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Job No:AR314
File: Adelong_Vol_1_Report [Rev 2.1].doc
Date: April 2014
Rev No: 2.1
Principal: SAB
Author: SAB
ADELONG
FLOOD STUDY
VOLUME 1 – REPORT
FINAL DRAFT REPORT
APRIL 2014
Prepared by:
Lyall & Associates
Consulting Water Engineers
Level 1, 26 Ridge Street
North Sydney NSW 2060
Tel: (02) 9929 4466
Fax: (02) 9929 4458
Email: lacewater@bigpond.com
Adelong Flood Study
Adelong_Vol_1_Report [Rev 2.1].doc i Lyall & Associates
April 2014 Rev.2.1 Consulting Water Engineers
FOREWORD
The State Government’s Flood Policy is directed at providing solutions to existing flooding
problems in developed areas and to ensuring that new development is compatible with the flood
hazard and does not create additional flooding problems in other areas.
Under the Policy, the management of flood liable land remains the responsibility of local
government. The State subsidises flood mitigation works to alleviate existing problems and
provides specialist technical advice to assist councils in the discharge of their floodplain
management responsibilities.
The Policy provides for technical and financial support by the Government through the following
four sequential stages:
1. Flood Study Determines the nature and extent of flooding.
2. Floodplain Risk Management Study Evaluates management options for the floodplain
in respect of both existing and proposed
development.
3. Floodplain Risk Management Plan Involves formal adoption by Council of a plan of
management for the floodplain.
4. Implementation of the Plan Construction of flood mitigation works to protect
existing development. Use of Local
Environmental Plans to ensure new development
is compatible with the flood hazard.
The Adelong Flood Study is jointly funded by Tumut Shire Council and the NSW Government, via
the Office of Environment and Heritage. The Flood Study constitutes the first stage of the
Floodplain Risk Management process (refer over) for this area and has been prepared for Tumut
Shire Council to define flood behaviour under current conditions.
ACKNOWLEDGEMENT
Tumut Shire Council has prepared this document with financial assistance from the NSW
Government through its Floodplain Management Program. This document does not necessarily
represent the opinions of the NSW Government or the Office of Environment and Heritage.
Adelong Flood Study
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FLOODPLAIN RISK MANAGEMENT PROCESS
Implementation of the
Plan will allow Council to
reduce the impact of
flooding on the
community through flood,
property, and response
modification measures.
The measures may
include structural works,
planning controls, flood
warnings, flood readiness
and response plans,
ongoing data collection
and monitoring.
Adelong Floodplain
Risk Management
Committee
Previous Studies,
1986 to Date
Flood Study
(in progress)
Established by Tumut Shire Council, and
includes community groups and State
Agency specialists
A reconnaissance study
was undertaken by the
then Water Resources
Commission of NSW (now
OEH) in 1986, which
documented the pattern of
flooding experienced in
Adelong during the 1984
flood. The NSW State
Emergency Service (SES)
later commissioned
studies to capture flood
intelligence following the
October 2010 and March
2012 floods.
Involves detailed
hydrologic and
hydraulic modelling of
the Adelong Creek
catchment and its
tributaries in the
Study Area.
Involves the
compilation of
existing data and the
collection of
additional data.
Data Collection
(in progress)
Preferred floodplain
management options
will be publicly
exhibited and the
responses from the
community
incorporated in the
Plan. The Plan will then
be formally approved
by Council following the
public exhibition period.
Floodplain Risk
Management
Study
(future activity)
Floodplain Risk
Management
Plan
(future activity)
The Floodplain Risk
Management Study will
determine options
which will seek to
reduce the impact of
flooding on the
community in
consideration of social,
ecological and
economic factors.
Implementation
of Plan
(future activity)
Technical
Sub-Committee
Adelong Flood Study
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TABLE OF CONTENTS
Page No.
SUMMARY .................................................................................................................................. 1
1 INTRODUCTION ............................................................................................................. 1
1.1 Study Background ............................................................................................... 1
1.2 Community Consultation and Available Data ........................................................ 1 1.3 Approach to Flood Modelling ............................................................................... 2
1.3.1. Hydrologic and Hydraulic Modelling ......................................................... 2 1.3.2. Model Calibration .................................................................................... 2 1.3.3. Design Flood Estimation .......................................................................... 3
1.4 Layout of Report .................................................................................................. 3
2 BACKGROUND INFORMATION ..................................................................................... 5
2.1 Adelong ............................................................................................................... 5
2.2 Future Growth Areas ........................................................................................... 6 2.3 Flood History and Analysis of Historic Rainfall ..................................................... 7
2.3.1. General ................................................................................................... 7
2.3.2. January 1984 Flood ................................................................................. 7 2.3.3. October 2010 Flood ................................................................................. 7 2.3.4. March 2012 Flood .................................................................................. 11
2.4 Analysis of Available Stream Gauge Record ...................................................... 13 2.4.1. General ................................................................................................. 13
2.4.2. Annual Flood Frequency Analysis .......................................................... 14
3 HYDROLOGIC MODEL DEVELOPMENT AND CALIBRATION ..................................... 17
3.1 Hydrologic Modelling Approach ......................................................................... 17 3.2 Hydrologic Model Layout ................................................................................... 17
3.3 Hydrologic Model Calibration ............................................................................. 17 3.3.1. General ................................................................................................. 17 3.3.2. RAFTS Model Parameters ..................................................................... 18 3.3.3. Results of Model Calibration .................................................................. 18
3.3.4. Parameters Adopted for Design Flood Estimation .................................. 19
4 HYDRAULIC MODEL DEVELOPMENT AND CALIBRATION ........................................ 20
4.1 The TUFLOW Modelling Approach .................................................................... 20
4.2 TUFLOW Model Setup ....................................................................................... 20 4.2.1. Model Structure ..................................................................................... 20 4.2.2. Model Parameters ................................................................................. 22
4.3 Model Boundary Conditions ............................................................................... 23
4.4 Hydraulic Model Calibration ............................................................................... 23 4.4.1. General ................................................................................................. 23 4.4.2. October 2010 Flood ............................................................................... 23 4.4.3. March 2012 Flood .................................................................................. 25 4.4.4. January 1984 Flood ............................................................................... 26
4.4.5. Batlow Road Stream Gauge Rating Curve ............................................. 26
Cont'd Over
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April 2014 Rev.2.1 Consulting Water Engineers
TABLE OF CONTENTS (Cont'd)
Page No.
5 DERIVATION OF DESIGN FLOOD HYDROGRAPHS ................................................... 28
5.1 Design Storms ................................................................................................... 28 5.1.1. Rainfall Intensity .................................................................................... 28
5.1.2. Areal Reduction Factors ........................................................................ 28 5.1.3. Temporal Patterns ................................................................................. 28
5.2 Probable Maximum Precipitation........................................................................ 29 5.3 Derivation of Design Discharges ........................................................................ 29
6 HYDRAULIC MODELLING OF DESIGN FLOODS ........................................................ 31
6.1 Presentation and Discussion of Results ............................................................. 31
6.1.1. Water Surface Profiles and Extents of Inundation .................................. 31 6.1.2. Accuracy of Hydraulic Modelling ............................................................ 31
6.1.3. Flooding Behaviour Along Adelong Creek Assuming No Blockage ......... 32 6.1.4. Flooding Behaviour Along Black Creek Assuming No Blockage ............. 33 6.1.5. Overland Flooding Behaviour in Adelong Assuming No Blockage .......... 34
6.2 Flood Hazard Zones and Floodways .................................................................. 36
6.2.1. Provisional Flood Hazard ....................................................................... 36 6.2.2. Floodways ............................................................................................. 36
6.3 Sensitivity Studies ............................................................................................. 38 6.3.1. General ................................................................................................. 38
6.3.2. Sensitivity to Hydraulic Roughness ........................................................ 38
6.3.3. Sensitivity to Partial Blockage of Bridges ............................................... 38
6.4 Climate Change Sensitivity Analysis .................................................................. 40 6.4.1. General ................................................................................................. 40
6.4.2. Sensitivity to Increased Rainfall Intensities ............................................ 40 6.5 Selection of Interim Flood Planning Levels ........................................................ 41 6.6 Flood Related Issues Associated with Future Growth Areas............................... 41
7 REFERENCES .............................................................................................................. 43
8 FLOOD-RELATED TERMINOLOGY .............................................................................. 44
ANNEXURES
(BOUND IN VOLUME 1)
A. Community Flyer
B. Details of Available Data
C. Photographs Showing Flooding Behaviour in Adelong – January 1984 Flood
D. Photographs Showing Flooding Behaviour in Adelong – October 2010 Flood
E. Photographs Showing Flooding Behaviour in Adelong – March 2012 Flood
F. Details of Herb Feint Bridge Depth Gauge
G. Batlow Road Stream Gauge Data
H. Comparison of Modelled Versus Recorded Peak Flood Heights - October 2010 Flood
I. Peak Flows Derived by TUFLOW Model
Adelong Flood Study
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APPENDICES
(BOUND IN VOLUME 2)
A. Rimmers Bridge Drawings
B. Herb Feint Bridge Work As Executed Drawings
C. Pedestrian Bridge Detailed Design Drawings
D. Field Survey Data for Bridges
E. Town of Adelong Flood Study Reference Plan (WRC, 1986)
Adelong Flood Study
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LIST OF FIGURES
(BOUND IN VOLUME 2)
1.1 Study Location Plan
2.1 Catchment Plan - Adelong Creek at Batlow Road Stream Gauge (GS 410061) and
Adelong Falls
2.2 Existing Stormwater System at Adelong
2.3 Typical Cross Sections - Adelong Bridge and Herb Feint Bridge
2.4 Cumulative Rainfall for Historic Storm Events
2.5 Isohyetal Map – Rain Days of 15-16 October 2010 – Raw BOM Data
2.6 Isohyetal Map – Rain Days of 15-16 October 2010 – Adjusted BOM Data
2.7 Intensity-Frequency-Duration Curves and Historic Storm Rainfalls
2.8 Isohyetal Map – Rain Days of 1-2 March 2012
2.9 Rating Curve and Cross Section – Batlow Road Stream Gauge (GS 410061)
2.10 Discharge Hydrographs - Adelong Creek at Batlow Road Stream Gauge (GS 410061)
2.11 Flood Frequency Relationship – Log-Pearson 3 Annual Series 1948-2012 – Adelong
Creek at Batlow Road Stream Gauge (GS 410061)
2.12 Flood Frequency Relationship – Log-Pearson 3 Annual Series 1948-2012 Including
Historic Event – Adelong Creek at Batlow Road Stream Gauge (GS 410061)
2.13 Flood Frequency Relationship – Generalised Extreme Value Annual Series 1948-2012 –
Adelong Creek at Batlow Road Stream Gauge (GS 410061)
3.1 Hydrologic Model Layout (2 Sheets)
4.1 TUFLOW Model Layout
4.2 TUFLOW Schematisation of Floodplain
4.3 Water Surface Profiles – Historic Storm Events (2 Sheets)
4.4 TUFLOW Model Results – October 2010 Flood (2 Sheets)
4.5 TUFLOW Model Results – March 2012 Flood
4.6 Batlow Road HEC-RAS Model Layout
4.7 HEC-RAS Water Surface Profiles at Batlow Road Stream Gauge (GS 410061)
6.1 Water Surface Profiles – Adelong Creek (2 Sheets)
6.2 Stage and Discharge Hydrographs – Design Flood Events
6.3 TUFLOW Model Results – 5 year ARI
6.4 TUFLOW Model Results – 10 year ARI
6.5 TUFLOW Model Results – 50 year ARI
6.6 TUFLOW Model Results – 100 year ARI
6.7 TUFLOW Model Results – 200 year ARI
6.8 TUFLOW Model Results – 500 year ARI
6.9 TUFLOW Model Results – PMF
6.10 Provisional Flood Hazard and Hydraulic Categorisation of Floodplain - 100 year ARI
Adelong Flood Study
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LIST OF FIGURES (Cont’d)
(BOUND IN VOLUME 2)
6.11 Sensitivity of Flood Behaviour to 20% Increase in Hydraulic Roughness Values –
100 year ARI 6 Hour Storm
6.12 Sensitivity of Flood Behaviour to a Partial Blockage of Rimmers Bridge – 100 year ARI
6 Hour Storm (3 Sheets)
6.13 Sensitivity of Flood Behaviour to a Partial Blockage of Herb Feint Bridge – 100 year ARI
6 Hour Storm (3 Sheets)
6.14 Sensitivity of Flood Behaviour to 10% Increase in Rainfall Intensity - 100 year ARI
6.15 Sensitivity of Flood Behaviour to 30% Increase in Rainfall Intensity - 100 year ARI
6.16 Impact of Increased Rainfall Intensities on Extent of Flooding - 100 year ARI
6.17 Interim Flood Planning Area – Main Stream Flooding Only
6.18 Considerations of Potential Changes in Floodway Extent Due to a Partial Blockage of
Bridge Structures and Increases in Rainfall Intensity – 100 year ARI (2 Sheets)
Adelong Flood Study
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NOTE ON FLOOD FREQUENCY
The frequency of floods is generally referred to in terms of their Annual Exceedance Probability
(AEP) or Average Recurrence Interval (ARI). For example, for a flood magnitude having 5%
AEP, there is a 5% probability that there will be floods of greater magnitude each year. As
another example, for a flood having a 5 year ARI, there will be floods of equal or greater
magnitude once in 5 years on average. The approximate correspondence between these two
systems is:
ANNUAL EXCEEDANCE
PROBABILITY
(AEP) %
AVERAGE RECURRENCE
INTERVAL
(ARI) YEARS
0.2
0.5
1
2
5
10
20
500
200
100
50
20
10
5
The report also refers to the Probable Maximum Flood (PMF). This flood occurs as a result of the
Probable Maximum Precipitation (PMP). The PMP is the result of the optimum combination of the
available moisture in the atmosphere and the efficiency of the storm mechanism as regards
rainfall production. The PMP is used to estimate PMF discharges using a model which simulates
the conversion of rainfall to runoff. The PMF is defined as the limiting value of floods that could
reasonably be expected to occur. It is an extremely rare flood, generally considered to have a
return period greater than 1 in 105 years.
NOTE ON QUOTED LEVEL OF ACCURACY
Peak gauge heights and flood levels have on occasion been quoted to more than 1 decimal place
in the report in order to identify minor differences in values. For example, to demonstrate minor
differences between peak heights reached by both historic and design floods and also minor
differences in peak flood levels which will result from, for example, a partial blockage of hydraulic
structures. It is not intended to infer a greater level of accuracy than is possible in hydrologic and
hydraulic modelling.
Adelong Flood Study
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ABBREVIATIONS
AEP Annual Exceedance Probability (%)
AHD Australian Height Datum
ALS Airborne Laser Scanning
AMC Antecedent Moisture Condition
ARF Areal Reduction Factor
ARI Average Recurrence Interval (years)
ARR Australian Rainfall and Runoff (IEAust, 1998)
BOM Bureau of Meteorology
DTM Digital Terrain Model
FDM Floodplain Development Manual (NSW Government, 2005)
FRMS Floodplain Risk Management Study
GEV General Extreme Value
IFD Intensity-Frequency-Duration
IFPA Interim Flood Planning Area
IFPL Interim Flood Planning Level
LGA Local Government Area
LP3 log-Pearson Type 3
NOW Department of Primary Industries – Office of Water
OEH Office of Environment and Heritage, Department of Premier and Cabinet (formerly
Department of Environment, Climate Change and Water [DECCW], formerly
Water Resources Commission of NSW [WRC])
PMF Probable Maximum Flood
PMP Probable Maximum Precipitation
RCP Reinforced Concrete Pipe
RFS Rural Fire Service
RMS Roads and Maritime Services (formally Roads and Traffic Authority [RTA])
NSWSES State Emergency Service
SRTM Shuttle Radar Topography Mission
TSC Tumut Shire Council
Chapter 8 of the report contains definitions of flood-related terms used in the study.
Adelong Flood Study
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SUMMARY
The study objective was to define flood behaviour in the Adelong Creek catchment at Adelong for
flood frequencies ranging between 5 and 500 year average recurrence interval ( ARI), as well as
for the Probable Maximum Flood (PMF). Figure 1.1 in Volume 2 of the report shows the extent
of the Adelong Creek catchment upstream of Adelong.
Flood behaviour was defined using a computer based hydrologic model of the Adelong Creek
catchment to generate flood flows and a hydraulic model of the inbank area of the creek and its
floodplain to convert flows into flood levels and velocities. The hydrologic model was a runoff -
routing model based on the RAFTS and DRAINS software, whilst the hydraulic model was based
on the TUFLOW software.
Limited information was provided by residents and business owners in response to a Community
Flyer that was distributed at the commencement of the study (refer Annexure A for copy). The
reason for this is likely due to the NSW State Emergency Service (NSWSES) having gathered
flood intelligence from the Adelong community immediately following floods that occurred in
October 2010 and March 2012. Details of the data that were available for the purpose of
undertaking the present study are contained in Chapter 2 and Annexure B. Several photos
showing flooding that occurred in Adelong during floods that occurred in January 1984,
October 2010 and March 2012 are contained in Annexures C, D and E, respectively.
A flood depth gauge was recently installed by the NSW Roads and Maritime Service (RMS) on
the left bank of Adelong Creek immediately downstream of the Herb Feint Bridge, readings from
which were taken by the Rural Fire Service (RFS) during the March 2012 flood. Details of the
depth gauge are given in Annexure F. The Department of Primary Industries – Office of Water
(NOW) also operate a telemetered stream gauge (Batlow Road - GS 410061) which is located
about 3 km (by river) upstream of Adelong on Adelong Creek. The stream gauge has been in
operation since 1947. A flood frequency analysis was undertaken using the annual peak flood
heights and flows that were recorded by the gauge (refer Annexure G for details of annual
maximums). Rainfall data recorded at several Bureau of Meteorology operated rainfall gauges
were also analysed. Both sets of data were used in combination with the flood intelligence
gathered by SES following the October 2010 and March 2012 floods to calibrate both the
hydrologic and hydraulic models. Figures 4.4 and 4.5 show flooding patterns along the modelled
reach of Adelong Creek for the October 2010 and March 2012 floods, respectively, whilst a
comparison between modelled versus recorded peak flood heights for the October 2010 flood is
contained in Annexure G.
Design model parameters were selected after consideration of the results of the model calibration
process. Design storms were then applied to the hydrologic model to generate discharge
hydrographs within the study area. The discharge hydrographs constituted the upstream
boundary and sub-catchment inputs to the hydraulic model.
Design water surface profiles along Adelong Creek are shown on Figures 6.1, whilst stage and
discharge hydrographs at key locations along Adelong Creek are shown on Figure 6.2.
