Racecourse Creek Flood Study and Options Assessment

INFRASTRUCTURE & ENGINEERING SERVICES

ATTACHMENT A

REPORT ON PUBLIC EXHIBITION -
RACECOURSE CREEK FLOOD STUDY AND

OPTIONS ASSESSMENT

ORDINARY MEETING
28 OCTOBER 2020

Racecourse Creek Flood Study and
Options Assessment

Report MHL2714
July 2020
Prepared for:

Department of Planning, Industry and Environment

Cover Photograph: Racecourse Creek Entrance, 2018 (photo courtesy of Wanda Amos)

Racecourse Creek Flood Study and
Options Assessment

Report MHL2714
July 2020

Bronson McPherson
Director of Engineering
110b King Street
Manly Vale NSW 2093
T: 02 9949 0200
E: [email protected]
W: www.mhl.nsw.gov.au

Document Control

Issue/ Author Reviewer Approved for Issue
Revision M Glatz
G Lewis Name Date
Progress M Glatz S Marshall
Report 1 G Lewis B McPherson 13/02/2020
M Glatz
Draft Report B McPherson 15/07/2020

© Crown in right of NSW through the Department of Planning, Industry and Environment 2020

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the Director, Manly Hydraulics Laboratory.

While this report has been formulated with all due care, the State of New South Wales does not
warrant or represent that the report is free from errors or omissions, or that it is exhaustive. The State
of NSW disclaims, to the extent permitted by law, all warranties, representations or endorsements,
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responsibility is accepted by the State of NSW for the accuracy, currency, reliability and correctness of
any information in the report provided by the client or third parties.

Report No. MHL2714

First published as draft in Dec 2019

Manly Hydraulics Laboratory is Quality System Certified to AS/NZS ISO 9001
110B King Street
Manly Vale NSW 2093
T 02 9949 0200 F 02 9948 6185 TTY 1300 301 181
ABN 20 770 707 468 www.mhl.nsw.gov.au

Executive Summary

Description of the study

The Racecourse Creek Flood Study and Option Assessment has been prepared in
accordance with the New South Wales Government’s Floodplain Development Manual
(2005). A flood study is the first step of the floodplain management process set up to reduce
flood risks and private/public losses resulting from flood while using eco-friendly solution
where possible.

Manly Hydraulic Laboratory (MHL) were engaged by MidCoast Council (Council) to undertake
the Racecourse Creek Flood Study and Option Assessment. The outcome of the study is to
develop and calibrate hydrologic and hydraulic models for the estimation of overland and
mainstream flood behaviour in the study area, taking into account the performance of the
stormwater drainage network including overflows from the drainage network. The study
outputs can also inform decision making for investing in the floodplain; managing flood risk
through prevention, preparedness, response and recovery activities; pricing insurance, and
informing and educating the community on flood risk and response to floods. Each of these
areas has different user groups, whose needs vary. Meeting the requirements of the identified
end user groups, which have been tailored to the context of the flood situation, is a key
objective of this study.

The study has been overseen and guided by the Coastal, Flooding and Drainage Group of
MidCoast Council.

Racecourse Creek Flood Study

The Racecourse Creek Flood Study and Option Assessment has been completed to provide
a detailed flooding assessment of Racecourse Creek and its catchment. The objective of this
study is to improve understanding of flood behaviour and impacts, and better inform
management of flood risk in the study area. The study also provides a sound technical basis
for any further flood risk management investigation in the area.

The key components of the flooding assessment included:

• Review of available data

• Community consultation

• Hydrological analysis and modelling

• Hydraulic analysis and modelling

• Sensitivity analysis including climate change impact

• Flood mapping

• Define flood planning area to determine flood control lots and hazard flood levels

• Description of consequences of flooding

• Assessment of flood damages

• Development of flood management options

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• Water quality analysis

• Development of a draft flood study followed by a final flood study that includes a draft
floodplain management plan.

The flood maps appended to this report are presenting the flood levels, depths and velocities
for the critical duration and rainfall pattern of a full set of events including the 20%, 10%, 5%,
2%, 1%, 1 in 500 AEP and PMF events and represent an envelope of the critical
duration/pattern of a selected representative township (upstream) catchment and the critical
duration/pattern at the approved golf course development upstream of Rose St pedestrian
footbridge. The township (upstream) catchments are very flashy with very short critical
durations of less than 30min to reach the peak level while the basin catchment has typical
critical durations ranging between 12h and 48h. The mapping is also based on a non-
mechanical breakout of the creek entrance and on the assumption that the entrance berm is
at the level as measured in the available LiDAR information.

Sensitivity analysis highlighted the following points:

• The catchment flooding is not very sensitive to the entrance berm level that could be
raised to approximately 3.5m AHD to provide protection to the properties along Pacific
Parade. Beyond this level, exacerbated flooding of Pacific Parade and properties along
Sunrise Cl may occur during severe storm events.

• Tailwater conditions (including sea level rise) typically have minimal impact on the
catchment flooding given the fact that the culvert under David Street is acting as a
control.

• Increase in rainfall intensity due to climate change may exacerbate the overland
flooding and would increase the level in the approved golf course area.

• Changes in roughness or antecedent conditions of the catchment (wet/dry catchment
leading to varying losses) could have minor to moderate impacts on the overland
flooding.

• Blockages of structures can have moderate impact in areas with no gravity flow that
mainly rely on the drainage network (e.g. ponding area such as Rushby Dr) and
maintaining the pits and pipes network is essential to avoid exacerbating the flooding in
such location.

The above results allowed the definition of the flood hazard (including provisional hazard and
flood life hazard categories) and hydraulic categories in the four catchments. These have
been created and mapped to inform development control planning.

Results of the model allow the identification of main flooding areas, key infrastructure
impacted by flooding and road closures around the catchments. Key infrastructure typically
may have access issues during severe flood events rather than flooding issues, except
during the PMF event.

Similarly, road closures predominantly occur on secondary roads with most of the major road
closures occurring for the PMF only. It is also noted that given the flashy behaviour of the
catchment, flooding and road closure in the upper catchment would be of relatively short
duration while flooding of the areas surrounding the golf course area may last for several
hours.

© Crown 2020 MHL2714 – ii

A preliminary flood damage assessment was also completed, and it was found that close to
100 properties are impacted by the 1% AEP flood event (including 10 over floor) and 266
properties are impacted by the PMF extent (including 154 over floor). The key flood issue
areas include the Oceanic Pl and Sunrise Cl, the properties along the creek at the southern
end of George St, the overland flow path leading from Joel Dr to the creek via Cross St and
Clerke St and some overland paths on the northern half of the Ocean Link Estate.

The results were also utilised to guide planning and emergency response by providing the
flood planning area and preliminary emergency response classifications mapping to assist
NSW SES during flood event. The majority of the properties are located in areas where
evacuation by car or by foot is possible or where flooding does not occur, but access is cut.

Option assessment and draft Floodplain Risk Management Plan (FRMP)

The draft Racecourse Creek FRMP is presented in Figure ES.1. The recommended
measures have been selected from a range of available options, after an assessment of the
impacts on flooding, as well as economic, environmental and social considerations.

The recommended measures are summarised below:
Flood modification measures

• Construction of Detention Basin / Artificial Wetland downstream of Rushby Dr

• Removal of existing GPT structure from within the creek
Property modification measures

• Prepare Council’s flood-proofing Guidelines as suggested; prepare a one-page,
graphic summary of the Guidelines

• Review and adopt the revised flood risk management provisions of MidCoast DCP
including freeboards for the study area

Response modification measures
• Improve emergency response planning:

- Update Local Flood Sub-Plan in view of the flood risk information in the Racecourse
Creek Flood Study and Option Assessment;

- Encourage and assist key floodplain community members who are likely to be
impacted by flooding to prepare and update their own flood emergency plans

• Improve flood warning system:

- By installing a new real-time rain gauge in the vicinity of Racecourse Creek

- Transition towards a system where people living or working in the floodplain can
stay informed via a web portal that allows access to data.

- Devise appropriate messages to accompany the rainfall alerts

• Flood Education:

- Develop a library or mobile display using historical flood photos, modelled flood
extents and appropriate messaging;

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- Develop an accessible flood emergency plan template suitable for use by
Racecourse Creek businesses, in conjunction with the MidCoast Business
Chamber;

- Conduct ‘meet-the-street’ type events for residents at the two locations listed above
(NSW SES);

- Engage with students at the local school to help them understand flood behaviour
near the school and to promote safe responses, including not to play in flooded
creeks and drains (Council in collaboration with NSW SES);

- Install flood depth indicators at low-points of Rushby Drive;

- Install signage in any detention basins such as the approved golf course where
flooding could pond to dangerous depths.

