heddon greta minor flood study and concept design – part 1

Heddon Greta Minor Flood Study and Concept Design – Part 1 Reference: R.N21198.001.03.docx Date: May 2020 Draft Report

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Document: R.N21198.001.03.docx

Title: Heddon Greta Minor Flood Study and Concept Design – Part 1

Project Manager: Stephanie Lyons

Author: Stephanie Lyons, Madelaine Broadfoot

Client: Cessnock City Council

Client Contact: Martin Conner

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Synopsis: Heddon Greta Minor Flood Study and Concept Design Part 1 Draft Report. REVISION/CHECKING HISTORY

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Heddon Greta Minor Flood Study and Concept Design – Part 1 iii Contents


Contents 1 Introduction 1

1.1 Study Area 1 1.2 Background 1 1.3 Study Objectives 1 1.4 About this Report 1

2 Model Development 4

2.1 Available Data 4 2.2 Hydrologic Model 4

2.2.1 Sub-catchment Delineation 4 2.2.2 Land Use Parameters 4 2.2.3 Flow Routing 6

2.3 Hydraulic Model 6 2.3.1 Topography and Extents 6 2.3.2 Stormwater Pipe Network 6 2.3.3 Hydraulic Roughness 8 2.3.4 Boundary Conditions 9

2.4 Model Validation 9 2.4.1 April 2015 Event 9 2.4.2 September 2019 Event 14

2.5 Historic Catchment Conditions 14

3 Design Flood Conditions 16

3.1 Rainfall-Runoff Analysis 16 3.1.1 Rainfall Depths 16 3.1.2 Areal Reduction Factors 17 3.1.3 Rainfall Losses 17 3.1.4 Temporal Patterns 18 3.1.5 Critical Duration 18

3.2 Sensitivity Assessment 21 3.2.1 Rainfall Losses 21 3.2.2 Manning’s Roughness 21 3.2.3 Structure Blockage 21 3.2.4 Climate Change 22 3.2.5 Downstream Boundary Condition 23

Heddon Greta Minor Flood Study and Concept Design – Part 1 iv Contents


3.2.6 Property Inundation 23

4 Design Flood Results 24

4.1 Existing Flood Conditions 24 4.2 Existing Drainage Capacity 24 4.3 Cross-Catchment Flow 24 4.4 Flood Hazard 27 4.5 Flood Function 28 4.6 Preliminary Flood Planning Area 29 4.7 Flood Damages Assessment 30 4.8 Property Inundation 32

5 Conclusions 34

6 References 35

Appendix A – Location of Utilities and Services 1

Appendix B – Existing Design Flood Depth Mapping 1

Appendix C – Existing Design Flood Hazard Mapping 1

Appendix D – Existing Design Flood Function Mapping 1

Appendix E – Preliminary Flood Planning Area 1

List of Figures Figure 1-1 Study Locality 2 Figure 1-2 Catchment Boundary and Topography 3 Figure 2-1 XP-RAFTS Hydrologic Model Sub Catchment Layout 5 Figure 2-2 Stormwater Drainage Network 7 Figure 2-3 Comparison of Recorded April 2015 Rainfall with IFD Relationships 11 Figure 2-4 Rainfall Gauge Localities and April 2015 48 hour Rainfall Totals 12 Figure 2-5 Simulated Flood Depth and Inundation Photos of the April 2015 Event 13 Figure 2-6 Simulated Flood Depth and Inundation Extents for the September 2019 Event 15 Figure 3-1 ARR 2019 XP-RAFTS 1% Critical Duration Assessment – Sub-catchment 6 19 Figure 3-2 ARR 2019 XP-RAFTS 1% Critical Duration Assessment – Sub-catchment 14 19 Figure 3-3 Modelled 1% AEP Design Hydrographs 20 Figure 4-1 Comparison between Historic and Present 1% AEP Design Flood Inundation

Extent 26 Figure 4-2 Combined Flood Hazard Curves – Vulnerability Thresholds 28 Figure 4-3 Types of Flood Damage 31

Heddon Greta Minor Flood Study and Concept Design – Part 1 v Contents


List of Tables Table 2-1 Impervious Fractions for XP-RAFTS Modelling 4 Table 2-2 PERN Values for XP-RAFTS Modelling 6 Table 2-3 Stormwater Drainage - Compendium of Dimension and Inverts 8 Table 2-4 Adopted Hydraulic Roughness Coefficients Based on Land Use 9 Table 2-5 Recorded Rainfall Totals 48-hours to 9 am 22nd April 2015 (mm) 10 Table 2-6 Comparison between Observed and Modelled Depths 11 Table 3-1 Design Rainfall Depth (mm) 17 Table 3-2 Rainfall Probability Neutral Burst Initial Loss (mm) 17 Table 3-3 Adopted Temporal Pattern and Peak Flow Conditions 20 Table 3-4 Adjusted Blockage as a Percent (ARR 2019) 22 Table 3-5 1% AEP Modelled Peak Depth- Sensitivity Testing Results 22 Table 4-1 Design Peak Flow Rates at Major Pipe Outlets, Heddon Greta 25 Table 4-2 Historic Peak Flow Rates 25 Table 4-3 Present Peak Flow Rates 25 Table 4-4 Peak Flow Rates Across the Study Site 26 Table 4-5 Peak Flood Levels at Cooper Street – Historic and Present 27 Table 4-6 Combined Flood Hazard Curves – Vulnerability Thresholds 27 Table 4-7 Flood Function Categories 29 Table 4-8 Summary of Total Existing Residential Tangible Flood Damages 32 Table 4-9 Existing Inundated Properties 33

Heddon Greta Minor Flood Study and Concept Design – Part 1 1 Introduction


1 Introduction

1.1 Study Area Heddon Greta is located in the north eastern region of Cessnock City Council Local Government Area (LGA), around 1.5 km north-east of Kurri Kurri and 14 km east of the major urban centre of Cessnock. Heddon Greta has a population of 2,047 people based on 2016 Census results. The study area locality is shown in Figure 1-1.

The catchment of Heddon Greta occupies an area of 0.8 km2, as shown in Figure 1-2. Elevations typically range between 15 and 30 m AHD, with the upper catchment located toward the south west. Two unnamed tributaries drain run-off within Heddon Greta. One drains to the north west into Swamp Creek and the other drains in an easterly direction into Wallis Creek. Both Wallis and Swamp Creek are tributaries of the Hunter River. A crest separating the two distinct drainage lines within the sub-catchment exists around Heddon Street and Adams Street.

1.2 Background When Heddon Greta was established in the early 1900s, little consideration was provided for drainage. Houses and allotments were positioned over natural water course alignments and some roads were constructed with insufficient grades. This has resulted in a long history of nuisance flooding, most recently during the April 2015 storm event.

Presently, Council is working towards resolving two critical drainage issues in Heddon Greta, being flooding of properties in Cooper Street and Clift Street.

1.3 Study Objectives This document forms Stage 1 (flood study investigation) of the Heddon Greta Minor Flood Study and Concept Design.