Figures 6.3 to 6.9 show the indicative extents of inundation for the full range of design storm
events. Annexure I contains a table which presents peak flows at selected locations throughout
the study area for the modelled design storm events.
Figure 6.10 shows the division of the floodplain into high and low hazard floodway and flood
fringe areas, noting that there were no major flood storage areas identified in the study area.
Adelong Flood Study
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Figure 6.11 shows the sensitivity of flood behaviour to a 20% increase in the best estimate
values of hydraulic roughness, whilst Figures 6.12 and 6.13 respectively show the impact a
partial blockage of the two road bridges at Adelong would have on flooding behaviour.
Figures 6.14 to 6.16 show the impact potential climate change related increases in rainfall
intensity could have on flooding behaviour at Adelong.
The key findings of the present study were:
NOW’s current rating curve for the Batlow Road stream gauge is considered to provide a
reasonable estimate of historic flood flows in Adelong Creek at Adelong.
The January 1984, October 2010 and March 2012 floods had ARI’s of about 50, 110 and 20
years, respectively.
Major floods on Adelong Creek carry a large amount of woody debris, especially the first in
a sequence of closely spaced events. As a result of this high debris load, several of the
bridges which cross Adelong Creek at Adelong (refer Figure 2.2 for location and
Appendices A, B, C and D for details) have historically experienced a partial blockage
during major flood events. This in turn has exacerbated flooding conditions in existing
development, especially that which is located along the main street of Adelong (i.e. Tumut
Street).
Flood flows are largely confined to the inbank and immediate overbank areas of Adelong
Creek for events up to about 50 year ARI. However, overbank flood runners commence to
form with further increases in flood magnitude. For example, a flood runner is shown to
form on the left overbank of Adelong Creek adjacent to Rimmers Bridge for a 100 year ARI
event and on the left overbank of Adelong Creek adjacent to the new Herb Feint Bridge for
a 200 year ARI event.
A partial blockage of the new Herb Feint Bridge would cause floodwater to surcharge the
left overbank of Adelong Creek during a 100 year ARI event, where it would impact on
existing development located along Tumut Street. A partial blockage of Rimmers Bridge
would also exacerbate flooding conditions in existing residential properties which are
located on the left bank of the creek downstream of the creek crossing.
Based on the above findings, a set of Interim Flood Planning Levels (IFPL’s) and an Interim
Flood Planning Area (IFPA) were developed for main stream flooding (refer Figures 6.17). The
IFPL’s and IFPA have been shown at a level equal to the peak 100 year ARI flood level plus an
allowance of 500 mm for freeboard assuming no blockage of the bridges which cross Adelong
Creek. Also shown on Figure 6.17 is an alternative set of IFPL’s and IFPA which reflect the
changes in flood behaviour which occur when Rimmers Bridge and the new Herb Feint Bridge
experience a partial blockage.
Whilst the future Floodplain Risk Management Study (FRMS) for Adelong will determine a final
set of Flood Planning Levels and a Flood Planning Area for the study area, consideration will
need be given in the interim to the change that occurs in the extent of the floodway when either a
partial blockage of the bridge structures or a flood larger than the 100 year ARI event occurs at
Adelong. Figure 6.18 (2 Sheets) shows the changes that would occur to the extent of the
floodway under partial blockage conditions or as a result of higher flows in the creek. Notable
increases in the extent of the floodway occur on the left overbank of Adelong Creek downstream
of Rimmers Bridge and along the main street of Adelong adjacent to the new Herb Feint Bridge.
It is advisable that Council limit future development in the areas identified as floodway on
Figure 6.18 until such time as the FRMS is completed, given the potentially hazardous nature of
the flow and the major impact that the blocking of these flow paths (either partial or total) would
have on flooding behaviour.
Adelong Flood Study
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1 INTRODUCTION
1.1 Study Background
This report presents the findings of an investigation of flooding in the Adelong Creek catchment at
Adelong and has been jointly sponsored by Tumut Shire Council (TSC) and the NSW
Government, via the Office of Environment and Heritage (OEH). Figure 1.1 shows the location of
Adelong west of Tumut in the Adelong Creek catchment.
The study objective was to define flood behaviour in terms of flows, water levels and flooding
patterns for floods ranging between 5 and 500 year ARI, as well as for PMF. The investigation
involved rainfall-runoff hydrologic modelling of the catchments and drainage systems to assess
flows in Adelong Creek, and application of these flows to a hydraulic model to assess peak water
levels and flow patterns. The model results were interpreted to present a detailed picture of
flooding under present day conditions.
The scope of the study included investigation of both main stream flood behaviour along the main
arm of Adelong Creek, as well as overland flooding which occurs as a result of surcharges of the
drainage system in Adelong.
The study forms the first step in the floodplain risk management process for Adelong (refer
process diagram presented in the Foreword), and is a precursor of the future FRMS sponsored by
TSC which will consider the impacts of flooding on existing and future urban development, as well
as potential flood mitigation measures.
1.2 Community Consultation and Available Data
At the commencement of the Flood Study, a Community Flyer was distributed to residents and
business owners advising that TRC had commenced the process of preparing a Floodplain Risk
Management Plan (FRMP) for the township of Adelong. A copy of the Community Flyer is
contained in Annexure A. Incorporated in the Community Flyer was reference to questionnaires
that were distributed by the NSWSES following the October 2010 flood. Information provided by
the 35 respondents to the questionnaire is documented in a report entitled “Flood Intelligence
Collection and Review for Towns and Villages in the Murray and Murrumbidgee Regions
Following the October 2010 Flood” (Bewsher, 2011).
The Community Flyer also stated that SES’s consultant was planning to contact a select number
of residents who responded to the post-October 2010 questionnaire to discuss flooding behaviour
which was observed in Adelong Creek during the March 2012 event. Information gathered by
SES’s consultant following the March 2012 flood is documented in a report entitled “Flood
Intelligence Collection and Review for 24 Towns and Villages in the Murray and Murrumbidgee
Regions Following the March 2012 Flood” (Yeo, 2013).
The Community Flyer invited residents and business owners to submit to TSC any additional
information that had not already been provided to SES and its consultant. However, no additional
information on flooding behaviour at Adelong was received by TSC in response to the Community
Flyer.
Annexure B contains details of the data that were available for the present study, whilst
Annexures C, D and E, respectively contain a select number of photographs showing flooding
behaviour in Adelong during the January 1984, October 2010 and March 2012 floods.
Adelong Flood Study
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1.3 Approach to Flood Modelling
1.3.1. Hydrologic and Hydraulic Modelling
Flood behaviour was defined using a two-staged approach to flood modelling involving the
running in series of:
1. The hydrologic models of the Adelong Creek catchment and the urbanised parts of
Adelong, based on the RAFTS and DRAINS rainfall-runoff software, respectively.
2. The hydraulic model of Adelong Creek and the drainage system in Adelong based on the
TUFLOW software.
The RAFTS and DRAINS models computed discharge hydrographs, which were then applied to
the TUFLOW hydraulic model at relevant sub-catchment outlets.
The TUFLOW model used a two-dimensional (in plan), grid-based representation of the natural
surface based on an Airborne Laser Scanning (ALS) survey of Adelong, as well as piped
drainage data provided by TSC. Field survey was used to derive cross sections (normal to the
direction of flow) along the inbank area of Adelong Creek, which was modelled as a one-
dimensional element within TUFLOW. Field survey was also used to confirm details of the three
bridges which presently cross Adelong Creek at Adelong (i.e. Rimmers Bridge, the recently
constructed Herb Feint Bridge and the pedestrian bridge which is located a short distance
downstream of the newly constructed road bridge).
1.3.2. Model Calibration
Streamflow data is available for Adelong Creek at Adelong via the NOW Batlow Road stream
gauge (GS 410061), which has been in operation since September 1947. As a result, it was
possible to formally “calibrate” the RAFTS hydrologic model to reproduce discharges recorded by
the stream gauge for a number of historic flood events.
Flood marks are available at Adelong for floods that occurred in January 1984 (WRC, 1986) and
October 2010 (Bewsher, 2011). Several photographs are also available which show flooding
patterns within the inbank area of Adelong Creek downstream of the Herb Feint Bridge for a flood
that occurred in March 2012 (Yeo, 2013).
Rainfalls recorded at Bureau of Meteorology’s (BOM) Batlow rainfall warning gauge (GS 72004.1)
during the October 2010 and March 2012 storms were applied to the RAFTS model to derive
discharge hydrographs in Adelong Creek at the location of the Batlow Road stream gauge.
These flows were then applied to the TUFLOW model to derive water surface profiles for
comparison with the recorded flood marks. The TUFLOW model parameters were varied until the
hydraulically modelled flows gave a reasonable correspondence between recorded and derived
flood levels.
Due to large variations in both the bed and bank geometry of Adelong Creek at the location of the
Batlow Road stream gauge, the HEC-RAS software was use to generated a series of closely
spaced cross sections to more accurately plot the rapidly changing water surface profile in the
vicinity of the gauge site. This allowed a check to be undertaken of NOW’s adopted rating curve
and to confirm that the flows recorded by the gauge for the historic floods are representative of
prototype conditions.
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Whilst there was no rainfall data available to test whether the calibrated RAFTS model could
reproduce the discharge hydrograph that was recorded during the January 1984 flood, the
calibrated hydraulic model was run using the recorded hydrograph to test whether it was able to
reproduce observed flood behaviour.
1.3.3. Design Flood Estimation
Design storms were derived using procedures set out in Australian Rainfall and Runoff
(ARR, 1998) and then applied to the RAFTS and DRAINS models to generate discharge
hydrographs. These hydrographs constituted input to the TUFLOW hydraulic model.
An “envelope” approach was adopted for defining design water surface elevations and flow
patterns throughout the study area. The procedure involved running the model for a range of
storm durations to define the upper limit (i.e. the envelope) of expected flooding for each design
flood frequency.
1.4 Layout of Report
Chapter 2 contains background information including a brief description of the Adelong Creek
catchment and its drainage system, details of previous flooding investigations, a summary of
community consultation undertaken as part of this present study, and a brief history of flooding at
Adelong.
Chapter 3 deals with the hydrology of the Adelong Creek catchment, and describes the
development and calibration of the RAFTS and DRAINS hydrologic models which were used to
generate discharge hydrographs for input to the hydraulic model.
Chapter 4 deals with the development and calibration of the TUFLOW hydraulic model which was
used to analyse flood behaviour at Adelong.
Chapter 5 deals with the derivation of design discharge hydrographs, which involved the
determination of design storm rainfall depths over the catchments for a range of storm durations
and conversion of the rainfalls to discharge hydrographs.
Chapter 6 details the results of the hydraulic modelling of the design floods. Results are
presented as water surface profiles and plans showing indicative extents of inundation for a
range of design flood events up to and including the PMF. A provisional assessment of flood
hazard and hydraulic categorisation is also presented. (The assessment of flood hazard
according to velocity and depth of floodwaters is necessarily “provisional”, pending a more
detailed assessment which includes other flood related criteria, to be undertaken during the future
FRMS.) The results of various sensitivity studies undertaken using the TUFLOW model are also
presented, including the effects of changes in hydraulic roughness, partial blockage of the piped
stormwater system, and potential increases in rainfall intensities due to future climate change.
This chapter also deals with the selection of interim Flood Planning Levels for the study area.
Chapter 7 contains a list of references, whilst Chapter 8 contains a list of flood-related
terminology that is relevant to the scope of the study.
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Annexure A contains a copy of the Community Flyer that was distributed to residents and
business owners in Adelong. Annexure B contains a list of data that were available for the
present investigation. Annexures C, D and E contain photographs showing flooding behaviour in
Adelong for the January 1984, October 2010 and March 2012 floods, respectively. Annexure F
contains details of the Herb Feint Bridge Gauge. Annexure G contains a list of historic peak
height and discharge data for the Batlow Road stream gauge (GS 410061), which is located a
short distance upstream of Adelong on Adelong Creek. Annexures H and I, respectively contain
tables which give a comparison of modelled versus recorded peak flood heights for the October
2010 flood and peak flows taken from the TUFLOW model.
Appendices A and B, respectively contain a select number of drawings for Rimmers Bridge and
the Herb Feint Bridge, whilst Appendix C contains a select number of drawings for the
pedestrian footbridge which crosses Adelong Creek immediately downstream of the Herb Feint
Bridge. Appendix D provides details of the field survey which was undertaken to confirm details
of the three bridges which cross Adelong Creek at Adelong. Appendix E contains a copy of the
Reference Plan contained in WRC, 1986. The drawings which are contained in the five
appendices are bound in Volume 2 of the report.
Figures referred to in both the main report and the appendices are bound in a separate volume of
the report (refer Volume 2).
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2 BACKGROUND INFORMATION
2.1 Adelong
The township of Adelong has a population of about 900 and is located about 15 km west of Tumut
on the Snowy Mountains Highway. The town is located on Adelong Creek, which has a
catchment area of about 160 km2 at the downstream limit of the town. Figure 2.1 shows the
extent of the Adelong Creek catchment at Adelong.
The headwaters of the Adelong Creek catchment are located near the township of Batlow, which
lies approximately 25 km to the south of Adelong. Flow in the creek discharges to the
Murrumbidgee River a distance of about 30 km downstream of Adelong. The Adelong Creek
catchment is characterised by a mixture of steep heavily wooded slopes and pastoral land.
Adelong Creek is characterised by relatively rapid drops in bed level at the location of exposed
rock bars, punctuated by level pools. The bed of the creek is generally devoid of vegetation at
the location of the level pools, however, it does encroach at the location of the exposed rock bars.
The banks of the creek are generally vegetated by dense stands of poplar trees.
The stormwater drainage system at Adelong generally comprises roadside table drains, with
piped crossings at road intersections. However, there are four piped drainage lines which
discharge to Adelong Creek downstream of the Herb Feint Bridge. These drainage lines control
overland flow which approaches Tumut Street from the south. The layout of the stormwater
drainage system at Adelong is shown on Figure 2.2.
Three new bridges have been constructed at Adelong in the past decade. The most upstream of
the three is located on Selwyn Street (M.R. 280) and is known locally as Rimmers Bridge. The
new four span reinforced concrete bridge replaced a wooden bridge which had the same number
of spans. Each span on the new bridge is 12 m in length. A select number of drawings for
Rimmers Bridge are contained in Appendix A. Drawings provided by TRC for the new bridge
show that the soffit level of the new bridge was set above the then computed 100 year ARI flood
level of RL 343.66 m AHD.1
The Herb Feint Bridge, which is located on the Snowy Mountain Highway (Hwy No. 4) crossing of
Adelong Creek, was designed and built by the RMS and replaced two in-series wooden bridges,
one which spanned the low flow section and the other the right (northern) inbank area of the
creek [The older twin bridges are referred to herein as the “Adelong Bridge”]. Whilst the soffit
and railing levels of the Herb Feint Bridge are similar to those of the Adelong Bridge, the older
bridge was characterised by a centrally located embankment and closely spaced bridge piers ,
several of which were skewed to the direction of flow. For example, the piers of the Adelong
Bridge, whilst smaller in diameter than those of the Herb Feint Bridge, were spaced at between
4.6 m and 8.5 m, whilst those of the new bridge are spaced at 15.5 m centres. The waterway
area beneath the Herb Feint Bridge is also much larger than the Adelong Bridge, due to the
removal of the central embankment and the positioning of the new bridge abutments behind those
of the old bridge. Figure 2.3 shows typical sections of the Adelong Bridge and Herb Feint Bridge,
the deck levels of which are higher than the adjacent left (southern) overbank of Adelong Creek.
A select number of Work As Executed drawings for the Herb Feint Bridge are contained in
Appendix B.
1 The drawings show that the peak 100 year ARI flood level at the bridge site was based on a peak flow of
328 m3/s.
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The Herb Feint Bridge was under construction at the time of the October 2010 flood, with the
eastern (upstream) portion of the new bridge having been completed on the day of the event. In
order to maintain access across Adelong Creek, the western (downstream) side of the Adelong
Bridge, including a portion of the centrally located embankment, was retained during the
construction of the eastern (upstream) portion of the new bridge. As a result, the waterway area
beneath the two bridges was obstructed by both sets of bridge piers (i.e. new and old), in addition
to the centrally located embankment. Further discussion on the impact the build-up of debris on
both bridges had on flood behaviour during the October 2010 event is contained in Section 2.3.3.
A three span steel truss type pedestrian bridge was constructed by TSC a short distance
downstream of the Herb Feint Bridge circa 2009. The central span of the pedestrian bridge
where it crosses the low flow section of Adelong Creek is about 20 m in length and has a soffit
level of RL 334.286 m AHD. A select number of detailed design drawings for the bridge are
contained in Appendix C.
Casey Surveying and Design were commissioned in 2012 as part of the present investigation to
survey the three above mentioned bridges in order to confirm details shown on the detailed
design drawings. Copies of the survey data are contained in Appendix D.
A fourth high level suspension bridge crosses Along Creek a short distance downstream of the
abovementioned pedestrian bridge. The suspension bridge, which provides pedestrian access
directly to the Golden Gully Caravan Park, has been denoted herein as the “Caravan Park
Suspension Bridge”. The left (southern) abutment of the bridge is located on the left overbank of
Adelong Creek in an area which was subject to relatively shallow inundation during the October
2010 flood. Further discussion on the amount of debris which accumulated around the bridge
abutment during the October 2010 flood is contained in Section 2.3.3.
Commercial development in Adelong is concentrated along Tumut Street west of Campbell Street
and at the northern end of Selwyn Street. The Golden Gully Caravan Park is located on the right
(northern) bank of Adelong Creek immediately downstream of the Herb Feint Bridge.
2.2 Future Growth Areas
The Tumut Shire Council Growth Strategy Planning Report – Adelong Investigation Areas
(SP, 2013) investigated land identified as potential growth areas for the purpose of residential
development in Adelong. The land was highlighted within the outcomes of the Tumut Shire Rural
Land Use Strategy 2008. Figure 2.2 shows the extent of the Adelong South-West Growth Area
and the Adelong South-East Growth Area.
In regard the Adelong South-West Growth Area, SP, 2013 identifies the significant impact
flooding has had on parts of Adelong (namely along Tumut Street), and notes that Adelong
Cemetery Road is inundated by floodwater which surcharges Black Creek during heavy rainfall
events. The report also notes that inundation of the roadway is typically of a shallow nature and
short duration. Reference to a new culvert being required under Todds Road is also contained in
the report.
