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Flood Study BSM Job2013 08 Date 15 Sep 2014 Revised
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Brisbane Stormwater Management Pty Ltd
VALKEV PTY LTD
STORMWATER DETENTION AND FLOODING
FOR SUBDIVISION OF LOT2 ON RP173883
AND LOTS 135, 136 AND 137 ON MCH978
AT ELM STREET AND LAKE MACDONALD DRIVE
COOROY
REVISED
15 SEPTEMBER 2014
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Full text of sections 2 to 30 (of 64)
Section 2
SUBDIVISION OF LOT 2 ON RP173883 AND LOTS 135, 136 AND 137 ON MCH 978
AT ELM STREET AND LAKE MACDONALD DRIVE, COOROY
STORMWATER DETENTION AND FLOODING REPORT
COMPILED FOR VALKEV P/L BY BRISBANE STORMWATER MANAGEMENT P/L
DOCUMENT CONTROL SHEET
Document Title: Stormwater Detention And Flooding For Subdivision Of Lot 2 On RP173883 And
Lots 135, 136 And 137 On MCH978, At Elm Street & Lake Macdonald Drive, Cooroy
Client: Valkev Pty Ltd
BSM Job No: 2013-08
Project Manager: Denis Ogle
Distribution:
Doc Filename Status Format Issued To Date
ValkevSDF.Doc Draft Pdf Callaghan & Toth P/L 28/4/14
BSM – Office 28/4/14
ValkevSDF.Doc Final Pdf & Callaghan & Toth P/L 30/4/14
Hardcopy BSM – Office Copy 30/4/14
ValkevSDF.Doc Revised Pdf & Callaghan & Toth P/L 15/9/14
Hardcopy BSM – Office Copy 15/9/14
Brisbane Stormwater Management P/L
92 Alkoomie St, PO Box 8119, Wynnum Nth, Q4178
Ph (07)33963383, Email [email protected]
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 1
Section 3
List Of Contents
1. Summary 3
2. Introduction 4
3. Response To Noosa Council Information Request 5
4. Response To Saunders Submission 8
5. Stormwater Detention Requirement 9
6. Stormwater Discharges To No 2 Pearsons Road 10
7. Runoff Rates To Floodway Model 12
7.1 Catchment Areas And Subdivision 12
7.2 Design Rainfall Data 12
7.3 Model Calibration 14
7.4 Linkage From Catchment Model To Floodway Model 15
8. Floodway Profiles, Pre-Development 17
9. Floodway Profiles, Post-Development 21
9.1 Mitigation Concept For Increased Stormwater Runoff 21
9.2 Model Simulation 21
9.3 Sensitivity Test 22
10. Certification 23
List Of Figures (following text)
1 Locality And Lot Plan
2 Urbs Models Of Stormwater Detention Requirement
3 Proposed Change In Discharge Scenarios To No 2 Pearsons Road
4 Catchment Subdivision For Pre-Development Urbs Model
5 Catchment Subdivision For Post-Development Urbs Model
6 XPStorm Hydraulic Model Network
7 Photographs Of Eastern Floodway On 25 November 2013
8 Photographs Of Western Floodway On 25 November 2013
9 Proposed Floodway Modifications, Bridge And Bio-Detention Ponds
10 Longitudinal Floodway Profiles
11 Floodway Cross-Section Changes – West Tributary
12 Footbridge And Bio-Basin Schematics
13 Flood Levels And Flood Boundaries
Appendices
A. Design Rainfall Data
B. Rational Method Calculations Of Floodway Inflows, Pre-Development
C. Rational Method Calculations Of Floodway Inflows, Post-Development
D. Floodway Storage Volumes
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Section 4
1.0 SUMMARY
This report has been revised in response to Noosa Council’s 18 July 2014 information request. Responses
to the relevant items in that information request are stated in Section 3 of the report. The applicant has
also received a copy of a 3 September 2014 submission to Council from the owners of an adjoining
property (JR & F Saunders), and comments on that submission are stated in Section 4 of the report.
This analysis has found that the development’s stormwater detention requirement can be achieved by
deliberately “roughening” the floodways (by revegetating the riparian zones, as required by Council), by
excavating a side-storage alongside the western tributary, and by constructing a restrictive footbridge
across the eastern tributary to back-up the flood profiles in that floodway, to augment the small amount of
conventional detention storage that can be gained in two bio-basins on the site (as in Figures 9 to 12).
In fact, the nett gains in flood storage volumes generated by these works will substantially exceed the
detention volumes that would otherwise be required of a fully conventional detention system, as shown in
Table 1, below.
Table 1: Comparison Of Pre-Development And Post-Development Floodplain Volumes (m3)
To Required Detention Volumes
1Year 2 Year 5 Year 10 Year 20 Year 50 year 100 Year
Pre-Development 5536 6311 8787 10202 11576 13125 14483
Floodway Storage
Post-Development 5909 6775 9607 11697 13389 15383 16849
Floodway Storage
Floodway Gain 373 464 820 1495 1813 2258 2366
Plus Bio-Basins 162 183 252 252 252 252 252
Less Park, Bund Fill -0 -0 -0 -0 -5 -40 -72
Nett Gain 535 647 1072 1747 2060 2470 2546
Req’d Extra Volume
(Conventional 395 525 770 910 1095 1370 1585
Detention)
The abovementioned works, together with a minor regrading of the invert profile around node 5 (as also
shown on Figure 10) will result in the development having practically nil adverse impact on flood levels
on adjoining properties, as summarised in Tables 19 to 21 of this report.
The works will also result in practically nil adverse impact on downstream peak flow rates, apart from a
slight increase in flows along the western tributary through the immediately adjoining downstream
property (No2 Pearsons Road). That increase in creek flows across the adjoining property results from the
deliberate diversion of local runoff away from that adjoining property, and directly to the western
floodway upstream of that property. Downstream of the natural point of inflow to the floodway for that
runoff, the flows revert to their pre-development rates.
The consequent flood levels for design of the development are summarised in Tables 19 to 21 and in
Figure 13 of this report. A sensitivity test has identified the impact of uncontrolled floodplain regrowth
on the abovementioned design levels. The consequent 100 year flood levels are summarised in Table 24
and in Figure 13, and appropriate account should be taken of these levels when considering appropriate
design levels for the development.
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Section 5
2.0 INTRODUCTION
This report describes Brisbane Stormwater Management P/L’s (BSM’s) stormwater detention and
flooding assessment of a proposed subdivision of Lot 2 on RP173883 and Lots 135, 136 and 137 on
MCH978, at Elm Street and Lake Macdonald Drive, Cooroy. The site locality and the proposed lot layout
are shown in Figure 1 of this report.
The site is traversed by two tributaries of Six Mile Creek, hereafter described as the eastern and western
tributaries. The catchment areas of the two tributaries are shown in Figure 4, and the tributary floodways
can be seen in more detail in Figure 6.
Some of the increased stormwater runoff from the proposed residential areas is proposed to be mitigated
by conventional detention volumes in two bio-retention basins, but most of the increased runoff is
proposed to be attenuated by an increase in the floodway storage on the site. The increased floodway
storage will be achieved by deliberately “roughening” the floodways (by revegetating the riparian zones,
as required by Council), by excavating a side-storage alongside the western tributary, and by constructing
a restrictive footbridge across the eastern tributary to back-up the flood profiles in that floodway.
Hydraulic model analyses of the interaction between increased site runoff and floodway through-flows
can be skewed by “hydrograph separation”, whereby the urbanisation of a particular site is predicted to
hasten the discharge of the increased site runoff down the floodway before the arrival of the major
floodway flows from the upstream catchment – which can lead to a perception that the peak flow-rates
along the floodway will not be significantly increased, which in turn leads to a perception that little or no
attenuation volume is required. This perception may be valid for a site development that is considered in
isolation, but it becomes invalid when the “equal opportunity” development of all such sites in the
catchment is collectively considered. To overcome that modelling difficulty without resorting to a
catchment-wide “equal opportunity” analysis, this assessment has firstly determined the detention volume
that would be required for a fully-conventional system of dedicated basins on the site’s stormwater
outlets. The assessment was thereafter focussed on achieving at least that volume from the proposed
combination of bio-basin detention and increased floodway volume, while also using a floodway model
to verify that the proposed changes to the floodway profiles on the site would not adversely affect the
flood profiles upstream and downstream of the site.
The development proposal is particularly cognisant of the need to not increase local stormwater
discharges (as distinct from tributary channel through-flows) onto an adjoining property to the north –
namely No 2 Pearsons Road (refer Figure 1), which is also referred to herein as the Saunders property.
This is proposed to be achieved by diverting part of the runoff from the relevant sub-catchment, directly
across to the western tributary floodway within the site. This diversion proposal had to be accounted in
the floodway modelling process.
The assessment process was therefore structured as follows:
- Determination of the required detention volume - as described in Section 5 of this report.
- Devising a diversion mechanism, in liaison with the applicant’s civil consultant (Callaghan & Toth),
to ensure no increase in stormwater discharges to No 2 Pearson Road – as described in Section 6.
- Calculation of the runoff rates to the floodways and running them though floodway models of the pre
and post-development scenarios – as described in Sections 7 to 9.
