Wollongong City Flood Study WMAwater 111022:WollongongCBD_FloodStudy_Final:17 April 2013 1 1. INTRODUCTION 1.1. Background The Wollongong City catchment area includes the suburbs of Wollongong (south of Crown Street), Coniston, and northern Port Kembla, and drains towards Wollongong Golf Course and JJ Kelly Park, ultimately discharging into Port Kembla Inner Harbour via the Gurungaty Waterway and Tom Thumb Lagoon. There have been several recorded instances of flood- producing storms in the catchment, including March 1978, March 1983, December 1990, March 1995, March 2011 and February 2012. A previous Flood Study of the lower catchment area, including modelling of open channel areas of Wollongong Golf Course and JJ Kelly Park, was undertaken in 1990 by GHD on behalf of the Water Board (Reference 3). The study did not estimate flood behaviour in residential areas of the catchment. The area was included in a study on the August 1998 storms that affected the greater Wollongong region (Reference 4), which found that rainfalls near the coast at Wollongong City were far less severe than those that fell higher on the Illawarra Escarpment. The present Flood Study has been commissioned by Wollongong City Council (WCC), with assistance from the NSW Office of Environment and Heritage (OEH). This study considers flooding in the entire Wollongong City catchment from local storm runoff, as well as backwater flooding from tidal influences in Port Kembla Inner Harbour. The Wollongong City catchment has a population of approximately 10,000 with strong population growth in the post-war years, and a stabilisation since the early 1990s. Continued growth may result in increased capitalisation of existing urban areas and re-development with more high-density housing, some of which is likely to be in flood-liable areas. It is therefore important that appropriate tools and information to assess flood risk are available to Wollongong City Council for planning future development in the area. 1.2. Objectives The key objective of this Flood Study is to develop a suitable hydraulic model that can be used as the basis for a Floodplain Risk Management Plan for the study area, which will assist Wollongong City Council to undertake flood-related planning decisions for existing and future developments. Previous hydraulic modelling of the study area was limited in extent, and did not estimate flood levels or flows in residential areas of the catchment. The primary objectives of the study are: • to determine the flood behaviour including design flood levels and velocities over the full range of flooding up to and including the PMF from storm runoff in the Wollongong City Catchment and from tidal influences; • to provide a model that can establish the effects on flood behaviour of future development;
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1. INTRODUCTION 1.1. Background - Wollongong · INTRODUCTION 1.1. Background The Wollongong City catchment area includes the suburbs of Wollongong (south of Crown Street), Coniston,
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Wollongong City Flood Study
WMAwater 111022:WollongongCBD_FloodStudy_Final:17 April 2013
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1. INTRODUCTION
1.1. Background
The Wollongong City catchment area includes the suburbs of Wollongong (south of Crown
Street), Coniston, and northern Port Kembla, and drains towards Wollongong Golf Course and
JJ Kelly Park, ultimately discharging into Port Kembla Inner Harbour via the Gurungaty
Waterway and Tom Thumb Lagoon. There have been several recorded instances of flood-
producing storms in the catchment, including March 1978, March 1983, December 1990, March
1995, March 2011 and February 2012.
A previous Flood Study of the lower catchment area, including modelling of open channel areas
of Wollongong Golf Course and JJ Kelly Park, was undertaken in 1990 by GHD on behalf of the
Water Board (Reference 3). The study did not estimate flood behaviour in residential areas of
the catchment. The area was included in a study on the August 1998 storms that affected the
greater Wollongong region (Reference 4), which found that rainfalls near the coast at
Wollongong City were far less severe than those that fell higher on the Illawarra Escarpment.
The present Flood Study has been commissioned by Wollongong City Council (WCC), with
assistance from the NSW Office of Environment and Heritage (OEH). This study considers
flooding in the entire Wollongong City catchment from local storm runoff, as well as backwater
flooding from tidal influences in Port Kembla Inner Harbour.
The Wollongong City catchment has a population of approximately 10,000 with strong
population growth in the post-war years, and a stabilisation since the early 1990s. Continued
growth may result in increased capitalisation of existing urban areas and re-development with
more high-density housing, some of which is likely to be in flood-liable areas. It is therefore
important that appropriate tools and information to assess flood risk are available to Wollongong
City Council for planning future development in the area.
1.2. Objectives
The key objective of this Flood Study is to develop a suitable hydraulic model that can be used
as the basis for a Floodplain Risk Management Plan for the study area, which will assist
Wollongong City Council to undertake flood-related planning decisions for existing and future
developments. Previous hydraulic modelling of the study area was limited in extent, and did not
estimate flood levels or flows in residential areas of the catchment.
