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Appendix I Intersection Flood Hazard Assessment - 2 of 2 · RAL25/0004
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Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
4.3 Model Roughness
The observed land uses for the 2D model surface for the existing catchment condition is shown in
Map 3.
The corresponding Manning’s n surface roughness values for each surface type are presented in
Table 4-1.
Table 4-1 Flood model surface roughness
Catchment Surface Assumed
Manning’s n
Openspace 0.08
Residential 0.12
Rural residential 0.06
Waterway 0.12
Roads 0.015
4.4 Validation
Design discharge hydrographs were exported from the local high-resolution flood model results
immediately upstream of the intersection and compared to design discharges extracted from the
Cooroy Creek flood model.
Design discharge hydrographs show good consistency across all flood events. Refer to Figure 1 to
Figure 6.
Figure 1 - 50% AEP design discharge comparison
7
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Section 2
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 2 - 10% AEP design discharge comparison
Figure 3 - 2% AEP design discharge comparison
Figure 4 - 1% AEP design discharge comparison
8
Section 3
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 5 - 1% AEP (2100) design discharge comparison
Figure 6 - 0.2% AEP design discharge comparison
9
Section 4
LAKE MACDONALD DR
DIANELLA CT
INTERSECTION
FLOOD ASSESSMENT
oa NOOSA
Council
@ Sensitivity Ref Pt copy
(__] Study area
[_] Model catchment
{__] Cooroy Subdivision
Cadastre
--- Breaklines
— Inflow locations
—— Regional tailwater boundary|
[) Model catchment
Study Area Landuse
(HN Openspace
[__] Residential
[__] Roads
[] Rural Res
Waterway
QLD Globe Imagery
0 100 200 m
kz €6=—CSCS
Hydra
Date Created : 03.09.2024
Created by : DGM
Projection : EPSG:28356 GDA94 / MGA zone 56
Scale at A3 Map Ref
1: 5,000 iy 3
Map Local Flood Model
Series Configuration
Section 5
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
5 FLOOD HAZARD
Australian Rainfall and Runoff (ARR) A Guide to Flood Estimation, Book 6 – Flood Hydraulics
presents a series of general flood hazard curves based on depth and velocity, see Figure 7. The flood
hazard classifications of H1 to H6 describe the six classifications of flood hazard detailed in Table 5-1.
The depth velocity flood hazard classification limits are presented in Table 5-2.
Figure 7 - General flood hazard curves
Table 5-1 Flood hazard classifications
Flood Hazard Description
Classification
H1 Generally safe for vehicles, people and buildings
H2 Unsafe for small vehicles
H3 Unsafe for vehicles, children and the elderly
H4 Unsafe for vehicles and people
Unsafe for vehicles and people. All building types vulnerable to structural damage. Some
H5 less robust
building types vulnerable to failure.
H6 Unsafe for vehicles and people. All building types considered vulnerable to failure.
Table 5-2 Depth and Velocity Vulnerability Threshold Classification Limits
11
Section 6
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Flood Hazard Classification limit Limiting still water depth Limiting velocity
Classification (D and V in combination) (D) (V)
m2/s m m/s
H1 D*V ≤ 0.3 ≤ 0.3 ≤ 2.0
H2 D*V ≤ 0.6 ≤ 0.5 ≤ 2.0
H3 D*V ≤ 0.6 ≤ 1.2 ≤ 2.0
H4 D*V ≤ 1.0 ≤ 2.0 ≤ 2.0
H5 D*V ≤ 4.0 ≤ 4.0 ≤ 4.0
H6 D*V > 4.0 - -
Flood hazards for the subject site intersection have been determined for the 10% AEP, 2% AEP, and
1% AEP design flood events in accordance with the guidance in ARR Book 6 and are presented in
Map 4 to Map 6.
For the 10% AEP design flood event the intersection is not inundated and hence no flood hazard
exists within the trafficable road corridor, see Map 4. For the 2% AEP design flood event, shallow
inundation at the peak of the flood event introduces a 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.