In regard the Adelong South-East Growth Area, SP, 2013 notes that flooding from Adelong Creek
will impact only a small portion of the land identified for future residential development. The
report also notes that access to the area was cut for a period of about 1 hour during the October
2010 flood due to the inundation of Selwyn Road. Reference to the same road not being cut in
the January 1984 flood event is also contained in the report. The report states that several
culverts under Wondalga Road and Rimmers Lane will need to be upgraded to service future
development.
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2.3 Flood History and Analysis of Historic Rainfall
2.3.1. General
The following discussion on the history of flooding at Adelong is taken principally from two reports
prepared by Bewhser Consulting and Stephen Yeo on behalf of the NSWSES following the
October 2010 and March 2012 flood events (Bewsher, 2011 and Yeo, 2013).
Table 2.1 over is taken from Bewsher, 2011 and provides a summary of the flood history at
Adelong, pre the October 2010 flood event. Of note is the major flood that occurred in January
1874, which is said to have been similar in terms of flood behaviour along Tumut Street, to that of
the October 2010 flood. Also mentioned is the high debris load which was characteristic of the
flood.
Details of available flood data are contained in Section 2.4, including the findings of an annual
flood frequency analysis which was undertaken on the Batlow Road stream gauge record.
2.3.2. January 1984 Flood
The January 1984 flood caused minor flooding in property located along Tumut Street, when
floodwater surcharge the left (southern) bank of Adelong Creek upstream of Adelong Bridge. As
noted in Table 2.1, a large amount of debris was observed to have built-up on Adelong Bridge
during the flood event, which would have probably exacerbated flooding conditions along Tumut
Street.
Appendix E contains a plan which was prepared by the Water Recourses Commission (WRC,
1986) showing the indicative extent of the January 1984 flood at Adelong. The plan shows that
floodwater was generally confined to the creek along most of its length, with the exception of the
following areas:
the left and right overbank of the creek upstream of Rimmers Bridge;
the left overbank of the creek at the northern end of Selwyn Street;
the left overbank of the creek along Tumut Street; and
the right overbank of the creek downstream of Havelock Street.
Several photographs showing areas which were impacted by floodwater during the January 1984
flood are contained in Annexure C.
2.3.3. October 2010 Flood
The October 2010 flood occurred as a result of heavy rain which fell over the upper reaches of
the Murrumbidgee River catchment, commencing in the Adelong Creek catchment at around
00:00 hours on 13 October 2010 and ending at around 21:00 hours on 15 October 2010.2
2 The October 2010 flood occurred only 5 weeks after a smaller flood event was experienced at Adelong,
the peak of which occurred on 4 September 2010. Based on data recorded by NOW’s Batlow Road stream
gauge, this earlier flood was of similar magnitude to the March 2012 flood. Video footage available on
YouTubeAU
shows there was freeboard to the soffit level of the partially demolished Adelong Bridge during
the flood event. No blockage of the structure can be observed in the footage, although large woody debris
can be observed being carried by the floodwater toward Rimmers Bridge (see “Images of Natwash –
AUSTRALIA….Adelong Creek Flooding 4th Sept’ 2010”).
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TABLE 2.1
FLOODING HISTORY AT ADELONG
Date Description Source
1860 Nov 23
(Fri)
Lower Adelong Creek rose in a few hours to a height greater than has ever been remembered and poured down a volume of water, the rush of which was truly terrible, sweeping over its banks huge logs, tearing down fences, desolating gardens, and bearing to indiscriminate ruin all the mining appliances of the upper streams... The bridge gathered enormous piles of driftwood; the girders sank at one end to nearly the bed of the stream... The flood reached the level of the race belonging to the Wheel of Industry; another foot or two and the whole would have been precipitated upon the lower portions of the township reaching as it then did to the door of the Adelong Hotel. Fortunately no loss of life is at present recorded... The flood was equally felt at Gilmore Creek but the damage done to the property was trivial...
SMH, 3 Dec 1860 p.6
1863 Jul 31
(Fri)?
Town visited by a flood unprecedented severity during the last 15 years. Bridge damaged by huge logs. Two gentlemen nearly swept away when water reached waist high as crossing the bridge; people worked to free the bridge from the stress of the logs which bore upon the foundation.
Freeman's Journal, 12 Aug 1863 (courtesy Adelong Alive Museum); Rockhampton Bulletin and Central Queensland Advertiser, 20 Aug 1863 p.2
1870 Jun 17
(Fri)
Creek rose rapidly; one or two families had to evacuate; large logs came rushing down the stream; people slung ropes around the logs and dragged them to the banks in order to protect the bridge which was slightly damaged; gold mine damaged.
SMH, Fri 24 Jun 1870 p.2
1870 Oct 26
(Wed) and
29 (Sat)
Wed: A tremendous flood; rose 5ft in less than 10 minutes; brought with it an immense quantity of debris and terrific large logs and stumps; water rose a foot over the floor of the bridges. Sat: water reached the same height as Wednesday but with less debris. One bridge badly damaged with approaches washed away and foundation undermined. Enormous damage to alluvial claims.
SMH Thurs 27 Oct 1870 p.5; Maitland Mercury & Hunter River General Advertiser Tues 8 Nov 1870 p.4
1874 Jan 17
(Sat)
Adelong Creek overflowed banks about 7p.m. and made a clean sweep through Tumut Street, "this divergence of the water being greatly occasioned by the channel being blocked and impeded by the logs and trees which came hurtling down." Little more than 15 minutes elapsed from the time the floodwaters overflowed the bank to the time they reached their greatest height. There was considerable danger of loss of life, with descriptions of near misses as people were being rescued. All the bridges over the creek have been nearly or quite destroyed. several business premises and houses destroyed.
SMH Tues 27 Jan 1874 p.5
1879 Oct 17
(Fri)
Approaches to the main bridge washed away. Fences near post office washed away. Man fell into creek and drowned while crossing one of the bridges.
SMH Tues 21 Oct 1879 p.5
1891 May have exceeded 1984 flood. WRC, 1986, p.6
1911 Mar 7
(Mon)
Terrific storm passed over Grahamstown. Rain gauge registered 51/2 inches for the 30 minutes of the worst of the storm. Vast flood swept down Sandy Gully. Animals carried helplessly down Adelong Creek.
Adelong and Tumut Express, Fri 11 Mar 1911 (courtesy Adelong Alive Museum)
1931 Jun 24
(Wed)
Experiencing most serious flood for 50 years; main approaches to Adelong Bridge carried away and whole structure in danger of collapsing.
SMH, Thurs 25 Jun 1931 p.9
1984 Jan 16
(Mon)
Dense debris built up at Adelong Bridge contributed to minor flooding in Tumut Street. 3 houses, 4 businesses and swimming pool flood affected, with depths over floor less than 0.5m except for the swimming pool.
WRC, 1986; OEH
2010 Sep 4
(Sat)
Swimming pool flooded. Video
Note: Reproduced from Table 12.1 of Bewsher, 2011, with minor edits to remove reference to tables and
appendices referred to in that report.
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Rainfall recorded in three hourly intervals by BOM at its Batlow rainfall warning gauge
(GS 72004.1) (refer Figure 2.4) indicates that the rain fell in two discrete bursts, with the second
burst resulting in the major flooding that was experienced in Adelong on the afternoon of
15 October 2010. However, the depths of rainfall recorded by BOM during the event are
significantly less than the official daily rainfall totals recorded at BOM’s Batlow Post Office daily
read rain gauge (GS 72004), as demonstrated in Table 2.2. As will be discussed in Chapter 3 of
the report, the depths of rainfall recorded by BOM’s rainfall warning gauge were not sufficient to
cause the hydrologic response from the catchment which was recorded by NOW’s Batlow Road
stream gauge (GS 410061) and it was only when the rainfall was factored up to match that
recorded at BOM’s Batlow Post Office rain gauge could similar flows to those recorded at the
stream gauge be generated by the hydrologic model. Figure 2.5 shows the two-day rainfall
depth contours (or isohyets) for the rain days of 15 and 16 October 2010 based on the raw data,
whilst Figure 2.6 shows rainfall depth contours for the same period after depths of rainfall at
Batlow were factored up to match that recorded at the Batlow Post Office.
TABLE 2.2
RECORDED DEPTHS OF RAINFALL
OCTOBER 2010 STORM
(mm)
Date/Time
Batlow Rainfall Warning Gauge
(GS 72004.1)
Batlow Post Office
(GS 72004)
3 hourly increments Daily total to 09:00 hrs Daily total to 09:00 hrs
14/10/2010 12:00 hrs 0
14/10/2010 15:00 hrs 0
14/10/2010 18:00 hrs 0
14/10/2010 21:00 hrs 0
15/10/2010 0:00 hrs 0
15/10/2010 3:00 hrs 0
15/10/2010 6:00 hrs 9
15/10/2010 9:00 hrs 23.6 32.6 41.8
15/10/2010 12:00 hrs 22.2
15/10/2010 15:00 hrs 21.8
15/10/2010 18:00 hrs 3.8
15/10/2010 21:00 hrs 0
16/10/2010 0:00 hrs 0
16/10/2010 3:00 hrs 0.8
16/10/2010 6:00 hrs 0.8
16/10/2010 9:00 hrs 0.2 49.6 81.6
Two-Day Total 82.2 82.2
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Further supporting the above finding is the comparison of the recorded rainfall data with design
intensity-frequency-duration curves for the gauge site. By inspection of Figure 2.7 (LHS), the
raw data suggests that the rainfall which fell at Batlow was equivalent to a storm with an ARI of
between 5 and 10 years, whereas the adjusted data has an ARI of about 25 years for a 6 hour
storm duration and about 100 years for storm durations of between 9 and 12 hours. The ARI of
the adjusted rainfall data is more consistent with the assessed frequency of the flood event at
Adelong (refer Section 2.4 for further details).3,4
It is further noted that the adoption of the raw
BOM data would result in a flood with a similar magnitude to that of the March 2012 event, which
did not cause major flooding in Adelong.
The damaging flooding that was experienced at Adelong on the afternoon of 15 October 2010
was primarily confined to commercial properties located along Tumut Street and at the northern
end of Selwyn Street. Several residential properties located on the left (southern) overbank of
Adelong Creek on Tumut Street also experienced above-floor flooding, as did a single residence
in Havelock Street. A single residence located immediately upstream of the Herb Feint Bridge on
the right (northern) bank of Adelong Creek was also inundated above its floor (Bewsher, 2011).
Damaging flooding in Adelong was principally a result of floodwater which surcharged Adelong
Creek on the upstream side of the Herb Feint Bridge which was under construction at the time of
the flood (refer Section 2.1 for details). Plates 9 and 14 in Annexure D show the pattern of
flooding which occurred on the left (southern) overbank of Adelong Creek near the time of the
peak of the October 2010 flood.
Several videos show relatively deep, fast moving floodwater traversing Tumut Street, whilst
relatively quiescent conditions are shown to have been present at the northern end of Selwyn
Street along the eastern (upstream) side of a large warehouse development (see for example
YouTubeAU
videos “Adelong Floods 1 (15 Oct 2010)” and “Adelong Floods 6”).
Similar to the previous major floods at Adelong, a large debris load was conveyed by the
floodwater during the October 2010 flood. The owner of the property which is located on the right
(northern) bank of Adelong Creek immediately upstream of the Herb Feint Bridge advised that a
large amount of woody debris lodged on the upstream side of the bridge during the event. The
owner also advised that a large amount of scour occurred beneath both the new and old sections
of bridge, which included the partial erosion of the centrally located embankment on Adelong
Bridge. A copy of the Work As Executed plan for the Herb Feint Bridge (refer Appendix B for
copy) shows the approximate extent of scour which occurred beneath the bridges as a result of
the flood.
3 By inspection of Figure 2.7 (RHS) the rain that fell to the west of Adelong was less intense than that which
fell in the upper reaches of the Adelong Creek catchment during October 2010.
4 The critical duration storm for the Adelong Creek catchment at Adelong was found to be 6 hours (refer
Chapter 6 for details), whilst the October 2010 flood was estimated to have an ARI of 110 years (refer
Table 2.4). Differences between the ARI of the recorded rainfall and peak flow in the creek could be
explained by the relatively wet nature of the catchment prior to the onset of the flood producing rainfall,
which resulted in an above normal amount of rainfall being converted to excess runoff. The catchment may
have also responded to more intense rainfall which may have occurred over a shorter period than was
implied by the 3 hour depth totals recorded by BOM, which in the case of the October 2010 storm are
believed to be under-estimates.
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Bewsher, 2011 also noted the following in regards blockage of the partially constructed Herb
Feint Bridge:
“Both photographs (Appendix L) and the questionnaires point to the significant
influence on flood behaviour of debris blockage of the Snow Mountains Highway
Bridge (cf. Table 12.1 [see Table 2.1 of this report] for reports of such in historical
floods). One observed at the S&C Club reported a 3.6 m long. 2.4 m diameter steel
tank trapped at the bridge about 1 pm. Others observed woody debris, fencing and
garden furniture. When the flood occurred, the upstream half-width of the new bridge
was completed whilst the downstream half-width of the old bridge was still in place,
which enhanced the opportunity for accumulation of debris since the bridges’ pylons
did not line up (RTA, pers. comm.). One respondent reported observing blockage
against the new bridge from as early as 11 a.m.”
Plates 15 to 25 in Annexure D, which are taken from Appendix L of Bewsher, 2011, show debris
lodged beneath both the partially constructed Herb Feint Bridge and the partially demolished
Adelong Bridge. The photographs show that the debris that lodged beneath the Herb Feint
Bridge was largely centred on the centrally located embankment associated with the Adelong
Bridge.
Woody debris also lodged on the upstream side of the western most bridge pier of Rimmers
Bridge (see YouTubeAU
video “Images of Natwash – AUSTRALIA….Adelong Flooding again,
15 Oct’ 2010”).
Plate 26, which is taken looking in the upstream direction, shows debris which settled
immediately downstream of the left (southern) abutment of the Caravan Park Suspension Bridge.
Whilst the bridge abutment is located on land which experienced only shallow inundation during
the October 2010 flood, it is believed that the debris likely settled as a result o f eddying that
occurred immediately downstream of it. Further details on flooding patterns in the vicinity of the
bridge abutment are contained in Chapter 4.
2.3.4. March 2012 Flood
The flood peak that occurred around noon on 1 March 2012 was a result of heavy rain which
commenced to fall around 00:00 hours on the same day. Rainfall depths recorded by the BOM’s
Batlow rainfall warning gauge (GS 72004.1) indicate that about 100 mm of rain fell near Batlow
over a 15 hour period commencing at 00:00 hours on 1 March 2012. By comparison with design
intensity-frequency-duration curves for the gauge site (refer LHS of Figure 2.7), the rainfall had
an equivalent ARI of about 2 year ARI for a storm duration of 6 hours, 5 year ARI for a storm
duration of 9 hours and between 10 and 20 years for storm durations of between 12 and
18 hours.5 Figure 2.8 shows the two-day rainfall depth contours (or isohyets) for the rain days of
1 and 2 March 2012.
5 Similar to the October 2010 storm, the ARI of the recorded rainfall for the critical storm duration of 6 hours
is not consistent with the ARI of the recorded peak flow, which was estimated to be about 18 years (refer
Table 2.4). The difference between the ARI of the recorded rainfall and peak flow in the creek is likely due
to the relatively wet nature of the catchment prior to the onset of heavy rainfall (e.g. a total of 233.6 mm was
recorded over the 5 consecutive raindays of 27 February 2012 to 2 March 2012), which resulted in an above
normal amount of rainfall being converted to excess runoff.
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Floodwater was generally confined to the inbank area of Adelong Creek , with no reported
occurrence of above-floor flooding (Yeo, 2013). Plates 27 and 28 in Annexure E show that the
water level in Adelong Creek probably did not reach the soffit level of the Herb Feint Bridge and
that at the time the photo in Plate 27 was taken, floodwater was on the point of surcharging the
left (southern) bank of Adelong Creek immediately downstream of the creek crossing. The same
observed flooding patterns downstream of the Herb Feint Bridge are supported by available video
footage (see YouTubeAU
video “Adelong River in flood in Adelong”).
Several water level readings were taken by the RFS at the depth gauge which RMS had only
recently installed on the left bank of Adelong Creek immediately downstream of the Herb Feint
Bridge (denoted herein as the “Herb Feint Bridge Depth Gauge”). Table 2.3 is a reproduction of
Table 18.2 in Yeo, 2013 which gives details of the levels which were recorded at both the Batlow
Road and Herb Feint Bridge gauges during the flood event. Details of the Herb Feint Bridge
Depth Gauge are provided in Annexure F.
TABLE 2.3
RECORDED FLOOD LEVEL DATA(1)
MARCH 2012 FLOOD
Date/Time
Gauge height
Description Batlow Road
(GS 410061)(2)
Adelong Bridge
(RFS)(3,4)
Tue 28th 1715 1.051m Flood peak (1st) at Batlow Road gauge
Thu 1st 1215 2.3m
Thu 1st 1315 2.7m Rising
Thu 1st 1515 3.106m Flood peak (2nd) at Batlow Road gauge
Thu 1st 1600 3.4m Flood peak (2nd) at Adelong (Herb Feint) Bridge
manual gauge
Sat 3rd 2000 Evacuation Warning for Low-lying Areas of Adelong
Creek issued
Sun 4th 0435 2.4m Steady at Bridge
Sun 4th 0620 Rising rapidly
Sun 4th 0700 2.807m Flood peak (3rd) at Batlow Road gauge
Sun 4th 0745 3.3m Flood peak (3rd) at Adelong (Herb Feint) Bridge
manual gauge
Sun 4th 0825
7 ML dam at Molineaux’s (77 Bleak Street) burst
banks impacting Bleak Street and taking out fencing
to Adelong electricity substation
Sun 4th 0840 3.1m Believes no further risk from Molineaux’s dam
Sun 4th 1115 2.9m
Fri 9th 1400 All Clear for Low-lying areas of Adelong Creek
issued
1. Reproduced verbatim from Table 18.2 in Yeo, 2013
2. Gauge zero = RL 351.52 m AHD.
3. Herb Feint Bridge Depth Gauge.
4. Gauge zero = RL 329.65 m AHD (Source: Survey undertaken by TSC in September 2013) . Refer Annexure F
for details.
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2.4 Analysis of Available Stream Gauge Record
2.4.1. General
NOW’s Batlow Road stream gauge (GS 410061) is located on Adelong Creek, approximately
3 km (by river) upstream of Rimmers Bridge. The stream gauge was first installed in September
1947 and later shifted in 1980 a distance of approximately 200 m upstream to its current location.
The relocated gauge is located on a rock bar and has a gauge zero of RL 351.52 m AHD. The
highest recorded stream gauging was taken on 26 September 1983, when the water level rose to
a peak height of 2.55 m. The recorded flow rate in Adelong Creek at this peak height was
105 m3/s.