The objectives of the Stage 1 investigation are to characterise existing and future flood behaviour in the catchment, to highlight issues with current catchment drainage.

Specifically, the Stage 1 study aims to:

• Determine the 50%, 20%, 5%, 2% and 1% AEP flood levels throughout the catchment for both current and historical scenarios.

1.4 About this Report This report documents the Study’s objectives, results and recommendations.

Section 1 introduces the study.

Section 2 outlines the general modelling approach, key assumptions and model validation to the April 2015 and September 2019 events.

Section 3 outlines the derivation of the design flood conditions.

Section 4 presents the design flood results.



Figure 1-1 Study Locality



Figure 1-2 Catchment Boundary and Topography

Heddon Greta Minor Flood Study and Concept Design – Part 1 4 Model Development


2 Model Development This section outlines the general modelling approach and provides details on the XP-RAFTS hydrologic model and TUFLOW hydraulic model developed for this current study.

2.1 Available Data The available data for this study includes:

• LiDAR survey collected by NSW LPI in 2012 – provides the most recent topographical representation of the catchment;

• Survey of the storm drainage network at Clift Street, Adams Street, Main Street, Heddon Street, and Averys Lane from Council (Red Wing Survey, 2019);

• April 2015 rainfall data – daily rainfall totals were recorded at the Kurri Kurri Golf Course; and

• April 2015- continuous rainfall data recorded at Williamtown Airport and Paterson (Tocal AWS), obtained from BoM; and

• April 2015 flood photographs provided by Council.

2.2 Hydrologic Model The XP-RAFTS hydrological model was developed for the study and was used to simulate the rate of runoff from rainfall and the attenuation of the flood wave as it travels down the catchment. This process is dependent on catchment area, slope and vegetation; variation in distribution, intensity and amount of rainfall; and antecedent conditions of the catchment.

2.2.1 Sub-catchment Delineation The catchment has been delineated into 12 sub-catchments. The sub-catchment layout is shown in Figure 2-1. In defining sub-catchment outlets, consideration has been given to the underlying pipe drainage network. Sub-catchment boundaries coincide with the location of the drainage system infrastructure and road crests, where appropriate.

2.2.2 Land Use Parameters The XP-RAFTS modelling utilises the two sub-catchment approach to individually reflect pervious and impervious contributions. The impervious percentages for each sub-catchment under existing conditions have been determined according to the proportional land use identified from aerial photography (Table 2-1).

Table 2-1 Impervious Fractions for XP-RAFTS Modelling

Land Use Typical Impervious %

Residential 40%



Figure 2-1 XP-RAFTS Hydrologic Model Sub Catchment Layout



The XP-RAFTS parameter PERN is input as a Manning’s 'n' representation of the average sub-catchment roughness. Table 2-2 summarises the adopted PERN value for various surface types within the catchment.

Table 2-2 PERN Values for XP-RAFTS Modelling

Surface Type PERN Value

Impervious 0.015

Pervious 0.060

The catchment storage modification coefficient Bx is taken as 1.0 for modelling. In the absence of flow data necessary to calibrate catchment response in terms of flow magnitude and timing, the default value has been adopted.

2.2.3 Flow Routing Due to the hydraulic influence of local topography controls during flood events, and the potential for cross-catchment overland flow to occur, all flow routing is to be undertaken within the TUFLOW hydrodynamic model. The derived local sub-catchment hydrographs from the XP-RAFTS model nodes are applied as direct sources to the TUFLOW model domain. As such, no direct channel routing is required for the XP-RAFTS model.

2.3 Hydraulic Model The overland flow regime in urban environments is characterised by large and shallow inundation of urban development with interconnecting and varying flow paths. Road networks often convey a considerable proportion of floodwaters due to the hydraulic efficiency of the road surface compared to developed areas (e.g. blocked by buildings), in addition to the underground pipe network draining to open channels. Given this complex flooding environment, an integrated 1D/2D modelling approach is warranted for the overland flooding areas. The TUFLOW software has been applied in this study which has the capability to simulate the dynamic interaction of in bank flows in open channels, major underground drainage systems, and overland flows through complex overland flow paths using a linked 1D/2D flood modelling approach.

2.3.1 Topography and Extents The 2012 LPI LiDAR was used to compute the Digital Elevation Model (DEM), which is read directly into TUFLOW to calculate the ground surface elevation of model grid points. high resolution DEM of 1m was created and a TUFLOW 2D model resolution of 2 m was adopted. The DEM of the study area generated from the 2012 LPI LiDAR is shown in Figure 1-2.

2.3.2 Stormwater Pipe Network The study requires the modelling of the trunk drainage system in the study catchment. The drainage network was developed from survey information provided by Council (Red Wing Survey, 2019) and the previous study on drainage works at Cooper Street, Heddon Greta, (BMT WBM, 2016).

The location of the existing drainage system, comprising the pit and pipe network, is shown in Figure 2-2. Dimensions and inverts are summarised in Table 2-3.



Figure 2-2 Stormwater Drainage Network



Table 2-3 Stormwater Drainage - Compendium of Dimension and Inverts

Pipe ID Type Upstream Invert

Downstream Invert

Width/ Diameter (m)

Height (m) No. of Pipes

A1-2 C 20.78 20.625 0.45 NA 2

B1-2 R 20.56 20.34 0.45 0.45 1

C1-2 C 19.65 19.56 0.6 NA 1

D1-2 C 17.73 16.89 0.75 NA 1

E1-2 C 23.25 19.8 0.375 NA 1

E2-3 C 19.8 19.56 0.45 NA 1

E3-4 C 19.56 19.45 0.825 NA 1

E4-5 C 19.45 18.64 0.825 NA 1

E5-6 C 18.61 17.96 0.825 NA 1

E6-7 C 17.97 16.92 0.825 NA 1

E7-8 C 16.93 16.62 0.75 NA 1

F1-E5 C 19.01 18.94 0.45 NA 1

G1-2 C 18.7 18.66 0.375 NA 1

G2-3 C 18.66 18.44 0.525 NA 1

G3-E6 C 18.44 17.97 0.525 NA 1

H1-2 C 17.55 17.5 0.375 NA 1

H2-3 C 17.5 17.4 0.375 NA 1

H3-4 C 17.4 17.2 0.825 NA 1

H4-5 C 17.1 16.8 0.9 NA 1

I1-H3 C 17.55 17.5 0.375 NA 1

J1-H4 C 17 17.1 0.375 NA 1

K1-2 C 17.89 17.17 0.45 NA 1

K2-H4 C 17 17.17 0.525 NA 1

L1-K2 C 17.22 17.17 0.45 NA 1

M1-2 C 20.37 17.84 0.45 NA 1

M2-H4 C 17.84 17 0.525 NA 1

The pipe network, represented as a 1D layer in the model, is dynamically linked to the 2D domains at specified pit locations for inflow and surcharging.

