integrated flood modelling and mitigation analysis -...

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Integrated flood modelling and mitigation analysis 8th Australasian Natural Hazards Management Conference (ANHMC2015)

Raymond Cohen | Mahesh Prakash | James Hilton | Yunze Wang | Fletcher Woolard

3:45-4:15pm Wednesday 14 October 2015

Background

ANHMC | Perth 2015 | Raymond Cohen

Motivations

• Australian councils are required to manage risk associated with areas that are prone to flooding from storm surge, heavy rainfall and catchment flooding

• Simultaneous combinations of these events are especially challenging

• Step 1: Identify present and future flood risk

• Computational simulations of worst case conditions now and in the future

• Make land use and planning decisions based on these results

• Step 2: Propose mitigation strategies to reduce impact of flood events

• Use computational modelling to determine delivered benefits of each strategy

• Evaluate cost/benefit options

• Make informed decisions for how to optimally allocate funds

– Could just be one option over another

– Attack / defend / withdraw


• Water depth << width • 2D formulation • Predicts water height, momentum • Shallow water finite volume model

Swift Hydrodynamic Model

width

depth

• Dam breaks • Flash flooding • Coastal inundation


Drainage flow direction

Normal conditions:

• Pressure head-based pipe network model • Assume sound speed >> gravity wave speed • Pipe network both helps and hinders flooding:

Swift Hydraulic Model

Storm tide

Drainage flow direction

Drainage reversal:

= =


Swift Flood Modelling Framework

• Integrated hydrodynamic/hydraulic flood modelling

• Modular & flexible

• Very fast - GPU accelerated code

• Capable of investigating storm surge, extreme rainfall, catchment flooding, sea level rise

• Able to study mitigation options in an integrated manner

• Developing software front end “C-FAST” aimed specifically for Australian councils and to be used by council engineers


Case studies

CSIRO been involved in a number of flood modelling studies

Today we will discuss:

• City of Port Phillip

• Shire of Murray

• City of Bunbury (current)

City of Port Phillip (CoPP)


Port Phillip Bay and CoPP

• Largest embayment in the world • CoPP is one of 10 cities around the bay • Legacy of engineering problems

CoPP


CoPP

Structural mitigation options: • Levees, basins and dams • Drainage networks

Elwood Swamp 1886

• 100,000 residents and over 22,000 businesses. • Population likely to double in next 15-20 years

due to development at Fishermans Bend


Project goals

Adaptation type (examples)

Mitigates what type(s) of flooding?

For what period?

At what cost?

Upstream retention/ detention & diversion

pipes ? ? ?

On-lot retention/detention

? ? ?

Offshore reef ? ? ?

Back valves ? ? ?

From a practical engineering or on-ground management perspective, want to understand what combinations of mitigation solutions work:


Model inputs

• Council wide terrain and bathymetry (LiDAR)

• Hydraulic and Hydrological (eg. drainage network)

• Storm surge with a peak of 1.3 m

• 1 in a 100 ARI rainfall for 3 hrs (also 1 in 5 and 1 in 10 ARI)

• Total simulation time = 24 hrs

• Rainfall starts at peak storm surge

• No upstream storm water reception (except in main drains)

• Sea level rise (SLR) = current day, 0.4, 0.8 and 1.1 m


• Extensive urban drainage network (~13k pipes) • Model effects of 1.3 m storm surge • Flow reversal back up the drainage network is critical feature to model for CoPP

0.1

m3/s

0

m3/s

Flo

w rate

N

Hydrodynamic Model City of Port Phillip


Flood visualisation – current day


Flood summary – current day

• Areas affected – Elwood, Middle Park, Beacon Cove, South Melbourne

• Flood type – mostly

nuisance (0 to 0.3 m for less than 1 hr) except small sections around Elwood and South Melbourne

Retention time


Flood visualisation – SLR 1.1 m


• Areas affected – Elwood, Middle Park, Beacon Cove, South Melbourne

• Flood type – severe flooding in all parts affected including very alarming long lived flooding (> 12 hrs+) around most parts of Elwood canal, South Melbourne and Middle Park

Retention time

Flood summary – current day


Mitigation options applied


Mitigation summary – SLR 0.0 m (current day)

