BCC BPD Stage 2
Brisbane City Council
11 May 2012
Stormwater Backflow
Devices Feasibility Study
Stage 2
Remaining Systems in Case Study Areas - Concept Report
AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
Stormwater Backflow Devices Feasibility Study Stage 2
Remaining Systems in Case Study Areas - Concept Report
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Brisbane City Council
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11 May 2012
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
Quality Information
Document
Stormwater Backflow Devices Feasibility Study Stage 2
Ref
60238110
Date
11 May 2012
Prepared by
Richard Hancock
Reviewed by
Ralf Sieberer
Revision History
Revision
2
Authorised
Revision
Date
Details
11 May 2012
Final For Issue
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Name/Position
Mark Gibbs
Segment Lead –
Water Resources and
Coastal Management
Signature
AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
Table of Contents
Executive Summary
1.0
Introduction
1.1
Project Background
1.2
Objectives
1.3
Structure
1.4
Selection of Three Case Study Areas
1.5
Selection of the Two Drainage Systems for Detail Design
1.6
Potential Sea Level Change Impacts
2.0
Factors affecting installation and operation of backflow prevention devices
2.1
River factors
2.2
Drainage network factors
2.3
Groundwater factors
2.4
Reduction of Flood Storage
3.0
Review and Evaluation of Backflow Prevention Devices
3.1
Conclusions
4.0
Maintenance Schedule
5.0
Study Area Investigation
5.1
Outline of process
5.2
Site Inspections
5.3
Summary of Design guidelines
5.4
New Farm System A: James St
5.4.1
Options for installation of BPDs within project objectives
5.4.2
Recommendation
5.5
New Farm System B: Sydney St
5.5.1
Options for installation of BPDs within project objectives
5.5.2
Recommendations
5.6
Brisbane CBD Systems
5.6.1
Recommendation
5.7
Milton System A: Castlemaine St
5.7.1
Recommendation
5.8
Auchenflower / Toowong System A: Lang Parade
5.8.1
Recommendation
5.9
Auchenflower System A: Coronation Drive
5.9.1
Recommendation
6.0
Approvals
6.1
Introduction
6.2
Local Approval Triggers
6.3
Riverine Protection Permit
6.4
Prescribed Tidal Works/Tidal Works
6.5
Waterway Barrier Works
7.0
Hydraulic and Hydrologic Modelling
7.1
General
7.2
Site Conditions
7.3
Methodology
7.3.1
Hydrology
7.3.2
Hydraulics
7.3.3
Assumptions
7.3.4
Limitations
7.4
Hydrology
7.4.1
Model Set Up
7.4.2
Results
7.4.3
Verification
7.5
Hydraulics
7.5.1
Model Set Up
7.5.2
Results
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8.0
9.0
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11.0
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
7.6
Modelling Conclusions
Cost Estimates
Concept Layouts
Conclusions
List of Abbreviations
33
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Appendix A
BPD Review Spreadsheet and Pictures
A
Appendix B
Modelling Maps
Catchment Maps
Model Extents
Levee Height Storage Extents
B
B
B
B
Appendix C
Head Loss Data
C
Appendix D
Cost Estimates
D
Appendix E
Concept Layouts
E
Appendix F
Sample Maintenance Plans
F
List of Tables
Table 1
Case Study Areas and Systems
Table 2
Summary of Outcomes of Investigations
Table 3
Case Study Areas and Systems
Table 4
Summary of Review and Evaluation of Backflow Prevention Devices
Table 5: Outfall details
Table 6: Estimated Peak Flood and Levee Heights (m, AHD)
Table 7: Outfall details
Table 8: Estimated Peak Flood and Levee Heights (m, AHD)
Table 9: Outfall details
Table 10: Estimated Peak Flood and Levee Heights (m, AHD)
Table 11: Outfall details
Table 12: Estimated Peak Flood and Levee Heights (m, AHD)
Table 13: Outfall details
Table 14: Estimated Peak Flood and Levee Heights (m, AHD)
Table 15: Outfall details
Table 16: Estimated Peak Flood and Levee Heights (m, AHD)
Table 17
Overview of review and potential triggers for each site
Table 18
Model Parameters
Table 19
Peak Local Discharges
Table 20
Peak Local Discharges
Table 21
BPDs for Each Trunk Line
Table 22
Change in Water Levels Due to BPDs options
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AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
i
Executive Summary
Following the January 2011 flood event, the Independent Flood Response Review Board recommended that
Brisbane City Council (BCC) study the feasibility of installing devices to prevent backflow from river flooding. BCC
has subsequently commissioned the study in multiple stages.
The Stage 1 investigations identified three case study areas at New Farm, Brisbane CBD and Milton, covering 8
discrete drainage systems. These are listed in Table 1. As a component of Stage 2 of the feasibility study, two of
the eight systems in the case study areas (New Farm C: Moray Street, and Milton B: Cribb Street) were identified
as suitable for initial installation of stormwater backflow prevention devices (BPDs) and were progressed to
detailed design in February 2012. This report presents the modelling and concept design for the remaining 6
systems in the case study areas.
Table 1
Case Study Areas and Systems
Systems
New Farm Case Study Area
CBD Case Study Area
Milton Case Study Area
New Farm A : James St
Milton A: Castlemaine St
New Farm B: Sydney St
Milton B: Cribb St
Detailed Design carried out
February 2012
CBD
New Farm C: Moray St
Detailed design carried out,
February 2012
Auchenflower / Toowong A:
Lang Parade
Auchenflower A: Coronation
Drive
The objectives of this study are to:
-
Identify which of the stormwater drainage networks in the three case study areas that have the hydraulic
capacity for retrofitting BPDs;
-
Investigate cost effective backflow prevention devices that will have minimum impact on flooding and will
require low maintenance.
Investigations have included:
-
A review of the previous phases of the study in Stage 1 and the detailed design undertaken for the two
systems already completed
-
A review of the spatial data provided by BCC, particularly the relationship between outfalls considered likely
to have contributed to backflow inundation, their associated levees and statistical and historical flood levels
-
Site inspections of outfalls which were accessible from public areas on dry land
-
A literature review of available, off-the-shelf BPDs, and consultation with suppliers to identify cost effective
devices which require low maintenance
-
High level, desktop evaluation of statutory and regulatory approval requirements for the selected study areas
-
Consultation with appropriate, nominated Council staff
-
Investigation into constructability constraints (plant access, condition of existing structures, etc.) for each of
the systems
-
Hydrological and hydraulic modelling of the systems to evaluate the impacts of installing BPDs on the
probability of flooding within the catchments of the systems.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
ii
A summary of the outcomes of the investigations are presented in Table 2 and explained in detail in the body of
the report. This report also includes concept layouts for installation of BPDs in each system, which are presented
in Appendix E.
The conclusions of this Concept Report are as follows:
-
Hydrologic and hydraulic modelling of the systems has shown that:

With penstocks on CBD 2 & 3, it is feasible to install either flap valves or duck bills to CBD1 & 4 with
minimal increase in peak water levels (less than 50mm rise) during a local storm event.

It is feasible to install either flap valves or duck bills to MR1 with minimal increase in peak water levels
(less than 50mm rise) during a local storm event

For systems NF1-2, NF4-7, MLT1-4 & AUCH1-3, installation of either flap valves or duck bills has a
significant impact on peak water levels during a local storm event.