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Section 6
3.0 RESPONSE TO NOOSA COUNCIL INFORMATION REQUEST
The following comments are offered in response to Noosa Council’s information request of 18 July 2014.
The requests are reproduced in italics, with a response immediately following each request:
1. The report details that in a Q100 event an additional 1,585m³ of storage is to be provided. Only
252 m³ is proposed to be provided in detention basins and the balance has been created by
floodplain roughness via the planting of vegetation. This equates to nearly 85% of the storage to
rely on vegetation plantings. Council will not accept this form of detention.
In the previous report, the 100 year volume in the bio-basins was, as noted above, 252 cubic metres.
However, the 1747 cubic metre gain in 100 year floodway volume included about 200 cubic metres of
“hard” storage from the headwater afflux at the proposed footbridge, which was not separately identified
in that previous report. This took the total “hard” storage increase to about 450 cubic metres, leaving the
vegetation (”soft” storage) to account for at least the remaining 1135 cubic metres (or about 72 percent)
of the 1585 cubic metre requirement.
In this revised report, an additional 424 cubic metre side-storage is proposed to be excavated into the
overbank area alongside the northern bank of the western channel at node 29.5. Furthermore, the opening
area of the footbridge across the eastern channel is proposed to be significantly reduced, which will in
turn raise the 10 year headwater level and consequently the deck level by 0.13 metre, which will further
raise the upstream afflux storage. The nett effect of these changes will be a significantly increased
proportion of “hard’ storage, and conversely a significantly reduced requirement for “soft” storage to
make up the required overall volume. The revised make-up of the volumes, for ARI’s of 1, 10 and 100
years, is summarised in Table 2, below.
Table 2: Relative Proportions of “Hard” and “Soft” Flood Storage
ARI (years) 1 10 100
Required Volume (m3) 395 910 1585
Proposed Hard Storage (m3)
- Footbridge 4 635 690
- Other (bio-basins, channel cut/fill, side-storage) 187 114 274
- Total hard storage 191 749 964
- Proportion of requirement 48% 82% 61%
Remainder, Soft Storage, Req’d (m3) 204 161 621
- Proportion of requirement 52% 18% 39%
Actual Soft Storage Proposed (m3) 344 998 1582
Given that the hard storage is now proposed to make up about half or more of the overall storage
requirement, and that the soft storage will provide about twice or more of the remaining overall
requirement, this outcome is suggested to be an appropriate strategy in the longer-term context.
The appropriateness of the strategy in the shorter-term horizon, relative to the timeframe required for the
revegetation to become reasonably well-established, can be seen in the context of the planned progress of
the development platform vis-à-vis the planned progress of the floodplain storage works. The next stage
of the development is Stage 5, which comprises the remainder of the area labelled “Model D” on Figure 2
of this report. The equivalent detention requirements for the “Model D” area (from Section 5 of this
report) are compared to the hard storage that will be constructed during that stage, in Table 3, on the
following page of this report. Apart from a slight shortfall at the 1 year ARI level, the hard storage
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Section 7
constructed during this stage will easily offset the detention requirement for this stage. The riparian
vegetation for the eastern tributary will be planted during this stage and, even when only newly planted, it
should substantially offset the shortfall in hard storage at the 1 year ARI level. The subsequent growth of
that vegetation will confirm the over-provision of storage for Stage 5 and provide a surplus of storage for
the start of subsequent stages.
Table 3: Detention Requirement –vs- Hard Storage Provision, For Stage 5
ARI (years) 1 10 100
Detention Requirement (m3) 75 190 350
Hard Storage In Stage 5
- Bio-Basin (m3) 49 63 63
- Footbridge (m3) 4 635 690
- Total (m3) 53 698 753
- % Of Requirement 73% 367% 215%
2. The report outlines 50mm of afflux upstream in a Q100 event relying on externalities to the site
for justification. Any afflux will not be supported.
The above request relates to flood levels at node 23 of the flood model, in the western tributary, upstream
of the site. This revision of the report is based on a revised floodway design, in which the western
tributary channel between nodes 23 and 25 of the flood model is now proposed to be revegetated to a
similar density as the existing channel (estimated Manning’s n value 0.055), rather than roughened to a
Manning’s n value of 0.068 as in the previous design. Consequently, the affluxes at node 23 of the flood
model are now less than zero (i.e. beneficial).
3. The 100mm impact reported at Node 29 results in localised peak flood level impacts at 2
Pearson’s Rd. Agreement by the owner should be provided acknowledging the flood impact on
their property.
The previously reported 100 mm impact was an anomalous prediction, given that the profile of the
floodway at this location was proposed to be lowered by about 200 mm. Testing of the model’s
sensitivities revealed some issues with respect to the manner in which the software simulated the effect of
a “hump” in invert of the existing channel, downstream of node 29, leading to an erroneous prediction of
the pre-development flood levels at node 29. The model has since been revised, with an additional node
(29.5) located on the crest of the existing “hump”, thereby correcting the anomalous prediction. The
model of the post-development scenario has also been revised in this area, with the addition of a 424
cubic metre side-storage to be excavated into the overbank area alongside the northern bank of the
channel at node 29.5. The nett result of these changes is that there is now predicted to be a – 140 mm
afflux, rather than the previous + 100 mm afflux, at node 29.
4. The western tributary is a minor waterway for fish passage. Given the extent of works in the
waterway and the proposed works are an integral part of the modelling, Council recommends the
applicant gain written support from DAFF for the proposed works.
Refer to Callaghan & Toth response.
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Section 8
5. Please confirm that the maximum flow through the bio retention swales in a Q100 event do not
exceed 1.0m/s in accordance with Healthy Waterways Design Guidelines.
Refer to Callaghan & Toth response.
6. Please confirm Q100 flood depths over the footbridge. It is unclear of the blockage factor that has
been applied to the bridge and handrail and its impact on flood levels.
The Q100 flood depth over the footbridge, as per the design in the previous (original) edition of this
report was 0.19 metre. This revised report includes a revised design for the footbridge, including the
removal of the land-spans and a 0.13 metre raising of the deck level. The revised Q100 headwater level is
0.20 metre above the revised deck level, resulting in about 0.16 metre depth of water across the deck and
a DxV product of 0.15. With 50 % blockage of the bridge opening and 100% blockage of the handrails,
the Q100 headwater level rises to 0.28 metre above the deck level and the DxV product increases to
0.255 – which is within safe limits.
A complete listing of 100 year upstream water levels, with 50% blockage of the bridge opening and
100% blockage of the handrails, is given in Table 24 of this report.
7. The hydrologic regime is proposed to be changed over 2 Pearson’s road in a minor event. Please
provide owners consent from 2 Pearsons Road for this change to hydrological flows on their
property.
a. No comment on the environmental impact of this (2 Pearsons Road) change to the
hydrological flow has been provided.
Refer to Callaghan & Toth response.
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Section 9
4.0 RESPONSE TO SAUNDERS SUBMISSION
J R and F Saunders wrote to Noosa Council on 3 September 2014, requesting clarification of particular
aspects of the original edition of this report, and expressing concern about possible local stormwater
impacts on their property. The Saunders property is marked on Figure 9 of this revision of this report.
Their requests are reproduced in italics, below, with a response immediately following each request:
On page 15 of the study, there are three groups of columns, but they are not labelled with recurrence
intervals. Please confirm there are from left to right: 10 Year, 20 Year and 50 Year.
The three columns should indeed have been labelled 10 Year, 20 Year and 50 Year. The oversight has
been corrected in this revised report.
Using Figure 6, we understand the nearest model node to our property is Node 29. On page 15, assumed
Q10, Node 29, the afflux is 1.07 metres with a flood level of 100.957 metres. This afflux seems to be
incorrect because:
1. assumed Q20 has an afflux of 0.09 meters with a flood level of 100.033 meters
2. for assumed Q10, the flood level does not fall from Nodes 28 > 29 > 30
The quoted afflux was indeed incorrect, and resulted from the incorrect transcription of the post-
development flood level as 100.957, when it should have been transcribed as 99.957, which would have
resulted in afflux of 0.07 metres. Incidentally, the modelling has since been revised, with more detail in
this particular area of the model network, and the affluxes at node 29 are now less than zero.
… we are concerned about the potential for scouring in our property if there is not an appropriate energy
dissipation structure on the overland flow path located near our side fence and between proposed lots 39
and 40. We believe that this structure should be located within the development.
The peak-rates of local runoff across the boundary at this location will be substantially reduced, by
diversion of flow eastward within the development, and this will substantially lessen the likelihood of
scouring. Any other works that may be required to ensure that the velocity and concentration of the
remaining flows across the boundary are no worse than existing, will be part of the normal operational
works design for such a scenario.
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Section 10
5.0 STORMWATER DETENTION REQUIREMENT
As already noted in Section 1, the increased rates of stormwater runoff from the site are proposed to be
mitigated partly by conventional detention storage in two bio-basins and partly by increased floodway
storage on the site, with a requirement that the combined volume of those two storages should be at least
equal to the detention volume that would be required for a fully-conventional system.