Figure 2.9 shows a plot of NOW’s current rating curve for the stream gauge (LHS) and a cross
section of Adelong Creek at the gauge site (RHS).6 Plotted against both are the peak heights
recorded by the gauge during the January 1984, October 2010 and March 2012 floods, as well as
the maximum gauged flow of 105 m3/s.
Table 2.4 lists the ten largest floods to have been recorded by the Batlow Road stream gauge in
terms of peak discharge. The peak heights recorded by the gauge are also given, but only for
those floods that have occurred since the gauge was relocated to its current position. Peak
height and discharge data for the full period of record are provide in Tables G1 and G2 in
Annexure G.
TABLE 2.4
HIGHEST RANKED ANNUAL FLOOD PEAKS AT ADELONG
1948 TO DATE
Date of Flood Peak Height (m) Peak Discharge (m3/s)
Approximate
Frequency
(year ARI)(1)
October 2010* 4.61 382.8 110
January 1984 3.52 212.4 50
March 2012(2)
3.11 162.6 20
October 1974 - 147.2 15
October 1975 - 129.8 10
October 1955 - 122.4 10
August 1983 2.63 112.9 9
October 1993 2.59 108.6 8
September 1960 - 100.2 7
December 1988 2.47 98.3 7
1. The frequency of the highest ranged historic floods is based on the findings of the flood frequency analysis
which incorporated the historic event of 1874 but omitted low flows (reefer Figure 2.12 (RHS)).
2. Note that a flood occurred in September 2010 which reached 3.03 m on the Batlow Road stream gauge,
indicating the event was of similar magnitude to that which occurred in March 2012.
6 Cross section compiled from inbank survey data provided by NOW on its website and ALS survey data of
the overbank areas.
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Figure 2.10 shows the discharge hydrographs which were recorded by the Batlow Road stream
gauge during the top three ranked floods. The discharge hydrograph which was recorded du ring
the flood which occurred in September 2010, one month prior to the larger October 2010 flood is
also shown for comparative purposes.
2.4.2. Annual Flood Frequency Analysis
A log-Pearson Type 3 (LP3) distribution was fitted to the annual series of flood peaks for the 65
year period of record. The resulting frequency curves, along with 5% and 95% confidence limits
are shown on Figure 2.11 (LHS).
Values at the low end of the observed range of flood peaks can distort the fitted probability
distribution and affect the estimates of large floods. Deletion of these low values may improve
the fitting of the remaining data. As the recorded flood peaks are only a small sample of peaks
actually occurring over a longer duration, an expected probability adjustment was also made
using the procedure set out in Australian Rainfall and Runoff (ARR, 1998). ARR, 1998
recommends implementing the expected probability adjustment to remove bias from the estimate.
Figure 2.11 (RHS) shows the results of omitting the nineteen annual flows less than 25 m3/s from
the analysis and applying the expected probability adjustment to the remaining data.
Bewsher, 2011 identified a historic flood that occurred in 1874 and resulted in damaging flooding
similar to that which was experienced in the October 2010 flood. Whilst the magnitude of the flow
in Adelong Creek and the degree of blockage that was experienced at the bridge crossing cannot
be determine, the inclusion of this flood in the flood frequency analysis, assuming that its peak
discharge was slightly larger than the October 2010 flood, increased the estimate of the 1% AEP
flood from 320 m3/s to 375 m
3/s (refer Figure 2.12 and Column E in Table 2.5 over).
7
The descriptions contained in Table 2.1 indicate that the 1874 flood was possibly the largest
flood that occurred over the period 1848 to date.8 A sensitivity analysis was carried out whereby
the length of the historic record was increased from 139 years (1874 to 2012) to 165 years (1848
to 2012) to account for this possibility. The analysis showed that the LP3 distribution described
above is not sensitive to the extension of the record by 26 years (refer blue and red dashed lines
on RHS of Figure 2.12).
Frequency analysis was also carried out fitting the annual peaks to the Genera l Extreme Value
(GEV) distribution using LH moments. Figure 2.13 shows the results for the both the full period
of record (LHS) and after the nineteen annual flows less than 25 m3/s are omitted from the
analysis (RHS).
Table 2.5 at the end of this chapter shows the estimates of peak flows for various probabilities of
occurrence as derived from the above analyses.
The results of the LP3 analyses show that the inclusion of low flows lead to a significant degree
of negative skew in the fitted distribution which reduces the estimate of peak flows for the larger,
less frequent floods. By comparison, the fitted probability distribution for the case where low
flows were omitted provides a better fit to the historic data.
7 The procedures set out in ARR, 1998 for incorporating large historic floods in the flood frequency analysis
were used in the derivation of the curves shown in Figure 2.12. The period of recorded adopted in the
analysis extended back to the time of the 1874 flood (i.e. 139 years).
8 The entry against the July 1863 flood states that this event was the largest that had been experienced in
Adelong over a 15 year period.
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The GEV distribution was found to be less sensitive to the inclusion of low flows for the larger,
less frequent floods. The estimated peak discharge of 290-300 m3/s for the 1% AEP flood is
similar to the LP3 distribution value given in Column C and about 25 per cent less than the
estimate given in Column E of Table 2.5. Note that the inclusion of the historic flood in the GEV
analysis, whilst not assessed as part of the present investigation, would increase the design peak
flow estimates given in Columns F and G.
Based on the above findings, the peak flow estimates given in Column E of Table 2.5, as well as
those derived from Figure 2.12 (RHS) should be given greatest weight when deriving design
discharge hydrographs for input to the hydraulic model. Refer Section 5.3 in Chapter 5 for
further discussion.
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TABLE 2.5
ESTIMATES OF PEAK FLOWS AT ADELONG
VALUES IN m3/s
Annual
Exceedance
Probability
% AEP
LP3 Distribution GEV Distribution
Full Period of
Record
Low Flows
Omitted(1)
Full Period of
Record Including
Historic Event(2)
Low Flows Omitted
Including Historic
Event(1,2)
Full Period of
Record
Low Flows
Omitted
[A] [B] [C] [D] [E] [F] [G]
5 166 163 176 185 160 151
2 206 245 223 270 226 224
1 233 320 256 375 287 297
0.5 256 430 285 500 359 394
1. With the expected probability adjustment, according to ARR, 1998.
2. Peak discharge of January 1874 flood assumed to equal 400 m3/s (i.e. a flood slightly larger than the October 2010 event).
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3 HYDROLOGIC MODEL DEVELOPMENT AND CALIBRATION
3.1 Hydrologic Modelling Approach
The present investigation required the use of a hydrologic model which is capable of representing
the rainfall-runoff processes that occur within the Adelong Creek catchment.
The hydrologic response of the Adelong Creek catchment upstream of Adelong was simulated
using the RAFTS software, as the catchment upstream of the Batlow Road stream gauge is
principally rural in nature. The discharge hydrographs generated by RAFTS were applied to the
TUFLOW hydraulic model as either point or distributed inflow sources.
The hydrologic response of the urban parts of Adelong was simulated using the DRAINS
software, which has been developed primarily for modelling the passage of a flood wave through
urban catchments and is therefore well suited to this present investigation. Discharge
hydrographs generated by DRAINS were applied to the TUFLOW hydraulic model as distributed
inflow sources.
3.2 Hydrologic Model Layout
Figure 3.1 (2 Sheets) shows the layout of the RAFTS and DRAINS models.
As the primary function of the hydrologic model was to generate discharge hydrographs for input
to the TUFLOW hydraulic model, individual reaches linking the various sub-catchments were not
incorporated in the model. However, in the upper reaches of the Adelong Creek catchment, it
was necessary to route the flow generated by several of the RAFTS sub-catchments to the
upstream boundary of the hydraulic model. The outlets of these sub-catchments were linked and
the lag times between each assumed to be equal to the distance along the main drainage path
divided by an assumed flow velocity of 2 m/s.
Careful consideration was given to the definition of the sub-catchments which comprise the
hydrologic model to ensure peak flows throughout the drainage system would be properly routed
through the TUFLOW model. In addition to using the ALS-based contour data, the location of
inlet pits and headwalls were also taken into consideration when deriving the boundaries of the
various sub-catchments.
Percentages of impervious area were assessed using TSC’s aerial photography and cadastral
boundary data. Sub-catchment slopes used for input to the RAFTS component of the hydrologic
model were derived using the vectored average slope approach, whilst the average sub-
catchment slope computed by the Vertical Mapper software was used for input to the DRAINS
component of the hydrologic model. The available contour data, which comprised both ALS
survey and 30 m SRTM data, was used as the basis for computing the slope for both methods.
3.3 Hydrologic Model Calibration
3.3.1. General
In the case of main stream flooding, rainfall and flood level data were available for the October
2010 and March 2012 events. Rainfalls recorded at BOM’s Batlow rainfall warning gauge (GS
72004.1) and Adelong (Etham Park) pluviometer were applied to the sub-catchments in RAFTS
model (refer Figures 2.6 to 2.8 for the adopted spatial allocation of historic rainfall) to obtain
discharge hydrographs which were then compared to those recorded by the Batlow Road stream
gauge (GS 410061).
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The discharge hydrographs generated by the RAFTS model for the October 2010 and March
2012 floods were then used in conjunction with the TUFLOW model to derive water surface
profiles for comparison with the available photographic record and recorded flood marks.
3.3.2. RAFTS Model Parameters
A Mannings n value of 0.08 was applied to the sub-catchments which describe the relatively
steep partially wooded areas of the Adelong Creek catchment upstream of the Batlow Road
stream gauge. Depending on the level of tree cover which could be observed in the aerial
photography, Mannings n values of either 0.05 or 0.1 were applied to the sub-catchments which
contribute to flow in the minor drainage lines which discharge to Adelong Creek downstream of
the stream gauge. Initial and continuing loss rates for impervious and pervious areas, as well as
the Bx factor in RAFTS, which were found to give good correspondence with the discharge
hydrographs recorded at the Batlow Road stream gauge are given in Table 3.1.
TABLE 3.1
ADOPTED RAFTS MODEL PARAMETERS
HISTORIC FLOOD EVENTS
Historic Event
Initial Loss (mm) Continuing Loss (mm/hr)
Bx Impervious
Area
Pervious
Area
Impervious
Area
Pervious
Area
October 2010 Flood 2 15(1)
0 2.5 1.0
March 2012 Flood 2 70(2)
0 2.5 1.2
1. Approximately 15 mm of rainfall was recorded in the catchment between 09:00 hours on 12 October 2010 (model
start time) and about 07:00 hours on the 13 October 2010, when water levels at the Batlow Road stream gauge
commenced to rise.
2. Approximately 70 mm of rainfall was recorded in the catchment between 09:00 hours on 28 February 2012
(model start time) and about 03:00 hours on the 1 March 2012, when water levels at the Batlow Road stream
gauge commenced to rise.
3.3.3. Results of Model Calibration
The discharge hydrographs generated by RAFTS for the October 2010 and March 2012 floods
were found to give a good match to those recorded at the Batlow Road stream gauge
(GS 410061) (refer Figure 2.9 for comparison), although in the case of the October 2010 flood,
only after the depth of rain recorded by BOM at its Batlow rainfall warning gauge (GS 72004.1)
was increased to match the reading at its adjacent Batlow Post Office daily rain gauge
(GS 72004) (refer Section 2.2.3 for further discussion).
The discharge hydrograph generated by the calibrated RAFTS model for the October 2010 f lood,
when applied to the TUFLOW hydraulic model, was found to provide a good match to the historic
flood marks, although a large degree of blockage needed to be applied to the model at the
location of the Herb Feint Bridge to reproduce observed flooding patterns along Tumut Street.
Similarly, the discharge hydrograph generated by the calibrated RAFTS model for the March
2012 flood, when applied to the TUFLOW hydraulic model, was found to provide a good match to
the available photographic record, which showed water levels were on the point of surcharging
the left (southern) bank of Adelong Creek immediately downstream of the Herb Feint Bridge. As
there was no reported flooding along Tumut Street, no blockage was applied to the Herb Feint
Bridge when modelling the March 2012 flood. Model calibration is discussed in detail in
Chapter 4.
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3.3.4. Parameters Adopted for Design Flood Estimation
The RAFTS model parameters found to give a good match to observed flood behaviour in the
case of the October 2010 flood (refer Section 3.3.2) were adopted for the design flood
estimation, although initial loss for pervious areas within the RAFTS model was varied for floods
of different ARI to provide reasonable comparison with the peak flow estimates derived by the
flood frequency analysis (refer Section 5.3 in Chapter 5 for further discussion).
Whilst historic flood data is not available to allow a formal calibration of the overland flow
generator in the hydrologic model (i.e. DRAINS) to be undertaken, the following parameters were
adopted for design flood estimation based on the findings of previous studies:
Soil Type = 3.0
AMC = 3.0
Paved area depression storage = 2.0 mm
Grassed area depression storage = 10.0 mm
Paved flow path roughness = 0.02
Grassed flow path roughness = 0.07
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4 HYDRAULIC MODEL DEVELOPMENT AND CALIBRATION
4.1 The TUFLOW Modelling Approach
TUFLOW is a true two-dimensional hydraulic model which does not rely on a prior knowledge of
the pattern of flood flows in order to set up the various fluvial and weir type linkages which
describe the passage of a flood wave through the system.
The basic equations of TUFLOW involve all of the terms of the equations of unsteady flow.
Consequently the model is "fully dynamic" and once tuned will provide an accurate represen tation
of the passage of the floodwave through the drainage system (both surface and piped) in terms of
extent, depth, velocity and distribution of flow.
TUFLOW solves the equations of flow at each point of a rectangular grid system which represent
overland flow on the floodplain and along streets. The choice of grid point spacing depends on
the need to accurately represent features on the floodplain which influence hydraulic behaviour
and flow patterns (e.g. buildings, streets, changes in floodplain dimensions and hydraulic
roughness, etc).
River, channel and piped drainage systems can be modelled as one-dimensional elements
embedded in the larger two-dimensional domain, which typically represents the wider floodplain.
Flows are able to move between the one and two-dimensional elements of the model, depending
on the capacity characteristics of the drainage system being modelled.
The TUFLOW model developed for the Adelong Creek catchment allows for the assessment of
potential flood management measures, such as detention storage, increased channel and
floodway dimensions, augmentation of culverts and bridge crossing dimensions, diversion banks
and levee systems. All of these measures will need to be considered in the future FRMS of the
catchment.
4.2 TUFLOW Model Setup
4.2.1. Model Structure
The layout of the TUFLOW model is shown on Figure 4.1. Within the township of Adelong, the
model comprises the pit and pipe drainage system, whilst the inbank area of Adelong Creek is
represented by a series of cross sections normal to the direction of flow. Both out-of-bank and
shallow “overland” flow are modelled by the rectangular grid.
The following sections provide further details of the model development.
Two-dimensional Model Domain
An important consideration of two-dimensional modelling is how best to represent the roads,
fences, buildings and other features which influence the passage of flow over the natural surface.
Two-dimensional modelling is very computationally intensive and it is not practicable to use a
mesh of very fine elements without excessive times to complete the simulation, particularly for
long duration flood events. The requirement for a reasonable simulation time influences the way
in which these features are represented in the model.
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A grid spacing of 5 m was found to provide an appropriate balance between the need to define
features on the floodplain versus model run times, and was adopted for the investigation. Ground
surface elevations for model grid points were initially assigned using a digital terrain model (DTM)
derived from ALS survey data, and updated using ground survey data where such data were
available.
The footprints of a large number of individual buildings located in the two-dimensional model
domain were digitised and assigned a high hydraulic roughness value relative to the more
hydraulically efficient roads and flow paths through allotments. This accounted for their blocking
effect on flow while maintaining a correct estimate of floodplain storage in the model.
It was not practicable to model the individual fences surrounding the many allotments in the study
area, even though they were observed to have a localised impact on flooding patterns during the
October 2010 event. For the purpose of the present investigation, it was assumed that there
would be sufficient openings in the fences to allow water to enter the properties, whether as flow
under or through fences and via openings at driveways. Individual allotments where development
is present were digitised and assigned a high hydraulic roughness value (although not as high as
for individual buildings) to account for the reduction in conveyance capacity which will result from
fences and other obstructions stored on these properties.
One-dimensional Model Elements
Cross section survey was obtained along the inbank area of Adelong Creek where it was
considered that ALS survey data were not adequate to define bed and bank levels. Figure 4.1
shows the location of the 41 cross sections that were surveyed by Casey Surveying and Design
in 2012. An additional 39 cross sections were also derived from the ALS survey data to improve
the definition of the waterway area along the modelled reach of Adelong Creek.
Details of the three bridges which presently cross Adelong Creek at Adelong (i.e. Rimmers
Bridge, the recently constructed Herb Feint Bridge and the pedestrian bridge which is located a
short distance downstream of the newly constructed road bridge) were surveyed by Casey
Surveying and Design in 2012 (refer Appendix D for copy of sketches). Details of the wooden
bridge that was replaced by the Herb Feint Bridge in early 2011 were taken from drawings
provided by RMS and TSC.
All of the piped elements contained in TRC’s asset database and which influence the passage of
shallow overland type flow were included in the TUFLOW model. Limited information was
available on pipe invert levels. Therefore an assumed cover of 700 mm was adopted for those
drainage elements where invert levels or depth measurements were not available. Adjustments
were made to the assumed invert levels where this approach resulted in a negatively graded
reach of pipe or culvert.
Several types of pits are identified on Figure 4.1, including junction pits which have a closed lid
and inlet pits which are capable of accepting overland flow. TSC’s asset database contained only
limited information in regard to inlet pit types and dimensions. Therefore it was not possible to
define inlet capacity relationships for incorporation in the TUFLOW model. Because of these
limitations, all inlet pits were assumed to have unlimited capacity, with the result that t he capacity
of the piped drainage system was based on the hydraulic capacity of the pipes as determined by
the model.
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Pit losses in the various piped drainage networks were modelled using the approach whereby
energy loss coefficients at pipe junctions are re-calculated at each timestep of the simulation.
The losses are based on a range of variables including the inlet/outlet flow distribution, the depth
of water within the pit, expansion and contraction of flow through the pit, the horizontal deflection
angle between inlet and outlet pipes, and the vertical drop across the pit.
4.2.2. Model Parameters
The main physical parameter for TUFLOW is the hydraulic roughness. Hydraulic roughness is
required for each of the various types of surfaces comprising the overland flow paths, as well as
for the cross sections representing the geometric characteristics of Adelong Creek. In addition to
the energy lost by bed friction, obstructions to flow also dissipate energy by forcing water to
change direction and velocity and by forming eddies. Hydraulic modelling traditionally represents
all of these effects via the surface roughness parameter known as “Manning’s n”. Flow in the
piped system also requires an estimate of hydraulic roughness.