2.3.3 Hydraulic Roughness The development of the TUFLOW model requires the assignment of different hydraulic roughness zones. These zones are delineated from aerial photography and cadastral data identifying different land-uses (e.g. cleared land, hardstand, roads, urban areas, etc.) for modelling the variation in flow resistance. The aerial imagery has been used to digitise a GIS layer of house / building polygons in the model area.



The adopted hydraulic roughness (Manning’s ‘n’) applied in the model according to land use type is shown in Table 2-4.

Table 2-4 Adopted Hydraulic Roughness Coefficients Based on Land Use

Land Use Manning’s ‘n’

Roads 0.03

Residential 0.06

2.3.4 Boundary Conditions The downstream model boundary condition applied is a constant water level on both tributary outlets (i.e. draining to Swamp Creek and Wallis Creek).

Local catchment inflows generated from the hydrological model (XP-RAFTS) are applied as inflows to the 2D/1D TUFLOW model. The inflows are either applied as point sources (e.g. to a 1D pipe) or as a source directly onto designated sub-catchment areas over the 2D domains. When the capacity of the pipe network is exceeded, surcharging through pit inlets on to the 2D domain will occur.

2.4 Model Validation In April 2015 severe weather conditions were caused in the Hunter Region by an intense East Cost Low (ECL). Early in the morning of the 22nd April 2015, the low-pressure system was situated just offshore of Newcastle and was moving slowly south. The system stayed close to the heavily populated coastal regions of the Paterson and Hunter Rivers and the Central Coast. The ECL produced widespread heavy rain and damaging wave conditions, with a localised “super storm” cell from Stroud and Dungog to Cessnock (WMA water, 2105). During the April 2015 event Heddon Greta experienced inundation of roads and residential lands, evident from in anecdotal and photographic evidence.

While undertaking the study, a small rainfall event occurred in September 2019. Rainfall was observed to fill roadside drains near the Golf Course, but inundation of yards and driveways did not occur.

The TUFLOW model was validated against the April 2015 and September 2019 events. The following sections summarise the model inputs and provides comparison between observed and simulated flood results.

2.4.1 April 2015 Event The 2015 event covered two days, with heavy rainfall recorded over the 48-hour period to 9 am on 22nd April. This event was widespread across the region, with serious flooding occurring elsewhere in the greater region, particularly in Dungog and Maitland.

A BoM daily rainfall gauge located in Heddon Greta at the Kurri Kurri Golf Club (061414) recorded a two-day cumulative total of 246 mm for the 48-hours to 9 am on the 22nd of April.

Two BoM pluviographs located near the study site are the Paterson (Tocal AWS) gauge (061250) located approximately 20 km north of the study area, and Williamtown RAAF gauge (061078) located



approximately 30 km north east of the study area. The 24-hour totals recorded over the same two-day period are summarised in Table 2-5.

Table 2-5 Recorded Rainfall Totals 48-hours to 9 am 22nd April 2015 (mm)

Date Kurri Kurri Golf Course

Paterson (Tocal) Williamtown RAAF

9 am 21st April ↓ 299.8 155.2

9 am 22nd April 246.0 175.6 114.0

48-hour Total 246.0 420.4 267.8

In the absence of temporal rainfall data closer to the study site, the half-hourly data recorded at Paterson (Tocal AWS) and Williamtown RAAF was utilised to distribute the two-day total recorded at Kurri Kurri Golf Club over the same 48-hour period. Two assess the sensitivity of modelled results on the adopted temporal pattern, two April 2015 scenarios were simulated.

For input into the XP-RAFTS model, the rainfall distribution recorded at the Patterson (Tocal AWS) pluviograph was scaled down to provide a total 48-rainfall equivalent to that recorded at the Kurri Kurri Golf Club i.e. the 48-hour rainfall total was scaled down from 420 mm to 250 mm. The Williamtown RAAF rainfall total did not require any scaling, as the 48-hour total recorded was similar to that recorded at Kurri Kurri. Hydrographs generated from the XP-RAFTS model were applied as local rainfall inflows to into the TUFLOW model.

To gain an appreciation of the relative intensity of the April 2015 event, the recorded rainfall depths for various storm durations recorded at Paterson (Tocal AWS) and Williamtown RAAF is compared with the design IFD data for Heddon Greta in Figure 2-3 (see Section 3.1.1 for further detail around the IFD data). Note that in Figure 2-3, the Paterson (Tocal) records have been scaled down to match the total rainfall recorded at Kurri Kurri.

As seen in Figure 2-3, the 2015 April storm temporal pattern recorded at Paterson (Tocal AWS) exceeds a 1% AEP design magnitude for a 3-hour duration and remains above a 5% AEP deign magnitude for longer durations, when scaled down to match the total rainfall recorded at Kurri Kurri. At the Williamtown RAAF, the 2015 April storm fluctuates between a 10% and 20% AEP design magnitude for shorter durations, exceeding a 5% AEP magnitude for durations longer than 18-hours.

Council has provided a series of photographs taken during the April 2015 event within the study catchment. In the absence of surveyed flood marks, these images provide a valuable reference of inundation patterns and relative depths of flooding, to enable comparison with the simulated model condition.

The simulated peak flood depths and inundation extents for the April 2015 event in the Heddon Greta catchment is shown in Figure 2-5. Available flood photographs and their respective locations within the model area are included for comparison between observed conditions and simulated results.

A comparison between the modelled depths and the depths observed in photographs is included in Table 2-6. The comparison between the observed and modelled depths has shown that in some regions of the study extent the model overestimated flood levels, however it is unknown when these images were taken and therefore may not of captured the peak flow.



Figure 2-3 Comparison of Recorded April 2015 Rainfall with IFD Relationships Overall the simulated flooding patterns of the scaled Maitland 18 data provided a good representation of the observed conditions in Heddon Greta. Significant inundation is simulated in known flood problem areas including Clift Street, Trenchard Street and Cooper Street.

The model configuration is considered to provide a suitable representation of the flood response in the catchment for assessing design event conditions and potential flood mitigation options, particularly for conditions where the stormwater network is not freely draining.

Table 2-6 Comparison between Observed and Modelled Depths

Image

Location

Observed Depth

[m]

Modelled Peak Depth

[m]

Additional Information

A 7 Clift St 0.10-0.15 0.15-0.25 Extent of flooding is consistent with photograph.

B 33 Main Rd NA 0.05-0.30 Extent of flooding to the right of the driveway inconsistent with photograph, while the flooding extent to the left of the driveway is consistent with the photograph. Approaching flood waters to Main Rd consistent with photograph.

C 25 Trenchard St 0.10-0.15 0.10-0.20 Depth was estimated from guttering.

D 10 Cooper St 0.10-0.25 0.10-0.35

E 16 Trenchard St 0.45 0.50-0.60 Depth estimated at driveway.



Figure 2-4 Rainfall Gauge Localities and April 2015 48 hour Rainfall Totals



Figure 2-5 Simulated Flood Depth and Inundation Photos of the April 2015 Event

2-5



The modelled results indicate that a small amount of rainfall is overtopping Clift Street with most driveways remaining free from inundation being consistent with Councils’ observations.

