future dwellings with freeboard would remain
free in the 1% (2100) AEP scenario with all culverts 100% blocked.
• Flood Risk is managed through ensuring building envelopes are located above the
PMF level to enable a safe flood refuge in extreme flood events (greater than the
1% AEP).
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Section 24
6. REFERENCES
Noosa Shire Council (2020), “Planning Scheme”.
Brisbane City Council (2000), “Natural Channel Design Guidelines.
Gold Coast City Council, “Interim approach for small scale developments at
risk of isolation due to flood”.
Queensland Government – Department of Transport and Main Roads (2015),
“Road Drainage Manual”.
BMT WBM (2020), “TUFLOW User Manual”.
Boyd et al (2019), “WBNM User Guide”.
ARR Data Hub, ‘https://data.arr-software.org/’
The Institute of Engineers Australia (2019), “Australian Rainfall and Runoff”.
21-0288-FS001-V5 | Flood Investigation | 60 Glenridge Drive, Cooroibah | 19
Section 25
APPENDICES [OCR of page images] APPENDICES
Section 26
Development Plans [OCR of page images] Appendix A Development Plans SD.
Section 27
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( BALANCE Section 28
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The text continues on section 29.