Manning’s n values along the channel and immediate overbank areas along the modelled length
of Adelong Creek were varied, with the values in Table 4.1 providing correspondence between
recorded and modelled flood levels. In relation to the two values of hydraulic roughness given for
the creek bed upstream of the Herb Feint Bridge, it was found that a value of 0.05 had to be
applied to three cross sections located immediately upstream of Rimmers Bridge in order to
reproduce observed flood behaviour.
TABLE 4.1
CALIBRATED HYDRAULIC ROUGHNESS VALUES
DERIVED FOR ADELONG CREEK
Surface Treatment Manning’s n
Value
Asphalt or concrete road surface 0.02
Well-maintained grass cover 0.03
Creek bed downstream of Herb Feint Bridge 0.040
Overbank area of Adelong Creek, including grass and lawns 0.045
Creek bed upstream of Herb Feint Bridge 0.05 and 0.065
Trees / Shrubs 0.09
Allotments (between buildings) 0.10
Densely vegetated creek bank and other areas 0.17
Buildings 10
The adoption of a value of 0.02 for the surfaces of roads, along with an adequate description of
their widths and centreline/kerb elevations, allowed an accurate assessment of their conveyance
capacity to be made. Similarly, the high value of roughness adopted for buildings recognised that
these structures will completely block the flow but are capable of storing water when flooded.
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Figure 4.2 is a typical example of flow patterns derived from the above roughness values. This
example applies to the October 2010 flood and shows flooding patterns on the left (southern)
overbank of Adelong Creek near the intersection of Tumut Street and the Snowy Mountains
Highway.
The left hand side of the figure shows the roads and inter-allotment areas, as well as the outlines
of buildings, which have all been individually digitised in the model. The right hand side shows
the resulting flow paths in the form of scaled velocity vectors and the depths of inundation. The
buildings with their high values of hydraulic roughness block the passage of flow, although the
model recognises that they store floodwater when inundated and therefore correctly accounts for
flood storage. The flow is conveyed via the road reserves and through the open parts of t he
allotments. Similar information to that shown on Figure 4.2 may be presented at any location
within the model domain (which is shown on Figure 4.1) and will be of assistance to TRC in
assessing individual flooding problems in the floodplain.
4.3 Model Boundary Conditions
The locations where sub-catchment inflow hydrographs were applied to the TUFLOW model are
shown on Figure 4.1. These comprise both point-source inflows at selected locations around the
perimeter of the two-dimensional model domain (RAFTS model only) and distributed inflows via
“Rain Boundaries” (RAFTS and DRAINS models).
The Rain Boundaries act to “inject” flow into the TUFLOW model, firstly at a point which has the
lowest elevation, and then progressively over the extent of the Rain Boundary as the grid in the
two-dimensional model domain becomes wet as a result of overland flow. The extent of each
Rain Boundary matches the sub-catchment area defined in the RAFTS and DRAINS hydrologic
models, resulting in the flows being applied as they would be in the real drainage system.
The downstream boundary of the TUFLOW model comprised a broadcrested weir arrangement,
the elevation of which was set equal to the invert level of Adelong Creek.
4.4 Hydraulic Model Calibration
4.4.1. General
The TUFLOW hydraulic model was calibrated to the October 2010 and March 2012 floods using
the available flood data. The calibrated hydraulic model was also run using the discharge
hydrograph that was recorded at the Batlow Road stream gauge for the January 1984 flood an d
the results compared to flood marks shown on WRC’s Reference Plan for Adelong (refer
Appendix B for copy).
4.4.2. October 2010 Flood
As previously mentioned, at the time of the October 2010 flood the upstream side of the Adelong
Bridge had been partially demolished and the corresponding piers and deck section (including the
traffic barrier) of the new Herb Feint Bridge installed. Whilst the underside of the new bridge
deck and top of the new traffic barrier closely matched that of the Adelong Bridge (refer
Appendix C for copy of bridge drawings), there would have been a minor reduction in waterway
area due to the presence of the new bridge piers which were offset from those of the existing
structure.
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For model calibration purposes, details of the old highway bridge were input to the hydraulic
model, since the narrower bridge opening and larger number of piers would have formed the
hydraulic control on flood behaviour immediately upstream of the crossing.
Initial runs of the hydraulic model showed that surcharge of the left (southern) bank of Adelong
Creek upstream of the crossing would not have occurred in the October 2010 flood had the full
waterway area been maintained during the event. However, the October 2010 flood was
characterised by a large amount of floating woody debris which was observed to lodge on the
bridge during the event.
A range of blockage scenarios were assessed as part of the present investigation, with a “top -
down” type blockage condition found to provide a best fit to observed flood behaviour. In order to
model the floating debris that was observed to have lodged on the upstream side of the bridge,
the soffit level of the Adelong Bridge was lowered by 1.33 m, from an elevation of 334.68 m AHD
to an elevation of 333.375 m AHD.
Figure 4.3 shows the modelled water surface profile along Adelong Creek for the October 2010
flood, as well as a number of flood marks which were surveyed by TRC along the main arm of
Adelong Creek following the event. By inspection of Figure 4.3, a good fit with recorded flood
levels was achieved, although it was necessary to reduce the hydraulic roughness value for the
inbank area of Adelong Creek immediately downstream of the Herb Feint Bridge from 0.065 to
0.035 in order to match recorded flood levels. The reduction in hydraulic roughness is attributed
to there being less dense stands of poplar trees along the creek bank, which in the upstream
reach tended to capture floating debris, thus increasing resistance to flow.
Whilst a good fit could be achieved with the recorded flood marks along the modelled reach of
Adelong Creek using the hydraulic roughness values given in Table 4.1, the TUFLOW model was
not able to reproduce the recorded flood peak of 356.13 m AHD (or 4.61 m) on the Batlow Road
stream gauge. A review of site conditions at the gauge site identified that the bed level of
Adelong Creek falls by about 3 m over a relative short distance immediately downstream of the
gauge and that a rock bar is also present in the bed of the creek downstream of the gauge.
Whilst additional cross sections were generated using on the ALS survey data in this area, the
model was not able to reproduce the rapidly varying water surface profile in the vicinity of the
gauge site. Further discussion on hydraulic modelling undertaken to verify NOW’s rating curve
for its Batlow Road stream gauge using the HEC-RAS hydraulic modelling software is contained
in Section 4.4.5.
Figure 4.4 (Sheet 1 of 2) shows indicative extents and depths of inundation along the modelled
reach of Adelong Creek, whilst Figure 4.4 (Sheet 2 of 2) shows flooding patterns along Tumut
Street in Adelong. Also shown on Figure 4.4 (Sheet 2 of 2) are the locations of a number of
photographs which were compiled by SES following the flood event and which are presented in
Annexure D.
The extents of inundation shown on Figure 4.4 are consistent with those shown on Figure 12.6 in
Bewsher, 2011, with the exception of the flooding that is shown to have occurred in the
Showground. Based on the results of the design flood modelling (refer Chapter 6 for details), it is
likely that the floodwater which was observed in the Showground originated from Black Creek,
rather than from Adelong Creek.
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Table H1 in Annexure H gives the difference between the recorded flood heights shown on
Figures 4.3 and 4.4 (2 sheets) and the TUFLOW model results, whilst Table I1 (Column D) in
Annexure I gives peak flows at selected locations along the modelled reach of Adelong Creek .
There are a number of locations where the difference in observed and modelled peak flood
heights is greater than 250 mm. The reasons for these relatively large differences are:
The recorded flood height is not consistent with adjacent heights. This issue is
considered to apply to flood marks 534, 571, 588, 590, 611, 627 and 651.
The TUFLOW model is not able to reproduce the localised effects that minor structures
such as fences and low walls had on flooding behaviour. This issue is considered to
apply to flood marks 562, 563, 594 and 595.
Based on the above, the calibrated TUFLOW model was considered to satisfactorily reproduce
flood behaviour which was observed during the October 2010 flood event.
4.4.3. March 2012 Flood
As the construction of the Herb Feint Bridge was completed at the time of the March 2012 f lood,
the TUFLOW model structure was altered to reflect the new structure, details of which were taken
from the design drawings (refer Appendix C for copy of bridge drawings).
There were no reports of a partial blockage being experienced at the new bridge during the flood
event, with the available photographic record showing water levels below the soffit level of the
bridge (refer Plate 26 in Annexure E).
Flood marks for the March 2012 flood are limited to the readings that were taken at the then
recently installed depth gauge which is located on the left bank of Adelong Creek immediately
downstream of the Herb Feint Bridge (refer Section 2.2.4 for further details). A photograph taken
by a local resident (refer Plate 27 in Annexure E) shows floodwater was on the cusp of
surcharging the left (southern) bank of Adelong Creek immediately downstream of the Herb Feint
Bridge. This observation was reproduced by the TUFLOW hydraulic model.
Hydraulic roughness values which were found to give good correspondence with observed flood
behaviour for the October 2010 flood (refer values given in Table 4.1) were also adopted for
modelling the March 2012 flood, with the exception that a higher value of 0.065 was adopted for
defining creek bed roughness immediately upstream of Rimmers Bridge.9
The modelled water surface profile for the March 2012 flood is shown on Figure 4.3, whilst
Figure 4.5 shows the indicative extents and depths of inundation along the modelled reach of
Adelong Creek. Table I1 (Column E) in Annexure I gives peak flows at selected locations along
the modelled reach of Adelong Creek.
Based on a review of the results of the model calibration process for the October 2010 and March
2012 floods, it was concluded that the version of the TUFLOW model which incorporates details
of the Herb Feint Bridge and the hydraulic roughness values given in Table 4.1 could be used for
defining flood behaviour at Adelong over the full range of design flood events.
9 By inspection of the available aerial photography, the reach of creek immediately upstream of Rimmers
Bridge is now more densely vegetated than adjacent reaches of creek. It was therefore considered prudent
to adopt the higher hydraulic roughness value, which was found to give good correspondence with observed
flood behaviour comparable reaches of the creek.
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4.4.4. January 1984 Flood
Whilst the January 1984 flood occurred nearly 30 years ago, it is the last major event which
caused flooding in Adelong (Bewsher, 2011). The discharge hydrograph recorded by the Batlow
Road stream gauge during the flood was input at the upstream boundary of the TUFLOW model
described in Section 4.4.2 (i.e. the model which incorporated the now demolished Adelong
Bridge).
Initial runs of the TUFLOW model showed that surcharge of Adelong Creek upstream of Adelong
Bridge would have only occurred had debris built up on the upstream side of the structure.
Table 12.1 in Bewsher, 2011 makes reference to the fact that dense debris was observed to have
built up on the bridge during the flood event, which contributed to minor flooding near the
intersection of Campbell Street and Tumut Street.
Whilst surcharge of Adelong Creek could be achieved by applying a blockage factor to the bridge,
modelled peak flood heights were generally lower than those shown on WRC’s Reference Plan
for Adelong (refer Appendix B for copy). The reason for the TUFLOW model producing lower
peak flood heights than were recorded may be attributed to a possible increase in hydraulic
roughness along the river bank at the time of this flood.
Given the time that has elapsed since the January 1984 flood and the close correlation ach ieved
between observed and modelled flood behaviour for the more recent October 2010 and
March 2012 floods, further consideration of the TUFLOW model results for this event is not
warranted.
4.4.5. Batlow Road Stream Gauge Rating Curve
In order to verify NOW’s rating curve for the Batlow Road stream gauge, and therefore provide
more confidence that the flows recorded by the gauge are representative of those that were
generated by the catchment during the three historic floods assessed as part of the present
investigation, a HEC-RAS model was developed of Adelong Creek in the vicinity of the gauge site
(denoted the ‘Batlow Road HEC-RAS Model’).
HEC-RAS is a one-dimensional hydraulic modelling package developed by the Hydrologic
Engineering Centre of the US Army Corps of Engineers and has seen widespread application in
Australia. HEC-RAS solves the momentum equation of open channel flow between user defined
grid arrangements (more typically, cross section locations) for given boundary conditions.
Typically, a peak discharge comprises the upstream boundary and the downstream boundary is
either a rating curve (stage versus discharge relationship) or the assumption of uniform flow
(friction slope equals the bed slope of the stream). The HEC-RAS modelling approach was
adopted to enable the rapidly varying water surface profile at the gauge site to be quickly and
more accurately defined through the use of closely spaced interpolated cross sections.
Peak flows for the January 1984 (215 m3/s), October 2010 (383 m
3/s) and March 2012 (163 m
3/s)
floods, together with the maximum gauged flow of 105 m3/s were used as input to the HEC-RAS
model. Hydraulic roughness values found to give good correspondence with the recorded flood
marks downstream of the gauge site were also used as input to the model.
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The cross sections of the inbank area of Adelong Creek that were surveyed by Casey Surveying
and Design in 2012 were extended using the ALS survey data. As mentioned, the HEC-RAS
software was also used to interpolate a number of cross sections along the modelled reach of
creek.
Figure 4.6 shows the layout of the Batlow Road HEC-RAS Model, whilst Figure 4.7 shows the
computed water surface and critical depth profiles along Adelong Creek as modelled in
HEC RAS. The resulting peak flood levels at the gauge site for the four modelled flow rates are
plotted against NOW’s rating curve which is shown on Figure 2.9.
By inspection of the water surface and critical depth profiles shown on Figure 4.6, a hydraulic
control is located immediately upstream of the gauge, where water levels rise rapidly due to a
constriction in the creek. Average flow velocities at the gauge site are around 2.5 m/s, increasing
to about 5 m/s at the location of the upstream constriction.
By inspection of Figure 2.8, the Batlow Road stream gauge HEC-RAS model generates peak
flood heights similar to NOW’s rating curve. Allowing for the relatively high velocity flow and the
likely presence of wave action at the gauge site which may affect recorded flood heights, it is
concluded that NOW’s rating curve provides a reasonable estimate of historic flood flows in
Adelong Creek at Adelong.
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5 DERIVATION OF DESIGN FLOOD HYDROGRAPHS
5.1 Design Storms
5.1.1. Rainfall Intensity
The procedures used to obtain temporally and spatially accurate and consistent intensity-
frequency-duration (IFD) design rainfall curves for the Adelong Creek catchment area are
presented in Book II of ARR, 1998. Design storms for frequencies of 5, 10, 50, 100, 200 and
500 year ARI were derived for storm durations ranging between 25 minutes and 18 hours. The
procedure adopted was to generate an IFD dataset for the catchment by using the relevant charts
in Volume 2 of ARR, 1998. These charts included design rainfall isopleths, regional skewness
and geographical factors.
5.1.2. Areal Reduction Factors
The rainfalls derived using the processes outlined in ARR, 2001 are applicable strictly to a point.
In the case of a catchment of over tens of square kilometres area, it is not realistic to assume that
the same rainfall intensity can be maintained. An areal reduction factor (ARF) is typically applied
to obtain an intensity that is applicable over the entire catchment. Chapter 2 of ARR, 2001 shows
curves relating the ARF to catchment area for various storm durations.
The Areal Reduction Factor (ARF) for a particular catchment area and given design rainfall burst
duration and annual exceedance probability (AEP), represents the ratio between the areal design
rainfall and the representative point duration rainfall for the catchment. ARR, 1998 recommended
ARF’s based on studies in the United States, whilst Jordan et al, 2011 describes the derivation of
ARF equations for NSW and ACT. Data from the record at over 6000 sites across those two
areas was used to derive ARF factors for durations between 1 and 5 days and AEP between 1 in
2 and 1 in 100. For durations less than 1 day, short duration equations based on studies
undertaken in Victoria were recommended.
The Adelong Creek catchment is 146 km2 in area. Using the Jordan et al, 2011 relationships,
ARF would range between 0.84 and 0.90, and the ARR, 1998 relationships between 0.94 and
0.97, for storm durations between 4.5 and 18 hours. Table 5.1 over gives peak 100 year ARI
flows generated by the RAFTS hydrologic model at the location of the Batlow Road stream gauge
for various ARF values. After comparison of the peak flows given in Table 5.1 with the findings of
the flood frequency analyses (refer Section 2.3.2 for details), no areal reduction in design point
rainfalls was made for this study.
5.1.3. Temporal Patterns
Temporal patterns for various zones in Australia are presented in ARR, 1998. These patterns are
used in the conversion of a design rainfall depth with a specific ARI into a design flood of the
same frequency. Patterns of average variability are assumed to provide the desired conversion.
The patterns may be used for ARI’s up to 500 years where the design rainfall data is extrapolated
to this ARI.
The derivation of temporal patterns for design storms is discussed in Book II of ARR, 1998 and
separate patterns are presented in Volume 2 for ARI < 30 years and ARI > 30 years. The second
pattern is intended for use for rainfalls with ARI’s up to 100 years, and to 500 years in those
cases where the design rainfall data in Book II of ARR are extrapolated to this ARI.
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TABLE 5.1
COMPARISON OF PEAK FLOWS FOR VARIOUS ARF VALUES
100 year ARI
(m3/s)
ARF Values
Storm Duration (hours)
4.5 6 9 12 18
No ARF 365
[1.0]
366
[1.0]
294
[1.0]
299
[1.0]
295
[1.0]
ARF Values as per
ARR, 1998
326
[0.94]
331
[0.94]
269
[095]
279
[0.96]
279
[0.97]
ARF Values as per
Jordan et al, 2011
270
[0.84]
279
[0.85]
231
[0.87]
239
[0.88]
249
[0.90]
Note: Numbers in [ ] refer to ARF values applied to RAFTS hydrologic model.
5.2 Probable Maximum Precipitation
Estimates of probable maximum precipitation were made using the Generalised Short Duration
Method (GSDM) as described in the Bureau of Meteorology’s update of Bulletin 53 (BOM, 2003).
This method is appropriate for estimating extreme rainfall depths for catchments up to 1000 km2
in area and storm durations up to 6 hours.
The steps involved in assessing PMP for the Adelong Creek catchment are briefly as follows:
Calculate PMP for a given duration and catchment area using depth-duration-area
envelope curves derived from the highest recorded US and Australian rainfalls.
Adjust the PMP estimate according to the percentages of the catchment which are
meteorologically rough and smooth, and also according to elevation adjustment and
moisture adjustment factors.
Assess the design spatial distribution of rainfall using the distribution for convective
storms based on US and world data, but modified in the light of Australian experience.
Derive storm hyetographs using the temporal distribution contained in Bulletin 53, which
is based on pluviographic traces recorded in major Australian storms.
Figure 3.1 (Sheet 1 of 2) shows the location and orientation of the PMP ellipses which were used
to derive the rainfall estimates for each individual sub-catchment.
