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From the archive FilesArchive.zip, inside RAL17/0511 - Information Request Response. File date in the archive: 2018-02-21 15:09.
MR M JONES
11 CHURCH STREET, POMONA
NOOSA COUNCIL RAL17/0511
INFORMATION REQUEST OF 8 DEC 2017
FLOOD LEVELS AND IMPACTS
21 FEBRUARY 2018
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Page 2
DEVELOPMENT OF 11 CHURCH STREET, POMONA
NOOSA COUNCIL RAL17/0511, INFORMATION REQUEST OF 8 DEC 2017
REPORT COMPILED FOR MR MARK JONES
BY BRISBANE STORMWATER MANAGEMENT PTY LTD
DOCUMENT CONTROL
BSM Job No: 2018-01
Doc Filename Status Format Issued To Date
ChurchSt.Doc Draft Pdf Mr Mark Jones 19/02/18
Haynes Consulting Engineers 19/02/18
BSM-Office
ChurchSt.Doc Final Pdf Mr Mark Jones 21/02/18
Haynes Consulting Engineers 21/02/18
BSM-Office
COPYRIGHT
This report is confidential and is proprietary to Brisbane Stormwater Management Pty Ltd and their client
Mr Mark Jones. The report is provided to Mr Jones and to his civil consultant Haynes Consulting
Engineers for the purpose of quantifying the flood levels and flood impacts of the proposed development
of 11 Church Street, Pomona. It is understood that the report will be submitted to Noosa Council as part
of a response to an information request regarding the proposed development. It may not be used by any
other party or for any other purpose whatsoever without the written permission of both Brisbane
Stormwater Management Pty Ltd and Mr Jones.
Brisbane Stormwater Management Pty Ltd and Mr Jones will not accept any responsibility for any losses
suffered by any other party as a result of the unauthorised use of this report.
CERTIFICATION
I hereby certify that the flood assessment described in this report have been undertaken using industry-
standard procedures and that the Noosa Council can rely on the findings described herein during their
consideration of the relevant matters for approval.
………………………………………………
Denis Ogle, RPEQ 2405, 21 February 2018
Director, Brisbane Stormwater Management P/L
92 Alkoomie St, PO Box 8119, Wynnum Nth, Q4178
Ph (07)33963383, Email: [email protected]
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List Of Contents
1. Introduction And Summary 3
1.1 Introduction 3
1.2 Summary 3
2. XPStorm Modelling - Existing Scenario 4
2.1 Adopted Software 4
2.2 Catchment Representation 4
2.3 Floodway Roughness 4
2.4 Storm Rainfall Inputs And Infiltration 4
2.5 Rational Method Sanity Check 5
2.6 Existing Flood Levels 5
3. XPStorm Modelling – Developed Scenario 10
3.1 Model Adjustment For Development Changes 10
3.2 Flood Impacts For Existing Climate 10
3.3 Design Flood Levels With Climate Change 11
3.4 100 Year Flood Boundaries 11
3.5 Impact Of Floodway Revegetation And Full Culvert Blockage 12
3.6 Depth x Velocity Parameters On Causeways 12
List Of Figures
1. Locality, Overall Catchment And XPStorm Model Extent 6
2. Downstream Area Of XPStorm Model 7
3. Photographs Of Existing Floodways On Site 8
4. Photographs Of Existing Floodway Downstream Of Site 9
5. Proposed Development Changes And 100 Year Flood Boundaries 13
List Of Tables
1. Comparison, Rational Method To XPStorm 5
2. Impacts On Flood Levels At Upstream Boundaries Of Site, For Existing Climate Regime 10
3. Impacts On Flood Levels Downstream Of Site, For Existing Climate Regime 11
4. 100 Year Flood Levels, With And Without Climate Change 11
5. Maximum 100 Yr Flood Levels With Climate Change, Revegetation & Fully Blocked Culverts 12
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1.0 INTRODUCTION AND SUMMARY
1.1 Introduction
Brisbane Stormwater Management (BSM) has been commissioned by Mr Mark Jones to address flooding
issues raised in an 8th December 2017 information request by Noosa Council in relation to Mr Jones’
proposed development of 11 Church Street, Pomona. The location of the site is defined in Figure 1 on
page 6 of this report. The development involves reconfiguring the site into four new lots as shown in
Figure 5 on page 10 of this report. The relevant part of Council’s information request is:
Issue
The site is affected by flooding and the extent of flooding is a critical element in determining if there is
sufficient building area within the lots and if safe access can be provided. In order to address the effects
of flooding the submitted plan indicates that the Defined Flood Level at the eastern boundary is RL 91.35
m. This level has been assumed to be the flood level across the site. It is noted that the Defined Flood
Level is the same as that indicated on a recent Flood Information Certificate issued by Council
(reference: CER17/2341 dated 10 August 2017). This certificate also included a flood level on the
western boundary (RL 91.85), a notation regarding the flow in the southern gully and an attached sketch
plan. The southern gully is noted as “Flow in this gully is not modelled so only backwater is shown.
