working paper potential schemes (volume 1 report)

Job No: CE344

File:/docs/O’Briens Tributary.doc

Date: April 2015

Rev No: 4.0

Author: BWL

Principal: BWL

WEDDIN SHIRE COUNCIL

O’BRIEN TRIBUTARY DRAINAGE IMPROVEMENTS

WORKING PAPER POTENTIAL SCHEMES

(VOLUME 1 – REPORT)

APRIL 2015

Prepared by: Lyall & Associates Consulting Water Engineers Level 1, 26 Ridge Street North Sydney NSW 2060 Tel: (02) 9929 4466 Fax: (02) 9929 4458 Email: [email protected]

Weddin Shire Council O’Brien Tributary Drainage Improvements

O’Brien Tributary.doc i Lyall & Associates

April, 2015 Rev. 4.0 Consulting Water Engineers

TABLE OF CONTENTS Page No

S1 KEY FINDINGS OF THE WORKING PAPER ....................................................................... S1

S1.1 Scope of Investigation ............................................................................................... S1 S1.2 Key Findings ............................................................................................................. S1

1 INTRODUCTION ...................................................................................................................... 1

1.1 Background ................................................................................................................. 1 1.2 Drainage Upgrade Options Considered in the Working Paper ................................... 2 1.3 Scope of Working Paper ............................................................................................. 3 1.4 The O’Brien Tributary Catchment and Drainage System ........................................... 3 1.5 Results of Flood Modelling – Present Day Conditions ................................................ 4

1.6 Impacts of Flooding on Urban Development in the Warraderry Street Area .............. 5 1.7 History of Flooding at the Oliver Court Development ................................................. 5

2 DATA COLLECTION AND FORMULATION OF POTENTIAL SCHEMES ............................ 7

2.1 Site Surveys ................................................................................................................ 7 2.2 Survey of Potential Retarding Basin ........................................................................... 7 2.3 Alternative Schemes for Drainage Improvements ...................................................... 7

2.4 Summary of Initial Screening of Schemes ................................................................ 11

3 HYDROLOGIC AND HYDRAULIC MODELLING ................................................................. 13

3.1 Hydrologic Modelling Approach ............................................................................... 13

3.2 Present Day Conditions ............................................................................................ 13

3.3 Retarding Basin at Brickfield Road ........................................................................... 13 3.3.1 Basin Objectives .......................................................................................... 13

3.4 Results of Hydrologic Modelling of the Retarding Basin ........................................... 13

3.5 Hydraulic Modelling of Post - Retarding Basin Conditions (Scheme 4) .................... 14 3.6 Reduction in Flooding in Warraderry Street Area - Post-Retarding Basin ................ 15

4 DESIGN CONCEPT FOR RETARDING BASIN (SCHEME 4) ............................................. 16

4.1 Layout of Basin and Indicative Cost .......................................................................... 16 4.1.1 Layout .......................................................................................................... 16

4.1.2 Indicative Cost of Basin ............................................................................... 16

5 CATERING FOR RESIDUAL FLOWS IN OLIVER COURT AND WARRADERRY STREET17

5.1 Scheme 5 – Retarding Basin Plus Swale and Pipeline in Warraderry Street ........... 17 5.2 Scheme 5A – Retarding Basin Plus Clearing Obstructions to Flow in 30 Warraderry

Street and Pipeline in Warraderry Street ............................................................................... 18 5.3 Scheme 6 – Retarding Basin + Pipe Scheme 3B (Lot 41 Through 30 Warraderry

Street to Camp Street) ........................................................................................................... 18 5.4 Scheme 7 – Retarding Basin + Pipeline from Lot 41 Through Oliver Court to Camp

Street ……………………………………………………………………………………………….20

6 ECONOMIC ANALYSIS OF SCHEMES ............................................................................... 21

6.1 Estimated Flood Damages ........................................................................................ 21 6.2 Present Worth Value of Scheme Benefits ................................................................ 21

6.3 Indicative Cost-Benefit Analysis ................................................................................ 22

Cont'd Over


O’Brien Tributary.doc ii Lyall & Associates


7 SUMMARY AND FINDINGS ................................................................................................. 24

7.1 Summary ................................................................................................................... 24 7.2 Findings ..................................................................................................................... 24

7.2.1 Piped Schemes 1 to 3B ............................................................................... 24 7.2.2 Retarding Basin in Isolation (Scheme 4) ..................................................... 25 7.2.3 Scheme 5 - Retarding Basin + Drainage Swale in 30 Warraderry Street +

Pipeline to Camp Street ............................................................................................ 25 7.2.4 Scheme 5A - Retarding Basin + Easement in 30 Warraderry Street + Pipeline

in Warraderry Street to Camp Street ........................................................................ 25 7.2.5 Scheme 6 - Retarding Basin + Pipeline Through 30 Warraderry Street to

Camp Street .............................................................................................................. 25 7.2.6 Scheme 7 - Retarding Basin + Pipeline Through Oliver Court to Camp Street

26

7.3 Recommendations for Consideration by Floodplain Management Committee ........ 26 7.4 Proceeding to Detailed Design ................................................................................. 26

ANNEXURES (BOUND IN VOLUME 1)

A. PHOTOGRAPHS OF THE STUDY AREA B. INDICATIVE COSTS OF SCHEMES


O’Brien Tributary.doc iii Lyall & Associates


LIST OF FIGURES (BOUND IN VOLUME 2)

1.1 Location Plan and Services in Warraderry Street

1.2 Pipeline Through Oliver Court (Scheme 1)

1.3 November 2010 Flooding in Oliver Court

1.4 Flooding at Oliver Court and Warraderry Street Present Day Conditions – 100 Year ARI

2.1 Site Survey in the Warraderry Street Area

2.2 Site Survey in the Brickfield Road Area

2.3 Potential Drainage Improvement Schemes (Sheets 1 to 3)

2.4 Longitudinal Section Scheme 1

2.5 Longitudinal Section Scheme 3B

2.6 Longitudinal Section Pipeline in Warraderry Street (Part of Scheme 5)

2.7 Longitudinal Section Pipeline Lot 41 via Oliver Court to Camp Street (Part of Scheme 7)

3.1 DRAINS Model Sub-Catchment Layout

3.2 Discharge Hydrographs in the Drainage System

3.3 Pre- and Post-Basin Discharge Hydrographs

3.4 Flooding at Oliver Court and Warraderry Street. Scheme 4 (Post-Basin) Conditions

– 100 year ARI

4.1 Plan of Potential Flood Retarding Basin Brickfield Road

4.2 Cross Section of Flood Retarding Basin Brickfield Road

5.1 Flooding at Oliver Court and Warraderry Street – Scheme 5 (Including Retarding Basin)

– 100 year ARI


– 100 year ARI


– 100 year ARI


O’Brien Tributary.doc S1 Lyall & Associates


S1 KEY FINDINGS OF THE WORKING PAPER

S1.1 Scope of Investigation

1. This Working Paper reviewed options for mitigating flooding in residential properties in Oliver

Court and Warraderry Street due to overland flows generated by storms on the O’Brien

Tributary catchment. The area is located in a “floodway”, according to the hydraulic

categorisation carried out in the Emu Creek Floodplain Risk Management Study. Figure 1.4

shows the results of hydraulic modelling and Table 1.1 shows the distribution of peak flows

under present day conditions for flood events ranging between 5 and 100 year Average

Recurrence Interval (ARI). At the 100 year ARI, two residential properties on the western side

of Warraderry Street, as well as all of the individual units in Oliver Court (No. 26 –

28 Warraderry Street) and the next door residence in 30 Warraderry Street would be flooded

to above–floor level. It is likely that residential development on the O’Brien Tributary overland

flow path downstream of Warraderry Street in the East Street area would also be flooded,

although without the benefit of detailed site survey it is not possible to provide quantitative

information on the number of properties affected.

2. The options considered comprised various piped schemes in the area between Lot 41 in

O’Brien Street and the intersection of Warraderry Street and Camp Street (ref. Figure 2.3,

Sheets 1 to 5). The potential reduction in downstream flows and depths of flooding achieved

by the construction of a retarding basin on the eastern side of Brickfield Road was also

assessed. Hydraulic modelling was undertaken using the TUFLOW two-dimensional

modelling software of present day and post-scheme conditions. Indicative cost estimates

were prepared for several piped options, as well as for the retarding basin (ref. Annexure B).

3. The economic benefits of the feasible schemes were computed. These benefits are

represented by the reduction in flood damages to the residential developments in Warraderry

Street, compared with present day conditions. Flood damages were assessed using

recommended Office of Environment and Heritage (OEH) procedures and converted to

present worth values using discount rates nominated in NSW Government Guidelines for the

economic analysis of infrastructure projects. Indicative cost-benefit analysis was then carried.

These results are shown in Tables 6.1 to 6.5.

S1.2 Key Findings

4. Peak flows generated by the catchment during major storms are too large to be conveyed by

pipelines through Oliver Court and along Warraderry Street. The feasible maximum pipe size

is limited to between 900 mm and 1200 mm, and is controlled by the available gradient and

minimum cover considerations, as well as by the presence of sewers in Warraderry Street.

Consequently, upgrading the drainage system by pipelines in isolation is not effective in

solving existing flooding problems in residential developments in the Warraderry Street area

and is not recommended. The reduction in downstream peak flows by the construction of a

retarding basin in the catchment headwaters is required for pipelines to be effective in

mitigating overland flows at Warraderry Street.

5. A large reduction in peak flows arriving at the eastern boundary of Oliver Court could be

achieved by construction of the basin at Brickfield Road (Scheme 4). The basin, in

conjunction with a drainage swale in the existing southern easement of Oliver Court would

convey flows up to 5 year ARI across Oliver Court, with a small residual flow over the




driveway, extending into 30 Warraderry Street. Figure 3.4 shows the results of hydraulic

modelling Scheme 4 and Table 3.1 shows the distribution of peak flows under post-basin

conditions for the range of flood events. (Table S2 over page summarises the reduction in

flows achieved by the various schemes for the 100 year ARI flood). Construction of the basin

would eliminate above-floor inundation in properties on the western side of Warraderry Street

under 100 year ARI conditions, although the properties would continue to experience shallow

overland flow at that frequency. One of the units in Oliver Court may be subject to shallow

above-floor flooding of up to 80 mm, which could be mitigated by some minor flood proofing

works at the entrance to the property. The indicative cost of the retarding basin is $716,000,

including an allowance of $320,000 for the purchase of land in the proposed storage area.

The retarding basin is a necessary component of the drainage improvements scheme for

O’Brien Tributary.

6. Although the retarding basin will achieve a major reduction in downstream flows as far as

Camp Street, in the event of a 100 year ARI flood supplementary measures would be

required to capture “residual” (i.e. post-basin) overland flows continuing to traverse Oliver

Court, 30 Warraderry Street and residential properties in Warraderry Street.