The primary objectives of the study are:
• to determine the flood behaviour including design flood levels and velocities over the full
range of flooding up to and including the PMF from storm runoff in the Wollongong City
Catchment and from tidal influences;
• to provide a model that can establish the effects on flood behaviour of future
development;
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• to assess the sensitivity of flood behaviour to potential climate change effects such as
increases in rainfall intensities and sea level rise; and
• to assess the provisional hydraulic categories and undertake mapping of provisional
hazard, preliminary emergency response planning classifications, and preliminary flood
planning extent areas.
This report details the results and findings of the Flood Study investigations. The key elements
include:
• a summary of available historical flood related data;
• establishment and validation of the hydrologic and hydraulic models;
• sensitivity analysis of the model results to variation of input parameters;
• potential implications of climate change projection; and
• the estimation of design flood behaviour for existing catchment conditions.
A glossary of flood related terms is provided in Appendix A.
1.3. Justification for Present Study
The present Flood Study has been initiated for the following reasons:
• There are currently no design flood estimates for the Wollongong City catchment;
• The availability of detailed topographic data from Airborne Laser Scanning (ALS) has
enabled the use of two-dimensional (2D) models of flood behaviour, and accurate
definition of topographic features in the floodplain such as flowpaths and storage
areas. This is important for the Wollongong City catchment as a large proportion of
runoff occurs as overland flow along road reserves and through properties;
• The continued development of computer technology and hydraulic modelling software
has enabled the more widespread use of 2D computer models which can provide
detailed flood extent and depth mapping for the entire study area; and
• The previous Flood Study (which covered a small portion of the lower catchment)
acknowledged but did not quantify potential risk or extent of backwater flooding from
tidal mechanisms.
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Diagram 1: Study Area – Key Features
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2. BACKGROUND
2.1. Catchment Description
The Wollongong City catchment (Diagram 1) lies immediately to the north of Port Kembla Inner
Harbour. The catchment is separated from the Pacific Ocean along its eastern boundary by a
high dune system. The general slope of the catchment is to the east and south, with the total
catchment area of approximately 730 ha draining into Port Kembla Inner Harbour via the
Gurungaty Waterway. The northern boundary of the catchment is traced by Crown Street, which
runs through Wollongong’s main commercial district. The catchment rises very steeply into the
suburbs of Coniston, Mangerton and Mt St Thomas to the west, reaching elevations of up to
around 100 mAHD.
The catchment is split by the South Coast railway line (historically the Illawarra Line) running
north-south, with the railway embankment obstructing overland flow (Figure 1) from the western
part of the catchment. There are four main pipe drainage crossings under the railway line,
draining the four principle sub-catchments upstream of the embankment (see Figure 5), the
largest of which is to the north and drains via a culvert close to Wollongong Train Station. In
large events, drainage under the railway line can also occur via a pedestrian underpass near
Union Street.
The catchment upstream of the railway line is comprised mainly of low to medium density
residential housing. In the lower part of the catchment between the railway line, Corrimal Street
and Springhill Road, land-use is a mixture of medium density residential, commercial and
industrial. The upper parts of the catchment to the north and north-east contains higher density
(residential and commercial) development, while the BlueScope steel works site occupies a
large portion of the southern catchment around Port Kembla Inner Harbour. The most low-lying
areas of the catchment to the south and east are primarily recreational parkland, including the
Wollongong Links Golf Course along the eastern catchment boundary, JJ Kelly Park, and
Wollongong Greenhouse Park.
2.2. Flood Behaviour
The drainage characteristics of the catchment have been significantly altered as a result of
urbanisation, particularly in the last 50 years. Most sections of creek and open channel have
been replaced with pipes and other man-made infrastructure. Isolated sections of open channel
remain in the upper catchment (particularly the northern branch, Figure 2), as well as within the
golf course and JJ Kelly Park. Urbanisation has resulted in the following impacts:
• an increase in the proportion of paved surfaces and consequent reduction in rainfall
infiltration, resulting in an increase in runoff (both in terms of peak flows and volumes);
• the introduction of embankments (road and rail) and other impediments to flow (such as
fences) along major drainage paths, creating trapped depressions where temporary
ponding of flood waters can occur; and
• an increase in development density within lower lying areas of the catchment where
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flood issues are more problematic.
For intense storm events, where runoff exceeds the capacity of the local subsurface drainage
system, overland flow occurs. In the lower catchment, this overland flow typically occurs along
roadways. In some areas, particularly the upper catchment (upstream of the railway line),
overland flow occurs both along roadways as well as along remnant natural flowpaths through
the back of private properties.
As the open channels in the Golf Course and JJ Kelly Park are at low elevations, with low
gradients, flooding in the lower catchment can occur via backwater from tidal action within Port
Kembla Inner Harbour, and flooding from stormwater can be exacerbated by coincident high
tides.
2.3. Floodplain Risk Management Concepts
Some broad concepts relating to floodplain risk management are discussed below. This
background information has been included to help clarify some issues raised during various
community consultation phases of the study.