2.4.2 September 2019 Event Whilst undertaking the current study, a storm event occurred on the 18th - 19th of September 2019. During the event, a 24-hour total of 87 mm recorded at the Kurri Kurri golf course on.

The September 2019 event provided additional validation data for the calibration of the model. In lieu of any temporal data for this event, we have applied the same 48 h rainfall distribution recorded at the Maitland 18 gauge for April 2015, with the total rainfall depth scaled down to 87 mm. The scaled down temporal data set results in a maximum half hour rainfall intensity of 20.6 mm. This was consistent with information provided by Council for this event.

The modelled results indicate that a small amount of rainfall is overtopping Clift Street with most driveways remaining free from inundation being consistent with Councils’ observations.

2.5 Historic Catchment Conditions The Heddon Greta catchment has undergone extensive development since the early 1990’s. To understand the expected historical flood behaviour prior to development, the TUFLOW model was modified to represent a more natural catchment condition. The historic conditions were constructed by removing dwellings, stormwater drainage and road crests from the model to simulate expected pre-development flood conditions within the region. Although smoothing over topographic controls associated with development (i.e. road works and fill for residential dwellings) will provide a reasonable approximation of the natural catchment topography, there remains some uncertainty as the modified catchment topography has not been confirmed against historic data such as survey or topographic maps.

The historic model was simulated for the 10% AEP and 1% AEP design events to determine the peak flow rates in the upper and lower catchments under historic conditions. A comparison between historic and present-day flood conditions is contained within Section 4.3.



Figure 2-6 Simulated Flood Depth and Inundation Extents for the September 2019 Event

2-6

Heddon Greta Minor Flood Study and Concept Design – Part 1 16 Design Flood Conditions


3 Design Flood Conditions

3.1 Rainfall-Runoff Analysis Local catchment rainfall-runoff has been considered for the determination of design flood conditions on the catchment.

The ARR 2019 update currently represents the best practice guideline for the industry. The updated procedures provide some significant changes to previous ARR 1987 procedures. Some of the most notable changes in ARR 2019 are summarised below:

• Rainfall depths – The revised IFD rainfall estimates (released in 2016) underpin the ARR 2019 release. The updated IFD analysis includes a significant period of additional rainfall data collected since the release of IFD 1987. Variation in rainfall between the 1987 and BoM 2016 IFDs is location dependent.

• Rainfall losses – The estimation of initial and continuing loss rates is provided in ARR 2019 as gridded spatial data. Representative losses for catchments are extracted from the database which is a significant change from ARR 1987 whereby basic loss ranges were recommended for broad areas i.e. eastern or western NSW.

• Areal reduction factors – New equations were developed as part of ARR 2019 with regionalised parameters to define the areal reduction factor for catchments based on area and storm duration, and

• Temporal patterns – Each design duration now has a suite of ten temporal patterns (opposed to a single temporal pattern) for each duration.

Input data for the design rainfall analysis can be obtained online through the ARR 2019 Data Hub.

There are two key locations of interest in this investigation - Copper Street and Clift Street / Averys Lane. Design hydrology has therefore been developed to provided critical flow conditions at these two locations.

3.1.1 Rainfall Depths Design rainfall depth is based on the generation of intensity-frequency-duration (IFD) design rainfall curves utilising the procedures outlined in ARR 2019. These curves provide rainfall depths for various design magnitudes (up to the 0.2% AEP) and for durations from 1-minute to 96-hours. Table 3-1 shows the design rainfall depths applicable to the centre of the Heddon Greta catchment, based on the BoM 2016 IFDs.

The Probable Maximum Precipitation (PMP) is used in deriving the Probable Maximum Flood (PMF) event. The PMP is defined as “the theoretical greatest depth of precipitation that is physically possible over a particular catchment” (ARR 2019). The PMP has been estimated using the Generalised Short Duration Method (GSDM). A range of durations were simulated in the XP-RAFTS model, with the 1-hour storm found to result in peak flow conditions at both key areas of interest in the catchment. The GSDM method for the estimation of the PMP provided a total rainfall depth of 350 mm for the 1-hour duration.



Table 3-1 Design Rainfall Depth (mm)

Duration Design Event Frequency (AEP)

50% 20% 10% 5% 2% 1% 0.5% 0.2%

30-min 21.5 30.6 37.3 44.3 54.2 62.4 70.1 83.0

45-min 25.1 35.6 43.3 51.4 62.7 71.9 81.0 95.8

1-hour 27.7 39.3 47.8 56.6 69.0 78.9 88.9 105

1.5-hour 31.7 45.0 54.6 64.6 78.6 89.8 101 120

2-hour 34.8 49.4 60.1 71.0 86.3 98.7 111 131

3-hour 39.8 56.7 68.9 81.6 99.3 114 128 151

3.1.2 Areal Reduction Factors Areal Reduction Factor (ARF) is to be applied to the design point rainfall depths and is dependent on catchment size. As previously noted, new equations have been developed as part of ARR 2016 with regionalised parameters to define an event specific areal reduction factor for catchments based on catchment size and storm duration.

Each area of interest (Cooper Street and Clift Street / Averys Lane), has a contributing upstream area of around 0.2 km2. Areal reduction factors are therefore not applicable and have not been applied for this investigation.

3.1.3 Rainfall Losses The recently released NSW-specific ARR 2019 guidance recommends a modified approach to design rainfall losses. A ‘Probability Neutral Burst Initial Loss’ is provided through the data hub rather than being derived through a storm initial loss and pre-burst rainfall approach. The continuing loss values are also recommended to be reduced to 40% of those extracted from the data hub. For the Heddon Greta catchment, this results in a continuing loss of 1.12 mm / h.

The rainfall Probability Neutral Burst Initial Loss for pervious areas for each of the modelled design events and durations are presented below in Table 3-2.

Table 3-2 Rainfall Probability Neutral Burst Initial Loss (mm)

Duration Design Event Frequency (AEP)

50% 20% 10% 5% 2% 1% 0.5% 0.2%

30-min 11.7 8.9 8.8 8.2 8.6 4.6 4.6 4.6

45-min 11.7 8.9 8.8 8.2 8.6 4.6 4.6 4.6

1-hour 11.7 8.9 8.8 8.2 8.6 4.6 4.6 4.6

1.5-hour 12.3 9.1 8.8 8.3 8.8 5.1 5.1 5.1

2-hour 12.8 9.9 8.8 8.2 8.6 4.6 4.6 4.6

3-hour 12.9 9.5 9.0 7.8 8.4 2.9 2.9 2.9



3.1.4 Temporal Patterns Temporal patterns are required to define what percentage of the total rainfall depth occurs over a given time interval throughout the storm duration. Under ARR 2019, ten temporal patterns are defined for each storm duration for each design event magnitude. The procedures for ARR 2019 provide for the selection of the temporal pattern that gives the peak flow closest to the mean of the peak flows from all ten temporal patterns. This method was followed to find the critical temporal pattern for each event duration.

