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HYDRAULIC MODELLING REPORT

NAMBUCCA RIVER AND

WARRELL CREEK

FINAL REPORT

NOVEMBER 2013

Level 2, 160 Clarence Street Sydney, NSW, 2000 Tel: 9299 2855 Fax: 9262 6208 Email: [email protected] Web: www.wmawater.com.au

HYDRAULIC MODELLING REPORT- NAMBUCCA RIVER AND WARRELL CREEK

FINAL REPORT

NOVEMBER 2013

Project Hydraulic Modelling Report- Nambucca River and Warrell Creek

Project Number 111036

Client Roads and Maritime Services

Client’s Representative Shane Green

Authors Monique Retallick Laura Wallis Mark Babister

Prepared by

Date 19 November 2013

Verified by

Revision Description Date

4 FINAL REPORT - REISSUED NOVEMBER 2013

3 FINAL REPORT OCTOBER 2012

2 DRAFT APRIL 2012

1 INITIAL DRAFT REPORT FOR DISCUSSION ONLY MARCH 2012

HYDRAULIC MODELLING REPORT- NAMBUCCA RIVER AND WARRELL CREEK

TABLE OF CONTENTS

PAGE

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

1.1. Overview .................................................................................................... 1

1.2. Report Outline ............................................................................................ 1

2. BACKGROUND ......................................................................................................... 3

2.1. Study Area .................................................................................................. 3

2.2. Previous Studies ......................................................................................... 3

3. AVAILABLE DATA .................................................................................................... 6

3.1. Rainfall Information ..................................................................................... 6

3.1.1. Historic Rainfall Data .................................................................................. 6

3.1.2. Design Rainfall Data ................................................................................... 6

3.2. Water Level Data ........................................................................................ 6

3.2.1. Time Series Water Level Data .................................................................... 6

3.2.2. Peak Flood Heights .................................................................................... 7

3.3. Selection of Calibration Events ................................................................... 8

3.4. Topographic Information ............................................................................. 9

3.5. Culvert and Structure Data ....................................................................... 10

4. ADOPTED MODELLING APPROACH ..................................................................... 11

5. HYDROLOGIC MODELLING ................................................................................... 12

5.1. Overview .................................................................................................. 12

5.2. Review of Bellinger, Kalang and Nambucca River Catchments Hydrology 12

5.3. 0.05% AEP Event ..................................................................................... 13

6. HYDRAULIC MODELLING ...................................................................................... 14

6.1. Model Configuration .................................................................................. 14

6.2. Boundary Conditions ................................................................................ 15

6.3. Model Calibration ...................................................................................... 15

6.4. Calibration Results and Discussion ........................................................... 16

6.4.1. Timing of Flood Peaks .............................................................................. 16

6.4.2. Results ..................................................................................................... 17

6.5. Model Verification ..................................................................................... 18

6.6. Verification Results and Discussion .......................................................... 18

7. DESIGN FLOOD BEHAVIOUR ................................................................................ 19

7.1.1. Boundary Conditions ................................................................................ 19

7.1.2. Design Event Results ............................................................................... 20

7.1.3. Sensitivity Analysis ................................................................................... 21

7.1.4. Climate Change ........................................................................................ 23

8. CONCLUSIONS ....................................................................................................... 25

9. REFERENCES ......................................................................................................... 26

LIST OF APPENDICES

Appendix A: Glossary

LIST OF TABLES

Table 1: Significant Peak Flood Levels at Bowraville................................................................... 7

Table 2: Significant Peak Flood Levels at Macksville .................................................................. 8

Table 3: Historic Information Availability ...................................................................................... 9

Table 4: Calibration and Verification Events .............................................................................. 11

Table 5: Adopted Manning’s “n” Values..................................................................................... 15

Table 6: Timing of Historical Flood Peaks Nambucca River – Bowraville, Utungun and

Macksville ................................................................................................................................. 16

Table 7: Calibration Events - Modelled vs Observed Flood Levels ........................................... 17

Table 8: Model Validation – Modelled vs Observed Levels –2009 event ................................... 18

Table 9: Ocean Boundary Peaks (mAHD) ................................................................................. 20

Table 10: Design Flood Levels at Key Locations ....................................................................... 20

Table 11: Comparison of 1% AEP Flood Levels at Macksville ................................................... 21

Table 12: Sensitivity Analyses ................................................................................................... 22

Table 13: Impacts of Blockage .................................................................................................. 22

Table 14: Climate Change Results ............................................................................................ 23

LIST OF FIGURES

Figure 1: Study Area

Figure 2: Available Survey Data

Figure 3: Hydrologic Model Layout

Figure 4: Hydraulic (TUFLOW) Model Layout

Figure 5: Manning’s n Values

Figure 6: Model Validation – Modelled vs Observed – 2009 Event – Macksville

Figure 7: Model Validation - Modelled vs Observed – 2009 Event – Stuarts Island

Figure 8: Model Calibration – Peak Flood Level - 1972 Event

Figure 9: Model Calibration – Peak Flood Level - 1977 Event

Figure 10: Model Validation – Peak Flood Level - 2009 Event

Figure 11: Timing of Flood Peaks Nambucca River - Macksville, Bowraville and Utungun

Figure 12: Stage Frequency Curve – Macksville

Figure 13: Design Event Flood Level Profiles – Nambucca River

Figure 14: Design Event Flood Level Profiles – Taylors Arm

Figure 15: Design Event Flood Level Profiles – Warrell Creek

Figure 16: Design Event – Peak Flood Depths and Level Contours – 5 Year ARI Event

Figure 17: Design Event – Peak Flood Depths and Level Contours – 10 % AEP Event



Figure 20: Design Event – Peak Flood Depths and Level Contours – 0.5 % AEP Event



Figure 23: Design Event – Peak Flood Depths and Level Contours – PMF Event

Figure 24: Design Event – Peak Flood Velocities – 5 Year ARI Event

Figure 25: Design Event – Peak Flood Velocities – 10 % AEP Event



Figure 28: Design Event – Peak Flood Velocities – 0.5 % AEP Event



Figure 31: Design Event – Peak Flood Velocities – PMF Event

Figure 32: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event with a

10% Rainfall Increase

Figure 33: Design Event - Peak Flood Depths and Level Contours – 1 % AEP Event with a


Figure 34: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event with a


Figure 35: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event 2050 Sea

Level Rise

Figure 36: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event 2100 Sea

Level Rise

Hydraulic Modelling Report- Nambucca River and Warrell Creek

WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013

1

1. INTRODUCTION

1.1. Overview

The objective of this study is to define the existing flood behaviour within the Nambucca River

and Warrell Creek. It is anticipated that the outcomes of this study will:

• Form the basis for the detailed design of the major river crossings for the Warrell Creek

to Urunga Pacific Highway upgrade project, and

• Be extended into a flood study under the NSW Flood Policy and will form the first stage

in the management process for these catchment areas by the Nambucca Shire Council.

The study has been funded by Roads and Maritime Services (RMS) and provided to Nambucca

Council for extension into a flood study under the NSW Flood Policy.

The model study areas were determined in consultation with Roads and Maritime Services,

Nambucca Shire Council, and Office of Environment and Heritage to ensure that it met the

needs of both the NSW flood program and the Warrell Creek to Urunga Pacific Highway

upgrade modelling study.

The Nambucca River and Warrell Creek study area is defined as:

• Upstream to Congarinni Road Bridge on Taylors Arm Road,

• Upstream to Bowraville Bridge on the Nambucca River,

• Upstream to near where Browns Crossing road bridges the railway on Warrell Creek

and

• Downstream -The Pacific Ocean

This report details the investigations, results and findings of the Hydraulic Modelling Study for

the Nambucca River and Warrell Creek. The key elements of which include:

• a summary of available data,

• hydraulic model development,

• calibration of the hydraulic model, and

• definition of the design flood behaviour through the analysis and interpretation of model

results.

The companion reports to this report are Hydraulic Modelling Report – Bellinger and Kalang

Rivers and Deep Creek Flood Study. A glossary of flood related terms is provided in Appendix

A.

1.2. Report Outline

This report provides background information on the catchment and previous studies in Sections

2. The available data used is described in Section 3. Section 4 describes the adopted modelling

approach.



2

Details of the hydrologic modelling that was undertaken to determine inflows to the hydraulic

model are contained in the earlier Review of Bellinger, Kalang and Nambucca Rivers Catchment

Hydrology (Reference 4) which investigates known hydrologic issues in the Bellinger, Kalang

and Nambucca River catchments. A summary of this report is included in Section 5. Also

included in Section 5 is the development of the 0.05% AEP flows.

