Improving the accuracy of breach modelling: why are we not progressing faster?

Improving theaccuracyof breachmodelling:whyarewenot progressing faster?Mark Morris1, Greg Hanson2 and Mohamed Hassan1

1 HR Wallingford Ltd, Wallingford, Oxfordshire, UK

2 Hydraulic Engineering Research Unit, ARS-USDA, Stillwater, OK, USA

Correspondence:

Mark Morris, HR Wallingford Ltd, Howbery

Park, Wallingford, Oxfordshire, OX10 8BA,

UK

Email: [email protected]

DOI:10.1111/j.1753-318X.2008.00017.x

Key words

Breach; breach formation; breach initiation;

breach modelling; breach research;

embankments; erodibility; moisture content;

research.

Abstract

Flood risk assessment and management often requires the prediction of potential

breaching of a flood defence embankment or dam in order to either assess

potential impacts or provide information to assist emergency planning, evacua-

tion, repair strategies and improve alternative future design strategies. There are

many different aspects of the overall breaching process, which are more, or less,

relevant to the wide range of potential end users of such information. Conse-

quently, the prediction of breach growth is an area where research has been

undertaken for many decades in an attempt to provide more reliable models and

predictions. However, despite many initiatives providing observations and recom-

mendations as to processes observed and how research might progress, more

detailed literature searches will often uncover conclusions and observations noted

a decade or two or three earlier that are similar to those being made today. In

particular, observations relating to material type, state (such as water content and

compaction) and properties are relevant here. This prompts the obvious question

as to why our ability to predict breach initiation and growth has not progressed

further over this period. Why are so many studies identifying similar issues and, in

effect, ‘reinventing the wheel’? With a programme of research into breach

initiation and growth under the EC FLOODsite Project and continued pressure

to improve tools and techniques following events such as those seen at New

Orleans in August 2005, this paper considers this question of apparent slow

progress and offers some suggestions as to why this may have occurred and what

direction might prove more effective in the future.

Introduction

Over the last 5–10 years, there has been a clear shift in the

management of flood events and flood defence assets

towards a system wide, risk-based approach. This approach

allows the performance of a flood defence system as a whole

to be considered and for the most effective measures to be

implemented in terms of overall flood risk. This may also be

undertaken within a wider integrated flood risk manage-

ment framework, allowing other uses and functions of the

flood plain and defences to be considered in the optimisa-

tion process. The overall analysis allows the optimisation of

limited resources in the construction, operation and main-

tenance of flood defence assets, which can also include dams

and reservoirs.

Flood risk assessment requires the consideration of hy-

draulic loading, defence asset performance, flood inunda-

tion and impact on ‘receptors’, such as people, property and

the environment. Figure 1 (upper) shows the source–

pathway–receptor model concept. Figure 1 (lower) shows

how probabilistic distributions of hydraulic loading, asset

performance and impact (flood inundation and damage)

may be used to assess flood risk at a system scale. Predicting

the breach initiation and growth process allows the pathway

component, representing asset performance, to be derived.

This performance curve can also be known as a fragility

curve.

However, optimisation of the systems and an accurate

assessment of risk including inundation requires the predic-

tion of a flood hydrograph resulting from a breach – i.e. a

reliable predictive breach model. However, traditional ana-

lysis of flood embankments and current system risk models

have typically focussed upon possible embankment failure

modes rather than the initiation and growth processes for

J Flood Risk Management 1 (2008) 150–161c� 2008 The AuthorsJournal Compilation c� 2008 Blackwell Publishing Ltd

breach. Failure mode analysis might include, for example,

an assessment of embankment stability (slip circle analysis),

susceptibility to piping or likely performance of grass or rip-

rap stone protection. Recent research under the FLOODsite

project has resulted in the first edition of a ‘failure modes

report’ (Allsop et al., 2007) detailing flood defence asset

failure modes for a wide range of structures and loading

conditions.

While these failure modes may be considered within a

fault tree analysis to give a prediction of the likelihood of

(partial or total) embankment failure through these pro-

cesses, these analyses do not provide a prediction of the

time-varying development of breach and hence the volume

and rate of release of flood water that might occur during

the breach. The failure mode analysis often assumes that

catastrophic embankment failure occurs as a result of one or

more specific failure modes taking place, which is not

necessarily the case. For example, failure of surface vegeta-

tion or rip-rap, or occurrence of a slip, does not necessarily

lead to complete or even partial breaching of the embank-

ment. The development of predictive breach models pro-

vides ‘missing’ information that allows a more reliable flood

risk assessment to be made. Integration of process knowl-

edge from the predictive breach models into embankment

performance or fragility curves is necessary to ensure that

maximum benefit is gained for the current approach to

system modelling for flood risk.

