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Flood Hazard Assessment
Intersection of Lake Macdonald Dr
and Dianella Crt
Cooroy
September 2024
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Document Control Sheet
Flood Hazard Assessment Intersection of Lake Macdonald Dr and Dianella Crt
Report Title
Cooroy
Affected Properties 62 Lake Macdonald Dr Cooroy
RP Description Lot 105 SP118458
Prepared For Noosa Council
Date 04 September 2024
Report Status Final v1.0
Author(s) Damian McGarry
Qualification BE Civil RPEQ MIEAust
Company HydraLogic
Phone 07 5444 7444
Certified by Damian McGarry
Industry Accreditation RPEQ No. 06045
Signed
Distribution Glen Conforti
C:\Users\Damian\Dropbox\002_HydraLogic\001_Projects\030_NSC\010_General Enquiries\029_Cooroy
Document Path:
Subdivision\006_Report\001_Draft\Flood Hazard Assessment Report Cooroy Subdivision dm 002.docx
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
CONTENTS
Executive Summary ................................................................................................................................. 1
1 Introduction .................................................................................................................................... 1
2 Available Data ................................................................................................................................. 3
3 Site Characteristics .......................................................................................................................... 4
3.1 Catchment Characteristics..................................................................................................... 4
4 Hydraulic Modelling ........................................................................................................................ 6
4.1 Model setup........................................................................................................................... 6
4.1.1 Model boundaries ............................................................................................................. 6
4.2 Future climate ....................................................................................................................... 6
4.3 Model Roughness .................................................................................................................. 7
4.4 Validation............................................................................................................................... 7
5 Flood Hazard ................................................................................................................................. 11
6 Sensitivity Analysis ........................................................................................................................ 17
6.1 Manning’s n roughness sensitivity ...................................................................................... 17
6.2 Culvert blockage sensitivity ................................................................................................. 19
7 Hydraulic Performance ................................................................................................................. 22
8 Summary of findings ..................................................................................................................... 23
Appendices .............................................................................................................................................. 1
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
FIGURES
Figure 1 - 50% AEP design discharge comparison................................................................................... 7
Figure 2 - 10% AEP design discharge comparison................................................................................... 8
Figure 3 - 2% AEP design discharge comparison..................................................................................... 8
Figure 4 - 1% AEP design discharge comparison..................................................................................... 8
Figure 5 - 1% AEP (2100) design discharge comparison ......................................................................... 9
Figure 6 - 0.2% AEP design discharge comparison.................................................................................. 9
Figure 7 - General flood hazard curves ................................................................................................. 11
Figure 8 - Duration of inundation Reference Point 1............................................................................ 13
Figure 9 - Duration of inundation Reference Point 2............................................................................ 13
Figure 10 - Sensitivity analysis +/- 25% Manning's n 2% AEP at Reference Pt A .................................. 17
Figure 11 - Sensitivity analysis +/- 25% Manning's n 1% AEP at Reference Pt A .................................. 18
Figure 12 - Sensitivity analysis +/- 25% Manning's n 0.2% AEP at Reference Pt A ............................... 18
Figure 13 - Sensitivity analysis 25% culvert blockage for 10% AEP at Reference Pt A ......................... 19
Figure 14 - Sensitivity analysis 50% culvert blockage for 2% AEP at Reference Pt A............................ 20
Figure 15 - Sensitivity analysis 50% culvert blockage for 1% AEP at Reference Pt A............................ 20
Figure 16 - Sensitivity analysis 50% culvert blockage for 0.2% AEP at Reference Pt A ........................ 21
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
EXECUTIVE SUMMARY
This report presents the findings of a flood hazard assessment undertaken for the road intersection
of Lake Macdonald Dr and Dianella Crt in Cooroy.
This report describes the configuration of the local high resolution flood model and summarises the
hydraulic performance of the intersection under a range of design flood conditions with Annual
Exceedance Probabilities of between 50% AEP and 0.2 % AEP.
Detailed ground and feature survey provided by Murray & Associates (see Appendix A) shows three
pipe culvert cells passing beneath Lake Macdonald Dr from the southeast corner of the intersection
with Dianella Ct to the northwest corner of the intersection adjacent to Wilgee Ct. Two of pipe
culverts are shown to be 1050 mm diameter and the third is 1350 mm diameter.