The detention volume that would be required for a fully-conventional system has been determined by
compiling hydrologic models of four distinct parts of the proposed development platform, notated as
models A, B, C and D in Figure 2 of this report.
The attenuation objective for each of the four models was set by Rational Method calculations of the pre-
and post-development peak flow rates, as summarised in Tables 1 and 2 hereafter. The contributing areas
shown for Area A in Tables 4 and 5 are slightly less than the actual area of 2.49 hectare, because of
partial-area criticality (refer Figure 2 for partial-area limits).
Table 4: Rational Method Calculations Of Pre-Development Peak Flow Rates
Area C10 Tc I (mm/hr) A Q (m3/s)
(min) 1 / 2 / 5 / 10 / 20 / 50 / 100 (Ha) 1 / 2 / 5 / 10 / 20 / 50 / 100
A 0.72 24 65 / 73 / 98 / 116 / 130 / 152 / 166 2.43 0.25 / 0.30 / 0.45 / 0.56 / 0.66 / 0.85 / 0.97
B 0.70 18 75 / 84 / 112 / 132 / 151 / 172 / 192 0.85 0.10 / 0.12 / 0.18 / 0.22 / 0.26 / 0.33 / 0.38
C 0.70 20 71 / 80 / 106 / 127 / 142 / 165 / 180 1.85 0.20 / 0.24 / 0.36 / 0.46 / 0.54 / 0.68 / 0.78
D 0.70 16 80 / 90 / 120 / 140 / 158 / 183 / 202 1.34 0.17 / 0.20 / 0.30 / 0.37 / 0.43 / 0.55 / 0.63
Table 5: Rational Method Calculations Of Post-Development Peak Flow Rates
Area C10 Tc I (mm/hr) A Q (m3/s)
(min) 1 / 2 / 5 / 10 / 20 / 50 / 100 (Ha) 1 / 2 / 5 / 10 / 20 / 50 / 100
A 0.78 18 75 / 84 / 112 / 132 / 151 / 172 / 192 2.34 0.30 / 0.36 / 0.54 / 0.67 / 0.80 / 1.00 / 1.17
B 0.80 13 89 / 100 / 133 / 155 / 175 / 201 / 221 0.85 0.13 / 0.16 / 0.24 / 0.29 / 0.35 / 0.44 / 0.50
C 0.80 14 86 / 96 / 129 / 150 / 169 / 194 / 214 1.85 0.28 / 0.34 / 0.50 / 0.62 / 0.73 / 0.92 / 1.06
D 0.80 11 93 / 105 / 141 / 163 / 186 / 215 / 235 1.34 0.22 / 0.27 / 0.40 / 0.49 / 0.58 / 0.74 / 0.84
Version 3.5g of the URBS software was used as the basis for the hydrological models of Areas A to D.
Each of those areas was modelled as a sub-area network, using the subdivision lines indicated on Figure
2. Each of the four models was based on post-development conditions and was calibrated to closely
match its peak outflow rates to the Rational Method calculations of the post-development flow rates (as
in Table 5). A hypothetical detention basin was then added to the outlet of each model, with its storage-
discharge characteristics adjusted so as to reduce the peak outflow rates to the Rational Method
calculations of the pre-development flow rates (as in Table 4). The resultant storage requirements are
summarised in Table 6, below:
Table 6: Required Detention Volumes (m3) For Fully-Conventional System
Area 1 Yr 2 Yr 5 yr 10 Yr 20 Yr 50 yr 100 Yr
A 130 170 245 290 355 445 520
B 60 80 110 130 150 195 220
C 130 175 260 300 360 425 495
D 75 100 155 190 230 305 350
Total 395 525 770 910 1095 1370 1585
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Section 11
6.0 STORMWATER DISCHARGES TO No 2 PEARSONS ROAD
A catchment area of 2.49 hectares presently drains across the northern boundary of the site and onto No 2
Pearsons Road, as shown for the pre-development scenario in Figure 3. Local stormwater discharges onto
the adjoining property will be maintained at or below their existing rates by diverting part of the runoff
from the relevant catchment, directly across to the western tributary floodway within the site, as shown
for the post-development scenario in Figure 3.
The existing discharge rates to the adjoining property, as calculated by the Rational Method for the
partial-area of 2.43 hectares shown in Figure 3, are summarised in Table 7 below:
Table 7: Rational Method Calculations Of Pre-Development Peak Flow Rates To No 2 Pearson Road
C10 Tc I (mm/hr) A Q (m3/s)
(min) 1 / 2 / 5 / 10 / 20 / 50 / 100 (Ha) 1 / 2 / 5 / 10 / 20 / 50 / 100
0.72 24 65 / 73 / 98 / 116 / 130 / 152 / 166 2.43 0.25 / 0.30 / 0.45 / 0.56 / 0.66 / 0.85 / 0.97
In the post-development scenario, runoff from the 2.40 hectare post-development catchment (excluding a
0.018 hectare drainage corridor connecting to the adjoining property) will be collected at the sag in the
proposed road, from which flows up to Q5 will be diverted eastward by piping along the road, to the
western tributary channel. Flows in excess of Q5 at the sag will discharge overland, through the
abovementioned drainage corridor, to the adjoining property.
The Q5 diversion capacity, as calculated by the Rational Method for a partial area of 2.25 hectares, is
summarised in Table 8, below:
Table 8: Rational Method Calculation Of Q5 Diversion Capacity
C10 Tc (min) I5 (mm/hr) A (Ha) Q5 (m3/s)
0.78 18 112 2.25 0.00278 x (0.95 x 0.78) x 112 x 2.25 = 0.52
The post-development discharge rates to the adjoining property up to Q5, being just the runoff from the
surface of the connecting drainage corridor, are calculated by the Rational Method as in Table 9 below.
These flow rates are all practically zero.
Table 9: Rational Method Calculations Of Post-Development Flows Up To Q5 To No 2 Pearson Road
C10 Tc I (mm/hr) A Q (m3/s)
(min) 1 / 2 / 5 (Ha) 1 / 2 / 5
0.90 10 97 / 109 / 147 0.018 0.003 / 0.004 / 0.006
The post-development discharge rates to the adjoining property from Q10 to Q100, as calculated by the
Rational Method for the partial area of 2.30 hectares shown in Figure 3, are summarised in Table 10
below:
Table 10: Rational Method Calculations Of Post-Development Flows Q10 - Q100 To No 2 Pearson Road
C10 Tc I (mm/hr) A Q (m3/s)
(min) 10 / 20 / 50 / 100 (Ha) 10 / 20 / 50 / 100
0.78 18 132 / 151 / 172 / 192 2.30 0.66 / 0.79 / 1.00 / 1.15
Less 0.52 m3/s Q5, =
0.14 / 0.27 / 0.48 / 0.63
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Section 12
The consequent changes in local discharges from the site, to No2 Pearsons Road, are collated from Tables 7, 9 and 10 and summarised in Table 11, below. The changes are seen to be non-adverse, from Q1 to Q100. Table 11: Changes To Local Stormwater Discharges (m3/sec) From Site To No 2 Pearsons Road Scenario Q1 Q2 Q5 Q10 Q20 Q50 Q100 Pre-Development 0.25 0.30 0.45 0.56 0.66 0.85 0.97 Post-Development 0.003 0.004 0.006 0.14 0.27 0.48 0.63 Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 11
Section 13
7.0 RUNOFF RATES TO FLOODWAY MODEL
7.1 Catchment Areas And Subdivision
Flood profiles along floodways such as the subject tributaries are affected by the hydraulic capacity of the
adjoining downstream reaches and by any waterway structures in those downstream reaches.
Accordingly, the Extran model extends some distance downstream of the site to the convergence of the
two tributaries just downstream of Pearsons Road, as shown in Figure 3. The catchment modelling
therefore includes not just the contributing areas alongside and upstream of the site, but also the areas
downstream of the site to that termination point downstream of Pearsons Road. The extent of the 151.7
hectare overall catchment is shown in Figures 4 and 5. The site occupies 8.0 hectare, or about 5 percent,
of the overall catchment area.
The catchment has been subdivided into 23 subareas for modelling of the pre-development scenario (as in
Figure 3), and 41 subareas for the post-development scenario (as in Figure 4). The subdivision of the
areas external to the site is identical between the two scenarios, whereas the subdivision of those areas
which discharge to the floodways within the site are treated differently between the two scenarios, to
allow for accurate modelling of the increased runoff from the proposed urban development of 65 percent
of the site area. The modelling of the post-development scenario also accounts for the Q5 splits of the
inflows to the bio-basins, which necessitates doubling-up of the relevant six subareas, taking the total
sub-area count to 47 as marked on Figure 5. The impervious fractions applied to each of the modelled
subareas are summarised in Table 12, on the following page of this report.
7.2 Design Rainfall Data
The Bureau of Meteorology has recently updated the rainfall intensity-frequency-duration data that was
previously published in Volume 1 of Australian Rainfall and Runoff (1987). The previous data was based
on rainfalls recorded up to 1985, whereas the latest data is based on rainfalls recorded up to 2010 / 2011.