Flood level and extent may not be correct”.
As the flooding in the southern gully is a critical element in the subject reconfiguration it must be
modelled and the potential impacts investigated. Please note that Council can provide access to its
existing flooding modelling in the area for a fee.
Information Required
1. Provide suitable flood modelling for the southern gully. Provide amended plans that are based on
the flood modelling for the site as a whole (northern and southern gullies) and include:
(a) Bearings and distances for the 1% AEP flood line with any areas below the flood line included in a
“Drainage Easement”
(b) The area available within the lots above the 1% AEP flood line.
2. Provide preliminary engineering designs of the access driveway to the house sites on Lots 2 & 3 and
the associated stormwater system. The designs must be based on the flood modelling of the site and
be in accordance with QUDM providing for a safe access for pedestrians and vehicles.
1.2 Summary
BSM’s contribution to the required information comprises the flood modelling, including the definition
of the relevant flood levels and flood boundaries together with the calculation of suitable culvert sizes
and depth-velocity parameters on the critical access points. The conversion of the calculated flood
boundaries to “bearings and distances”, the calculation of flood-free areas and the preliminary design of
the access driveways and associated stormwater systems are being undertaken by others.
The calculated flood boundaries and the recommended culvert sizes are shown in Figure 5 on page 10 of
this report. The culverts have been dimensioned to reduce upstream affluxes to practically zero (i.e. a
maximum of 0.01 metre, or “within the accuracy of the modelling”. Downstream impacts are practically
zero. The relevant design flood levels are summarised in Tables 4 and 5 on pages 11 and 12. The highest
depth-velocity parameter is 0.12 – i.e. safe.
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2.0 XPSTORM MODELLING – EXISTING SCENARIO
2.1 Adopted Software
The hydrologic routing of runoff from the catchment surfaces and the hydraulic modelling of flow
profiles along the floodways have been undertaken by the XPStorm software, using Interface Version
9.52A and Engine Version 9.28 – a particularly “robust” version of that software.
2.2 Catchment Representation
The extent and arrangement of the XPStorm model are shown in Figures 1 and 2 on pages 6 and 7 of this
report, with further detail in Figure 5 on page 13. The model was initially compiled for another of Mr
Jones’ developments, on the upstream side of Pottery Street, which included only cursory floodway
details between Pottery Street and its end-point at Church Street. The model has been extended to the
downstream side of Rectory Street to provide adequate run-in for this study and the cursory details in the
downstream end of the initial model have been replaced by field-survey data for all of the cross-sections
in this area. The model has also been extended sideways to include the overflow in Red Street.
The overall area of the modelled catchment is 41.13 hectares, including the 1.19 hectare site.
The modelled catchment includes future land-use zones of Open Space / Conservation, Rural Settlement
and Detached Housing, as delineated on Figure 1. For the purposes of this study, all of those parts of the
catchment which are external to the site have been modelled as being already developed to their future
zoning, with appropriate impervious fractions in accordance with QUDM, in both the existing and
developed scenarios.
