www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1

WP4.4 A Rapid Embankment Breach Assessment (AREBA)

Myron van DammeSept 2011

Goal of the workWP4.4 (University of Oxford and HR Wallingford): Rapid breach assessment - will develop simplified equations for the rapid prediction of breach size for a limited range of embankment structures. The methods will be directly applicable to practicing engineers and will replace the default and very approximate breach modelling methods currently included within the RASP family of tools.

www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1

Context – system risk needs• System risk models simulate flood risk

from many combinations of potential defence failures, inundation etc.

• Assumptions for breach are currently crude.

• Hence, the need for a more accurate, fast breach model – simplified breach

www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1

Context – Physical processes

• Failure mechanism for breach depends upon hydraulic loading and soil erodibility

• Surface erosion• Headcut• Piping (internal erosion)

• Failure mechanism for breach depends upon hydraulic loading and soil properties (erodibility, cohesion)

• Surface erosion• Head cut• Piping (internal erosion)...

Study physics Study HR Breach

Creation of the simplified breach model

Create datasets with HR Breach

Derive simplified physical equations

Estimate inaccuracies input parameters

Collect breach data

Organize data into datasets

Validate model against datasets

Estimate inaccuracies datasets Create the simplified model

Are the model predictions within

the uncertainty band?

yes

no

Check the code

Write up the method behind the development of the simplified model.

Underlying assumptions (1) 1D flow behaviour

No equilibrium transport conditions (HR BREACH) Instantaneous failure grass cover (HR BREACH) Soil erodibility is equal along the embankment and constant in time (BRES, HR BREACH)

Source: TAW (1999)

Source: TAW (1999)

Downstream slope retreats through the embankment before widening (surface erosion) f.e. BRES model (Visser 1997)

No lowering of the crest level before the headcut has reached the upstream slope. (SIMBA) (Temple & Hanson (2005))

Breach widening rate is a function of the rate at which the crest lowers. (HR BREACH, SIMBA) Mohamed (2002)

Underlying assumptions (2) Slope gradient of the inland slope is limited (BRES) Fixed side slope assumptions (BRES, SIMBA) Headcut starts at the top of the inland slope (SIMBA) No erosion below the foundation of the embankment (HR BREACH) An initial pipe diameter widens in an equal rate due to the flow through the pipe

After slumping of the soil above the pipe, potential further failure of the embankment is described by the surface erosion failure process

Global description: surface erosion

Global description Headcut erosion

Performance and validation

• AREBA gives promising results when being bench marked against HR BREACH

• Validating AREBA against the IMPACT experiments showed that the model prediction lie within the bounds of uncertainty following from the uncertainty in input parameters

• Run speed AREBA is approximately 0.2s per run.

Conclusions• A fast running simplified breach model has been developed suitable for the use

in system risk models that deals with grass protection failure, surface erosion failure, head cut failure, and piping.

• The model is able to predict a flood hydrograph that falls within the bounds of uncertainty given by the input parameters.

• A comparison between the outcomes for system risk analysis using the current method applied in RASP versus this new approach is currently underway.

• Deliverable = report detailing the modelling methodology (Jan 2012)

www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1

AcknowledgementThe research reported in this presentation was conducted as part of the Flood Risk Management Research Consortium with support from the:

• Engineering and Physical Sciences Research Council • Department of Environment, Food and Rural Affairs/Environment Agency Joint

Research Programme • United Kingdom Water Industry Research• Office of Public Works Dublin• Northern Ireland Rivers Agency

Data were provided by the EA and the Ordnance Survey.Thank you to:

Mark Morris, Alistair Borthwick for their ongoing supervision. Mohamed Achmed Ali Mohamed Hassan for his assistance