The higher tailwater elevation scenario generally results in a greater increase in water levels than the
lower tailwater elevation scenario.
-
Locations have been proposed for installation of appropriate BPDs for each system and concept layouts
developed which are presented in Appendix E.
-
In general, flap gates installed on the outlet are the preference, due to low construction cost, ease of
maintenance, and reliable operation in Council experience. Chambers are chosen when access to the outlet
for construction or maintenance is difficult. Inline valves are chosen in order to avoid construction work at
the outlet. Duck bills are chosen where siltation is an issue. Penstocks are chosen where zero head loss is
necessary, or for very large outlets (e.g. over 2400mm), but the requirements for increased maintenance,
activation and de-activation must be considered.
-
The exact alignments and pipe sizes, as well as location of other services will need to be confirmed at
detailed design stage.
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Table 2
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
iii
Summary of Outcomes of Investigations
System
Recommendation
Notes
New Farm A: James St: NF1 and 2
-
Installation of penstocks fitted
to headwalls of outfalls.
-
Large size pipes, so flap gates
not recommended
New Farm B: Sydney St: NF3-7
-
NF3 be disconnected from the
system, following on-site
investigations
Penstocks be fitted to the
headwalls of outfalls NF6 and
MF7
-
Large size pipes, so flap gates
not recommended
Installation of penstocks in
chambers on CBD1, 2 and 3
Installation of flap gate on the
outlet of CBD4
-
Large size pipes, so flap gates
not recommended for CBD 1-3
Installation of penstocks to the
headwalls of the outfalls of
MILT 1, 2 and 2A.
Installation of flap gates to the
headwalls of the outfalls of
MILT 3 and 4
-
Large size pipes, so flap gates
not recommended for MILT 1,
2 & 2A.
Installation of a number of flap
valves to a structure
constructed across the outlet
of Milton Drain to manage
backflow to the level of the
pedestrian walkway.
Installation of a system of
penstocks to provide
protection from backflow
above the level of the flap
valve structure, including a
penstock device to close off
the pedestrian walkway during
a flood event.
-
Need to provide access under
bridge so penstocks provide
protection in extreme events
Installation of penstocks to the
headwalls of the outfalls of
AUCH 1-3
-
Large size pipes, so flap gates
not recommended
-
CBD
-
Milton A: Castlemaine St: MILT 1-4,
2A
-
-
Auchenflower / Toowong A: Lang
Parade: MR1
-
-
Auchenflower A: Coronation Drive:
AUCH 1-3
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AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
1.0
Introduction
1.1
Project Background
1
Following the January 2011 flood event, the Independent Flood Response Review Board recommended that
Brisbane City Council (BCC) study the feasibility of installing devices to prevent backflow from river flooding. BCC
has subsequently commissioned the study in multiple stages.
The Stage 1 investigations identified three case study areas at New Farm, Brisbane CBD and Milton, covering 8
discrete drainage systems. These are listed in Table 1. As a component of Stage 2 of the feasibility study, two of
the eight systems in the case study areas (New Farm C: Moray Street, and Milton B: Cribb Street) were identified
as suitable for initial installation of stormwater backflow prevention devices (BPDs) and were progressed to
detailed design in February 2012. This report presents the modelling and concept design for the remaining 6
systems in the case study areas.
Table 3
Case Study Areas and Systems
Systems
New Farm Case Study Area
CBD Case Study Area
Milton Case Study Area
New Farm A : James St
Milton A: Castlemaine St
New Farm B: Sydney St
Milton B: Cribb St
Detailed Design carried out
February 2012
CBD
New Farm C: Moray St
Detailed design carried out,
February 2012
Auchenflower / Toowong A:
Lang Parade
Auchenflower A: Coronation
Drive
Figure 1 lists the reports which make up the outputs of the Stage 2 studies. The Summary Report provides a brief
overview of all the studies and their conclusions.
Figure 1
Flowchart of reports in Stage 2
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1.2
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
2
Objectives
The objectives of the Concept Phase are:
-
Identify stormwater drainage networks in the three case study areas that have the hydraulic capacity for
retrofitting BPDs;
-
Investigate cost effective backflow prevention devices that will have minimum impact on flooding and will
require low maintenance.
1.3
Structure
Throughout this report, the following definitions are applied:
-
An ‘area’ is taken to be a large geographical area, often based on suburb boundaries, which may include a
large number of drainage systems
-
A ‘drainage system’ is taken to be a discrete system of pipes and / or channels which are connected
together, and which can be investigated with respect to the flooding, drainage network and topographical
features.
This Concept Report presents the investigations undertaken on the remaining systems in the three case study
areas.
Investigations have included:
-
A review of the previous phases of the study in Stage 1 and the detailed design undertaken for the two
systems already completed
-
A review of the spatial data provided by BCC, particularly the relationship between outfalls considered likely
to have contributed to backflow inundation, their associated levees and statistical and historical flood levels
-
Site inspections of outfalls which were accessible from public areas on dry land
-
A literature review of available, off-the-shelf BPDs, and consultation with suppliers to identify cost effective
devices which require low maintenance
-
High level, desktop evaluation of statutory and regulatory approval requirements for the selected study areas
-
Consultation with appropriate, nominated Council staff
-
Investigation into constructability constraints (plant access, condition of existing structures, etc.) for each of
the systems
-
Hydrological and hydraulic modelling of the systems to evaluate the impacts of installing BPDs on the
probability of flooding within the catchments of the systems.
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1.4
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
3
Selection of Three Case Study Areas
The three case study areas selected for examination for backflow prevention (New Farm, Brisbane CBD and
Milton / Rosalie) were identified in the Stage 1 Investigation as set out in the MWA Pre-feasibility Study Report,
which states that these areas were apparently inundated by backflows during the January 2011 flood event.
The reasons for the selection of the case study areas were summarised as follows:
-
There was potential for backflows to have caused a significant amount of inundation of these areas during
the January 2011 flood event.
-
The level of development in these areas is relatively extensive and long-standing, adding to the probability
that the benefits of providing backflow protection are likely to outweigh the difficulties and costs.
-
They provide a range of scenarios for the various technologies to be assessed against during Stage 1of the
investigation.
-
The preliminary assessments in the three case study areas will then provide a basis for more detailed
assessments to be made during Stage 2, as well as a methodology which can be applied to classifying other
areas apparently inundated by backflows in January 2011
-
Council had previously undertaken drainage relief studies in each of these areas which provides guidance to
the local drainage system’s characteristics and the extent to which each area may be subject to surcharging
of the existing trunk drainage systems.
(MWA, Pre-Feasibility Study Report)
1.5
Selection of the Two Drainage Systems for Detail Design
The previous phases of the Backflow Devices Study investigated all the drainage systems in the three case study
areas, through desktop reviews and field investigations. These investigations identified that the New Farm NF811 system and the Milton MLT5-8 system were feasible for installation of BPDs within the project objectives.
These investigations were detailed in the Detail Design of Stormwater Backflow Devices - Concept Report.
1.6
Potential Sea Level Change Impacts
While not specifically addressed in the pre-feasibility study, the impacts of a sea level rise due to climate change
on the effectiveness of backflow prevention devices were examined. Although a sea level rise implies higher tide
levels on average, this should not affect the operation of the backflow prevention devices, but it may mean they
are effective in preventing backflow more often. Sea level rise could also mean that some areas become more
suitable for BPD’s in the future. There is the possibility of higher river levels increasing the closing pressure on the
devices, but current BPD specifications allow for a range of closing pressures.
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2.0
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
4
Factors affecting installation and operation of backflow
prevention devices
In order to select the most effective locations and devices for backflow prevention, this study has assessed a
range of factors for each drainage system in the three case study areas. These factors are discussed here in
general terms, and where a factor is relevant to a particular location, the role of that factor in the selection process
is addressed in the sections relating to that location below.
2.1
River factors
In order to check that a backflow prevention device will function effectively in preventing river flood water from
flowing up drainage systems, it is necessary to identify the relative heights of the levee between the river and the
inundated area and the potential peak flood heights. For each area studied, the levee heights have been
determined using aerial survey data provided by Council and these have been compared to estimated peak flood
levels derived from Council’s Floodwise Property Reports.
Other characteristics such as tidal levels, river flow velocities and siltation potential have been included in the
assessment of appropriate backflow prevention devices.
2.2
Drainage network factors
Identification of the drainage networks which may form a potential route for backflow has been carried out using
data supplied by Council, combined with visual and surface inspections of the case study areas. Comparison of
the topography of the catchments using contour data (0.25m vertical resolution) has been carried out to identify
areas which may be below estimated peak flood levels.
The locations of outfalls, inlets, manholes and other structures have been compared with contours to identify
those which are on the river side of levees which informs the location of BPDs
It should be noted that Council does not maintain information on private drainage connections, for instance roof
water connections. It is conceivable that such connections may exist in situations where they could contribute to
backflow inundation issues. Confirmation of this is outside the scope of this project.
2.3
Groundwater factors
While BPD’s are designed to block the flow of water from the river into the stormwater drainage system, there are
other possible routes which may cause backflow inundation.
The MWA report in Stage 1 stated that, in the January 2011 Brisbane flood, “There appear to have been a
significant number of instances where river floodwater backflowed either up the stormwater drains or overland and
gained entry to the basements via service ducts (e.g. Energex), car park entrances or by bursting the basement
walls under the increased hydrostatic pressure of water.” It is recommended that Council forward this report to
utility authorities for their information.
This project is focused on investigating backflow prevention devices, and so has not investigated the issue of
water infiltration via service conduits, or due to increased hydrostatic pressure. It is recommended that Council
forward this report to utility authorities for their information.
It is understood that Council has been working to address such issues via building codes.
Council has identified potential issues with infiltration of groundwater into the stormwater system during
installation of BPD chambers which will need to be considered during the construction process.
2.4
Reduction of Flood Storage
Installation of BPDs on a system prevents that drainage system being used as part of the flood storage volume for
that flood event. Removal of this volume of flood storage is considered minor for the systems currently under
consideration when compared with the volume of water stored in the main river system, and so any consequent
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
increase in the Brisbane River flood level is also considered to be minor. The flood storage would be captured in
any flood event where the flood overtops the levee. Consideration should be given to the volume of storage
removed by installation of BPDs on further systems when they are planned.
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AECOM
3.0
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
6
Review and Evaluation of Backflow Prevention Devices
As part of the earlier phases of the Backflow Devices Study, a literature review was carried out of available on offthe-shelf BPDs, consultation with suppliers and with council staff to understand current practises, observed
behaviour and previous history of BPD’s under council control. The results of that review are duplicated here as
they have been used to inform the choices of backflow prevention devices in this concept report.
Consideration has been given to the different types of BPDs available and their possible installation locations.
Particular attention has been given to:
-
the head loss rating curve of the device (typically relating flow rate with head loss)
-
the ability to install the device simply and effectively
-
whether the device is best installed at an outlet or at some point further upstream
-
reducing required monitoring and maintenance of the device
-
the ability of the device to withstand high external pressures and high lateral velocities
-
suitability for installation in the intertidal zone
-
types of devices already installed within BCC
-
the procurement and installation cost of the device.
The aim of this review was to identify a number of types of BPD which could be installed, in order to understand
their operating parameters. This data has then been used in the modelling phase to evaluate the impact of
installing different types of BPDs in the networks with respect to introduced head loss which may lead to
increased risk of flooding from local storm inundation, as well as identify which BPDs would provide the highest
efficiency and the lowest maintenance requirements.
The addition of a valve to the drainage system reduces its capacity to convey stormwater, and this reduction in
capacity is described in terms of a ‘head loss’. This head loss varies depending on how the device operates as
well as operating conditions such as whether or not the device is submerged. Head loss data which has been
used in the modelling was sourced from device manufacturers, and is included in Appendix C.
The results of the BPD review and evaluation are presented in the spreadsheet included as Appendix A, including
pictures of a number of the BPDs reviewed and a summary shown in Table 4.
3.1
Conclusions
A number of suppliers, both manufacturers and distributors were identified, offering a range of devices. The
devices can be separated into the types shown in Table 4.
The self-regulating tide gate has been excluded as being overly complicated for this situation, leading to the
potential for both high maintenance costs and a higher risk of damage from vandalism.
None of the systems in the three case study areas are suitable for installation of an inflatable rubber dam due to
the temporary nature of the dam.
All devices will require assessment of the peak river height likely to be experienced by the device, in the form of
the hydraulic pressure the device can withstand.
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Table 4
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
7
Summary of Review and Evaluation of Backflow Prevention Devices
Device
Type
Flap gates
Notes
A number of
options for
material, hinges,
installation