Flood risk management also requires information about

potential breach development to assist with, for example,

emergency planning, evacuation or repair strategies. Specific

questions that are likely to be asked include:

� Where will a breach most likely occur? How can I prevent

this?

� If the embankment shows signs of distress, how long will

it be before the embankment fails?

� If the embankment starts to fail, how can I stop it? What is

the best method? How large will the breach become? How

can this be repaired?

� If the embankment fails, what will happen in terms of

flooding? How quickly will the failure occur? How quickly

will flood water spread and to what extent? How will this

affect access within the area?

� Can I even optimise failure? Are there better locations for

failure than others? Can I design my system to take

advantage of these considerations?

It can be seen that there are many different aspects of the

overall breaching process that are relevant to a wide range of

potential end users. Consequently, the prediction of breach

growth is an area where research has been undertaken for

many decades in an attempt to provide more reliable models.

However, despite many initiatives providing observations

and recommendations as to processes observed and how

research might progress, more detailed literature searches

will often uncover similar conclusions noted a decade or two

or three earlier. Bossut and Viallet (1764) even give descrip-

tions of how soil type affects embankment performance and

how care should be taken in selecting and placing soils,

which would not be out of place today! This prompts the

obvious question as to why our ability to predict breach

initiation and growth has not progressed further over this

period. Why are so many studies identifying similar issues

and, in effect, ‘reinventing the wheel’?

Under the EC FLOODsite Project (http://www.floodsite.net),

there is a programme of research into breach initiation and

growth. In addition, following the catastrophic events that

occurred in New Orleans in August 2005, there has been

pressure within the United States to ensure that flood risk

assessment and management procedures are state of the art.

Among many other actions, this prompted a workshop

aimed at facilitating links between European and US

Pathway(e.g. beach, defence and floodplain)

Source(River or sea)

Receptor(e.g. people in the floodplain)

Load Load

f(load exceede)P (fail)

Flooddepth Total damage (£k)

P(depth exceeded) P(damage exceeded)

Figure 1 Source, pathway, receptor model for considering the flood risk process and system modelling (adapted from Sayers et al., 2002).

J Flood Risk Management 1 (2008) 150–161 c� 2008 The AuthorsJournal Compilation c� 2008 Blackwell Publishing Ltd

151Improving the accuracy of breach modelling

research and practice, held in Budapest 7–9 November 2006

(International Workshop: Research Perspectives on Flood-

ing and Flood Risk Management in United States and

Europe). These events have helped raise the question as to

how best to progress our ability to reliably predict breach

growth.

This paper first provides an overview of breaching

processes to give clarification on the complexity of the

subject. The authors then consider why progress has not

been faster over the last five decades and subsequently offer

suggestions as to how we might progress faster in the future.

How complicated is the process?

Many different researchers have analysed the breach pro-

blem and tried to define distinct stages in breach develop-

ment. Whether quoted as stages or phases of development,

none of these definitions have taken on universal use,

probably due to disagreement and confusion arising from

the differences in process that occurs through breaching of

different material types and structures. By considering

breach first in relation to the shape of the outflow hydro-

graph and secondly by breach formation mechanism, as

determined by majority material type and state, and then a

logical structure for classifying stage and type of breach

development can be established. This form of describing

recognised physical processes should help to reduce confu-

sion when describing different breach processes within a

range of embankment materials and states.

Generic breach outflow hydrograph

Figure 2 shows the shape of a breach flow hydrograph that

might occur for a breach forming through a flood embank-

ment or embankment dam. In practice, one of the factors

that determines the shape and duration of the hydrograph is

the type of hydraulic loading (i.e. the volume of water

retained behind the defence and the variation in loading

such as storm, tides, etc.). However, the features demon-

strated in this example will be common to all breach

hydrographs to varying degrees. A series of time markers

indicates different stages of breach activity.

The different stages of breach activity (in relation to the

time markers shown on Figure 2) may be broadly sum-

marised as follows:

T0–T1 Apparently stable – no breach initiation.

T1 Start of breach initiation. Seepage through, or flow

over, the embankment initiates the breach. Cata-

strophic failure can be prevented.

T1–T2 Progression of breach initiation. The breach flow

increases slowly through increased loading and/or

progressive removal of material. Flow is typically

small and the rate of change is slow. The time

period may be minutes, hours, days or months.

Catastrophic failure can be stopped.

T2–T3 Transition to breach formation. A critical stage

where steady (and relatively slow) erosion cuts

through to the upstream face of the embankment

initiating relatively rapid and often unstoppable

breach formation (but noting that certain scenarios

of reservoir storage and embankment erosion

resistance can result in erosion stopping short of

complete breach formation – see Photo 1).

