Flood Forecasting and a case-study of the Bristol Avon

BHS SW Young Hydrologist’s Meeting25 October 2006

Andy Barnes, Halcrow

Presentation Structure

• Flood Forecasting techniques• The Bristol Avon• Particular challenges• Model set-up• Inundation maps

Flood Forecasting techniques

• Flood Intelligence Files (FIF’s)• Level-to-level correlations• Rainfall vs response analysis• Rainfall-runoff models• Hydrodynamic forecasting models

Techniques: Flood Intelligence Files

Based on analysis of past events, including:• Peak levels/flows• Event rainfall information• Rate of rise & time-to-peak data• Typical response hydrographs

Techniques: Level-to-level correlations

Site A

Site B

Peak level at Site B = f (Peak level at Site A)

Techniques: Rainfall vs response analysis

Time

Flowor

level

T

Flood Warning threshold

Techniques: Rainfall-runoff models

Techniques: Hydrodynamic models

Inflow 1

Inflow 2

Inflow 3

Town

Forecast

BA catchment: Location

BA catchment: Main tributaries

BA catchment: Topography

BA catchment: Hydrology - SAAR

BA catchment: Main flood risk areas

BA catchment: October 2000 flood event

Approximate numberof properties flooded

• Bath 3

• Bradford-on-Avon 27

• Chippenham 3

• Malmesbury 22

• Trowbridge 1

BA catchment: July 1968 flood event

Pensford, River Chew

Challenges: Information

Challenges of flood forecasting model:

• Pilot study for hydrodynamic forecasting model

• Large catchment• 9 models• Tidal / fluvial interaction

And……………

Challenges: Rating reviews

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Discharge (m3/s)

Stag

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Bye modelAgency ratingSpot gaugingsHalcrow ISISRecommended rating

Datum = 67.80m AOD

Challenges: Structures

Challenges: Netham Weir

Challenges: Inundation maps

Challenges: Upstream storage

Significant upstream storage in some catchments

e.g. Chew Valley Lake

Model: Separate models

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Bitton

Dauntsey

Calstone

Startley

Fosseway

Hallatrow

Rodbourne

Semington

Chew Magna

Hawkeridge

Crabb Mill

MiddlehillAshton Vale Brislington

Ladden Bows

Stoke Bottom

Witham Friary

Matford Bridge

Midsomer Norton

Chipping Sodbury

Chew Valley Lake

Holwell

ChewStoke

Brokenborough

Frome

Calne

Vallis

Wellow

Midford

Stanley

Twerton

Bathford

Frenchay

Saltford

Avonmouth

Tellisford

Trowbridge

Wallbridge

ChippenhamNetham Weir

Tubbs Bottom

Compton Dando PulteneyWeir

Great Somerford

Bradfordon Avon

FramptonCotterell

Bath Ultrasonic

Middle Avon

Upper Avon

Lower Avon

River Chew

Midford Brook

Somerset Frome

Bristol Frome

River Biss

RiverMarden

Bristol Avon Flood Forecasting ModelCatchment map showing location ofseparate models and level/flow stations

# Error correction# Inflow# Monitoring

# Structure# Tidal

Level/Flow station type

Somerset FromeRiver BissRiver Marden

Upper AvonMiddle Avon

Midford BrookBristol FromeRiver ChewLower Avon

Model

Model: Routing and hydrodynamic reaches

# Error correction# Inflow# Monitoring

# Structure# Tidal

Level/Flow station type

RoutingHydrodynamic

Model type

Bristol Avon Flood Forecasting ModelCatchment map showing location of routing andhydrodynamic models and level/flow stations

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Bitton

Dauntsey

Calstone

Startley

Fosseway

Hallatrow

Rodbourne

Semington

Chew Magna

Hawkeridge

Crabb Mill

MiddlehillAshton Vale Brislington

Ladden Bows

Stoke Bottom

Witham Friary

Matford Bridge

Midsomer Norton

Chipping Sodbury

Chew Valley Lake

Holwell

ChewStoke

Brokenborough

Frome

Calne

Vallis

Wellow

Midford

Stanley

Twerton

Bathford

Frenchay

Saltford

Avonmouth

Tellisford

Trowbridge

Wallbridge

ChippenhamNetham Weir

Tubbs Bottom

Compton DandoPulteneyWeir

Great Somerford

Bradfordon Avon

FramptonCotterell

Bath Ultrasonic

Middle Avon

Upper Avon

Lower Avon

River Chew

Midford Brook

Somerset Frome

Bristol Frome

River Biss

RiverMarden

Model: Conversion of existing models

• Existing models used where possible• HecRas to ISIS• Simplifications• Run-time & stability improvements

Model: Other info

• Gate control rules• Cross-section extension• Ungauged tributaries

Model: Calibration and verification

Flows at Great Somerford - October 2000

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Flow

(cum

ecs)

Observed (revisedrating)

Modelled (improvedfloodplainrepresentation)

Flows at Great Somerford - January 2003

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Flows at Great Somerford - December 2000

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Flow

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ecs)

Observed (revisedrating)Modelled

Model: NFFS

• NFFS ‘forecasting platform’• Delft• National consistency

Maps: MIP areas

Maps: Information

• Scenarios• Return period estimation• DeltaMapper• LiDAR data

Maps: Example output

Maps: Real-time inundation maps

Questions……

Any questions?