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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: [email protected]

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

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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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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






6.9 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



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.

Adelong Flood Study



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.

Adelong Flood Study



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).

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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”).

Adelong Flood Study



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.

Adelong Flood Study



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

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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).

Adelong Flood Study


April 2014 Rev. 2.1 Consulting Water Engineers

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).

Adelong Flood Study



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.

Adelong Flood Study



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

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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).

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study



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



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).

Adelong Flood Study



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.

Adelong Flood Study



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.

Adelong Flood Study

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Plate 7 - Looking west along Tumut St towards Broadhurst

Motors.

Plate 8 - Adelong Falls.

ANNEXURE D


OCTOBER 2010 FLOOD

(Source: Bewsher, 2011)

Adelong Flood Study

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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.

Adelong Flood Study



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.



Adelong Flood Study










Flood Channel Adelong Bridge, looking upstream.


Bridge, looking upstream.

Plate 26 - Evidence of debris build up pedestrian

Bridge, looking upstream.

ANNEXURE E


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

Adelong Flood Study

Adelong_Vol_1_Report [Rev 2.1].doc Page H1 Lyall & Associates


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

Adelong Flood Study

Adelong_Vol_1_Report [Rev 2.1].doc Page I1 Lyall & Associates


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


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


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


Adelong Creek Bleak Street - - 2.4 180 3.3 180 6.9 60 7.8 60 13.3 60 16.9 60 76 180


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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