Capture of the residual overland flow in a drainage swale in 30 Warraderry Street, together

with piping northwards along Warraderry Street to the intersection with Camp Street, would

reduce overland flow in the street. This scheme is denoted Scheme 5 and would require the

voluntary acquisition of the 30 Warraderry Street property by Council. Figure 5.1 shows the

results of hydraulic modelling this scheme and Table 5.1 shows the distribution of peak flows.

The indicative cost of the Scheme 5 swale and downstream pipeline is $784,000. Together

with the basin, the total cost would be $1,500,000. Scheme 5 would eliminate above-floor

inundation in properties on the western side of Warraderry Street under 100 year ARI

conditions, although several of the properties would continue to experience shallow overland

flow at that frequency. No units in Oliver Court would be subject to above-floor flooding1.

7. Other schemes have also been considered for catering for residual flows in the event that the

voluntary purchase required for Scheme 5 does not proceed. They are denoted

Schemes 5A, 6 and 7 and are summarised in Tables S1 and S2.

8. Scheme 5A would replace Scheme 5’s swale in 30 Warraderry Street by an easement for

drainage along the northern side of the property which, together with the existing easement in

Oliver Court, would be cleared of obstructions to flow (including removal of the existing

diversion bank in 30 Warraderry Street – ref. Figure 2.1) to allow residual flows to be

conveyed overland to Warraderry Street and then piped to Camp Street. It is estimated to

cost $1,246,000, including the cost of the retarding basin. Scheme 5A has not been

hydraulically modelled, but patterns of flow would be similar to those shown on Figure 5.1 for

Scheme 5.

9. Scheme 6 involves construction of a 900 mm diameter pipeline from Lot 41 in O’Brien Street

through 30 Warraderry Street to Camp Street, following the same route as Scheme 3B

1 The voluntary purchase and removal of flood prone residential property from floodway areas (as required for

Scheme 5), is supported by NSW Government flood policy, as set out in the Floodplain Development Manual.

Scheme 5 was incorporated in the scope of work for this report by the Technical Committee for the project.

However, Council has decided at this time not to proceed with negotiations for voluntary purchase of the property

in 30 Warraderry Street. This has led to the consideration of alternative Schemes 5A, 6 and 7.




(Figure 2.3, Sheet 4). It would reduce the portion of residual flow which would be conveyed

overland across properties downstream of Lot 41. Results of hydraulic modelling of

Scheme 6 are shown on Figure 5.2 and Table 5.2 shows the distribution of peak flows. It is

estimated to cost $1,619,000, including the cost of the retarding basin.

10. Scheme 7 is the most expensive of the schemes for catering for residual flows and involves

duplicate pipelines from Lot 41 through Oliver Court to Camp Street (Figure 2.5, Sheet 5).

Results of hydraulic modelling of Scheme 7 are shown on Figure 5.3 and Table 5.3 shows

the distribution of peak flows. It is estimated to cost $2,160,000, including the cost of the

retarding basin. Scheme 7 requires duplicate pipelines through the Oliver Court property.

Potholing to locate services and building foundations is required to confirm there is sufficient

room for laying the pipelines.

S1.3 Recommendations for Consideration by the Floodplain Management Committee

11. The retarding basin (Scheme 4) is an essential component of the drainage improvements for

O’Brien Tributary and will achieve a substantial reduction in downstream flooding, particularly

for the minor (5 year ARI) and medium (20 year ARI) floods. Proceeding to the geotechnical

investigation and design phases for the basin is recommended at the first stage of the

Drainage Improvements Project.

12. The decision whether or not to adopt the drainage swale through 30 Warraderry Street and

pipeline in Warraderry Street for controlling residual flows (i.e. Scheme 5), would depend

upon negotiations with the owner of 30 Warraderry Street, as purchase and clearing of this

property is required. In the event that voluntary purchase does not proceed, Scheme 5A

could be considered, but requires an easement for conveying overland flow through the

30 Warraderry Street property as well as the clearing of obstructions to flow in that property

and the existing easement in Oliver Court.

13. Alternatively, Scheme 6 which comprises the retarding basin plus the single 900 mm

diameter pipeline associated with Scheme 3B could be considered, but at greater cost than

Scheme 5A. This scheme reduces overland flows through properties downstream of Lot 41

O’Brien Street as far as Camp Street but not by the same amount as Scheme 7 below.

14. Scheme 7 (which includes the duplicate pipeline from Lot 41, through Oliver Court to Camp

Street) is the most expensive scheme, but has the highest hydraulic capacity and is the best

option for mitigating overland flooding from Brickfield Road to Camp Street. The technical

feasibility of constructing dual pipelines through the restricted space available in Oliver Court

would need to be confirmed by accurate location of the building foundations and services

prior to commencing detailed design.




TABLE S1

SUMMARY OF POTENTIAL SCHEMES

Scheme

Elements of Scheme Indicative Cost Comment Report Reference

4

Retarding Basin at Brickfield Road. $716,000

The basin is an essential component of the drainage upgrade. It will reduce the incidence of

above-floor flooding under 100 year ARI conditions from: all units in Oliver Court, 2 residences

in Warraderry Street west side and 30 Warraderry Street (present day); to: shallow above-floor

flooding in one unit in Oliver Court (post-basin).

Chapters 3 and 4.

Figure 3.4 (hydraulic modelling).

Figures 4.1, 4.2 (layout drawings).

Table 6.1 (economic analysis).

Table 3.1 (peak flows)

5

Retarding Basin + Drainage Swale in

30 Warraderry Street + Pipeline in Warraderry

Street, extending to Camp Street.

$1,500,000

The swale and pipeline is the best technical method of controlling post-basin “residual flows”.

With this scheme in place no properties will be subject to above-floor inundation at the

100 year ARI level. However, voluntary purchase of 30 Warraderry Street is required.

Section 5.1.


Figure 2.3, Sheet 3 and Figure 2.6 (pipeline layout).


Table 5.1 (peak flows).

5A

Retarding Basin + Drainage Easement for

conveying residual overland flow across

30 Warraderry Street; clearing the easement

and existing easement in Oliver Court of

obstructions to flow + Pipeline in Warraderry

Street, extending to Camp Street.

$1,246,000

This easement and pipeline scheme is suitable for controlling post-basin “residual flows” in the

event that Scheme 5 does not proceed. With this scheme in place no properties will be subject

to above-floor inundation at the 100 year ARI level. However, there will be a nuisance overland

flow through the properties at the 100 year ARI.

Section 5.2.

Scheme not modelled hydraulically, but would perform

similarly to Scheme 5.

Pipeline in Warraderry Street as for Scheme 5.


6

Retarding Basin + Pipeline from Lot 41 O’Brien

Street through 30 Warraderry Street, continuing

to Camp Street + clearing easement on

southern boundary of Oliver Court to assist with

the conveyance of residual overland flows.

$1,619,000

This scheme is more expensive than Schemes 5 or 5A, but will reduce overland flows in

properties between Lot 41 and Warraderry Street compared with those schemes. However,

there will still be nuisance overland flow through the properties at the 100 year ARI, as it is not

practicable to pipe all of the residual flow under post-basin conditions.

Section 5.3.


Pipeline as for Scheme 3B (Figure 2.3, Sheet 4).



7

Retarding Basin + Pipeline from Lot 41 O’Brien

Street through Oliver Court, continuing to

Camp Street + clearing easement on southern

boundary of Oliver Court to assist with the

conveyance of residual overland flows.

$2,160,000

This scheme is the most expensive of the schemes considered. The duplicate pipeline would

convey post-basin flows up to 20 year ARI across through Oliver Court. Residual overland

flows between Lot 41 and Camp Street are less than for the other schemes. However, the

technical feasibility of constructing duplicate pipelines in the driveway of Oliver would need to

be confirmed by accurate location of building foundations and services prior to commencing

detailed design.

Section 5.4.

Pipeline extends from Lot 41 O’Brien Street to Camp Street

through Oliver Court

Figure 2.3, Sheet 5 and Figure 2.7 (pipeline layout).






TABLE S2

PEAK FLOWS OLIVER COURT AND WARRADERRY STREET AREA

100 YEAR ARI

(m3/s)

ID(1) Location Present Day

Conditions(1) Scheme 4(1) Scheme 5(1) Scheme 6(1) Scheme 7(1)

A Flow Through Lot 41

O’Brien St. 8.9 3.4 3.4 3.4 3.4

B

Southern Easement

26-28 and 30

Warraderry St.

4.5 1.7 3.4 2.0 0.7

C Driveway 26-28

Warraderry St. 2.0 1.0 0 0.0 0.4

D Northern Easement

26-28 Warrderry St. 1.8 0.5 0 0.0 0.2

E

Surcharge through 27,

29, 31, 33 and 35

Warraderry St.

8.2 3.9 3.2 2.9 2.3

F

Surcharge through Lot

2 and west along

Camp St.

0.4 0.2 0.8 1.1 2.0

G

(Piped

Flow)

Flow in existing 900

RCP in Camp St. 1.9 1.8 2.0 1.9 1.9

H

(Piped

Flow)

Flow in new pipe in

Warraderry St - - 1.2 1.2 2.0

Notes:

(1) Refer Figures 1.4, 3.4 and 5.1 to 5.3 for Locations and results of hydraulic modelling.


O’Brien Tributary.doc 1 Lyall & Associates


1 INTRODUCTION

1.1 Background

Weddin Shire Council issued the Brief for the design of improvements to the stormwater drainage

system of the O’Brien Tributary at the Oliver Court residential unit development (No. 26 –

28 Warraderry Street), located on the eastern side of that street about 100 m south of the intersection

with Camp Street (ref. Figure 1.1). The works which were the subject of the Brief comprised

Measure 5 of the Emu Creek Floodplain Risk Management Study (FRMS) and involved the collection

and piping of flows across the unit development, with provision for safely conveying residual flood

flows which exceeded the capacity of the upgraded system as overland flow across the property.

The scheme was intended to solve local drainage problems in the development and initially terminate

at Council’s drainage system at the front of the property in Warraderry Street. It comprised the first

stage of the upgrade of the O’Brien Tributary drainage system. A future stage would extend

northwards along Warraderry Street to the intersection with Camp Street. Ultimately, it was intended

that the upgrade would extend downstream of Camp Street to the junction of O’Brien Tributary with

Emu Creek.

The proposal developed in the FRMS and incorporated in Council’s Brief (refer Figure 1.2, taken from

the FRMS report) comprised the following elements and is denoted Scheme 1 in this Working Paper:

1. An earth mound and drainage swale to capture overland flow and direct it to a large grated

intake pit about 2.5 m deep in private property on the eastern boundary of Oliver Court.