T3–T5 Completion of breach formation. Rapid erosion of

the embankment vertically; continued erosion of

the embankment vertically and laterally. Extent

and rate is dependent upon the volume of available

flood water and the design and condition of the

embankment.

T4 The peak discharge, Qp, is a function of a number

of factors including available flood water and

embankment design and condition.

T4T5 Continued breach growth. After erosion of the

embankment vertically to (or beyond) the em-

bankment bed, a continued supply of flood water

will continue erosion (widening) of the breach

laterally.

Figure 2 Generic breach outflow hydrograph. Photo 1 Partial breach – not enough water to get the job done!


152 Morris et al.

In a tidal situation the progression from stage to stage

may take more than one tidal cycle. The advantage of time

available between cycles to undertake emergency repairs and

to restrict further breach progression is offset by an infinite

tidal volume that will continue to drive the breach progres-

sion if left unchecked.

Breach initiation

There are many different factors that can influence and lead

to breach initiation. These can include, for example, wave

overtopping, overflow and seepage, leading to surface ero-

sion of vegetation, other surface protection measures or the

embankment soil. These are important aspects of the overall

breaching process, but are not considered further in detail

here.

Basic breach formation mechanisms

The breaching process is complex, but due as much to the

wide range of materials, conditions, designs and hydraulic

loading as to variations in specific physical processes. By

understanding how some basic growth mechanisms work

for different materials and states, behaviour under varying

loading conditions may be deduced.

At the simplest level and from a material type perspective,

there appears to be three broad categories of material that

trigger differences in the breach growth process. These are

rock fill, noncohesive and cohesive materials, which are

considered below (see Figure 3). The exact division between

these types is indistinct, particularly the cohesive–noncohe-

sive interface, which will be affected by soil suction pressures

giving apparent cohesive behaviour within classically de-

fined noncohesive materials.

Within the rock fill material range, it can be imagined

that the particle sizes become so large that the interlocking

nature of the rock begins to have a significant effect upon the

process of erosion. This might delay the onset of initial

erosion but may subsequently speed the removal of material

once motion is initiated. The focus of discussion within this

paper addresses breaching processes within cohesive and

noncohesive materials rather than breaching through rock

fill material.

Within the noncohesive material range, breach behaviour

suggests a progressive surface type of erosion where material

is eroded progressively. From the point of overflow across an

embankment, for example, material tends to be eroded from

the embankment surface between the crest and the toe. This

cuts a relatively narrow channel into the embankment, with

widening of this channel both by surface erosion and by

discreet failure of the breach sides through undercutting

(Mohamed, 2002). This relates to Stage T1–T2 on Figure 2.

As soon as erosion of the upstream face/crest interface

occurs, then flow increases rapidly leading to complete

breaching (Stage T2–T3 and Figure 2). While the material

may be noncohesive in nature, moisture content within the

material will result in internal suction pressures that will

allow steeper material slopes than would otherwise be

expected. This typically results in vertical sides to the

eroding breach rather than trapezoidal (Mohamed, 2002;

Pickert et al., 2004), as can be seen in Photo 2. Vegetation

and scale may also play a role in the observed erosion

mechanism. Vegetation may influence surface conditions at

certain scales enough to cause discontinuous erosion (i.e.

head cut formation and migration) to occur.

Within the cohesive range, breach behaviour suggests a

head cutting type of erosion whereby a number of discreet

steps are eroded on the downstream embankment face

(Stage T1–T2, Figure 2). The initial location of these steps is

triggered by weaknesses in the soil surface or surface

protection and may also be exacerbated by the layering of

?

? ?

?

SILT SAND GRAVEL ROCKCLAY

Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse CO

BB

LES

2 6 20 60 0.2 0.6 2 6 20 60 200

µm <? ?> mm [British Soil Classification System (BS 5930: 1981)]Cohesive ~ ~ ~ ~ Non cohesive Rock fill

Interlocking

Progressive Surface Erosion

Head Cutting

Figure 3 Broad division of breach behaviour by material type.



soil placed during construction of an embankment. The

head cut steps tend to migrate upstream and progressively

merge into fewer, but larger steps (Hanson et al., 2007). The

net effect is a steepening of the downstream slope. This

process cuts a relatively narrow channel into the embank-

ment, with widening of this channel by discreet failure of the

breach sides through undercutting. As soon as erosion of the

upstream face/crest interface occurs, then flow increases

rapidly leading to complete breaching (Stage T2–T3 and

Figure 2) (Photo 3).