The design point temporal pattern approach was used to assess the entire catchment due to its small size of 0.2 km². The design point temporal pattern approach uses an ensemble of ten temporal patterns which are defined for each AEP bin (i.e. frequent, intermediate and rare events), duration and region.

3.1.5 Critical Duration The critical duration is the storm duration for a given event magnitude that provides for the peak flood conditions at the location of interest. For example, small catchments are more prone to flooding during short duration storms, while for large catchments longer durations will be more critical.

Utilising the Storm Injector software and the XP-RAFTS model, a range of design events and durations (including the 50% 20%, 10%, 5%, 2% and 1% AEP design events and durations from 30 min to 3-hours) were simulated, identifying the critical duration and mean peak flow rate for each design event.

The outputs of the XP-RAFTS model simulations are shown in Figure 7-4 and Figure 7-5, presenting the critical mean flow for the 1% AEP design storm across six durations (30-minutes, 45-minutes, 1-hour, 1.5-hour, 2-hours and 3-hour) at sub-catchment 6 (draining to the north western stream i.e. Copper Street) and sub-catchment 14 (draining to the easterly stream i.e. Averys Lane), respectively. The box-whisker plot indicates the median peak flow (horizontal line), mean peak flow (indicated by the “x”) and first and third quartile peak flows (remainder of boxes). The whiskers above and below the box represent the lowest and highest peak flows. The XP-RAFTS model outputs indicate that for both sub-catchment 6 and sub-catchment 14, the 45-minute duration event produces the peak flow rate.

Figure 7-8 shows the flow hydrographs of the selected design temporal patterns for the critical events of sub-catchment 6 and sub-catchment 14 (45-minute) for the 1% AEP event.

The peak flow to be simulated within the TUFLOW model is the mean of all ten temporal patterns. Where the temporal pattern selected did not give a peak flow exactly equal to the mean value, peak flows were scaled to match. The adopted critical storm duration, temporal pattern ID and mean peak flow rates adopted for each design flood vent are shown in Table 3-3.



Figure 3-1 ARR 2019 XP-RAFTS 1% Critical Duration Assessment – Sub-catchment 6

Figure 3-2 ARR 2019 XP-RAFTS 1% Critical Duration Assessment – Sub-catchment 14



Figure 3-3 Modelled 1% AEP Design Hydrographs Table 3-3 Adopted Temporal Pattern and Peak Flow Conditions

Catchment Event Critical

Duration (minutes)

Temporal ID Peak Flow (m³/s)

C6

50% AEP 120 4640 0.7

20% AEP 90 4605 1.3

10% AEP 60 4565 1.8

5% AEP 60 4475 2.3

2% AEP 60 4405 3.1

1% AEP 45 4528 3.8

0.5% AEP 45 4496 6.0

0.2% AEP 45 4496 5.5

C14

50% AEP 120 4640 1.0

20% AEP 90 4605 1.8

10% AEP 60 4565 2.4

5% AEP 60 4475 3.0

2% AEP 60 4405 4.1

1% AEP 45 4528 5.2

0.5% AEP 45 4496 6.0

0.2% AEP 45 4496 7.4



3.2 Sensitivity Assessment

3.2.1 Rainfall Losses The sensitivity of modelled peak flood levels to the adopted initial and continual loss values was tested for the 1% AEP design event. The initial and continuing loss values applied to the catchment were increased and decreased. Rainfall losses were increased by adopting an initial loss of 10 mm and a continual loss of 2.5 mm/h at the 1% AEP. Rainfall losses were decreased by adopting an initial loss of 0 mm. For the reduced loss scenario, the continuing adopted for the design simulation of 1.12 mm/h remained unchanged.

The sensitivity of modelled peak flood levels to changes to the initial and continual losses is minimal, with modelled peak flood levels within 0.02 m of the baseline 1% AEP result across the catchment. Modelled peak flood levels across the catchment for the range of sensitivity tests undertaken is summarised in Table 3-5 at the end of this section.

3.2.2 Manning’s Roughness The sensitivity of modelled peak flood levels to the adopted Manning’s ‘n’ roughness values were assessed for the 1% AEP design event. Roughness values for all materials types within the channel and floodplain were increased and decreased by 25%.

The sensitivity of modelled peak flood level to changes to the Manning’s roughness values is minimal, with modelled peak flood levels within 0.05 m of the baseline 1% AEP result across the catchment. Modelled peak flood levels across the catchment for the range of sensitivity tests undertaken is summarised in Table 3-5 at the end of this section.

3.2.3 Structure Blockage During flood events, blockages can significantly increase local flood levels. The adopted methodology for determining appropriate consideration of blockages is that proposed in Chapter 6: Blockage of Hydraulic Structures, Book 8 in Australian Rainfall and Runoff - A Guide to Flood Estimation (ARR, 2019)

Under the revised ARR 2019 guidelines, appropriate blockages to consider for design flood conditions determine an at-site debris potential, based on criteria relating to the nature of the source catchment. The availability, mobility and transportability of debris within the study catchment were all deemed to fall within the “low” category, resulting in a 1% AEP debris potential of “low”. The 1% AEP debris potential is then adjusted to account for the influence of the inlet clear width of the structure on actual blockage potential. Adjustments are based on the relationship between the likely length of debris (L10) and the width of the structure (W) and have been reproduced in Table 3-4. The value for L10 is determined as the average length of the longest 10% of debris reaching the site. For this investigation, L10 has been estimated as a large tree branch, being approximately 1 m in length.

With reference to the values in Table 3-4, when the 1% AEP debris potential is “low”, the blockage percentage to be adopted for design simulation ranges from between 0% - 25%, depending on the width of the individual structure.



Table 3-4 Adjusted Blockage as a Percent (ARR 2019)

Control Dimension 1% AEP Debris Potential at Structure

High Medium Low

W < L10 100% 50% 25%

L10 >= W<= 3*L10 20% 10% 0%

W > 3*L10 10% 0% 0%

The modelled design flood extent for the blockage scenario remains similar to the baseline for much of the study area, with peak flood levels typically less than 0.02 m higher than the baseline scenario around Clift Street, Adams Street and Averys Lane. Downstream of Main Road, the impact of blocked pipes becomes more significant, with the extent of inundation increasing between Main Road and Trenchard Street and peak flood levels along Trenchard Street increasing by up to 0.1 m.

Modelled peak flood levels across the catchment for the range of sensitivity tests undertaken is summarised in Table 3-5 at the end of this section.

3.2.4 Climate Change The potential impacts of future climate change in the form of increased rainfall intensities were considered for the catchment. The NSW Government guideline for Practical Consideration of Climate Change (DECCW, 2007) in the floodplain management process advocated for consideration of increased design rainfall intensities of up to 30%.

Analysis of the ARR 2019 IFDs indicated that the 0.5% AEP and 0.2% AEP can be adopted as surrogates for projected increases in rainfall intensities on the 1% AEP of around 10% and 30%.

For the study area an increase in rainfall intensity by approximately 10% resulted in increase in peak flood levels of up to 0.06 m for the 1% AEP event. For a 30% increase, peak flood levels increased by up to 0.2 m. Design flood mapping for the 0.5% AEP and 0.2% AEP events is included in Apendix A.