There has also been a change of nomenclature, whereby the previous description of severity in terms of
“average recurrence interval” has been updated to “annual exceedence probability” for storms from 2-
year to 100 year severity. Rainfall intensities for the 1 year severity are still quoted for an average
recurrence interval of once per year. The updating has increased the rainfall intensities at higher ARI’s
(or lesser exceedence probabilities) by about 4 percent, compared to the previous data. The updated
intensity-frequency-duration data for Cooroy is reproduced in Appendix A of this report.
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Section 14
Table 12 : Impervious Fractions Assigned To Urbs Model Subareas
Pre-Development Scenario Post-Development Scenario
Subarea Impervious Subarea Impervious
(Figure 1) Fraction (Figure 2) Fraction
1 0.10 1 0.10
2 0.15 2 0.15
3 0.15 3 0.15
4 0.35 4 0.35
5 0.15 5 0.15
6 0.50 6 0.50
7 0.20 7 0.30
8 0.06 8 0.60
9 0.10 9 0.00
10 0.30 10 0.60
11 0.30 11 0.50
12 0.58 12 0.50
13 0.65 13 0.50
14 0.28 14 0.50
15 0.45 15 0.50
16 0.45 16 0.50
17 0.00 17 0.50
18 0.04 18 0.50
19 0.08 19 0.60
20 0.31 20 0.00
21 0.35 21/22 0.30
22 0.25 23 0.37
23 0.45 24 0.58
25 0.65
26 0.28
27 0.45
28 0.45
29/39 0.15
30/40 0.15
31/41 0.50
32/42 0.50
33/43 0.55
34 0.55
35 0.55
36 0.45
37 0.08
38 0.60
44 0.39
45 0.35
46 0.25
47 0.45
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Table 12: Impervious Fractions Assigned To Urbs Model Subareas
Pre-Development Scenario
Post-Development Scenario
Subarea Impervious Subarea Impervious
(Figure 1) Fraction (Figure 2) Fraction
1 0.10 1 0.10
2 0.15 2 0.15
3 0.15 3 0.15
4 0.35 4 0.35
5 0.15 5 0.15
6 0.50 6 0.50
7 0.20 7 0.30
8 0.06 8 0.60
9 0.10 9 0.00
10 0.30 10 0.60
11 0.30 11 0.50
12 0.58 12 0.50
13 0.65 13 0.50
14 0.28 14 0.50
15 0.45 15 0.50
16 0.45 16 0.50
17 0.00 17 0.50
18 0.04 18 0.50
19 0.08 19 0.60
20 0.31 20 0.00
21 0.35 21/22 0.30
22 0.25 23 0.37
23 0.45 24 0.58
25 0.65
26 0.28
27 0.45
28 0.45
29/39 0.15
30/40 0.15
31/41 0.50
32/42 0.50
33/43 0.55
34 0.55
35 0.55
36 0.45
37 0.08
38 0.60
44 0.39
45 0.35
46 0.25
47 0.45
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Section 15
7.3 Model Calibration
The catchment runoff processes have been modelled with the “basic” variant of Version 3.5g of the Urbs
software, with the “Rational Method” procedure invoked so as to improve the routing simulation within
individual subareas, and with rainfalls converted to runoff via the initial/continuing loss procedure. The
operating parameters of the model which affect the outcome of the simulation are the storage lag
parameter (alpha), the non-linearity parameter (m), the initial rainfall loss (IL) and the continuing rainfall
loss (CL).
Ideally, the operating parameters should be calibrated to observed rainfall and runoff data in the subject
catchment, using either data from particular storm events or derived statistical data via a frequency curve
of recorded flows. In this instance, there is a streamgauge further downstream in the broader catchment
(at Cooran, where the contributing catchment area is 180 square kilometres, or about 120 times larger
than the subject catchment). This gauge has been recording flood flows for 60 years, but the accuracy of
its height/flow rating curve has been confirmed by current-metering to only just above the 1-in-2 year
flood height, which means that its usefulness for model calibration to flows of 5 year or greater severity
is very questionable.
The calibration has therefore reverted to an alternative process involving three stages:
- Calibration to Rational Method calculations of the peak flow rates from individual subareas and groups
of subareas.
- Comparing the calibrated parameters from the first stage to the corresponding parameters from
previous calibrations to other morphologically similar catchments with more highly rated streamgauge
data.
- Comparing the predicted peak flow rates at particular locations in the modelled area to the
corresponding flow rates in the 2002 L&T floodway model.
The Rational Method calibration has been applied to the peak flow rates entering the floodway model
near the upstream boundaries of the site and to the peak inflow rates from each of the subareas which
discharge to the floodway within the subject site. The relevant calculations are summarised in
Appendices B and C of this report, for the pre- and post-development scenarios respectively. An
acceptably close calibration to these calculated peak flow rates was achieved, as in Table 13 below and in
Table 14 on the following pages of this report, using an alpha parameter of 0.85 an m parameter of 0.8,
an initial rainfall loss of 16 mm, and a continuing rainfall loss of 2 mm/hour. The alpha parameter for
individual small subareas which discharge overland flow mostly as shallow sheet flow was increased by
arbitrarily applying reach factors of up to 3.
Table 13 : URBS / Rational Method Peak Flow Calibration (m3/sec) For Pre-Development Scenario
(Refer Figure 4 For Subarea Locations)
Subareas 1 year 2 year 5 Year 10 Year 20 year 50 year 100 Year
1–5 5.2 / 6.0 6.7 / 7.6 11.2/ 11.5 14.5 / 14.2 17.4 / 16.9 21.4 / 21.5 24.8 / 24.6
7 0.29 / 0.29 0.36/ 0.35 0.54 / 0.52 0.64 / 0.63 0.78 / 0.75 0.96 / 0.95 1.10 / 1.10
8 0.24 /0.25 0.30 /0.30 0.47 / 0.45 0.56 / 0.55 0.68 / 0.65 0.83 / 0.83 0.96 / 0.96
9 0.21 / 0.23 0.26 / 0.27 0.40 / 0.40 0.49 / 0.49 0.60 / 0.59 0.74 / 0.74 0.85 / 0.85
12 – 16 4.8 / 4.8 6.1 / 5.8 9.3 / 8.6 11.2 / 10.8 13.6 / 12.8 16.5 / 16.2 18.8 / 18.7
17 0.19 / 0.22 0.24 / 0.26 0.39 / 0.38 0.48 / 0.48 0.59 / 0.57 0.72 / 0.71 0.82 / 0.83
18 0.20 / 0.21 0.25 / 0.26 0.39 / 0.38 0.47 / 0.47 0.57 / 0.56 0.70 / 0.71 0.81 / 0.81
19 0.19 / 0.21 0.23/ 0.25 0.37 / 0.37 0.46 / 0.46 0.56 / 0.54 0.69 / 0.70 0.79 / 0.79
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Section 16
Table 14 URBS / Rational Method Peak Flow Calibration (m3/sec) For Post-Development Scenario
(Refer Figure 5 For Subarea Locations)
Subareas 1year 2 year 5 Year 10 Year 20 year 50 year 100 Year
7 0.30 / 0.30 0.37 / 0.36 0.56 / 0.54 0.68 / 0.67 0.81 / 0.79 1.00 / 1.00 1.14 / 1.15
8 0.05 / 0.05 0.06 / 0.06 0.10 / 0.09 0.12 / 0.11 0.14 / 0.13 0.17/ 0.17 0.19 / 0.19
9 0.13 /0.14 0.17 /0.16 0.27 / 0.25 0.33 / 0.30 0.39 / 0.36 0.47 / 0.46 0.53 / 0.53
10-18 0.19 / 0.20 0.24 / 0.24 0.37 / 0.37 ---- /----- ---- / ---- ---- / ---- ---- / ----
19 0.06 / 0.06 0.08 / 0.08 0.12 / 0.11 0.14 / 0.14 0.17 / 0.17 0.20 / 0.21 0.23 / 0.24
20 0.19 / 0.20 0.23 / 0.22 0.39 / 0.34 0.47 / 0.42 0.55 / 0.50 0.66 / 0.64 0.74 / 0.74
25-35 0.37 / 0.39 0.47 / 0.46 0.71 / 0.68 ---- / ---- ---- / ---- ---- / ---- ---- / ----
36 0.21 / 0.21 0.26 / 0.25 0.39 / 0.38 0.48 / 0.43 0.56 / 0.56 0.68 / 0.70 0.77 / 0.80
37 0.10 / 0.11 0.13 / 0.13 0.20 / 0.19 0.24 / 0.24 0.29 / 0.28 0.36 / 0.35 0.41 / 0.41
38 0.06 / 0.06 0.07 / 0.07 0.11 / 0.11 0.14 / 0.13 0.16 / 0.16 0.19 / 0.20 0.22 / 0.23
The calibrated parameters compare favourably with those inferred from other morphologically similar
catchments having more highly rated streamgauge data. In this instance, data have been inferred from
previous model calibrations to streamgauge records in Moggill Creek and Bulimba Creek in the outer
