www.floodrisk.org.uk epsrc grant: ep/fp202511/1 predicting breach formation mark morris hr...

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

Predicting Breach Formation

Mark MorrisHR Wallingford

The Science of Asset ManagementLondon – 9th December 2011

“An overview of current options, practice and future direction for predicting breach formation”

1. What are we talking about?

2. Current options / Latest developments

3. Practice

4. Future direction

5. Research gaps

[1] What are we talking about?

What are we talking about?

What are we talking about?

Why do we need to predict breach?

The way and rate at which an embankment breaches can affect the timing of the breach, the rate and magnitude of the flood water released and the size of the breach itself.

Therefore, breach affects the analysis of flood risk (ie. FRAs) and can change the way in which flood events might be managed.

Understanding the degree of uncertainty within the process and any prediction is a very important aspect of using breach predictions

Why do we need to predict breach?

Consider that different aspects of breach prediction are important for different users:

Flood risk assessmentPlanning

Scheme design

Emergency planning

Flood event management

Emergency repairs

Peak discharge? Flood volume? Rate of flooding?

Time to catastrophic failure? Size of breach?

Different types of breach processes

The stages in breach development comprise:• Initiation

– Surface protection (grass) cover fails, soil starts to erode

• Formation – Significant erosion of material through embankment body and

down to base

• Growth (widening)– Open breach; flow continues to widen the breach

Can be driven by wave overtopping, water overflowing or seepage through

Different types of breach processes

Soil type and state affects the soil erodibility

Soil erodibility affects the overall physical process

...which (for external erosion) may be headcut erosion or surface erosion (or a combination / transition between the two).

Typically:• Sandy, non cohesive soil = surface erosion• Cohesive, clayey soil = headcut

[not forgetting that soil state significantly affects the process...]

Different breach processes

Different breach processes

Photos: Greg Hanson - USDA-ARS

What’s the point here?

1. Rate of flow through a breach is controlled by the upstream crest level– Breach formation occurs when this control erodes since flow

can then increase, so increasing erosion– The timing of crest erosion for headcut or surface erosion

varies– Hence the nature of flood hydrograph depends upon the form

of erosion, hence the soil erodibility, hence soil type and state

What’s the point here?

2. The nature of the flood hydrograph also depends upon the reservoir stage/volume relationship and the soil erodibility– Assuming the crest erodes (surface erosion)– Hydrograph shape = function of rate crest erosion and

reservoir surface area• Can the reservoir level drop to match the rate of crest

erosion? Effect of Soil Erodibility on Breach - Outflow

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (Seconds)

Flo

w (

m3

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M1 - Kd=0.2M1 - Kd=18M1 - Kd=0.9M1 - Kd=35M1 - Kd=3.5M1 - Kd=88M1 - Kd=17.7M1 - Kd=177

[2a] Current options for predicting breach

Different methods include:

1.Judgement (guess work?)

2.Simple predictive equations

3.Simple predictive models

4.More complex predictive models

5.Integrated breach and flow models

6.Probabilistic breach models

Current options for predicting breach

1. Judgement (guess work?)– Can vary from reasonable judgement to a blind guess hence

can be highly inaccurate– Use of historic records from same catchment / soil / structure

type helps

2. Simple predictive equations– Typically developed for dams or non cohesive soils– Regression analysis on limited data (and hence specific

conditions)– Can have large uncertainties– Typically give peak discharge, maybe breach width – not

hydrograph

Current options for predicting breach

3. Simple predictive models– Predefines and simplifies the breach process

• Is the simplification acceptable?

4. More complex predictive models– Greater complexity – less uncertainty – typically a slower model

5. Integrated breach and flow models– Allows for drowning of breach – important but don’t overlook

what the model actually does

6. Probabilistic breach models– Gives more information regarding range of possible behaviour

[2b] Some recent developments in predicting breach

i. Advances in rapid simulation – FRMRC2

ii. Conclusions from the DSIG breach modelling project

iii. Advances in more detailed modelling – HR BREACH

iv. Advances in understanding soil erodibility

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.

Researcher = Myron van Damme – University of Oxford

[i] Advances in rapid simulation: FRMRC2 WP4.4 research

Goal of the research

In summary...

Myron has:

1.Reviewed breaching processes and models

2.Chosen to develop simple, physically based equations– NOT regression analysis; Not emulating other models

3.Equations simulate average erosion rates for the different types of breach process (surface / headcut / internal erosion)

4.Assumes predefined physical processes for breach

5.Validated the model [AREBA] using HR BREACH and DSIG case study data [ builds upon international best]

General assumptions

1D flow behaviour

Rectangular spatially constant cross section breach

No equilibrium transport conditions

Constant soil erodibility

Instantaneous failure grass cover

No erosion below the foundation level of the embankment

Widening rate is proportional to the downward erosion rate

Underlying assumptions (1/4)

Assumptions surface erosion

Landside slope remains equal to the initial slope gradient

Depth along landside slope approaches the normal depth starting from the critical depth

Landside slope retreats at a spatially averaged erosion rate

Crest erodes downward while landside slope retreats

Underlying assumptions (2/4)

Assumptions headcut erosion

No downward erosion due to flow over the crest

Headcut starts at the top of landside slope

Underlying assumptions (3/4)

Assumptions piping

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

The grass cover is assumed to have failed with failure of the pipe

Underlying assumptions (4/4)

• AREBA - A Rapid Embankment Breach Assessment• AREBA gives promising results when being bench marked

against HR BREACH• Validating AREBA against the DSIG data showed that the model

prediction lie within the bounds of uncertainty following from the uncertainty in the input parameters

• Run speed AREBA is approximately 0.2s per run.