Typical embankment material

Because embankment failure modes and breach growth

behaviour is significantly affected by embankment material

type, it is important to know what an embankment is

constructed from. However, many embankments were in-

itially constructed decades or even centuries ago, and have

often been modified and raised. In the absence of construc-

tion records, it is often useful to look at the local geology,

because embankments are typically constructed from locally

available material rather than the theoretically ideal material

and the cost of transporting material from a distance will be

balanced against the performance of different locally avail-

able materials. The result is that many embankments are

often constructed from local material and hence the local

geology can give an indication of likely material, related

processes and problems. Considering an example distribu-

tion of materials along a river network from mountain to

the sea also gives an indication of likely variations in local

material types (Figure 4).

Why are we not progressing faster?

Because engineers and researchers alike have tried to develop

reliable breach prediction methods for many decades, one

questions why our current ability to predict breaching

processes remains quite ‘crude’. Research under the Eur-

opean IMPACT Project (Morris, 2005; Morris and Hassan,

2005a) suggested that breach modelling accuracy could

perhaps predict the peak discharge arising from a breach to

� 30%, with the accuracy of predicting other parameters

such as timing and breach dimensions being considerably

worse. More recent research under projects such as the Dam

Safety Interest Group breach modelling project (Courivaud,

2007; Kahawita, 2007; Wahl, 2007) and the European

FLOODsite project (Morris, 2008a; Morris et al., 2008)

provide state-of-the-art reviews and highlight some signifi-

cant advances over the last 5 years.

Nevertheless, when considering progress over the past few

decades, advances have, on the whole, been slow. Possible

reasons for a lack of significant and rapid progress may be

divided broadly into two categories; firstly a series of more

general observations and factors and secondly specific

factors relating to the influence that material properties

and condition have on the breaching process.

General factors influencing progression ofbreach modelling capability

While similar observations relating to beach growth pro-

cesses may be seen in selected papers dating back over the

last six decades (White and Gayed, 1948; Ralston, 1987;

Wahl, 1997), there have also been significant changes in

Photo 2 Breach formation through a non cohesive material. showing

vertical breach sides (IMPACT Project field test).

Photo 3 Breach formation through a cohesive material (Stage T1–T2,

Figure 2) (IMPACT Project field test #1).


154 Morris et al.

science and technology affecting, and hopefully improving,

our ability to model or predict breach growth. Two key

factors are firstly the development of concepts, tools and

techniques in the field of geotechnical engineering and

secondly the development of computers to aid predictive

modelling. It is clear that both of these areas play an integral

part in our present ability to predict breach growth and that

any advances or limitations in these areas will have influ-

enced the rate at which we could refine and improve our

prediction techniques. However, improving capabilities

within both of these disciplines are not similarly reflected

in breach modelling progression over the last 10–20 years;

other factors must be dominant here.

A factor that should not be dismissed is confusion

propagated by research reported as offering a ‘generic’

solution to breach but in reality being quite dependent on

the specific test cases used for the research. It has been

shown above that the overall breaching process is quite

complex, with a number of different stages and processes

that are dependent on the type of embankment design,

material and construction, and which are more or less

relevant to different types of end user. Key differences

between breach initiation and growth processes related to

material type (primarily rock fill, noncohesive and cohesive

earth fill) have not always been clearly identified by re-

searchers. This has resulted in dissemination of models or

methods that should have clear limitations on applicability,

but which do not, and which are subsequently misapplied or

dismissed by later researchers who typically identify that the

earlier model performs poorly in comparison with others

when applied to a different data set. In short, a failure to

recognise the importance of material state as well as type

when developing and validating models.

A further problem with some models is the inappropriate

use of case study data to validate the model. A good example

of this is use of the Teton Dam failure, which is often cited

because it is one of a few rare data sets that exist regarding

the real failure of a large dam. However, the Teton failure

arose through a very specific process of seepage across a rock

abutment, leading to pipe formation and complete dam

failure. The significance of breach development against a

rock abutment rather than free formation in the body of the

dam often appears overlooked. As reported by Mohamed

(2002) and the IMPACT Project (http://www.impact-pro

ject.net, Morris & Hassan, 2005a, b) the location of a breach

does significantly affect the breach evolution rate and hence

flood hydrograph. An IMPACT Project laboratory test

showed a measurable difference in peak discharge (for a

given test geometry) of approximately 13%. A similar effect

might be envisaged for Teton, which had nonerodible rock

abutments; hence, validation of a breach model assuming a

central free forming breach but using the Teton data to

validate will lead to an inherent error in the model.