Hydraulic modelling of the Nambucca River and Warrell Creek are detailed in Section 6. This

includes model calibration, verification and sensitivity analysis. Design flood behaviour is

discussed in Section 7.



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2. BACKGROUND

2.1. Study Area

The study area (refer to Figure 1) includes Nambucca River and Warrell Creek catchments

located in Nambucca Shire. Warrell Creek joins with the Nambucca River and discharges to the

ocean at Nambucca Heads. Taylors Arm is the other main tributary of the Nambucca, which has

its confluence upstream of Macksville. The catchment area of the combined Nambucca River

and Warrell Creek and their tributaries is 1315 km2. The catchment area of the Nambucca River

and Taylors Arm to their junction is approximately equal though they have very different shapes.

The Warrell Creek catchment is narrow and wraps around the other catchments to the south.

The headwaters of both catchments are located in the Great Dividing Range and characterised

by steep topography. The lower reaches of the Nambucca River are characterised by broad

floodplains and farmland. The lower reaches of Warrell Creek is characterised by a narrow

meandering channel with dense overbank vegetation.

Residential development within the catchments is generally characterised by small settlements.

Major centres exist at Macksville, Bowraville and Nambucca Heads on the Nambucca River.

The small settlement of Warrell Creek exists on Warrell Creek.

The Study area is defined as:

• Approximately 2km upstream Congarinni Road Bridge on Taylors Arm Road on Taylors

Arm,

• Upstream to Bowraville Bridge on the Nambucca River,

• Upstream to near where Browns Crossing road bridges the railway on Warrell Creek

and

• Downstream - The Pacific Ocean.

2.2. Previous Studies

Nambucca River Flood History 1843-1979 (PWD, 1980)

This study (Reference 11) was undertaken to document flood data in the tidal section of the

Nambucca River (up to approximately three kilometres downstream of Lanes Bridge, Bowraville)

to be used in preparing flood maps for Macksville. A flood frequency analysis was conducted

using recorded data for Macksville and Bowraville. Floods with peak heights greater than 2.3

mAHD and 8.9 mAHD were included for Macksville and Bowraville respectively. Flood levels for

the 1%, 2% and 5% AEP events at Macksville were determined. The report details personal

recollections of residents about significant historical events. Flood reference points for significant

flood events including the 1972 and 1977 events are reported which were used for model

calibration in the current study.



4

New South Wales Coastal Rivers Floodplain Management Studies the Nambucca Valley

(Flood Plain Management Studies Steering Committee, July 1981)

Part of a series of reports on NSW coastal rivers, this report (Reference 12) details floodplain

management measures within the Nambucca valley and makes recommendations on policy.

The report contains recorded water levels of historical floods.

Macksville Flood Study (Department of Public Works, 1983)

This report (Reference 13) investigates a levee system to protect Macksville. It divides the area

of Macksville into three parts; Central Macksville, North Macksville and Kings Point. A “SAMOD”

Model was used for input data. The model was calibrated to the October 1972 flood and verified

against the May 1980 flood. The Design flood was defined as the 100 Year ARI. The report

includes maps and hydrographs for the 100 Year ARI event.

Lower Nambucca River Flood Study (PWD, 1994)

The Lower Nambucca River Flood Study (1994, Reference 14) investigated flooding in the

Nambucca River downstream of Wirrimbi Island, Congarini Bridge and the Pacific Highway

Bridge on Warrell Creek. A MIKE 11 hydraulic model was developed to determine flood levels

for the 1%, 2%, and 5% Annual Exceedance Probability (AEP) and extreme design flood events.

The 1972 and 1977 historical events were used for model calibration and verification. The

effects of ocean levels and bed scour were incorporated into the model. A RAFTS hydrological

model was developed of the Nambucca River catchments to convert rainfall to flow

hydrographs. Model results from the current study were compared to the 1994 Flood Study.

Compilation of Flood Marks, Rainfall Data and Observed Flood Behaviour for the

March/April Flood Event of 2009 (Nambucca Shire Council, 2009)

Following severe flooding in 2009 a data collection exercise was undertaken to collect all useful

information for a future flood study. This report (Reference 15) documents the findings of the

data collection. This data has been used in the current study to inform the calibration of the

hydraulic model.

Warrell Creek to Urunga Upgrade Environmental Assessment (RTA, 2010)

The Warrell Creek to Urunga Upgrade Environmental Assessment (2010) (Reference 6)

assessed the impact of the proposed pacific highway upgrade on flood levels. The study

adopted the layout of the Lower Nambucca Flood Study RAFTS model with some modification.

In order to fit the flood frequency analysis results the study adopted the Australian Rainfall and

Runoff (ARR) temporal patterns for zone 3 rather than zone 1. The study also used a very high

Bx value which would have a similar effect as a large aerial reduction factor as it would lead to

significant hydrograph attenuation. The study modelled the Nambucca River and Warrell Creek

separately using MIKE Flood.



5

Nambucca Heads Flood Study (SKM, 2011)

The study (Reference 16) focused on the entrance of the Nambucca River. A 2D TUFLOW

hydraulic model of the study area was produced to model flood behaviour. The terrain

information used for the (Reference 6) was used to define the model grid outside of the study

area. Inflows to the hydraulic model were derived from the RAFTS model developed as part of

Reference 6 for the 5, 20, 50, 100 and 200 year ARI flood events and Probable Maximum Flood

(PMF). No model calibration and verification was undertaken as part of the study.

Review of Bellinger, Kalang and Nambucca Rivers Catchment Hydrology (WMAwater,

2011)

The Review of Bellinger, Kalang and Nambucca Rivers Catchment Hydrology (Reference 4)

investigates known hydrologic issues in the Bellinger, Kalang and Nambucca River catchments.

This area of the NSW north coast has presented a range of challenges for a number of studies

where problems have been encountered matching rainfall runoff modelling with flood frequency

results. As part of the study WBNM hydrologic models were developed for each catchment and

calibrated to historical events. The hydrology developed for the Nambucca River Catchment as

part of the study has been used for the current study.



6

3. AVAILABLE DATA

3.1. Rainfall Information

3.1.1. Historic Rainfall Data

Historical rainfall data was obtained at a number of locations within the study area and

surrounds. Daily rainfall and pluviograph data was obtained for a number of gauges within the

region from a number of sources including the Bureau of Meteorology (BoM) and Manly

Hydraulics Laboratory (MHL).

Historic rainfall data available for the 1972, 1977, and 2009 events on the Nambucca River and

Warrell Creek is documented in Reference 4. For the 1972 and 1977 events no pluviograph

information was available for the catchment though several pluviometers were located in

adjacent catchments. While multiple events occurred during 2009 over the study area only the

April event was used for consistency with the Bellinger/Kalang system and availability of

information. There was little difference in peak heights at Macksville between the events. This

event was the largest on the Kalang River of the 2009 events. This is discussed in more detail in

Reference 4.

3.1.2. Design Rainfall Data

Design rainfall data available for the Nambucca River and Warrell Creek is documented in

Reference 4. All of the BoM long term daily and pluviograph gauges within and near the

catchment were analysed on a 24hr 9am restricted basis to produce new IFD estimates. This

was supplemented by at site analysis of other gauges which was incorporated into the surface

mapping.

3.2. Water Level Data

3.2.1. Time Series Water Level Data

Manly Hydraulics Laboratory (MHL) and Office of Water operates a number of water level

recorders in the Nambucca River catchment. These being:

• Utungun,

• Stuarts Island,

• Macksville, and

• Bowraville.

Stage hydrograph data was obtained from the MHL operated water level stations. The recorded

time-series of water levels was used for model calibration purposes. It should be noted that

these water level recorders are located within the tidal limit. The opportunity for the water level

record to be translated into a corresponding flow hydrograph is therefore limited except for

Bowraville which is at the very upper limit of the tidal limit and for which rating curves exist.



7

However, the recorders do provide a valuable record of flood level behaviour during an actual

flood.

3.2.2. Peak Flood Heights

The Nambucca River Valley has a long history of flooding. Flood records for Bowraville date

back to the 1890’s (Reference 11). In comparison to Bowraville, there is less observed peak

flood height data available for Macksville (starting at 1894). No information was available for the

1890 flood for Macksville. The 1890 flood was the most significant event at Bowraville and is

therefore likely to be the most significant at Macksville. A summary of significant events which

have occurred in the area is presented in Table 1 (Bowraville) and Table 2 (Macksville). The

more recent events for which significant data is available for calibration and validation purposes

occurred in 1972, 1977 and March/April 2009.