suburbs of Brisbane, with a comparison as in Table 15 below:
Table 15 Comparison Of Adopted Operating Parameters With Other Calibrations
Operating Parameter Adopted Values Moggill, Bulimba Creeks
Alpha 0.85 0.80 – 0.83
m 0.80 0.75 – 0.80
IL (mm) 16 15 – 20
CL (mm/hr) 2 2-3
The predicted peak inflow rates to the upstream end of the floodway model also compare favourably to
the peak flow rates in L&T’s Mike 11 floodway model, as in Table 16, below:
Table 16 : Comparison of BSM / CLT Peak Flows (m3/sec) At Upstream Boundaries Of Site
BSM Subareas CLT Mike XS 10 Year 20 Year 50 Year 100 Year
1–5 T3-789 14.4 / 12.1 17.3 / 16.1 21.3 / 21.8 24.8 / 25.7
12 – 16 T2-457 11.2 / 11.0 13.5 / 13.2 16.5 / 16.5 18.8 / 18.7
7.4 Linkage From Catchment Model To Floodway Model
BSM’s in-house ‘UrbsInt’ and “AsciXP” subroutines were used to convert flow outputs from particular
subareas or groups of subareas within the Urbs model to inflow hydrographs at particular nodes within
the floodway model, as in Table 17, hereafter:
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Section 17
Table 17 : Discharge Points From Urbs Hydrologic Model To XPStorm Hydraulic Model
Pre-Development Scenario Post-Development Scenario
Urbs Subareas XPStorm Node Urbs Subareas XPStorm Node
(Location, Figure 4) (Location, Figure 6) (Location, Figure 5) (Location, Figure 6)
1 –5 1 1–5 1
6 3 6 3
7 4 7 4
8 8 8 7
9 13 9 8
10 16 10 - 18 10
11 20 19 11
12 – 16 23 20 13
17 25 21 16
18 28 22 20
19 - 20 31 24 - 28 23
21 32 29 - 36 27
22 35 37 28
23 38 3823 29
44 31
45 32
46 35
47 38
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Table 17 :
Discharge Points From Urbs Hydrologic Model To XPStorm Hydraulic Model
Pre-Development Scenario
Post-Development Scenario
Urbs Subareas
(Location, Figure 4)
XPStorm Node
(Location, Figure 6)
Urbs Subareas
(Location, Figure 5)
XPStorm Node
(Location, Figure 6)
1-5 1 1-5 1
6 3 6 3
7 4 7 4
8 8 8 7
9 13 9 8
10 16 10 - 18 10
11 20 19 11
12-16 23 20 13
17 25 21 16
18 28 22 20
19 - 20 31 24 - 28 23
21 32 29 - 36 27
22 35 37 28
23 38 3823 29
44 31
45 32
46 35
47 38
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Section 18
8.0 FLOOD PROFILES, PRE-DEVELOPMENT
Version 9.28 of the Extran software, as embedded in the XPStorm modelling suite, has been used to
convert the catchment runoff hydrographs to flood profiles along the floodways. The extent and detail of
the model are shown in Figure 3 of this report. Te model provides approximately three times the detail of
the Mike11 model which L&T used in 2002 to model these floodways. The model network also includes
a number of interconnecting links to provide for simulation of overflows between the two tributaries
where they are aligned in close proximity, downstream of the site.
This version of the Extran software includes a facility to specify different internodal distances for the left
overbank, channel and right overbank segments, which in turn provides for a reasonably accurate
calculation by the software of the internodal flood volumes up to the peak flood level profile. This feature
was vital for checking that the reshaping of the floodway in the post-development scenario achieves the
required stormwater detention volumes, over-and-above the existing floodway volumes. Accordingly,
this facility was invoked for this analysis and separate internodal distances were measured and applied to
each connecting link of the model network.
Field-surveyed spot heights, which can be seen in the background of Figure 6 were used to define the
cross-section shape of each of the links between adjoining nodes of the model. The Manning’s n values
of the channel and overbank sectors of the floodways were perceived by site inspection. The adopted
values ranged from 0.035 for open grassed paddock or park, to 0.150 in densely timbered areas.
Examples of the adopted values are shown in Figures 7 and 8 of this report.
The starting point for the simulations was the tailwater level just downstream of the convergence of the
two tributaries. Initial runs of the model, based on a “normal-depth” criterion at node 39 at the
downstream end of the model, simulated a 100 year tailwater level substantially higher than L&T’s
predicted tailwater level at this location. The slope of the outfall reach, between nodes 38 and 39, was
therefore artificially steepened so as to induce a simulated tailwater level much closer to the L&T level.
Initial runs of the model, for ARI’s of 2 year and 100 year, and for storm durations from 20 minutes to 9
hours, revealed that the critical storm durations varied from 1.5 hours to 3 hours along these floodways.
Accordingly, all subsequent simulations included three storm durations: - namely, 1.5, 2 and 3 hours.
The set-up of the model resulted in simulated flood levels near the upstream and boundaries of the site
that, apart from node 23, were generally at or slightly above the co-located levels from L&T’s Mike 11
model, as indicated in Table 18 below. Overall, the consistency between the two models appears to be
quite good, despite the considerable difference in network density between the two models.
Table 18 : Comparison of BSM / CLT Flood Levels (m) In Vicinity Of Site Boundaries
BSM Node CLT X-Sect 10 year 20 Year 50 Year 100 Year
3 T3-811 101.61 / 101.38 101.65 / 101.54 101.72 / 101.69 101.81 / 101.71
16 T3-1199 99.60 / 99.58 99.68 / 99.61 99.74 / 99.68 99.80 / 99.71
23 T2-389 100.98 / 101.33 101.04 / 101.39 101.12 / 101.47 101.17 / 101.51
29.5 T2-611 100.13 / 99.82 100.17 / 99.89 100.21 / 99.97 100.23 / 100.05
The predicted pre-development peak flood levels at each node of the model, for ARI’s from 1to 100
years, are summarised in Tables 19 to 21 on the following pages of this report, where they are compared
to the corresponding post-development flood levels. The corresponding 100 year pre-development flood
profile is shown on Figure 10, and the pre-development 100 year boundary is mapped on Figures 9 and
13.
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Section 19
Table 19 : Predicted 1 Year To 5 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
1 YEAR 2 YEAR 5 YEAR
NODE PRE-DEV POST-DEV AFFLUX PRE-DEV POST-DEV AFFLUX PRE-DEV POST-DEV AFFLUX
EASTERN TRIBUTARY
1 102.259 102.254 0.00 102.625 102.635 0.01 102.977 102.978 0.00
2 102.213 102.207 -0.01 102.614 102.624 0.01 102.969 102.969 0.00
3 101.350 101.162 -0.19 101.386 101.200 -0.19 101.542 101.368 -0.17
4 101.327 101.112 -0.22 101.351 101.134 -0.22 101.479 101.214 -0.27
5 101.250 100.766 -0.48 101.287 100.784 -0.50 101.395 100.877 -0.52
6 100.761 100.763 0.00 100.776 100.780 0.00 100.846 100.874 0.03
7 100.755 100.755 0.00 100.768 100.770 0.00 100.829 100.856 0.03
8 100.084 100.151 0.07 100.145 100.229 0.08 100.295 100.518 0.22
9 100.056 100.140 0.08 100.121 100.229 0.11 100.298 100.508 0.21
10 100.061 100.144 0.08 100.131 100.234 0.10 100.323 100.504 0.18
11 100.012 100.097 0.08 100.079 100.186 0.11 100.269 100.461 0.19
12 99.915 99.963 0.05 99.983 100.039 0.06 100.195 100.268 0.07
13 99.644 99.698 0.05 99.725 99.996 0.27 99.961 99.991 0.03
14 99.554 99.605 0.05 99.647 99.688 0.04 99.888 99.923 0.03
15 99.439 99.433 -0.01 99.551 99.544 -0.01 99.800 99.789 -0.01