Suitable installation
environment

Clearance below gate
required to reduce
likelihood of debris
preventing gate from
closing, resulting in
increased construction
cost in some cases.
Large sizes (above
1800mm) subject to higher
wear and tear due to
movement of flap gate
under normal river and tide
conditions
Poor experience with
fibre glass, steel and
single hinged. Better
results with aluminium,
double hinged.
Effective when well
designed.
Low

BCC experience
Relative
head loss
Duckbill
Vertical or
horizontal
opening

Able to cope with some
siltation in front of BPD so
less requirement for
vertical clearance below
BPD.
Some problems with
debris preventing
duckbill from closing
High
Penstock
Vertical opening
via manual
control
Requires decision
to be made as to
when the
penstock is to be
closed

Requires manual control
above gate
Must be manually
activated in flood situation
and de-activated
afterwards
Requires more regular
maintenance program to
ensure gate is operational
Not suitable where likely to
suffer siltation under gate.
Implications for high
maintenance
requirements need to
be taken into account
Zero
Does not protrude from
pipe
May be suited to
installation into existing
manholes
No information
provided.
Medium



Insert fitted with
rubber flap to
provide one way
flow

Self
regulating
tide gate
Gate has device
fitted with
flotation bulbs
attached

Easily subject to damage,
so requires vandal free
environment
No information
provided.
Unknown
Inflatable
rubber dam
Dam is inflated in
place when
required

Able to dam large open
outfalls
Must be manually
activated in flood situation
and de-activated
afterwards.
Easily subject to damage,
so requires vandal free
environment
No information
provided.
Zero as
temporary
Inline
rubber flap



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4.0
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
8
Maintenance Schedule
Reliable and efficient operation of backflow prevention devices can be affected by numerous events, such as
mechanical damage due to wear or vandalism, debris or wildlife interfering with operation, sedimentation or sand
build up from normal environmental processes. While some of these events are unavoidable, an appropriate
maintenance schedule can reduce the likelihood of these events affecting operation of the BPDs leading to
damage from floodwaters.
Council’s current maintenance program for existing BPDs in tidal areas has been operating for many years. The
maintenance is scheduled prior to ‘king’ tides for devices in the tidal zone, and as part of the response to major
flood events. Maintenance inspections are carried out at the lowest available tide to allow best access to the
BPDs. Maintenance inspections comprise a visual inspection of the hinges, seals and other parts, removal of
debris or silt, and an assessment of the structure generally. For BPDs for floodwaters, further maintenance is
recommended for the period at the end of the dry season, prior to the start of the wet season, as the wet season
is when operation of BPDs is most important.
Manufacturers will also recommend particular maintenance schedules and these should be provided to Council by
suppliers for incorporation into their maintenance schedule. Examples of two plans provided by suppliers to
Council are included in Appendix F.
It is recommended that an initial inspection period, featuring more frequent visual inspections be implemented for
the BPDs being installed in order to gain an understanding of the particular characteristics of the BPDs in these
locations. A suggested frequency would be every 3 months for the first 2 years, with more frequent inspections if
issues are discovered.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
5.0
Study Area Investigation
5.1
Outline of process
9
After consultation with council staff the stormwater systems within the study areas have been evaluated based
upon their:
-
relationship of invert and surface levels to river heights and tidal levels
-
local tidal levels at the outfall location
-
frequency of inundation based upon the flood events that cause them to backflow, coincident flooding events
and the tidal range
-
existing structure geometry that may allow for ease of installation
-
construction cost
-
ability to provide measurable reductions in back flow flooding to justify their installation
Estimated flood levels have been sourced from BCC Floodwise Property Reports for properties near to the
outfalls.
Within each of the study areas, there are a number of shorter systems which do not contribute to the backflow
process, because the inlets and / or outfalls are not within the river flood zone. A desktop review of these
additional short systems has been carried out to exclude them.
Assessment of conflicts with existing infrastructure and services has been carried out through a visual inspection
and Dial Before You Dig enquiries. Details of conflicts are included for each system below. Further assessment
of service conflicts will be required during the design phases.
5.2
Site Inspections
Site inspections were undertaken to provide a better perspective of the site, and to identify design opportunities
and constraints and/or limitations of natural or constructed features. The site inspections also identified existing
site constraints including services, mangroves, trees, outlet structures and other council and community assets.
Local, regional and citywide knowledge and context setting was also obtained from BCC.
Initial site visits were undertaken within 2 hours either side of low tide on the 9th and 10th November 2011. Further
site visits were undertaken as required on a number of other occasions. Access to outfalls was limited to publicly
accessible sites on dry land. Site inspections were supplemented by photographic evidence provided by BCC.
5.3
Summary of Design guidelines
The choice of valve and location is subject to a number of constraints that can be conflicting. Often there is not a
single clear choice, but a number of choices with conflicting advantages and disadvantages, resulting in different
costs and risks. Detailed consultation with appropriate, nominated Council officers has taken place to identify
previous experience and future preferences. Council will need to decide whether the proposed concept layouts
meet its needs.
The following guidelines have been applied to develop the concept designs for the installation of BPDs, not
necessarily in any order:
-
Head loss impact on upstream flooding in local storm events must be minimised
-
BPDs fitted to outlets are generally cheaper to install due to reduced construction works
-
BPDs only to be fitted to outlets where access for maintenance is feasible utilising Council’s existing
maintenance resources. Generally this would require vehicular and personnel access to the outlet without
additional equipment.
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BCC BPD Stage 2
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-
Where access to outlets is not suitable, BPDs should be fitted in chambers constructed at an appropriate
location.
-
Selection of chamber locations should consider:

Access to chamber for maintenance with minimum of traffic management, i.e. off roads etc.