2.3.1. Flood Prone Land
The NSW state government definition says any property affected by the PMF is "flood liable."
The PMF is the Probable Maximum Flood, resulting from Probable Maximum Precipitation,
which is defined as "...the greatest depth of precipitation for a given duration meteorologically
possible for a given size storm area at a particular location at a particular time of the year, with
no allowance made for long-term climatic trends." In simpler words, it is the biggest storm that
has a realistic probability of happening in any given location. Its probability is very slim (chances
of happening in any given year around 1 in 100,000). The NSW government Floodplain
Development Manual (Reference 2) recognises the need to consider the full range of floods up
to the PMF. It defines flood prone land as land susceptible to flooding in the PMF, however the
1% AEP (1 in 100 chance of happening each year) is the flood often used for planning controls
on properties. The PMF is primarily used to assess potential risk to life from extreme floods.
2.3.2. Identification of Flood Prone Property and Development Controls
It should be recognised that although land inundated in the PMF is categorised as “flood prone,”
this does not directly translate into significant restrictions being placed on development of the
land, or a significant total flood risk. Total flood risk incorporates consideration of both the
likelihood and consequences of flooding. The PMF has a very low likelihood, and the
consequences vary across the catchment. In areas only affected by shallow flow during the
PMF, the total flood risk is very low. Properties in this category will be subject to minor flood-
related development controls.
Properties for which flood-related development controls may apply are those indicated as being
within the “Flood Planning Extent,” which is based on the 1% AEP (one in a hundred chance of
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happening each year) flood level plus some freeboard (a factor of safety). Wollongong City
Council also applies some flood-related development controls for land in Low Flood Risk
Precincts (above the 1% AEP + 0.5 m level), but minimum floor level requirements in this
category are only stipulated for critical utilities, not residential development. Being identified as
within the Flood Planning Extent does not necessarily indicate that a property is likely to be
significantly damaged by flooding, as the existing buildings on the property may be sufficiently
high above ground level to prevent above-floor inundation.
The purpose of this tagging is to ensure that any future modifications or developments on the
property will be appropriate to the flood risk at the property. Chapter E13 of Council’s
Development Control Plan (Reference 5) specifies flood-related development controls. This
policy has been used to reduce flood damages to residential development and infrastructure
throughout the LGA. Such policy is effective at preventing development that will be subject to a
high likelihood of flood damage.
2.3.3. Flood Insurance
There have been major changes to the way the insurance industry in Australia approaches flood
risk as a result of the widespread flooding and major flood damages that occurred in
Queensland and Victoria in early 2011, and which caused major losses for insurers. Many
Australian residents have been charged higher home insurance premiums in the aftermath of
this event, in some cases several times higher than what they were previously paying.
It should be noted that the quantification of flood risk for insurance purposes is not a direct
objective of this study. Any insurance agency seeking to use the information in this Flood Study
to determine flood risk, and to calculate flood-related components of insurance premiums, is
strongly advised to consider the total flood risk at each property (i.e. both likelihood and
consequences of flooding), including an assessment of the height of habitable floor levels at the
property relative to the modelled flood levels, for a wide range of flood events. WMAwater do
not consider it appropriate to base insurance premiums on a single design flood event such as
the 1% AEP or PMF event. Insurers are therefore encouraged to base decisions on a
comprehensive assessment of total flood risk across a range of flood magnitudes.
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3. AVAILABLE DATA
3.1. Topographic and Bathymetric Survey
Airborne Light Detection and Ranging (LiDAR) survey of the catchment and its immediate
surroundings was provided for the study by Wollongong City Council. Council indicated that the
data were collected between 2005 and 2007. These data typically have accuracy in the order of:
• +/- 0.15m in the vertical direction (to one standard deviation); and
• +/- 0.25m in the horizontal direction (to one standard deviation).
Bathymetric survey was provided for Port Kembla Inner Harbour (collected in 2008). The LiDAR
and bathymetric survey data points were combined and a Triangular Irregular Network (TIN)
was generated. This TIN was sampled at a regular spacing of 2m to create a Digital Elevation
Model (DEM), which formed the basis of hydraulic modelling for the study.
LiDAR survey levels of areas inundated by water or covered by very dense vegetation are
generally unreliable, so the bathymetric dataset was used in preference to LiDAR in the Inner
Harbour and other areas generally below the water surface. In tidal areas that were not covered
by the bathymetric survey extent, the LiDAR terrain was checked against additional cross-
section survey at key locations. Cross-section data were used in preference to the LiDAR data
to provide a more accurate representation of the Gurungaty Waterway channel geometry, with
interpolation of cross-sections at intermediate locations (refer to Figure 4).