16 99.128 99.120 -0.01 99.230 99.223 -0.01 99.471 99.459 -0.01
17 98.873 98.865 -0.01 98.971 98.964 -0.01 99.188 99.183 0.00
18 98.687 98.679 -0.01 98.776 98.770 -0.01 99.053 99.050 0.00
19 98.620 98.614 -0.01 98.681 98.678 0.00 98.830 98.828 0.00
20 97.962 97.959 0.00 97.993 97.991 0.00 98.086 98.085 0.00
21 97.702 97.703 0.00 97.837 97.838 0.00 98.031 98.029 0.00
22 97.601 97.602 0.00 97.686 97.687 0.00 97.839 97.837 0.00
WESTERN TRIBUTARY
23 100.727 100.625 -0.10 100.790 100.698 -0.09 100.922 100.849 -0.07
24 100.576 100.438 -0.14 100.627 100.506 -0.12 100.723 100.650 -0.07
25 100.127 100.230 0.10 100.195 100.297 0.10 100.339 100.434 0.09
26 100.079 100.090 0.01 100.140 100.158 0.02 100.268 100.299 0.03
27 99.985 100.009 0.02 100.048 100.078 0.03 100.189 100.215 0.03
28 99.935 99.834 -0.10 100.000 99.905 -0.09 100.144 100.041 -0.10
29 99.902 99.725 -0.18 99.967 99.795 -0.17 100.114 99.914 -0.20
29.5 99.865 99.673 -0.19 99.927 99.742 -0.19 100.078 99.843 -0.23
30 99.442 99.450 0.01 99.462 99.476 0.01 99.531 99.544 0.01
31 98.909 98.923 0.01 99.039 99.047 0.01 99.262 99.263 0.00
32 98.857 98.870 0.01 98.988 98.995 0.01 99.218 99.218 0.00
33 98.563 98.575 0.01 98.675 98.684 0.01 99.032 99.029 0.00
34 98.521 98.529 0.01 98.601 98.609 0.01 98.830 98.828 0.00
35 97.744 97.746 0.00 97.876 97.879 0.00 98.083 98.081 0.00
36 97.702 97.702 0.00 97.837 97.838 0.00 98.031 98.029 0.00
37 97.620 97.621 0.00 97.708 97.709 0.00 97.924 97.922 0.00
38 97.414 97.414 0.00 97.518 97.519 0.00 97.719 97.716 0.00
39 96.815 96.814 0.00 96.917 96.918 0.00 97.121 97.119 0.00
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Table 19 :
NODE
Predicted 1 Year To 5 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
PRE-DEV POST-DEV AFFLUX
EASTERN TRIBUTARY
WESTERN TRIBUTARY
23
24
25
26
27
28
29
29.5
30
31
32
33
34
35
36
37
38
39
102.259
102.213
101.350
101.327
101.250
100.761
100.755
100.084
100.056
100.061
100.012
99.915
99.644
99.554
99.439
99.128
98.873
98.687
98.620
97.962
97.702
97.601
100.727
100.576
100.127
100.079
99.985
99.935
99.902
99.865
99.442
98.909
98.857
98.563
98.521
97.744
97.702
97.620
97.414
96.815
1 YEAR
102.254
102.207
101.162
101.112
100.766
100.763
100.755
100.151
100.140
100.144
100.097
99.963
99.698
99.605
99.433
99.120
98.865
98.679
98.614
97.959
97.703
97.602
100.625
100.438
100.230
100.090
100.009
99.834
99.725
99.673
99.450
98.923
98.870
98.575
98.529
97.746
97.702
97.621
97.414
96.814
0.00
-0.01
-0.19
-0.22
-0.48
0.00
0.00
0.07
0.08
0.08
0.08
0.05
0.05
0.05
-0.01
-0.01
-0.01
-0.01
-0.01
0.00
0.00
0.00
-0.10
-0.14
0.10
0.01
0.02
-0.10
-0.18
-0.19
0.01
0.01
0.01
0.01
0.01
0.00
0.00
0.00
0.00
0.00
PRE-DEV POST-DEV AFFLUX
102.625
102.614
101.386
101.351
101.287
100.776
100.768
100.145
100.121
100.131
100.079
99.983
99.725
99.647
99.551
99.230
98.971
98.776
98.681
97.993
97.837
97.686
100.790
100.627
100.195
100.140
100.048
100.000
99.967
99.927
99.462
99.039
98.988
98.675
98.601
97.876
97.837
97.708
97.518
96.917
2 YEAR
102.635
102.624
101.200
101.134
100.784
100.780
100.770
100.229
100.229
100.234
100.186
100.039
99.996
99.688
99.544
99.223
98.964
98.770
98.678
97.991
97.838
97.687
100.698
100.506
100.297
100.158
100.078
99.905
99.795
99.742
99.476
99.047
98.995
98.684
98.609
97.879
97.838
97.709
97.519
96.918
0.01
0.01
-0.19
-0.22
-0.50
0.00
0.00
0.08
0.14
0.10
0.14
0.06
0.27
0.04
-0.01
-0.01
-0.01
-0.01
0.00
0.00
0.00
0.00
-0.09
-0.12
0.10
0.02
0.03
-0.09
-0.17
-0.19
0.01
0.01
0.01
0.01
0.01
0.00
0.00
0.00
0.00
0.00
PRE-DEV POST-DEV AFFLUX
102.977
102.969
101.542
101.479
101.395
100.846
100.829
100.295
100.298
100.323
100.269
100.195
99.961
99.888
99.800
99.471
99.188
99.053
98.830
98.086
98.031
97.839
100.922
100.723
100.339
100.268
100.189
100.144
100.114
100.078
99.531
99.262
99.218
99.032
98.830
98.083
98.031
97.924
97.719
97.121
5 YEAR
102.978
102.969
101.368
101.214
100.877
100.874
100.856
100.518
100.508
100.504
100.461
100.268
99.991
99.923
99.789
99.459
99.183
99.050
98.828
98.085
98.029
97.837
100.849
100.650
100.434
100.299
100.215
100.041
99.914
99.843
99.544
99.263
99.218
99.029
98.828
98.081
98.029
97.922
97.716
97.119
0.00
0.00
-0.17
-0.27
-0.52
0.03
0.03
0.22
0.21
0.18
0.19
0.07
0.03
0.03
-0.01
-0.01
0.00
0.00
0.00
0.00
0.00
0.00
-0.07
-0.07
0.09
0.03
0.03
-0.10
-0.20
-0.23
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
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Section 20
Table 20 : Predicted 10 Year To 50 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
10 YEAR 20 YEAR 50 YEAR
NODE PRE-DEV POST-DEV AFFLUX PRE-DEV POST-DEV AFFLUX PRE-DEV POST-DEV AFFLUX
EASTERN TRIBUTARY
1 103.013 103.013 0.00 103.045 103.044 0.00 103.083 103.082 0.00
2 103.000 103.001 0.00 103.028 103.027 0.00 103.059 103.058 0.00
3 101.605 101.509 -0.10 101.654 101.593 -0.06 101.723 101.696 -0.03
4 101.533 101.304 -0.23 101.611 101.350 -0.26 101.679 101.474 -0.20
5 101.447 100.945 -0.50 101.484 100.966 -0.52 101.530 101.015 -0.52
6 100.920 100.942 0.02 100.983 100.961 -0.02 101.009 101.009 0.00
7 100.903 100.919 0.02 100.963 100.933 -0.03 100.979 100.972 -0.01
8 100.415 100.728 0.31 100.552 100.817 0.26 100.634 100.886 0.25
9 100.404 100.709 0.31 100.538 100.802 0.26 100.604 100.877 0.27
10 100.428 100.713 0.28 100.532 100.808 0.28 100.627 100.889 0.26
11 100.375 100.683 0.31 100.476 100.772 0.30 100.572 100.845 0.27
12 100.305 100.391 0.09 100.404 100.482 0.08 100.519 100.576 0.06
13 100.003 100.043 0.04 100.041 100.135 0.09 100.162 100.229 0.07
14 99.956 99.997 0.04 100.002 100.039 0.04 100.068 100.128 0.06
15 98.879 99.876 1.00 99.935 99.922 -0.01 100.014 99.993 -0.02
16 99.604 99.596 -0.01 99.681 99.674 -0.01 99.744 99.732 -0.01
17 99.345 99.337 -0.01 99.440 99.436 0.00 99.542 99.532 -0.01
18 99.178 99.172 -0.01 99.251 99.249 0.00 99.318 99.313 0.00
19 98.930 98.924 -0.01 99.020 99.017 0.00 99.121 99.113 -0.01
20 98.172 98.166 -0.01 98.270 98.267 0.00 98.380 98.372 -0.01
21 98.110 98.104 -0.01 98.199 98.196 0.00 98.299 98.291 -0.01
22 97.936 97.928 -0.01 98.064 98.060 0.00 98.209 98.196 -0.01
WESTERN TRIBUTARY
23 100.976 100.919 -0.06 101.044 100.995 -0.05 101.122 101.080 -0.04
24 100.970 100.719 -0.25 100.824 100.790 -0.03 100.885 100.871 -0.01
25 100.403 100.504 0.10 100.461 100.577 0.12 100.524 100.661 0.14
26 100.330 100.365 0.03 100.386 100.438 0.05 100.449 100.519 0.07
27 100.250 100.278 0.03 100.304 100.346 0.04 100.362 100.422 0.06
28 100.197 100.104 -0.09 100.245 100.172 -0.07 100.296 100.245 -0.05
29 100.162 99.972 -0.19 100.205 100.035 -0.17 100.249 100.099 -0.15
29.5 100.126 99.896 -0.23 100.170 99.955 -0.22 100.211 100.016 -0.19
30 99.600 99.612 0.01 99.658 99.666 0.01 99.722 99.732 0.01
31 99.378 99.372 -0.01 99.491 99.487 0.00 99.645 99.634 -0.01
32 99.345 99.338 -0.01 99.446 99.443 0.00 99.556 99.547 -0.01
33 99.152 99.147 0.00 99.215 99.213 0.00 99.273 99.269 0.00
34 98.933 98.927 -0.01 99.024 99.020 0.00 99.125 99.116 -0.01
35 98.171 98.164 -0.01 98.270 98.267 0.00 98.380 98.372 -0.01
36 98.109 98.104 0.00 98.198 98.196 0.00 98.298 98.291 -0.01
37 98.041 98.033 -0.01 98.161 98.158 0.00 98.279 98.271 -0.01
38 97.821 97.814 -0.01 97.933 97.930 0.00 98.045 98.035 -0.01
39 97.223 97.215 -0.01 97.332 97.328 0.00 97.444 97.434 -0.01
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 19