Table 1: Significant Peak Flood Levels at Bowraville

Date Gauge Height (m)

Flood

Height

(mAHD)

Comment

March 1890 11.47 11.9 Nambucca River Flood history 1890-1979

Feb-54 10.57 11 Nambucca River Flood history 1890-1979

Jun-50 10.47 10.9 Nambucca River Flood history 1890-1979

2009 10.42 10.85 PINNEENA

1974 10.28 10.71 PINNEENA (used instead of Nambucca River

Flood History)

Feb 1893 9.97 10.4 Nambucca River Flood history 1890-1979


Flood History)

May-48 9.37 9.8 Nambucca River Flood history 1890-1979

Aug-49 9.37 9.8 Nambucca River Flood history 1890-1979

1989 9.36 9.79 PINNEENA

Apr-62 9.27 9.7 Nambucca River Flood history 1890-1979


Mar-46 9.17 9.6 Nambucca River Flood history 1890-1979


Flood History)

Jul-21 9.07 9.5 Nambucca River Flood history 1890-1979

Nov-59 9.07 9.5 Nambucca River Flood history 1890-1979


1985 9.05 9.48 PINNEENA

1996 8.96 9.39 PINNEENA

2001 8.94 9.37 PINNEENA

1975 8.68 9.11 PINNEENA


Jan-68 8.67 9.1 Nambucca River Flood history 1890-1979

March 1894 8.47 8.9 Nambucca River Flood history 1890-1979




8

Jun-67 8.47 8.9 Nambucca River Flood history 1890-1979

1988 8.22 8.65 PINNEENA

1980 8.1 8.53 PINNEENA

Table 2: Significant Peak Flood Levels at Macksville

Year Flood Level (m AHD) Comment

1890 ? No Information

Jun-1950 3.4 Nambucca River Flood history 1890-1979

Mar-1964 3.2 Nambucca River Flood history 1890-1979

Feb-1954 3.15 Nambucca River Flood history 1890-1979

Jul-1921 2.95 Nambucca River Flood history 1890-1979

Apr-1962 2.95 Nambucca River Flood history 1890-1979




Nov-1959 2.65 Nambucca River Flood history 1890-1979

May-1977 2.65 Nambucca River Flood history 1890-1979

Jun-2011 2.62 MHL readings

Oct-1972 2.6 Nambucca River Flood history 1890-1979




Aug-1949 2.5 Nambucca River Flood history 1890-1979

Apr-1963 2.45 Nambucca River Flood history 1890-1979

Jun-1967 2.4 Nambucca River Flood history 1890-1979


Feb-1929 2.35 Nambucca River Flood history 1890-1979

Mar-2001 2.33 MHL readings

Feb-2009 2.28 MHL readings

May-2009 2.26 MHL readings

Apr-2009 2.24 MHL readings

Jan-1968 2.22 Nambucca River Flood history 1890-1979

Jul-1962 2.15 Nambucca River Flood history 1890-1979

Jul-1999 2.11 MHL readings

3.3. Selection of Calibration Events

A review of previous studies and available data found some observed peak flood heights at a

number of locations within or near the study area. Data for the 1972 and 1977 events are

presented in Reference 14. Reference 15 contains data for the March/April 2009 event. Several

data collection exercises have been undertaken to collect peak flood levels and anecdotal

evidence of significant floods in the area (Refer Section 2).

Significant events for which sufficient data (peak flood heights and rainfall data) existed for use

in the calibration validation purposes were the 1972, 1977, and 2009. Table 3 summarises the



9

available historic information.

Other historical events were not included in the calibration due to:

• No pluviographs within the catchment at the time of the event, and

• Limited observed data available including limited water level recorders within the

catchment.

Table 3: Historic Information Availability

Event Rainfall Observed levels (peaks) Streamflow data Ocean levels

1962

Pluviograph records for

Bellbrook, Kempsey

and Coffs Harbour

Variable quality

1963 None

Various locations between the old

Pacific Hwy crossing to Scotts

Head, indirect evidence of level at

Warrell Creek

1972*


Bellbrook and

Kempsey (61 daily)

Sufficient for Nambucca River not

for Warrell Creek

Stage Hydrographs at 3

locations

Information

available

1974

insufficient pluviograph

records from Bellbrook

(56 daily)



Head, only one level for Warrell

Creek


locations

1977*


Bellbrook and Coffs

Harbour (52 daily)



Head, sufficient for Nambucca

River not for Warrell Creek


locations

Information

available

1979

Detailed

information at

several locations

1980

0 (3 hour rain fall totals

for Bellbrook, Kempsey

and Coffs Harbour)


locations

1991



Head

2009 6 pluviographs (51

daily) Sufficient

1 location plus water

level timeseries at

Macksville and Stuarts

Island

Detailed

timeseries Coffs

Harbour

*Indicates used in 1994 Lower Nambucca Flood Study for calibration

3.4. Topographic Information

There is a considerable amount of topographic data available for the study area. However, the

accuracy and suitability of these existing datasets for use in the present study varies. This

includes contours, hydrosurvey, cross sections and Airbourne Laser Scanning.

Council provided topographic contours of the study area in GIS format. These were at 10m

intervals for the majority of the catchment and 2m contours for a limited set of highly populated



10

coastal areas.

Hydrosurvey of the estuary was available from OEH. The hydrosurvey provides waterway cross

sections for the estuarine reaches of Nambucca River and Warrell Creek. The hydrosurvey was

collected between November 2008 and August 2009. During this time several large flood events

occurred. The hydrosurvey was undertaken in several sections:

• Macksville to 2-3 km upstream of entrance done in Nov 2008

• Upper Warrell creek done after Feb 2009

• Lower Warrell ck to Scotts Head done before Feb 2009

• Entrance done after Feb 2009

Historic hydrosurvey from 1979 was also available. This shows the entrance with a less

conveyance than the 2009 survey which was taken shortly after the 2009 flood event. A

sensitivity analysis is recommended as part of a future flood study.

Cross sections and details of culverts along the Macksville Town drain were surveyed by

Nambucca Council surveyors and local surveyors.

Aerial photography collected by the Lands and Property Management Authority was also

available within the catchment boundary.

Airbourne Laser Scanning (ALS) ground levels were provided for the study area. The ALS

collection was part of the Coastal capture program by the Lands and Property Management

Authority. It captures from the coast to the 10m contour interval. Spatial accuracy of the ALS in

the horizontal and vertical directions was reported to be 0.8m and 0.3m respectively.

A DEM (Digital Elevation Model) at a 1m grid resolution was used in order to:

• confirm sub-catchment and catchment watershed boundaries; and

• inform the 2D model used in the study.

Due to issues with the data processing used to produce the original grids provided, the raw LAS

files were obtained. The non ground strikes were filtered from this data set. Within a 60m buffer

of the waterway the ground strikes and hydrosurvey were tinned and a DEM produced. This

DEM, the ALS grid (outside of the 60m buffer) and gridded 10m contours (in areas within the

hydraulic model extent where ALS wasn’t available) were combined to create a DEM for use in

the 2D model.

3.5. Culvert and Structure Data

Details of culverts and structures along the existing highway were obtained from RTA works as

executed plans and culvert database. For local roads details of culverts and bridge structures

were collected on a site visit and based on council records. Stuarts Island Causeway details

were obtained from Council’s design plans. Culvert details along Gumma Road were based on

council survey. Where culvert details were not available a reasonable estimate was made based

on upstream culverts (particularly for railway culverts). Included culverts are shown on Figure 4.



11

4. ADOPTED MODELLING APPROACH

The key purpose of this study was to develop a more detailed hydraulic model of the Nambucca

River and Warrell Creek, to better define flood behaviour which could then be used to assess

the impacts of the Pacific Highway Upgrade and also be used to set develop control levels as

part of a flood study and future floodplain management study under the NSW Flood program.

The approach adopted for this study has been influenced by the study objectives, accepted

practice and the quality and quantity of available data. There are two basic approaches to

determining design flood levels namely:

• a flood frequency approach based upon a statistical analysis of the flood record, and

• using a rainfall/runoff routing approach (hydrologic modelling) to obtain flows, and then

inputting these flows into a hydraulic model of the floodplain.

The flood frequency approach was undertaken as part of an earlier study for the Bellinger,

Kalang, and Nambucca River catchments. The results of Reference 4 were used to inform the

current study. Flood frequency analysis was undertaken at Bowraville.

A hydrologic (WBNM, Watershed Bounded Network Model, Reference 5) model was established

for each catchment to determine inflows into the hydrodynamic model. A combined one and two

dimensional hydrodynamic (TUFLOW) model was established for each system to define the

flood behaviour using ALS and hydrosurvey.

The TUFLOW models were calibrated and verified to a range of historical events (Table 4).