Figure 5 shows modelling results from the HR BREACH

model assuming central (Figure 5a) and then side (Figure 5b)

breach growth predictions for the Teton failure. The four

lines within each plot show breach model results using three

different sediment relationships (Yang, 1979; Visser, 1995;

Visser, 1998 and Chen & Anderson, 1986) compared against

a line representing observed peak outflow discharge. Ob-

served discharge is shown as a line because there is uncer-

tainty as to the actual value that occurred during the failure

and the line indicates the range within which the true value

probably sits. It can then be seen that results for the side

breach simulation offer a better potential fit to the observed

data range than the results from the central breach simula-

tion. A comparison of peak discharge values between the two

scenarios shows that the side breach predictions are approxi-

mately 20–25% lower than the central breach predictions.

Photo 4 shows an example of breach along a rock abutment.

mountainous

River valleys, fluvial terraces,flood plains, alluvium

Lakes, lacustrine, estuaries,estuarine deltas, deltaic

Coastline, clifferosion, beachdeposits

Seas, marineOceans, oceanic

boulders, cobbles gravels gravels

sandssiltsclays

Organics, plant remainsColloids,

muds, ooze,skeletalremains

Figure 4 Simplified deposition environment (after Barnes, 2000).



Material condition influencing progression ofbreach modelling capability

The points discussed above offer a range of factors that are

likely to have had a general influence on our ability to

develop more reliable models. The factors would have

slowed and confused scientific development. However, more

recent research into the effect of material properties on

breach, particularly for cohesive materials, highlights some

specific factors that could explain why many models initially

appear to perform well, but are subsequently discredited by

others who find their performance not to be as good in

comparison with (yet) another new breach model.

As early as the 1940s, it was observed that clay and water

content had a significant effect on physical modelling of

breach failure of small-scale embankments (White and

Gayed, 1948). At that point no conclusions physically or

numerically were made because it was deemed too complex

to offer a solution. Research over the last decade or so by

Hanson et al. (2001), Hanson and Cook (2004), Hanson

et al. (2005a, b), Hanson et al. (2005), Hanson and Hunt

(2007) and Hunt et al. (2005) at USDA-ARS has focussed on

the erodibility of cohesive material and how this affects

breach growth. Three key factors affecting erodibility have

been identified, namely material texture, compaction moist-

ure content and compaction energy. The influence that these

parameters have on erodibility and hence the rate of breach

initiation and growth is extremely high, extending to orders

of magnitude for relatively small changes in moisture

content and/or compaction. Erodibility studies conducted

by Hanson and Hunt (2007) on a series of soil material

samples prepared at three compactive efforts over a range of

water contents provide an insight into the importance and

complexity of compaction (Figure 6). They observed that

the erodibility coefficient, kd, can clearly vary by several

orders of magnitude as compaction water content and effort

is varied. Additional observations from Figure 6 include:

(1) each compaction effort results in a unique curve dry of

the optimum water content (wopt); (2) each curve merges

at water content values greater than wopt; and (3) small

changes in water content dry of wopt can result in very

Teton Breach Outflow Prediction(Central Breach Simulation)

0

20000

40000

60000

80000

100000

(a) (b)

0 2500 5000 7500 10000 12500 15000 0 2500 5000 7500 10000 12500 15000

Time (s)

Flo

w (

m3/

s)

Prediction - Yang

Prediction - Visser

Prediction - Chen

Observed Range

Teton Breach Outflow Prediction(Side Breach Simulation)

0

20000

40000

60000

80000

100000

Time (s)

Flo

w (

m3/

s)

Prediction - Yang

Prediction - Visser

Prediction - Chen

Observed Range

Figure 5 Comparing breach model predictions for the Teton failure assuming free format central breach growth or restricted side breach growth (after

Morris and Hassan, 2005a).

Photo 4 Breach formation along a rock abutment.

Water Content w, %8 10 12 14 16 18 20 22 24

k d (

cm3 /

N-s

)

0.01

0.1

1

10

100

1000Std. Comp.9 B/L5 B/L

wopt

Figure 6 Measured variation of erodibility for a soil over a range of

compaction water contents and compaction effort (Hanson and Hunt,

2007).


156 Morris et al.

dramatic changes in erodibility. The variations of erodibility

are a result of the influence of compaction effort and water

content on the complex interaction of soil particles, material

structure and water. It is recognised that the extent of

erodibility sensitivity to variations in these parameters will

vary according to material type and tests are currently being

performed to widen the range of available data. Hanson and

Hunt (2007) also observed that material properties (i.e.

gradation) also have an important influence on erodibility.

These observations have important practical implications

relative to construction practices of earthen embankments

and the resulting erosion resistance, and for assessing (i.e.

testing and measuring erodibility resistance) and under-

standing the state of existing embankments.