5.3 Derivation of Design Discharges
The RAFTS and DRAINS hydrologic models were run with the adopted parameters
(Section 3.3.4) to obtain design hydrographs for ARI’s ranging between 5 and 500 years,
together with the PMF for input to the TUFLOW hydraulic model. As mentioned in Section 3.3.4,
the initial loss value for pervious areas within the RAFTS model was varied for floods of different
ARI to provide reasonable comparison with the peak flow estimates derived by the flood
frequency analysis. Table 5.2 over gives a comparison of peak flows derived by the flood
frequency analysis (refer Figure 2.12 (RHS)) and those generated by the calibrated RAFTS
model for design storms of varying ARI. The initial loss value which gave the best fit to the peak
flow data is also given.
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TABLE 5.2
COMPARISON OF DESIGN PEAK FLOWS
Design Storm Event
Peak Flow (m3/s)
Initial Loss Value(2)
(mm) Flood Frequency
Analysis(1)
RAFTS
5 year ARI 84 80 25
10 year ARI 120 117 25
50 year ARI 270 270 20
100 year ARI 375 366 15
200 year ARI 500 432 15
1. Peak flows taken from Figure 2.12 (RHS)
2. Initial loss values apply to pervious areas in RAFTS model. Refer Section 3.3.4 for other adopted RAFTS model
parameters.
The values of initial loss which were found to give good correspondence with the flood frequency
analysis are in general agreement with those recommended in Walsh et al, 1991 for practical
flood estimation in NSW. Based on this finding, the initial loss values given in Table 5.2 were
adopted for design flood estimation, noting that an initial loss of 15 mm was adopted for
modelling the 500 year ARI and PMF events.
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6 HYDRAULIC MODELLING OF DESIGN FLOODS
6.1 Presentation and Discussion of Results
6.1.1. Water Surface Profiles and Extents of Inundation
Water surface profiles along Adelong Creek are shown on Figures 6.1 for the modelled design
floods events, whilst Figure 6.2 shows stage and discharge hydrographs at the Batlow Road
stream gauge, Rimmers Bridge and the Herb Feint Bridge for floods up to 500 year ARI.10
The
results confirm the “flash flood” nature of the Adelong Creek catchment, with flood levels
generally peaking less than 6 hours after the commencement of rainfall.
Figures 6.3 to 6.9 show the TUFLOW model results for the 5, 10, 50, 100, 200 and 500 year ARI
floods and the PMF. These diagrams show the indicative extents of inundation along the main
arm of the creek, as well as the overland flow paths and depths of inundation. Table I1
(Columns F to S) in Annexure I gives peak design flows at selected locations throughout the
study area.
In order to create realistic results which remove most of anomalies caused by inaccuracies in the
LiDAR (which has a design accuracy such that 68 per cent of the points have an accuracy in level
of +/- 150 mm), a filter is sometimes applied to remove depths of inundation over the natural
surface less than 50 mm. This has the effect of removing the very shallow depths which are
more prone to be artifacts of the model. However, in the present case modelled depths of
inundation less than 50 mm have been displayed to allow a clearer representation to the reader
of the various overland flow paths, particularly those located in the urbanised areas in Adelong.
6.1.2. Accuracy of Hydraulic Modelling
The accuracy of results depends on the precision of the numerical finite difference procedure
used to solve the partial differential equations of flow, which is also influenced by the time step
used for routing the floodwave through the system and the grid spacing adopted for describing
the natural surface levels in the floodplain. Open channels are described by cross-sections
normal to the direction of flow, so their spacing also has a bearing on the accuracy of the results.
The results are also heavily dependent on the size of the two-dimensional grid, as well as the
accuracy of the ALS data, which as noted above has a design accuracy based on 68% of points
within +/- 150 mm.
Given the uncertainties in the ALS data and the definition of features affecting the passage of
flow, maintenance of a depth of flow of at least 200 mm is required for the definition of a
“continuous” flow path in the areas subject to shallow overland flow approaching the main arm of
the creek. Lesser modelled depths of inundation may be influenced by the above factors and
therefore may be spurious, especially where that inundation occurs at isolated locations and is
not part of a continuous flow path. In areas where the depth of inundation is greater than
200 mm threshold and the flow path is continuous, the likely accuracy of the hydraulic modelling
in deriving peak flood levels is considered to be between 100 and 150 mm.
Use of the flood study results when applying flood related controls to development proposals
should be undertaken with the above limitations in mind. Proposals should be assessed with the
benefit of a site survey to be supplied by applicants, in order to allow any inconsistencies in
results to be identified and given consideration. This comment is especially appropriate in the
areas subject to shallow overland flow, where the errors in the ALS or obstructions to flow would
10
Note results assume ideal flow conditions at the two bridge crossings (i.e. zero blockage conditions).
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have a proportionally greater influence on the computed water surface levels than in the deeper
flooded main stream areas.
Minimum floor levels for residential and commercial developments should be based on the
100 year ARI flood level plus appropriate freeboard (this planning level is defined as the “Flood
Planning Level” (FPL)), to cater for uncertainties such as wave action, effects of flood debris
conveyed in the creek and overland flow streams and precision of modelling. Selection of interim
FPL’s, pending completion of the FRMS for the catchment, is presented in Section 6.5.
The sensitivity studies and discussion presented in Section 6.3 provide guidance on the
suitability of the recommended allowance for freeboard under present day climatic conditions.
In accordance with OEH recommendations (DECCW, 2007), sensitivity studies have also been
carried out (refer Section 6.4) to assess the impacts of future climate change. Increases in flood
levels due to future increases in rainfall intensities may influence the selection of FPL’s.
However, final selection of FPL’s is a matter for more detailed consideration in the future FRMS.
6.1.3. Flooding Behaviour Along Adelong Creek Assuming No Blockage
Floodwater is generally confined to the inbank area of Adelong Creek for floods up to 20 year
ARI,11
with surcharge of the overbank area occurring at the following locations for floods of
greater ARI:
Floodwater commences to surcharge the left bank of Adelong Creek at the northern end
of Selwyn Street for a flood with an ARI between 10 and 50 years.12
Commercial
development located adjacent to the bend in the creek is impacted by floodwater which
surcharges the left bank of Adelong Creek and flows across Selwyn Street in a 50 year
ARI flood.
Floodwater commences to inundate low lying areas located on both the left and right
overbank of Adelong Creek immediately upstream of Rimmers Bridge in a 50 year ARI
flood. Floodwater also commences to inundate Cromwell Street near its intersection with
Selwyn Street. Floodwater also commences to backwater toward the extension of
Gundagai Street on the left overbank of Adelong Creek in a 50 year ARI flood event.
Floodwater which backs up into this area is joined by floodwater which surcharges the left
bank of Adelong Creek upstream of Gundagai Street in a 100 year ARI flood event. No
existing development is located in this area.
Floodwater surcharges Cromwell Street near its intersect ion with Selwyn Street in a
100 year ARI flood where it follows the line of a natural overbank runner north to Oberne
Street. Several residences located along the western side of Selwyn Street north of
Cromwell Street are affected by floodwater which breaks out of Adelong Creek at this
location. One of these residences was observed to comprise slab-on-ground type
construction. Floodwater also extends to the rear of several buildings which are located
along the northern side of Tumut Street between Wyndam Street and Neill Street in a 100
year ARI flood event. Floodwater would extend out to Tumut Street within several of
these properties in a 200 year ARI flood event.
11
Based on modelling of the March 2012 flood, which was equivalent to about a 20 year ARI
event (refer Figure 4.5 for TUFLOW model results).
12 By inspection of Figure 4.5, surcharge of the left bank of Adelong Creek at this location is likely
to occur during floods larger than about 20 year ARI.
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Floodwater commences to surcharge the left bank of Adelong Creek upstream of the Herb
Feint Bridge for floods greater than 200 year ARI. Floodwater which surcharges Adelong
Creek at this location generally flows in a westerly direction along Tumut Street, where it
discharges between several buildings before re-joining flow in the creek (Figure 4.2
shows typical flooding patterns in the vicinity of the break out and along the downstream
section of Tumut Street).
There is about 0.5 m freeboard to the soffit level of Rimmers Bridge, and zero freeboard to that of
the new Herb Feint Bridge in a 100 year ARI flood.
The Golden Gully Caravan Park, which is located on the right bank of Adelong Creek a short
distance downstream of the Herb Feint Bridge (refer Figure 2.2 for location), is located above the
100 year ARI flood.
Table 6.1 over gives the peak flood levels and corresponding gauge heights at the Batlow Road
stream gauge as modelled in HEC-RAS, as well as the corresponding peak flows, as generated
by the RAFTS model for events up to 500 year ARI. Details of historic floods are also given for
comparison purposes. Also given in Table 6.1 over are peak heights and corresponding peak
flows for the Herb Feint Bridge depth gauge, along with the approximate travel time between the
two gauge sites, all of which have been extracted from the TUFLOW model results.
A common feature in areas where there are incised valleys with narrow floodplains (as is the case
in Adelong), is a relatively large flood range, especially for the more extreme flood events. For
example, the flood range at Adelong between the 5 and 100 year ARI events is about 2 m,
increasing to about 5 m between the 100 year ARI and PMF events (refer Figure 6.1). Whilst the
relatively large flood range does not translate into a large increase in the extent of flood affected
land (i.e. because of the steep sided nature of the floodplain), consideration will need to be given
during the preparation of the future FRMS to this relatively large flood range, especially when
determining appropriate flood related planning controls and flood evacuation routes.
6.1.4. Flooding Behaviour Along Black Creek Assuming No Blockage
Black Creek upstream of Todds Road has limited capacity and floodwater surcharges both its left
and right bank during relatively frequent storm events. Depths of overbank flow are however
relatively shallow, in the range 0-300 mm for floods up to 100 year ARI, with isolated areas of
deeper flooding generally centred on the creek.
Floodwater surcharges the culverts under Adelong Cemetery Road for events as frequent as
5 year ARI. Floodwater which surcharges the left bank of the creek at this location flows in an
easterly direction along the upstream side of Oberne Street before crossing the road west of Neill
Street. Depths of flow across the low point in the road increase from between 100-200 mm in a
5 year ARI event to 300-400 mm in a 100 year ARI event.
Floodwater also surcharges the left bank of Black Creek immediately upstream of the Todds
Road culvert for events as frequent as 5 year ARI. Floodwater which surcharges the creek at this
location ponds along the northern (upstream) side of the road east of the creek, before
surcharging Todds Road during storms larger than about 10 year ARI. Floodwater which
surcharges Todds Road east of Black Creek contributes to flooding in the Adelong Showground
and in the rear of several residential properties which are located along the western side of
Cromwell Street.
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Whilst there is no existing residential development impacted by floodwater which surcharges
Black Creek, consideration will need to be given to flooding behaviour during the rezoning and
subdivision of land as part of the Adelong South-West Future Growth Area (refer Section 6.6 for
further details).
6.1.5. Overland Flooding Behaviour in Adelong Assuming No Blockage
Surcharge of the existing piped drainage system occurs for flows as frequent as 5 year ARI.
Depths of overland flow are generally in the range 0-300 mm in most areas, with greater depths
of flow shown to occur in property located along the western side of Wyndham Street north of
Lockhart Street and between Neill and Havelock Streets north of Lockhart Street.
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TABLE 6.1
DESIGN AND HISTORIC FLOOD DATA AT GAUGE SITES
Flood Event
Batlow Road Stream Gauge Herb Feint Bridge Depth Gauge Travel Time
Between Gauge
Sites(10)
(Minutes)
Peak Flow(1,2)
(m3/s)
Peak Flood Level(3,6)
(m AHD)
Corresponding Gauge
Height(4,5,6)
(m)
Peak Flow
(m3/s)(7)
Peak Flood
Level
(m AHD)(6,7)
Corresponding
Gauge Height(6,8,9)
(m)
[A] [B] [C] [D] [E] [F] [G] [H]
5 year ARI 80 353.65 2.13 93 332.27 2.62 60
10 year ARI 117 354.06 2.54 131 332.61 2.96 50
March 2012 163 354.43 [354.63] 2.91 [3.11] 160 332.96 [333.05] 3.31 [3.4] 40
January 1984 215 354.92 [355.04] 3.40 [3.52] - - - -
50 year ARI 270 355.45 3.93 289 333.80 4.15 40
100 year ARI 366 356.23 4.71 388 334.43 4.78 40
October 2010 383 356.36 [356.13] 4.84 [4.61] - - - -
200 year ARI 432 356.69 5.17 451 334.81 5.16 40
500 year ARI 520 357.23 5.71 527 335.18 5.53 30
1. Peak flow for design floods generated by RAFTS model, whilst those for historic floods are based on NOW’s current rating curve for the stream gauge.
2. Peak flows are for design storm of 6 hours duration, which is generally critical for maximising flow in Adelong Creek at Adel ong
3. Peak flood levels for both historic and design floods computed using HEC-RAS software. Refer Section 4.4.5 for description of hydraulic model.
4. Gauge zero = 351.52 m AHD.
5. Corresponding gauge heights based on computed peak flood level given in Column C.
6. Values in [ ] are actual gauge heights recorded by stream gauge for each historic flood.
7. Peak flood levels and flows extracted from TUFLOW model results.
8. Gauge zero = 329.65 m AHD.
9. Corresponding gauge heights based on computed peak flood level given in Column F.
10. Modelled travel time between gauge sites rounded to the nearest 10 minutes.
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6.2 Flood Hazard Zones and Floodways
6.2.1. Provisional Flood Hazard
Flood hazard categories may be assigned to flood affected areas in accordance with the
procedures outlined in the FDM. Flood prone areas may be provisionally categorised into Low
Hazard and High Hazard areas depending on the depth of inundation and flow velocity. Flood
depths as high as a metre, in the absence of any significant flow velocity, could be considered to
represent Low Hazard conditions. Similarly, areas of flow velocities up to 2.0 m/s, bu t with small
flood depths could also represent Low Hazard conditions.
A Provisional Hazard diagram for the 100 year ARI event in the study area based on Diagram L2
of the FDM is presented on Figure 6.10.
For the 100 year ARI, high hazard flooding in the study area is generally confined to the inbank
area of Adelong Creek and several of its minor tributaries. Isolated areas of high hazard are also
located on the overbank area of Adelong Creek, for example at the northern end of Selwyn
Street, where depths of flow exceed 1 m on the left (southern) overbank of the creek.
Further discussion on the impact a partial blockage of both Rimmers Bridge and the Herb Feint
Bridge will have on flood behaviour, and the resulting changes in flood hazard is contained in
Section 6.3.2.
The Flood Hazard assessment presented herein is based on considerations of depth and velocity
of flow and is provisional only. As noted in the FDM, other considerations such as rate of rise of
floodwaters and access to high ground for evacuation from the floodplain should also be taken
into consideration before a final determination of Flood Hazard can be made. These factors
would be taken into account in the FRMS for the catchment.
6.2.2. Floodways
According to the FDM, the floodplain may be subdivided into the following three hydraulic
categories:
Floodways;
Flood storage; and
Flood fringe.
Floodways are those areas of the floodplain where a significant discharge of water occurs during
floods. They are often aligned with obvious naturally defined channels. Floodways are the areas
that, even if only partially blocked, would cause a significant re-distribution of flow, or a significant
increase in flood level which may in turn adversely affect other areas. They are often, but not
necessarily, areas with deeper flow of areas where higher velocities occur.
Flood storage areas are those parts of the floodplain that are important for the temporary
storage of floodwaters during the passage of a flood. If the capacity of a flood storage area is
substantially reduced by, for example, the construction of levees or by landfill, flood levels in
nearby areas may rise and the peak discharge downstream may be increased. Substantial
reduction of the capacity of a flood storage area can also cause a signif icant redistribution of
flood flows.
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Flood fringe is the remaining area of land affected by flooding, after floodway and flood storage
areas have been defined. Development in flood fringe areas would not have any significant effect
on the pattern of flood flows and/or flood levels.
Flood storage effects are not significant on Adelong Creek as there is very little storage in the
overbank areas. Peak flood levels are primarily determined by the conveyance capacity of the
waterway and by definition, most of the conveyance is located within the floodway. For this
reason, the floodplain was sub-divided into floodway and flood fringe areas only.
Floodplain Risk Management Guideline No. 2 Floodway Definition, offers guidance in relation to
two alternative procedures for identifying floodways. They are:
Approach A. Using a qualitative approach which is based on the judgement of an
experienced hydraulic engineer. In assessing whether or not the area under consideration
was a floodway, the qualitative approach would need to consider; whether obstruction
would divert water to other existing flow paths; or would have a significant impact on
upstream flood levels during major flood events; or would adversely re-direct flows
towards existing development.
Approach B. Using the hydraulic model, in this case TUFLOW, to define the floodway
based on quantitative experiments where flows are restricted or the conveyance capacity
of the flow path reduced, until there was a significant effect on upstream flood levels
and/or a diversion of flows to existing or new flow paths.
One quantitative experimental procedure commonly used is to progressively encroach across
either floodplain towards the channel until the designated flood level has increased by a
significant amount (for example 0.1 m) above the existing (un-encroached) flood levels. This
indicates the limits of the hydraulic floodway since any further encroachment will intrude into that
part of the floodplain necessary for the free flow of flood waters – that is, into the floodway.
The quantitative assessment associated with Approach B is technically difficult to implement.
Restricting the flow to achieve the 0.1 m increase in flood levels can result in contradictory
results, especially in unsteady flow modelling, with the restriction actually causing reductions in
computed levels in some areas due to changes in the distribution of flows along the main
drainage line.
Accordingly the qualitative approach associated with Approach A was adopted, together with
consideration of the portion of the floodplain which conveys approximately 80% of the total flow
and also the findings of Howells et al, 2004 who defined the floodway based on velocity of flow
and depth. Howells et al suggested the following criteria for defining those areas which operate
as a “floodway” in a 100 year ARI event:
Velocity x Depth greater than 0.25 m2/s and Velocity greater than 0.25 m/s; or
Velocity greater than 1 m/s.
The portion of the overland flow path which did not reach the above threshold values would be
denoted the “flood fringe”.
Flood storage areas would normally be identified as those areas which do not operate as
floodways in a 100 year ARI event but where the depth of inundation exceeded 1 m. However, in
Adelong there are only small isolated pockets where this criterion applies.
The hydraulic categorisation of the floodplain for the 100 year ARI is shown on Figure 6.10. The
hydraulic categorisation includes both floodway and flood fringe areas, but not flood storage
areas given their limited extent.