Suitable for excavation and construction of the chamber

At a suitable point on the network to be effective
10
-
Flap gates generally have lower head loss impact than duck bills, while inline valves are in between.
Penstocks have zero head loss impact as they are fully open when not activated.
-
Duck bills are considered to be more suitable where there is a likelihood of silt, sand or sediment deposition
-
Inline valves are suitable where construction work at the outlet is difficult or where a valve on the outlet
would be aesthetically inappropriate.
-
Flap gates larger than 1800 mm will be subject to higher wear and tear due to movement of flap gate under
normal river and tide conditions, but may be necessary in some situations, for instance where the head loss
impacts prevent use of a duckbill or penstocks are not appropriate.
-
Flap gates generally should have minimum 150mm clearance between the invert of the pipe and the surface
of the apron to allow debris to fall out.
-
The mass of the device should be considered during detailed design, including assessing the suitability of
the existing structure to anchor the device.
-
The visual impact of the devices has been considered, but it is recommended that alternative designs from
suppliers be considered where the visual impact is a critical factor
-
Environmental impacts of particular valves, and construction requirements should be considered at detail
design stage.
In general, flap gates installed on the outlet are the preference, due to low construction cost, ease of
maintenance, and reliable operation in Council experience. Chambers are chosen when access to the outlet for
construction or maintenance is difficult. Inline valves are chosen in order to avoid construction work at the outlet.
Duck bills are chosen where siltation is an issue. Penstocks are chosen where zero head loss is necessary, or for
very large outlets (e.g. over 2400mm), but the requirements for increased maintenance, activation and deactivation must be considered.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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5.4
11
New Farm System A: James St
Table 5: Outfall details
Location
Outfall type and size
Outlet Invert Level
A: James St: NF1
2100 RCP
Unknown
A: James St: NF2
3000 x 2100 RCBC
-0.7
Table 6: Estimated Peak Flood and Levee Heights (m, AHD)
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee Height
2.5
2.2
2.1
1.9
2.9
3.75
NF1 and NF2 are interconnected. Therefore any BPD on either of these outfalls would require a BPD on both or
an integrated backflow prevention solution.
Backflow from these systems contributed to a maximum flood depth of approx. 0.65 m and an average flood
depth of approx. 0.40 m with an area of inundation of approximately 3.87ha in the January 2011 event.
The upstream common trunk line that runs through the area of inundation is generally below Mean Sea Level
(MSL). However the pipe depths vary at that location between 2.0 m and 3.5 m and have a slope of approximately
1:503. For the outlets (m AHD), HAT:1.65, MSL:0.10, MHWS:1.06.
Both of outlets NF1 and 2 are considered suitable for installation of BPD’s attached to the face of the headwalls,
providing minor repairs are undertaken to the face of NF1. Construction of a chamber upstream is possible under
the end of Merthyr Road nearest the river. Construction work anywhere on these two pipes will be complicated by
the large size of the pipes, the age and unknown condition of the circular brick pipe, as well as the pipes being
partially submerged at all times.
There are two gully inlets situated on the river side of the levee, between 2.5m and 2.75m, and one between
2.75m and 3.0m. Since the 2011 flood height at this point was 2.9m at least the lower two of these inlets would
have been inundated. If BPDs are installed on the outfalls of NF1 and NF2, then these inlets will need to be
temporarily blocked whenever they are likely to be inundated, such as when a flood level above Q100 is
anticipated. This could be achieved with commonly available materials such as plywood and sandbags, which
would be sufficient to restrict flow due to the low levels, up to 400mm in January 2011.
Access to the area above the outlets is available from the Merthyr Road reserve, with the only infrastructure
evident in the immediate area being BCC controlled, such as the ferry depot walkway above NF2.
No service conflicts are immediately evident, but this would need to be confirmed before installation.
5.4.1
Options for installation of BPDs within project objectives
Option
Description
Option
One
Penstocks fitted to headwall of outfalls
Opportunities



Option
Two
Construct chamber between end of
Merthyr Road and river with penstocks
fitted to pipes inside chambers

Issues
Short
construction
time
Simple
installation
process
Lower cost

Good access
to
construction
site




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Floods above Q100
require field inlets to
be blocked
Activation and deactivation risks
More complex
construction
Possible service
conflicts
Higher cost
AECOM
5.4.2
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
Recommendation
-
New headwalls cast in situ in front of existing headwalls to allow installation BPDs
-
Installation of penstocks fitted to headwalls of outfalls.
NF1
-
Concept layouts are shown in Drawing 60238110-4002 in Appendix E.
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AECOM
NF2
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
13
AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
5.5
14
New Farm System B: Sydney St
Table 7: Outfall details
Location
Outfall type and size
Outlet Invert Level
NF3
525 RCP
Unknown
NF4
1650 RCP
-0.69
NF5
1950 RCP
-1.00
NF6
2100 RCP
-1.6
NF7
3000 x 1500 RCBC
~-0.15
Table 8: Estimated Peak Flood and Levee Heights (m, AHD)
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee Height
2.5
2.2
2.1
1.9
2.9
3.75
Outfalls NF3 to NF7 are identified as a single system, due to being connected either by overland flow, or via
stormwater pipes, and therefore any backflow management strategy will need to take account of all of these
outfalls.
Backflow from these systems contributed to a maximum flood depth of approx. 0.90 m (in numerous locations)
and an average flood depth of approx. 0.65 m with an area of inundation of approximately 23.23ha (approximately
1/3 of that is New Farm Park) in the January 2011 event.
Outfall NF3 appears to be connected to a short system which may not be affected by backflow inundation. The
GIS information received from BCC shows that the pipe is not connected to NF4, however, there are two
upstream manhole structures and there is a small line from the NF4 system that would indicate they are
connected, but could also be for roof drainage. Therefore it is inconclusive as to whether the NF3 system
contributes to backflow flooding in the local area west of Grey St. It may also be possible to isolate this system to
remove its potential impact on backflow. This will require further investigation to be confirmed. The pipe has a
slope of approximately 1:60.
The NF3 outfall is located approximately 2m underneath a concrete boardwalk, making access very difficult.
Furthermore, the lower surface of the concrete boardwalk projects over the top of the pipe by approximately
40mm. The bottom of the pipe is similarly obstructed, although this may be loose material. Further investigation
would be required to identify a method of fitting a BPD or extension pipe to the BPD. Access to the site itself is
reasonable, via the public riverside footpath above. No service conflicts are immediately evident, but this would
need to be confirmed before installation. Installation of a BPD upstream in a chamber or manhole is also
possible.
Outfall NF4 already has a single hinged, fibreglass flap gate installed, and NF5 has a double hinged aluminium
flap valve installed, dating from the redevelopment of the site. While a visual inspection provided no evidence of
damage, Council should confirm the current performance of these valves.
There are a further two outfalls along this stretch of the river at New Farm, designated EW/7 located near the
former easterly end of Sydney Street and EW/2 off Hollis Crescent. Both outfalls service short networks
unconnected to others and unaffected by backflow inundation due to the relative height of the surface levels for
these networks. Both already have a BPD fitted. These outfalls have not been considered as part of this project,
but further information would be required from Council as to the current performance of these flaps which would
inform the need for further investigation into bringing them up to current engineering and BCC standards.
Outfalls NF6 and NF7 are located in New Farm Park and are interconnected at a manhole about 200 m upstream
of the outlet. Both outfalls appear in good condition. At the time of site inspections in November 2011, both
outfalls had lids, with small gates attached, presumably as part of the desilting operations being carried out from a
work site in the centre of New Farm Park. The installation of these lids suggests that installation of flood gates
would also be feasible. There is also potential to install BPDs in chambers under New Farm Park.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
15
Access to the site is straightforward from New Farm Park. No service conflicts are immediately evident, but this
would need to be confirmed before installation.
The trunk lines which run through New Farm Park are generally below MSL. However the pipe depths vary at that
location between 3.0 m and 5.5 m and has a slope of approximately 1:626. For the outlets (m AHD), HAT:1.65,
MSL:0.10, MHWS:1.06
5.5.1
Options for installation of BPDs within project objectives
Option
Description
Option
One
Penstocks fitted to headwall of outfalls NF6
and NF7
Opportunities



Option
Two
5.5.2
Construct chamber under New Farm Park,
between first junction and levee with BPDs
fitted to pipes inside chambers

Short
construction
time
Simple
installation
process
Lower cost

Activation and
de-activation
risks
Good access
to construction
site

More complex
construction
Higher cost
Recommendations
-
NF3 be disconnected from the system, following on-site investigations
-
Penstocks be fitted to the headwalls of outfalls NF6 and NF7
-
Concept layouts are shown in Drawing 60238110-4003 in Appendix E.
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Issues