2. A pipeline (preliminary sizing 1200 mm diameter) about 100 m total length traversing the

Oliver Court allotment and connecting to the existing piped drainage system in Warraderry

Street. For initial planning, the proposed route followed the driveways of Oliver Court, as there

appeared to be insufficient space available to locate the pipeline in either of the narrow 4 and

3 m wide drainage easements on the northern and southern boundaries of the property

respectively. These easements are denoted “de 4w” and “de 3w” on Figure 1.2.

3. A large grated pit in Warraderry Street at the front of the property, with a short section of line

connecting the new pipeline to Council’s existing stormwater system. Because of the

restricted capacity of the existing stormwater line in Warraderry Street, which is only 525 mm

diameter, the pit would initially function as a “surcharge pit” during periods of heavy rainfall

pending completion of the next stage to Camp Street.

Flows in the new pipeline which exceed the capacity of the existing 525 mm pipeline in Warraderry

Street would travel northwards along Warraderry Street as overland flow and pond in the low lying

area between that street and Camp Street, as occurs at present. The ponding will be evacuated over

time via the existing grated pit on the eastern gutter of Warraderry Street at the intersection. During

significant storms, the overland flow along Warraderry Street is likely to surcharge the western

boundary of the street, with the flows being conveyed overland through residential allotments and the

service station at the north-west corner of Warraderry and Camp Street (Lot 2). Figure 1.1 shows the

existing drainage system and the directions of overland flow approaching Oliver Court and in the

Warraderry Street area.

The overland flow which surcharges the drainage system in Warraderry Street will continue to East

Street and across Camp Street, before turning westwards and flowing through the commercial part of

town along Melyra Street to Emu Creek. Council’s piped stormwater system from East Street to Emu




Creek is of uncertain size and therefore it is not possible to assess its capacity. It is known, however,

that the system surcharged during the April 1990 storm resulting in severe flooding in the commercial

area of Grenfell.

The intended hydraulic capacity of the proposed pipeline through Oliver Court in the FRMS was equal

to the 20 year Average Recurrence Interval (ARI), leaving a “gap flow” for the 100 year ARI, to be

conveyed as overland flow through the allotment. As the driveway would need to be reinstated after

the pipeline is laid, it was intended that the design could incorporate some adjustments to its profile to

assist with the conveyance of overland flows which surcharged the capacity of the new pipeline.

A modification to Scheme 1 involved locating the route of the pipeline through Oliver Court as above,

in conjunction with a drainage swale in the southern easement to carry flows exceeding the pipe

capacity. However, the hydraulic capacity of the swale would be limited by the narrow (3 m) width of

the easement. This scheme is denoted Scheme 1A in this Working Paper. Figure 2.3, Sheet 1 shows

the works comprising Schemes 1 and 1A. The works comprising Scheme 1 are shown in a solid

yellow colour and the works comprising Scheme 1A are shown in orange.

1.2 Drainage Upgrade Options Considered in the Working Paper

This Working Paper has been prepared following the Inception Meeting of the Floodplain

Management Committee (FMC), where objections to Scheme 1 by one of the landowners on the

eastern boundary to Oliver Court were discussed. In correspondence to Council, the landowner had

objected to the possible location of the large intake pit required for this scheme either within or near to

his property.

In particular, the landowner was concerned about potential blockage of the pit due to the debris load

associated with flood events and the resulting water logging of his property. The owner identified the

occurrence of three recent flood events which had resulted in water logging in his property due to

overland flows from the O’Brien Tributary and considered that this would be exacerbated following the

construction of the proposed works (i.e. either Scheme 1 or Scheme 1A).

Accordingly, it was decided at the meeting to extend the scope of the investigation to cover the reach

of the O’Brien Tributary from Brickfield Road upstream of Oliver Court, to the intersection of

Warraderry Street and Camp Street. The intention was to investigate the feasibility of schemes which

eliminated the need for a large intake pit on the eastern side of Oliver Court. These schemes could

involve the capture of overland flows and their conveyance to Warraderry Street via a pipeline through

allotments to the south (i.e. upstream) of Oliver Court. The construction of a retarding basin on the

headwaters of O’Brien Tributary which would result in a reduction in downstream peak flows and

hence the size of pipeline required was also to be investigated.

Bringing forward the upgrading of the stormwater system in Warraderry Street by continuing the new

line to the Camp – Warraderry Street intersection, rather than terminating the works at the front of

Oliver Court, was also added to the scope of the investigation.




1.3 Scope of Working Paper

The Working Paper has been prepared to provide information necessary for the FMC to make a

decision on the scheme to be adopted for detailed design and documentation. It covers the following

items:

1. The results of initial data collection involving site inspections and field survey and appraisal of

the feasibility of Schemes 1 and 1A.

2. Development of alternative piped schemes to Schemes 1 and 1A, extending both upstream

of the proposed intake pit at the eastern boundary of Oliver Court (shown on Figure 2.3,

Sheet 1) and downstream of the proposed surcharge pit in Warraderry Street.

3. Review of the feasibility of providing a retarding basin on the O’Brien Tributary upstream of

Oliver Court to reduce downstream flows and hence reduce the size of drainage works

required to control “residual” overland flows.

4. Screening of the potential schemes (advantages and disadvantages) and selection of feasible

schemes for more detailed evaluation.

5. Hydraulic modelling of feasible schemes, together with the preparation of design concepts,

costs and benefits (in terms of reduction in flooding in residential properties) for consideration

by the FMC.

1.4 The O’Brien Tributary Catchment and Drainage System

The O’Brien Tributary catchment rises in the rural area to the east of Brickfield Road. Runoff from the

catchment flows westwards as overland flow across rural land, before turning northwards to flow

along the eastern boundary of the residential properties in Warraderry Street. (ref. Figure 1.1 and

also Figure 3.1 of Chapter 3 which deals with flood hydrology. These diagrams show the layout of

the various sub-catchments contributing to flows in the O’Brien Tributary.)

Flows are conveyed as shallow overland flow to the eastern boundary of Oliver Court where the

catchment contributing to flows comprises sub-areas A to F of Figure 3.1, amounting to a total area of

80.8 ha. They then flow through the allotment as surface flow to Warraderry Street along the internal

driveway and the drainage easements on the northern and southern property boundaries. The piped

drainage system only caters for runoff generated by the allotment and is therefore of very limited

capacity. Consequently, overland flows through the property are frequent occurrences during periods

of heavy rainfall. Hydraulic modelling undertaken during this investigation (see Figure 1.4 and

discussion below) shows that a portion of the overland flow arriving at the eastern boundary of the

properties in Warraderry Street would also discharge to that street as overland flow through the

property on the southern side of Oliver Court (No 30 Warraderry Street).

Photographs showing the most recent occurrence of overland flows through the property on

29 November 2010 are attached as Figure 1.3. From the hydrologic analysis of this storm, reported in

Appendix D of the FRMS report, the intensity of rainfall varied across the Grenfell creek system.

Peak flows were greater than 20 year ARI on Company Dam Overflow to the west of O’Brien

Tributary, about 20 year ARI on Star Gully and the upper reaches of Emu Creek to the north and of

lesser magnitude on O’Brien Tributary. The best estimate of the severity of flows evident in the

photographs was that they approximated a 5 year ARI. Considerable overland flow was reported

through Oliver Court and along Warraderry Street for this (comparatively minor) flood event. Under




100 year ARI conditions, the peak flow would be expected to be three times the 5 year ARI flow and

would result in above-floor inundation in residential properties in Oliver Court, 30 Warraderry Street

and on the western side of Warraderry Street.

1.5 Results of Flood Modelling – Present Day Conditions

Figure 1.4 shows the indicative extents and depths of inundation resulting from the design 5 year and

100 year ARI floods under present day conditions. These diagrams were derived from TUFLOW two-

dimensional hydraulic modelling, which used the site survey undertaken for this present investigation

to describe natural surface levels and features controlling the pattern of flooding (see Section 2.1 and

Figure 2.1 for details). The survey extended to the front boundary of the residential properties on the

western side of Warraderry Street and therefore provided data on the magnitude of flows entering

these properties. Hydrologic modelling of the catchment based on the RAFTS – DRAINS rainfall -

runoff catchment modelling system assessed discharge hydrographs approaching Oliver Court and

travelling north along Warraderry Street, which were applied to the upstream boundaries of the

TUFLOW model (ref. Chapter 3).

The TUFLOW results show the colour coded depths of inundation over the modelled area, as well as

the directions of flow, which are indicated by “velocity vector” arrows. The peak flows along the

various flow paths at locations A to F of Figure 1.4 are presented in Table 1.1. The model predicts

overland flows being conveyed through Oliver Court via three flow paths: the southern and northern

easements (B and D) and along the internal driveway (C). Flow path B also includes flow in

30 Warraderry Street. The TUFLOW model did not include the various fences and other obstructions

to flow in the easements in Oliver Court and may therefore slightly overestimate the magnitude of

flows in the easements and underestimate the flow in the driveway.

TABLE 1.1

PEAK FLOWS OLIVER COURT

AND WARRADERRY STREET AREA

PRESENT DAY CONDITIONS

(m3/s)

ID (Figure 1.4)

Location

5 year ARI

20 year ARI

100 year ARI

A Flow Through Lot

41 O’Brien St. 2.6 5.1 8.9

B

Southern Easement

26-28 and 30

Warraderry St.

1.3 2.6 4.5

C Driveway 26-28

Warraderry St. 0.7 1.3 2.0

D Northern Easement

26-28 Warrderry St. 0.4 0.8 1.8

E

Surcharge through

27, 29, 31, 33 and

35 Warraderry St.

1.4 4.0 8.2

F

Surcharge through

Lot 2 and west

along Camp St.

0.1 0.3 0.4

G Flow in existing 900

RCP in Camp St. 1.6 1.8 1.9




It is also to be noted from Table 1.1 that at the 5 year ARI level of flooding about 1.4 m3/s flows over

the western gutter into residential properties in Warraderry Street (at location E), increasing to

8.2 m3/s for the 100 year ARI event.

The escape of overland flows westwards through the residential properties reduces the overland flow

arriving at the northern end of Warraderry Street at the intersection with Camp Street and the

magnitude of residual overland flow discharging through the service station at the north-west corner

(location F of Figure 1.4).

Landowners adjoining Oliver Court on the eastern side of Warraderry Street have over the years

undertaken works to divert these overland flows away from their properties, thereby increasing the

magnitude of flows traversing Oliver Court. For example, as shown on the survey plan of Figure 2.1

and Photo 10 of Annexure A, a diversion bank constructed in No. 30 Warraderry Street was intended

to protect residential development in that property, but directs overland flow towards the Oliver Court

allotment. However, under design flood conditions Figure 1.4 shows that the bank would be

overtopped and flooding would occur in No. 30 Warraderry Street.