Table 4: Calibration and Verification Events

Catchment Calibration and Verification Events

Nambucca River and Warrell Creek 1972, 1977, 2009

The calibrated hydraulic models were then used to assess the flood levels and hydraulic flood

hazard for the 5 Year ARI, 10%, 2%, 1%, 0.5%, 0.2%, 0.05% AEP and Probable Maximum

Flood (PMF) design events.



12

5. HYDROLOGIC MODELLING

5.1. Overview

Hydrologic models of the Bellinger, Kalang and Nambucca and Warrell Creek catchments were

established as part of Review of Bellinger, Kalang and Nambucca River Catchments Hydrology

Report (Reference 4). All models were developed using the Watershed Bounded Network Model

(WBNM).

WBNM (Reference 5) is widely used throughout Australia and particularly NSW. WBNM

simulates a catchment and its tributaries as a series of sub-catchment areas linked together to

replicate the rainfall and runoff process through the natural stream network. Input data includes

the definition of physical catchment characteristics including surface area of sub-catchments,

proportion of impervious surfaces, stream length adjustments, initial and continuing losses,

temporal and spatial patterns over the catchment.

Key parameters for WBNM represent the physical characteristics of the catchment. Typical

model parameters include;

• Rainfall Losses: two values, initial and continuing loss, modify the amount of rainfall

excess to be routed through the model sub-catchments;

• Lag Parameter: this affects the timing of the runoff response to the rainfall and is subject

to catchment size, shape and slope; and

• Non Linearity Exponent: adjustment of the non-linearity of catchment response.

The parameters adopted for this study were based on those recommended in ARR 1987

(Reference 1), previous experience and calibration. Some of the information is summarised

briefly below and further details on the parameters used for each of the catchments can be

found in Reference 4). A good fit to observed data was achieved with default lag and non

linearity parameters.

5.2. Review of Bellinger, Kalang and Nambucca River Catchments

Hydrology

Full details of the development of the WBNM model can be found in Reference 4. Figure 3

shows the hydrological model layout for the Nambucca River and Warrell Creek. As part of the

Review of Bellinger, Kalang and Nambucca River Catchments Hydrology Report (Reference 4)

historical rainfall data was obtained at a number of locations within and surrounding the study

area. Historic stream flow data was also obtained for a number of gauges within the catchments.

A number of stream flow gauges within the catchment was investigated to provide an indication

of the reliability of the rating curves used. Due to the unreliability of the extrapolation techniques

used in extending rating curves for many of the stream flow gauges new rating curves were

estimated in locations where cross sections at the gauge location were available.

Flows for historical events for 1972, 1977 and 2009 were estimated using the hydrologic model



13

and calibrated against known flood levels and flood flows at a number of gauging locations. As

with the previous models, temporal and spatial patterns were established from recorded rainfall

data in the region.

Due to concerns over the ARR 1987 design rainfall estimates, revised estimates were produced

for a range of design events in an approach consistent with that being proposed for the new

version of ARR. Design rainfalls for events up to the 0.2% AEP event were established from the

revised IFDs whilst the PMP estimates were made using the Generalised Tropical Storm

Method as Revised (GTSMR).

Temporal patterns were also applied to the design rainfall as described in Reference 4. Initial

and continuing losses were varied with event size. Losses for the 0.5% and 0.2% AEP were

calculated in accordance with ARR Book IV (Reference 17) by interpolating between the 1%

AEP and PMF losses. More detail on the hydrologic model development can be found in

Reference 4. For design events, the 48 hour storm was found to be the critical duration for the 5

year, 10%, and 2% AEP events. For the 1%, 0.5% and 0.2% AEP events the 36 hours storm

was found to be critical and the 24 hour storm critical for the PMF.

Design losses consistent with the 2% and 1% AEP were adopted for the 5 year ARI and 10%

AEP instead of those used in Reference 4 as discussed in Section 7.1.2. The adopted initial loss

was 40mm and continuing loss of 2.5mm/hr.

5.3. 0.05% AEP Event

The 0.05% AEP event rainfall depths were calculated using the guidance in Australian Rainfall

and Runoff Book VI (ARR, Reference 17) by interpolating between the 2% AEP, 1% AEP and

PMP depths. ARR Book VI recommends that the GTSMR spatial patterns are applied for an

extreme event such as the 0.05% AEP. However these were found to produce inconsistent peak

flows when compared to the 0.2% AEP and PMF events and therefore the ARR patterns as

used in the smaller design events were applied to the 0.05% AEP also. More details can be

found in Reference 4.



14

6. HYDRAULIC MODELLING

A model of the study area was developed in the hydrodynamic modelling package (TUFLOW).

TUFLOW (Reference 2) is widely used in Australia and internationally for assessing flood

behaviour and hydraulic hazard. TUFLOW is a finite difference numerical model which is

capable of solving the depth averaged shallow water equations in both the one and two

dimensional domains.

The model extent for each catchment was determined in conjunction with the Roads and

Maritime Services (RMS), Nambucca Shire Council and Office of Environment and Heritage

(OEH). The purpose of these models is to both meet the needs of Council and OEH in terms of

the NSW Flood Study Program and the RMS to assess the impacts of proposed waterway

crossings.

A combined one and two dimensional hydrodynamic model (TUFLOW) model of the Nambucca

River and Warrell Creek was established. Incorporating both systems into one model allowed for

interaction between Gumma Swamp and Warrell Creek to be properly accounted for in larger

events.

6.1. Model Configuration

The model consists of a combined one and two dimensional hydrodynamic model. A 2D 20m

grid was used to define the overbank and the channel for the Nambucca River, Warrell Creek

and its tributaries. One dimension network was used to define the main channel on the Upper

Nambucca River, mid Warrell Creek, Tilly Willy Creek and the Macksville Town Drain. The

extent of the TUFLOW model is shown in Figure 4.

The model extends a sufficient distance upstream and downstream of the study area such that

the imposed boundary conditions do not influence the model results in the region of interest.

The TUFLOW model limits were:

• Approximately 2km upstream Congarinni Road Bridge on Taylors Arm Road on Taylors

Arm,

• Upstream to Bowraville on the Nambucca River,

• Upstream to near where Browns Crossing road bridges the railway on Warrell Creek,

and

• Downstream - The Pacific Ocean.

A 20 metre grid resolution digital terrain model (DTM) was created using the topographic data

outlined in Section 2. No major topography changes occurred between the 1970’s and 2000’s.

Culverts under a number of roads were incorporated in the TUFLOW model including culverts

under the Pacific Highway, and Stuarts Island Causeway. Bridges on the Pacific Highway,

North Coast Railway and a number of local roads were modelled.



15

6.2. Boundary Conditions

Inflows and boundary conditions for the TUFLOW model consist of a number of time varying

flow hydrographs developed from the WBNM model. At the downstream boundary of the model,

a tailwater level defining the creek entrance was used. The tailwater conditions were based on

recorded tide levels at Coffs Harbour, experience on nearby catchments and OEH guidelines

(Reference 8). Figure 4 shows the inflow locations and boundary condition types.

6.3. Model Calibration

Model calibration was undertaken using historical data for the 1972 and 1977 flood events. This

historical flood was selected for calibration as observed data was available in the catchment.

The 1979 hydrosurvey was used in the vicinity of the entrance for these events.

The hydraulic efficiency of the creeks is represented (in part) within the TUFLOW model by the

roughness or friction factor, Manning’s “n” value. Manning’s “n” is used to describe the influence

of the following factors on flow behaviour:

• channel roughness,

• channel sinuosity,

• vegetation and other debris/obstructions in the channel

• bed forms and shapes.

As part of the calibration process the Manning’s “n” roughness value (Figure 5) was adjusted

within reasonable limits to best match the recorded flood water levels along the creek system.

Adopted values were selected based on an assessment of the ground cover types and

vegetation density within the floodplain. The adopted values (Refer to Table 5) were then used

for the hydraulic modelling of the design events and assessment of the proposed highway

upgrade (refer to Reference 10).

Table 5: Adopted Manning’s “n” Values

Description Manning’s n Value

Low Density Residential, Farms 0.04

Medium Density Residential / Overbank 0.06

Dense/Thick vegetation 0.08

Grass/open space 0.04

Channel 0.02, 0.025, 0.03, 0.035

Vegetated Creeks 0.045

Roads/railway line/culverts 0.02

Mangroves and Dense Timber 0.3



16

6.4. Calibration Results and Discussion

6.4.1. Timing of Flood Peaks

Flood levels at Macksville are very dependent on the relative timing of the flood peaks on the

Nambucca River and Taylor's Arm. The catchment area of both rivers to their confluence just

upstream of Macksville are similar (458 and 459 km2 respectively). While the catchment areas

are very similar the catchments have very different shapes with Taylor's Arm being long and

narrow while the Nambucca River catchment is much more compact. The Taylor's Arm

catchment also wraps around the Nambucca River on the south western and north western

sides. The differences in catchment shape result in very different response times.

