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Refining Flood Model by Replacing

Approximations and Assumptions with Real

Data in Narellan Creek Catchment

2017 Floodplain Management Australia National Conference Preparing for the Next Great Flood

Newcastle Exhibition & Convention Centre (NEX), Newcastle NSW

16–19 May 2017

Svetlana Cvetkovic & Leon Collins

Project Engineers

NSW Public Works, Sydney, NSW

Maria Pinto

Senior Stormwater and

Floodplain Engineer

Camden Council, Oran Park, NSW

Comparison of TUFLOW Model Results

for two scenarios

� Flood Study with approximations,

estimations & assumptions; and

� Refined Model by replacing approximations

with actual data

Introduction

Narellan Creek Flood Study – Focusing on

Harrington Park

Delineation of Hydraulic Roughness Zones

Vegetation Survey

Corresponding Manning’s Roughness: Vegetation Survey

Narellan Creek Flood Model

Material Depth ‘n’ 1(1m)

Depth ‘n’ 2(2m)

‘n’ at1m

‘n’ at2m

‘n’ at3m

Cumberland Plain High 0.4 0.2 2 0.12 0.17 0.12 0.12

Cumberland Plain Low 0.4 0.07 2 0.17 0.07 0.07 0.07

Riverflat Eucalypt High 0.4 0.2 2 0.12 0.17 0.12 0.12

Riverflat Eucalypt Low 0.4 0.1 2 0.07 0.089 0.07 0.07

Revegetated Eucalypt H 0.4 0.2 2 0.12 0.17 0.12 0.12

Revegetated Eucalypt L 0.4 0.1 2 0.06 0.085 0.06 0.06

Swamp Oak High 0.4 0.2 2 0.12 0.17 0.12 0.12

Swamp Oak Low 0.4 0.1 2 0.09 0.096 0.09 0.09

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Corresponding Manning’s Roughness: Vegetation Survey, (cont.)

Narellan Creek Flood Model

Material Depth ‘n’ 1(1m)

Depth ‘n’ 2(2m)

‘n’ at1m

‘n’ at2m

‘n’ at3m

Slashed Grassland

High

0.1 0.075 0.5 0.05 0.05 0.05 0.05

Slashed Grassland Low 0.05 0.075 0.25 0.03 0.03 0.03 0.03

Parkland with trees 0.05 0.075 0.25 0.04 0.04 0.04 0.04

Material Depth ‘n’ 1(1m)

Depth ‘n’ 2(2m)

‘n’ at1m

‘n’ at2m

‘n’ at3m

All Overbank Area 1 0.2 5 0.1 0.2 0.175 0.15

Waterway / Channel 0.5 0.1 2.5 0.075 0.094 0.081 0.075

Estimated Manning’s Roughness: Flood Study Model

1% AEP Flood Profile Comparison Along Narellan Creek – Harrington Park

Comparison of Model Results – Estimated & Surveyed Roughness

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0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000

Wate

r L

evel (m

AH

D)

Chainage (m)

Comparison of Model Results – Estimated & Surveyed Roughness

• Flood levels are lower varying from 0.2 m to 0.5 m in this section, up to Fairwater Garden.

• comparable between the two models from the upstream end of Fairwater Gardens through to Camden Valley Way.

1% AEP Flood Level Difference Between Roughness Updated (2016) and Roughness Estimated (2015) Flood Models

Further Improvement to the Model

Updates: Inclusion of bathymetry, low flow pipes and culverts

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Further Improvement to the Model

• Harrington Park Bridge dimensions and form loss coefficients

were replaced with detailed survey

• Form loss Equation; ∆ℎ = � �2/2 (kinetic energy, form loss

factor, weighted depth)

• Form loss coefficients of 4 & 2.5 for bridge deck and railing were

too high and were adjusted based on surveyed data (Austroads1994).

• 1D HEC-RAS model was used to verify TUFLOW model flood

behavior at the bridge.

Bridge Dimensions and foam loss coefficient

Further Improvement to the Model

(Source: AWACS 1994)

Harrington Park Lake low level outlet structure physical model rating curve

• Empirical labyrinth weir equations used for Harrington

Park Lake were replaced by a

rating curve derived from physical modeling

• Flow behavior is very difficult to

predict using theoretical weir equations

• Physical model results best

represent the structure

Further Improvement to the Model

Comparison of labyrinth weir behavior with physical model rating curve and

validation of Updated Model discharge rating

1% AEP Flood Profile Comparison Along Narellan Creek – Harrington Park

Comparison of Model Results – updated and existing models

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AH

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Chainage (m)

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Comparison of Model Results – Existing and Refined Models

Difference in Peak 1 % AEP Flood Levels

Comparison of Model Results – Existing and Refined Models

• directly upstream of Harrington Parkway bridge is 0.75 m lower

• Other areas up to Fairwater Gardens are 0.15 m to 0.4 m lower

• In addition to Harrington Park the flood level reduction in the

entire catchment varies from 0.1 m to 0.65m

1% AEP Flood Level Difference Between Updated Flood Modeland Existing Flood Model with approximations and estimations

• Flood Level decrease is significant for roughness changes based on vegetation

survey.

• Flood Level decrease for all other model improvements are more localised.

Comparison of Model Results – Existing and Refined Models

• Increased discharge at the low level outlet of Harrington Park Lake with the application of physical model discharge rating curve

• Reduced afflux at the bridge with improved bridge structural details and validated form loss coefficients

• Increased conveyance through Narellan Creek and WQMS due to modified bathymetry

• Inclusion of low flow pipes

• Changes to hydraulic roughness associated with riparian vegetation

survey, resulting in lower roughness in overbank areas.

Difference in peak flood levels & extents are attributed to:

In Conclusion

• there is always a balance between budget,

level of survey acquisition, model accuracy and

reliability of model results in undertaking a

flood study .

• This project illustrates the value of capturing

survey and other detailed data in the

development of flood models, from which will

later inform management decisions including

investment in mitigation schemes.

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Thank You

Acknowledgement

• Council staff supported by reviewing this presentation

• Wafaa Wasif, Office of Environment & Heritage (OEH)