AECOM
NF3
NF4
NF5
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16
AECOM
NF6
NF7
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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17
AECOM
5.6
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
18
Brisbane CBD Systems
Table 9: Outfall details
Location
Outfall type and size
Outlet Invert Level
CBD1
2700 RCP
Unknown
CBD2
1350 RCP
Unknown
CBD 3
1900 x 2000 RCBC
Unknown
CBD4
900 RCP
Unknown
Table 10: Estimated Peak Flood and Levee Heights (m, AHD)
Location
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee
Height
CBD1 / 2
3.9
3.2
2.1
n/a
4.4
3.75
CBD 3 / 4
3.9
3.2
2.1
n/a
4.4
4.75
The drainage systems in Brisbane CBD are based on a number of very old systems, dating back to at least 1875,
modified and expanded over time. Because of this, the systems are interconnected and will require a detailed
analysis of the effectiveness of BPD’s as a management strategy for backflow prevention.
The CBD systems are interconnected, so all pipes in the system need to be considered to offer any protection to
the area.
CBD 1, 2 and 3 were not able to be located from publicly accessible areas, and have not been inspected, but
BCC records show that they are located under the skyscrapers and boardwalks which have been constructed
along the riverside. It is unlikely that access to these outlets will be appropriate for installation of BPDs
CBD4 is suitable for installation of a BPD following removal of the gross pollutant trap currently attached to the
outfall. The headwall is in apparently good condition, and there is good clearance around the pipe, which projects
up to 0.5m. Access is possible from the public road above the outfall, footpaths on 2 sides or the access road to
the adjacent hotel.
CBD1, 2 and 3 will require BPDs fitted inside chambers upstream under the roads. CBD 3 appears to have a
chamber suitable for installation of BPDs with some alteration. CBD1 and 2 will both require new chambers to be
constructed.
The levee height at the outfalls of CBD 1 and 2 of 3.75m is below the heights of both the 2011 flood and the 100
year ARI flood, resulting in reduced protection by BPDs.
All CBD sites are located near a significant number of other services and confirmation of the location of these
services will be required in the design process.
5.6.1
Recommendation
-
Installation of penstocks in chambers on CBD1, 2 and 3
-
Installation of flap gate on the outlet of CBD4
-
Concept layouts are shown in Drawing 60238110-4004 to 4007 in Appendix E.
-
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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19
CBD4
5.7
Milton System A: Castlemaine St
Table 11: Outfall details
Location
Outfall type and size
Outlet Invert Level
MILT1
2400 RCP
-0.877
MILT2
3000 RCP
-3.77
MILT2A
1500 RCP
-0.013
MILT3
375 RCP
Unknown
MILT4
375 RCP
Unknown
Table 12: Estimated Peak Flood and Levee Heights (m, AHD)
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee Height
5.4
4.3
3
n/a
6
8
The MILT1 and 2 system services a large area which stretches from the Brisbane River outfall to the Musgrave
Rd-Enoggera Terrace intersection and to the Musgrave Rd-Petrie Terrace intersection. It is inconclusive from a
study of the Council GIS data as to whether MILT3 and MILT4 are also connected to this system – this study has
assumed they will require BPDs to be fitted.
Backflow from this system contributed to a maximum flood depth of approx. 5.50 m (at rear of the Boys Town
Business Centre) and an average flood depth of approx. 3.00 m with an area of inundation of approximately
15.32ha in the January 2011 event. The MILT3 and 4 system contributed to backflow inundation with an area of
approximately 310 m2. There is little data available regarding the invert levels of the MILT3 and 4 system. For
the outlets (m AHD), HAT:1.64, MSL:0.05, MHWS:1.06.
MILT1, 2, 2A and 3 outlet at one headwall below the Bicentennial Bikeway. MILT 2 is a 3000 RCP which was
completely submerged at the time of inspection, close to low tide. There is also a short 375 pipe which is a short
connection to a gully inlet on the bikeway and does not contribute to backflow inundation and so no further action
is recommended.
MILT3 and 4 appear to be a roof water connection and further investigation is required to identify whether they
contribute to backflow inundation or are connected to other systems. Both outfalls appear to be suitable for
installation of BPDs on their outlets.
Some services are apparent along the bikeway and the location of these will need to be confirmed during the
design process.
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AECOM
5.7.1
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
Recommendation
-
Installation of penstocks to the headwalls of the outfalls of MILT 1, 2, 2A and 3.
-
Installation of flap gates to the headwalls of the outfalls of MILT 4 and 5.
-
Concept layouts are shown in Drawing 60238110-4008 to 4010 in Appendix E.
MILT1,
MILT2 and
MILT2A
MILT3
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20
AECOM
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
21
MILT4
5.8
Auchenflower / Toowong System A: Lang Parade
Table 13: Outfall details
Location
Outfall type and size
Outlet Invert Level
A: Lang Parade: MR1
Trapezoidal Channel (Area 29.7m²)
-1.2
Table 14: Estimated Peak Flood and Levee Heights (m, AHD)
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee Height
5.4
4.3
3
n/a
6.1
6.2
MR1 is the Milton Drain, an open, trapezoidal channel carrying runoff from a significant catchment. The levee at
this point is the embankment carrying Coronation Drive, which narrowly avoided being overtopped in January
2011. The open drain runs under the Bicentennial Bikeway, Coronation Drive and the John Oxley building before
changing to a closed pipe system under Kilroe Street approximately 300m from the river.
The water level in the January 2011 flood event was very close to the bottom of the road deck at 6.1m. In order to
provide protection from a similar level of river flooding, the installation of BPDs will be required to manage the
flood up to this level. The pedestrian walkway underneath Coronation Drive will need to be maintained, which
would constrain the design of BPDs. The level of this walkway is approximately RL3.0m near the door. Providing
protection to the level of the walkway here would provide protection to Q20. Tide levels in this area are
approximately (m AHD), HAT:1.68, MSL:0.04, MHWS:1.09, so protection to the level of the walkway would
provide protection from tides too.
The point at which Milton Drain passes under the John Oxley Centre may be suitable for installation of BPDs
offering a lower level of protection. While accurate levels are not available at this location, it is estimated that the
protection provided would be to a level of approximately RL 2.0m, before the water would overtop and run through
the car park. This would provide protection above MHWS (1.09m) and HAT (1.68m), but it would not provide
protection to the Q20 level of 3.0m. While this approach has not been recommended for this location as it does
not provide sufficient protection, it may provide a lower cost, lower benefit alternative.
In a major flood event, this system provides storage for a significant volume of flood water. It is recommended
that Council investigate the impacts of removing the flood storage provided by this system on the river system as
a whole.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
22
Detailed design of this BPD should incorporate sufficient study and design to appropriately manage the risk of
failure of the device.
5.8.1
Recommendation
-
Installation of a number of flap valves to a structure constructed across the outlet of Milton Drain to manage
backflow to the level of the pedestrian walkway.
-
Installation of a system of penstocks to provide protection from backflow above the level of the flap valve
structure, including a penstock device to close off the pedestrian walkway during a flood event.
-
Concept layouts are shown in Drawing 60238110-40011 in Appendix E.
MR1
5.9
Auchenflower System A: Coronation Drive
Table 15: Outfall details
Location
Outfall type and size
Outlet Invert Level
AUCH1
3000 RCP
-2.08
AUCH2
2 x 2100 x 1950 RCBC
Unknown
AUCH3
1950 RCP
Unknown
Table 16: Estimated Peak Flood and Levee Heights (m, AHD)
100 Year ARI
50 Year ARI
20 Year ARI
5 Year ARI
2011 Flood
Height
Levee Height
5.4
4.4
3
n/a
6.5
4.75
AUCH1 and AUCH2 are interconnected at Coronation Drive. The system services an area that extends up to
Mount Coot-tha Rd and Frederick St. Flooding during the January 2011 event occurred mostly from the river
overtopping at this location and extending past Kellett St.
Backflow from these systems contributed to flooding in the park and at a few residential properties along Sylvan
Rd. The flooding from backflow had a maximum flood depth of approx. 3.00 m and an average flood depth of
approx. 1.50 m with an area of inundation of approximately 5.07ha in the January 2011 event.
The trunk lines are generally below MSL downstream of Coronation Dr. However the pipe depths vary at that
location between 4.8 m and 5.3 m. There was not enough information within the GIS data to comment on the
general slope for the trunk system. For the outlets (m AHD), HAT:1.68, MSL:0.04, MHWS:1.09
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
23
AUCH1, 2 and 3 are all suitable for installation of BPD’s, having headwalls in good condition and straightforward
access to the outfalls.
It should be noted that the levee height of 5m is insufficient to retain the Q100 flood of 5.4m or the 2011 level of
6.5m. Because of this, installation of BPDs would provide protection against inundation in flood events up to Q50,
but would be rendered ineffective if the flood level exceeds the levee height of 4.75m. However, any BPDs would
remain effective if the levee height was increased in future.
5.9.1
Recommendation
-
Installation of penstocks to the headwalls of the outfalls of AUCH 1-3
-
Concept layouts are shown in Drawing 60238110-4012 to 4013 in Appendix E.
AUCH3
AUCH1 & 2
.
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
6.0
Approvals
6.1
Introduction
24
As part of the earlier AECOM Concept Report (Detail Design of Stormwater Backflow Devices Concept Report, 22
December 2011) a high level desktop assessment of the potential approvals triggers was undertaken for the study
areas of Brisbane CBD, Rosalie/Milton and New Farm. This review investigated:
-
Local Approval Triggers (i.e. NALL, local heritage register)
-
Riverine Protection Permit
-
Prescribed Tidal Works/Tidal Works
