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Attached to the e-mail REC13/0146 - Response to 2nd Information request.
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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Page 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]
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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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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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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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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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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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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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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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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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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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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
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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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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 Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 13
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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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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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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 Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 16
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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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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 Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 18
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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
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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
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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
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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
Page 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
Page 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
Page 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
Page 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
Page 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
Page 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
Page 30
SCALE BSM— 200m 10 Sep 2014 | VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY | XPSTORM HYDRAULIC MODEL NETWORK | [FIGURE 6 |
Page 31
LOOKING DOWNSTREAM FROM NODE 10, THROUGH NODE 11 TO NODE 12
LOOKING DOWNSTREAM FROM NODE 12, THROUGH NODE 13 TO NODE 14
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
PHOTOGRAPHS OF EASTERN FLOODWAY ON 25 NOVEMBER 2013 FIGURE 7
Page 32
LOOKING DOWNSTREAM FROM NODE 25, TO NODE 26
LOOKING UPSTREAM FROM NODE 27, TO NODE 26
28 Apr 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
PHOTOGRAPHS OF WESTERN FLOODWAY ON 25 NOVEMBER 2013 FIGURE 8
Page 33
NORTH
SAUNDERS PROPERTY
FO
O
TP
AT
H
SW
SW
OVERALL PLAN
0 SCALE 100 m
EXG TREE
x100.04x (0.13 above NS)
1-
1-
on
on
-6
-2
-6
0
on
1-
1 - in - 100
99.09x
.2
NS
x x
99
100.19 (0.17 above NS)
1-
.3
SAUNDERS PROPERTY in - EXG TREE
99
ETR
10 -6
0
on
T R/P B 4 0 0 /8 T R/P B 4 0 0 /8 1-
x
99.28 DETAIL OF SIDE-STORAGE
0 50
15 Sept 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
PROPOSED FLOODWAY MODIFICATIONS, BRIDGE AND BIO-DETENTION PONDS FIGURE 9
Page 34
WESTERN TRIBUTARY
EASTERN TRIBUTARY
10 Sep 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
LONGITUDINAL FLOODWAY PROFILES FIGURE 10
Page 35
SIDE-STORAGE
SIDE-STORAGE
DETAIL OF CHANNEL RESHAPING
TYPICAL CROSS-SECTION DETAILS
10 Sep 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
FLOODWAY CROSS - SECTION CHANGES, WEST TRIBUTARY FIGURE 11
Page 36
DECK LEVEL 100.68 (10 YR FLOOD LEVEL)
0.4 M
10 YR FLOOD LEVEL 10 YR FLOOD LEVEL
APPROACH FILL APPROACH FILL
IL 98.63
0.45 M DIA PILE
PROPOSED FOOTBRIDGE, LOOKING DOWNSTREAM
(8.8 M CLEAR SPAN) (BRIDGE OPENING AND ENTRY / EXIT TO BE ROCK-PROTECTED)
Q5 STORAGE LEVEL
Q1 STORAGE LEVEL
0.3 M BIO-STORAGE 0.7 M 0.4 M 0.1 M
WEIR
N-3
1-O
SAND SURFACE AREA 200 M2
1 M x 1M (INT) RISER BOX
0.7 M WIDE x 0.4 M DEEP NOTCH IN ONE SIDE
OUTLET PIPE, DIA AS REQ'D SAFETY / TRASH RACK AS REQ'D
TO CONVEY Q5 = 0.63 M3/SEC
BIO-BASIN SCHEMATIC, WESTERN TRIBUTARY
Q5 STORAGE LEVEL
0.2 M BIO-STORAGE
0.2 M 0.07 M Q1 STORAGE LEVEL
WEIR
1- ON - 5
SAND SURFACE AREA 150 M2
1 M x 1M (INT) RISER BOX
1.0 M WIDE x 0.2 M DEEP NOTCH IN ONE SIDE
OUTLET PIPE, DIA AS REQ'D SAFETY / TRASH RACK AS REQ'D
TO CONVEY Q5 = 0.35 M3/SEC
BIO-BASIN SCHEMATIC, EASTERN TRIBUTARY
15 Sep 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
FOOTBRIDGE AND BIO - BASIN SCHEMATICS FIGURE 12
Page 37
NORTH
SAUNDERS PROPERTY
0 SCALE 50 m
15 Sep 2014
VALKEV PTY LTD - ELM STREET AND LAKE MACDONALD DRIVE, COOROY
FLOOD LEVELS AND FLOOD BOUNDARIES FIGURE 13
Page 38
Appendices
A. Design Rainfall Data
B. Rational Method Calculations Of Floodway Inflows, Pre-Development
C. Rational Method Calculations Of Floodway Inflows, Post Development
D. Floodplain Storage Volumes
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 37
Page 39
Appendix A
Design Rainfall Data
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 38
Page 40
Mgt Restate’ ureau of Meteorology Location Label: Cooroy Latitude: 26° 24' 26" S)] Longitude: 152° 54! 51" [Nearest grid cell: 152.9125 (E)] mm) Rainfall depth for Durations, Exceedance per Year (EY), and Annual Exceedance Probabilities (AEP). EY Duration | AEY 50% © 20% «= 10% «= 5% = 2% =| 1% 1 min 27 a4 4.2 49| . 5.6 6.4 71 2 min 48 5.4 7.4 88 101 119 133 3 min 68 76 104 12.2 141 165 18.4 4 min a 8.5 96 130 153 175 205 22.7 5 min 10.1. 114 154 180 206 240 265 10min 161 182 245 285 32.4 37.3 40.9 15 min 20.4 +«-23.0's«30.9si36.0—i«.9—“ié‘iWCt*«*SiCSS 30 min 289 326 439 514 586 67.8 74.7 ahour “303 444 603 71.2 817 95.7 1064 2 hour | 523,S—«i59.4.s«BSsi«BSOC«dAB. “Cs: s151.0 3 hour 61.6 =—70.4.=Ss«984. «18.0 «137.6 164.3. 185.3 6 hour 814 93.9 134.3 162.8 191.6 231.0 262.3 12 hour | 106.3. 124.0 181.5 222.5 264.1 321.7 367.6 24 hour 134.9 158.8 237.6 2948 353.6 436.1 502.8 48 hour - 164.5 194.4 295.7 371.2 450.7 564.9 659.8 72 hour | 481.6 «214.5. 327.8 413.8 505.7, 640.0 753.4 96 hour 194.2. 229.1 350.0 442.8 542.9 690.6 816.5 120 hour 204.9 241.2, 367.6 «465.2, «570.8 727.5 861.7 144 hour 214.8 252.3. 383.1 484.2, 593.7, 756.2 895.5 168 hour 204.5 -263.2, «397.6 «501.3 613.4 779.5 —:921.5 Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 39