For the 10% AEP design flood event the intersection is not inundated and hence no flood hazard
exists within the trafficable road corridor. For the 2% AEP design flood event, shallow inundation at
the peak of the flood event introduces a low (H1) flood hazard classification across the intersection
with areas of higher classification where flow across the road concentrates and returns to the main
waterway, see Map 5. The flood hazard classifications are similar for the 1% AEP design flood event,
however, a more prominent corridor of higher (H5) flood hazard exists across the intersection, see
Map 6.
The duration of inundation across the intersection for the 2% AEP and 1% AEP design flood events is
anticipated to be less than 1.5 hours. The actual duration of inundation will vary for each major
rainfall event experienced in the local catchment; however, the approximate duration of the design
flood event provides a reasonable indication of the duration of inundation which is also consistent
with the size of the upstream catchment that contributes flood flows to the intersection location.
The intersection is flood free and trafficable for events up to the 10% AEP.
Detailed flood modelling of the catchment and subject intersection has demonstrated the capacity
of the existing culverts provides 10% AEP design flood immunity to the intersection. For larger and
rarer flood events of less than 2% AEP the model results show the intersection will experience water
over the road to depths of less 0.3 m for a 1% AEP flood event for relatively short durations of less
than 1.5 hours.
The flood modelling has also demonstrated that should a significant blockage of the culverts occur
during a major rainfall flood event the depths of inundation across the culvert would not be
expected to significantly change, however, the inundation across the intersection would persist for
longer periods.
The hydraulic performance of the intersection culverts is summarised in Table E -1 below.
E1
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Table E 1 - Hydraulic performance of intersection culverts
Discharge Depth Velocity Water
AEP surface
Culverts Weir Weir Weir elevation*
(%) (1 in x) (m3/s) (m3/s) (m) (m/s) (m AHD)
0.2 500 11.1 18.8 0.16 1.79 103.8
1 (2100) 100 (2100) 10.7 15.8 0.14 1.69 103.77
1 100 10.3 8.2 0.07 1.36 103.67
2 50 10.1 5.7 0.05 1.21 103.6
10 10 8.7 0 0.00 0.00 103.3
50 2 3.7 0 0.00 0.00 102.5
* Upstream face
E2
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
1 INTRODUCTION
Noosa Council approached HydraLogic to provide a summary of the design flood conditions at the
intersection of Lake Macdonald Drive and Dianella Court in Cooroy.
The crossing is portrayed as being inundated in Council’s strategic planning and disaster
management maps.
The watercourse and catchment associated with the culvert crossing at this location lie within the
2018 Six Mile Creek flood model prepared by WMA Water for Noosa Council.
An initial review of the flood model results revealed the local tributary was only defined in the flood
study model to determine design flood inflows to the main branch of Six Mile Creek downstream of
Cooroy township. The road culvert crossings along this local tributary were not defined in the flood
model.
To enable a detailed summary of the design flooding conditions of the intersection to be prepared a
higher resolution flood model was developed for the local tributary. This local flood model includes a
higher resolution digital terrain model and the pipe culverts at Lake Macdonald Dr, Curlew Cresent,
and Swift Drive.
This report describes the configuration of the local high resolution flood model and summarises the
hydraulic performance of the intersection under a range of design flood conditions with Annual
Exceedance Probabilities of between 50% AEP and 0.2 % AEP.
The intersection and local catchment are shown in Map 1.
1
Page 8
LAKE MACDONALD DR DIANELLA CT INTERSECTION FLOOD ASSESSMENT vd > NOOSA Council (_] Study area [__] Cooroy Subdivision [_] Model catchment Cadastre Watercourses [) Model Catchment QLD Globe Imagery 0 100 200 m = Hydral_ogic Date Created : 03.09.2024 Created by : DGM Projection : EPSG:28356 GDA94 / MGA zone 56 Scale at A3 Map Ref 1: 5,000 i 1 Map Series Location
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
2 AVAILABLE DATA
Table 2-1 presents a summary of the data utilised to inform this investigation.