In order to determine the relative timing of the two rivers historical recorded hydrographs at the

Bowraville, Utungun and Macksville gauges were compared. Historical hydrographs for a

number of events are shown on Figure 11.

For most small to moderate events from 1997 to present, flood levels at Utungun peak after

Macksville even though Macksville is located downstream. However this is somewhat

complicated by the tide interaction at Macksville. The difference between Macksville and

Utungun is shown in Table 6 and varies from -9 to +10 hrs (Calculated as the time the first

location (Macksville) occurs after the second location (Utungun)) with an average of -4.2 hrs (ie.

Macksville 4.2hrs earlier than Utungun). A timing difference of between -8.25 and -20 hours

was found to exist between the peaks at Bowraville and Utungun. These characteristics mean

that flooding at Macksville is usually caused by the peak flow on the more responsive Nambucca

catchment interacting with the rising limb of the Taylor's Arm hydrograph.

As the flood hydrograph at Macksville is complicated by the tides, the best locations to look at

for timing is Bowraville to Utungun (which are a similar distance upstream of the junction of the

Nambucca River and Taylors Arm).

This timing difference was not reproduced using a fixed design storm in the WBNM model. The

coinciding timing produced unrealistic flood levels at Macksville (for example the 100yr level was

0.7 m higher than those presented in Table 10). In order to reproduce this historic timing

difference, a 11 hour delay was adopted. Figure 12 shows stage vs frequency at Macksville for

historic events along with the design flood levels produced by this study. Also shown is the 1%

AEP flood level if a timing delay was not applied, which is significantly higher than the historical

trend.

Table 6: Timing of Historical Flood Peaks Nambucca River – Bowraville, Utungun and Macksville

Event Timing Difference of Peaks between locations (hours)

Bowraville - Utungun Macksville - Bowraville Macksville - Utungun

Mar-2009 -15.75 7.75 -8

Jul-1999 -8.25 -0.75 -9

Feb-2009 -13 6 -7

May-2009 -20.75 30.75 10

Feb-2001 -13.5 6.5 -7



17

Oct-2009 -6 8.5 -14.5

NB. Calculated as the time the flood peak level at the first location occurs after the second location. Timing at

Macksville is effected by tide.

6.4.2. Results

Peak flood levels and depths for the 1972 and 1977 events are shown in Figure 8 and Figure 9.

No calibration points were available on Warrell Creek for either event. In order to match

observed levels between just upstream of the junction of the Nambucca River and Taylor’s Arm

and Goats Island, a Manning’s n value of 0.02 was adopted for the main channel. It is noted that

the 1994 Lower Nambucca Flood Study similarly had to lower the Manning’s n value within this

reach to match observed levels.

The model calibrated well to observed flood levels (Table 7). For the 1992 event, the modelled

levels were generally within 0.2m of the observed values, with one value within 0.4m. The model

showed a bias towards being slightly high.

The model also calibrated well to observed flood levels for the 1977 event (Table 7). Modelled

flood levels were generally within 0.3m of the observed flood levels. The observed point just

upstream of the Joffre St bridge on Taylors arm is approximately 0.6m lower than the modelled

level. This observed point appears to be in conflict with the observed level a similar distance

upstream of the junction on the Nambucca River. The modelled results are slightly lower than

observed in the vicinity of the entrance and slightly high in the vicinity of Macksville.

Table 7: Calibration Events - Modelled vs Observed Flood Levels

Event Location

Observed

Level

(mAHD)

Modelled

Level

(mAHD)

Difference

(m)

1972

Cnr River And Princess Sts, Macksville 2.5 2.61 0.11

1.8km Downstream Of Tewinga School. 3.8 3.93 0.13

Congarinni Bridge 3.4 3.8 0.4

Nambucca River And Gumma Gumma Creek Junction 2.3 2.3 0

Watt Creek (Mr Sc Lumsden) (Backwater) 2 2.2 0.2

1977

Macksville Showground. 2.9 3.13 0.23

Nambucca Marine, Nambucca Heads. 2.05 1.99 -0.06

Nambucca Heads Golf Club. 2.25 2.17 -0.08

Hacienda Motor Inn, Nambucca Heads 2.4 2.4 0

Pacific Highway, Macksville (Backwater) 2.4 2.64 0.24

Cnr River And Princess Streets, Macksville 2.65 2.95 0.3

Cnr East And River Sts, Macksville (Backwater) 2.65 2.69 0.04

Cnr West And Mckay Sts, Macksville (Backwater) 2.6 2.71 0.11

Wilson Rd Adjacent To Bridge (Backwater) 2.6 3.17 0.57

Newee Creek, 0.4km Upstream Nambucca River Junction

(Backwater) 2.5 2.82 0.32

Pacific Highway At Watt Creek. 2.45 2.53 0.08

Macksville Nursing Home (Backwater) 2.4 2.62 0.22

Pacific Highway At Teaques Creek. 2.25 2.27 0.02

Pacific Highway 0.5km North Of Hacienda Motor Inn (Backwater) 2.25 2.38 0.13



18

6.5. Model Verification

Model verification was conducted, using the calibrated model and parameters (as discussed in

Section 6.3) for the March/April 2009 event. This event while slightly lower than the October

event on the Nambucca River was associated with a significant data collection exercise and so

provided calibration data. Time varying water level data was also available for the lower estuary

(at Macksville and Stuarts Island) for this event. Inflows for the March/April 2009 event were

developed as part of Reference 4.

6.6. Verification Results and Discussion

Peak flood levels for the March/April 2009 event are shown in Figure 10. The model calibrated

well to observed flood levels (Table 8). The modelled levels were generally within 0.3 m of the

observed values for the 2009 event. The model showed a bias towards being slightly high. Very

little information was available for Warrell Creek.

Table 8: Model Validation – Modelled vs Observed Levels –2009 event

River Location Observed Level

(mAHD)

Modelled Level

(mAHD)

Difference

(m)

Warrell Ck

Scotts Head Road. Warrell

Creek 3.125 3.34 0.215

Scotts Head Road. Barbers

Flat 3.1 2.79 -0.31

Nambucca

River

Congarrinni Road, Macksville 2.27 2.43 0.16

Macksville Gauge 2.24 2.26 0.02

Stuarts Island Gauge data 1.48 1.33 -0.15



19

7. DESIGN FLOOD BEHAVIOUR

7.1.1. Boundary Conditions

7.1.1.1. Design Inflows

As with the historical events the TUFLOW inflows for the 5 Year ARI, 10, 2, 1, 0.5, 0.2 and 0.05

% AEP and Probable Maximum Flood (PMF) design events were obtained from a number of

time varying flow hydrographs taken from the WBNM model (refer to Section 5 and Reference

4). These inflow hydrographs were then applied to the calibrated TUFLOW hydraulic model to

produce design flood levels. The timing delay identified in the calibration events was applied to

the design events.

7.1.1.2. Tailwater Conditions

In addition to runoff from the catchment, the lower reaches of the estuary can also be influenced

by backwater effects resulting from elevated ocean levels. Hence, the height of the tide at the

time of the arrival of the peak runoff from the catchment can also have an influence on flood

levels in the lower reaches. However, these two distinct flooding mechanisms may or may not

result from the same storm. Consideration must therefore be given to accounting for the joint

probability of coincident flooding from both catchment runoff and backwater effects due to

elevated ocean levels.

A full joint probability analysis is beyond the scope of the present study. Traditionally, it is

common practice to estimate design flood levels in these situations using a ‘peak envelope’

approach that adopts the highest of the predicted levels from the two mechanisms.

Design tidal hydrographs in this study were based on the experience in nearby catchments,

previous studies in the area, and OEH guidelines. Reference 8 recommends the use of a 2.6

mAHD 1% AEP tide (including wave run up, wave setup etc) for a small untrained narrow and

shallow entrance. For large entrances it suggests a site specific assessment be undertaken. The

recommended 2.6mAHD includes ocean components that would not be present for the

Nambucca River entrance such as wave set up and runup. These effects are of a short term

nature and due to the large storage volume in the estuary and the long period of flooding, wave

setup would not persist for long enough to effect flood level on the Nambucca and in Warrell

Creek.

A 1% AEP tide level of 2.2mAHD and 2.4mAHD has been adopted by previous studies on the

catchment. A 1% AEP peak tide of 2.1 – 2.2 mAHD is considered more appropriate given the

trained nature of the entrance. A sensitivity analysis was undertaken to determine the extent of

the influence of the tide level. The tide level was found to have an influence up to Stuarts Island.