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The floodway area is generally confined to the inbank area of Adelong Creek due to the relatively
high hydraulic capacity of the channel. However, the floodway does extend onto the immediate
overbank of Adelong Creek at several locations along its length. Most of the areas in Adelong
subject to shallow overland flow are “Flood Fringe” zones. The high hazard floodway area was
found to closely correlate with the extent of the 50 year ARI flood, the peak flow of which is about
74% that of the 100 year ARI, whilst the low hazard floodway generally comprised areas where
major break outs of flow occur due to the higher flow rate (e.g. at the northern and southern end s
of Selwyn Street and on the right overbank of Adelong Creek upstream of Rimmers Bridge).
It is also to be noted in the context of defining the floodway for the planning flood (100 year ARI)
that a partial blockage of the bridge structures or slightly higher flows in Adelong Creek will result
in the development of new flow paths, for example along Tumut Street. Consideration of these
changes in flood behaviour will need to be taken into consideration when determining an
appropriate freeboard to be applied to future development bordering Adelong Creek (refer
Section 6.5 for further discussion).
6.3 Sensitivity Studies
6.3.1. General
The sensitivity of the hydraulic model was tested to variations in model parameters such as
hydraulic roughness and the partial blockage of Rimmers Bridge and the Herb Feint Bridge by
woody debris. The main purpose of these studies was to give some guidance on the freeboard to
be adopted when setting floor levels of development in flood prone areas, pending the completion
of the future FRMS for Adelong. The results are summarised in the following sections.
6.3.2. Sensitivity to Hydraulic Roughness
Figure 6.11 shows the difference in peak flood levels (i.e. the “afflux”) for the 100 year ARI
6 hour duration storm resulting from an assumed 20% increase in hydraulic roughness (compared
to the values given in Table 4.1) along the main arm and overbank area of Adelong Creek. The
typical increase in peak flood level along the main arm of Adelong Creek is in the range 100 to
500 mm, with lesser values of 10 to 50 mm in the tributaries. The increase in extents of
inundation in land bordering Adelong Creek would not be significant, although floodwater would
commence to surcharge the left (southern) bank of Adelong Creek immediately upstream of the
Herb Feint Bridge.
6.3.3. Sensitivity to Partial Blockage of Bridges
Given the high debris load which has been observed during major floods on Adelong Creek, it is
likely that Rimmers Bridge and the new Herb Feint Bridge will experience a partial blockage
during future flood events. Whilst the degree of blockage experienced during the October 2010
flood at the new Herb Feint Bridge is considered a-typical (i.e. because the central embankment
of the partially demolished Adelong Bridge acted to trap debris beneath the partially constructed
Herb Feint Bridge), debris is likely to accumulate on the upstream side of the bridge piers during
a flood. If the flood is of sufficient magnitude, debris could also accumulate on the upstream side
of the bridge deck, with a partial reduction in waterway area experienced below the soffit level of
the bridge.
EA, 2013 includes guidance on modes of blockage which are likely to be experienced for different
hydraulic structures. In regards bridge structures, those with clear opening heights less than 3 m
(e.g. Herb Feint Bridge) are said to be susceptible to blockage in streams where large floating
debris is conveyed by floodwater, presumably due to large woody debris becoming lodged in the
clear opening of the bridge. For bridges of all heights, EA, 2013 considers that debris is likely to
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also wrap around the bridge piers. The guidance contained in EA, 2013 is therefore consistent
with modes of blockage which have been observed at both Rimmers Bridge and the Herb Feint
Bridge during recent flood events.
The impact an accumulation of debris on Rimmers Bridge and the Herb Feint Bridge on flood
behaviour was assess as part of the present study assuming the following three modes of
blockage:
Blockage Mode 1: Assumes a 1 m thick raft of debris lodges beneath the underside of
the bridge deck.
Blockage Mode 2: Assumes a 4 m wide raft of debris lodges on the upstream side of
each bridge pier over the full height of clear opening.
Blockage Mode 3: Combination of Modes 1 and 2.
The above blockage scenarios represent a reduction in waterway area which is less than that
which occurred during the October 2010 flood (at least in the case of the Adelong/Herb Feint
Bridges). The above blockage scenarios are considered to be more realistic of the conditions
which might arise at the new bridge in the future, given the reduced number and improved
alignment of the bridge piers, combined with the increased waterway area which has resulted
from the removal of the central island which formed part of the old Adelong Bridge.
Figure 6.12 (3 sheets) shows the impact a partial blockage of Rimmers Bridge would have on
100 year ARI flooding patterns. Blockage Model 1 (refer Sheet 1) has a greater impact on
flooding patterns when compared to Blockage Mode 2 (refer Sheet 2), with peak flood levels
increased by more than 500 mm immediately upstream of the br idge. All modes of blockage
result in an increase in the magnitude of flow which would surcharge the left bank of Adelong
Creek (in the order of 30 m3/s in the case of Blockage Mode 3 (refer Sheet 3)), with the result
that peak flood levels would be increased in several residential properties. Whilst floor level data
is not presently available, increased depths of above-floor inundation are likely to be experienced
in two residential properties that are located on the western side of Selwyn Street immediately
north of, and another on the western side of Cromwell Street closest to, the creek crossing.
Figure 6.13 (3 sheets) shows the impact a partial blockage of the Herb Feint Bridge would have
on 100 year ARI flooding patterns. Whilst the accumulation of debris around the bridge piers will
increase flood levels upstream of the creek crossing, it will not result in a major break out flow
along Tumut Street (refer Sheet 1). Similarly, the accumulation of debris beneath the underside
of the bridge deck will also result in only a minor surcharge of the left bank of Adelong Creek
(refer Sheet 2). It is only when both modes of blockage are experienced at the bridge that major
surcharge of Adelong Creek occurs, with a depth of flow of between 300-500 mm experienced
along Tumut Street (refer Sheet 3). The peak flow rate in Tumut Street under these conditions is
about 16 m3/s.
Based on the above findings, it is recommended that during the preparation of the FRMS,
consideration be given to incorporating an additional freeboard allowance in the Flood Planning
Levels (FPL’s) for Adelong to take account of the impacts a partial blockage of both Rimmers
Bridge and the Herb Feint Bridge has on flood behaviour. In regard to residential development,
this would relate to the setting of minimum floor level controls, whereas for commercial
development, it could relate to setting the elevation of a separate mezzanine area where goods
could be stored during a flood event, with finished floor levels set at street level for amenity
reasons.
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6.4 Climate Change Sensitivity Analysis
6.4.1. General
Scientific evidence shows that climate change will lead to sea level rise and potentially increase
flood producing rainfall intensities. The significance of these effects on flood behaviour w ill vary
depending on geographic location and local topographic conditions. Climate change impacts on
flood producing rainfall events show a trend for larger scale storms and resulting depths of rainfall
to increase. Future impacts on sea levels are likely to result in a continuation of the rise which
has been observed over the last 20 years.
OEH recommends that its guideline Practical Considerations of Climate Change, 2007 be used
as the basis for examining climate change induced increases in rainfall intensities in projects
undertaken under the State Floodplain Management Program and the FDM. The guideline
recommends that until more work is completed in relation to the climate change impacts on
rainfall intensities, sensitivity analyses should be undertaken based on increases in rainfall
intensities ranging between 10 and 30 per cent. On current projections the increase in rainfalls
within the service life of developments or flood management measures is likely to be around 10
per cent, with the higher value of 30 per cent representing an upper limit. Under present day
climatic conditions, increasing the 100 year ARI design rainfall intensities by 10 per cent would
produce a 200 year ARI flood; and increasing those rainfalls by 30 per cent would produce a 500
year ARI event.
The impacts of climate change and associated effects on the viability of floodplain risk
management options and development decisions may be significant and will need to be taken into
account in the FRMS for the Adelong Creek catchment, using site specific data.
At the present flood study stage, the principal issue regarding climate change is the potential
increase in flood levels throughout study area. In addition it is necessary to assess whether the
patterns of flow will be altered by new floodways being developed for key design events, or
whether the provisional flood hazard will be increased.
In the FRMS it will be necessary to consider the impact of climate change on flood damages to
existing development. Consideration will also be given both to setting floor levels for future
development and in the formulation of works and measures aimed at mitigating adverse effects
expected within the service life of development.
Mitigating measures which could be considered in the FRMS include the implementation of
structural works such as levees and channel improvements, improved flood warning and
emergency management procedures and education of the population as to the nature of the flood
risk.
6.4.2. Sensitivity to Increased Rainfall Intensities
As mentioned, the investigations undertaken at the flood study stage are mainly seen as
sensitivity studies pending more detailed consideration in the FRMS. For the purposes of the
investigation, the design rainfalls for 200 and 500 year ARI events were adopted as being
analogous to flooding which could be expected should present day 100 year ARI rainfall
intensities increase by 10 and 30 per cent, respectively.
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Figure 6.14 shows the afflux resulting from a 10 per cent increase in 100 year ARI rainfall
intensities. The increase in peak flood levels along the main arm of Adelong Creek would vary
between 100 to 500 mm with lesser values in the tributaries. An increase in rainfall intensity of
10 per cent does not result in a major break out of floodwater from Adelong Creek upstream of
the Herb Feint Bridge.
Figure 6.15 shows the afflux for a 30 per cent increase in 100 year ARI rainfall intensities. The
increase in peak flood levels along the main arm of Adelong Creek would be greater than 500 mm
with lesser values up to 100 mm in the tributaries. An increase in rainfall intensity of 30 per cent
results in a major breakout of flow occurring upstream of the Herb Feint Bridge, with depths of
flow in Tumut Street shown to reach between up to about 800 mm west of Neill Street. The peak
flow rate in Tumut Street under these conditions is about 21 m3/s.
Whilst the afore mentioned increases peak flood levels do not translate into a large increase in
the extent of inundation in Adelong for a 100 year ARI event (refer Figure 6.16), flooding
behaviour along Tumut Street would alter significantly, from low hazard overland type flow to
more hazardous main stream type flooding, similar to that experienced during the October 2010
flood (refer Plates 12 to 14 in Annexure D).
Consideration of these possible changes in flood behaviour will need to be given during the
preparation of the future FRMS.
6.5 Selection of Interim Flood Planning Levels
After consideration of the TUFLOW results and the findings of sensitivit y studies outlined in
Section 6.3, a freeboard allowance of 500 mm was adopted for determination of IFPL’s.
IFPL contours developed on that basis and the associated IFPA for main stream flooding along
Adelong Creek and several of its minor tributaries are shown on Figure 6.17.
Whilst the future FRMS for Adelong will determine a final set of Flood Planning Levels and a
Flood Planning Area for the study area, consideration will need be given in the interim to the
change in the extent of the floodway that occurs as a result of a partial blockage of the bridge
structures or a slightly higher flow in Adelong Creek. Figure 6.18 (2 Sheets) shows the changes
that would occur to the extent of the floodway under partial blockage conditions and as a result of
a higher flow in the creek. Notable increases in the extent of the floodway occur on the left
overbank of Adelong Creek downstream of Rimmers Bridge and adjacent to the new Herb Feint
Bridge along Tumut Street. It is advisable that Council limit future development in the areas
identified as floodway on Figure 6.18 until such time as the FRMS is completed, given the
potentially hazardous nature of the flow and the major impact that the blocking of these flow
paths (either partial or total) would have on flooding behaviour.
6.6 Flood Related Issues Associated with Future Growth Areas
The findings of the present investigation show that the future growth areas in Adelong are subject
to both main stream flooding and local overland flow. Consideration will need to be given to the
nature of flooding affecting the two future growth areas during both the rezoning and subdivision
process in order to properly manage the flood risk.
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In regards the Adelong South-West Future Growth Area, the main source of flooding is due to
surcharge of Black Creek. Rezoning of the land will need to incorporate an appropriately sized
corridor along the line of the creek which encompasses as a minimum the extent of the low and
high hazard floodway areas. Provision for overland flow will need to be incorporated in the
subdivision layout, with designated flow paths provided along the three principal drainage lines
which lie to the south of Adelong Cemetery Road. The hydrologic standard of both Adelong
Cemetery Road and Todds Road will need to be improved to service future development, as the
former is inundated by storms with ARI’s less than 5 years (refer Figure 6.3) and the latter by a
storm with an ARI of 10 years (refer Figure 6.4).
In regards the Adelong South-East Future Growth Area, the main source of flooding is a result of
runoff which originates from the hills which lie to its south. Provision will therefore need to be
incorporated in the future subdivision layout for the conveyance of this flow through the future
growth area to Adelong Creek. Whilst the minor/major approach to controlling this flow could be
adopted in most areas, an easement for drainage should be created over the watercourse which
runs through the eastern portion of the future growth area. A series of catch drains, possibly in
combination with earth bunding will also need to be incorporated at the foot of the hills to divert
uncontrolled surface runoff away from residential development and toward the designated
drainage lines. As identified in SP, 2013, the existing culverts under both Rimmers Lane and
Wondalga Road will need to be upgraded to service future development in the area. Access to
the area via Selwyn Road and Cromwell Street will be cut temporarily during major flood events,
as occurred in the October 2010 flood (refer Figure 4.4). Options for improving the hydrologic
standard of these roads should be investigated as part of the future FRMS.
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7 REFERENCES
Austroads, 1994. “Waterway Design. A Guide to the Hydraulic Design of Bridges , Culverts and
Floodways”.
BOM (Bureau of Meteorology), 2003. “The Estimation of Probable Maximum Precipitation in
Australia: Generalised Short-Duration Method”.
Bewsher (Bewsher Consulting), 2011. “Flood Intelligence Collection and Review for Towns and
Villages in the Murray and Murrumbidgee Regions Following the October 2010 Flood”
CSIRO, 2007. “Climate Change in the Hawkesbury Nepean Catchment”.
DECC (Department of Environment and Climate Change), 2007. “Practical Consideration of
Climate Change”. Floodplain Risk Management Guideline.
EA (Engineers Australia), 2013. “Australian Rainfall & Runoff - Revision Projects – Project 11 –
Blockage of Hydraulic Structures”. Stage 2 Report P11/S2/021 dated February 2013.
IEAust (The Institution of Engineers Australia), 1998. “Australian Rainfall and Runoff – A Guide
to Flood Estimation”, Volumes 1 and 2.
NSWG (New South Wales Government), 2005. “Floodplain Development Manual – The
Management of Flood Liable Land”.
O’Loughlin, 1993. “The ILSAX Program for Urban Stormwater Drainage Design and Analysis
(User’s manual for Microcomputer Version 2.13)”, Civil Engineering Monograph 93/1, University
of Technology, Sydney (5th
printing, 1st version 1986).
SP (Salvestro Planning), 2013. “Tumut Shire Council Growth Strategy Planning Report –
Adelong Investigation Areas”
Walsh et al (Walsh, M.A, Pilgrim, D.H, Cordery, I) (1991). “Initial Losses for Design Flood
Estimation in New South Wales” Intn’l Hydrology & Water Resources Symposium, Perth.
WRC (Water Resources Commission), 1986. “Adelong Flood Study Report”
Yeo (Dr Stephen Yeo), 2013. “Flood Intelligence Collection and Review for 24 Towns and
Villages in the Murray and Murrumbidgee Regions Following the March 2012 Flood”
Adelong Flood Study
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8 FLOOD-RELATED TERMINOLOGY
Note: For an expanded list of flood-related terminology, refer to glossary contained within the
Floodplain Development Manual, NSW Government, 2005).
TERM DEFINITION
Afflux Increase in water level resulting from a change in conditions. The
change may relate to the watercourse, floodplain, flow rate, tailwater
level etc.
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 events
occurring in any one year (see average recurrence interval).
Australian Height Datum (AHD) A common national surface level datum approximately corresponding
to mean sea level.
Average Recurrence Interval
(ARI)
The average period in years between the occurrence of a flood of a
particular magnitude or greater. In a long period of say 1,000 years, a
flood equivalent to or greater than a 100 year ARI event would occur
10 times. The 100 year ARI flood has a 1% chance (i.e. a one-in-100
chance) of occurrence in any one year (see annual exceedance
probability).
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.
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 (e.g. metres per second [m/s]).
Flood fringe area The remaining area of flood prone land after floodway and flood
storage areas have been defined.
Flood Planning Area (FPA) The area of land inundated at the Flood Planning Level.
Flood Planning Level (FPL) A combination of flood level and freeboard selected for planning
purposes, as determined in floodplain risk management studies and
incorporated in floodplain risk management plans.
Flood prone land Land susceptible to flooding by the Probable Maximum Flood. Note
that the flood prone land is synonymous with flood liable land.
Flood storage area 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.
Floodplain Area of land which is subject to inundation by floods up to and
including the probable maximum flood event (i.e. flood prone land).
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TERM DEFINITION
Floodplain Risk Management
Plan
A management plan developed in accordance with the principles and
guidelines in the Floodplain Development Manual, 2005. Usually
includes both written and diagrammatic information describing how
particular areas of flood prone land are to be used and managed to
achieve defined objectives.
Floodway area 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 flow, or a significant
increase in flood levels.
Freeboard A factor of safety typically used in relation to the setting of floor levels,
levee crest levels, etc. It is usually expressed as the difference in
height between the adopted Flood Planning Level and the peak height
of the flood used to determine the flood planning level. Freeboard
provides a factor of safety to compensate for uncertainties in the
estimation of flood levels across the floodplain, such and wave action,
localised hydraulic behaviour and impacts that are specific event
related, such as levee and embankment settlement, and other effects
such as “greenhouse” and climate change. Freeboard is included in
the flood planning level.
High hazard Where land in the event of a 100 year ARI flood is subject to a
combination of flood water velocities and depths greater than the
following combinations: 2 metres per second with shallow depth of
flood water depths greater than 0.8 metres in depth with low velocity.
Damage to structures is possible and wading would be unsafe for able
bodied adults.
Low hazard Where land may be affected by floodway or flood storage subject to a
combination of floodwater velocities less than 2 metres per second
with shallow depth or flood water depths less than 0.8 metres with low
velocity. Nuisance damage to structures is possible and able bodied
adults would have little difficulty wading.
Mainstream flooding Inundation of normally dry land occurring when water overflows the
natural or artificial banks of a stream, river, estuary, lake or dam.
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.
Overland flooding Inundation by local runoff rather than overbank discharge from a
stream, river, estuary, lake or dam.
Peak discharge The maximum discharge occurring during a flood event.
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TERM DEFINITION
Peak flood level The maximum water level occurring during a flood event.
Probable Maximum Flood (PMF) The largest flood that could conceivably occur at a particular location,
usually estimated from probable maximum precipitation 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
(i.e. the floodplain). The extent, nature and potential consequences of
flooding associated with events up to and including the PMF should be
addressed in a floodplain risk management study.
Probability A statistical measure of the expected chance of flooding (see annual
exceedance probability).
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 stream flow, also
known as rainfall excess.