Table 2-1 Available Data
Data Source Comments
2022 LiDAR Digital Elevation
Topographic details Provided by Noosa Council
Model
2018 Six Mile creek Flood Study
Catchment Boundaries
2022 DEM
Determined from aerial image
Land Use Google Satellite
assessment
Watercourse flow hydrographs
Hydrologic design flood runoff extracted from flood model
hydrographs results from the 2018 Six Mile
Creek Flood Study
Watercourse stage hydrographs
Hydraulic tailwater boundary extracted from flood model
hydrographs results from the 2018 Six Mile
Creek Flood Study
Contour & Detail Survey of
Lot 105 on SP 118458 Dated 05.04.24
Survey and infrastructure plans 62 Lake McDonald Drive, Job number 100988 DTM
Cooroy prepared by Murray & See Appendix A
Associates
Issue A
62 Lake Macdonald Drive, Cooroy Dated 10.01.24
Road upgrade details
prepared by Covey Associates Ptd Job number 233467 C250 – 252
Ltd See Appendix B
3
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
3 SITE CHARACTERISTICS
The subject site is the intersection of Lake Macdonald Dr and Dianella Ct in Cooroy.
Detailed ground and feature survey provided by Murray & Associates shows three pipe culvert cells
passing beneath Lake Macdonald Dr from the southeast corner of the intersection with Dianella Ct
to the northwest corner of the intersection adjacent to Wilgee Ct. Two of pipe culverts are shown to
be 1050 mm diameter and the third is 1350 mm diameter.
The Swift Drive culvert is a 1350 mm diameter and the Curlew Crs crossing is a 750 mm diameter.
Invert levels for these pipe culverts were determined from the 2022 digital elevation model (DEM).
3.1 Catchment Characteristics
Upstream of the subject site, the local catchment extends to the southeast and northeast and covers
an area of approximately 70 hectares. The catchment is comprised of three sub-tributaries. The
larger two of these sub-tributaries have a confluence downstream of Swift Dr and Curlew Crs. A third
minor tributary drains from southwest and joins the main tributary watercourse approximately 450
m upstream of the subject site. There is also a minor inflow to the main tributary from the north just
upstream of the subject site. The tributary waterway corridor is well defined by adjacent elevated
topography.
4
Page 11
LAKE MACDONALD DR
DIANELLA CT
INTERSECTION
FLOOD ASSESSMENT
SY
ae a
[__] Cooroy Subdivision
(__] Study area
{__] Upstream catchment
Cadastre
~—— Watercourses
[) Model Catchment
DEM 2022 (mAHD)
Hi 93
[J 112
[| 126
[J 140
Hg 155
QLD Globe Imagery
0
|
Hydral_ogic
Date Created : 03.09.2024
Created by : DGM
Projection : EPSG:28356 GDA94 / MGA zone 56
Scale at A3
1: 5,000
Digital
Elevation
Model
Map
Series
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
4 HYDRAULIC MODELLING
4.1 Model setup
To determine the potential flooding conditions of the Lake Macdonald Dr and Dianella Crt
intersection a high-resolution HEC RAS (Version 6.5) 2D flood model was developed for the local
catchment and watercourse.
The terrain base for the flood model was a 2022 digital elevation model provided by Noosa Council
derived from aerial laser survey (LiDAR).
The flood model included the entire contributing local catchment area upstream of the subject site
and extended approximately 450 m downstream to a confluence with another minor tributary.
A 2D model grid spacing of 5 m was adopted to ensure the accuracy of the terrain representation
throughout the waterway corridor. The 5 m grid was tested against a 2m grid and produced
consistent results.
Breaklines were incorporated to improve the model’s representation of significant changes in
surface elevation, land use and flow paths.
4.1.1 Model boundaries
To maintain consistency with the Six Mile Creek flood model, tailwater boundary conditions and
upstream tributary flows were extracted from the model and applied to the high-resolution local
flood model.
The downstream boundary of the local flood model was established as a time varying water level
hydrograph. Upstream of the subject site time varying discharge hydrographs were also extracted
from the Six Mile Creek flood model at five reference locations and used as inflow boundaries in the
local model, see Map 3.
Design tailwater level and instream flow hydrographs were extracted from the Six Mile Creek flood
model results for the following design flood events:
50% AEP (1 in 2 AEP)
10% AEP (1 in 10 AEP)
1% AEP (1 in 100 AEP)
1% AEP (1 in 100 AEP) 2100
0.2% AEP (1 in 500 AEP)
4.2 Future climate
As a part of the Six Mile Creek flood study, the Cooroy Creek flood model adopted a climate
change/variability rainfall allowance of a 20% increase in rainfall intensity at year 2100.
6