-
Waterway Barrier Works
A review of the remaining systems in the case study areas has been undertaken to explore the likely approval
requirements for the installation of the proposed BPDs.
Table 17 provides an overview of this review and any potential approval triggers for the relevant sites.
6.2
Local Approval Triggers
Potential local approval triggers requires consideration of the following factors:
-
Impact to vegetation protected by Brisbane City Council Natural Asset Local Law (NALL)
-
Impact to a local heritage place
-
The proposed works being triggered assessable by Brisbane City Plan
Overall the likelihood of the proposed works requiring a local approval or permit is considered relatively low,
however Table 17 provides a high level consideration of these factors. In relation to the impact on NALL
vegetation it is noted that it is assumed that there will be not loss of vegetation as a consequence of any BPD
works.
Where physical works not captured by the following section is required to be undertaken as part of installing the
BPD’s (i.e works within pathways or road reserve) it is considered that item 1, table 4 of schedule 4 (Operational
work by or on behalf of a public sector entity) of the Sustainable Planning Regulation 2009 can be reasonably
applied, whereby these works cannot be made assessable. It is noted that in previous consultation with Council
has confirmed the application of this section of the regulation suitable for the aforementioned purpose, however,
where works are within a tidal area or on unallocated state land all relevant approvals are still required to be
obtained by Council, as outlined in the following sections.
6.3
Riverine Protection Permit
As works are proposed to be undertaken by a Local Government Entity the requirement for a Riverine Protection
Permit is not likely to be triggered on the basis that the DERM Guideline – Activities in a watercourse, lake or
spring carried out by an entity can be used, and the thresholds within this guideline are not exceeded.
It is recommended that the DERM Guideline – Activities in a watercourse, lake or spring carried out by an entity
be consulted throughout the detailed design and construction process to ensure that Council’s obligations and
duties are undertaken in accordance with the requirements of the guideline.
6.4
Prescribed Tidal Works/Tidal Works
The Brisbane River within each of the study areas is identified within a Coastal Management District; therefore
where works are proposed within this area an application for Prescribed Tidal Works (PTW) will generally be
required. Tidal works are taken to be ‘prescribed’ in the basis that they are wholly within a Local Government Tidal
Area. Where an application for PTW is required the local authority, Brisbane City Council, is the assessment
manager for the application. This remains the same, even when the PTW are proposed to be undertaken by or
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behalf of the Local Government authority. Where an application for PTW is required a resource entitlement will be
required to be obtained from DERM to support the lodgement of any application.
As part of previous advice from Council it is understood that the Council’s historical drainage infrastructure is
taken to be lawfully in place, and in light of this that the attachment of proprietary devices (such as the BPDs) to
pipe or headwall and any associated works on the riverwalls, is classified as maintenance operations not requiring
approval for prescribed tidal works. Works classified as maintenance would include minor modifications to a
structural element in order to repair an existing outlet structure. Examples would include replacing a cracked
apron with a new apron at a lower level, or building a new headwall in front of the existing headwall to limit
disturbance to the rest of the wall where the headwall is cracked or uneven.
Where the proposed works require the reconstruction of elements, or new elements it is considered that an
application for PTW will be required.
6.5
Waterway Barrier Works
It is possible that the proposed BPD’s may trigger the requirement for approval for Waterway Barrier Works
(WWBW). This is only triggered where the BPD would be considered to limit fish access and movement along a
waterway. While the risk of triggering this may be low it has been included for completeness as a specific
application process with similar timeframes to the PTW application would be required to be undertaken and
submitted to Fisheries Queensland (DEEDI).
Table 17
Overview of review and potential triggers for each site
Site
(Pipe
Label)
Local Approvals
Riverine Protection Permit
Prescribed Tidal Works
NF1
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline – Activities in a
watercourse, lake or spring carried
out by an entity (DERM Guideline)
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
NF2
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
NF6
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
NF 7
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
CBD 1
N/A
N/A
Works should be undertaken in
accordance with the DERM
Guideline
N/A
Works are proposed within
road reserve/pathway
outside the CMD
N/A
CBD 2
N/A
N/A
Works should be undertaken in
N/A
Works are proposed within
N/A
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Local Approvals
Riverine Protection Permit
Prescribed Tidal Works
accordance with the DERM
Guideline
road reserve/pathway
outside the CMD
Waterway
Barrier
Works
CBD 3
N/A
N/A
Works should be undertaken in
accordance with the DERM
Guideline
N/A
Works are proposed within
road reserve/pathway
N/A
CBD 4
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MILT 1
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MILT 2
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MILT
2A
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MILT 3
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MILT 4
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
MR 1
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
AUCH 1
N/A
Identified NALL
features – however
no impact to
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
N/A
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Riverine Protection Permit
vegetation
assumed
Prescribed Tidal Works
27
Waterway
Barrier
Works
Council
AUCH 2
N/A
Identified NALL
features – however
no impact to
vegetation
assumed
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
AUCH 3
N/A
N/A
Works should be undertaken in
accordance with the DERM
Guideline
Yes
Application for PTW is
likely to be required to be
lodged with Brisbane City
Council
N/A
7.0
Hydraulic and Hydrologic Modelling
7.1
General
Hydraulic models were generated to assess the impacts of the BPD’s on water levels within the local catchment
for local stormwater events. As the purpose of the modelling was to assess the impact of BPD’s on local
stormwater levels, this assessment does not report on local flood water levels for the storm events.
Six stormwater pipe systems were assessed as part of the hydraulic modelling within this study. They are as
follows:
System
Outfall
New Farm A
NF1 & NF2
New Farm B
NF 3-7
CBD
CBD 1-3, CBD4
Milton A
MLT 1,2,3 & 4
Auchenflower/ Toowong A
MR1
Auchenflower/ Coronation Drive A
AUCH 1,2 &3
For the purposes of discussion these systems are referred to as systems NF1-7, CBD1-4, MLT1-4, and MR1 &
AUCH1-3 respectively.
7.2
Site Conditions
NF1 and NF2 are interconnected. Therefore any BPD on either of these outfalls would require a BPD on both or
an integrated backflow prevention solution.
Backflow from these systems contributed to a maximum flood depth of approximately 0.65 m and an average
flood depth of approximately 0.40 m with an area of inundation of approximately 3.87ha in the January 2011
event. The upstream common trunk line that runs through the area of inundation is generally below Mean Sea
Level (MSL).
Outfalls NF3 to NF7 are identified as a single system, due to being connected either by overland flow, or via
Stormwater pipes. Backflow from these systems contributed to a maximum flood depth of approximately 0.90 m
(in numerous locations) and an average flood depth of approximately 0.65 m with an area of inundation of
approximately 23.23ha (approximately 1/3 of that is New Farm Park) in the January 2011 event.
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The drainage systems in Brisbane CBD are based on a number of very old systems, dating back to 1875,
modified and expanded over time. The levee height at the outfalls of CBD 1 and 2 of 3.75m is below the level of
both the 2011 flood and the 100 year ARI flood, resulting in reduced protection by BPDs.
The MILT1 - 4 system services a large area which stretches from the Brisbane River outfall to the Musgrave RdEnoggera Terrace intersection and to the Musgrave Rd-Petrie Terrace intersection. MILT1, 2, 2A and 3 outlets
are located at one headwall below the Bicentennial Bikeway.
MR1 is the Milton Drain, a trapezoidal channel carrying runoff from a significant catchment area.
AUCH1 -3 system services an area that extends up to Mount Coot-tha Rd and Frederick St. Flooding during the
January 2011 event occurred mostly from the river overtopping at this location and extending past Kellett St.
Maps showing the systems, catchments and storage areas it are presented in Appendix A.
7.3
Methodology
The methodology followed for the hydraulic modelling is as follows:
7.3.1
Hydrology
-
Assess catchment conditions such as slope and land use for the local stormwater catchment associated with
the systems described above
-
Calculate the local catchment runoff using Brisbane City Council duration independent storms (DIS) using an
XP-RAFTS runoff routing model
-
Calculate catchment runoff utilising the Rational Method as outlined in the Queensland Urban Drainage
Manual (QUDM, 2007)
-
Compare peak runoff generated from the XP-RAFTS model to the Rational Method peak discharges to
validate the XP-RAFTS model.
7.3.2
Hydraulics
-
Set up 2D-1D hydraulic TUFLOW model based on topographical data supplied by Council, trunk stormwater
lines for each system, inflow hydrographs from the XP-RAFTS model and tailwater levels as per the
discussion in section 6.5;
-
Generate model results for the existing system
-
Generate model results for scenarios which include various BPDs
-
Report changes in water levels due to the BPDs
7.3.3
Assumptions
The TUFLOW hydraulic model was chosen to model the systems due to its ability to account for floodplain
storage and 1-dimensional pipe flow in the same model. As mentioned above the trunk lines only were
