1. INTRODUCTION 1.1. Background - Wollongong · INTRODUCTION 1.1. Background The Wollongong City...

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;



2

• 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.



3

Diagram 1: Study Area – Key Features



4

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



5

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



6

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.



7

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).



8

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).



9

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.



10

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

Thumb Road).

Table 1: Previous Study Peak Catchment Flow Estimates – 1% AEP

METHOD Flow (m3/s)

RAFTS (medium losses) 155

RAFTS (high losses) 150

RAFTS (low losses) 160

RAFTS/SWMM 125

ILSAX 180

Rational Method 200

The estimated 1% AEP flows obtained by WMAwater as part of the present study are

significantly lower again than those in Table 1 above (refer to Section 9), due to the inclusion of

additional detail throughout the catchment of features which delay runoff from the catchment to

the outlet. The most significant of these features is the Illawarra Railway Line, as well as the

cumulative effect of localised obstructions such as fences and buildings throughout the

catchment, and the effect of Council’s Conduit Blockage Policy.

Reference 3 found that there were several contributing factors to flood issues in the vicinity of

the Wollongong Link Golf Course, including:

• Low grades of drainage channels and pipes;

• The presence of constrictions and obstructions (causeways) in the downstream

waterways;

• Backwater influences from other catchments and tidal effects;

• Trapped detention storage areas within the golf course; and

• A lack of formal overland flow paths.



11

The study involved preliminary investigation of several potential drainage management options,

and identified the following as the most feasible:

• construction of two retarding basins and the hydraulic improvement and realignment of

the Golf Links drainage branch;

• construction of a diversion drain around the proposed development site, (bounded by

Corrimal, Banks and Stewart Streets) with a small pump-out retarding basin; and

• minimisation of impacts from proposed development (between Banks and Stewart

Streets) on existing drainage and flood behaviour.

It was recommended that a study of the total catchment be undertaken, and the identified

drainage options be re-evaluated within a broader framework addressing issues such as

flooding, water quality and sedimentation of Port Kembla Inner Harbour.

3.5.2. August 1998 Wollongong Storms (Reference 4)

This report, prepared by the Institution of Engineers Australia, was essentially a compilation of

accounts from various sources of the storms which devastated the Wollongong region between

15 and 19 August 1998. The report covers several broader issues arising from the storms, such

as emergency services response, insurance, and the impacts of the storms on the community.

The main aspect of the report relevant to the present Flood Study is the investigation of rainfall

patterns by the Bureau of Meteorology (BoM), which identified substantial spatial variation in the

intensity of rainfall during the storm.

In particular, the authors identified that at several locations (Rixons Pass, Mt Ousley, Balgownie

Reservoir, Bulli Pass and Beth Salem) recorded rainfall intensities were greater than 1% AEP

for durations between 30 minutes and 6 hours. However, east of the escarpment, rainfall

intensities were considerably lower, with an estimated probability of between 50% to 20% AEP

“near the coast at Wollongong City.”

This finding is supported by rainfall analysis undertaken by WMAwater (Section 3.6.2).



12

3.6. Rainfall

Table 2 lists the daily read stations within a 10 km radius of Wollongong Train Station.

Table 2: Daily Rainfall Stations

Station Name Operating

Authority

Distance from

Wollongong

Station (km)

Elevation

(mAHD)