Funding

The total capital cost of implementing the Plan is about $2.4M, comprised mainly of the two
flood modification measures. Construction of the bioretention basin downstream of Rushby
Dr would have a Benefit Cost Ratio (BCR) of 0.34 while the GPT removal would have a BCR
of 0.18.

© Crown 2020 MHL2714 – iv

Figure ES.1 – Recom

© Crown 2020 MHL2714 – v

mmended measures

Contents

EXECUTIVE SUMMARY I

CONTENTS VI

1. INTRODUCTION 1
2
1.1 Scope and objectives
3
2. BACKGROUND 3
3
2.1 Study area 5
5
2.2 Previous studies 7
10
2.3 Discussion of relevant policies, legislation and guidance 15
15
2.3.1 National Provisions 15
2.3.2 State Provisions 15
2.3.3 Local Provisions 15

2.4 Flooding behaviour 16

2.4.1 Mainstream flooding 16
2.4.2 Overland flooding 16
2.4.3 Coastal inundation 16
2.4.4 Observed flood prone areas 16
18
3. AVAILABLE DATA 19
19
3.1 Historic data
20
3.2 Rainfall and water level data
20
3.2.1 Water level data 20
3.2.2 Rainfall Data
3.2.3 “At-Site” IFD Comparison 21
21
3.3 Topographic, aerial, and imagery 21
21
3.4 Pits and pipes 21
23
4. COMMUNITY AND STAKEHOLDER CONSULTATION 23
24
4.1 Consultation via Council 24
25
4.2 Local community 26
26
5. HYDROLOGICAL ANALYSIS 26
26
5.1 Hydrologic behaviour 28

5.2 Model selection 30
30
5.3 Model setup 31

5.3.1 Catchment delineation
5.3.2 Spatial patterns
5.3.3 Basin and entrance representation
5.3.4 Rainfall

5.4 Design events

5.5 Probable Maximum Flood (PMF) event

5.6 Parameter selection

5.6.1 Impervious areas
5.6.2 Losses
5.6.3 Lag

5.7 Critical duration

6. HYDRAULIC ANALYSIS

6.1 Model selection

6.2 Model setup

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6.2.1 Grid size 31
6.2.2 Digital elevation model 31
6.2.3 Modelling approach 33
6.2.4 Hydraulic roughness 33
6.2.5 Structures 35
6.2.6 Boundary conditions 35
6.2.7 Entrance behaviour 36
6.2.8 Blockages 38

6.3 Model assumptions and limitations 39

7. MODEL CALIBRATION AND VALIDATION 40

7.1 Methodology 40

7.2 Calibration results 40

8. MODEL SENSITIVITY 42

8.1 Entrance breakout behaviour 42

8.2 Losses and roughness sensitivity analysis 42

8.3 Blockage sensitivity analysis 43

9. FLOOD MODELLING RESULTS 44

9.1 Flood modelling description 44

9.2 Flood mapping 44

9.2.1 Mapping filtering 44
9.2.2 Flood maps 44

9.3 Flood levels in the golf course basin 44

10. POST-PROCESSING OF RESULTS 45

10.1 Preamble 45

10.2 Flood hazard 45

10.3 Flood function (or hydraulic categorisation) 46

11. CONSEQUENCES OF FLOODING ON THE COMMUNITY 48

11.1 Preamble 48

11.2 Flood behaviour 48

11.3 Key infrastructures 49

11.4 Road closure 50

11.5 Property flooding over floor 50

12. INFORMATION TO SUPPORT DECISIONS ON ACTIVITIES IN THE FLOODPLAIN AND MANAGING

FLOOD RISK 54

12.1 Flood emergency response classification 54

12.2 Flood planning area 54

12.3 Land-use planning considerations 55

12.4 Duration of flooding and time to peak 56

13. PRELIMINARY MANAGEMENT OPTIONS ASSESSMENT 57

13.1 Identification of potential management options 57

13.2 Options selected for preliminary modelling 57

13.3 Preliminary options modelling results 58

14. FLOOD DAMAGE ASSESSMENT 60

14.1 General approach 60

14.2 Floor level survey 60

14.2.1 Floor survey criteria 60
14.2.2 Survey methodology 60

14.3 Type of flood damage 60

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14.4 Basis of flood damages calculations 61
14.5 Economic analysis 63
14.6 Summary of flood damages 65

15. FLOOD MODIFICATION OPTIONS 68

15.1 Introduction 68
15.2 Structural options 68
68
15.2.1 Detention basin 69
15.2.2 Bridge upgrade 70
15.3 Creek modification options 70
15.3.1 Creek bank regrading 71
15.3.2 Removal of GPT 72
15.4 Combination option 72
15.5 Discussion on impact of options on creek entrance scour 73
15.6 Multi-criteria analysis 76
15.7 Summary 76
15.8 Recommendations
78
16. PROPERTY MODIFICATION OPTIONS 78
16.1 Voluntary House Purchase (VP) 79
16.2 Voluntary House Raising (VHR) or Redevelopment 81
16.3 Flood-proofing 82
16.4 Advice on land-use planning 82
16.4.1 Zoning suitability 84
16.4.2 General land use considerations
85
17. RESPONSE MODIFICATION OPTIONS
85
17.1 Flood Warning Systems 85
17.1.1 General 86
17.1.2 Evaluation 87
87
17.2 Emergency Response Planning 87
17.2.1 Prepare Local Flood Sub-Plan 88
17.2.2 Prepare and update private flood plans 88
89
17.3 Flood Education 89
17.3.1 General
17.3.2 Messages 92
17.3.3 Methods 92
92
18. IMPLICATION OF CLIMATE CHANGE 92
18.1 Climate Change Impacts Relevant to Flood Risk 93
18.1.1 Sea Level Rise 93
18.1.2 Frequency and Intensity of Heavy Rainfall Events 96
18.1.3 MidCoast Council Approaches 98
18.2 Impact of Climate Change on Local Flood Behaviour and Impacts 98
18.3 Influence on Flood Modification Options
18.4 Influence on Property Modification Options 99
18.5 Influence on Response Modification Options 99
99
19. WATER QUALITY ANALYSIS 100
19.1 Introduction 100
19.2 Assessment Methodology 103
19.3 MUSIC Modelling
19.3.1 Modelling Parameters
19.3.2 Treatment Measures

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19.3.3 MUSIC Model Network 106
19.3.4 Results 107
20. FLOODPLAIN MANAGEMENT PLAN
20.1 Objective 108
20.2 Recommended Measures 108
21. PEER REVIEW 108
REFERENCES
112
CALIBRATION RESULTS 114
SENSITIVITY RESULTS
DESIGN MAPS
FLOOD HAZARD
HYDRAULIC CATEGORIES
EMERGENCY RESPONSE CLASSIFICATION
FLOOD PLANNING AREA
PRELIMINARY MANAGEMENT OPTIONS RESULTS
FLOOD DAMAGE ASSESSMENT
CLIMATE CHANGE SENSITIVITY
TIME TO PEAK AND DURATION OF FLOODING

TABLES

Table 5-1 – Golf Course Basin details and discharge curve 23

Table 5-2 – Design Event Terminology as per AR&R 2019 24

Table 5-3 – GSDM summary for Racecourse Creek Catchment 25

Table 5-4 – Adopted impervious area percentage per sub-catchment 26

Table 5-5 – Critical durations for each design event 28

Table 6-1 – Adopted Manning’s ‘n’ Hydraulic Roughness Coefficients 35

Table 6-2 – Combination of catchment flooding and oceanic inundation scenarios 36

Table 6-3 – Summary of peak design levels for various categories and locations 36

Table 9-1 – Summary of Peak Flood Levels in the Golf Course basin 44

Table 10-1 – Hydraulic category criteria 47

Table 11-1 – List of Key Infrastructures 49

Table 11-2 – Peak depth, duration of flooding over 0.3m and time to depth above 0.3m at

road closure location 52

Table 14-1 – Input variables for residential damages assessment 62

Table 14-2 – Summary of flood damage by design event for Racecourse Creek catchment 66

Table 14-3 – Components of flood damage for Racecourse Creek catchment to floor level