1.6 Impacts of Flooding on Urban Development in the Warraderry Street Area

The results of the more detailed TUFLOW modelling undertaken for the Working Paper confirm the

analysis undertaken in the FRMS investigation which showed that the Oliver Court – Warraderry

Street area was located in the “floodway” of the O’Brien Tributary, according to the procedures

outlined in the Floodplain Development Manual, 2005 for the hydraulic categorisation of the floodplain.

It is predicted from the results of the flood modelling that damaging flooding would commence at the

5 year ARI level of flooding with two residences in Warraderry Street experiencing above-floor

inundation. At the 100 year ARI, the two residential properties on the western side of Warraderry

Street, as well as all of the individual units in Oliver Court and the residence in 30 Warraderry Street

would be flooded to above–floor level. It is likely that residential development on the O’Brien Tributary

overland flow path downstream of Warraderry Street in the East Street area would also be flooded,

although without the benefit of detailed site survey it is not possible to provide quantitative information

on the number of properties affected.

1.7 History of Flooding at the Oliver Court Development

The Warraderry Street area has suffered from problems due to overland flows from the O’Brien

Tributary catchment for many years. The approval of the Oliver Court development, which lay astride

the historic overland flow path, was subject to easements being provided (4 m and 3 m wide

respectively) along the northern and southern boundaries for the drainage of stormwater. Council also

supported the construction by the developer of a pipeline and drainage swale along the southern

easement to capture overland flows from upstream. However, these measures were not incorporated

in the finished development. The as-constructed allotment stormwater system comprises a small

intake pit in the grassed area at the north-east side of the site, with lines of 150 mm diameter to

capture allotment and roof runoff. The hydraulic capacity of this system is insignificant compared with

the magnitude of overland flows developed from the O’Brien Tributary catchment.

Over time, the residents of Oliver Court have constructed works in the easements such as a shed at

the north-east boundary, colour-bond dividing fences between the units, a water tank on the southern

easement and paving along the northern boundary. These existing works, together with difficulties of




access for construction equipment in the narrow easements and the potential impact on the building

foundations, which according to the design drawings of the building, comprise strip footings about

600 mm deep along the southern boundary, would preclude the retro-fitting of the piped drainage

works formerly proposed by Council. Access for construction of the pipeline along the southern

boundary of Oliver Court would require the easement being cleared and extended in width and

combined with a new easement taken along the northern boundary of the adjacent property,

No. 30 Warraderry Street.




2 DATA COLLECTION AND FORMULATION OF POTENTIAL SCHEMES

2.1 Site Surveys

The preliminary site survey of Oliver Court undertaken for the FRMS report (shown on Figure 1.2)

was refined to provide natural surface levels in the property on its eastern boundary where the

approaching overland flow would be collected by the proposed diversion bank/ inlet pit system. The

survey which is presented on Figure 2.1 was extended southwards to the presently undeveloped

Lot 41. The survey also extended into 30 Warraderry Street. The bank diverting overland flows

towards Oliver Court is located in the north-east corner of this allotment and extends to the southern

boundary of Oliver Court.

The survey was extended along Warraderry Street to the intersection with Camp Street to allow the

assessment of the direction and depth of surcharge flow exceeding the capacity of the existing

525 mm diameter stormwater line.

2.2 Survey of Potential Retarding Basin

Figure 2.2 shows the contour survey of the potential retarding basin site in the rural area upstream of

Brickfield Road. The site is located on the overland flow path of the O’Briens Tributary in the five

portions of land totalling 3.8 ha in area on the eastern side of the road and is bounded by laneways to

the north and south. The results of hydrologic modelling of the catchment to assess the potential

reductions in peak flow achieved by the temporary storage of floodwaters in the basin are presented

in Chapter 3 of the Working Paper.

2.3 Alternative Schemes for Drainage Improvements

A preliminary evaluation of the schemes considered for the upgrade of the drainage system, listing the

components of the schemes and their advantages and disadvantages is presented in this section of

the report. Figure 2.3, Sheets 1 to 5 shows the locations of the elements comprising the schemes.

Table 2.1 at the end of this Chapter summarises the results. The following discussion supplements

information presented in Table 2.1.

1 Schemes 1, 1A and 2 (ref. Figure 2.3, Sheet 1) all terminate at the front of Oliver Court, with

a surcharge pit in the street and a temporary connection to the existing piped stormwater

system in Warraderry Street. These schemes would only provide localised improvements to

the drainage system, at Oliver Court. In the interim period pending future amplification of the

system in Warraderry Street, overland flows in that street may be slightly higher than under

present day conditions due to the concentration of flows resulting from the upstream drainage

improvements. The length of pipeline required would range between 106 m for Schemes 1

and 1A, and 91 m for Scheme 2.

2 Schemes 1 and 1A have been strongly opposed by residents in O’Brien Street in responses

to the exhibition of the FRMS. Scheme 2 which involves construction of the pipeline in the

adjacent property to the south (No. 30 Warraderry Street) rather than in Oliver Court would be

similarly opposed. The basis for their objections is the increase in flows across their properties

which results from the closure of the existing “escape” routes for overland flows into upstream

properties fronting Warraderry Street prior to their arrival at the large intake pit to the new

pipe. The potential for increased ponding and waterlogging due to potential blockage of the

inlet pit grate would also be a concern of landowners.




3 Scheme 2 is hydraulically more efficient than the route through Oliver Court envisaged for

Schemes 1 and 1A as there are fewer bends in the line. The longitudinal section of

Scheme 1 shown on Figure 2.4 shows that Council’s sewer in Warraderry Street could act as

a constraint on the grading of the pipe. The sewers would similarly constrain the grading of

Scheme 2. None of these schemes is capable of conveying more than about 1.2 m3/s out of

the 8.2 m3/s of overland flow presently traversing Oliver Court under 100 year ARI conditions

and would not mitigate existing flooding problems on the western side of Warraderry Street.

Tables B1 and B2 in Annexure B show indicative costs for Schemes 1 and 2 ($424,000 and

$467,000 respectively). None of these three schemes is cost-effective and they are not

recommended for further consideration.

4 Scheme 3 (ref. Figure 2.3, Sheet 2) is intended to mitigate the objections of residents in

O’Brien Street to the above schemes by the capture and piping of the overland flows at a

location further to the south, by relocating the intake to the pipeline to Lot 41 O’Brien Street,

which is privately owned but undeveloped land. The overland flow across the eastern

boundaries of the properties in O’Brien Street would therefore be reduced by piping some of

the flow. Easements would need to be taken over the route of the pipeline, which may raise

objections by affected landowners (i.e. the owners of Lot 41, 5 and 7 O’Brien Street; and

30 Warraderry Street). The total length of pipeline required for Scheme 3, which also

terminates in a surcharge pit in Warraderry Street, is 145 m.

The hydraulic head required to drive the flow through the pipe would be provided with a

headwall and banking in Lot 41 to capture the approaching overland flow. The capacity of the

pipeline would be up to 1.5 m3/s. However, as the 100 year ARI overland flow approaching

the headwall amounts to 8.2 m3/s (ref. Table 1.1), most of the flow would surcharge the pipe

and would be conveyed as overland flow, as occurs at present. As is the case with the above

schemes, Scheme 3 is not cost-effective and is not recommended for further consideration.

5 Scheme 3A (ref. Figure 2.3, Sheet 2) commences at Lot 41 O’Brien Street and runs

westwards through 32 Warraderry Street. After provision for a surcharge pit in the street, it

connects with the existing 450 mm stormwater pipe. It does not require works in 5 and

7 O’Brien Street. However, it is less hydraulically efficient than Scheme 3 due to the bends in

the pipeline through the property. An easement would need to be taken over the route of the

pipeline, which may raise objections by the affected landowner (the owner of 32 Warraderry

Street). The total length of pipeline required is about 115 m. As for the previous schemes,

most of the 100 year ARI discharge would surcharge the pipe and would be conveyed as

overland flow as at present. Scheme 3A is not cost effective and is not recommended for

further consideration.

6 Schemes 3B (ref. Figure 2.3, Sheet 2) is an extension of Scheme 3, by continuing the

pipeline along Warraderry Street to connect with the existing drainage pit in the eastern gutter

near the intersection with Camp Street. There is no interim connection required to the existing

pipeline in Warraderry Street with this scheme, because the pipeline would be constructed in

a single stage. The total length of pipeline from Lot 41 O’Brien Street to Camp Street would

be about 258 m.

A preliminary longitudinal section is shown on Figure 2.5. There are two crossings of

Council’s sewer for this scheme. The first crossing is a constraint on the grade of the line. The

pipeline could be raised to miss the pipe, at the expense of a reduction in cover. The

hydraulic capacity of the pipeline is about 1.5 m3/s. Most of the 100 year ARI discharge would

therefore surcharge the pipe and be conveyed as overland flow along Warraderry Street.

Table B3 in Annexure B gives an indicative cost estimate of $903,000 for the scheme.




Scheme 3B in isolation (i.e. without the reduction in flows achieved by an upstream retarding

basin) is not cost-effective and is not recommended for further consideration.

7 Scheme 4 (ref. Figures 4.1 and 4.2) involves the construction of the retarding basin at

Brickfield Road. The intention is to reduce downstream flood peaks by the temporary storage

of runoff in the basin storage, followed by a slow release via the low level outlet pipe in the

basin embankment. From the results of modelling of the catchment described in Chapter 3, a

considerable reduction in downstream peak flows in the Oliver Court – Warraderry Street area

could be achieved. A preliminary layout of the basin is presented in Chapter 4. It has an

indicative cost estimate of $716,000, including the cost of purchase of land in the storage

area, (Table B4 of Annexure B). The retarding basin is a cost-effective measure for

mitigating the downstream flooding problem and is considered an essential component of the

drainage upgrade (subject to a geotechnical investigation of the site).

Analysis and concept design of the basin is presented in Chapters 3 and 4 of the Working

Paper. Supplementary drainage works downstream of the basin (e.g. Scheme 5 below) were

evaluated which would control residual “post-basin” flows and further reduce downstream

flooding conditions in Oliver Court and Warraderry Street.

8 Scheme 5 (ref. Figure 2.3, Sheet 3) comprises the retarding basin at Brickfield Road in

conjunction with downstream measures for controlling residual flow, as follows:

8.1 Council’s voluntary purchase of the flood prone residential property No. 30 Warraderry

Street located on the southern side of Oliver Court and its conversion to an “open space”

with a drainage swale conveying overland flows from the eastern side of the property to

Warraderry Street. The alternative solution of creating the drainage swale by purchasing

and demolishing three units on the southern side of Oliver Court could also be

considered. This would be a more expensive solution than purchasing No. 30 Warraderry

Street and would require extensive amendments to the layout of the existing driveway

and infrastructure in the Oliver Court allotment. Council officers would need to undertake

initial discussions with the landowners of both properties to gauge reactions regarding the

proposal.