The software was configured to hydrologically route the runoff from each subarea by the SWMM Runoff
Non-Linear Reservoir Method. This allowed for each subarea to be represented as two subareas, firstly
representing the directly connected impervious area (roofs and road carriageways) and secondly
representing the grassed areas and indirectly connected impervious areas (garden sheds, pool surrounds,
footpaths, driveways). Directly connected impervious areas were broadly assumed to occupy about two
thirds of the total impervious area within each subarea.
Directly connected impervious areas forming the first subarea were assumed to have a flow path
equivalent to a 20 metre length of 15 degree pitched roof – this inherently allowed for about 5 metres of
flow length down an actual roof or across a 3 metre half-road width, with the remaining 15 or so metres
representing the equivalent formalised (kerb/pipe) flow time to the subarea outlet. Accordingly, the
equivalent width of each impervious area was specified to the model by dividing the area by 20 metres.
The equivalent width of the pervious and indirectly connected impervious areas within each of the second
subareas was specified to the model by dividing the area by the average flow length to the subarea outlet.
Manning’s n values for the catchment surfaces were set at 0.025 for the impervious areas (tiled /
corrugated roofs and bitumen) and 0.100 for the pervious areas (forest in the conservation areas and
grass/landscaping with fences and other obstructions in the developed areas).
2.3 Floodway Roughness
Various areas of the existing floodways upstream of the site maintained their Manning’s n values of
0.035 to 0.150 from the model compilation for the previous study. The value of 0.035 was generally
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applied to the dams and flowing channels, while values around 0.045 and 0.050 were applied to grassed
overbank areas and values of 0.100 to 0.150 were applied to timbered area.
The existing floodways through the site and downstream areas, with their assigned n values, are depicted
in the photographs in Figures 3 and 4 on pages 8 and 9 of this report.
2.4 Storm Rainfall Inputs And Infiltration
Since the model is basically an extension of a model from a previous flood study, the design rainfall
inputs were from the ARR87 IFD data as per that study. However, the inclusion of initial rainfall losses
from 20 to 30 mm across the range of ARI in the previous study, so as to achieve a Rational Method
calibration, was replaced by a universal initial loss of zero in this extended model. The previous
continuing loss rate of 2 mm/hr was maintained in the extended model. The critical storm durations for
peak flow rates and flood levels across the site were found to be 25 minutes and 1 hour in the southern
tributary, 1 hour in the main floodway, and 25 minutes in the northern tributary.
The allowance for climate change involved a 15 percent increase in rainfall intensities at a 60 year
horizon.
2.5 Rational Method Sanity Check
The model’s predictions of the 1, 10 and 100 year peak flow rates at model nodes N48, N54 and N56
were compared to Rational Method calculations of the same flow rates, as summarised in Table 1, below.
The XPStorm predictions are consistently higher than the Rational Method predictions, with the disparity
ranging from 8 to 10 percent at 100 year ARI, to as much as 40 percent at 1 year ARI.
Table 1: Comparison, Rational Method To XPStorm
Location 1 Year ARI 10 Year ARI 100 Year ARI
(XPStorm) Rat Method. XPStorm Rat Method. XPStorm Rat Method. XPStorm
N48 0.44 0.56 1.00 1.28 1.81 1.98
N54 2.81 3.93 6.05 6.56 11.2 12.3
N56 0.23 0.27 0.53 0.68 0.94 1.03
2.6 Existing Flood Levels
The full complement of the XPStorm model’s existing flood levels through and downstream of the site,
assuming 20% blockage at all culverts, are summarised in Tables 2 to 4 on pages 10 and 11 of this report.
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NORTH
STRY
H ST
POTTE
ST
OVERALL CATCHMENT BOUNDARY
RY
CHURC
RECTO
... . N55 N56 N63
.... .
.....DETACHED HOUSING ....... . ...
N1
N58
1 09
0 .00 0
N67
..... . . . .. .. . 94.
41 90.
NS
72
N69
N54 91.