(AAD) 67

Table 15-1 – Option assessment criteria 74

Table 15-2 – Preliminary option assessment matrix shown from highest to lowest ranking 75

Table 15-3 – Flood modification options BCR assessment & premises protected 77

Table 16-1 – List of potential candidates for VHR or flood-proofing 81

Table 17-1 – Bureau of Meteorology warning services of potential benefit in flash flood

catchments 86

Table 18-1 – Number of residential and non-residential buildings affected by above floor

depth in a 1% AEP event (with approved Golf Course) 95

Table 18-2 – Flood modification options BCR assessment & premises protected for the 1%

AEP under 2100 climate change conditions (30% increase in rainfall intensity and 0.9m

sea level rise) 97

Table 19-1 – Reduction Rate Targets for Water Quality Treatment 99

© Crown 2020 MHL2714 – ix

Table 19-2 – Meteorological data used in MUSIC modelling 101
Table 19-3 – Catchment Definition 102
Table 19-4 – Rainfall Runoff Parameters 103
Table 19-5 – Pollutant Concentration Parameters 103
Table 19-6 – Biofiltration Properties 105
Table 19-7 – Treatment Train Effectiveness - Receiving Node 107
Table 20-1 – Draft Racecourse Creek Floodplain Risk Management Plan 109
Table 21-1 Peer Review findings 112

FIGURES

Figure 2.1 – Location map 4

Figure 2.2 – Extract from Greater Taree LEP 2010 Clause 7.2 11

Figure 3.1 – Rainfall data around the Old Bar area from BoM 17

Figure 3.2 – Comparison of “At Site” IFD and 2016 IFD from BoM 18

Figure 3.3 – Comparison of 1987 IFD and 2016 IFD from BoM 19

Figure 5.1 – Hydrological model catchment delineation 22

Figure 5.2 – Land Uses in Racecourse Creek Catchment 27

Figure 5.3 – Critical duration and design storm selection 29

Figure 6.1 – Digital elevation model 32

Figure 6.2 – Roughness applied in hydraulic model 34

Figure 6.3 – Structures and boundary conditions 37
Figure 7.1 – Rainfall hyetograph for Old Bar June 7th-8th
40

Figure 7.2 – June 2007 Observed Flood Marks 41

Figure 10.1 – General flood hazard vulnerability curves 45

Figure 11.1 – Road Closure Map 51

Figure 11.2 – Property flooding above floor level 53

Figure 14.1 – Types of flood damage 62

Figure 14.2 – Randomly occurring flood damage as annual average damage 65

Figure 14.3 – Example of AAD estimation based on events annual exceedance probability

and associated damages at Racecourse Creek 65

Figure 15.1 – Conceptual model of detention basin between Rushby Dr and Racecourse

Creek (Option 10) 69

Figure 15.2 – Conceptual model of David Street bridge upgrade (Option 1) 70

Figure 15.3 – Conceptual model of creek banks regrading (Option 4) 71

Figure 15.4 – Conceptual model of GPT removal (Option 9) 72

Figure 16.1 – Land use zoning within PMF and FPA 83

Figure 18.1 – Depths of above floor inundation in 1% AEP event, residential sector 95

Figure 18.2 – Depths of above floor inundation in 1% AEP event, non-residential sector 95

Figure 19.1 – Sub-catchment Boundaries 100

Figure 19.2 – Rainfall & PET Data Presented via Meteorological Template Builder 101

Figure 19.3 – Typical Biofiltration Basin Cross-section 105

Figure 19.4 – MUSIC Model Network for the Study Area 106

Figure 20.1 – Recommended measures 111

© Crown 2020 MHL2714 – x

1. Introduction

The Racecourse Creek Flood Study and Options Assessment has been prepared in
accordance with the New South Wales Government’s Floodplain Development Manual
(2005). The manual guides implementation of the NSW Government’s Flood Prone Land
Policy (2005), the primary objective of which is to:

“reduce the impact of flooding and flood liability on individual owners and occupiers of
flood prone property, and to reduce private and public losses resulting from floods,
utilising ecologically positive methods wherever possible.”

Under the policy, primary responsibility for floodplain risk management rests with local
government. Financial and technical assistance is provided to councils by the NSW
Government’s Environment, Energy and Science group (EES).

Manly Hydraulic Laboratory (MHL) were engaged by MidCoast Council (Council) to
undertake this study. The outcome of the study is to develop and calibrate hydrologic and
hydraulic models for the estimation of overland and mainstream flood behaviour in the study
area, taking into account the performance of the storm-water drainage network including
quality and quantity of overflows from the drainage network into the Creek. The study outputs
can also inform decision making for investing in the floodplain; managing flood risk through
prevention, preparedness, response and recovery activities; pricing insurance, and informing
and educating the community on flood risk and response to floods. Each of these areas has
different user groups, whose needs vary. Meeting the requirements of the identified end user
groups, which have been tailored to the context of the flood situation, is a key objective of
this study.

The key end-user groups that this study aims to support are:

• High-level strategic decision makers

• Community

• Flood risk management professionals

• Engineers involved in designing, constructing and maintaining mitigation works

• Emergency management planners

• Land-use planners (strategic planning and planning controls)

• Hydrologists and meteorologists involved in flood prediction and forecasting

• Insurers

© Crown 2020 MHL2714 – 1

1.1 Scope and objectives

The objective of this study is to improve understanding of flood behaviour and impacts, and
better inform development strategies, management, and planning for flood risk in the study
area.

The key objectives of this study are to:

• improve understanding of flood behaviour combined with coastal processes

• detail flood impacts

• determine flood risk in the study area in consideration of the available information and
relevant standards and guidelines

• recommend prioritised management solutions to the identified risks with consideration
given to improved water quality outcomes

• detail how management solutions will impact the coastal process of Old Bar Beach

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2. Background

2.1 Study area

Racecourse Creek is located within the township of Old Bar, approximately 20km east of
Taree and 75km south of Port Macquarie. The Creek is located within a 3.2km2 catchment
and extends about a kilometre south-west from the township into a flood storage area
henceforth referred to as the “basin”. In the town, the Creek continues through some
constrictions until it reaches David Street, where it passes through a box culvert and does a
U-turn, moving back south-west parallel to the coastline before discharging over Old Bar
beach.

As recently as 10 years ago the discharge location of the Creek was 200m further down the
coastline where a gabion wall and geotextile mattress was built in 1992 to train the entrance
channel and prevent beach erosion due to stormwater discharge. However, in the interim
period, ongoing beach erosion has compromised the dune of the beach structure and
Racecourse Creek now discharges typically around 200m along Pacific Parade. Old Bar
beach has been identified as a NSW coastal erosion hotspot by EES.

There is little record of flooding on Racecourse Creek, however floods in March 2000 and
June 2007 were observed by locals and caused property inundation. Further, storm-water
discharge from the Creek has been attributed to beach erosion and used as justification for
capital works historically and so there is a public perception that the Creek adds to erosion
on the beach. Since the training wall was built in 1992 there have been a number of flood-
related studies, culminating in a floodplain risk-management study by Sinclair-Knight Merz
(SKM) in 2009.

A location map is presented in Figure 2.1.

2.2 Previous studies

Previous studies were reviewed and relevant information has been adopted in the
development of this flood study. The following studies have been reviewed and are
summarised below:

• Racecourse Creek Entrance at Old Bar – A Management Strategy (AWACS, 1991)

• South Old Bar Rezoning Flood Assessment (Bewsher Consulting, 1999)

• Old Bar/Wallabi Point Development Strategy (GTCC, 2001)

• Precinct 3 Old Bar Coastline Hazard Assessment (Worley Parsons, 2008)

• Precinct 3, Old Bar Provisional Floodplain Risk Management Plan (SKM, 2009)

• Erosion Analysis of the Manning Valley Coastal Sediment Compartment (MHL, 2017)

© Crown 2020 MHL2714 – 3

N

Threadfin Cres Forest Ln
Bluehaven Dr
Rosier Pl Breezeway
Albatross Wy

Flagtail Ave Yellowfin Ave
Greenbank Wy Mackerel Wy

Albatross Wy Trevally Ave
Mariner Ave
Albatross Wy

Saltwater Rd

Albatross Wy

© Crown 2020 / Aerial Imagery Source: NSW Land & Property Information

Joel Dr Connell St David St Figure 2.1
Smith St
Clerke St Location Map
Sunrise Cl
Legend

Study Area
Cadastre

Wyden St Cross St
Pryor Cres Oceanic Pl

Rushby Dr Hall St
Pacific Pde
CrGeeeokrge St Rose St
Racecourse
Lewis St

Pacific Ocean

Report MHL2714

Racecourse Creek Flood
Study and Options
Assessment

2.3 Discussion of relevant policies, legislation and guidance

Appropriate land use planning is one of the most effective measures available to floodplain
managers, especially to control future risk but also to reduce existing flood risks as
redevelopment occurs. The following sections discuss existing planning legislation and
policies that affect the development of land within the MidCoast Council Local Government
Area.