Depth hydrographs were extracted from two reference points to determine the approximate
duration of inundation that could be anticipated for inundation of the intersection. The depth
hydrographs for the reference points shown on Map 6 are presented in Figure 8 and Figure 9. The
figures show the duration of inundation for the 2% AEP and 1% AEP design flood events 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.
12
Section 7
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 8 - Duration of inundation Reference Point 1
Figure 9 - Duration of inundation Reference Point 2
13
Section 8
45 4 40 4 35 4 3.0 4 25 4 Depth (m) 20 4 15 4 10 4 Os 4 H6 - unsafe for vehicles and people. All building types considered vulnerable to failure. HS - unsafe for vehicles and people. All buildings vulnerable to structural damage. Some less robust building types vulnerable to failure. H4 - unsafe for people and vehicles. H8 - unsafe for vehicles, children and the elderly = He - unsafe for small vehicles 0.0 0 H1- generally safe | for people, vehicles and buildings 9 10 20 3.0 Velocity (m/s) 40 5.0 LAKE MACDONALD DR DIANELLA CT INTERSECTION FLOOD ASSESSMENT v.¢ NOOSA | #®& Council Cadastre () Study area Hazard 10% AEP [J H1 [| H2 [#3 [] H4 [] H5 [] H6 QLD Globe Imagery 0 10 20 30 40m EZ) Hydral_ogic Date Created : 03.09.2024 Created by : DGM Projection : EPSG:28356 GDA94 / MGA zone 56 Scale at A3 Map Ref 1: 900 Map 10% AEP Series Flood Hazard
Section 9
LAKE MACDONALD DR DIANELLA CT INTERSECTION FLOOD ASSESSMENT SY 1 ae NOOSA Cadastre () Study area Hazard Classification [J H1 [| H2 [#3 [] H4 [] H5 [] H6 QLD Globe Imagery 45, -| H6- unsafe for vehicles and people Alll building types considered vulnerable to failure. 0 10 20 30 40m EZ) 3.0 + H5 - unsafe for vehicles and people. All buildings vulnerable to structural damage. Some less robust building types. Depth (m) 25 7 vuinerable to failure Hydral_ogic 20 4 Date Created : 03.09.2024 . Created by : DGM H4 - unsafe Projection : EPSG:28356 GDA94 / MGA zone 56 15 4 forpeople ° and vehicles: Scale at A3 Map Ref 1.0 +) H3 - unsafe» for vehicles, children and the elderly os 4 = H2 - unsafe for small vehicles H1- generally safe ee for people, vehicles and buildings 00 10 20 3.0 40 5.0 Velocity (m/s) 1: 900 Map 2% AEP Series Flood Hazard 00
Section 10
4.5 4 46-unsafe for vehicles and people. ° All building types considered vulnerable to failure. 35 4 3.0 7 HS - unsafe for vehicles and people. All buildings vulnerable to structural damage. Some less robust building types 25 7 \uinerable to failure Depth (m) 20 4 H4 - unsafe J for people 15 1 and vehicles [J H1 [] H2 [J 43 [| 4 [] H5 [__] H6 QLD Globe Imagery 0 10 LAKE MACDONALD DR DIANELLA CT INTERSECTION FLOOD ASSESSMENT SY 7 ae NOOSA @ Road Reference Pt Cadastre (_] Study area Hazard001 copy EZ) Hydral_ogic Date Created : 03.09.2024 Created by : DGM Projection : EPSG:28356 GDA94 / MGA zone 56 20 30 40m 1.0 + H3-unsafe» for vehicles, children and the elderly os 4 — He - unsafe for small vehicles H1- generally safe | pumas for people, vehicles and buildings 7 00 0.0 10 20 3.0 40 5.0 Velocity (m/s) Scale at A3 Map Ref 1: 900 6 Map 1% AEP Series Flood Hazard
Section 11
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
6 SENSITIVITY ANALYSIS
To test the sensitivity of the high-resolution local flood model to modest changes in adopted
parameter values, two sensitivity tests were conducted. A reference point (A) was adopted at the
road entrance to Dianella Crt where hydrographs were extracted from the flood model and
compared to the base case results. See Map 3 for reference point location.