Given the 2011 Nambucca Heads study has been adopted for planning purposes, a 1% AEP

tide level of 2.4mAHD has been adopted to be consistent with Council policy.



20

Table 9: Ocean Boundary Peaks (mAHD)

Event Peak Ocean Level (mAHD)

5 Year ARI 1.64

10% AEP 1.86

2% AEP 2.26

1% AEP 2.4

The influence of these varying tailwater assumptions were mainly confined to the lower reaches.

In addition to the above it is reasonable to expect that the effects of a severe storm in terms of

ocean levels and runoff could be coincident for a catchment of this size. Hence to establish the

design flood levels in the present study, the relative phasing of the ocean levels was adjusted

such that the peak of the tidal hydrograph would approximately coincide with the peak of the

catchment runoff.

For the 0.5%, 0.2%, and 0.05% AEP and PMF events, the 1% AEP design tidal hydrograph was

adopted. For events of 1% AEP or more frequent river dominated events were enveloped with

the corresponding ocean tide case at the river entrance.

7.1.2. Design Event Results

Peak flood level profiles for the 5 Year ARI, 10, 2, 1, 0.5, 0.2 and 0.05 % AEP and Probable

Maximum Flood (PMF) design events are presented in Figure 13 to Figure 15. Peak flood levels

and depths within the study area for the design events are presented in Figure 16 to Figure 23.

Peak velocities are depicted in Figure 24 to Figure 31. Table 10 documents design flood levels

at key locations.

Design losses consistent with the 2% and 1% AEP were adopted for the 5 year ARI and 10%

AEP were modified from those adopted in Reference 4, as when the inflow hydrographs were

applied to the TUFLOW model the resultant flood levels at Macksville did not fit the on historical

trend (Figure 12).

Table 10: Design Flood Levels at Key Locations

ID Location

Flood Level (mAHD)

5YR

ARI

10%

AEP

2%

AEP

1%

AEP

0.5%

AEP

0.2%

AEP

0.05%

AEP PMF

1 D/S Lanes Bridge, Bowraville 8.67 9.28 10.49 11.20 11.72 12.36 13.31 16.17

2

Railway Bridge, Macksville / near

Nambucca and Taylors Arm

Confluence

2.15 2.35 3.48 3.89 4.46 5.20 6.17 9.44

3 Pacific Highway Bridge, Macksville 2.09 2.25 3.31 3.67 4.20 4.92 5.90 9.17

4 Goat Island 1.98 2.09 3.05 3.41 3.91 4.51 5.45 8.68

5 Stuarts Island 1.73 1.91 2.30 2.42 2.57 3.05 3.75 6.21

6 Congarinni Road Birdge, Taylors 3.40 3.97 5.11 5.51 5.82 6.26 7.06 10.11



21

Arm

7 Gumma Gumma Swamp 1.71 1.94 3.01 3.39 3.92 4.53 5.48 8.73

8 Near Sawmill, Warrell Creek 6.39 6.86 7.90 8.74 9.45 10.09 11.33 13.82

9 Pacific Highway Near Scotts Head

Road, Warrell Creek 3.20 3.79 5.03 5.71 6.30 7.08 8.56 12.42

10 New Pacific Highway Crossing

(approved alignment) 2.03 2.13 3.13 3.47 3.97 4.57 5.51 8.75

Table 11 compares estimates of the 1% AEP flood level at Macksville from the current and

previous studies. Differences between the 1994 Flood Study and the current study are a result

of:

• The use of a two dimensional model (current study) compared to a one dimensional

model (1994 study)

• Improved IFD estimates in the current study

• Change in hydrologic model to one with more realistic parameters.

Table 11: Comparison of 1% AEP Flood Levels at Macksville

Study Existing Pacific Highway

Bridge at Macksville (mAHD)

New Pacific Highway Crossing (Approved Alignment) (mAHD)

1994 Lower Nambucca Flood Study 3.55 3.35 - 3.4

RTA 2010 - 3.77

Current Study 3.67 3.47

Current Study if timing delay not applied 4.32 -

7.1.3. Sensitivity Analysis

The model established for this study relies on a number of assumed parameters, the values of

which are considered to be the most appropriate for the study area. A range of sensitivity

analysis was undertaken on different key parameters in order to quantify potential variations

corresponding to different modelling assumptions.

7.1.3.1. Modelling Scenarios and Assumptions

The following scenarios were considered to represent the envelope of likely parameter values:

• ± change in loss rates in the WBNM hydrologic model,

• ± 20% change in the C storage routing parameter in the WBNM hydrologic

model,

• ± 20% change in Manning’s “n” value, and

• Blockage of culverts and bridges (50%),

• Ocean Boundary Conditions.

For the scenarios listed above the hydrologic/hydraulic models were run for the 1% AEP design

storm and the results are provided in Table 12.



22

Change in loss values was found to have approximately ± 0.1m impact on flood level. Varying

the storage routing parameter was found to have up to 0.4m impact on the flood level.

A 20% increase and decrease in manning’s value was found to have minimal impact on flood

levels other than at Bowraville. Of more importance than the roughness value for flood levels in

the vicinity of Macksville is the relative roughness of the channel and overbank. The relative

roughness determines how much water is conveyed by the channel and the volume that spills

into Gumma Swamp.

All bridges with spans less than 6m and all culverts were blocked by 50% to determine

sensitivity to blockage. The impacts of blockage are localised and significant impacts are

summarised in Table 13.

Table 12: Sensitivity Analyses

Location

1% AEP

Flood

Level

(mAHD)

Flood Level (mAHD)

+ Loss -Loss +20%

C

-20%

C

+20%

Mannings

-20%

Mannings

50%

Blockage

D/S Lanes Bridge,

Bowraville 11.20 -0.08 0.07 -0.32 0.31 0.53 -0.60 0.00

Railway Bridge,

Macksville / near

Nambucca and Taylors

Arm Confluence

3.89 -0.12 0.12 -0.19 0.19 0.28 -0.32 0.00

Pacific Highway Bridge,

Macksville 3.67 -0.11 0.11 -0.17 0.18 0.29 -0.33 0.00

Goat Island 3.41 -0.11 0.11 -0.17 0.18 0.27 -0.31 0.00

Stuarts Island 2.42 -0.28 -0.13 -0.30 -0.10 0.00 0.00 0.00

Congarinni Road Birdge,

Taylors Arm 5.51 -0.05 0.05 -0.21 0.21 0.18 -0.23 0.00

Gumma Gumma Swamp 3.39 -0.12 0.12 -0.17 0.19 0.28 -0.32 0.00

Near Sawmill, Warrell

Creek 8.74 -0.10 0.13 -0.44 0.39 0.37 -0.48 0.00

Pacific Highway Near

Scotts Head Road,

Warrell Creek

5.71 -0.10 0.11 -0.19 0.20 0.24 -0.28 0.00

Table 13: Impacts of Blockage

Location Impact (m)

Railway Over Stony Creek 0.22

Cockburns Lane Over Small Creek 0.04

Railway Over Butchers Creek 0.04

Scotts Head Road Over Way Way Creek Tributary 0.08



23

7.1.4. Climate Change

The 2005 Floodplain Development Manual (Reference 7) requires that Flood Studies and

Floodplain Risk Management Studies consider the impacts of climate change (sea level rise and

rainfall increase) on flood behaviour. Both RMS and the State Government have a similar

policy. Both rainfall increases and sea level rise could affect the Nambucca River and Warrell

Creek. In accordance with the Department of Environment, Climate Change and Water

(DECCW) – Floodplain Risk Management Guideline 2007 (Reference 9), the following climate

change scenarios are considered in this climate change assessment:

• Increase in peak rainfall and storm volume(rainfall by the year 2070):

- low level rainfall increase = 10%,

- medium level rainfall increase = 20%,

- high level rainfall increase = 30%.

• Sea level rise:

- a 0.4m increase in level by year 2050

- a 0.9m increase in level by year 2100

A high level rainfall increase of up to 30% is recommended for consideration due to the

uncertainties associated with this aspect of climate change. It is understood that work currently

being undertaken by Engineers Australia, CSIRO and the Bureau of Meteorology as part of the

revision of Australian Rainfall and Runoff which will provide better direction on the possible

impacts of climate change on rainfall.