2.3.1 National Provisions
Australian Rainfall & Runoff, 2019

Australian Rainfall and Runoff (AR&R) is a national guideline document, data and software
suite that is used for the estimation of design flood characteristics in Australia. This is the 4th
edition of AR&R, after the 1st edition was released by Engineers Australia in 1958.
Geoscience Australia supports AR&R as part of its role to provide authoritative, independent
information and advice to the Australian Government and other stakeholders to support risk
mitigation and community resilience.

AR&R is pivotal to the safety and sustainability of Australian infrastructure, communities and
the environment. It is an important component in the provision of reliable and robust
estimates of flood risk. Consistent use of AR&R ensures that development does not occur in
high risk areas and that infrastructure is appropriately designed.

Building Code of Australia

The 2016 edition of the Building Code of Australia (BCA) introduced new requirements
related to building in Flood Hazard Areas (FHAs), which provide a minimum construction
standard across Australia for specified building classifications in FHAs up to the Defined
Flood Event (DFE).

The DFE is analogous to the planning flood event and is most commonly the 1% AEP
(Annual Exceedance Probability) flood. FHAs are defined in the BCA as encompassing land
lower than the flood hazard level (FHL), which in turn is defined as ‘the flood level used to
determine the height of floors in a building and represents the DFE plus the ‘freeboard’.
Therefore, FHAs would typically be defined as those areas falling within the flood planning
area.

Volume One, BP1.4 and Volume Two, P2.1.2 specify the Performance Requirements for the
construction of buildings in FHAs. They only apply to buildings or parts of buildings of
Classes 1, 2, 3, 4 (residential), 9a (health-care) and 9c (aged-care). These Performance
Requirements require a building in an FHA to be designed and constructed to resist flotation,
collapse and significant permanent movement resulting from flood actions during the DFE.
The actions and requirements to be considered to satisfy this performance requirement
include but are not limited to:

• Flood actions;

• Elevation requirements;

• Foundation and footing requirements;

• Requirements for enclosures below the flood hazard level;

© Crown 2020 MHL2714 – 5

• Requirements for structural connections;

• Material requirements;

• Requirements for utilities; and

• Requirements for occupant egress.

The Deemed-to-Satisfy (DTS) provisions of Volume One, B1.6 and Volume Two, 3.10.3.0
require buildings in the classes described above and located in FHAs to comply with the
ABCB Standard for Construction of Buildings in Flood Hazard Areas 2012 (the ABCB
Standard).

The ABCB Standard specifies detailed requirements for the construction of buildings to which
the BCA requirements apply, including:

• Resistance in the DFE to flood actions including hydrostatic actions, hydrodynamic
actions, debris actions, wave actions and erosion and scour;

• Floor height requirements, for example that the finished floor level of habitable rooms
must be above the Flood Hazard Level (FHL);

• The design of footing systems to prevent flotation, collapse or significant permanent
movement;

• The provision in any enclosures of openings to allow for automatic entry and exit of
floodwater for all floods up to the FHL;

• Ensuring that any attachments to the building are structurally adequate and do not
reduce the structural capacity of the building during the DFE;

• The use of flood-compatible structural materials below the FHL;

• The siting of electrical switches above the FHL, and flood proofing of electrical conduits
and cables installed below the FHL; and

• The design of balconies etc. to allow a person in the building to be rescued by
emergency services personnel, if rescue during a flood event up to the DFE is required.

Building Circular BS13-004 (NSW Department of Planning and Infrastructure, 2013)
summarises the scope of the BCA and how it relates to NSW planning arrangements. The
scope of the ABCB Standard does not include parts of FHA that are subject to flow velocities
exceeding 1.5 m/s or are subject to mudslide or landslide during periods of rainfall and runoff
or are subject to storm surge or coastal wave action.

It is particularly noted that the Standard applies only up to the DFE, which typically will
correspond to the level of the 1% AEP flood plus 0.5 m freeboard. The Building Circular
emphasises that because of the possibility of rarer floods, the BCA provisions do not fully
mitigate the risk to life from flooding.

The ABCB has also prepared an Information Handbook for the Construction of Buildings in
Flood Hazard Areas. This Handbook provides additional information relating to the
construction of buildings in FHA but is not mandatory or regulatory in nature.

In the NSW planning system, the BCA takes on importance for complying development
under the State Environmental Planning Policy (Exempt and Complying Development

© Crown 2020 MHL2714 – 6

Codes) 2008. Certain development on the floodplain is also required to satisfy the
requirements of the BCA under the Greater Taree Development Control Plan 2010 (currently
being revised). The Building Circular also indicates that following development approval, an
application for a construction certificate (CC) will require assessment of compliance with the
BCA.

2.3.2 State Provisions
Environmental Planning and Assessment Act 1979

General

The NSW Environmental Planning and Assessment Act 1979 (EP&A Act) creates the
mechanism for development assessment and determination by providing a legislative
framework for development and protection of the environment from adverse impacts arising
from development. The EP&A Act outlines the level of assessment required under State,
regional and local planning legislation and identifies the responsible assessing authority.

Prior to development taking place in NSW a formal assessment and determination must be
made of the proposed activity to ensure it complies with relevant planning controls and,
according to its nature and scale, conforms with the principles of environmentally sustainable
development.

Section 7.11 Development Contributions

Section 7.11 (previously Section 94) of the EP&A Act enables councils to collect
contributions from developers for the provision of infrastructure that is necessary as a
consequence of development. This can include roads, drainage, open space and community
facilities. Each council must develop a Section 94 Contributions Plan which demonstrates a
quantifiable link between the development intensification and the need for the additional
infrastructure as well as a detailed costing of such infrastructure and formulae to be used to
determine contributions from each type of development.

Section 9.1 Directions – Direction No. 4.3 (Flood Prone Land)

NSW flood-related planning requirements for local councils are set out in Ministerial Direction
No. 4.3 Flood Prone Land, issued in 2007 under the then Section 117 (now Section 9.1) of the
EP&A Act. It requires councils to ensure that development of flood prone land is consistent
with the NSW Government’s Flood Prone Land Policy as set out in the Floodplain Development
Manual (NSW Government, 2005). It requires provisions in a Local Environmental Plan on
flood prone land to be commensurate with the flood hazard of that land. In particular, a planning
proposal must not contain provisions that:

• Permit development in floodway areas;

• Permit development that will result in significant flood impacts to other properties;

• Permit a significant increase in the development of that land;

• Are likely to result in a substantially increased requirement for government spending
on flood mitigation measures, infrastructure or services; and

• Permit development to be carried out without development consent except for the
purposes of agriculture, roads or exempt development.

© Crown 2020 MHL2714 – 7

The Direction also requires that councils must not impose flood related development controls
above the residential flood planning level (FPL, typically the 1% AEP flood plus 0.5m
freeboard) for residential development on land, unless a relevant planning authority provides
‘adequate justification’ for those controls to the satisfaction of the Director-General.

Section 10.7 Planning Certificates

Planning certificates are a means of disclosing information about a parcel of land. Two types
of information are provided in planning certificates: information under Section 10.7(2) and
information under Section 10.7(5) of the EP&A Act. (Note that previously this clause was
Section 149).

A planning certificate under Section 10.7(2) discloses matters relating to the land, including
whether or not the land is affected by a policy that restricts the development of land. Those
policies can be based on identified hazard risks (Environmental Planning and Assessment
Regulation 2000, Clause 279 and Schedule 4 Clause 7), and whether development on the
land is subject to flood-related development controls (EP&A Regulation, Schedule 4 Clause
7A). If no flood-related development controls apply to the land (such as for residential
development in so-called ‘low’ risk areas above the FPL, unless ‘adequate justification’ has
been satisfied), information describing the flood affectation of the land would not be indicated
under Section 10.7(2). A lot that is a ‘flood control lot’ under the Codes SEPP is a prescribed
matter for the purpose of a certificate under section 10.7(2).