Performance and validation

• AREBA predicts breach through simple, homogeneous embankments– Does not simulate composite or complex structures– But speed of simulation does allow the user to ‘play’

• AREBA requires definition of:– Upstream load (time varying water level)– Embankment geometry– Embankment soil erodibility, Kd– Assumed failure mode (surface, headcut, internal erosion)– Downstream volume / stage (time varying water level)

Data Requirements / Applicability

• International project – dam owners – industry needs focus• Breach model review 3 selected for evaluation (as greatest

potential for industry uptake) = SIMBA / HR BREACH / FIREBIRD• Data review to assist model performance validation

– USDA ARS data / IMPACT project data / 2 dam failures

• FIREBIRD eventually rejected due to lack of usability and hence testing

[ii] Conclusions from the DSIG breach modelling project

• SIMBA = Headcut; HR BREACH = surface erosion• Performance of both models against the various test cases

varied, but neither was consistently better or worse• Conclusions:

– Still to be formally reported by PM (Tony Wahl – USBR) ( not formal conclusions)

– Develop an industry model using both headcut and surface erosion processes (ie SIMBA + HR BREACH)

– Modeller understanding and ability with the models is important• For example, models simulate breach through simple structures; expertise is needed

to know how to apply the models to more complex, real structures

– Available data to support research and model validation is limited– Uncertainty / lack of knowledge in soil erosion processes / simulation

DSIG Conclusions (Cont.)

[iii] Advances – HR BREACH model

• HR Wallingford programme of research over past decade– National / EU / International projects plus company R&D

HR BREACH model – Mohamed (2002); Integrated InfoWorks (Wallingford Software

Innovyze); Morris (2011)

• 1D flow; sections through embankment; 2D section erosion plus block failure

• Focus here on one area of advance – breach formation through zoned flood embankments / embankment dams

Trying to predict breach through real rather than idealised structures

How significant is this for flood risk assessment and hence asset management practice?

Zoned embankments

Representing zones in the model

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Effect of Kd versus As• Soil erodibility versus reservoir stage area

Homogeneous embankment breach – two different erodibilities

Peaky

Hydrograph

Flat

Hydrograph

Effect of zoned embankment (2 layers)• 2 flat layers – different erodibility (also fn of reservoir area)• Extremes show peaky versus flat hydrograph (trends in-between)

Homogeneous embankment breach – two layers with different erodibilities

Erodible top layer

Resistant top layer

Effect of layers on outflow hydrograph• 2 layers – different erodibility

Advances – HR BREACH model

Conclusions:• The combined effects of zoning and soil erodibility

can significantly affect the breach flood hydrograph• Layering can provide significant degrees of protection

against more erodible material beneath– Ie. the Dutch / German embankment design

• Erodibility plus reservoir area affects flood hydrograph

• Each of these points is significant for design / construction / maintenance / risk assessment

Greg Hanson

[iv] Advances in understanding soil erodibility

[4] Industry Practice

Industry practice: • Often guesswork / judgement or simple equations

Why?• Confusion over choice of best (appropriate) method?• Avoid the cost

– By avoiding perceived complexity of analysis– By avoiding purchase of software

• Blame uncertainty

Systems analysis:• Risk models run many thousands of simulations need

simple and / or fast methods of prediction

[5] Future Direction1. Industry versus academic solutions – the challenges are

different– Fast – simple Complex – slow(er)?

2. Need to integrate flow, with soil erosion, with structure response / behaviour– Complex / significant interdependencies– Can reduce the uncertainty from flow by moving to a 2D, 3D flow

model (using existing models and improved computer power)– Need to keep a balance in terms of analysis accuracy & relevance

for different processes

3. Need to improve understanding of soil erodibility

[5] Future Direction4. Need to improve understanding of natural and man

made variability in soils / structures in order to assess likely erosion / failure processes

5. Different soils erode by head cut or surface erosion – currently as separate models merge into a single model with analysis sufficient to determine and flip between processes

[5] Future Direction

5. Putting model speed aside, use more accurate flow models combined with a better analysis of soil behaviour... merge a soils analysis model (ie. Levee stability analysis) with a breach model to provide the complete assessment of levee response and failure

6. Given the uncertainty in soil parameters / distribution, consider how this may be done probabilistically as well as deterministically

[6] Research Gaps

(i) Soil Erodibility:

1. Understand which parameters affect erodibility (close)

2. Understand how those parameters vary in-situ and under climate change conditions

3. Link parameters to field observations for asset management

4. Link parameters to design and construction specifications

[6] Research Gaps

(ii) Breach model:

1. Meshing headcut and surface erosion, with analysis to flip between processes

2. Meshing ‘geotechnical analysis’ model with a ‘breach’ model

3. Dealing with uncertainty distribution?

[6] Research Gaps

(iii) Data:

We always seem to be chasing better data!

1. Despite the frequency of dam and levee failure there remains very little high quality, field scale data against which the different processes can be studied and models validated. Solution?

2. Soil erodibility data – If we accept process dependence upon soil erodibility, and we build models using measures of erodibility, how can we provide best guidance on value selection?

The EU FP6 Integrated Project – FLOODsite - Tasks 4 & 6:

www.floodsite.net/html/work_programme2.asp?taskID=4

www.floodsite.net/html/work_programme2.asp?taskID=6

The CEATI Dam Safety Interest Group breach modelling project:

www.ceati.com/collaborative-programs/generation/dam-safety

The FRMRC II project website: www.floodrisk.org.uk

Contact: Mark Morris mark.morris@samui.co.uk

More information on breach? Website information