A rainfall increase of 10% is likely to increase 1% AEP flood levels over most of the catchment

by 0.35-0.5m. A rainfall increase of 30% is likely to increase 1% AEP flood levels over the

catchment by 0.5-1.1m. Smaller increases occur near the entrance. A sea level rise of 0.4m will

result in an increase of in 1% AEP flood levels of approximately 0.02m in the mid reaches of the

Nambucca River and Warrell Creek. Closer to the entrance increases in the flood level will be

higher with 0.4m expected at Stuarts Island. Table 14 summarises the impacts of climate

change on the 1% AEP flood level.

Table 14: Climate Change Results

Location

1% AEP

Flood Level

(mAHD)

Change in Flood Level (m)

10%

Rainfall

Increase

20%

Rainfall

Increase

30%

Rainfall

Increase

2050 Sea

Level

Rise

2100 Sea

Level

Rise

D/S Lanes Bridge,

Bowraville 11.20 0.38 0.73 1.06 0.00 0.00

Railway Bridge, Macksville /

near Nambucca and

Taylors Arm Confluence

3.89 0.38 0.75 1.08 0.02 0.09

Pacific Highway Bridge,

Macksville 3.67 0.34 0.69 1.02 0.02 0.08

Goat Island 3.41 0.34 0.62 0.90 0.02 0.09

Stuarts Island 2.42 0.03 0.25 0.46 0.38 0.86



24

Congarinni Road Birdge,

Taylors Arm 5.51 0.20 0.40 0.61 0.00 0.01

Gumma Gumma Swamp 3.39 0.36 0.65 0.93 0.02 0.10

Near Sawmill, Warrell

Creek 8.74 0.49 0.87 1.18 0.00 0.00

Pacific Highway Near

Scotts Head Road, Warrell

Creek

5.71 0.39 0.78 1.13 0.02 0.06



25

8. CONCLUSIONS

A detailed hydraulic model (TUFLOW) has been developed to quantify the flood behaviour of the

Nambucca River and Warrell Creek, making best use of the data currently available. This model

has been used to reproduce the historical flood behaviour from events in 1972, 1977 and 2009.

The TUFLOW model has been used to define flood behaviour for a range of design events (5

Year ARI, 10, 2, 1, 0.5, 0.2 and 0.05 % AEP and Probable Maximum Flood).

The model developed for the current study is suitable for use in a subsequent Flood Study and

design assessment for the Pacific Highway Upgrade.



26

9. REFERENCES

1. Pilgrim DH (Editor in Chief)

Australian Rainfall and Runoff – A Guide to Flood Estimation

Institution of Engineers, Australia, 1987.

2. WBM BMT

Tuflow User Manual – GIS Based 2D/1D Hydrodynamic Modelling

2010

3. SKM

Warrell Creek to Urunga Environmental assessment Volume 3 –Working paper 5

Water (flooding and water quality)

January 2010

4. WMAwater

Review of the Bellinger, Kalang and Nambucca River Catchments Hydrology

July 2011

5. Boyd M, Rigby T, VanDrie R, and Schymitzek I

WBNM User Guide

2007

6. RTA

Warrell Creek to Urunga Upgrading the Pacific Highway Environmental

Assessment - Volume 1 Environmental Assessment

January 2010

7. NSW Government

Floodplain Development Manual: The management of flood liable land

April 2005

8. Department of Infrastructure, Planning and Natural Resources

Floodplain Management Guideline No 5 – Ocean Boundary Conditions

9. NSW Government

Draft Sea Level Rise Policy Statement

2009



27

10. WMAwater

Warrell Creek to Urunga – Pacific Highway Upgrade Modelling

2012

11. PWD

Nambucca River Flood History 1843-1979

1980

12. Flood Plain Management Studies Steering Committee

New South Wales Coastal Rivers Floodplain Management Studies the

Nambucca Valley

July 1981

13. Department of Public Works

Macksville Flood Study

1983

14. PWD

Lower Nambucca River Flood Study

1994

15. Nambucca Shire Council

Compilation of Flood Marks, Rainfall Data and Observed Flood Behaviour for

the March/April Flood Event of 2009

2009

16. SKM

Nambucca Heads Flood Study

2011

17. Nathan, RJ and Weinmann, E,

Estimation of Large to Extreme Floods, Book VI in Australian Rainfall and

Runoff - A Guide to Flood Estimation,

The Institution of Engineers, Australia, Barton, ACT, 1999.


WMAwater

111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A1

APPENDIX A: GLOSSARY

Taken from the Floodplain Development Manual (April 2005 edition)

acid sulfate soils

Are sediments which contain sulfidic mineral pyrite which may become extremely

acid following disturbance or drainage as sulfur compounds react when exposed

to oxygen to form sulfuric acid. More detailed explanation and definition can be

found in the NSW Government Acid Sulfate Soil Manual published by Acid Sulfate

Soil Management Advisory Committee.

Annual Exceedance

Probability (AEP)

The chance of a flood of a given or larger size occurring in any one year, usually

expressed as a percentage. For example, if a peak flood discharge of 500 m3/s

has an AEP of 5%, it means that there is a 5% chance (that is one-in-20 chance)

of a 500 m3/s or larger event occurring in any one year (see ARI).

Australian Height Datum

(AHD)

A common national surface level datum approximately corresponding to mean

sea level.

Average Annual Damage

(AAD)

Depending on its size (or severity), each flood will cause a different amount of

flood damage to a flood prone area. AAD is the average damage per year that

would occur in a nominated development situation from flooding over a very long

period of time.

Average Recurrence

Interval (ARI)

The long term average number of years between the occurrence of a flood as big

as, or larger than, the selected event. For example, floods with a discharge as

great as, or greater than, the 20 year ARI flood event will occur on average once

every 20 years. ARI is another way of expressing the likelihood of occurrence of

a flood event.

caravan and moveable

home parks

Caravans and moveable dwellings are being increasingly used for long-term and

permanent accommodation purposes. Standards relating to their siting, design,

construction and management can be found in the Regulations under the LG Act.

catchment

The land area draining through the main stream, as well as tributary streams, to a

particular site. It always relates to an area above a specific location.

consent authority

The Council, government agency or person having the function to determine a

development application for land use under the EP&A Act. The consent authority

is most often the Council, however legislation or an EPI may specify a Minister or

public authority (other than a Council), or the Director General of DIPNR, as

having the function to determine an application.

development

Is defined in Part 4 of the Environmental Planning and Assessment Act (EP&A

Act).

infill development: refers to the development of vacant blocks of land that are

generally surrounded by developed properties and is permissible under the

current zoning of the land. Conditions such as minimum floor levels may be

imposed on infill development.

new development: refers to development of a completely different nature to that

associated with the former land use. For example, the urban subdivision of an

area previously used for rural purposes. New developments involve rezoning and

typically require major extensions of existing urban services, such as roads, water

supply, sewerage and electric power.


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redevelopment: refers to rebuilding in an area. For example, as urban areas

age, it may become necessary to demolish and reconstruct buildings on a

relatively large scale. Redevelopment generally does not require either rezoning

or major extensions to urban services.

disaster plan (DISPLAN)

A step by step sequence of previously agreed roles, responsibilities, functions,

actions and management arrangements for the conduct of a single or series of

connected emergency operations, with the object of ensuring the coordinated

response by all agencies having responsibilities and functions in emergencies.

discharge

The rate of flow of water measured in terms of volume per unit time, for example,

cubic metres per second (m3/s). Discharge is different from the speed or velocity

of flow, which is a measure of how fast the water is moving for example, metres

per second (m/s).

ecologically sustainable

development (ESD)

Using, conserving and enhancing natural resources so that ecological processes,

on which life depends, are maintained, and the total quality of life, now and in the

future, can be maintained or increased. A more detailed definition is included in

the Local Government Act 1993. The use of sustainability and sustainable in this

manual relate to ESD.

effective warning time

The time available after receiving advice of an impending flood and before the

floodwaters prevent appropriate flood response actions being undertaken. The

effective warning time is typically used to move farm equipment, move stock,

raise furniture, evacuate people and transport their possessions.

emergency management

A range of measures to manage risks to communities and the environment. In

the flood context it may include measures to prevent, prepare for, respond to and

recover from flooding.

flash flooding

Flooding which is sudden and unexpected. It is often caused by sudden local or

nearby heavy rainfall. Often defined as flooding which peaks within six hours of

the causative rain.

flood

Relatively high stream flow which overtops the natural or artificial banks in any

part of a stream, river, estuary, lake or dam, and/or local overland flooding

associated with major drainage before entering a watercourse, and/or coastal

inundation resulting from super-elevated sea levels and/or waves overtopping

coastline defences excluding tsunami.

flood awareness

Flood awareness is an appreciation of the likely effects of flooding and a

knowledge of the relevant flood warning, response and evacuation procedures.