Modelled peak flood levels across the catchment for the range of sensitivity tests undertaken, including climate change, is summarised in Table 3-5.

Table 3-5 1% AEP Modelled Peak Depth- Sensitivity Testing Results

Location Base 1% AEP

Higher Losses

Lower Losses

- 25% Manning’s

‘n’

+25% Manning’s

‘n’ Structure Blockage

∼10% Rainfall Increase

∼30% Rainfall Increase

Clift St 21.71 21.71 21.71 21.69 21.72 21.71 21.73 21.75

Adams St 19.93 19.92 19.93 19.90 19.94 19.94 19.95 19.97

Main Rd 19.28 19.27 19.28 19.28 19.28 19.30 19.31 19.33

Trenchard St 18.81 18.80 18.81 18.81 18.81 18.91 18.90 19.05

Bowden St 16.99 16.99 16.99 16.96 17.01 16.98 17.02 17.08

Cooper St 16.18 16.18 16.18 16.15 16.21 16.14 16.20 16.23



3.2.5 Downstream Boundary Condition The model boundaries are located around 800 m downstream of Averys Lane and 400 m downstream of Hall Street. For design runs, the downstream boundaries were assumed to be representative of a free flowing tailwater condition with the constant water level set to the lowest topographic elevation at the boundary. The boundaries are located sufficiently far from the area of interest such that they will not be influencing modelled flood behaviour.

As Heddon Greta is located on the fringe of the Hunter River floodplain, there is potential for elevated Hunter River tailwater levels to inundate the study area. Hunter River flood conditions were derived in the Hunter River Branxton to Green Rocks Flood Study (WMA, 2010). The peak 1% AEP design Hunter River flood level at the site is 9.7 m AHD, at which only the very edge of the catchment area identified on Figure 1-2 would be inundated. Due to the vastly different size of the Hunter River and Heddon Greta catchments, it is unlikely that a 1% AEP Hunter River flood event would occur simultaneously with a 1% AEP design rainfall event in Heddon Greta. It would be more appropriate to adopt a coincident 10% AEP Hunter River flood condition (5.5 m AHD) coincident with a 1% AEP local rainfall event in Heddon Greta. This is similar to the condition adopted for the design modelling presented in this report.

3.2.6 Property Inundation When considering the range of sensitivity simulations undertaken, the rainfall losses and Manning’s ‘n’ values had a minimal impact on modelled flood extents and levels at the 1% AEP design event. The structural blockage assessment identified that the area between Main Road and Trenchard Street was the most susceptible to increased inundation and peak flood levels when incorporating blockage factors into the stormwater drainage network. Two additional properties on Trenchard Street would become inundated above floor level at the 1% AEP design event if structural blockages are assumed, when compared to the baseline 1% AEP scenario. The potential for increased rainfall intensities associated with future climate change would also increase the number of properties inundated above floor level (further detail is provided in Section 4.8).

Heddon Greta Minor Flood Study and Concept Design – Part 1 24 Design Flood Results


4 Design Flood Results The design flood events are hypothetical floods used for planning and floodplain management investigations. The 50% AEP, 20%, 10% AEP, 5% AEP, 2% AEP, 1% AEP, 0.5% AEP, 0.2% AEP and Probable Maximum Flood (PMF) events have been simulated for this study. The adopted model parameters and design rainfall conditions were discussed in Section 2 and Section 0, respectively.

4.1 Existing Flood Conditions The simulation of existing design event conditions is required to determine the nature and extent of the existing flood problem, to identify potential works to reduce the flood impact and to provide a platform for assessing the performance of suggested upgrade works.

Peak flood depths for the 50% AEP, 20%, 10% AEP, 5% AEP, 2% AEP,1 % AEP, 0.5% AEP, 0.2% AEP and PMF results are contained in Appendix A.

The catchment draining through Heddon Greta has two outlets – one at Averys Lane (south east) and the other through Copper Street (north west). Runoff from the Golf Course is currently intercepted by roadways and stormwater drainage to be discharged to the Averys Lane outlet. When capacity of stormwater drainage and roadway conveyance is exceeded, floodwaters will run overland to the north, into the Copper Street channel.

The simulated inundation patterns for the 1% AEP event shows flooding at Clift Street, Adams Street, Heddon Street, Main Street, Averys Lane Trenchard Street, and the Bowden Street and Cooper Street alignments. Flooding is mostly contained to the gutter with some spillage onto the road, particularly on Trenchard Street with depths of greater than 0.2 m. During the 10% AEP event, the flooding extent decreases and most flood inundation is contained to the guttering. At the PMF event the flooding depth and extent increases significantly across the entire study area.

4.2 Existing Drainage Capacity The performance of the existing drainage line has been assessed under a range of design flood magnitudes to gain an appreciation into its relative capacity. Design peak flow rates at the at major Pipe outlets are shown in Table 4-1.

4.3 Cross-Catchment Flow Under present-day conditions, a portion of runoff from the upper catchment is discharged to the Averys Lane outlet to the east of the catchment. Development within the catchment is likely to have modified the natural drainage regime of the catchment, as much of this runoff is captured and conveyed via the stormwater networks and roadways. The TUFLOW model developed to represent historic catchment conditions (see Section 2.5) was simulated for the 10% AEP and 1% AEP design events to determine the expected cross-catchment flow under historic conditions.

Peak flow rates at the Copper Street (north west) and Averys Lane (south east) outlets are summarised in Table 4-2 and Table4-3 for both the historic and present day catchment conditions.



Table 4-1 Design Peak Flow Rates at Major Pipe Outlets, Heddon Greta

Design Event Peak Flow Rate (m³/s)

A1-2 B1-2 D1-2 E7-8 H4-5

50% AEP 0.5 0.0 0.5 0..3 0.4

20% AEP 0.5 0.3 0.8 0.6 0.7

10% AEP 0.5 0.3 0.9 0.9 0.9

5% AEP 0.5 0.4 0.9 1.3 1.0

2% AEP 0.6 0.4 0.9 1.3 1.3

1% AEP 0.6 0.4 0.9 1.4 1.4

0.5% AEP 0.6 0.4 0.9 1.4 1.4

0.2% AEP 0.6 0.4 0.9 1.4 1.5

PMF 0.7 0.5 1.1 1.4 1.9

Table 4-2 Historic Peak Flow Rates

Outlet Peak Flow Rate (m³/s)

10% AEP 1% AEP

Copper Street 2.5 5.8

Averys Lane 0.6 1.3

Table 4-3 Present Peak Flow Rates

Outlet Peak Flow Rate (m³/s)

10% 1% AEP

Copper Street 1.5 2.8

Averys Lane 1.9 3.9

Development of the catchment has decreased the peak flow rate reaching the Copper Street channel. The total flow reaching both outlets has slightly decreased as a result of development and can be attributed to attenuation and storage of floodwaters due to topographic changes associated with catchment development. Historically, approximately 80% of floodwater from the catchment was discharged to Cooper Street. Under present day conditions, around 40% of floodwater is discharged to Cooper Street.