A planning certificate may also include information under Section 10.7(5). This allows a
council to provide advice on other relevant matters affecting land. This can include past,
current or future issues.

Inclusion of a planning certificate containing information prescribed under section 10.7(2) is a
mandatory part of the property conveyancing process in NSW. The conveyancing process
does not mandate the inclusion of information under section 10.7(5) but any purchaser may
request such information be provided, pending payment of a fee to the issuing council.

State Environmental Planning Policies (SEPPs)

SEPPs are the highest level of planning instrument and generally prevail over Local
Environmental Plans.

SEPP (Housing for Seniors or People with a Disability) 2004

State Environmental Planning Policy (Housing for Seniors or People with a Disability) 2004
aims to encourage the provision of housing (including residential care facilities) that will
increase the supply of residences that meet the needs of seniors or people with a disability.
This is achieved by setting aside local planning controls that would prevent such
development.

Clause 4(6) and Schedule 1 indicate that the policy does not apply to land identified in
another environmental planning instrument as being, amongst other descriptors, a floodway
or high flooding hazard.

SEPP (Infrastructure) 2007

State Environmental Planning Policy (Infrastructure) 2007 aims to facilitate the effective
delivery of infrastructure across the State by identifying development permissible without

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consent. SEPP (Infrastructure) 2007 allows Council to undertake stormwater and flood
mitigation work without development consent.

SEPP (Exempt and Complying Development Codes) 2008

A very important SEPP is State Environmental Planning Policy (Exempt and Complying
Development Codes) 2008, which defines development which is exempt from obtaining
development consent and other development which does not require development consent if
it complies with certain criteria.

Clause 1.5 of this ‘Codes’ SEPP defines a ‘flood control lot’ as ‘a lot to which flood related
development controls apply in respect of development for the purposes of dwelling houses,
dual occupancies, multi dwelling housing or residential flat buildings (other than development
for the purposes of group homes or seniors housing)’. These development controls may
apply through a LEP or DCP. Exempt development is not permitted on flood control lots but
some complying development is permitted.

Clause 3.5 states that complying development is permitted on flood control lots where a
Council or professional engineer can certify that the part of the lot proposed for development
is not a flood storage area, floodway area, flow path, high hazard area or high-risk area. The
Codes SEPP specifies various controls in relation to floor levels, flood compatible materials,
structural stability (up to the PMF if on-site refuge is proposed), flood affectation, safe
evacuation, car parking and driveways.

In addition, Clause 1.18(1)(c) of the Codes SEPP indicates that complying development must
meet the relevant provisions of the Building Code of Australia.

SEPP (Coastal Management) 2018

SEPP (Coastal Management) 2018 aims to promote an integrated and co-ordinated
approach to land use planning in the coastal zone. For areas mapped as ‘coastal wetland
and littoral rainforests’ – including sizeable areas in the study area near the three lakes –
development consent is required for the clearing of native vegetation, and for earthworks,
construction of a levee, draining the land and environmental protection works, and for any
other development. For areas mapped as ‘coastal environment areas’ – covering much of the
study area – development consent must not be granted unless the consent authority has
considered whether the proposed development is likely to cause an adverse impact on “the
integrity and resilience of the biophysical, hydrological (surface and groundwater) and
ecological environment” amongst other factors. The development must be designed, sited
and managed to either avoid, minimise or mitigate adverse impacts.

NSW Flood Related Manuals

Floodplain Development Manual, 2005

The Floodplain Development Manual 2005 (the Manual) was gazetted on 6 May 2005 and
relates to the development of flood liable land. It incorporates the NSW Flood Prone Land
Policy, which aims to reduce the impacts of flooding and flood liability on individual owners
and occupiers of flood prone property and to reduce private and public losses resulting from
floods, using ecologically positive methods wherever possible. To implement this policy and
achieve these objectives, the Manual espouses a merit approach for development decisions
in the floodplain, taking into account social, economic, ecological and flooding

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considerations. The Manual confirms that responsibility for management of flood risk remains
with local government. It assists councils in their management of the use and development of
flood prone land by providing guidance in the development and implementation of local
floodplain risk management plans.

At the time of preparing this report, the Floodplain Development Manual is being updated.

Guideline on Development Controls on Low Flood Risk Areas, 2007

The Guideline on Development Controls on Low Flood Risk Areas – Floodplain Development
Manual (the Guideline) was issued on 31 January 2007 as part of Planning Circular PS 07-
003 at the same time as the Section 117 (now Section 9.1) Directive described previously.
The Guideline is intended to be read as part of the Floodplain Development Manual.

It stipulates that unless there are exceptional circumstances, councils should adopt the 100-
year flood as the flood planning level (FPL) for residential development and that ‘unless there
are exceptional circumstances, councils should not impose flood related development
controls on residential development on land … that is above the residential FPL’.

An adequate freeboard should then be applied to the 100-year flood level to allow for
potential inaccuracies in available data and limitation of the flood models.

Flood related development controls are not defined but would include any development
standards relating to flooding applying to land, that are a matter for consideration under
Section 4.15 (previously Section 79C) of the EP&A Act.

The Guideline states that councils should not include a notation for residential development
on Section 10.7 (previously Section 149) certificates for land above the residential FPL if no
flood related development controls apply to the land. However, the Guideline does include
the reminder that councils can include ‘such other relevant factors affecting the land that the
council may be aware [of]’ under Section 10.7(5) of the EP&A Act.

In proposing a case for exceptional circumstances, a council would need to demonstrate that
a different FPL was required for the management of residential development due to local
flood behaviour, flood history, associated flood hazards or a particular historic flood.
Justification for exceptional circumstances would need to be agreed by relevant State
Government departments prior to exhibition of a draft local environmental plan or a draft
development control plan that proposes to introduce flood related development controls on
residential development above the default FPL.

At the time of preparing this report, the Guideline is being reviewed.

2.3.3 Local Provisions
In NSW, local government councils are responsible for managing flood risk within their Local
Government Areas (LGAs). A Local Environmental Plan (LEP) is used to establish what land
uses are permissible and/or prohibited on land within the LGA and sets out high level flood
planning objectives and requirements. A Development Control Plan (DCP) sets the
standards, controls and regulations that apply when carrying out development or building
work on land.

A merger between Greater Taree City Council, Great Lakes Council and Gloucester Shire
Council to form the MidCoast Council was announced in May 2016. At the time of preparing
this report (January 2020), development applications within the study area continue to be

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assessed based on the former Greater Taree City Council planning controls.

This section briefly describes and reviews the flood-related controls within the Greater Taree
policies, with a view to flood behaviour in the Racecourse Creek study area.

Greater Taree Local Environmental Plan 2010
Greater Taree Local Environmental Plan 2010 (Greater Taree LEP 2010) outlines the zoning
of land, what development is allowed in each land use zone and any special provisions
applying to land.

Flood planning is addressed in clauses 7.2. This clause is reproduced in Figure 2.2.
Clause 7.2 relates to land at or below the flood planning level (FPL), sometimes called the
‘flood planning area’. The FPL is defined in Greater Taree LEP 2010 as ‘the level of a 1:100
ARI (average recurrent interval) flood event plus 0.5 metre freeboard’.

The appropriateness of the existing Greater Taree LEP 2010 for managing flood risk in the
Racecourse Creek catchments is considered under the following headings:

• Flood planning area definition

• Evacuation challenges

Figure 2.2 – Extract from Greater Taree LEP 2010 Clause 7.2
Note: version dated 15 January 2019

Flood planning area definition

Flood planning levels (FPLs) and the flood planning area (FPA) are important tools in the
management of flood risk. The FPA is used to define the area where flood-related

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development controls apply. For those areas contained within the FPA, the FPLs are
frequently used to establish the elevation of critical components of a development, such as
minimum floor levels.

The FPL is typically derived by adding a freeboard to a specific design flood. This specific
design flood is frequently referred to as the “planning” flood. The freeboard is intended to
account for any uncertainties in the derivation of the planning flood level.

The adoption of the 1% AEP flood for setting the flood planning level (FPL) is considered
appropriate for the Racecourse Creek local catchments. A more frequent design flood would
expose communities to too great a risk, while a rarer event is not considered warranted given
the modest flood height range between the 1% AEP flood and the PMF across most of the
catchment.