25
. ....... ....
N45 93.
39
I L/ 1.2 RC P
.
79
92.
TB
N15 N44
90.
40
90.
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36
N64
N43
78
90.
BB
.
84
90.
N38
89 N S/
90. BB
23
90.
53
89.
N53
BB
N42
RURAL SETTLEMENT . N32
N48
.
SUBJECT
SITE
. N31
. N29
N30
OPEN SPACE
.
N28
OVERALL CATCHMENT BOUNDARY
CONSERVATION
0 SCALE 200 m
LOCALITY, OVERALL CATCHMENT AND XPSTORM MODEL EXTENT FIGURE 1
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NORTH
R ED ST
. .
.
N55 N56
N63
. N57
ST
. . ..
RY
..
RECTO
N1 N2
N3
. N4
N5
..
N58
..
H ST
N59
.. .
N60
ST
.. .
N67
CHURC
N6 SUBJECT N61 N68 N69
RY
N7 SITE
90
NS
.72
.. ... . .
POTTE
N8
.
.25
91
N10 N47 N54 N66
..
N9 N65
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.
N11
.79
N64
92
TB
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.. . . . . . . N44
. . . N45
.
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.
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.53
.
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89
N15 N43
.
N25
.
N14 N24 N26 N38
. N51 N52
. ..
N33 N36
N37
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.. N50
N49
. N48 N50
N35
.
N34
N32
MEMOR
IAL AVE
NUE
N31 .
ST
RANGE
0 SCALE 100 m
DETAIL OF DOWNSTREAM AREA OF XPSTORM MODEL FIGURE 2
Brisbane Stormwater Management File ChurchSt.Doc 19/02/18 Page 7
[OCR of page images]
7 “a — NI
XPSTORM MODEL FIGURE 2
gQ_______scaLe__100m DETAIL OF DOWNSTREAM AREA OF
Brisbane Stormwater Management File ChurchSt.Doc 19/02/18 Page 7
Page 9
MANNINGS N 0.06 / 0.05 / 0.06
N48 .
. N44
MANNINGS N 0.08 / 0.07 / 0.10
LOOKING UPSTREAM TO SOUTHERN BOUNDARY AT NODE N48 LOOKING UPSTREAM TO WESTERN BDY AT NODE N44
. .. N51
N53 N52
MANNINGS N 0.05 / 0.05 / 0.05
N47 .
MANNINGS N 0.05 / 0.05 / 0.05
N54
.
LOOKING UPSTREAM ALONG SOUTHERN TRIBUTARY AND MAIN CREEK FROM JUNCTION AT NODE N54
PHOTOGRAPHS OF EXISTING FLOODWAYS FIGURE 3
Brisbane Stormwater Management File ChurchSt.Doc 19/02/18 Page 8
Page 10
. N59
.
MANNING'S N 0.06 / 0.05 / 0.05
N60
LOOKING UPSTREAM ALONG NORTHERN TRIBUTARY
. N65
MANNING'S N 0.05 / 0.06 / 0.05
. N64
LOOKING DOWNSTREAM FROM CHURCH STREET
PHOTOGRAPHS OF EXISTING FLOODWAYS FIGURE 4
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Page 11
3.0 XPSTORM MODELLING – DEVELOPED SCENARIO
3.1 Model Adjustment For Development Changes
The XPStorm model of the developed scenario was compiled by making the appropriate development
changes to a copy of the existing scenario model. Those changes, mostly shown on Figure 5, comprised:
- Altering the pervious/impervious assignments of those model subareas which contained part of the
site area.
- Adding the driveway access to Lots 2 and 3, with an associated northward shift of the creek channel
between nodes N50/N51 to offset the intrusion of the driveway batter into the existing creek bed.
- Addition of two sets of box culverts under the abovementioned driveway, between nodes N49/N50
and nodes N51/N52. Given that most of the runoff to these culverts comes from Range Street and
Memorial Avenue, which are mostly residential areas, these culverts were assumed in the design-
case modelling to be unblocked.