flood education

Flood education seeks to provide information to raise awareness of the flood

problem so as to enable individuals to understand how to manage themselves an

their property in response to flood warnings and in a flood event. It invokes a

state of flood readiness.

flood fringe areas

The remaining area of flood prone land after floodway and flood storage areas

have been defined.

flood liable land

Is synonymous with flood prone land (i.e. land susceptible to flooding by the

probable maximum flood (PMF) event). Note that the term flood liable land

covers the whole of the floodplain, not just that part below the flood planning level


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(see flood planning area).

flood mitigation standard

The average recurrence interval of the flood, selected as part of the floodplain risk

management process that forms the basis for physical works to modify the

impacts of flooding.

floodplain

Area of land which is subject to inundation by floods up to and including the

probable maximum flood event, that is, flood prone land.

floodplain risk

management options

The measures that might be feasible for the management of a particular area of

the floodplain. Preparation of a floodplain risk management plan requires a

detailed evaluation of floodplain risk management options.

floodplain risk

management plan

A management plan developed in accordance with the principles and guidelines

in this manual. Usually includes both written and diagrammetic information

describing how particular areas of flood prone land are to be used and managed

to achieve defined objectives.

flood plan (local)

A sub-plan of a disaster plan that deals specifically with flooding. They can exist

at State, Division and local levels. Local flood plans are prepared under the

leadership of the State Emergency Service.

flood planning area

The area of land below the flood planning level and thus subject to flood related

development controls. The concept of flood planning area generally supersedes

the Aflood liable land@ concept in the 1986 Manual.

Flood Planning Levels

(FPLs)

FPL=s are the combinations of flood levels (derived from significant historical

flood events or floods of specific AEPs) and freeboards selected for floodplain risk

management purposes, as determined in management studies and incorporated

in management plans. FPLs supersede the Astandard flood event@ in the 1986

manual.

flood proofing

A combination of measures incorporated in the design, construction and alteration

of individual buildings or structures subject to flooding, to reduce or eliminate flood

damages.

flood prone land

Is land susceptible to flooding by the Probable Maximum Flood (PMF) event.

Flood prone land is synonymous with flood liable land.

flood readiness

Flood readiness is an ability to react within the effective warning time.

flood risk

Potential danger to personal safety and potential damage to property resulting

from flooding. The degree of risk varies with circumstances across the full range

of floods. Flood risk in this manual is divided into 3 types, existing, future and

continuing risks. They are described below.

existing flood risk: the risk a community is exposed to as a result of its location

on the floodplain.

future flood risk: the risk a community may be exposed to as a result of new

development on the floodplain.

continuing flood risk: the risk a community is exposed to after floodplain risk

management measures have been implemented. For a town protected by levees,

the continuing flood risk is the consequences of the levees being overtopped. For

an area without any floodplain risk management measures, the continuing flood

risk is simply the existence of its flood exposure.


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flood storage areas

Those parts of the floodplain that are important for the temporary storage of

floodwaters during the passage of a flood. The extent and behaviour of flood

storage areas may change with flood severity, and loss of flood storage can

increase the severity of flood impacts by reducing natural flood attenuation.

Hence, it is necessary to investigate a range of flood sizes before defining flood

storage areas.

floodway areas

Those areas of the floodplain where a significant discharge of water occurs during

floods. They are often aligned with naturally defined channels. Floodways are

areas that, even if only partially blocked, would cause a significant redistribution of

flood flows, or a significant increase in flood levels.

freeboard

Freeboard provides reasonable certainty that the risk exposure selected in

deciding on a particular flood chosen as the basis for the FPL is actually provided.

It is a factor of safety typically used in relation to the setting of floor levels, levee

crest levels, etc. Freeboard is included in the flood planning level.

habitable room

in a residential situation: a living or working area, such as a lounge room, dining

room, rumpus room, kitchen, bedroom or workroom.

in an industrial or commercial situation: an area used for offices or to store

valuable possessions susceptible to flood damage in the event of a flood.

hazard

A source of potential harm or a situation with a potential to cause loss. In relation

to this manual the hazard is flooding which has the potential to cause damage to

the community. Definitions of high and low hazard categories are provided in the

Manual.

hydraulics

Term given to the study of water flow in waterways; in particular, the evaluation of

flow parameters such as water level and velocity.

hydrograph

A graph which shows how the discharge or stage/flood level at any particular

location varies with time during a flood.

hydrology

Term given to the study of the rainfall and runoff process; in particular, the

evaluation of peak flows, flow volumes and the derivation of hydrographs for a

range of floods.

local overland flooding

Inundation by local runoff rather than overbank discharge from a stream, river,

estuary, lake or dam.

local drainage

Are smaller scale problems in urban areas. They are outside the definition of

major drainage in this glossary.

mainstream flooding

Inundation of normally dry land occurring when water overflows the natural or

artificial banks of a stream, river, estuary, lake or dam.

major drainage

Councils have discretion in determining whether urban drainage problems are

associated with major or local drainage. For the purpose of this manual major

drainage involves:

$ the floodplains of original watercourses (which may now be piped,

channelised or diverted), or sloping areas where overland flows develop along

alternative paths once system capacity is exceeded; and/or


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$ water depths generally in excess of 0.3 m (in the major system design storm

as defined in the current version of Australian Rainfall and Runoff). These

conditions may result in danger to personal safety and property damage to

both premises and vehicles; and/or

$ major overland flow paths through developed areas outside of defined

drainage reserves; and/or

$ the potential to affect a number of buildings along the major flow path.

mathematical/computer

models

The mathematical representation of the physical processes involved in runoff

generation and stream flow. These models are often run on computers due to the

complexity of the mathematical relationships between runoff, stream flow and the

distribution of flows across the floodplain.

merit approach

The merit approach weighs social, economic, ecological and cultural impacts of

land use options for different flood prone areas together with flood damage,

hazard and behaviour implications, and environmental protection and well being

of the State=s rivers and floodplains.

The merit approach operates at two levels. At the strategic level it allows for the

consideration of social, economic, ecological, cultural and flooding issues to

determine strategies for the management of future flood risk which are formulated

into Council plans, policy and EPIs. At a site specific level, it involves

consideration of the best way of conditioning development allowable under the

floodplain risk management plan, local floodplain risk management policy and

EPIs.

minor, moderate and major

flooding

Both the State Emergency Service and the Bureau of Meteorology use the

following definitions in flood warnings to give a general indication of the types of

problems expected with a flood:

minor flooding: causes inconvenience such as closing of minor roads and the

submergence of low level bridges. The lower limit of this class of flooding on the

reference gauge is the initial flood level at which landholders and townspeople

begin to be flooded.

moderate flooding: low-lying areas are inundated requiring removal of stock

and/or evacuation of some houses. Main traffic routes may be covered.

major flooding: appreciable urban areas are flooded and/or extensive rural areas

are flooded. Properties, villages and towns can be isolated.

modification measures

Measures that modify either the flood, the property or the response to flooding.

Examples are indicated in Table 2.1 with further discussion in the Manual.

peak discharge

The maximum discharge occurring during a flood event.

Probable Maximum Flood

(PMF)

The PMF is the largest flood that could conceivably occur at a particular location,

usually estimated from probable maximum precipitation, and where applicable,

snow melt, coupled with the worst flood producing catchment conditions.

Generally, it is not physically or economically possible to provide complete

protection against this event. The PMF defines the extent of flood prone land,

that is, the floodplain. The extent, nature and potential consequences of flooding

associated with a range of events rarer than the flood used for designing

mitigation works and controlling development, up to and including the PMF event


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should be addressed in a floodplain risk management study.

Probable Maximum

Precipitation (PMP)

The PMP is the greatest depth of precipitation for a given duration

meteorologically possible over 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

(World Meteorological Organisation, 1986). It is the primary input to PMF

estimation.

probability

A statistical measure of the expected chance of flooding (see AEP).

risk

Chance of something happening that will have an impact. It is measured in terms

of consequences and likelihood. In the context of the manual it is the likelihood of

consequences arising from the interaction of floods, communities and the

environment.

runoff

The amount of rainfall which actually ends up as streamflow, also known as

rainfall excess.

stage

Equivalent to Awater level@. Both are measured with reference to a specified

datum.

stage hydrograph

A graph that shows how the water level at a particular location changes with time

during a flood. It must be referenced to a particular datum.

survey plan

A plan prepared by a registered surveyor.

water surface profile

A graph showing the flood stage at any given location along a watercourse at a

particular time.

wind fetch

The horizontal distance in the direction of wind over which wind waves are

generated.

HYDRAULIC MODELLING REPORT NAMBUCCA RIVER AND … · 2014-07-28 · HYDRAULIC MODELLING REPORT-...

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