[OCR of page images]
Table 20 :
NODE
Predicted 10 Year To 50 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
PRE-DEV POST-DEV AFFLUX
EASTERN TRIBUTARY
ON OanR WD =
MMM eee ean nnn an gg
NBA CO AN DAA RWNY +0
WESTERN TRIBUTARY
23
24
25
26
27
28
29
29.5
30
31
32
33
34
35
36
37
38
39
103.013
103.000
101.605
101.533
101.447
100.920
100.903
100.415
100.404
100.428
100.375
100.305
100.003
99.956
98.879
99.604
99.345
99.178
98.930
98.172
98.110
97.936
100.976
100.970
100.403
100.330
100.250
100.197
100.162
100.126
99.600
99.378
99.345
99.152
98.933
98.171
98.109
98.041
97.821
97.223
10 YEAR
103.013
103.001
101.509
101.304
100.945
100.942
100.919
100.728
100.709
100.713
100.683
100.391
100.043
99.997
99.876
99.596
99.337
99.172
98.924
98.166
98.104
97.928
100.919
100.719
100.504
100.365
100.278
100.104
99.972
99.896
99.612
99.372
99.338
99.147
98.927
98.164
98.104
98.033
97.814
97.215
0.00
0.00
-0.23
-0.50
0.02
0.02
0.31
0.31
0.28
0.31
0.09
0.04
0.04
1.00
-0.01
-0.01
-0.01
-0.01
-0.01
-0.01
-0.01
-0.06
-0.25
0.10
0.03
0.03
-0.09
-0.19
-0.23
PRE-DEV POST-DEV AFFLUX
103.045
103.028
101.654
101.611
101.484
100.983
100.963
100.552
100.538
100.532
100.476
100.404
100.041
100.002
99.935
99.681
99.440
99.251
99.020
98.270
98.199
98.064
101.044
100.824
100.461
100.386
100.304
100.245
100.205
100.170
99.658
99.491
99.446
99.215
99.024
98.270
98.198
98.161
97.933
97.332
20 YEAR
103.044
103.027
101.593
101.350
100.966
100.961
100.933
100.817
100.802
100.808
100.772
100.482
100.135
100.039
99.922
99.674
99.436
99.249
99.017
98.267
98.196
98.060
100.995
100.790
100.577
100.438
100.346
100.172
100.035
99.955
99.666
99.487
99.443
99.213
99.020
98.267
98.196
98.158
97.930
97.328
0.00
0.00
-0.26
-0.52
-0.02
-0.03
0.26
0.26
0.28
0.30
0.08
0.09
0.04
-0.01
-0.01
0.00
0.00
0.00
0.00
0.00
0.00
-0.05
-0.03
0.12
0.05
0.04
-0.07
-0.17
-0.22
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
PRE-DEV POST-DEV AFFLUX
103.083
103.059
101.723
101.679
101.530
101.009
100.979
100.634
100.604
100.627
100.572
100.519
100.162
100.068
100.014
99.744
99.542
99.318
99.121
98.380
98.299
98.209
101.122
100.885
100.524
100.449
100.362
100.296
100.249
100.211
99.722
99.645
99.556
99.273
99.125
98.380
98.298
98.279
98.045
97.444
50 YEAR
103.082
103.058
101.696
101.474
101.015
101.009
100.972
100.886
100.877
100.889
100.845
100.576
100.229
100.128
99.993
99.732
99.532
99.313
99.113
98.372
98.291
98.196
101.080
100.871
100.661
100.519
100.422
100.245
100.099
100.016
99.732
99.634
99.547
99.269
99.116
98.372
98.291
98.271
98.035
97.434
0.00
0.00
-0.20
-0.52
0.00
-0.01
0.25
0.27
0.26
0.27
0.06
0.07
0.06
-0.02
-0.01
-0.01
0.00
-0.01
-0.01
-0.01
-0.01
-0.04
-0.01
0.14
0.07
0.06
-0.05
0.15
-0.19
0.01
0.00
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc
Page 19
Section 21
Table 21 : Predicted 100 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
100 YEAR
NODE PRE-DEV POST-DEV AFFLUX
EASTERN TRIBUTARY
1 103.110 103.110 0.00
2 103.080 103.079 0.00
3 101.805 101.743 -0.06
4 101.731 101.534 -0.20
5 101.561 101.068 -0.49
6 101.025 101.062 0.04
7 100.981 101.022 0.04
8 100.720 100.927 0.21
9 100.667 100.926 0.26
10 100.702 100.942 0.24
11 100.642 100.892 0.25
12 100.594 100.627 0.03
13 100.235 100.295 0.06
14 100.136 100.192 0.06
15 100.066 100.036 -0.03
16 99.797 99.782 -0.02
17 99.606 99.596 -0.01
18 99.358 99.351 -0.01
19 99.188 99.176 -0.01
20 98.453 98.439 -0.01
21 98.365 98.352 -0.01
22 98.310 98.292 -0.02
WESTERN TRIBUTARY
23 101.172 101.137 -0.03
24 100.928 100.924 0.00
25 100.564 100.715 0.15
26 100.490 100.573 0.08
27 100.400 100.473 0.07
28 100.328 100.292 -0.04
29 100.276 100.138 -0.14
29.5 100.234 100.053 -0.18
30 99.813 99.809 0.00
31 99.765 99.752 -0.01
32 99.622 99.610 -0.01
33 99.308 99.302 -0.01
34 99.191 99.179 -0.01
35 98.453 98.439 -0.01
36 98.365 98.352 -0.01
37 98.352 98.338 -0.01
38 98.112 98.098 -0.01
39 97.510 97.497 -0.01
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 20
[OCR of page images]
Table 21 :
NODE
EASTERN TRIBUTARY
1 103.110
2 103.080
3 101.805
4 101.731
5 101.561
6 101.025
7 100.981
8 100.720
9 100.667
10 100.702
an} 100.642
12 100.594
13 100.235
14 100.136
15 100.066
16 99.797
17 99.606
18 99.358
19 99.188
20 98.453
21 98.365
22 98.310
WESTERN TRIBUTARY
23 101.172
24 100.928
25 100.564
26 100.490
27 100.400
28 100.328
29 100.276
29.5 100.234
30 99.813
31 99.765
32 99.622
33 99.308
34 99.191
35 98.453
36 98.365
37 98.352
38 98.112
39 97.510
Predicted 100 Year Flood Levels (m) And Affluxes (m)
(Yellow shading indicates areas within site)
PRE-DEV POST-DEV AFFLUX
100 YEAR
103.110
103.079
101.743
101.534
101.068
101.062
101.022
100.927
100.926
100.942
100.892
100.627
100.295
100.192
100.036
99.782
99.596
99.351
99.176
98.439
98.352
98.292
101.137 0.03
100.924
100.715
100.573
100.473
100.292
100.138
100.053
99.809
99.752
99.610
99.302
99.179
98.439
98.352
98.338
98.098
97.497
0.00
0.00
-0.20
-0.49
0.04
0.04
0.21
0.26
0.24
0.25
0.03
0.06
0.06
-0.03
-0.02
-0.01
-0.01
-0.01
-0.01
-0.01
-0.02
0.00
0.15
0.08
0.07
-0.04
-0.14
-0.18
0.00
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc
Page 20
Section 22
9.0 FLOOD PROFILES, POST-DEVELOPMENT
9.1 Mitigation Concept For Increased Stormwater Runoff
The impact of the increased site runoff on flood profiles along the floodways is proposed to be mitigated
mostly by increasing the floodway storage on the site, and partly by conventional detention storage in
two of the proposed bio-basins. The increase in the floodway storage will be achieved by deliberately
roughening the floodways (by revegetating the riparian zones, as required by Council), by excavating a
side-storage alongside the western tributary, and by constructing a restrictive footbridge across the
eastern tributary to back-up the flood profiles in that tributary floodway.
A small additional amount of storage is also required to offset the minor intrusions of the development
platform into the pre-development 100 year flood boundary, and for some minor regrading of an artificial
hump in the existing low-flow channel around node 5 of the Extran model (as seen on Figure 10).
The full suite of the proposed modifications to the flood storage system is shown on Figure 9, with
additional detail on Figures 10 to 12.
The revegetation of the riparian zones, by planting of shrubs (typically, Lomandra sp) and trees at
densities of one item per 1.5 to 2 square metres, is presumed to increase the Manning’s n of the presently
grassed areas from 0.03/0.05 to 0.07.
The detention storages in the bio-basins are accounted as only the “live” storages, in the narrow range of
elevation between the riser overflow level and the weir level. The overflow level in the riser will be
controlled by an overflow slot in one side of the riser, with details as in Figure 12.
9.2 Model Simulation
An Extran model of the post-development scenario was compiled by adjusting a copy of the pre-
development model so as to include the reshaped and revegetated floodways and associated increases in
Manning’s n values, together with the inclusion of the proposed footbridge (with zero blockage), the
minor regrading of the invert around Node 5, and hypothetical vertical walls at the boundaries of the
proposed allotments.