8.2 Capture of the overland flows leaving the downstream end of the swale and their

conveyance in a new 900 mm diameter pipeline about 135 m in length, running along the

eastern gutter of Warraderry Street, from Oliver Court to connect with the existing system

at the intersection with Camp Street. Overland flows would be collected and directed into

the pipe via several pits in the street. Council’s sewer in Warraderry Street

(ref. Figure 2.3, Sheet 3) would have to be crossed. The “best estimate” of the elevation

of the sewer is shown in the longitudinal section Figure 2.6, but would have to be

confirmed by potholing to be carried out by Council prior to the commencement of

detailed design.

8.3 Addition of the amounts in Tables B4 and B5 in Annexure B gives a total indicative cost

estimate of $1,500,000 for Scheme 5. This amount includes the costs of the retarding

basin (including purchase of the land in the storage area), the purchase of

No. 30 Warraderry Street and the cost of constructing the swale in that property, together

with the cost of the pipeline in Warraderry Street. On technical grounds Scheme 5 would

capture all of the post-basin residual flow between the eastern boundary of Oliver Court

and Warraderry Street. However, it requires the voluntary purchase of the

No. 30 Warraderry Street property by Council.




9 Scheme 5A is a variation of Scheme 5 which could be considered in the event that Council

does not acquire 30 Warraderry Street and converts the allotment to open space for the

swale. The scheme would comprise the retarding basin at Brickfield Road in conjunction with

downstream measures for controlling residual flow, as follows:

9.1 Council’s acquisition of an easement along the northern side of 30 Warraderry Street

and removal of existing obstructions to the conveyance of overland flow, including the

diversion bank which currently directs flow towards Oliver Court (ref. Figure 2.1).

The new easement, together with the existing easement on the southern boundary of

Oliver Court, which would also need to be cleared of obstructions, would form a flow

path for the conveyance of overland flows to Warraderry Street. As for Scheme 5,

Council officers will need to undertake initial discussions with the landowners of both

properties to gauge reactions regarding the proposal.

9.2 Capture of the overland flows leaving the downstream end of the easement and their

conveyance in a new 900 mm diameter pipeline about 135 m in length, running along

the eastern gutter of Warraderry Street, from Oliver Court to connect with the existing

system at the intersection with Camp Street. Overland flows would be collected and

directed into the pipe via several pits in the street. The longitudinal profile of the

pipeline in Warraderry Street would be similar to that of Scheme 5 (shown on

Figure 2.6).

9.3 Addition of the amounts in Tables B4 and B6 in Annexure B gives an indicative cost

estimate of $1,246,000 for Scheme 5A. This amount includes the costs of the

retarding basin (including purchase of the land in the storage area), the easement in

No. 30 Warraderry Street and removal of obstructions to create a surface overland

flow path in that property and Oliver Court, together with the cost of the pipeline in

Warraderry Street.

10 Schemes 6 and 7 are variations of the piped schemes considered above and are intended to

reduce the magnitude of overland flow through 30 Warraderry Street and Oliver Court under

post-basin conditions by capturing the “residual flows”. Under 100 year ARI conditions, there

is a “residual flow” of about 3.2 m3/s travelling as overland flow downstream of the basin and

arriving at the eastern boundaries of these properties.

11 Scheme 6 follows the same route as piped Scheme 3B and is shown on Figure 2.3, Sheet 4.

Due to the limited capacity of the pipeline it is not practicable to capture all of the “residual

flow”. Under 100 year ARI conditions there will be a flow of around 1.5 to 2 m3/s which will still

flow overland to Warraderry Street. Some of this flow could be captured by formalising an

overland flow path in the existing drainage easement on the southern boundary of Oliver

Court. The remainder would flow along the driveways of the two properties, but is not likely to

result in damaging flooding. The scheme is considerably more expensive than Schemes 5

and 5A.

12 Scheme 7 also commences at Lot 41 in O’Brien Street, but follows the same route as

Scheme 1 through Oliver Court and is shown on Figure 2.3, Sheet 5. Scheme 7 involves the

construction of duplicate pipelines under the driveways of Oliver Court. No “as constructed”

information is available to confirm the locations of building foundations and services.

Potholing would be required if this scheme proceeds. However, it has been retained in this

screening process for further consideration.




2.4 Summary of Initial Screening of Schemes

From the results of the preliminary screening, Scheme 4, which comprises the retarding basin at

Brickfield Road is an essential component of the drainage improvements. Schemes 5 to 7 include

supplementary pipework to capture post-basin residual flows. They are subject to further analysis

and evaluation in subsequent Chapters of the report. Table 2.1 (at the end of this Chapter)

summarises indicative costs and benefits in terms of mitigation of flooding in residential

properties. Indicative cost-benefit analysis is presented in Chapter 6.

As an alternative to conveying post-basin “residual flows” overland through 30 Warraderry Street,

flows could be the piped through the property.This scheme includes the retarding basin and is

denoted Scheme 6 in Table 2.1. However, it is considerably more expensive than the other

options and as mentioned in the previous section, it is not practicable to capture all of the flow in

the pipeline under 100 year ARI conditions. The indicative cost of Scheme 6 including the basin

is $1,619,000. Further details of its hydraulic performance are given in Section 5.2

Alternatively, construction in 30 Warraderry Street may be avoided with duplicate pipelines

commencing at Lot 41, running through the Oliver Court unit block to Warraderry Street and then

along that street to Camp Street. This scheme includes the retarding basin and is denoted

Scheme 7 in Table 2.1.The pipeline through Oliver Court follows the route of Scheme 1 (ref.

Figure 2.4, Sheet 5) with two 90 degree bends. In order to compensate for the resulting loss in

hydraulic capacity, it is proposed to duplicate sections of the 900 mm diameter pipe, upgrading

section of the line to 1200 mm diameter and clearing the easement on the southern side of the

property to achieve additional capacity. This scheme would be the most expensive of the

schemes considered and as mentioned, there is uncertainty about its constructability in the

confined spaces through the Oliver Court allotment. Scheme 7 is estimated to cost $2,160,000.




TABLE 2.1

O’BRIEN TRIBUTARY DRAINAGE IMPROVEMENT

SCREENING OF POTENTIAL SCHEMES

Scheme

Elements of Scheme Indicative

Cost Comment

4

Retarding Basin at Brickfield Road. $716,000

The basin is an essential component of the

drainage upgrade. It will reduce the incidence

of above-floor flooding under 100 year ARI

conditions from: all units in Oliver Court,

2 residences in Warraderry Street west side

and 30 Warraderry Street (present day) to

shallow above-floor flooding in one unit in

Oliver Court (post-basin).

5

Retarding Basin + Drainage Swale in

30 Warraderry Street + Pipeline in

Warraderry Street, extending to Camp

Street (Figure 2.3, Sheet 3).

$1,500,000

The swale and pipeline is technically the best

method of controlling post-basin “residual

flows”. With this scheme in place no properties

will be subject to above-floor inundation at the

100 year ARI level. However, voluntary

purchase of 30 Warraderry Street is required

for this scheme to proceed.

5A

Retarding Basin + clearing

obstructions for conveying residual

overland flow across

30 Warraderry Street and clearing the

existing easement in Oliver Court of

obstructions + Pipeline in Warraderry

Street extending to Camp Street.

$1,246,000

The easement and pipeline is suitable for

controlling post-basin “residual flows” if

30 Warraderry Street is not able to be

purchased by Council. With this scheme in

place no properties will be subject to above-

floor inundation at the 100 year ARI level.

However, there will be a nuisance overland

flow through the properties at the 100 year

ARI.

6

Retarding Basin + Pipeline from Lot

41 O’Brien Street through 30

Warraderry Street, continuing to

Camp Street + clearing easement on

southern boundary of Oliver Court to

assist with the conveyance of residual

overland flows (Figure 2.3, Sheet 4).

$1,619,000

This scheme is more expensive than

Schemes 5 or 5A, but will reduce overland

flows in properties between Lot 41 and

Warraderry Street compared with those

schemes. However, there will still be nuisance

overland flow through the properties at the

100 year ARI, as it is not practicable to pipe all

of the residual post-basin flow.

7

Retarding Basin +duplicate pipeline

from Lot 41 O’Brien Street through

Oliver Court, continuing to Camp

Street + clearing easement on

southern boundary of Oliver Court to

assist with the conveyance of residual

overland flows (Figure 2.3, Sheet 5).

$2,160,000

This scheme is the most expensive of the

schemes considered but has a hydraulic

capacity greater than Scheme 6 and

eliminates above-floor inundation to the 100

year ARI level of flooding. The technical

feasibility of constructing duplicate pipelines in

the driveway of Oliver Court would need to be

confirmed by potholing building foundations

and services prior to detailed design.




3 HYDROLOGIC AND HYDRAULIC MODELLING

3.1 Hydrologic Modelling Approach

The RAFTS - DRAINS rainfall-runoff modelling software was adopted for deriving discharge

hydrographs generated by the O’Brien Tributary catchment. Figure 3.1 shows the sub-catchments

comprising the O’Brien Tributary catchment. Discharge hydrographs were required for input to the

TUFLOW hydraulic model to determine flooding patterns in the Oliver Court – Warraderry Street area

under present day conditions. The RAFTS - DRAINS model was also used to route discharge

hydrographs through the storage of a possible retarding basin at Brickfield Road and determine post-

basin hydrographs at the upstream boundary of the TUFLOW model.

3.2 Present Day Conditions

Figure 3.2 shows discharge hydrographs generated by the RAFTS – DRAINS model for flood

frequencies ranging between 5 and 100 year ARI. Although the RAFTS – DRAINS model extends to

the northern end of Warraderry Street and incorporates all of the sub-catchments upstream of that

location, it does not incorporate the ability of modelling the flows passing over the western side of

Warraderry Street. The TUFLOW hydraulic model which converts flows to flood levels and flooding

patterns based on the site survey achieves this objective. Consequently, the discharge hydrographs

shown in Figure 3.2 overestimate the flow arriving at the intersection with Camp Street. The

distribution of peak flows downstream of the eastern boundary of Oliver Court is shown on Table 3.1,

(over page) which is based on the TUFLOW results.

3.3 Retarding Basin at Brickfield Road

3.3.1 Basin Objectives

Retarding basins provide a temporary storage of floodwaters additional to that contained in the natural

floodplain, which can reduce the flood peak in downstream reaches of the creek. The basin should be

located in the reaches of the catchment, sufficiently close to the area intended to be protected, that its

attenuating effects over flood peaks is not negated by downstream tributary inflows. Typically the

basin should command in excess of 50 per cent of the total catchment at the urban centre to be

protected. The basin site on the eastern side of Brickfield Road controls a catchment of 55 ha, about

68 per cent of the 80.8 ha catchment at the eastern boundary of Oliver Court.

Another requirement is that the basin be of sufficient size to store a significant percentage of runoff

from the design storm. Basins attenuate the flood peak (i.e. reduce the downstream peak rate of

runoff) by temporarily storing the incoming discharge hydrograph and releasing it at a controlled rate.