- Addition of a shallow waterbody around nodes N52/N53 to alleviate future grass maintenance
issues (this waterbody would be the third waterbody on the site, being additional to two existing
shallow waterbodies around node N45 and between nodes N46/N47.
- Adding a driveway access to Lot 4, with a 100-year capacity box culvert and a causeway across the
northern tributary.
3.2 Flood Impacts For Existing Climate
The XPStorm model of the developed scenario was firstly used to design the driveway culverts for
practically zero impact at the site’s upstream boundaries, assuming that all reaches of the floodway other
than the proposed waterbody will remain in their present condition. Effectively, this meant sizing the
culverts under the driveway accesses to Lots 2 and 3 for the minimum possible afflux at the site’s
southern boundary at node N48. Two 3 metre wide boxes at each of these crossings, 450 mm high
between nodes N49/N50 and 825 mm high between nodes N51/N52, with causeways barely covering the
culvert crowns, resulted in 0.01 metre afflux (or within the accepted model accuracy) across the range of
ARI at node N48.
The model was then used to size the culvert and causeway level for a 100 year access to Lot 4, resulting
in a 1200 x 450 box culvert, with a causeway level of 91.38.
The consequent affluxes at the site’s upstream boundaries are summarised in Table 2, below:
Table 2: Impacts On Flood Levels At Upstream Boundaries Of Site, For Existing Climate Regime
ARI Western Bdy, Node N44 Southern Bdy, Node N48 Northern Bdy, Node N58
(yrs) Exg Dev Afflux Exg Dev Afflux Exg Dev Afflux
100 91.28 91.28 0.00 91.17 91.18 0.01 92.65 92.65 0.00
50 91.22 91.22 0.00 91.12 91.13 0.01 92.62 92.62 0.00
20 91.14 91.14 0.00 91.06 91.07 0.01 92.61 92.61 0.00
10 91.02 91.03 0.01 90.95 90.96 0.01 92.58 92.58 0.00
5 90.88 90.88 0.00 90.92 90.93 0.01 92.57 92.57 0.00
2 90.76 90.76 0.00 90.88 90.89 0.01 92.55 92.55 0.00
1 90.67 90.67 0.00 90.84 90.85 0.01 92.53 92.53 0.00
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The consequent affluxes downstream of the site are summarised in Table 3, below. The affluxes are zero,
apart from +/- 0.01 on the 20 year and 2 year flood levels at the upstream side of Church Street.
Table 3: Impacts On Flood Levels Downstream Of Site, For Existing Climate Regime
ARI U/S Side Church St, N62 Midway Church/Rectory, N65 U/S Side Rectory St, N67
(yrs) Exg Dev Afflux Exg Dev Afflux Exg Dev Afflux
100 91.16 91.16 0.00 89.88 89.88 0.00 89.82 89.82 0.00
50 91.12 91.12 0.00 89.82 89.82 0.00 89.77 89.77 0.00
20 91.05 91.06 0.01 89.75 89.75 0.00 89.71 89.71 0.00
10 90.95 90.95 0.00 89.67 89.67 0.00 89.64 89.64 0.00
5 90.75 90.75 0.00 89.64 89.64 0.00 89.62 89.62 0.00
2 90.40 90.39 -0.01 89.58 89.58 0.00 89.56 89.56 0.00
1 90.06 90.06 0.00 89.37 89.37 0.00 89.35 89.35 0.00
3.3 100 Year Flood Levels With And Without Climate Change
The 100 year flood levels on the existing and developed site, with and without climate change (15%
increase in rainfall intensities), are summarised in Table 4, below:
Table 4: 100 Year Flood Levels With And Without Climate Change
Node Existing Developed
Without Climate Change With Climate Change
N44 91.28 91.28 91.35
N45 91.24 91.24 91.31
N46 91.19 91.20 91.26
N47 91.17 91.17 91.22
N48 91.17 91.18 91.23
N49 91.16 91.18 91.23
N50 91.16 91.17 91.22
N51 91.16 91.17 91.22
N52 91.16 91.16 91.21
N53 91.16 91.16 91.21
N54 91.16 91.16 91.21
N58 92.65 92.65 92.69
N59 91.83 91.83 91.84
N60 91.31 91.38 91.54
N61 91.16 91.16 91.21
N62 91.16 91.16 91.21
3.4 100 Year Flood Boundaries
The 100 year flood boundaries on the existing and developed site are marked on Figure 5 on page 13 of
this report. Since the balance lot (Lot 4) is regarded as a temporary entity which will almost certainly be
subdivided further in the next few years, and may have a different driveway configuration, with any
required adjustments to flood modelling completed at that time, the 100 year flood boundary marked in
the area upstream of the driveway on Lot 4 in this report is therefore appropriate to a short-term scenario
– i.e. without climate change. All other areas of the marked 100 year flood boundaries are applicable to a
longer-term scenario – i.e. with climate change.