If kd, as a measure of erodibility, is a dominant factor

contributing to soil behaviour during erosion (i.e. determin-

ing whether erosion is by surface erosion or head cut

processes), then redrawing Figure 3 using erodibility along

the x-axis might allow identification of the exact transition

point between these two erosion processes. This could have

significant implications for embankment performance and

hence design if it can be shown that the timing and nature of

catastrophic breach for a given embankment geometry

differs significantly between breach arising from surface

erosion and breach arising from head cut erosion. Research

comparing these processes in relation to geometry, and the

degree of process dependency on a measure of erodibility,

continues but is not yet at a level where a revised Figure 3

type plot may be produced. Clarifying which processes will

occur when for different material types and state should

help to reduce confusion, focus breach model research and

hence allow more rapid progress towards improved model

performance.

Having established the importance of understanding

material type and state in relation to erodibility and hence

breach growth, a number of questions may then be con-

sidered – as discussed below.

Does embankment size affect the breachingprocess?

A further issue that arises in considering potential transi-

tions between or conditions relating to head cut and surface

erosion is how embankment size affects erosion processes.

For example, is there a transition in process when the head

cut height on large embankments approaches the limits of

the physical strength of the material? Does this result in a

process of catastrophic movement or does the process

continue as a form of steep sloped movement dependent

upon hydraulic stress and erodibility? Dependencies upon

scale such as this will affect the applicability of models

developed using smaller scale tests or field data. In this

context, ‘large embankments’ might be considered to be in

the range of 20–100 m; hence, ‘smaller scale tests or field

data’ refers to most, if not all, tests undertaken to date and

result in another aspect of uncertainty and another reason

for the present slow rate of progress in this field! Aspects

such as these are only likely to be addressed once researchers

recognise and agree upon the relationship between materi-

als, erodibility and breaching processes.

Do existing models use the right parameters/material characterisation?

If one accepts the significance of these parameters (material

type, compaction moisture content; compaction effort,

erodibility) in addition to soil strength as relevant to the

breach formation process, then an immediate question that

logically arises is whether or not they are adequately

reflected within the various breach models that are pre-

sented by researchers. The answer to this is not always

simple, because although a majority of models do not

directly incorporate these parameters, some do include

other parameters that can be related, and hence infer some

degree of effect on the modelling results. However, a

subjective description of the situation might be that a lot of

past models do not include any such soil or condition

parameters, a few current models do and a few might

include related parameters.

Some breach models include an implicit representation of

material type, moisture content and compaction energy in

other soil parameters such as soil strength, density and

porosity. That raises the question as to whether this is

enough to represent the real effects of varying material state

on failure processes. To answer this, we have to look at the

failure processes that are associated with breach develop-

ment and that may be simulated by a model. We can broadly

split the failure process into two main categories, namely

soil erosion and slope stability. If the above representation

cannot account adequately for the soil state reflected by

moisture content and compaction energy in the calculation

of each of these processes, then the representation is

insufficient. It is clear that the soil strength and density form

key parts in slope stability analysis, but does strength,

density or any other soil parameter account well for the

erodibility of the material? If yes, how do they reflect this

large effect? If not, what can be used to reflect this large

effect? Looking at traditional sediment transport equations

(which have been typically used within a majority of breach

models), we can see that the effect of density or porosity on

the rate of transport is not significant as the equations are

mostly based on applied shear stress, water velocity and

particle diameter and specific gravity, which do not reflect

soil strength, density, porosity or erodibility. Therefore, we

could say that at least one of the main breach processes is not

well represented by the above implicit modelling.



If the effect of material type, compaction moisture con-

tent and compaction effort is so great, it is clear that any

models developed without these parameters or direct input

of erodibility based on specific measurements will struggle

to provide reliable predictions over a range of embankment

types and conditions. While these parameters are particu-

larly relevant to cohesive materials (of which many flood

embankments and embankment dams are constructed), the

concepts are also relevant to breaching through noncohesive

materials. Analysis of the European IMPACT Project field

test data (Morris, 2008b) shows both head cut and surface

erosion processes occurring in noncohesive materials, aris-

ing as a result of the material state.

Why is there a frequent cycle of new modeldevelopment?

A third question that then arises is why models initially

appear to work well, and are subsequently criticised by

others? It is suggested that this relates to the way in which

models are often calibrated and validated. Breach models are

often calibrated to a data set rather than developed purely

on process theory. Calibration is often done using a specific

set of data arising from laboratory or, in some cases, field

tests. Where directly calibrated, the model should invariably

perform well against the data sets. Fundamentally, if materi-

al state arises, for example, from intentional or default

compaction effort, and moisture content conditions are not

recorded, then the value of the data reduces significantly

because there is no means to determine where within the

differing orders of magnitude of erodibility the test data

resides. While the calibrated model may reproduce those test

conditions well, there can be no guarantee of prediction for

other material types or conditions.