6.1 Manning’s n roughness sensitivity
The first sensitivity test was conducted using a +/- 25% variation in the adopted Manning’s n surface
roughness value for the model. The adopted Manning’s n surface roughness values are shown in
Table 4-1.
Water level (Depth) hydrographs were extracted from the results of each sensitivity analysis at
reference point A and compared to the base case results for the 2% AEP, 1% AEP, and 0.2% AEP
design flood events. See Figure 10 to Figure 12.
The results show the design flood levels at the intersection are not sensitive to reasonable changes
in the adopted Manning’s n surface roughness.
Figure 10 - Sensitivity analysis +/- 25% Manning's n 2% AEP at Reference Pt A
17
Section 12
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 11 - Sensitivity analysis +/- 25% Manning's n 1% AEP at Reference Pt A
Figure 12 - Sensitivity analysis +/- 25% Manning's n 0.2% AEP at Reference Pt A
18
Section 13
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
6.2 Culvert blockage sensitivity
The second sensitivity test assumed an amount of blockage of the available waterway area of the
three pipe culverts which pass beneath the subject site intersection. A blockage factor was applied
based on the Australian Rainfall and Runoff Book 6 Chapter 6 guidelines. An assumed Debris
Potential (DP) of medium was adopted and an L10 value of 1.5 m assumed. The smaller culvert pipe
diameter of 1050 mm was applied as the inlet width (W) control dimension. Based on guidelines a
blockage factor of 25% was applied to the 50% AEP and 10% AEP flood event, and 50% to the 2%
AEP, 1% AEP, and 0.2% AEP flood events.
Water level (Depth) hydrographs were extracted from the results of each sensitivity analysis at
reference point A and compared to the base case results for the 10% AEP, 2% AEP, 1% AEP, and 0.2%
AEP design flood events. See Figure 13 to Figure 16.
The results show that shallow inundation to a depth of approximately 100 mm is introduced at
reference point A for the 10% AEP design flood event, see Figure 13. For the 2% AEP, 1% AEP, and
0.2% AEP design flood events inundation depths across the intersection at reference point A
increase by approximately 120 mm to 150 mm with the durations of inundation also increasing to
between 2 hours and 6 hours respectively.
The results demonstrate that for flood events with AEPs less than 10% flooding conditions across the
intersection do not change significantly in depth, however, durations of inundation may be
extended.
Figure 13 - Sensitivity analysis 25% culvert blockage for 10% AEP at Reference Pt A
19
Section 14
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 14 - Sensitivity analysis 50% culvert blockage for 2% AEP at Reference Pt A
Figure 15 - Sensitivity analysis 50% culvert blockage for 1% AEP at Reference Pt A
20
Section 15
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
Figure 16 - Sensitivity analysis 50% culvert blockage for 0.2% AEP at Reference Pt A
21
Section 16
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
7 HYDRAULIC PERFORMANCE
The hydraulic performance of the waterway crossing intersection is summarised in Table 7-1.
The hydraulic performance is based on the adopted flood model parameters and does not include
potential significant blockage of the culverts.
Table 7-1 - Hydraulic performance of intersection culverts
Discharge Depth Velocity Water
AEP surface
Culverts Weir Weir Weir elevation*
(%) (1in 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
22
Section 17
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
8 SUMMARY OF FINDINGS
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 may persist for longer periods.
23
Section 18
Flood Hazard Assessment
Intersection of Lake Macdonald Dr and Dianella Crt
APPENDICES
Appendix A - Contour & Detail Survey prepared by Murray & Associates
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The text continues on section 21.
Used in these records
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