represented in the models. The inlets to the system were taken as the lowest pit location within the system. In
order to adequately assess the impact of BPDs in upstream water levels, the inlets were assumed to have no
impact on discharges in transferring flows from the 2d domain to the pipe network. This allowed the pipe capacity
to be the controlling element in the upstream portion of the system.
7.3.4
Limitations
Only the change in water surface elevation for areas already inundated in the existing case are presented in this
report. Calibration data and a more detailed representation of roughness and hydraulic structures (including the
pipe networks) would be needed to report flood depths, water surface elevations, and changes in overland flow
paths due to local stormwater runoff.
7.4
Hydrology
7.4.1
Model Set Up
The runoff from the local catchments was calculated using the XP software Runoff Analysis and Flow Training
Simulation (XP-RAFTS) model. The rainfall hyetographs used to generate the runoff calculated by this model
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were derived according to Brisbane Intensity Frequency Distribution (IFD) data and the BCC DIS. IFD data was
calculated as per the methods outlined in the Australian Rainfall and Runoff manual (AR&R, 1989). As per the
request from BCC the DIS was then generated using the Brisbane IFD data.
Catchment parameters within the XP-RAFTS model were set as follows
Table 18
Model Parameters
Parameter
Value
General
Storage Multiplication Factor
1
Initial Impervious Loss
0 mm
Continuing Impervious Loss
2.5 mm
Initial Pervious Loss
10 mm
Continuing Pervious Loss
2.5 mm
Impervious Area Manning’s (n) Coefficient
0.015
Pervious Area Manning’s (n) Coefficient
0.04
Average Catchment Slope
NF1&2
3.1%
NF3-5
1.6%
NF6&7
3.1%
CBD1-3
1.4%
CBD4
2.6%
MLTU_1
2.1%
MLTU_2
3%
MLTD_1
2.6%
MLTD_2
3.6%
MLTD_3
2.6%
MR1
1.5%
AUCH1
3.21%
AUCH2
3.12%
AUCH3
2.42%
The catchments for all the stormwater system are presented in Appendix A.
7.4.2
Results
The XP RAFTS models were run for the 2, 10 and 50 year Average Return Interval (ARI) events. Table 19
contains a summary of the peak discharges generated for each catchment modelled.
Table 19
Peak Local Discharges
Peak Discharge (m3/s)
Catchment
NF1-7
CBD1-4
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Sub-Catchment
2 ARI
10ARI
50 ARI
NF1&2
17.5
26.6
33.6
NF3-5
13.6
20.6
26
NF6&7
6.9
10.8
13.9
CBD1-3
26.1
38.9
48.5
29
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CBD4
2.1
3.6
4.9
MLTU_1
15.3
23.1
29.2
MLTU_2
9.46
14.5
18.5
MLTD_1
9.7
14.8
18.8
MLTD_2
5.6
8.6
10.8
MLTD_3
4.4
6.1
8.3
MR1
MR1
91.2
127.3
172.2
AUCH1-3
AUCH1
12.3
19.5
26.2
AUCH2
19.8
32.2
45.9
AUCH3
11.4
17.8
23
MLT1-4
7.4.3
Verification
Rational Method calculations were undertaken to compare with the XP-RAFTS peak discharges. The purpose of
this comparison is to ensure the XP-RAFTS peak discharges are within a reasonable range. Rational method
calculations were undertaken using the methods outlined in QUDM for urban catchments and the same IFD data
used for the hydrologic model. Table 20 shows comparisons of the Rational Method calculations to XP-RAFTS
peak discharges.
Table 20
Peak Local Discharges
Catchmen
t
NF1-7
CBD1-4
MLT1-4
MR1
AUCH1-3
SubCatchmen
t
XP RAFTS Peak
Discharge (m3/s)
10AR
2 ARI
50 ARI
I
Rational Method Peak
Discharge (m3/s)
Difference (%)
2 ARI
10ARI
50 ARI
2 ARI
10ARI
50 ARI
NF1&2
17.5
26.6
33.6
12.2
21.1
33.3
22%
9%
7%
NF3-5
13.6
20.6
26
9.5
16.4
25.9
30%
21%
1%
NF6&7
6.9
10.8
13.9
6.7
11.5
16.1
30%
20%
0%
CBD1-3
26.1
38.9
48.5
16.5
28.0
44.0
6%
4%
7%
CBD4
2.1
3.6
4.9
2.6
4.5
7.2
36%
28%
8%
MLTU_1
15.3
23.1
29.2
11.2
19.3
30.5
13%
15%
22%
MLTU_2
9.46
14.5
18.5
6.9
11.9
18.8
26%
16%
4%
MLTD_1
9.7
14.8
18.8
7.1
12.2
19.3
26%
18%
2%
MLTD_2
5.6
8.6
10.8
3.9
6.8
10.6
26%
17%
3%
MLTD_3
4.4
6.1
8.3
3.1
5.3
8.1
29%
21%
2%
MR1
91.2
127.3
172.2
71.0
116.0
184.0
30%
14%
2%
AUCH1
12.3
19.5
26.2
10.6
18.4
29.2
14%
5%
12%
AUCH2
19.8
32.2
45.9
22.1
38.7
61.9
11%
19%
32%
AUCH3
11.4
17.8
23
10.6
18.2
28.8
7%
3%
26%
Table 20 demonstrates that the peak discharges generated by XP-RAFTS form majority of the catchment are
matching with Rational Method discharges. However, for some of the catchment, XP RAFTS calculated peak
discharges are higher or vice versa. This could be because a storage factor of one has been adopted for the
modelling.
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7.5
Hydraulics
7.5.1
Model Set Up
31
The hydraulic model was set up to incorporate the storage areas and trunk stormwater lines up to the lowest pit
for each of the six systems.
The key hydraulic model inputs for these models are the inflows, tail water, topography and roughness. Inflow
boundaries for the hydraulic models were taken from the XP-RAFTS model outputs. The hydraulic models for
each system were run for the 2, 10, and 50 year ARI events. For each system and ARI event, two tailwater
conditions were considered:
1.
Low tailwater elevation equal to the Mean High Water Springs (MHWS) for each system. MHWS for
NF1-7, MLT1-4 & CBD1-4 is 1.06 m AHD. Similarly, MHWS for AUCH1-3 & MR1 is 1.09m AHD
2.
High tailwater elevation equal to the lowest pit elevation in the model. For systems NF1-7, MLT1-4,
CBD1-4 , AUCH1-3 & MR1 the high tailwater elevation was 1.8m, 3.1m, 2.1m, 2.1m & 2.3m respectively.
This tailwater condition was chosen as an extreme tailwater condition where local stormwater will still
discharge from the pipe system.
The topographic data for the hydraulic models was taken from the 2m digital terrain model supplied by BCC. The
key Manning’s ‘n’ roughness values used in the model are as follows:
-
Roads
0.025
-
Residential
0.100
-
Hospital/Commercial
0.150
-
Water
0.025
-
Parkland
0.050
The model extents and trunk stormwater representation of the hydraulic models for all systems are presented in
Appendix A
The hydraulic models were set up for two key scenarios: 1) base case; 2) BPDs installed on the trunk lines.
Table 21 contains details of the BPD installed on each of the trunk lines modelled as part of this study.
Table 21
BPDs for Each Trunk Line
System
Trunk Line
BPD1
(Scenario 1)
BPD2
(Scenario 2)
NF1-7
NF1&2
Flap
Duckbill
NF3-5
Flap
Duckbill
NF6&7
Flap
Duckbill
CBD1-3
Flap ( CBD 1 only)
Duckbill ( CBD 1 only)
CBD4
Flap
Duckbill
MLTU_1
Flap
Duckbill
MLTU_2
Flap
Duckbill
MLTD_1
Flap
Duckbill
MLTD_2
Flap
Duckbill
MLTD_3
Flap
Duckbill
MR1
MR1
Flap
Duckbill
AUCH1-3
AUCH1
Flap
Duckbill
CBD1-4
MLT1-4
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System
Trunk Line
BPD1
(Scenario 1)
BPD2
(Scenario 2)
AUCH2
Flap
Duckbill
AUCH3
Flap
Duckbill
32
Note: It is assumed that CBD 2 & 3 will have penstock valves installed.
Losses for these devices were obtained from suppliers and are presented in Appendix B.
For the duck bill BPDs losses for both submerged and unsubmerged flow were accounted for as appropriate. It
was assumed that river velocity components were taken into account for the submerged scenarios.
For the flap gates losses due to river velocity for the two systems analysed were considered negligible. This is
due to the outlets being perpendicular to the Brisbane River. Therefore velocity vectors due to high river flows
were assumed to be perpendicular to the outlets and therefore create negligible resistance on the opening of the
flaps.
For pipes larger than 2.5m diameter and the box culverts and channel, outlets were modelled with multiple
combinations of smaller diameter valves and losses in each outlet were calculated accordingly.
For all BPD scenarios, flow in the negative direction (back flow) for the pipe systems was disabled to account for
the effects of the BPDs. For the existing scenarios, however, back flow was allowed as to represent the existing
conditions.
7.5.2
Results
The hydraulic models were run for the 2, 10, and 50 year ARI events for both high tailwater and low tailwater
conditions. Table 22 shows a summary of the water level increases due to the duck bill BPD and flap gate BPD
installation options at the outfalls.
Table 22
Change in Water Levels Due to BPDs options
Increase in water level (mm)
System
NF1-2
NF4-7
CBD1-4
MLT1-4
MR1
AUCH1-3
Rainfall
Event
Low Tailwater
BPD1
BPD2
(Scenario 1(Scenario 2Flap)
Duck bill)
High Tailwater
BPD1
BPD2
(Scenario 1 (Scenario 2 –
Flap)
Duck bill)
2 yr
0
0
0
0
10 yr
0
0
0
0
50 yr
125
130
0
80
2 yr
5
10
5
15
10 yr
5
40
5
20
50 yr
240
280
0
80
2 yr
0
0
0
40
10 yr
0
0
0
0
50 yr
0
0
5
15
2 yr
0
5
0
5
10 yr
115
120
115
115
50 yr
60
60
150
165
2 yr
30
30
20
20
10 yr
5
5
0
0
50 yr
5
5
5
5
2 yr
0
5
0
560
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Increase in water level (mm)
System
Rainfall
Event
Low Tailwater
BPD1
BPD2
(Scenario 1(Scenario 2Flap)
Duck bill)
High Tailwater
BPD1
BPD2
(Scenario 1 (Scenario 2 –
Flap)
Duck bill)
10 yr
5
400
5
800
50 yr
70
85
40
550
The results presented in Table 22 demonstrate that the proposed BPDs (flap valves or duck bill) for CBD1-4 and
MR1 will have an acceptable impact on water levels (less than 50mm rise) due to local stormwater runoff.
However, for systems NF1-2, NF4-7, MLT1-4 & AUCH1-3, the backwater caused by installation of either flap
valves or duck bills will have a significant impact on flood level up to 800 mm.
It should be noted that the tailwater level used in the high tailwater scenarios for both sites is considerably higher
than the HAT (1.65 m AHD for NF8-NF11 and 1.64 m AHD for MLT5-MLT8). This therefore represents a
combined river flood or storm surge and local flood condition. In this condition the outlet will be submerged. It
should also be noted that for submerged flow many assumptions regarding losses were made such as the
accuracy of supplier data and the assumptions stated in Section 7.5.1 regarding river velocities.
7.6
Modelling Conclusions
The following conclusions can be drawn from the hydraulic modelling:
-
It is feasible to install either flap valves or duck bills to systems CBD1-4 & MR1 with minimal increase in
peak water levels (less than 50mm rise) during a local storm event.
-
For systems NF1-2, NF4-7, MLT1-4 & AUCH1-3, installation of either flap valves or duck bills has a
significant impact on peak water levels during a local storm event.
-
The higher tailwater elevation generally results in a greater increase in water levels than the lower tailwater
elevation.
8.0
Cost Estimates
Cost Estimates are presented in Appendix D.
9.0
Concept Layouts
Concept Layouts are presented in Appendix E.
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Conclusions
The conclusions of this Concept Report are as follows:
-
Hydrologic and hydraulic modelling of the systems has shown that:

