Flood Forecasting and a Case-Study of the Bristol Avon

Flood Forecasting and a case-study of the Bristol Avon

BHS SW Young Hydrologist’s Meeting 25 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

Flood Warning threshold Flow or level

Time Techniques: Rainfall-runoff models Techniques: Hydrodynamic models

Inflow 1

Inflow 2

Inflow 3

Forecast

Town 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

• Bath 3 • Bradford-on-Avon 27 Approximate number • Chippenham 3 of properties flooded • 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

Datum = 67.80m AOD

3.5

2.5

Stage (m) 1.5

1 Bye model Agency rating 0.5 Spot gaugings Halcrow ISIS Recommended rating

0 0 102030405060708090100 Discharge (m3/s) 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

Bristol Avon Flood Forecasting Model Catchment map showing location of separate models and level/flow stations

Brokenborough #$ Upper Avon Fosseway #$ #$ Crabb Mill Ladden Bows Rodbourne #$ $ Great Somerford Tubbs Bottom # #$ ## # # Matford Bridge # #$ # # $ #$ St artley # # Frampton Chipping Sodbury Dauntsey Avonmouth Cotterell ## Bristol Frome Frenchay ##

Netham Weir Stanley River Chippenham ## # ## #% Marden Lowe r Avon Bit ton ## #$ $ # # #$ Middlehill Calne Calstone Brislington As hton Vale #$ #$ Bat hford # % # # Bath Ultrasonic # Compton Dando Salt ford # %%# ## ##Pulteney Middle Avon Chew Magna Twerton Weir #$ Chew River Chew Bradf ord #$ #$ Midford St oke Chew Valley Lake ## on Avon $ ## # Midford Brook Sem ington Wellow Hallatrow ## ## Trowbridge #$ # Tellisford ## Midsomer Norton River Biss $ #$ # Hawkeridge

Somerset Frome Model Vallis Frome Upper Avon

# ## #

$ Middle Avon # ## Wallbridge Stoke Bottom Lower Avon Level/Flow station type #$ River Chew Holwell # Error correction Bristol Frome # Inflow Midford Brook #$ # Witham Friary Monitoring River Marden # Tidal River Biss # Structure Somerset Frome Model: Routing and hydrodynamic reaches

Bristol Avon Flood Forecasting Model Catchment map showing location of routing and hydrodynamic models and level/flow stations

Brokenborough #$ Upper Avon Fosseway #$ #$ Crabb Mill Ladden Bows Rodbourne #$ Tubbs Bottom #$ Great Somerford #$ ## ## # # Matford Bridge # #$ ## # $ #$ St artley # Frampton Chipping Sodbury Dauntsey Avonmouth Cotterell ## Bristol Frome Frenchay ##

Netham Weir Stanley River Chippenham # # ## #% Marden Lowe r Avon Bit ton ## #$ $ # # #$ Middlehill Calne Calstone Brislington Ashton Vale #$ #$ Bat hford # % # # Bat h U ltras onic # Compton Dando Salt ford # %%# ## ##Pulteney Middle Avon Chew Magna Twerton Weir #$ # River Chew Chew Bradf ord #$ #$ Midford St oke Chew Valley Lake ## on Avon ## #$ Midford Brook Semington Wellow Hallatrow ## ## Trowbridge #$ Tellisford ## Midsomer Norton River Biss $ #$ # Hawkeridge

Somerset Frome Vallis Frome

# ## #

$ # ## Wallbridge Stoke Bottom

#$ Level/Flow station type Holwell # Error correction Model type

#$ # # Inflow Routing Witham Friary # Monitoring Hydrodynamic # Tidal # Structure 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 Flows at Great Somerford - January 2003

100 60 Observed (revised Observed (revised rating) 50 rating) ) 80 ) Modelled (improved Modelled 40 60 floodplain representation) 30 40 20 Flow (cumecs Flow Flow (cumecs Flow 20 10 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 140 Time (hours) Time (hours)

Flows at Great Somerford - December 2000 60 Observed (revised rating) 50

) Modelled

30 Flow (cumecs Flow 20

10 0 20 40 60 80 100 120 Time (hours) 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?