Traditionally a 0.5 metre freeboard has been added to the planning flood to define the FPL
and this FPL has been extended laterally until it encounters higher ground to define the FPA.
While this approach is suitable for areas bound by higher ground (e.g., lakes, creek, rivers), it
is not necessarily appropriate for urban catchments where the FPL may not be contained by
higher ground.

In recognition of the challenges involved in mapping the FPA in an urban catchment, studies
for other catchments along the NSW coastline have defined the FPA by incorporating a
rainfall intensity increase to the 1% AEP event and using the inundation extent from this
simulation to define the FPA. The rainfall intensity increase serves as a factor or safety (i.e.,
freeboard), thereby incorporating an allowance for uncertainty while ensuring a hydraulically
realistic FPA is provided. For this study, a similar approach was adopted whereby the FPA
was defined by re-simulating the 1% AEP flood with a 30% increase in rainfall to account for
uncertainties.

For FPLs relevant to minimum floor levels, a variable freeboard (i.e. 0.3 metre freeboard
across the majority of the study area affected by overland flows with modest flood height
ranges, and 0.5 metre freeboard across localised areas and the lakes’ foreshores) may be
appropriate.

However, the model LEP clause taken up in Greater Taree LEP 2010 – stipulating only a 0.5
metres freeboard – does not allow this flexibility. As MidCoast Council consolidates the
Greater Taree, Great Lakes and Gloucester LEPs into a single instrument, and as it
considers the diversity of flood mechanisms across the LGA, it is possible that even more
flexibility will be considered appropriate to define flood planning areas. It is therefore
recommended that Council seek to amend the definition of flood planning level to cater for
flexible requirements. For example:

‘Flood planning level means the level of a 1:100 ARI (average recurrent interval) flood event
plus 0.5 metres freeboard, or other freeboard as determined in relevant studies and plans.’

Evacuation challenges

Flood modelling undertaken as part of this study identifies a number of features of flood
behaviour that indicate evacuation in advance of, or during, a flood is likely to be impractical,
and that on-site refuge may be an acceptable or safer emergency response:

• The worst flooding in these local catchments results from short storms (60 minutes)

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• Roads may be flooded only minutes after the commencement of a storm. As a result,
there is unlikely to be sufficient time to evacuate from parts of the catchments before
roadways are inundated

• Roadways may be impassable for a relatively short time, which means a limited period of
isolation

• Depths of inundation across most of the study area are typically shallow and the flood
hazard indicates that most buildings are unlikely to suffer structural damage.

Greater Taree Development Control Plan 2010
Supporting Greater Taree LEP 2010 is the Greater Taree Development Control Plan 2010
(Greater Taree DCP 2010), which at the time of writing continues to set the design and
construction standards that apply when carrying out development within the Racecourse
Creek study area.

This section considers controls that may be appropriate to manage overland flow inundation
risks in the Racecourse Creek catchment, which along with recommendations for similar
overland flow catchments, could be considered as a new MidCoast DCP is prepared.
Floor level

Given the modest flood height range, a freeboard of 0.3m rather than the normal 0.5m is
considered appropriate for setting the flood planning level (FPL) across the majority of the
study area. The FPL, in turn, sets minimum habitable floor levels for new dwellings.

Historically, concessions to floor level controls were sometimes permitted for commercial or
industrial land uses, reasoning that businesses have capacity to tolerate more risk. Recent
floods however have shown that flooding can cause severe damage to modern equipment
and to livelihoods that depend on that business, which argues against lower floor levels for
these uses.

Sensitive uses and critical infrastructure typically have the PMF level as the minimum
habitable floor level, which is considered appropriate.

Given the observation from past floods that significant damage to precious contents can
occur in garages, sheds or “storage areas”, it is also considered appropriate to set minimum
floor levels for non-habitable buildings or rooms. This could be to a lesser standard such as
the 5% AEP flood. For example:

Floor levels to be 300mm above the finished ground level or equal to or greater
than the 5% AEP flood level (whichever is higher).

Parts of the Racecourse Creek overland flow floodplains that are also subject to flooding
from rising water from the proposed golf course area acting as a basin should be subject to
the higher FPL that applies to the land.
Building components

It is considered appropriate that any part of buildings constructed below the FPL should be
installed with flood-compatible components. This is also consistent with the requirement in
the Codes SEPP.

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Structural soundness

It could be argued that in areas of shallow overland flow, a requirement to demonstrate the
structural soundness of a building is unnecessary. However, since such a provision is
contained in the Codes SEPP, it would be inconsistent to apply a lesser standard in the DCP
for land below the flood planning level.

Inundation effects

It is considered appropriate that new buildings should not worsen inundation on adjacent
properties. This also is consistent with a requirement in the Codes SEPP. However, there is
an argument for defining what constitutes a significant adverse flood impact (e.g. >20 mm
rise).

Car parking and driveway access

Car parking controls are important given the ease with which vehicles can become buoyant
and float and then become floating debris with potential to block culverts and pose
environmental hazards. Carport floor levels could arguably be set at the 5% AEP level or
300mm above the ground level, whichever is higher.

Driveway access controls are considered less critical (for single dwellings) for land subject to
short-lived, shallow overland flows since there may be negligible warning of floods, no
opportunity for safe evacuation, and relatively short durations of isolation—suggesting that
for this catchment, on-site refuge above the PMF may be safer than evacuation.

Evacuation

Given the impracticality and perhaps even the danger of evacuation – if flood conditions on
roads are worse than those encountered at a property – and the relatively short duration of
isolation – having controls for the Racecourse Creek overland flow catchments that require
evacuation may be inappropriate. In addition, the incremental difference in flood depths
between the 1% AEP event and the PMF suggest that requiring a proportion of floor space
within new dwellings to be above the PMF level to serve as an on-site refuge in extreme
floods is not essential at every location within this catchment. It would, however, be a
desirable feature, given the fickleness of human behaviours during floods, which could see
people get into difficulties if their houses commence to flood and result in a burden for
rescuers. The cost of providing a higher floor space may not be prohibitive, and would be a
sensible long-term resilience measure.

Fencing

Fencing can have a significant impact on overland flows. Ideally, it should not impede the
flow of floodwaters so as to result in additional flood impacts on surrounding land, and should
be able to withstand flooding or to collapse in a controlled manner to prevent a ‘wave’
causing additional problems downstream. Council could consider introducing specific
controls for fencing on land below the FPL, such as prohibiting brick/masonry fences (likely
to create impediments). It is recognised however that implementing fencing controls can be
difficult.

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2.4 Flooding behaviour

The Racecourse Creek catchment is susceptible to inundation from both the ocean and
stormwater. Flood behaviour exhibited within the study is summarised as follows.

2.4.1 Mainstream flooding
Mainstream flooding is a result of relatively high flows which overtop the natural or artificial
banks along any part of a watercourse (creeks, tributaries), lake, dam or lagoon. Floods in
the tributaries draining to the basin and across the township result from intense and short
duration storms, typically less than three hours. The small catchment area results in short
flood response time to rainfall. The short response time coupled with the confined nature of
the creek channel leads to spilling of floodwaters over the bank. Flooding of the basin region
results from rainfall of much longer durations, typically 12-36 hours or longer. The large area
of the basin requires a considerable volume of runoff to raise the water level.

2.4.2 Overland flooding
Overland flooding is caused by heavy rainfall flowing across the ground or overflowing pipes,
pits and gutters. It is inundation as a result of local runoff rather than inundation created by
overbank flows discharging from a watercourse, lake or dam. Local overland flooding is often
characterised by a rapid rise in flood levels, particularly where the local catchment is
relatively steep or small. The nature of the catchments within the study area are susceptible
to this type of flooding particularly in urbanised areas, on roads and through property.

2.4.3 Coastal inundation
Generally elevated ocean levels occur in combination with increased wave activity. The level
in the Creek could be raised as a result of wave runup and overtopping of the beach berm.
However, this action is also likely to lower the berm, reducing the water level in the Creek.
Furthermore, the extent of this inflow is likely to be short due to the perched nature of the
Creek and the flow-controlling culvert at David St.

2.4.4 Observed flood prone areas
Details of specific flood prone areas have been collated from the previous studies outlined in
Section 2.2 and from photographic accounts obtained as part of this study:

• Oceanic Place – This area may experience flooding as a result of overland flows
backing up against elevated creek levels. Ponding of water has been observed on the
road, sidewalk, in front yards, and in a few properties.

• George Street – Similar to Oceanic Place, George St becomes flooded when overland
flows have nowhere to drain to when the basin and creek are elevated due to long-
duration rainfall. Further, the western-most properties which back along the Creek can
be inundated from mainstream flooding alone.