Opened Closed

68069 Wollongong Post Office BOM (AUS) 0.8 30 30-Jul-1870 29-Dec-53

68074 Smiths Hill BOM (AUS) 1.3 15.2 1938 1972

68188 Wollongong University BOM (AUS) 2.9 25 30-Aug-70

68169 Mount Keira Yates Ave BOM (AUS) 3.2 90 30-Jan-66

68046 Mount Pleasant BOM (AUS) 3.3 198.1 1907 1964

68153 Rosemount BOM (AUS) 3.3

29-Sep-13 29-Dec-34

68171 Wollongong Obriens Rd BOM (AUS) 3.5 48.8 30-Jan-66 29-Dec-71

68131 Port Kembla (Bhp Central Lab) BOM (AUS) 4.5 9 4-May-63

68103 Mount Keira Summit BOM (AUS) 4.7 304.8 30-Oct-62 29-Dec-66

68086 Mount Keira Scout Camp BOM (AUS) 4.9 310 30-Jan-44 29-Jul-92

68119 Towradgi BOM (AUS) 5.1 9.1 30-Oct-62 29-Dec-75

68053 Port Kembla Signal Stn BOM (AUS) 6 11 30-May-50 29-Oct-77

68172 Mount Kembla 2 BOM (AUS) 6.6 137.2 30-Jan-66 29-Dec-67

68201 Mount Kembla BOM (AUS) 6.6

1908 1922

68060 Unanderra BOM (AUS) 6.6 12.2 1903 1969

68146 Kembla Heights BOM (AUS) 6.7 320 30-Jul-56 29-Dec-73

68149 Mount Kembla BOM (AUS) 6.7

1895 1918

68110 Berkeley Northcliffe Drive BOM (AUS) 6.8 5 30-Oct-62

68126 Nidgee Stud BOM (AUS) 7 131.1 30-Oct-62 29-Dec-72

568080 Kembla Heights SCA (NSW) 7.1 320 1956

68147 Kembla Heights 1 BOM (AUS) 8.1

Sep-1895 29-Dec-19

68170 Mount Nebo Colliery BOM (AUS) 8.1 207 30-Jan-66 29-Dec-79

68173 Kembla Heights Cordeaux Rd BOM (AUS) 8.1 300 30-Mar-67 29-Dec-68

68237 Kembla Grange Racecourse BOM (AUS) 8.1 7 30-Jan-94 22-Feb-04

68049 Obriens Gap BOM (AUS) 8.6

30-Mar-25 29-Dec-54

68108 Woonona (Popes Rd) BOM (AUS) 9.6 45 30-Mar-29

68019 Cordeaux No.2 Dam BOM (AUS) 9.6 300 29-Jun-15 29-Dec-67



13

Table 3 lists the automated continuous stations (pluviometers) within a 20 km radius of

Wollongong Train Station.

Table 3: Pluviometer Stations

Station Name Operating

Authority

Distance from

Wollongong

Station (km)

Elevation

(mAHD)

Opened Closed

68188 Wollongong University BOM (AUS) 2.9 25 16-Feb-80 10-Aug-85

68131 Port Kembla (BHP Central Lab) BOM (AUS) 4.5 9 4-May-63

568116 Balgownie (Brokers Rd) BOMNS (NSW) 4.4 100 20-May-98

568043 Figtree (O'Briens Rd) BOMNS (NSW) 4.7 140 20-May-98

68086 Mount Keira Scout Camp BOM (AUS) 4.9 310 10-Jul-74 29-Jul-92

68172 Mount Kembla 2 BOM (AUS) 6.6 137.2 6-Feb-66 22-Apr-67

68173 Kembla Heights Cordeaux Rd BOM (AUS) 8.1 300 31-May-67 28-Apr-68

568097 Mount Keira(Kentish No.2) SCA (NSW) 9.0 430 5-Feb-64

568068 Upper Cordeaux (No.2 Dam) SCA (NSW) 10.3 330 21-Sep-73

568067 Beth Salem SCA (NSW) 10.8 366 30-Aug-66

68104 Tallawarra Power Stn BOM (AUS) 12.7 10 30-Oct-71 29-May-89

568175 Wongawilli Reservoir BOMNS (NSW) 13.5

7-Jan-02

568071 Upper Avon SCA (NSW) 14.7 330 30-Jan-64

68187 Cataract C BOM (AUS) 16.4 476 30-Jan-64 7-Dec-98

568061 Browns Rd SCA (NSW) 16.5 442 4-Feb-64

568049 Cordeaux Quarters SCA (NSW) 17.5 365 25-May-64

68241 Wollongong Airport BOM (AUS) 17.6 8 1-Jun-99

568065 Letterbox Tower SCA (NSW) 18.1 440 6-Dec-64

68000 Albion Park Post Office BOMNS (NSW) 19.0 8 1-Oct-99

568048 Cataract Dam SCA (NSW) 19.6 340 1904

Key rainfall gauge locations are indicated on Figure 7.