• Focus area – Elwood canal • Mitigation achieved – Flooding at intersection of Barkly and Meredith street

completely stopped

Retention time

With mitigation Without mitigation

Shire of Murray (SoM)


SoM Project

• Murray (and Serpentine) river delta • Series of eight low lying islands and

coastal areas • At the confluence of two rivers (the

Murray and the Serpentine) where they flow into the Peel estuary in South Yunderup WA

• Areas of cultural, heritage and future infrastructure value

• A combination of several factors puts the delta at high risk, including • Sea level rise • Increased frequency and severity of

storm surges • Greater tidal influence and salinity • Poorly planned protection measures


SoM topography and bathymetry


SoM simulation cases

Case Sea level

(above 2014 AHD)

Simulation description

1 0.0 m Current day with storm surge 2 0.14 m 2030 with storm surge 3 0.4 m 2070 with storm surge 4 0.8 m 2100 with storm surge


SLR 0.0 m SLR 0.14 m

SLR 0.8 m SLR 0.4 m


Findings

• Heritage listed Mill on the western Tip of Culeenup

• SLR 0.0 m - only nuisance flooding

• SLR 0.14 m - significantly flooded - 0.6 m persisting for 8+ hours

• Money would be better spent protecting the Mill prior to investing in restoration

• Homes in the Murray Delta in the three inhabited islands

• SLR 0.0 m, 0.14 m - inundation minimal

• SLR 0.4 m - 40% homes inundated

• SLR 0.8 m - whole of Yunderup Island is inundated

• Mitigation options prove ineffective from a cost/benefit perspective due to the scale and extent of flooding. Might be some benefit in investigating basic mitigation such as a levee or barrier perhaps for Yunderup Island where the extent of flooding is less dramatic

City of Bunbury (CoB)


• City of Bunbury (CoB) is a developing low-lying coastal region especially susceptible to flooding from combined storm surge and rainfall events

• Areas around the Preston River and the Leschenault Inlet historically worst hit

• WA-NDRP project to study flooding for future planning needs and to assess mitigation options

• Investigation into extreme present and future (with SLR) flooding events and the impact of potential mitigations including

• Retention/detention schemes

• Changes to the drainage network – e.g. back valves, additional pipes

• Changes to pumping stations

• Sea walls

CoB – Flood mitigation analysis


Lidar, bathymetry, land usage Drainage network, pumps

Rainfall

Tides

Joint probability analysis

Statistical extreme tide & rainfall scenarios

Data collection Input data types


Scenarios to be considered

A. Extreme rainfall into 5 Mile Brook sub-catchment

B. Storm surge into northern Bunbury

C. Preston River response to heavy rainfall


A. Extreme rainfall Five Mile Brook sub-catchment

• Issue: • Extreme rainfall from new

Dalyellup development will drain into Five Mile Brook. Impact on Bunbury pipe network is unknown.

• Mitigation option • Prescribe maximum inflow

allowance


B. Storm surge Impact on northern Bunbury

• Issue: • Storm surge and SLR has the

capacity to overtop Koombana Drive and to flood Leschenault Inlet / northern Bunbury

• Mitigation options • New sea wall

• Permanently close storm surge barrier and lower water level of Leshenault Inlet


C. Preston river Response to heavy rainfall

• Issue • Low lying housing along the Preston

River are susceptible to flooding from extreme rainfall / levee bank failure

• Mitigations • More accurately define the

potential flood plain

• Improve levee banks

• Redirect river course into the Leschenault Inlet

Future directions


Swift

• CSIRO is developing a tool “C-FAST” based on Swift which can model urban flooding

• A number of case studies have been undertaken which have provided detailed requirements from a diverse range of councils across Australia

• In the next 18 months the intention is to deploy the tool for use internally by the early adopter councils we have already worked with

• Council engineers will be empowered to run new simulations with incrementally updated input data and to rapidly experiment with proposed mitigation options

www.csiro.au

Computational Modelling & Simulation Dr. Raymond Cohen Research Scientist

t +61 3 9545 8064 e [email protected] w www.csiro.au/data61

Thank you

integrated flood modelling and mitigation analysis -...

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