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3. Available data

3.1 Historic data

Significant rainfall or storm events occurred around the study area on or around the following
dates:

• May 1883 • March-April 1963 • January 2006

• January 1895 • March 1976 • June & August
• December 1897 • March 1978 2007
• July & September • 1997
• March 2000 • April 2008
1922 • March & May 2001 • February 2009
• April 1927 • May 2003 • April 2014
• February 1929 • October & • January 2015
• January 1938 • February 2020
• February 1954 November 2004

Historical data was collated from various sources including news articles and historical
newspapers as well as the storm history from the Bureau of Meteorology (BoM) in the vicinity

of the study area.

3.2 Rainfall and water level data

3.2.1 Water level data
Limited quantitative flood data is available for the Racecourse Creek catchment. There is no
water level monitoring infrastructure installed on the Creek. For the June 2007 event, six
flood marks were recorded by the local community.

3.2.2 Rainfall Data
The three nearest rainfall monitoring location are the following gauges administered by the
Bureau of Meteorology (BoM):

• Station 060146 at Old Bar (Ondarro Crest) that includes daily-read data from May 2003
to present.

• Station 060030 at Taree (Patanga Cl) that includes daily-read data from October 1881
to present.

• Station 060141 at Taree Airport AWS that includes half-hourly data from July 1997 to
present, 1-minutes data from January 2010 to present and 6-min pluviograph (rainfall
intensity) data from May 2005 to October 2016 (45% capture).

Rainfall data are presented in Figure 3.1.

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Figure 3.1 – Rainfall data around the Old Bar area from BoM

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3.2.3 “At-Site” IFD Comparison
The 2016 Intensity-Frequency-Duration (IFD) curves from the Bureau of Meteorology were
compared to the IFD created using the “At Site” rainfall gauges available in the vicinity of the
study and the results are presented in Figure 3.2. The Taree Airport AWS (060030) had the
highest sampling frequency and was used to determine the “At Site” IFD for events up to the
5% AEP. The relatively short period of record did not allow estimates of rarer events. It is
noted that the 2016 and the “At Site” IFDs are typically consistent with the largest difference
observed for durations of less than 1 hour.

The 2016 IFD were also compared to the 1987 IFD and the results are presented in Figure
3.3. It is observed that the 2016 IFD rainfall intensities are consistent with the 1987 IFD for
frequent event and for long durations. The 2016 IFD rainfall intensities are higher than the
1987 IFD intensities from 10% AEP. This is particularly true for shorter durations.

The 2016 IFD were adopted for this study as per AR&R 2019 requirements.

Figure 3.2 – Comparison of “At Site” IFD and 2016 IFD from BoM

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Figure 3.3 – Comparison of 1987 IFD and 2016 IFD from BoM

3.3 Topographic, aerial, and imagery

Two digital elevation maps (DEMs) were used in this study: A 1m resolution grid produced
from LiDAR by NSW Spatial Services in August 2012 was used as the primary dataset, and
a 5m DEM produced from LiDAR by EES in 2017 was used to patch in recent topographic
changes at Ocean Links Estate and the entrance.

No bathymetric data was available for this study, however creek-bottom data was
interpolated from cross-sectional data in the 2009 flood study, see Section 2.2.

3.4 Pits and pipes

Council’s ‘pit and pipe’ GIS layers were reviewed for completeness and quality for flood
modelling purposes. The review identified missing data required for modelling the storm-
water network as follows:

• Pipe data including locations and pipe sizing was available for the majority of the
catchment.

• Locations of pits, culverts, headwalls, and gross pollutant traps (GPTs) were mostly
available, however invert levels and pit sizing were not present in the data provided.

• Ocean Links Estate did not have a spatial dataset for its pits and pipes.
MHL assumed a standard cover depth between 300 and 900mm for all provided pits using
standard pipe grading as a guide to ensure hydraulic continuity. All kerb-type pits were
assumed to be 1800mm wide and 200mm high.

Construction drawings for Ocean Links Estate, Forest Lane, and Rushby Drive were digitised
to produce spatial layers to be used for hydraulic modelling.

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4. Community and stakeholder consultation

4.1 Consultation via Council

MidCoast Council received a number of suggestions for flood management options from the
community (e.g. construction of a detention basin near Rushby Dr). These options were
incorporated into the management options assessment described in Sections 13 and 15.
A number of photographs from the 2007 event were also provided by Council to assist with
the calibration of the model at the commencement of the project.

4.2 Local community

During the site inspection, the project team met with a local resident that highlighted a
number of anecdotal information about flooding along the creek such as approximate level at
the property or impact of blockage at the David St culvert and described the vegetation along
the creek as a corridor heavily used by native birds and wildlife to cross old bar while being
sheltered.

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5. Hydrological analysis

A direct rainfall method was employed in this study. This method applies rainfall directly to
the 2D hydraulic model cells which then determine the quantity, direction and velocity of flow
on a per-grid basis using provided surface material and topographic information. Therefore,
development of a traditional hydrologic model was not required to complete the study.

Despite this lack of necessity, there are benefits which a hydrological model provides to flood
modelling which cannot yet be replicated by the 2D approach. These benefits include:

• Identification of critical design durations from the ensemble of events specified by
AR&R 2019

• Spatial and temporal pattern development for these design durations

• Validation of hydrodynamic model

• Rapid deployment and assessment for flood forecasting
Therefore, a hydrological model was developed as a part of this flood study.

5.1 Hydrologic behaviour

Based on the previous XP-RAFTS model, it was anticipated that two critical durations would
need to be selected: a longer duration for the large detention basin which acts as flood
storage for the majority of the catchment and one shorter duration for Old Bar township. The
basin has a much longer critical duration for all design events compared to the township
catchment. Therefore, design mapping in this study represents an envelope of the
maximums of results of modelling each duration.

5.2 Model selection

The hydrological model selected for this study is WBNM (version 2017). This model is very
robust and has been validated against numerous catchments in NSW.

Moreover, this version of the model has been developed to include the 2016 Intensity-
Frequency-Duration (IFD) diagrams that are at the basis of the AR&R 2019 guideline
requirements.

5.3 Model setup

5.3.1 Catchment delineation
Catchments were produced from LiDAR data and validated against the previous model from
SKM. Due to the small catchment size and limited amount of developments in the upstream
catchments, the catchment behaviour was able to be simulated using a small number of sub-
catchments. Figure 5.1 shows the hydrological model conceptual layout.

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N

© Crown 2020 / Aerial Imagery Source: NSW Land & Property Information

Figure 5.1

Hydrological
model catchment
delineation

Legend

Catchment
Sub-Catchments
Wetland area
Hydrologic flowpaths

Elevation (m AHD)

0
7.5
15
22.5
30

Report MHL2714

Racecourse Creek Flood
Study and Options
Assessment

5.3.2 Spatial patterns
AR&R 2019 guidelines mention that catchments with areas up to and including 20 km2 are
sufficiently small that there is little available data to derive a spatial pattern. For these
catchments, it is usually acceptable to adopt a uniform spatial pattern unless there is
sufficient density of rainfall information to derive alternative (non-uniform) design spatial
patterns. Since the total catchment area is only 3km2, a single uniform spatial pattern was
adopted for all design events.

5.3.3 Basin and entrance representation

The existing wetland area presented in Figure 5.1 acts as a detention basin. This area was
modelled as an outlet basin in WBNM in sub-catchment 2. This approach allowed the
storage volume within the wetland to be considered prior to the basin spilling into
Racecourse Creek.

Storage vs. volume information was determined from the DEM and validated against the
previous model. Initially, the discharge ratings were adopted as per the XP-RAFTS model,
however it was found that the resulting outflows were significantly higher than those from
preliminary hydraulic analysis. Therefore, a new height vs. flow relationship was developed
from the calibrated hydraulic model to more accurately represent the true basin behaviour.

The discharge curve of the basin is described in Table 5-1.

The ocean entrance was modelled both as an open channel and as a scourable weir in
WBNM. Due to the flashy nature of this sub-catchment it was found that the critical duration
at the ocean entrance did not differ depending on whether the channel entrance was open or
closed due to David Street acting as a control. Therefore, an open entrance was adopted for
hydrological design to simplify the outputs and keep the model consistent with previous work.

