Transcript Breach growth - rapid and more complex models

WP4.4 A Rapid Embankment Breach
Assessment (AREBA)
Myron van Damme
Sept 2011
www.floodrisk.org.uk
EPSRC Grant: EP/FP202511/1
Goal of the work
WP4.4 (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.
Context – system risk needs
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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
• Failure
mechanism
for breach
depends
upon
loading and
soil properties
(erodibility,
cohesion)
loading and soil erodibility
• hydraulic
Surface erosion
•• Surface
Head cuterosion
Piping (internal erosion)...
•• Headcut
• Piping (internal erosion)
www.floodrisk.org.uk
EPSRC Grant: EP/FP202511/1
Creation of the simplified breach model
Study physics
Study HR Breach
Create datasets with HR Breach
Collect breach data
Derive simplified physical equations
Organize data into datasets
Estimate inaccuracies input parameters
Create the simplified model
Estimate inaccuracies datasets
Validate model
against datasets
Write up the
method behind
the
development of
the simplified
model.
yes
Are the model
predictions within
the uncertainty
band?
no
Check the code
Underlying assumptions (1)
 1D flow behaviour
 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))
Source: TAW (1999)
 Breach widening rate is a function of the
rate at which the crest lowers. (HR
BREACH, SIMBA) Mohamed (2002)
 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)
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)
Acknowledgement
The research reported in this presentation was conducted as part of the Flood Risk Management
Research Consortium with support from the:
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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
www.floodrisk.org.uk
EPSRC Grant: EP/FP202511/1