The peak flow rates at key locations under present day conditions are summarised in Table 4-4.

Peak flood inundation extents for the 1% AEP design event are shown in Figure 4-1 for the historic (dark blue) and present day (light blue) scenarios. The lots identified as bold have a higher maximum peak flood level within the lot boundary under present day conditions when compared to the historic condition.



Table 4-4 Peak Flow Rates Across the Study Site

Design Events Cooper St Bowden St Clift St Heddon St Adams St Averys Ln Main St

50% AEP 0.5 0.4 0.0 0.0 0.0 0.5 0.0

20% AEP 0.8 0.7 0.5 0.0 0.0 0.9 0.0

10% AEP 1.1 0.9 0.8 0.0 0.2 0.9 0.0

5% AEP 1.3 1.1 1.2 0.1 0.5 0.9 0.0

2% AEP 1.7 1.4 1.7 0.3 0.5 0.9 0.0

1% AEP 1.9 1.4 2.4 0.6 1.1 1.0 0.1

0.5% AEP 2.0 1.6 2.9 0.7 1.3 1.0 0.3

0.2% AEP 2.3 2.1 3.7 1.0 2.0 1.0 0.8

PMF 14.2 12.7 15.0 3.9 11.6 1.7 3.9

Figure 4-1 Comparison between Historic and Present 1% AEP Design Flood Inundation Extent



With reference to Figure 4-1, historically there would have been a greater extent of inundation along the Cooper Street overland flow path. Peak flood levels at key locations along this flow path are summarised in Table 4-5.

Table 4-5 Peak Flood Levels at Cooper Street – Historic and Present

Location Historic Present Day

10% AEP 1% AEP 10% AEP 1% AEP

10-12 Cooper St 16.3 16.5 16.2 16.3

4.4 Flood Hazard The Flood Hazard Guideline 7-3 of the Australian Disaster Resilience Handbook 7 Managing the Floodplain: A Guide to Best Practice in Flood Risk Management in Australia (AIDR, 2017) represents the current industry best practice with regards to defining flood hazard. The guideline considers a holistic approach to consider flood hazards to people, vehicles and structures. It recommends a composite six-tiered hazard classification, reproduced in Figure 4-2. The six hazard classifications are summarised in Table 4-6.

Table 4-6 Combined Flood Hazard Curves – Vulnerability Thresholds

Hazard Classification Description

H1 Relatively benign flow conditions. No vulnerability constraints.

H2 Unsafe for small vehicles.

H3 Unsafe for all vehicles, children and the elderly.

H4 Unsafe for all people and vehicles.

H5 Unsafe for all people and all vehicles. Buildings require special engineering design and construction.

H6 Unconditionally dangerous. Not suitable for any type of development or evacuation access. All building types considered vulnerable to failure.

It can be seen that the flood hazard level is determined on the basis of the predicted flood depth and velocity. This is conveniently done through the analysis of flood model results. A high flood depth will cause a hazardous situation while a low depth may only cause an inconvenience. High flood velocities are dangerous and may cause structural damage while low velocities generally have no major threat.

Provisional hazard mapping based on the above criteria is included in Appendix B for the each of the design flood events considered.

For the 1% AEP event, flooding within the Heddon Greta catchment is mostly classed as hazard category H1 to H2 and is indicative of relatively benign flow conditions that would not pose a significant flood risk to people, animals and vehicles. However, some properties at Clift Street, Main Road and Cooper Street have recorded a hazard category of H3 at the 1% AEP with conditions becoming unsafe for vehicles, children and the elderly.



Figure 4-2 Combined Flood Hazard Curves – Vulnerability Thresholds

4.5 Flood Function The flood function (or hydraulic categorisation) of a floodplain helps describe the nature of flooding in a spatial context and from a flood planning perspective can determine what can and can’t be developed in areas of the floodplain. The hydraulic categories as defined in the Floodplain Development Manual are:

• Floodway - Areas that convey a significant portion of the flow. These are areas that, even if partially blocked, would cause a significant increase in flood levels or a significant redistribution of flood flows, which may adversely affect other areas.

• Flood Storage - Areas that are important in the temporary storage of the floodwater during the passage of the flood. If the area is substantially removed by levees or fill it will result in elevated water levels and/or elevated discharges. Flood storage areas, if completely blocked would cause peak flood levels to increase by 0.1 m and/or would cause the peak discharge to increase by more than 10%.

• Flood Fringe - Remaining area of flood prone land, after floodway and flood storage areas have been defined. Blockage or filling of this area will not have any significant effect on the flood pattern or flood levels.



There are no prescriptive methods for determining what parts of the floodplain constitute floodways, flood storages and flood fringes. Descriptions of these terms within the Floodplain Development Manual are essentially qualitative in nature. Of difficulty is the fact that a definition of flood behaviour and associated impacts is likely to vary from one floodplain to another depending on the circumstances and nature of flooding within the catchment. Flood function criteria was adopted from the Greta Flood Study (WMA, 2019).

The adopted flood function categorisation for the 1% AEP design event is summarised in Table 4-7.

Based on the flood function mapping for the 1% AEP event, the floodway is largely contained within creek channels and adjacent to the Golf Course boundary. The remainder of the inundation area is predominately flooded by depths of less than 0.5 m and is therefore classed as flood fringe.

Table 4-7 Flood Function Categories

Hydraulic Category

Categorisation Criteria Description

Floodway (V x D) > 0.25 m2/s, AND peak velocity > 0.25 m/s; OR peak velocity > 0.6 m/s and peak depth > 0.3 m

Areas and flowpaths where a significant proportion of floodwaters are conveyed (including all bank-to-bank creek sections).

Flood Storage peak depth > 0.5 m Areas where floodwaters accumulate before being conveyed downstream. These areas are important for detention and attenuation of flood peaks.

Flood Fringe peak depth < 0.5 m Areas that are low-velocity backwaters within the floodplain. Filling of these areas generally has little consequence to overall flood behaviour.

4.6 Preliminary Flood Planning Area Flood Planning Levels (FPLs) are used for planning purposes and can also be used to determine the extent of the Flood Planning Area (FPA), which is the area of land subject to flood-related development controls. The FPL is the level below which a Council places restriction on development due to the hazard of flooding.

Traditional floodplain planning has relied almost entirely on the definition of a singular FPL, which has usually been based on the 1% AEP flood level. In accordance with the NSW Floodplain Development Manual (DIPNR, 2005), the standard definition for setting the FPL in NSW is the 1% AEP design flood level with a 0.5 m allowance for freeboard. The FPL is used as the minimum requirement for habitable floor levels for standard residential development.

To determine the FPA, a common method is to apply a freeboard of 0.5 m to the 1% AEP design level and extrapolate the surface outward to intersect with the underlying terrain. This approach works well for areas subject to mainstream flood inundation but can prove difficult in overland flow environments. The FPA should not extend beyond the PMF extent as this represents the maximum extent of the floodplain.