With penstocks on CBD 2 & 3, it is feasible to install either flap valves or duck bills to CBD1 & 4 with
minimal increase in peak water levels (less than 50mm rise) during a local storm event.

It is feasible to install either flap valves or duck bills to MR1 with minimal increase in peak water levels
(less than 50mm rise) during a local storm event

For systems NF1-2, NF4-7, MLT1-4 & AUCH1-3, installation of either flap valves or duck bills has a
significant impact on peak water levels during a local storm event.

The higher tailwater elevation scenario generally results in a greater increase in water levels than the
lower tailwater elevation scenario.
-
Locations have been proposed for installation of appropriate BPDs for each system and concept layouts
developed which are presented in Appendix E.
-
In general, flap gates installed on the outlet are the preference, due to low construction cost, ease of
maintenance, and reliable operation in Council experience. Chambers are chosen when access to the outlet
for construction or maintenance is difficult. Inline valves are chosen in order to avoid construction work at
the outlet. Duck bills are chosen where siltation is an issue. Penstocks are chosen where zero head loss is
necessary, or for very large outlets (e.g. over 2400mm), but the requirements for increased maintenance,
activation and de-activation must be considered.
-
The exact alignments and pipe sizes, as well as location of other services will need to be confirmed at
detailed design stage.
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11.0
BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
Case Study Areas - Concept Report
List of Abbreviations
Abbreviation
Meaning
ARI
Average Recurrence Interval
AUCH1-3
Auchenflower Drainage Outlets 1-3
BCC
Brisbane City Council
BPD
Backflow Prevention Device
CBD
(Brisbane) Central Business District
CBD1-4
Central Business District Drainage Outlets 1-4
DBYD
Dial Before You Dig
DEEDI
Department of Employment, Economic Development
and Innovation
DERM
Department of Environmental Resource Management
FRRB
Independent Flood Response Review Board
HAT
Highest Astronomical Tide
IDAS
Integrated Development Assessment System
m
metres
m, AHD
metres, Australian Height Datum
MHWS
Mean High Water Springs
MILT1-8
Milton Drainage Outlets 1-8
MR1
Milton Rosalie Trapezoidal Open Channel Outlet
MSL
Mean Sea Level
MWA
Max Winders Associates
NALL
Natural Assets Local Law
NF1-11
New Farm Drainage Outlets 1-11
PTW
Prescribed Tidal Works
Q100
Interchangeable with 100 year ARI flood
RCBC
Reinforced Concrete Box Culvert
RCP
Reinforced Concrete Pipe
WWBW
Waterway Barrier Works
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Appendix A
BPD Review Spreadsheet
and Pictures
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Appendix A
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BCC BPD Stage 2
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BPD Review Spreadsheet and Pictures
A-1
AECOM
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Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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Appendix B
Modelling Maps
-
Catchment Maps
-
Model Extents
-
Levee Height Storage Extents
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Appendix B
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Modelling Maps
B-1
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Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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B-2
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Case Study Areas - Concept Report
Appendix C
Head Loss Data
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Appendix C
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Head Loss Data
C-1
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BCC BPD Stage 2
Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in Case Study Areas - Concept Report
KEY
fl – RCP with flap valve installed;
db - RCP with duckbill valve installed;
dbsub - RCP with duckbill valve installed under submerged conditions;
nl – RCP flow with no losses due to valves applied;
sa – 50% of RCP flows for sensitivity analysis;
ws – RCP with wastop valve installed
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E
AECOM
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F
AECOM
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G
AECOM
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H
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I
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J
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K
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L
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M
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N
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O
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P
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Appendix D
Cost Estimates
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Appendix D
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Cost Estimates
D-1
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Stormwater Backflow Devices Feasibility Study Stage 2 - Remaining Systems in
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D-2
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Appendix E
Concept Layouts
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Appendix E
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Concept Layouts
E-1
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Appendix F
Sample Maintenance
Plans
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Appendix F
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Sample Maintenance Plans
MAINTENANCE MANUAL
ALL-RUBBER DUCK BILL CHECK
VALVE
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F-1
INDEX
MAINTENANCE
Line pressure should flush the valve clean of debris in most cases. Periodic inspections for trapped debris should
be conducted.
In vacation seashore areas, quart size plastic bottles have a tendency to float on top and not flush through except
during a major storm.
A feathered 1” x 4”, 1-1/2” x 12”, or suitable plank inserted into the bill of the valve and turned 90 degrees is a
simple method of clearing the Check Valve of small debris which may be trapped between the lips.
CAUTION: Sharp objects should not be used on the Check Valve as there is a chance of cutting the rubber and
damaging the protective fabric covering.
Any gouges in the cover wrap that occur should be sealed to safeguard against ozone or chemical attack. This is
best done with rubber cement or a good brand of silicone or polyurethane rubber sealer made by the major
manufacturers.
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F-2
OPERATION AND
MAINTENANCE MANUAL
MARINE GRADE ALUMINIUM
FLAP GATE
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F-3
OPERATION AND MAINTENANCE INSTRUCTIONS FOR FLAP GATES
Prior to Operation
1.
The flap gate should be brushed off to clear all dirt, grit, grout etc particularly around the seal faces. A
high pressure wash down with clean water is also recommended.
2.
Ensure the flap gate is clear of any obstructions that could impede in its range of motion or prevent the
lid from seating and sealing.
3.
Ensure the flap gate seats evenly and tightly around the perimeter of the seal and there are no gaps.
General Operation
1.
The primary function of the flap gate is to prevent backflow of water and upstream flooding.
2.
The flap gate will open automatically when the upstream water level is higher than the downstream water
level. The degree of opening will be dependent on the difference in these water levels and the discharge
flow rate.
3.
The flap gate will close automatically when the downstream water level is higher than the upstream
water level.
4.
The flap gate will also automatically close with no water upstream or downstream.
5.
To minimise and dampen slamming of the lid due to wave impact a lid mounted bucket has been
incorporated into the design. The rising tide will fill the bucket via the four fill/drain holes and provide
temporary additional weight to the lid. As the tide falls the bucket will empty automatically via the four
fill/drain holes.
General Maintenance
1.
The front mounted lid bucket should be cleaned out thoroughly every 3 months ensuring the bottom fill/drain
holes are not obstructed.
2.
Hinge pins and bushes should be inspected and cleaned at 6 monthly intervals.
3.
It is recommended that the flap gate be washed down and cleaned from any sediment and debris build up at
12 monthly intervals. This opportunity should also be used to inspect seals and other components for
wear and damage and check bolt tension.
4.
Upon each inspection it is important to ensure that the flap gate will open and close fully and seat correctly
and tightly onto the seal. Any build up of sediment in front of the flap gate restricting it from opening
should be removed.
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