Model validation is also typically accomplished using a

different data set (but often drawn from the same test series)

or by using case study data. Test data from the same

experimental setup is likely to use material in a similar state

(i.e. similar compaction effort or moisture control processes

during model construction) as the data used for initial

numerical model calibration. Hence, the importance of

material state is unlikely to be highlighted here. This is

demonstrated well by the recent DSIG project review of

breach modelling data sets (Courivaud, 2007), which shows

that very few recorded laboratory experiment data sets

include details of material state (such as moisture content

and compaction detail); real dambreak or breach case study

data sets contain even less information and often do not

include a clear record of material type and in some cases

little information on embankment geometry.

As with the earlier Teton Dam example, the error that

would arise in model prediction is a result of generally

applying a model that, in fact, is only really valid for a

limited set of circumstances.

How useful are case study data?

Validation against case study data then raises a fourth

question, namely that if construction history, defining

material state, plays such a significant role in determining

erodibility and breach growth, how can model performance

be validated against case study data where this information

typically does not exist? The answer has to be that models

cannot be reliably validated against such data, yet that is

exactly what is done! It seems that the attraction of compar-

ing model performance against a real event is strong and

tends to blind many to the question of the real uncertainties

associated with the case study data.

A more general problem with case study data is related to

the uncertainty surrounding the event itself. This is due to

uncertainty in obtaining reliable data related to the as-built

embankment materials, construction history, condition,

geometry, reservoir inflow and storage failure timing, pro-

cess and outflow. Often, the combined magnitude of these

uncertainties makes detailed model performance evaluation

very difficult, if not impossible. This is one of the challenges

facing the current, Dam Safety Interest Group project on

breach model evaluation (Courivaud, 2007; Wahl et al.,

2008).

Without better certainty in the actual case study data, and

the limited number of such cases, there can then be a

reluctance to improve physically based breach models or to

rely on the results of such models. In order to solve this

problem there is a need for improved data gathering and

forensics of actual failures and increased physical modelling

of breach processes at large rather than laboratory scales.

The way forward -- breaking the cycle

There are a number of goals to achieve if we are to make

significant and rapid advances in the prediction of breach

growth. These include:

1. The recognition by researchers of the clear differences

between breach initiation and formation processes aris-

ing, in particular, among rock fill, noncohesive and

cohesive earth fill materials.

2. Understanding the significant role that material type,

compaction moisture content and effort plays in erod-

ibility and hence breach growth – in particular for

cohesive materials (which form a very large proportion

of flood defence embankments and earth dams) and

hence the implications this has for:

(a) suitability of existing model codes;

(b) usability of existing laboratory data; and

(c) usability of existing case study data


158 Morris et al.

3. The need to understand and integrate hydraulic, geo-

technical and structural processes when considering the

overall breaching process. This includes clear agreement

and classification of, for example, breach growth geome-

try (e.g. earlier Teton example; common and incorrect

assumption of a trapezoidal breach shape) and the

importance of simulating key physical processes such as

block failures in the lateral growth of a breach.

4. The need to understand how compaction, moisture

content and hence embankment state varies in practice.

This includes spatial and temporal variations arising in

the short term leading up to and during storm events,

and longer term effects arising from local climate and

climate change effects. The impact of weathering or

deterioration effects such as fissuring are also important

factors that need to be considered if we are to make direct

practical links between predictive breach models and

asset management practice.

5. The recognition that erodibility of cohesive materials is as

important a soil parameter as soil strength or perme-

ability in embankment design. This also implies the

important challenge of developing appropriate geotech-

nical tools to measure erosion resistance of soil materials.

Implications for future benchmark testing

The points raised above have significant implications for

model application or programmes of model development

and benchmarking. Where data sets exist, but do not

contain details of material types and/or material state then

it will be difficult to assess model performance because the

data sets could relate to a wide range of potential breach

scenarios. With models invariably dealing with material type

and state in different ways, if at all, it would be difficult to

assess the most reliable performer within this band of

uncertainty.

The issue of model performance calibrated to data sets

has been highlighted. Ideally, models should not be directly

calibrated against specific data sets where they are intended

for use against a wide range of conditions. Any benchmark

tests should recognise where models have been calibrated or

validated against data and avoid inappropriate comparisons.

Where models have been calibrated to specific conditions,

these should be made clear so that the user is aware of any

limitations in model applicability.