3.6.1. Analysis of Daily Read Data

As flooding in the catchment is typically caused by intense rainfall over periods of sub-daily

duration, analysis of daily totals is of limited use for identifying historically significant rainfall

events. There was found to be very little correlation with high daily rainfall totals in the area and

known instances of flooding in the catchment. Some of the highest daily totals in the record were

the result of reasonably uniform rainfall falling consistently throughout the day. In depth analysis



14

of historical daily totals has therefore not been undertaken.

3.6.2. Analysis of Pluviometer Data

As indicated on Figure 7, there are no continuous (pluviometer) rainfall recorders currently

operating within the study area. The gauges at Figtree and Balgownie were established in

1998, while the Port Kembla gauge has been operating since May 1963 (although there are

some gaps in the record, including the early 1990s). Representatives from Wollongong City

Council indicated that previously there had been a rainfall gauge in operation on top of the

Council building on Burelli Street, but its use had been discontinued and records were not

available for this study.

The available pluviometer records in and around the catchment display substantial variation in

recorded rainfall for many of the large historical storms. In particular, the closest long term

pluviometer to the catchment (Port Kembla gauge) consistently showed substantially less rainfall

than other gauges further inland (and at higher elevations). This strong orographic rainfall

gradient introduces significant uncertainty into the estimation of an appropriate catchment

average rainfall intensity, both for calibration and design storms events. There is also a

possibility that the Port Kembla gauge is in a “rain shadow” from nearby buildings.

Rainfall data from the dates corresponding to known flood levels in the catchment were

analysed in detail for the Port Kembla, Mount Keira and Upper Avon gauges. Figure D-4 to

Figure D-13 show rainfall hyetographs for these periods and comparisons with the design IFD

information (Section 3.6.3). The analyses for March 2011 (Figure D-4) and August 1998 (Figure

D-6) show particular variability in the recorded rainfall.

This analysis highlights that rainfall is subject to significant spatial and temporal variation within

the Wollongong City catchment, particularly as a result of orographic effects due to the steep

elevation increase up to the Illawarra escarpment. Even though pluviometer rainfall records are

available around the study area, the density of gauges is not sufficient to fully define this

variability. It is therefore not possible to develop a precise spatial and temporal definition of the

historical rainfall patterns for known flood-producing storms in the Wollongong City catchment.

3.6.3. Design Rainfalls

Design rainfall depths and temporal patterns for various storm durations at the study area were

obtained from Australian Rainfall and Runoff 1987 (ARR87), for events up to and including the

1% AEP event. Probable Maximum Precipitation estimates were derived according to Bureau of

Meteorology (BoM) guidelines (Reference 16). A summary of the design rainfall depths is

provided in Table 4.

There is some variation in design rainfall depths across the catchment, due to the influence of

the Illawarra Escarpment. The implications of this variation are discussed in Section 10.2.

Design flood modelling for this study uses design rainfall depths estimated at the catchment

centre.



15

Table 4: Rainfall Intensity-Frequency-Duration Data at Centre of Catchment

DURATION

Design Rainfall Intensity (mm/hr)

1 yr ARI 2 yr ARI 5 yr ARI 10 yr ARI 20 yr ARI 50 yr ARI 100 yr ARI

5 minute 114 145 179 198 224 257 282

10 minute 87.9 112 140 156 177 205 226

20 minute 64.5 82.7 105 118 135 158 174

30 minute 52.6 67.7 87 98.2 113 132 147

1 hour 35.9 46.5 60.7 69.1 80.1 94.5 106

2 hour 23.6 30.8 40.9 47 54.8 65.2 73.1

3 hour 18.3 24 32.1 37.1 43.4 51.9 58.4

6 hour 11.9 15.6 21.2 24.6 29 34.9 39.5

12 hour 7.73 10.2 14.1 16.5 19.5 23.6 26.8

24 hour 5.1 6.76 9.41 11.1 13.2 16 18.3

48 hour 3.32 4.41 6.2 7.31 8.75 10.7 12.2

72 hour 2.5 3.33 4.7 5.57 6.68 8.18 9.38

Date post:	26-Aug-2018
Category:	Documents
Upload:	duongdien
View:	218 times
Download:	0 times

1. INTRODUCTION 1.1. Background - Wollongong · INTRODUCTION 1.1. Background The Wollongong City...

Documents