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3.5 Impact Of Floodway Revegetation And Full Culvert Blockage
A final model run was undertaken for a worst-case scenario involving climate change, completely
blocked culverts within and downstream of the site, and full revegetation of the floodways (to a
Manning’s n of 0.15) through and downstream of the site. The consequent 100 year flood levels on the
site are summarised in Table 5, below, to indicate the minimum required house floor levels:
Table5: Maximum 100 Yr Flood Levels With Climate Change, Revegetation And Fully Blocked Culverts
Node 100 Year Worst-Case Flood Level
N44 91.77
N45 91.68
N46 91.55
N47 91.40
N48 91.49
N49 91.45
N50 91.44
N51 91.39
N52 91.31
N53 91.31
N54 91.31
N58 92.82
N59 91.98
N60 91.50
N61 91.37
N62 91.30
3.6 Depth -Velocity Parameters On Causeways
The critical points for access immunity and safety are the two culverts under the driveway leading to Lots
2 and 3, with driveway sag levels of 91.035 over both culverts. The XPStorm model results indicate a
flood immunity of 15 to 20 year for these crossings, or comfortably more than the 10 year minimum
standard required, and with DxV parameters of 0.07 / 0.08 during a 100 year storm. In the climate change
scenario, these crossings will be overtopped by about 0.03 metre (i.e. 3 centimetres) during a 10 year
storm, with the 100 year DxV parameters increasing to 0.11 / 0.12, easily satisfying the requirements of
QUDM/ARR.
The driveway causeway to Lot 4 will be set at the 100 year flood level and hence will have a DxV of zero
in a 100 year storm.
.
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10 9 8 7 6
31 RP71186 RP71186 RP71186 RP71186 RP71186
RP862439
N58 . .
..
N59
N60
.
100 YEAR FLOOD BOUNDARY
N607
59
9 3.
WITHOUT CLIMATE CHANGE
NS
23
9 2.
CAUSEWAY EL 91.38
NS
N61
STREET
4 PROPOSED 1200 x 450 BC
USIL 90.60 DSIL 90.50
7507m²
100 YEAR FLOOD BOUNDARY
WITH CLIMATE CHANGE
. . . . N47
N54
N62
.
CHURCH
N46
.
1
RP66341
N45
. 2 N53 92
..
2012m² s
1000m² above ce
r vi
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Defined Flood Event
REALIGNED CREEK BANK
N51 N52ss & PROPOSED WATERBODY 93
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N44 for 100 YEAR FLOOD BOUNDARY
t WITH CLIMATE CHANGE
91
92 3 N50
9 0.
56
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1 94
93 1218m²
N49 1118m²
.
1000m² above
Defined Flood Event CAUSEWAY EL 91.035
PROPOSED 2 / 3000 x 825 BC
N48 USIL 89.96 DSIL
S 89.93
94
93 94
92 CAUSEWAY EL 91.035
PROPOSED 2 / 3000 x 450 BC
USIL 90.35 DSIL 90.33
5 4
94
1
2
92
RP71186 RP71186 RP71186 RP71186
RP71186
0 10 20 30 40 50 m
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