A problem of data shortage will invariably arise, because

there is already a recognised shortage of good quality, large-

scale data relating to breach initiation and formation. If data

sets omitting pertinent geotechnical information (i.e. mate-

rial type, compaction moisture content and effort data) are

removed, this leaves even less data upon which objective

testing and development can be made. While initiatives such

as the European IMPACT Project (Morris, 2005) and the

Dam Safety Interest Group breach project (Wahl et al., 2008)

have reviewed and collected large-scale data sets including

data on soil state, these data sets still represent a very small

sample in relation to the wide range of materials, designs

and embankment states that breach modellers are required

to simulate. More large-scale reliable data are required to

refine model performance over a wider range of design and

material types.

Understanding moisture content andcompaction effort

Following identification of compaction moisture content

and compaction effort as key factors affecting soil state and

hence erodibility (clearly for cohesive soils, but also for a

range of noncohesive soils), it is then logical to try and

‘bound’ the problem by understanding how these condi-

tions vary in nature, in time, by design and by material. This

is by no means a simple task but seems inevitable given the

potential significance of influence in relation to breach

prediction. Variables here will include moisture content,

compaction and state variation:

� across different materials;

� through construction process;

� through the embankment structure;

� through deterioration (i.e. in time); and

� through other factors such as vegetation, drainage, etc.

By understanding the likely normal state and range of

conditions that might occur, we can then be sure that

research and model development remains focussed within

the practical range of application that reflects real operating

state and conditions.

A common approach?

Focussing attention upon soil state, specifically including

soil moisture content, is further validated when looking at

the wider range of recent and ongoing research relating to

embankment performance. Some examples of this include

work investigating:

� Soil suction and its effect on breach growth (Pickert et al.,

2004);

� Fine fissuring and its effect on embankment performance

(Dyer et al., 2007);

� Wave overtopping in relation to embankment fissuring

and failure mechanisms observed during the 1953 floods

in the United Kingdom, Netherlands and Germany

(Marsland & Cooling, 1958); and

� Erodibility as a function of soil moisture content and

compaction energy, and its effect on breach growth (Hunt

et al., 2005; Hanson et al., 2006; Hanson & Hunt, 2007).

Each of these research areas is focussing upon specific

processes, with moisture content/ingress of water being a



common driver prompting failure or poor embankment

performance. As such, it would seem logical to use moisture

content as a key parameter from which a number of failure

modes/processes may be identified.

By establishing how these failure processes are influenced

by initial and subsequent variations in soil state, both

predictive models and risk-based representations of em-

bankment performance (e.g. fragility curves) may be im-

proved. A specific example of this for breach modelling is

the introduction of fissuring as an embankment state

affecting breach formation. By introducing variable soil

erodibility (as a function of fissuring) through embankment

depth, the effect of regions of fissuring (or layered or

compacted soils) may be simulated by the breach model.

Because fissuring affects the erodibility of surface soils

(Dyer et al., 2007), this may have a significant effect on

overall breach growth through the preferential erosion

of embankment crest material, which typically controls

the rate of breach flow during the breach initiation and

formation stages. This then demonstrates the practical

importance of soil state and avoidance of fissuring in

relation to embankment performance and flood risk

management.

By linking field measurement or observation with soil

state, a more dynamic approach to predicting potential

breach formation may be adopted, which is also consistent

with the ongoing development of more complex risk-based

system models for flood risk and asset management.

The existing body of knowledge relating embankment

condition (typically from visual inspection) to known fail-

ure modes may offer a useful starting point for relating

possible embankment soil state to likely breaching

processes.

Conclusions

A range of reasons for the apparent slow improvement in

our ability to predict breach growth over the last few decades

(and further) have been identified. Some of these factors

relate to the maturing of knowledge within the geotechnical

field and the development of computers, but some equally

relate to misunderstanding of basic processes still propa-

gated today by researchers in this field. However, an over-

riding issue (particularly for cohesive materials) appears to

be the significance of material type, compaction moisture

content and compaction effort on erodibility and hence

breach growth. The significance of these parameters appears

only to have been more widely publicised within the flood

risk management community during the last 5–10 years, but

the implications for model calibration/validation using

existing laboratory and field data do not seem to be widely

recognised.

To make significant advances in predictive capabilities for

breach modelling, a number of areas of research focus are

recommended and where existing data sets are used for

model development, recognition of potential limitations

and inherent uncertainty is essential.

Acknowledgements

The work described in this publication was supported by the

European Community’s Sixth Framework Programme

through the grant to the budget of the Integrated Project

FLOODsite, Contract GOCE-CT-2004-505420.

This paper reflects the authors’ views and not those of

the European Community. Neither the European Commu-

nity nor any member of the FLOODsite Consortium is liable

for any use of information in this paper.

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Improving the accuracy of breach modelling: why are we not progressing faster?

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