To be effective basins storage volumes about 40 per cent of the volume of runoff of the incoming flood

event are required. A storage volume of 19,000 m3

is capable of being developed at the Brickfield

Road site. This volume is equivalent to 35 mm of surface runoff from the 55 ha catchment. This

storage is about 80 per cent of the runoff generated by a 100 year ARI storm of 1 hour’s duration.

3.4 Results of Hydrologic Modelling of the Retarding Basin

Figure 3.3 shows pre- and post-basin discharge hydrographs for 20 and 100 year ARI flows

assuming a low level pipe outlet of 450 mm diameter. At the upstream boundary of Oliver Court –

location Q2 on Figure 3.1, peak flows would be reduced to about 3.4 m3/s for the 100 year ARI flood.




3.5 Hydraulic Modelling of Post - Retarding Basin Conditions (Scheme 4)

The discharge hydrographs at location Q2, as well as at Q3 which represents flow from sub-

catchments H and I flowing northwards along Warraderry Street, were applied to the TUFLOW model

to determine post-basin flood levels and flooding patterns.

The results are presented in Figure 3.4. Peak flows at key locations are shown on Table 3.1. A

comparison with corresponding values on Table 1.1 shows the effects of the basin in attenuating

downstream flows. The following effects are to be noted:

Overland flows would be considerably reduced at Oliver Court and 30 Warraderry Street.

Under 5 year ARI conditions the total discharge flowing through Oliver Court (locations B, C

and D) would be reduced from 2.4 m3/s to 0.9 m

3/s; and under 100 year conditions from

8.3 m3/s to 3.2 m

3/s. Construction of a drainage swale in the 3 m wide easement on the

southern boundary of Oliver Court would reduce the flow in the driveway and northern

easement (locations C and D), at the expense of a slightly larger discharge at location B (the

southern easement).

Flows over the western gutter of Warraderry Street into the residential properties (location E)

would be similarly reduced. Under 5 year ARI conditions, the flow would be reduced from

1.4 m3/s to 0.5 m

3/s; and under 100 year conditions from 8.2 m

3/s to 3.9 m

3/s.

TABLE 3.1

SCHEME 4 - (POST- RETARDING BASIN) PEAK FLOWS

OLIVER COURT AND WARRADERRY STREET AREA

(m3/s)

ID (Figure 3.4)

Location

5 year ARI

20 year ARI

100 year ARI

A Flow Through Lot

41 O’Brien St. 1.0 2.0 3.4

B

Southern Easement

26-28 and 30

Warraderry St.

0.5 1.0 1.7

C Driveway 26-28


D Northern Easement

26-28 Warrderry St. 0.1 0.3 0.5

E

Surcharge through

27, 29, 31, 33 and

35 Warraderry St.

0.5 1.7 3.9

F

Surcharge through

Lot 2 and west

along Camp St.

0.0 0.1 0.2



Notes: (1) The above results apply for construction of retarding basin (Scheme 4) with no additional works.

(2) Compare these flows with Present Day flows shown in Table 1.1.




3.6 Reduction in Flooding in Warraderry Street Area - Post-Retarding Basin

It is predicted from the results of the flood modelling that construction of the basin would eliminate

above-floor inundation in properties on the western side of Warraderry Street under 100 year ARI

conditions, although the allotments may continue to experience shallow overland flow at that

frequency. One of the units in Oliver Court may be subject to shallow above-floor flooding of around

80 mm depth which could be mitigated by some minor flood proofing at the entrance.




4 DESIGN CONCEPT FOR RETARDING BASIN (SCHEME 4)

4.1 Layout of Basin and Indicative Cost

4.1.1 Layout

The preliminary layout for a retarding basin is shown on Figures 4.1 and 4.2. It involves the

construction of an embankment running along the eastern side of Brickfield Road for about 175 m

before turning eastwards and continuing for about 90 m.

The storage characteristics of the basin were estimated from the recent survey of the site. For initial

sizing, the basin was sized to store runoff to a peak depth of about 2.3 m. An allowance of 1.0 m in

height would be provided between the 100 year ARI storage level and the embankment crest

elevation. This freeboard incorporates allowances for the head over the spillway crest necessary to

convey floods larger than the 100 year ARI, plus an allowance for wave action. An embankment up to

3.3 m in height would therefore be required.

The volume of storage could be increased by excavation within the storage area, which would also

lower the peak storage level and reduce the potential for backwater flooding in areas upstream of the

basin site. This excavation would also provide fill for the embankment, subject to future geotechnical

testing if the basin proceeded beyond the concept stage. The geotechnical investigation would also

be needed to confirm that the embankment and storage did not lie within the extent of former mining

activities, which are evident downstream of Brickfield Road.

4.1.2 Indicative Cost of Basin

The indicative cost of the basin (Scheme 4), including an allowance for land acquisition and

geotechnical investigation is $716,000. Table B4 in Annexure B provides further details.




5 CATERING FOR RESIDUAL FLOWS IN OLIVER COURT AND WARRADERRY STREET

5.1 Scheme 5 – Retarding Basin Plus Swale and Pipeline in Warraderry Street

Figure 5.1 shows the results of hydraulic modelling of the post-basin case together with the drainage

swale through 30 Warraderry Street and the 900 mm diameter pipeline from the downstream end of

the swale to Camp Street. Table 5.1 shows the corresponding distribution of peak flows in the

modelled area.

TABLE 5.1

PEAK FLOWS

OLIVER COURT AND WARRADERRY STREET

SCHEME 5 – RETARDING BASIN + SWALE IN 30 WARRADERRY STREET +

PIPELINE IN WARRADERRY STREET

(m3/s)

ID (Figure 5.2)

Location

5 year ARI

20 year ARI

100 year ARI

A Flow Through Lot

41 O’Brien St. 1.0 2.0 3.4

B Drainage Swale in

30 Warraderry St. 1.0 2.0 3.4

C Driveway 26-28


D Northern Easement

26-28 Warrderry St. 0.0 0.0 0.0

E

Surcharge through

27, 29, 31, 33 and

35 Warraderry St.

0.4 1.2 3.2

F

Surcharge through

Lot 2 and west

along Camp St.

0.0 0.4 0.8



H Flow in new pipe in

Warraderry Street 1.0 1.1 1.2

Notes:

(1) The above results apply for retarding basin (Scheme 4) plus swale in 30 Warraderry Street and pipeline in Warraderry

Street.

(2) Compare these flows with Present Day flows shown in Table 1.1 and retarding basin only (Scheme 4) in Table 3.1.

It is predicted from the results of the flood modelling that construction of the basin and Scheme 5

swale and pipeline would eliminate above-floor inundation in properties on the western side of

Warraderry Street under 100 year ARI conditions, although the properties may continue to experience

shallow overland flow at that frequency. All of the units in Oliver Court would be above flood level.

This scheme is estimated to cost $1,500,000 made up of $716,000 for the retarding basin and

$784,000 for downstream works, including the purchase of 30 Warraderry Street and construction of

the pipeline in Warraderry Street to Camp Street (ref. Table B5).




5.2 Scheme 5A – Retarding Basin Plus Clearing Obstructions to Flow in 30 Warraderry

Street and Pipeline in Warraderry Street

As noted in Section 2.3, Scheme 5A is a variation of Scheme 5 which could be considered in the

event that Council does not acquire 30 Warraderry Street and converts the allotment to open space

for the swale.

The scheme would comprise the retarding basin at Brickfield Road in conjunction with Council’s

acquisition of an easement along the northern side of 30 Warraderry Street and its conversion to an

“open space”. The new easement, together with the existing easement on the southern boundary of

Oliver Court, which would need to be cleared of obstructions, would form a flow path for the

conveyance of overland flows to Warraderry Street.

Overland flows leaving the downstream end of the easement would be captured by a large inlet pit at

the front of 30 Warraderry Street and conveyed in a new 900 mm diameter pipeline about 135 m in

length, running along the eastern gutter of Warraderry Street, to connect with the existing system at

the intersection with Camp Street.

The longitudinal profile of the pipeline would be similar to that of Scheme 5. As for Scheme 5, Council

officers will need to undertake initial discussions with the landowners of both properties to gauge

reactions regarding the proposal.

The scheme would perform similarly to Scheme 5 in terms of mitigating flooding.

The indicative cost of the scheme is $1,246,000, made up of $716,000 for the retarding basin and

$530,000 for clearing obstructions in 30 Warraderry Street and construction of the pipeline in

Warraderry Street to Camp Street (ref. Table B6).

5.3 Scheme 6 – Retarding Basin + Pipe Scheme 3B (Lot 41 Through 30 Warraderry Street to

Camp Street)

Although the basin associated with the retarding basin (Scheme 4) would be very effective in

throttling flows from its upstream catchment, there is an uncontrolled catchment of about 25 ha

downstream of Brickfield Road (sub-areas D, E and F of Figure 3.1) which will generate “residual

flows” at Oliver Court. In addition there will be flows from the 16.2 ha uncontrolled catchment

discharging to the northern end of Warraderry Street (sub-areas H and I).

Figure 5.2 shows the results of hydraulic modelling of the post-basin case together with the

900 mm diameter pipeline from Lot 41 to Camp Street. Table 5.2 over the page shows the

corresponding distribution of peak flows in the modelled area.




TABLE 5.2

PEAK FLOWS


SCHEME 6 – RETARDING BASIN +

PIPE SCHEME 3B LOT 41 TO CAMP STREET

(m3/s)

ID (Figure 5.1)

Location

5 year ARI

20 year ARI

100 year ARI

A Flow Through Lot

41 O’Brien St. 1.0 2.0 3.4

B

Southern Easement

26-28 and 30

Warraderry St.

0.0 0.7 2.0

C Driveway 26-28


D Northern Easement

26-28 Warrderry St. 0.0 0.0 0.0

E

Surcharge through

27, 29, 31, 33 and

35 Warraderry St.

0.4 1.0 2.9

F

Surcharge through

Lot 2 and west

along Camp St.

0.0 0.5 1.1





Notes:

(1) The above results apply for construction of retarding basin (Scheme 4) plus Pipeline 3B.


With this scheme, there would be no flow across Oliver Court for minor flood events (5 year ARI) and

only minor flows at the 20 year ARI level, which could be captured by a swale along the southern

easement of that property. Similarly the flows escaping over the western footpath of Warraderry Street

into the residential properties would be reduced to levels below floor level for the 100 year ARI flood.

However, the pipeline does not have sufficient capacity to capture all of the overland flow traversing

30 Warraderry Street and the depth of flow would just surcharge the floor level. Minor re-shaping of

natural surface levels would be required to direct flows away from the building.

As shown on Table B3 of Annexure B, the indicative cost of the Scheme 3B pipeline is $903,000,

giving a total indicative cost of $1,619,000 with the cost of the basin included.