Table 5-1 – Golf Course Basin details and discharge curve

Elevation (m) Storage (thousands m3) Discharge (m3/s)

4.1 0.000 0.00

4.2 0.786 0.10

4.9 81.022 0.10

5 102.354 2.00

5.1 125.493 2.64

5.2 150.113 3.60

5.3 175.916 5.49

5.4 202.469 8.79

5.5 229.482 13.70

5.6 256.780 19.00

5.7 284.300 25.68

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5.3.4 Rainfall
The 2016 IFDs from the Bureau of Meteorology were used as input for this study. The
procedure described in AR&R 2019 was applied to determine the appropriate temporal
pattern and duration to use in the flood modelling.

5.4 Design events

The design events modelled in this study include:

• Frequent events - 20% AEP and 10% AEP

• Rare events - 5% AEP, 2% AEP and 1% AEP

• Very rare events - 1 in 500 AEP

• Extreme event - Probable Maximum Flood (PMF)

The terminology of these events is defined as per the AR&R 2019 guidelines presented in
Table 5-2. All events but the PMF events use spatial and temporal patterns provided by the
AR&R 2019 Data Hub.

Table 5-2 – Design Event Terminology as per AR&R 2019

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5.5 Probable Maximum Flood (PMF) event

The Probable Maximum Precipitation (PMP) rainfall depth has been estimated using the
Generalised Short Duration Method (GSDM) derived by the Bureau of Meteorology.
Durations of up to 6-hours have been considered for the PMP in accordance with the GSDM.
A summary of the GSDM approach is provided in Table 5-3.

Table 5-3 – GSDM summary for Racecourse Creek Catchment

The temporal patterns used to derive the probable maximum flood (PMF) should be selected
from an ensemble of patterns appropriate for use with the Generalised Probable Maximum
Precipitation (PMP).

At present, the best source of ensemble temporal patterns for use with short duration Very
Rare to Extreme events are those derived by Jordan et al (2005); these patterns were
derived specifically from storms associated with thunderstorm or deeply convective events.

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These ten patterns were therefore adopted in this study and applied to the calculated PMP
rainfall depth. The critical pattern was determined as per the typical AR&R 2019 guidelines
applied to the other design events.

5.6 Parameter selection

Parameters required by the WBNM model include sub-catchment area and linkage, pervious
and impervious percentage, runoff lag factor, stream routing lag factor, rainfall input, initial
losses and continuing losses. Adopted parameters were assigned based on spatial data, XP-
RAFTS model parameters, and calibration of results. Each parameter is also detailed in this
section.

5.6.1 Impervious areas

Impervious areas were derived by adopting impervious percentages for various land uses
defined by Councils GIS layer for LEP zoning. A land use map is presented in Figure 5.2
and additional information on land use is provided in Section 16.4. Based on land use
areas, a weighted average was calculated for each sub-catchment. The residential areas
were assumed to be 60% impervious, roadway corridors 90% and wet areas/basins 100%
impervious while other areas were assumed as 0% impervious. Table 5-4 summarises the
percentage imperviousness used for each sub-catchment as per Figure 5.1.

Table 5-4 – Adopted impervious area percentage per sub-catchment

Sub-Catchment Percentage Impervious
1 56.5%
2 4.9%
3 18.5%
4 10.2%
5 4.6%

5.6.2 Losses

For the initial and continuing rainfall losses, values of 9.1mm and 2.9mm/hr were used for
pervious areas and 1 mm and 0 mm/hr for impervious areas. These values were selected
based on the AR&R 2019 recommended values. Initial loss value for the calibration of 15mm
was adopted from previous XP-RAFTS model.

The initial conditions for calibration and design vary due to different antecedent conditions.

5.6.3 Lag
A lag parameter of C = 1.6 was adopted for the WBNM model. This value is recommended
for use by WBNM mentioning that typical values range between 1.3 and 1.8 with an average
value of 1.6. It is also the recommended value for use on ungauged catchments for NSW
(Boyd and Bodhinayake 2006).

A stream lag routing Type R with a value of 1 was adopted. This is the recommended natural
channel routing value.

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N

© Crown 2020 / Aerial Imagery Source: NSW Land & Property Information

Figure 5.2

Land Uses in
Racecourse Creek
Catchment

Legend

Study Area
Cadastre

Land Use

B1 - Neighbourhood Centre
E2 - Environmental
Conservation
E3 - Environmental
Management
IN2 - Light Industrial
R1 - General Residential
R5 - Large Lot Residential
RE1 - Public Recreation
RE2 - Private Recreation
RU1 - Primary Production
RU3 - Forestry
SP2 - Infrastructure

Report MHL2714

Racecourse Creek Flood
Study and Options
Assessment

5.7 Critical duration

For each design AEP event, 24 different durations were modelled ranging from 10 minutes to
168 hours, except for the PMF which had eight durations up to 6 hours. Within each duration,
10 specific rainfall events were modelled (as recommended in AR&R 2019) which varied the
rainfall temporal pattern, though not the magnitude, over that period. This led to 240
individually modelled rainfall events per design intensity which were then analysed to pick the
one event to use as design rainfall.

Critical durations were selected based on the methodology described in AR&R 2019. This
methodology consists of selecting, for each duration, the rainfall temporal pattern that is the
closest to the average flow obtained from the 10 specific patterns provided in the AR&R 2019
database. This provides an automated approached that can then be adjusted for consistency
in durations between the various events. Figure 5.3 presents box-plots for the 1% AEP at
both basin and township locations. Adopted critical durations and temporal pattern (from
AR&R 2019 Data Hub) for each event are presented in Table 5-5. The design storm for each
AEP is the rainfall event which results in the median flow for the critical duration (Figure
5.3c).

The selection of the critical duration for the basin was based on the peak flow out of the sub-
catchment rather than the peak inflow into the basin. This approach was adopted to consider
the significant attenuating effect of the storage on flows through catchment. Since the
outflows from the basin are directly proportional to its water level, this approach is equivalent
to considering the peak water level in the basin.

Table 5-5 – Critical durations for each design event

Catchment Event Adopted Critical Adopted Temporal
Duration Pattern from AR&R

2019 Data Hub

20% AEP 48 hr 4949

10% AEP 36 hr 4803

5% AEP 36 hr 4915

Basin 2% AEP 36 hr 4688

1% AEP 36 hr 4908

1 in 500 AEP 12 hr 4747
PMF 4 hr BR97*

20% AEP 20 min 4451

10% AEP 20 min 4440
5% AEP 20 min 4440

Township 2% AEP 20 min 4404

1% AEP 20 min 4404

1 in 500 AEP 20 min 4404

PMF 30 min DA74*

*Temporal pattern ID from Jordan et al. (2005) rather than the data hub as recommended for
PMF analysis in AR&R 2019

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Figure 5.3 – Critical duration and design storm selection

(a) Top-Left. Temporal pattern distribution for the 1% AEP event at the
basin. NB: 36-hour duration was selected for this AEP despite 12-hour
duration being highlighted following testing of the critical pattern of
the various duration in the TUFLOW model.

(b) Top-Right. Temporal pattern distribution for the 1% AEP event at
the township (sub-catchment 1).

(c) Bottom-Right. Design storm selection from 10 temporal patterns for
1% AEP, 36-hour storm.

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MHL271

14 – 29

6. Hydraulic analysis

6.1 Model selection

TUFLOW HPC has been used for hydraulic modelling in this study to simulate flood
behaviour across the study area. TUFLOW is robust and widely accepted unsteady-state
flood simulation software with combined 1D and 2D capabilities. The use of a TUFLOW
model allows integrated investigation of local overland flow flooding, mainstream creek
flooding, foreshore flooding and tidal influences, and the inclusion of storm-water drainage
infrastructure.

The GIS data layers and control files used to drive the model can be easily modified for use
in the options assessment, including modelling the impact of mitigation measures, or
assessment of development applications. MHL flood modelling processes follow guidance
provided in AR&R 2019.

The dynamically linked 1D/2D model requires a number of GIS data layers to represent the
study area. These include, for example:

• 1D Domain
- Pits & headwalls GIS layer

- Pipe network GIS layer

- Culverts GIS layer
• 2D Domain

- 2D grid / digital elevation model (DEM)

- Topographic modifications and break lines (e.g. to incorporate channel cross-
sections)

- Materials layer (specifies surface roughness and infiltration)

- Rainfall on the grid

- Layered flow constrictions layer for 2D bridges

- Tidal boundary condition

- Entrance berm conditions

- Initial water level polygons

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