3.2. Detail Survey
Wollongong City Council provided an asset database including dimensions and invert elevations
for the majority of stormwater conduits within the study area. WMAwater have relied upon this
information to model stormwater pipes and pits and other key structures as part of modelling
undertaken for this study. Some gaps in the database were found, and where possible these
were supplemented with additional survey information undertaken by WCC at WMAwater’s
request. At some locations, where pipe sizes and/or invert levels were unavailable they were
estimated using information from adjacent assets. The following datasets were used to define
stormwater infrastructure in modelling for this study:
• pipe asset database “gi08398-SWLine-Asset.shp” (received 6 April 2011);
• pit asset database “gi08398-SWPit-Asset.shp” (received 6 April 2011);
• catchment plan “CityFloodStudy.dwg” (received 30 March 2011); and
• drawings of detail survey from June/July 2010 – Drawing No. 2011319-FS01 Issue A (36
sheets) by LandTeam Australia Pty Ltd, provided in Appendix C; and
• cross section survey from “WOLLONGONG CATCHMENT FLOOD STUDY INDEX
PLAN.dwg” (received 30 March 2011).
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3.3. Design Plans
Wollongong City Council provided WMAwater with plans for the building footprints and site
levels of a proposed development on Gladstone Avenue (between Rowland Avenue and
Osborne Street), which already has DA approval but which had not yet been constructed at the
time of model establishment. These plans were used to incorporate the approved development
in the model for “current conditions.”
3.4. Flood Levels
Wollongong City Council provided historical records of flooding within the study area. Additional
flood records were obtained as part of the community consultation process (refer to Section 4 for
details). Generally, attention was focused on trying to find data records corresponding to dates
of known floods in the Wollongong City catchment:
• March 1978;
• March 1983;
• December 1990; and
• March 1995.
Extremely heavy rainfall occurred on 17 August 1998 in the greater Wollongong area,
particularly along the Illawarra escarpment. However in the Wollongong City catchment Closer
to the coast at Wollongong City the rainfall was less intense, and flooding was relatively minor
(see Sections 3.5.2 and 3.6.2 for more discussion in relation to the August 1998 storm).
In March 2011, just prior to the commencement of the study, a reasonably intense storm was
experienced in the study area catchment, causing nuisance flooding at several intersections and
elevated flows in the main trunk drainage flowpaths. Council staff obtained several photographic
and video records shortly after this event, documenting the flood extents and levels. These data
were used extensively for the model calibration/validation process (refer to Section 8 for details).
In February 2012, when the model calibration had been finalised, a severe storm produced
flooding of residential and commercial properties in the lower parts of the catchment.
Continuous rainfall data for this event were not available for this study, due to delays in the
release of data from the Bureau of Meteorology, so an assessment of the storm magnitude has
not been made. A flood level of 2.7 mAHD was reported at 34 Swan Street and 200 Kembla
Street (see photograph below), approximately consistent with estimates for the 5% AEP flood
level obtained in this study (Figure E-8B).
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Photograph 1: Flooding at 200 Kembla St on 9 February 2012 (source: Illawarra Mercury – Frebruary 11/12 2012 edition).
Other flood data for the February 2012 storm were also collected as part of the community
consultation during the public exhibition phase of this study. It is recommended that further
analysis of this storm be undertaken in conjunction with the Floodplain Risk Management Study
& Plan.
3.5. Previous Studies
3.5.1. Wollongong Flooding and Drainage Management Study
(Reference 3)
WMAwater understand that the only previous modelling study in the catchment was undertaken
by Gutteridge Haskins & Davey (GHD) in 1990, on behalf of the Water Board (now Sydney
Water). The study focused on a small section of the lower catchment east of Corrimal Street and
south of Stewart Street. At the time the study area was partially developed as the Wollongong
Links Golf Course, with proposals to introduce new residential/tourist development between
Bank Street and Stewart Street (which has subsequently been completed).
The primary objectives of the study were to estimate flood behaviour in the area (a flood study)
and investigate potential drainage management options to alleviate known historical flood issues
in and around the Golf Course.
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A hydrologic model of the full Wollongong City catchment was created using the RAFTS
package. Results were verified against ILSAX (DRAINS) and the Rational Method
(Reference 1). Due to low grades and the influence of channel storage in the lower catchment,
an unsteady flow hydraulic model of the lower channel system was established to estimate the
open channel flood behaviour in the lower catchment. The hydraulic model (in SWMM) included
the study area east of Corrimal Street and extended south close to the system outlet at Port
Kembla Inner Harbour.
The peak flows from the catchment at Port Kembla Inner Harbour for the 1% AEP design event
estimated in the study, utilising various methods, are reproduced in Table 1.
The study observed that the flow estimate from the combined hydrologic/hydraulic modelling
(RAFTS/SWMM) was significantly lower than for the hydrologic modelling alone, due mainly to
the retarding effects of causeways across the Gurungaty Waterway (near what is now Tom