5.4 Scheme 7 – Retarding Basin + Pipeline from Lot 41 Through Oliver Court to Camp Street

Figure 5.3 shows the results of hydraulic modelling of the post-basin case together with the duplicate

900 mm diameter pipelines from Lot 41 to Camp Street. Table 5.3 shows the corresponding

distribution of peak flows in the modelled area.

TABLE 5.3

PEAK FLOWS


SCHEME 7 – RETARDING BASIN +

DUPLICATE PIPELINE LOT 41 THROUGH OLIVER COURT TO CAMP STREET

(m3/s)

ID (Figure 5.1)

Location

5 year ARI

20 year ARI

100 year ARI

A Flow Through Lot

41 O’Brien St. 1.0 2.0 3.4

B

Southern Easement

26-28 and 30

Warraderry St.

0.0 0.0 0.7

C Driveway 26-28


D Northern Easement

26-28 Warrderry St. 0.0 0.0 0.2

E

Surcharge through

27, 29, 31, 33 and

35 Warraderry St.

0.4 1.0 2.3

F

Surcharge through

Lot 2 and west

along Camp St.

0.0 0.6 2.0





Notes:

(1) The above results apply for construction of retarding basin (Scheme 4) plus Pipeline from Lot 41 to Camp Street.


With this scheme, there would be no flow across Oliver Court for medium flood events (20 year ARI)

and only minor flows at the 100 year ARI level, which could be captured by a swale along the

southern easement of that property. Flows escaping over the western footpath of Warraderry Street

into the residential properties would be reduced compared with Scheme 6. The duplicate pipeline

reduces overland flows through properties downstream of Lot 41and the depth and extent of

inundation. The depth of overland flow traversing 30 Warraderry Street would be reduced to below the

floor level. As shown on Table B7 of Annexure B, the indicative cost of the Scheme 3B pipeline is

$903,000, giving a total indicative cost of $2,160,000 with the cost of the basin included.




6 ECONOMIC ANALYSIS OF SCHEMES

6.1 Estimated Flood Damages

Estimation of flood damages to residential development bordering the O’Brien Tributary system in

the Warraderry Street area was carried out to assess the impact of flooding under present day

and post-scheme conditions. The objectives were to allow an economic assessment of the

schemes to be carried out. Damages were assessed for floods ranging between the 5 year ARI

and 100 year ARI events. The 5 year ARI is the threshold flood at which significant damages are

experienced in the area.

There were no quantitative data available on historic flood damages at Grenfell. The analysis was

carried out using the residential flood damages model attached to “Floodplain Risk Management

Guideline No 4 - Residential Flood Damages”. This publication was prepared by DECCW (now

OEH) to allow the consistent assessment of residential damages across NSW for the economic

comparison of flood management projects.

In Guideline No 4, damage assessments undertaken after major flooding in other urban centres

were adjusted and used to estimate damages likely to be experienced to typical residential

development in NSW. Data for the flood damages model comprised the depths of inundation over

the floodplain, as well as information on the unit values of damages to residential property. The

depths of inundation were determined from the results of the TUFLOW hydraulic modelling and

from surveyed floor levels and representative natural surface levels at each allotment. The type

of structure and potential for property damage were assessed from a visual inspection.

The procedures in Guideline Number 4 allow for structural damage to the building, damage to

internals and contents, external damages and clean-up costs. The level of flood awareness and

available warning time are taken into account by factors which are used to reduce “potential”

damages to contents to “actual” damages. “Potential” damages represent losses likely to be

experienced if no action were taken by residents to mitigate impacts. A reduction in the potential

damages to "actual" damages is usually made to allow for property evacuation and raising

valuables above floor level, which would reduce the damages actually experienced. The ability of

residents to take action to reduce flood losses is mainly limited to reductions in damages to

contents, as damages to the structure and clean-up costs are not usually capable of significant

mitigation.

6.2 Present Worth Value of Scheme Benefits

The areas under the damage versus frequency curves for both present day and post -scheme

conditions were integrated and converted to present worth values for the three discount rates

nominated in NSW Government Treasury Guidelines for the economic assessment of projects.

These discount rates are 7 per cent (best estimate) with sensitivity studies for 3 per cent and

10 percent.

The benefits of each project are represented by the reduction in flood damages, which are

calculated by subtracting post-project damages from present day values. From a knowledge of

the cost of each project, the benefit-cost ration may be calculated and the projects ranked in a

strictly economic sense. The results are summarised in Tables 6.1 to 6.5 over page.




6.3 Indicative Cost-Benefit Analysis

TABLE 6.1

ECONOMIC ANALYSIS – SCHEME 4 (RETARDING BASIN)

($Million)

Discount Rate % 4 7 10

Present Worth Value of Benefits

(Damages Prevented) $ x 106

0.58 0.45 0.36

Cost of scheme $ x 106 0.716 0.716 0.716

Benefit/Cost Ratio 0.81 0.63 0.51

TABLE 6.2

ECONOMIC ANALYSIS – SCHEME 5

RETARDING BASIN + DRAINAGE SWALE IN 30 WARRADERRY ST

+ PIPELINE TO CAMP ST

($Million)




0.82 0.64 0.52

Cost of scheme $ x 106 1.50 1.50 1.50


TABLE 6.3

ECONOMIC ANALYSIS – SCHEME 5A

RETARDING BASIN + CLEARING OBSTRUCTIONS TO FLOW IN 30 WARRADERRY ST

+ PIPELINE TO CAMP ST

($Million)




0.82 0.64 0.52

Cost of scheme $ x 106 1.246 1.246 1.246





TABLE 6.4


RETARDING BASIN + SCHEME 3B PIPELINE TO CAMP ST

($Million)




0.8 0.64 0.52

Cost of scheme $ x 106 1.619 1.619 1.619


TABLE 6.5


RETARDING BASIN + DUPLICATE PIPELINE

LOT 41 THROUGH OLIVER COURT TO CAMP ST

($Million)




0.8 0.62 0.50

Cost of scheme $ x 106 2.16 2.16 2.16





7 SUMMARY AND FINDINGS

7.1 Summary

This Working Paper reviewed potential measures for mitigating flooding in residential properties in

Oliver Court and Warraderry Street due to overland flows generated by the O’Brien Tributary

catchment. The area is located in a “floodway” according to the hydraulic categorisation carried out in

the Emu Creek Floodplain Risk Management Study.

At present there is no formal stormwater system to capture and convey flows across the Oliver Court

property. The stormwater system commences in Warraderry Street and comprises a pipeline of only

450 and 525 mm diameter in that street, which is of limited hydraulic capacity and capable of

conveying runoff from only minor storms . Historically Oliver Court and Warraderry Street area have

been subject to frequent flooding, most recently in the November 2010 storm which had a return

period of about 5 year ARI on the O’Brien Tributary. Figure 1.1 shows overland flow paths.

The Working Paper formulated and reviewed options for capturing overland flows on or upstream of

the eastern boundary of Oliver Court and conveying them through the property to Warraderry Street.

The options comprise various piped schemes in the reach between Lot 41 in O’Brien Street and the

intersection of Warraderry Street and Camp Street (ref. Figure 2.3, Sheets 1 to 5). The potential

reduction in downstream flows and depths of flooding achieved by the construction of a retarding

basin on the eastern side of Brickfield Road was also assessed. Figures 4.1 and 4.2 show the

concept design of the basin.

Hydraulic modelling was undertaken using the TUFLOW two-dimensional modelling software of

present day and post-scheme conditions. Indicative cost estimates were prepared for several piped

options, as well as for the retarding basin (ref. the various tables in Annexure B).

The economic benefits of the more viable schemes were computed. These benefits are represented

by the reduction in flood damages to the residential developments in Warraderry Street compared

with present day conditions. Flood damages were assessed using recommended OEH procedures

and converted to present worth values using discount rates nominated in NSW Government

Guidelines for the economic analysis of infrastructure projects. Indicative cost-benefit analysis was

then carried out for feasible schemes.

7.2 Findings

7.2.1 Piped Schemes 1 to 3B

Peak flows generated by the catchment during major storms are too large to be conveyed by pipelines

through Oliver Court and along Warraderry Street. The feasible maximum pipe size is limited to

900 mm, and is controlled by the available gradient and minimum cover considerations, as well as by

the presence of Council’s sewers in Warraderry Street.

Consequently, upgrading the drainage system by pipelines in isolation is not cost-effective in solving

existing flooding problems in residential developments in the Warraderry Street area and is not

recommended.




7.2.2 Retarding Basin in Isolation (Scheme 4)

A large reduction in peak flows arriving at the eastern boundary of Oliver Court could be achieved by

the retarding basin at Brickfield Road. The basin in conjunction with a drainage swale in the existing

southern easement of Oliver Court would convey 5 year ARI flows across Oliver Court with a small

surcharge over the driveway. Figure 3.4 shows the results of hydraulic modelling and Table 3.1

shows the distribution of peak flows under post-basin conditions. The indicative cost of the basin is

$716,000, including an allowance of $320,000 for the purchase of land in the proposed storage area.

Construction of the basin would eliminate above-floor inundation in properties on the western side of

Warraderry Street under 100 year ARI conditions, although the properties may continue to experience

shallow overland flow at that frequency. One of the units in Oliver Court may be subject to shallow

above-floor flooding of up to 80 mm which could be mitigated by some minor flood proofing works at

the entrance to the property.

7.2.3 Scheme 5 - Retarding Basin + Drainage Swale in 30 Warraderry Street + Pipeline to

Camp Street

Although the retarding basin will achieve a major reduction in downstream flows as far as Camp

Street, supplementary measures would be required to capture “residual” overland flows continuing to

traverse Oliver Court, 30 Warraderry Street and residential properties in Warraderry Street.

The retarding basin plus capture of overland flows in a drainage swale in 30 Warraderry Street (which

would be voluntary acquired and cleared) and piped northwards along Warraderry Street to the

intersection with Camp Street (Figure 2.3, Sheet 3) would reduce overland flooding. Figure 5.1

shows the results of hydraulic modelling and Table 5.1 shows the distribution of peak flows. The

indicative cost of the Scheme 5 swale and pipeline works is $784,000. In conjunction with the basin,

the total cost would be $1,500,000.

Construction of the basin and the downstream works would eliminate above-floor inundation in

properties on the western side of Warraderry Street under 100 year ARI conditions, although several

of the properties may continue to experience shallow overland flow at that frequency. No units in

Oliver Court would be subject to above-floor flooding.

7.2.4 Scheme 5A - Retarding Basin + Easement in 30 Warraderry Street + Pipeline in

Warraderry Street to Camp Street

The retarding basin plus a drainage easement in 30 Warraderry Street in conjunction with the existing

easement in Oliver Court, together with a pipeline in Warraderry Street to Camp Street (Scheme 5A)

would also reduce overland flooding similar to Scheme 5, but at less cost. The indicative cost of the

easement and pipeline is $530,000. In conjunction with the basin, the total cost would be $1,246,000.