Page 41
Appendix B
Rational Method Calculations Of Floodway Inflows, Pre-Development
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 40
Page 42
Concentration Times
Subareas 1 - 5
Average Stream Velocity: 5-6% catchment slope = hilly; T = 1206 / 0.9 x 60 = 22 mins
Components Of Flow: 40 m sheet flow @ 10% = 9 mins
530 m steep [email protected] m/sec = 7 mins
730 m [email protected] – 1.1 m/sec = 12 mins
Total: 28 mins
Bransby-Williams: T = 58 L / A0.1 Se0.2 = 58 x 1.206 / 66.8 0.1 x 1.04 0.2 = 46 mins
From 22, 28, 46 mins (average 32) : Adopt Tc = 30 mins
Subarea 7
55 m sheet flow @ 4-5% = 12 mins
230 m swale/channel @ 5% = 4 mins
Total: 16 mins
Subarea 8
130 m sheet flow, partly lawn @ 7% = 16 mins
Subarea 9
85 m sheet flow @ 8% = 13 mins
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 41
Page 43
Subareas 12 — 16 Std inlet time for 10% grade = 9 mins 150 m pipe/kerb along Tewantin Rd@ 6% = 1.2 min 60 m pipe/kerb along Kauri St@ 4% grade = 0.6 mins 115 m pipe/kerb along Pearl St @ 4% grade = 1.2 mins 115 m pipe flow to sapphire [email protected]% grade = 1.8 mins 260 m open easement [email protected]% grade = 6.0 mins 370 m natural floodway @ hydraulic model 0.8 m/sec = 7.7 mins Total: = 27 mins Subarea 17 130 m sheet flow @ 4% = 19 mins Subarea 18 90 m sheet flow @ 4%, partly lawn = 16 mins Subarea 19 85 m sheet [email protected]%, average grass = 19 mins 85 m natural [email protected] m fall = 5 mins Total: 24 mins Q=0.00278 x FxCxIx A (F = 0.80 / 0.85 / 0.95 / 1.00 / 1.05 / 1.15 / 1.20) Subs Fi | 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 1-5 | 0.21 | 0.74 | 28 60/67 / 90 / 107 / 123 / 140 / 155 | 66.8 | 6.0/7.6/11.5/14.2/16.9 /21.5 /24.6 7 0.20 | 0.74 | 16 80/90 /120/ 140 / 158 / 183 / 202 | 2.2 | 0.29/0.35/0.52/0.63 / 0.75/ 0.95/ 1.10 8 0.06 | 0.71 16 80/90 /120/ 140 / 158 / 183 / 202 | 2.0 | 0.25/0.30/0.45 /0.55 / 0.65 / 0.83/ 0.96 9 0.10 | 0.72 | 13 89 /100/133/ 155 / 175 / 201 / 221 | 1.64 | 0.23/0.27/ 0.40 / 0.49 / 0.59 / 0.74 / 0.85 12-16 | 0.50 | 0.80 | 24 61/73/ 98 / 116 / 130 / 153 / 165 | 44.4 | 4.8/5.8 /8.6 /10.8 /12.8 /16.3/18.7 17 0.00 | 0.70 | 19 73: 182/109 / 129 / 148 / 167 / 186 | 1.9 | 0.22/0.26/ 0.38 / 0.48 / 0.57 / 0.71 / 0.83 18 0.04 | 0.71 16 80/90/120/ 140 / 158 / 183 / 202 | 1.7 | 0.21/0.26/0.38/0.47/0.56/0.71/0.81 19 0.09 | 0.72 | 24 65/ 73/ 98/ 116/ 130/ 152 / 166 | 1.99 | 0.21/0.25 / 0.37 / 0.46 / 0.54 / 0.70 /0.79 Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 42
Page 44
Appendix C
Rational Method Calculations Of Floodway Inflows, Post-Development
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 43
Page 45
Concentration Times Subarea 7 As per “Pre-Development”, Tc = 16 mins Subarea 8 QUDM std inlet time, 6 – 7% slope = 10 mins Subarea 9 Most of the area is within the 100 yr flood boundary: adopt Tc = 5 mins. Subareas 10 - 18 50 m sheet flow across lawn @ 8% = 13 mins 50 m swale/pipe @ 1% = 1 min Total: = 14 mins Subarea 19 30 m sheet flow across lawn @ 3% = 10 mins 45 m pipe / swale @ 2% = 1 min Total: = 11 mins Subarea 20 Most of the area is within the 100 yr flood boundary: adopt Tc = 5 mins Subarea 29 - 35 70 m sheet flow across lawn @3% = 17 mins 65 m pipe/kerb @ 2 m fall = 1 min 160 m pipe @ 6 m fall = 2 mins Total: = 20 mins Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 44