Stage Equivalent to water level (both measured with reference to a specified
datum).
ANNEXURE A
COMMUNITY FLYER
ANNEXURE B
DETAILS OF AVAILABLE DATA
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B1. COLLECTION OF MISCELLANEOUS DATA
B1.1 Airborne Laser Scanning Survey and Aerial Photography
The Adelong LGA was flown by Land and Property Information in January 2012 for the purpose of
preparing a DTM for TSC based on Airborne Laser Scanning (ALS) survey. The LGA was flown
at an altitude of 1800 m to the International Committee on Surveying and Mapping (ICSM)
guidelines for digital elevation data with a 95% confidence interval on horizontal accuracy of
±800 mm and a vertical accuracy of ±300 mm.
B1.2 Stormwater Pit and Pipe Network
At the commencement of the study, TSC provided a copy of its then current stormwater pit and
pipe database in MAPINFO format. The database was generally limited to pipe/culvert
dimensions and pit type. No information on grate or pipe invert levels were contained in the
database. Figure 2.2 shows the extent of the stormwater pit and pipe network in Adelong.
A review of the database showed that there were gaps in the data provided by TSC. Missing pits
and pipes were located and measured in the field during the data collection phase of the study.
B1.3 Cross Sectional Survey
Casey Surveying and Design were engaged to undertake survey at regular intervals along
Adelong Creek, as well as at Rimmers Bridge, Herb Feint Bridge and the pedestrian foot bridge.
Cross section data was provided as tabulations of offset versus elevation in an Excel
spreadsheet. An AutoCAD file was also provided in the MGA co-ordinate system showing the
extent of each cross section. A photographic record of each cross section was also compiled by
the surveyor.
Upon review of initial hydraulic model results, additional cross-sections were sampled from the
ALS to add further definition to the model.
The channel reaches comprising the study area have been colour coded on Figure 4.1 to
differentiate between those reaches of channel that were surveyed by Casey Surveying and
Design (refer channel reaches coloured cyan) and those where cross section data were sampled
from the ALS survey (refer channel reaches coloured orange).
B1.4 Stormwater Drainage Works
TSC provided Work as Executed plans of recently constructed Herb Feint Bridge that included
details of the original Adelong Bridge. These plans provided information on the change that
occurred in the cross sectional area of Adelong Creek as a result of the October 2010 flood.
B1.5 Historic Stream Data
NOW’s Batlow Road stream gauge (GS 410061) is located on Adelong Creek, approximately
3 km (by river) upstream of Rimmers Bridge. Historic flows in Adelong Creek have been
recorded at the gauge site since it was first installed in September 1947, although the gauge was
shifted about 200 m upstream to its current location in 1980.
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B1.6 Historic Rainfall Data
Rainfall Data was available for one BOM operated rainfall pluviograph in the vicinity if the study
area. The gauge, Adelong (Etham Park) (GS 72159), is located approximately 11 km west of
Adelong in the Yaven Yaven Creek catchment. Rainfall data in 5 minute intervals were available
at Adelong (Etham Park) between January 1993 and present day.
Rainfall data were also available for three rain gauges operated by BOM as part of its flood
warning network. These gauges, the locations of which are shown on Figure 2.8, are Batlow
(GS 72004.1) (located 24 km south of Adelong), Belmore Bridge (GS 572010) (located 30 km
south west of Adelong) and Argalong (GS 72152.1.1) (located 34 km east of Adelong). Rainfall
data in 3 hourly intervals were recorded at each of these sites.
Daily rainfall totals were available for various gauge sites in the vicinity of Adelong which helped
provided helped define the special distribution of rainfal l for historic events.
B1.7 Historic Flood Marks
At the commencement of the study, TSC provided a copy of surveyed flood marks which were
surveyed immediately following the October 2010 flood in MAPINFO format. The database
included surveyed levels along Adelong Creek at various times during the flood.
Historic flood data for the January 1984, October 2010 and March 2012 floods were also taken
from the three reports listed below.
B1.8 Previous Reports
Historic flood data, including several photos were extracted from the following reports:
Adelong Flood Study Report (WRC, 1986).
Flood Intelligence Collection and Review for Towns and Villages in the Murray and
Murrumbidgee Regions Following the October 2010 Flood (Bewsher, 2011) .
Flood Intelligence Collection and Review for 24 Towns and Villages in the Murray and
Murrumbidgee Regions Following the March 2012 Flood (Yeo, 2013) .
ANNEXURE C
PHOTOGRAPHS SHOWING FLOODING BEHAVIOUR IN ADELONG –
JANUARY 1984 FLOOD
(Source: Bewsher, 2011)
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Plate 1 - Adelong Pool kiosk. Plate 2 - View across Snowy Mountains Hwy
towards No. 46 Tumut St.
Plate 3 - View across Snowy Mountains Hwy towards
No. 46 Tumut St.
Plate 4 - View from Adelong Bridge towards Royal Hotel,
45 Tumut St.
Plate 5 - Old Pharmacy, 88 Tumut St. Plate 6 - Inundation at Broadhurst Motors, 96 Tumut St.
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Plate 7 - Looking west along Tumut St towards Broadhurst
Motors.
Plate 8 - Adelong Falls.
ANNEXURE D
PHOTOGRAPHS SHOWING FLOODING BEHAVIOUR IN ADELONG –
OCTOBER 2010 FLOOD
(Source: Bewsher, 2011)
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Plate 9 - Flooded garages at rear of Nos. 38 and 40
Tumut Street, with damaged fences.
Plate 10 - View of 46 Tumut Street from Tumut
Street.
Plate 11 - View of 46 Tumut Street from Adelong
Bridge.
Plate 12 - View northwest from corner of Tumut and
Campbell Streets towards Bendigo Bank.
Plate 13 - View southeast up Tumut Street. Plate 14 - View northwest down Tumut Street
towards Adelonia Theatre.
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Plate 15 - View south across new half of Adelong
Bridge, showing creek capacity exceeded 1pm Fri
15th Oct.
Plate 16 - View south across new half of Adelong
Bridge, showing inundation of the bridge deck and
debris trapped on upstream side.
Plate 17 - View from 46 Tumut St north towards
new Adelong Bridge as flood recedes, shows
accumulation of debris and influence in elevating
water levels.
Plate 18 - Middle section of the remaining downstream
half-width of the old Adelong Bridge collapsed.
Plate 19 - Evidence of debris blockage at Adelong
Bridge, looking downstream.
Plate 20 - Evidence of debris blockage at Adelong
Bridge, looking downstream.
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Plate 21 - Evidence of debris blockage at Adelong
Bridge, looking downstream.
Plate 22 - Evidence of debris blockage at Adelong
Bridge, looking downstream.
Plate 23 - Evidence of debris blockage at Adelong
Bridge, looking downstream.
Plate 24 - Evidence of debris blockage at Adelong
Flood Channel Adelong Bridge, looking upstream.
Plate 25 - Evidence of debris blockage at Adelong
Bridge, looking upstream.
Plate 26 - Evidence of debris build up pedestrian
Bridge, looking upstream.
ANNEXURE E
PHOTOGRAPHS SHOWING FLOODING BEHAVIOUR IN ADELONG –
MARCH 2012 FLOOD
(Source: Yeo, 2013)
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Plate 27 - View south across Adelong Creek with
Herb Feint Bridge on left, Thursday 1 March 2012.
Plate 28 - View downstream of Herb Feint Bridge
with floodwater on the cusp of surcharging the left
(southern) bank of Adelong Creek, Thursday
1 March 2012.
ANNEXURE F
DETAILS OF HERB FEINT BRIDGE DEPTH GAUGE (Source: Survey undertaken by TSC in September 2013)
ANNEXURE G
BATLOW ROAD STREAM GAUGE DATA
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TABLE G1
RECORDED PEAK HEIGHT AND DISCHARGE DATA IN DATE ORDER
BATLOW ROAD STREAM GAUGE(1, 2, 3)
Year Peak Height
(m)
Peak Discharge
(m3/s) Year
Peak Height
(m)
Peak Discharge
(m3/s)
1874 Unknown Unknown 1980 1.33 21.0
1948 0.90 5.1 1981 2.11 68.6
1949 2.44 90.9 1982 0.68 2.4
1950 2.13 70.0 1983 2.63 112.9
1951 1.83 48.4 1984 3.52 212.4
1952 2.42 89.4 1985 1.55 32.8
1953 1.68 39.1 1986 2.12 69.2
1954 1.45 24.9 1987 1.74 43.8
1955 2.90 122.4 1988 2.47 98.3
1956 1.83 48.4 1989 1.87 52.1
1957 0.63 3.4 1990 1.98 59.4
1958 1.91 53.8 1991 1.20 15.3
1959 1.22 14.9 1992 2.07 65.9
1960 2.57 100.2 1993 2.59 108.6
1961 1.54 30.2 1994 1.24 15.4
1962 1.00 9.2 1995 2.22 76.7
1963 1.52 29.5 1996 1.65 37.8
1964 1.83 48.4 1997 1.20 14.4
1965 0.79 5.3 1998 1.73 42.8
1966 2.01 61.0 1999 1.50 28.9
1967 0.38 1.1 2000 2.37 88.8
1968 1.73 42.4 2001 1.32 20.3
1969 2.27 79.3 2002 1.05 10.9
1970 2.51 95.8 2003 1.65 38.2
1971 1.51 28.6 2004 1.18 15.0
1972 1.77 44.6 2005 2.02 62.1
1973 1.70 40.6 2006 0.54 1.1
1974 3.24 147.2 2007 0.87 5.6
1975 3.00 129.8 2008 0.93 7.1
1976 1.75 43.1 2009 0.79 4.1
1977 1.69 40.1 2010 4.61 382.8
1978 1.62 35.1 2011 1.82 48.7
1979 1.71 41.0 2012 3.11 162.6
1. Stream gauge shifted approximately 200 m upstream in 1980.
2. Gauge zero prior to relocation of stream gauge (i.e. pre-1980) = Assumed Datum
3. Gauge zero following relocation of stream gauge (i.e. post-1980) = RL 351.52 m AHD
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TABLE G2
RECORDED PEAK HEIGHT AND DISCHARGE DATA IN ORDER OF MAGNITUDE
BATLOW ROAD STREAM GAUGE(1, 2, 3)
Year Peak Height
(m)
Peak Discharge
(m3/s) Year
Peak Height
(m)
Peak Discharge
(m3/s)
1874 Unknown Unknown 1998 1.73 42.8
2010 4.61 382.8 1968 1.73 42.4
1984 3.52 212.4 1979 1.71 41.0
2012 3.11 162.6 1973 1.70 40.6
1974 3.24 147.2 1977 1.69 40.1
1975 3.00 129.8 1953 1.68 39.1
1955 2.90 122.4 2003 1.65 38.2
1983 2.63 112.9 1996 1.65 37.8
1993 2.59 108.6 1978 1.62 35.1
1960 2.57 100.2 1985 1.55 32.8
1988 2.47 98.3 1961 1.54 30.2
1970 2.51 95.8 1963 1.52 29.5
1949 2.44 90.9 1999 1.50 28.9
1952 2.42 89.4 1971 1.51 28.6
2000 2.37 88.8 1954 1.45 24.9
1969 2.27 79.3 1980 1.33 21.0
1995 2.22 76.7 2001 1.32 20.3
1950 2.13 70.0 1994 1.24 15.4
1986 2.12 69.2 1991 1.20 15.3
1981 2.11 68.6 2004 1.18 15.0
1992 2.07 65.9 1959 1.22 14.9
2005 2.02 62.1 1997 1.20 14.4
1966 2.01 61.0 2002 1.05 10.9
1990 1.98 59.4 1962 1.00 9.2
1958 1.91 53.8 2008 0.93 7.1
1989 1.87 52.1 2007 0.87 5.6
2011 1.82 48.7 1965 0.79 5.3
1951 1.83 48.4 1948 0.90 5.1
1956 1.83 48.4 2009 0.79 4.1
1964 1.83 48.4 1957 0.63 3.4
1972 1.77 44.6 1982 0.68 2.4
1987 1.74 43.8 1967 0.38 1.1
1976 1.75 43.1 2006 0.54 1.1
1. Stream gauge shifted approximately 200 m upstream in 1980.
2. Gauge zero prior to relocation of stream gauge (i.e. pre-1980) = Assumed Datum
3. Gauge zero following relocation of stream gauge (i.e. post-1980) = RL 351.52 m AHD
ANNEXURE H
COMPARISON OF MODELLED VERSUS RECORDED PEAK FLOOD HEIGHTS
OCTOBER 2010 FLOOD
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TABLE H1
COMPARISON OF MODELLED VERSUS RECORDED PEAK FLOOD HEIGHTS
OCTOBER 2010 FLOOD
Flood Mark Identifier
Recorded Peak
Height (m AHD)
Modelled
Peak
Height
(m AHD)
Difference
(m) Flood Mark Identifier
Recorded Peak
Height (m AHD)
Modelled
Peak
Height
(m AHD)
Difference
(m)
527 347.742 347.684 -0.058 626 336.046 336.119 0.073
526 347.572 347.396 -0.176 584 332.123 332.094 -0.029
530 347.049 347.054 0.005 586 332.262 332.111 -0.151
533 346.75 346.686 -0.064 587 332.132 332.134 0.002
534 346.659 346.418 -0.241 588 332.72 332.144 -0.576
543 344.762 344.718 -0.044 590 332.673 332.281 -0.392
547 344.785 344.714 -0.071 593 333.601 333.519 -0.082
548 344.665 344.692 0.027 594 333.15 332.838 -0.312
549 344.572 344.551 -0.021 595 333.049 332.778 -0.271
550 344.275 344.403 0.128 597 333.708 333.518 -0.19
562 344.148 343.534 -0.614 599 333.178 333.013 -0.165
563 343.038 343.279 0.241 600 333.272 333.124 -0.148
567 343.968 344.097 0.129 601 333.293 333.108 -0.185
569 343.086 342.913 -0.173 602 333.702 333.537 -0.165
571 342.236 341.98 -0.256 603 333.816 333.583 -0.233
574 342.703 342.574 -0.129 611 333.752 333.435 -0.317
575 342.062 342.191 0.129 622 333.596 333.534 -0.062
579 338.145 338.157 0.012 627 335.686 335.96 0.274
580 337.439 337.377 -0.062 633 335.648 335.574 -0.074
581 337.458 337.371 -0.087 651 333.436 333.059 -0.377
623 336.367 336.343 -0.024 652 332.738 332.752 0.014
624 336.317 336.238 -0.079 654 332.433 332.484 0.051
625 336.216 336.144 -0.072 659 335.929 335.967 0.038
634 338.727 338.861 0.134
Refer Figure 4.4 (2 sheets) for location of recorded peak flood heights.
ANNEXURE I
PEAK FLOWS DERIVED BY TUFLOW MODEL
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TABLE I1
PEAK HISTORIC AND DESIGN FLOOD FLOWS(1)
Peak Flow
Location Identifier
(2)
Tributary Location
Historic Flood Events
Design Flood Events
October 2010
March 2012
5 year ARI 10 year ARI 50 year ARI 100 year ARI 200 year ARI 500 year ARI PMF
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
D
ura
tio
n
(min
ute
s)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
Du
rati
on
(min
ute
s)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
Du
rati
on
(m
inu
tes
)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
D
ura
tio
n
(min
ute
s)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
Du
rati
on
(min
ute
s)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
Du
rati
on
(m
inu
tes
)
Pe
ak
Flo
w
(m3/s
)
Cri
tica
l S
torm
D
ura
tio
n
(min
ute
s)
[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S]
Q01 Adelong Creek Upstream Extent of Model 383 163 80 360 118 360 270 360 366 360 432 360 527 270 3304 180
Q02 Adelong Creek Upstream of Adelong
Showground 394 165 83 360 119 360 273 360 374 360 437 360 527 270 3408 180
Q03 Adelong Creek Downstream of Rimmers
Bridge 367 158 90 360 128 360 289 360 390 360 460 360 552 270 3418 180
Q04 Adelong Creek Downstream of Gundagai
Street 391 167 94 360 133 360 293 360 394 360 462 360 557 270 3470 180
Q05 Adelong Creek Upstream of Herb Feint Bridge 399 166 93 360 131 360 289 360 388 360 451 360 527 270 3517 180
Q06 Adelong Creek Quartz Street 362 158 96 360 133 360 283 360 380 360 447 360 531 270 3345 180
Q07 Adelong Creek Downstream Extent of Model 379 158 94 360 133 360 305 360 410 360 475 360 565 270 3450 180
Q08 Black Creek Upstream Extent of Model - - 4.1 180 5.4 180 9.9 120 12.1 60 15.1 60 19.6 60 119 180
Q09 Black Creek Inglis Street - - 5.9 180 8.0 180 14.9 60 18.8 60 23.5 60 31.1 60 177 180
Q10 Black Creek Neill Street - - 7.0 180 9.3 180 17.0 60 21.2 60 27.1 60 35.6 60 199 180
Q11 Minor Tributary Arm of
Adelong Creek Upstream Extent of Model - - 4.9 180 6.6 180 12.4 60 15.4 60 19.1 60 24.4 60 104 180
Q12 Minor Tributary Arm of
Adelong Creek 350 m Upstream of Adelong
Creek Confluence - - 5.4 180 7.4 180 14.5 60 18.1 60 22.4 60 28.6 60 120 180
Q13 Minor Tributary Arm of
Adelong Creek Flow into Adelong Creek - - 6.0 180 8.1 180 15.6 60 19.4 60 23.9 60 30.5 60 117 180
Q14 Minor Tributary Arm of
Adelong Creek Bleak Street - - 2.4 180 3.3 180 6.9 60 7.8 60 13.3 60 16.9 60 76 180
Q15 Minor Tributary Arm of
Adelong Creek Camp Street - - 4.1 180 5.6 180 9.9 180 12.8 120 15.9 120 19.4 120 141 180
Q16 Overland Flow Wyndham Street - - 0.3 180 0.4 60 1.4 60 1.9 25 2.5 25 3.4 25 4.1 180
Q17 Overland Flow Campbell Street - - 0.2 25 0.3 25 1.1 25 1.7 25 2.3 25 3.2 25 2.3 180
Q18 Overland Flow Neill Street between Lockhart
Street and Lynch Street - - 0.4 25 0.6 25 1.6 25 2.0 25 2.4 25 3.0 25 1.4 180
Q19 Overland Flow Havelock Street - - 0.4 25 0.7 25 1.9 25 2.4 25 2.9 25 3.7 25 1.5 180
1. Peak flows less than 100 m3/s have been quoted to the first decimal place in order to show minor differences.
2. Refer relevant figures in Volume 2 for location of Flow Location Identifiers.
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