Construction of the basin and associated works would have the same mitigation benefits as

Scheme 5 above.

7.2.5 Scheme 6 - Retarding Basin + Pipeline Through 30 Warraderry Street to Camp Street

As an alternative to conveying post-basin flows overland through 30 Warraderry Street, flows

could be the piped through the property with the Scheme 3B pipeline, as described in

Section 5.3. Figure 5.2 shows the results of hydraulic modelling and Table 5.2 shows the




distribution of peak flows. However, it is considerably more expensive than the other options. The

indicative cost of that scheme including the basin is $1,619,000, including the retarding basin.

7.2.6 Scheme 7 - Retarding Basin + Pipeline Through Oliver Court to Camp Street

Construction in 30 Warraderry Street may be avoided with a pipeline commencing at Lot 41

O’Brien Street, continuing to Oliver Court and then running through the unit block to Warraderry

Street and then on to Camp Street. The pipeline through Oliver Court follows the route shown on

Figure 2.3, Sheet 5. Figure 5.3 shows the results of hydraulic modelling and Table 5.3 shows the

distribution of peak flows. In order to compensate for the loss in hydraulic capacity resulting from

the two 90 degree bends in Oliver Court and reduce overland flows, it is proposed to duplicate

the 900 mm diameter pipe, upgrade section of the line to 1200 mm diameter and clear the

easement on the southern side of the property to maximise hydraulic capacity. This scheme

would be the most expensive of the schemes considered and would cost around $2,160,000

including the retarding basin.

7.3 Recommendations for Consideration by Floodplain Management Committee

The retarding basin (Scheme 4) is an essential component of the drainage improvements for O’Brien

Tributary and as noted above will achieve a substantial reduction in downstream flooding, particularly

for the minor (5 year ARI) and medium (20 year ARI) floods. Proceeding to the geotechnical

investigation and design phases for the basin is recommended.

The decision whether or not to adopt the drainage swale through 30 Warraderry Street and pipeline in

Warraderry Street as the favoured option for controlling residual flows (i.e. Scheme 5), would depend

upon negotiations with the owner of 30 Warraderry Street, as purchase and clearing of this property is

required.

In the event that voluntary purchase of 30 Warraderry Street is not achievable, Scheme 5A could be

considered which requires an easement for overland flow through the property, well as clearing the

existing easement in Oliver Court from obstructions to flow.

Alternatively, Scheme 6 or Scheme 7 could be considered at greater cost (and noting the

requirement for accurate location of building foundations and services for Scheme 7).

7.4 Proceeding to Detailed Design

Detailed design of the basin and measures to control downstream residual flows will involve the

following sequence of activities:

13 Floodplain Management Committee to confirm the scheme to be adopted.

Geotechnical Investigation and testing of the basin foundation and in the storage area to

assess the suitability of the material as fill for the embankment.

Potholing in Warraderry Street (and Oliver Court in the case of Scheme 7) to confirm the

depths of the sewers at the crossing locations of the new drainage pipeline.

Refining the design concepts included in this Working Paper to the stage of a detailed design

and the preparation of working drawings for the basin.




Negotiation with the owner of the basin site once the requirements for land acquisition have

been determined for the design studies.

Negotiation with the owner of 30 Warraderry Street to acquire the property for the drainage

swale - Scheme 5, (or alternatively, negotiation with the owner for an easement –

Schemes 5A and 6), or negotiations with the owners of Oliver Court (Scheme 7).

Design of the adopted scheme and preparation of working drawings.

Refinement of indicative costings to the standard of pre-construction cost estimates.

Provision of standard clauses in Contract Documentation for the works.

Supply of Specifications for the various work items.

Preparing a Review of Environmental Factors for the selected scheme.

ANNEXURE A

PHOTOGRAPHS OF THE STUDY AREA

ANNEXURE B

INDICATIVE COSTS OF SCHEMES

Table B1 Indicative Cost of Pipeline - Scheme 1

Item Description Unit Rate Quantity Amount

1

Establish Easements along Route of pipeline in Private Property in O'Brien St. and in Oliver Court Item 20,000.00$ 1 20,000$

2 Drainage swale in existing easement along Southern Boundary of Oliver Court Item 20,000.00$ 1 20,000$

3Establishment and Traffic Control in Warraderry Street Item 2,500.00$ 1 2,500$

4 Intake Pit and banking in 5 O'Brien Street Item 10,000.00$ 1 10,000$

5Demolish pavment and landscaping in driveway along route of pipeline m 2 40.00$ 400 16,000$

6 Supply, Lay, Joint 900 RCP m 1,570.00$ 106 166,420$

7 Provide 2 junction pits along route of pipeline Item 2,500.00$ 2 5,000$

8Surcharge Pit in Warraderry Street and connection to existing pipeline Item 8,000.00$ 1 8,000$

9Reinstate driveway and landscaping in Oliver Court m2 80.00$ 400 32,000$

10 Reinstate Services Item 15,000.00$ 1 15,000$

Sub-Total 294,920$

Unestimated Items and Contingencies 25% 73,730$

Sub-Total 368,650$

Survey, Investigation and Design 15% 55,298$

Total 423,948$

Table B2

Indicative Cost of Pipeline - Scheme 2


1

Establish Easements along Route of pipeline in

private Property in O'Brien St., 30 Warraderry

St.and in Oliver Court Item 20,000.00$ 1 20,000$

2

Banking and Drainage swale in existing easement

along Southern Boundary of Oliver Court Item 20,000.00$ 1 20,000$

3

Establishment and Traffic Control in Warraderry

Street Item 2,500.00$ 1 2,500$

4 Intake Pit in 7 O'Brien Street Item 10,000.00$ 1 10,000$

5

Demolish pavment and landscaping along route

of pipeline in 30 Warraderry St. m2

40.00$ 450 18,000$


7

Surcharge Pit in Warraderry Street and

connection to existing pipeline Item 8,000.00$ 1 8,000$

8

Reinstate pavement and landscaping along route

of pipeline m2

200.00$ 450 90,000$


Sub-Total 324,800$


Sub-Total 406,000$


Total 466,900$

Table B3 Indicative Cost of Pipeline - Scheme 3B


1

Establish Easements along Route of pipeline in private property in O'Brien St. and 30 Warraderry St. & flood prone portion of Lot 41 Item 45,000.00$ 1 100,000$



4 Headwall and Banking in Lot 41 Item 10,000.00$ 1 10,000$

5Demolish pavment and landscaping in private property along route of pipeline m 2 40.00$ 450 18,000$


7Reinstate pavement and landscaping in private property along route of pipeline m 2 80.00$ 450 36,000$


9Reinstate pavement in Warraderry Street along route of pipeline m2 30.00$ 400 12,000$


Sub-Total 628,560$


Sub-Total 785,700$


Total 903,555$

Table B4

Indicative Cost of Detention Basin

Brickfield Road

(Scheme 4)


1 Acquisition of Land Item 1 320,000$

2

Geotechnical Investigation and

testing Item 1 15,000$

3

Site Establishment and Set out

works by registered surveyor Item 1 2,500$

4

Erosion Control and Care of Creek

during construction Item 1 5,000$

5

Strip topsoil (300mm) over

embankment footprint m2

1.50$ 5,500 8,250$

6

Excavate additional earthworks for

cut off trench m3

5.00$ 300 1,500$

7

Proof roll embankment foundation

and cutoff trench m2

0.60$ 5,500 3,300$

8

Excavate storage area; place and

compact fill for embankment m3

18.00$ 6,300 113,400$

9

Place topsoil over embankment

surface, grass seed and rehabilitate m2

5.00$ 6,000 30,000$

10 Scour Protection Spillway Item 20,000$

11

Supply, Lay Joint RCP Low Level

Outlet Pipes: 1 x 450 mm, Class 4 m 450.00$ 30 13,500$

12 RC Headwalls Low Level Outlets Item 1,000.00$ 2 2,000$

13

Energy Dissipation at Outlets (300

mm Reno mattress) m2

60.00$ 30 1,800$

Sub-Total 536,250$

Unestimated Items and

Contingencies 25% 134,063$

Sub-Total 670,313$

Survey, Investigation and Design

(excluding acquisition of land) 15% 45,347$

Total 715,659$

Table B5Indicative Cost

Drainage Swale in 30 Warraderry Street and Pipeline in Warraderry Street(Part of Scheme 5)


1 Purchase 30 Warraderry Street Item 300,000.00$ 1 300,000$

2Clear site and construct Drainage Swale in 30 Warraderry Street Item 25,000.00$ 1 25,000$


4Headwall to 900 RCP at western end of Drainage Swale in 30 Warraderry Street Item 8,000.00$ 1 8,000$

5 Supply, Lay, Joint 900 RCP in Warraderry Street m 1,570.00$ 135 211,950$



8 Reinstate services Item 15,000.00$ 1 15,000$

Sub-Total 584,450$


Sub-Total 730,563$

Survey, Investigation and Design (excluding purchase of 30 Warraderry Street property) 15% 53,334$

Total 783,897$

Table B6Indicative Cost

Easement to Create Overland Flow Path in 30 Warraderry Street+ Pipeline in Warraderry Street ( Part of Scheme 5A)


1 Negotiate Easement in 30 Warraderry Street Item 30,000.00$ 1 30,000$

2Demolish pavment and landscaping in 30 warraderry Street Item 15,000.00$ 1 15,000$

3Reinstate pavement and landscaping in 30 Warraderry Street m2 80.00$ 450 36,000$

4

Clear and incorporate existing easement on southern boundary of Oliver Court with easement in 30 Warraderry Street into one drainage path m2 80.00$ 360 28,800$

5Provide large entry pit and grating at front of Oliver Court and 30 Warraderry Street Item 15,000.00$ 1 15,000$


7 Supply, Lay, Joint 900 RCP in Warraderry Street m 1,570.00$ 135 211,950$

8Provide 3 junction pits along route of pipeline in Warraderry Street Item 5,000.00$ 3 15,000$


Sub-Total 368,750$


Sub-Total 460,938$

Survey, Investigation and Design 15% 69,141$ Total 530,078$

Table B7 Indicative Cost of Pipeline - (Part of Scheme 7)


1

Establish Easements along Route of pipeline in private property in O'Brien St., Oliver Court and flood prone portion of Lot 41 Item 45,000.00$ 1 100,000$



4 Headwall and Banking in Lot 41 Item 10,000.00$ 1 10,000$

5Demolish pavment and landscaping in private property along route of pipeline m 2 40.00$ 600 24,000$



8Reinstate pavement and landscaping in private property along route of pipeline m 2 80.00$ 600 48,000$




Sub-Total 1,004,900$


Sub-Total 1,256,125$


Total 1,444,544$

working paper potential schemes (volume 1 report)

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