The achievement of the required increase in floodplain volume, to offset the development’s stormwater
detention requirement, is summarised in Table 22, below:
Table 22: Comparison Of Pre-Development And Post-Development Floodplain Volumes (m3)
To Required Detention Volumes
1Year 2 Year 5 Year 10 Year 20 Year 50 year 100 Year
Pre-Development 5536 6311 8787 10202 11576 13125 14483
Floodway Storage
Post-Development 5909 6775 9607 11697 13389 15383 16849
Floodway Storage
Floodway Gain 373 464 820 1495 1813 2258 2366
Plus Bio-Basins 162 183 252 252 252 252 252
Less Park, Bund Fill -0 -0 -0 -0 -5 -40 -72
Nett Gain 535 647 1072 1747 2060 2470 2546
Req’d Extra Volume
(From Table 6) 395 525 770 910 1095 1370 1585
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 21
Section 23
The impact of the proposed development on peak flow rates downstream of the site is summarised in
Table 23, below. Flow rates leaving the site via the eastern tributary (nodes 15 – 16) will be slightly
reduced. Flow rates leaving the site via the western tributary (nodes 29.5 - 30) will increase, and this is a
result of the diversion of local stormwater runoff away from No 2 Pearsons Road and directly to the
western floodway. Further downstream along the western floodway, past the junction with the remaining
local runoff from that adjoining property (nodes 31-32), the post-development flow rates will be close to
the pre-development flow rates. Further downstream, at the downstream end of the model (nodes 38-39),
where the two tributaries have converged, the flow-rates will be unchanged or slightly reduced.
Table 23 : Pre / Post-Development Peak Flow Rates (m3/sec) Through Downstream Properties
Between 1 Year 2 Year 5 Year 10 Year 20 Year 50 Year 100 Year
Nodes
15 - 16 6.8 / 6.7 8.4 / 8.3 13.8 /13.4 17.4 / 17.3 20.7 / 19.9 23.8 / 22.6 27.0 / 25.0
29.5 - 30 5.0 / 5.3 6.3 / 6.7 9.6 / 10.3 11.7 / 12.3 13.7 / 14.9 16.0 /18.1 17.8 / 20.5
31 - 32 5.4 / 5.6 6.7 / 6.8 9.9 / 10.3 12.5 / 12.4 17.7 / 17.7 26.3 / 26.1 31.5 / 31.2
38 - 39 12.7/12.7 15.8 / 15.8 24.2 / 24.1 30.1 / 29.6 37.9 / 37.6 47.9 / 47.0 55.1 / 53.7
The flood levels predicted by the post-development model have been compared to those for the pre-
development scenario in Table 19 to 21 on preceding pages of this report, for ARI’s from 1 to 100 years.
Yellow highlighting is used in those tables to identify nodes that are within the site boundaries. The flood
level affluxes on all external properties as a result of the proposed development are seen to be practically
zero (maximum 0.01 metre) or negative (beneficial), when quoted to the nearest centimetre.
The corresponding 100 year post-development flood profiles are shown on Figure 10, and the post-
development 100 year boundary is mapped on Figure 13.
9.3 Sensitivity Tests
The sensitivity of the development to uncontrolled regrowth along the floodways was examined by
applying a Manning’s n of value of 0.15 to the overbank riparian zones of all the cross-sections of the
model. The additional impact of the footbridge being blocked by debris was simulated by assuming that
the handrails could be completely blocked by debris and the under-bridge aperture could be 50 percent
blocked. The 100 year flood levels predicted by these tests are summarised in Table 24, on the following
page of this report, and in Figures 10 and 13. The design levels for the development platform should take
due account of these flood levels.
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 22
Section 24
Table 24: 100 Year Flood Levels (m) With High Manning’s n And Footbridge Blockage (50% blockage of bridge opening and 100% blockage of handrail) XPStorm Node High Manning’s N High Manning’s N + Bridge Blockage 3 101.78 101.79 4 101.68 101.68 5 101.20 101.22 6 101.18 101.20 7 101.15 101.18 8 101.05 101.10 9 101.04 101.08 10 101.04 101.08 11 100.97 101.02 11.3 100.93 100.99 (Bridge) (Bridge) (Bridge) 11.4 100.88 12 100.82 13 100.56 14 100.44 1S 100.17 23 101.53 24 101.27 25 101.00 26 100.83 27 100.71 28 100.51 29 100.32 29.5 100.23 3 100.01 10.0 CERTIFICATION I hereby certify that the findings described in this report have been deduced by industry-standard analytical procedures and that the Noosa Council can rely on those findings in their considerations of the relevant development application. She AEDES oo yancie 72s 2s Seine dea serene eee Denis Ogle, RPEQ 2405, 15 September 2014 Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 23
Section 25
NORTH
CORDWO
OD
DRIVE
M A SONRY
16
8
15
9 1
AD
RP173883
RO
NS
10
ST RE
SO
14
AR
ET
PE
11
7
32
No 2, PEARSONS ROAD 12
6
44 31
43 13
42
5
41
40 30
45 39
38
37
E LM
36 33 28 27
35 100
29 PARK
34
STREE
46 26
T
47 48 49
50 20
51 100 25
52 24
53 56 PARK
54
55 21
19
22 4
57 23 BIO BASIN
3
17 2
E
BLUE R EN
18 1
D RIV
ALD
ON
CD
W
A
E M
PLA LA K
C E
0 SCALE 100 m
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
LOCALITY AND LOT PLAN FIGURE 1
Section 26
NORTH
CORDWO
OD
DRIVE
M A SONRY
PARTIAL-AREA LIMITS, FOR CALCULATION OF
PRE- AND POST-DEVELOPMENT FLOW RATES 16
8
15
9 1
AD
RP173883
RO
NS
10
S T RE
SO
14
AR
ET
PE
11
7
32
No 2, PEARSONS ROAD 12
6
44 31
43 13
42
5
41
E LM
40 30
45 39
38
37 33 27
36 28
35 100
29 PARK
STREE
34
46 26
T
47 48 49
50 20
51 100 25
52 24
53 56 PARK
54
55 21
19
22 4
57 23 BIO BASIN
3
17 2
E
BLUE R EN
18 1
D RIV
ALD
ON
CD
W
A
E M
PLA LA K
C E
0 SCALE 100 m
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
URBS MODELS OF STORMWATER DETENTION REQUIREMENT FIGURE 2
Section 27
AD
RO
NORTH
S
ON
S
AR
PE
PARTIAL-AREA LIMIT, FOR CALCULATION OF
PRE-DEVELOPMENT FLOW RATES
No 2, PEARSONS ROAD
ELM
STREET
PRE-DEVELOPMENT SCENARIO
AD
RO
S
S ON
AR
PE
PARTIAL-AREA LIMIT, FOR CALCULATION OF
POST-DEVELOPMENT FLOW RATES
No 2, PEARSONS ROAD
44
43 FLOW
42 IN EXCESS S
OF Q5
39
ELM
45 38
37 33
36
35 BIO
STREET
34
46
FLOWS UP
TO Q5
47 48 49
50
51
52
53 56
54
55
57
POST-DEVELOPMENT SCENARIO
0 SCALE 100 m
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
PROPOSED CHANGE IN DISCHARGE SCENARIOS TO No 2 PEARSONS ROAD FIGURE 3
Section 28
NORTH
T
E C
GE
W IL
E
IV
DR
A LD
ON
CD
MA
KE
LA
ST RE ET
G EM
COOROY
EL M
CEMETERY
S TREE
T
E
DRIV
A LD
ON
CD
E M
LA K
COOROY
ET
S TRE
KAURI
ST
NE
I VI
OL
SAPPHIRE STREET
STREET
PEARL STREET
RD
N
TI
AN
MA P
ELM
T EW
LE
STRE ET
DIAMOND ST
0 SCALE 500 m
KAURI ST
EM ERA LD
ST
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
CATCHMENT SUBDIVISION FOR PRE-DEVELOPMENT URBS MODEL FIGURE 4
Section 29
NORTH
T
E C
GE
WIL
E
IV
DR
LD
T
E C
NA
GE
O
W IL
E
CD
MA
IV
DR
KE
D
AL
ON
KE
MA
CD
LA
LA
T
ST RE E
G EM
ET
COOROY
E M STRE
G
EL M
CEMETERY
S TREE
COOROY
T
E
RIV
EL M
D D
CEMETERY
AL
ON
CD
E M
ST R E
LA K
COOROY
ET
ET
S TRE
KAURI
ST
E
IN
OL
IV
DETAIL PLAN - SITE AREA 0 SCALE 200 m
SAPPHIRE STREET
STREET
PEARL STREET
RD
N
TI
AN
MA P
ELM
W
TE
LE
STRE ET
DIAMOND ST
0 SCALE 500 m
KAURI ST
EM ERA LD
ST
OVERALL PLAN
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
CATCHMENT SUBDIVISION FOR POST-DEVELOPMENT URBS MODEL FIGURE 5
Section 30
NORTH
5
.4
5
99
.2
7
5 98
.7
.9
97
98
7
.0 .1 3
3
98 .0 4
4
.5
9998
.6
99
98
5
.7
99
MASON
8
.3
699
8
9 1
.7
.8
2
8
.8 .1
99
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The text continues on section 31.