In considering an appropriate FPL and FPA for the study area, the following points are noted:

• North of Main Road – The standard approach of applying 0.5 m freeboard on the 1% AEP level is applicable for defining the FPA (due to presence of a gully line within the topography) and resulted in an FPA very similar to the PMF extent. It is considered appropriate to adopt an FPL of the 1% AEP design flood level plus 0.5 m freeboard for properties located north of Main Road and all properties fronting Main Road (both sides of roadway).

• South of Main Road – The nature of flooding upstream of Main Road is shallow overland flow, such that the approach to extrapolate an FPL surface does not produce sensible results. Peak PMF flood levels here are typically 0.2 – 0.3 m higher than the 1% AEP design flood level. It may therefore be appropriate to consider a reduced freeboard for this upper catchment area when defining the FPL. Adopting a freeboard of 0.3 m above the 1% AEP peak flood surface is typical for overland flow environments.

The preliminary FPA is based on the PMF flood extent and has been mapped for the study area (see Appendix E).

4.7 Flood Damages Assessment The definitions and methodology used in estimating flood damage are summarised in the Floodplain Development Manual. Figure 4-3 summarises the “types” of flood damages as considered in this study. The two main categories are 'tangible' and 'intangible' damages. Tangible flood damages are those that can be more readily evaluated in monetary terms, while intangible damages relate to the social cost of flooding and therefore are much more difficult to quantify.

Flood damages have been calculated using a database of potentially flood affected properties and stage-damage curves derived for different types of property within the catchment. These curves relate the amount of flood damage that would potentially occur at different depths of inundation, for a particular property type. Residential damage curves are based on the DPIE guideline stage-damage curves for residential property.



Figure 4-3 Types of Flood Damage The floor levels for 105 dwellings located within the PMF extent were included in the property database. The Red Wing Survey (5/07/19) provided floor level details for 45 properties around Clift Street, Adams Street, Main Road and Trenchard Street. Surveyed floor levels for an additional 5 properties near Cooper Street was obtained from previous investigations completed by BMT. Where survey information was not available, floor levels were assumed to be 0.3 m above the average DEM level of the building footprint.

The preliminary damage estimates derived in this study are for the tangible damages only. Whilst intangible losses may be significant, these effects have not been quantified, due to difficulties in assigning a meaningful dollar value.

Estimation of Direct Damages

The peak depth of flooding was determined at each property for the 50% AEP, 20% AEP, 10% AEP, 5% AEP, 2% AEP, 1% AEP, 0.5%, 0.2% and PMF event. The associated direct flood damage cost to each property was then estimated from the stage-damage relationships. The flood damage curves include a flat $11,725 cost of external damages for any level of flood inundation incurred below floor level. For instances where the property is not inundated above floor level and the external flood depth is below 0.3 m, this value is considered to be overly conservative. Therefore, a nominal $1,000 value has been adopted for external flood damages for below floor flooding of less than 0.3 m. Total



damages for each flood event were determined by summing the predicted damages for each individual property.

The Average Annual Damage (AAD) is the average damage in dollars per year that would occur in a designated area from flooding over a very long period of time. In many years there may be no flood damage, in some years there will be minor damage (caused by small, relatively frequent floods) and, in a few years, there will be major flood damage (caused by large, rare flood events). Estimation of the AAD provides a basis for comparing the effectiveness of different floodplain risk management measures (i.e. the reduction in the AAD), investigated in Section 4.8.

Estimation of Indirect Damages

The indirect damages are more difficult to determine and would vary for each flood event, particularly with the duration of the flood inundation. Previous studies detailing flood damages from actual events have found that the indirect damages for residential properties are typically in the order of 20% of the direct damages. Given the relative uncertainty associated with the indirect damages a value of 20% of the direct damages has also been adopted for this study.

Total Tangible Flood Damages

The total tangible flood damages for residential properties in Heddon Greta are presented in Table 4-8. From this data, the combined AAD was calculated as being $120,600.

Table 4-8 Summary of Total Existing Residential Tangible Flood Damages

Design Event Direct Flood Damages ($)

Indirect Flood Damages ($)

Total Tangible Flood Damages ($)

50% AEP 2,000 400 2,400

20% AEP 167,800 33,600 201,400

10% AEP 289,500 57,900 347,400

5% AEP 401,400 80,300 481,600

2% AEP 415,800 83,200 499,000

1% AEP 485,500 97,100 582,500

0.5% AEP 565,500 113,100 678,600

0.2% AEP 779,400 155,900 935,300

PMF 3,052,600 610,500 3,663,100

AAD 100,500 20,100 $120,600

4.8 Property Inundation The number of properties inundated above floor level and above ground level are summarised in Table 4-9. It should be noted that floor level survey was available for all properties identified as being flooded above floor level.



Table 4-9 Existing Inundated Properties

Design Event Above Floor Above Ground

50% AEP 0 2

20% AEP 3 15

10% AEP 5 18

5% AEP 7 24

2% AEP 7 32

1% AEP 8 47

0.5% AEP 10 49

0.2% AEP 13 59

PMF 51 96

Heddon Greta Minor Flood Study and Concept Design – Part 1 34 Conclusions


5 Conclusions This study has defined existing design flood conditions in Heddon Greta, through development of appropriate computer modelling tools. The XP-RAFTS hydrologic and TUFLOW hydraulic models developed for the study were validated against recent rainfall events within the catchment, occurring in April 2015 and September 2019.

Understanding the flood behaviour in the catchment, particularly through definition of the design flood events, is used to inform future floodplain risk management activities in the study area.

Heddon Greta Minor Flood Study and Concept Design – Part 1 35 References


6 References BMT WBM (2016). Cooper Street, Heddon Greta (L.N20218.007_cooperSt_HeddonGreta.docx).

BMT WBM (2017). Drainage Investigation at Clift Street, Heddon Greta (L.N20218.000_Clift.St_Pipe.docx).

Department of Environment, Climate Change and Water (DECCW) (2007). Floodplain Risk Management Guideline: Practical Consideration of Climate Change.

DIPNR. (2005). Floodplain Development Manual: the management of flood liable land.

FEMA (2007). FEMA 551 Selecting Appropriate Mitigation Measures for Flood Prone Structures.

NSW Department of Infrastructure, Planning and Natural Resources (DIPNR) (2005) Floodplain Development Manual.

Red Wing Survey (2019). Heddon Greta Clift Street Detail Survey (Drawing number H C CCC002 001 002).

WMA (2015a). Black Creek Flood Study Stage Two – Nulkaba to Branxton.

WMA (2019). Greta Flood Study- Final Report Volume 1.

WMA (2010). Hunter River Braxton to Green Rocks.

A-1

Appendix A – Location of Utilities and Services

B-1

Appendix B – Existing Design Flood Depth Mapping

C-1

Appendix C – Existing Design Flood Hazard Mapping

D-1

Appendix D – Existing Design Flood Function Mapping

E-1

Appendix E – Preliminary Flood Planning Area

heddon greta minor flood study and concept design – part 1

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