Post on 10-Oct-2019
transcript
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: wma@wmawater.com.au 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 18: Design Event – Peak Flood Depths and Level Contours – 2 % AEP Event
Figure 19: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event
Figure 20: Design Event – Peak Flood Depths and Level Contours – 0.5 % AEP Event
Figure 21: Design Event – Peak Flood Depths and Level Contours – 0.2 % AEP Event
Figure 22: Design Event – Peak Flood Depths and Level Contours – 0.05 % 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 26: Design Event – Peak Flood Velocities – 2 % AEP Event
Figure 27: Design Event – Peak Flood Velocities – 1 % AEP Event
Figure 28: Design Event – Peak Flood Velocities – 0.5 % AEP Event
Figure 29: Design Event – Peak Flood Velocities – 0.2 % AEP Event
Figure 30: Design Event – Peak Flood Velocities – 0.05 % 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
20% Rainfall Increase
Figure 34: Design Event – Peak Flood Depths and Level Contours – 1 % AEP Event with a
30% Rainfall Increase
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
3
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
1977 9.6 10.03 PINNEENA (used instead of Nambucca River
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
May-63 9.27 9.7 Nambucca River Flood history 1890-1979
Mar-46 9.17 9.6 Nambucca River Flood history 1890-1979
1972 9.1 9.53 PINNEENA (used instead of Nambucca River
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
Mar-64 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
Mar-53 8.67 9.1 Nambucca River Flood history 1890-1979
Jan-68 8.67 9.1 Nambucca River Flood history 1890-1979
March 1894 8.47 8.9 Nambucca River Flood history 1890-1979
May-1913 8.47 8.9 Nambucca River Flood history 1890-1979
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Mar-1974 2.95 Nambucca River Flood history 1890-1979
Mar-1953 2.8 Nambucca River Flood history 1890-1979
Mar-1946 2.7 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
May-1913 2.55 Nambucca River Flood history 1890-1979
May-1921 2.5 Nambucca River Flood history 1890-1979
May-1948 2.5 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
Mar-1894 2.35 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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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*
Pluviograph records for
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)
Various locations between the old
Pacific Hwy crossing to Scotts
Head, only one level for Warrell
Creek
Stage Hydrographs at 3
locations
1977*
Pluviograph records for
Bellbrook and Coffs
Harbour (52 daily)
Various locations between the old
Pacific Hwy crossing to Scotts
Head, sufficient for Nambucca
River not for Warrell Creek
Stage Hydrographs at 3
locations
Information
available
1979
Detailed
information at
several locations
1980
0 (3 hour rain fall totals
for Bellbrook, Kempsey
and Coffs Harbour)
Stage Hydrographs at 1
locations
1991
Various locations between the old
Pacific Hwy crossing to Scotts
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater 111036:WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater
111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A2
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater
111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A3
(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.
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater
111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A4
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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater
111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A5
$ 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
Hydraulic Modelling Report- Nambucca River and Warrell Creek
WMAwater
111036 :WC2U_Hydraulic_Modelling_Report_Nambucca_reissued:19 November 2013 A6
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.