Page 46
Subarea 36 QUDM std inlet time for 8% grade = 10 mins 70 m wq-swale & pipes @ 1% = 2 mins Total : = 12 mins Subarea 37 As per “Pre-Development” : 16 mins Subarea 38 QUDM std inlet time for 1.5% grade = 15 mins 5-10 m pipe = 0 mins Total: = 15 mins Q=0.00278 x FxCxIx A (F = 0.80 / 0.85 / 0.95 / 1.00 / 1.05 / 1.15 / 1.20) Subs Fi 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 0.30 | 0.76 16 80/90/ 120/140 / 158/ 183/202 | 2.25 | 0.30/ 0.36/ 0.54 / 0.67 / 0.79 / 1.00/ 1.15 0.60 | 0.82 10 97/109 / 147/171 / 194/224/245 | 0.29 | 0.05/0.06/ 0.09 / 0.11 / 0.13 / 0.17 /0.19 9 0.00 | 0.70 5 121/137 /185 / 216 / 247 / 288 / 318 | 0.72 | 0.14/0.16/ 0.25 / 0.30 / 0.36 / 0.46 / 0.53 10-18 | 0.50 | 0.80 14 84/96 / 128 / - / - / -/ - | 1.35 | 0.20/0.24/0.37/ ----/ --- -- | ---- 19 0.60 | 0.82 11 94/106 /143 / 166 / 189 / 218/238 | 0.37 | 0.06/0.08 / 0.11 /0.14/0.17/0.21 /0.24 20 0.00 | 0.70 5 121/137 / 185 / 216 / 247 / 288/318 | 0.99 | 0.20/0.22 / 0.34 / 0.42 / 0.50 / 0.64 /0.74 29-35 | 0.46 0.79 20 71/80/106/ --- / --- / --- /--- | 3.09 | 0.39/0.46/0.68/ --- / --- / --- /--- 36 0.45 0.79 12 92 /103 /138/ 160 / 183/211 /231 | 1.32 | 0.21/0.25/0.38/0.48 / 0.56 /0.70/0.80 37 0.08 0.72 16 80/90/ 120/140 / 158/ 183/202 | 0.84 | 0.11/0.13 /0.19 /0.24/ 0.28 / 0.35 /0.41 38 0.60 | 0.82 15 82/92 /124/ 144/ 164/ 188/206 | 0.40 | 0.06/0.07 / 0.11 / 0.13 / 0.16 / 0.20 / 0.23 oo; Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 45
Page 47
Appendix D
Floodplain Storage Volumes
Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc Page 46
Page 48
PRE-DEVELOPMENT FLOODPLAIN STORAGE VOLUMES (CUBIC METRES) USNODE DSNODE 1YEAR 2YEAR 5YEAR 10YEAR 20 YEAR 50 YEAR 100 YEAR 3 4 273 286 356 398 438 490 549 4 5 127 136 178 204 225 250 273 5 6 583 612 724 815 900 957 988 6 7 648 669 776 910 1031 1075 1089 7 8 139 178 341 465 535 560 702 8 9 197 227 317 392 499 563 635 9 10 175 200 280 340 422 497 565 10 11 300 345 491 592 712 842 948 11 12 247 282 403 480 561 668 751 12 13 314 362 522 590 667 840 971 13 14 470 558 865 981 1071 1285 1480 14 15 310 386 615 719 798 922 1022 24 25 116 138 198 231 268 313 346 25 26 324 366 463 514 567 630 675 26 27 188 226 332 388 445 511 558 27 28 263 316 455 518 579 648 693 28 29 473 565 805 900 991 1089 1152 29 30 389 459 666 765 867 985 1086 TOTAL 5536 6311 8787 10202 11576 13125 14483 POST-DEVELOPMENT FLOODPLAIN STORAGE VOLUMES (CUBIC METRES) USNODE DSNODE 1YEAR 2YEAR 5YEAR 10YEAR 20 YEAR 50 YEAR 100 YEAR 3 4 208 217 254 293 321 383 422 4 5 85 91 115 147 162 206 238 5 6 646 668 794 891 919 991 1072 6 7 650 673 825 946 978 1067 1176 7 8 146 190 437 665 932 1081 1181 8 9 234 279 472 666 779 876 940 9 10 206 242 400 579 670 751 805 10 11 356 422 685 993 1154 1304 1404 11 12 283 327 484 611 719 834 909 12 13 347 397 565 654 787 944 1046 13 14 522 606 930 1067 1211 1465 1646 14 15 323 399 627 737 807 939 1044 24 25 150 178 247 286 331 389 430 25 26 286 332 445 511 590 685 750 26 27 237 281 383 438 502 580 636 27 28 229 282 400 463 535 619 675 28 29 328 403 558 641 738 843 912 29 30 404 480 629 730 854 1008 1139 Ss 29.5 269 308 357 379 400 418 424 TOTAL 5909 6775 9607 11697 13389 15383 16849 Brisbane Stormwater Management Pty Ltd Document File: WalkevSDF.Doc Page 47