MORRIS & HASSAN 1
Breach Formation through Embankment Dams & Flood DefenceEmbankments: A State of the Art Review
M W MORRIS, HR Wallingford, UK ([email protected])MAAM HASSAN, HR Wallingford, UK ([email protected])
IMPACT Project WorkshopHR Wallingford, UK 16/17th May 2002
MORRIS & HASSAN 2
SUMMARY
This paper offers a review of the state-of-the-art for predicting breach formation throughembankment dams and flood defence embankments. The problem to be solved is firstreviewed, followed by a summary of past work and existing tools and methods. Currentinitiatives in this field are then considered, and compared against suggested needs for todayand the future.
1 INTRODUCTIONA key aspect of managing any flood defence or flood control structure is an understanding ofhow that structure performs – both under normal (design) load conditions and under extremeflood conditions. Predicting failure conditions and failure processes is part of this process. Forembankment dams and flood defence embankments the failure process is inevitably throughbreach formation.
Predicting breach formation is not a new concept! It is also not as simple as might firstappear. The large number of processes involved and the lack of reliable field data againstwhich to study processes and develop modelling tools has probably contributed greatly to ourinability to reliably predict these processes.
Over the years many people have developed and applied different techniques in a quest tounderstand and quantify the various formation processes involved. The success, or accuracy,with which this has been achieved in the past is questionable, with many methods or modelscalibrated against a limited number of data sets, for which the model subsequently performs.However application of these methods or models to non-calibrated events typically thenreveals a much poorer performance.
The purpose of this paper is to present a picture of the current state of the art in this field.Current trends in flood management are towards risk identification and management, fuelledby a number of extreme flood events world-wide during the last decade and the suggestion ofworsening flood conditions through climate change effects. Such an approach to managingfloods requires a quantification of failure risk, for which a clear understanding of the breachformation process is required. Consequently many researchers around the world continue toresearch and develop new techniques in this field.
Are we making progress towards a better understanding of breaching processes?
To provide a concise overview of breach formation work, this paper tries to address thefollowing questions:
• What is the difficulty in predicting breach formation, and why do we need to?• What has been done in the past to predict breach formation?• What tools exist already and what are our current capabilities?• What needs to be done?• What is being done?• Where does the future take us?
MORRIS & HASSAN 3
2 WHAT IS THE PROBLEM?The problem that needs to be solved here is simply the reliable prediction of breach formationthrough embankment dams and flood defence embankments.
Simple? Not really!
The prediction process is complicated by the number of processes involved in breachformation and the needs of various ‘end users’ of this information, for which the emphasisand accuracy of data requirements vary. Table 1 below lists some of the likely end users,applications for the data and processes requiring quantification.
Potential End UsersDam owners / managersFlood defence owners / managersEmergency plannersLand use plannersInsurance companiesEngineering consultantsResearch organisationsResidents / public interest groups
Potential ApplicationsDam safety risk assessmentAsset / risk management (maintenance and operation)Public liability assessmentLand use planningEmergency planningReal time flood managementEmbankment design / assessment
Breaching ProcessesBreach locationBreach initiationBreach formation
Table 1 Likely end users, applications for the data and processes requiringquantification
The three items listed under breaching processes summarises the needs of many of the ‘endusers’, namely, tell me where the breach will occur, when it will occur and what will happenif it does occur.
To answer each of these questions requires the analysis of a host of other issues and processesas summarised in Table 2 below:
MO
RR
IS &
HA
SSA
N 4
Val
ley
dam
Bun
ded
Res
ervo
irL
inea
r F
lood
Def
ence
s(C
oast
al &
Flu
vial
)M
ain
func
tion
To
reta
in
and
stor
e w
ater
fo
rsu
pply
and
/ or
flo
od c
ontr
olT
o st
ore
wat
er f
or s
uppl
yT
o pr
otec
t la
nd,
peop
le,
asse
tsfr
om e
xtre
me
wat
er l
evel
s an
dhe
nce
floo
ding
Mai
n T
ypes
Ear
th f
ill d
ams
Roc
k fi
ll da
ms
Ear
th –
non
coh
esiv
eE
arth
– c
ohes
ive
Ear
th –
com
posi
te s
truc
ture
s
Ear
th –
non
coh
esiv
eE
arth
– c
ohes
ive
Ear
th –
com
posi
te s
truc
ture
sT
ypic
ally
po
orly
co
nstr
ucte
dfr
om v
arie
ty o
f m
ater
ials
Pri
mar
y L
oads
Wat
er p
ress
ure
Self
wei
ght
See
page
Wav
e ac
tion
Ero
sion
Wat
er p
ress
ure
See
page
Self
wei
ght
Wav
e ac
tion
(li
mit
ed)
Exp
osed
wav
e ac
tion
(coa
stal
)W
ater
pre
ssur
eS
eepa
geSe
lf w
eigh
t
Prin
cipl
e Fa
ilure
Mod
esO
vert
oppi
ngPi
ping
Slop
e in
stab
ilit
yFo
unda
tions
Pipi
ngSl
ope
inst
abil
ity
Foun
datio
ns
Ove
rtop
ping
Pipi
ngE
rosi
onSl
ope
inst
abil
ity
Foun
datio
ns
Wat
er T
ype
Fres
hFr
esh
Fres
h / S
alin
e
Wat
er V
olum
eFi
nite
res
ervo
ir +
sto
rm v
olum
eFi
nite
res
ervo
irFi
nite
sto
rm (
fluv
ial)
Infi
nite
& p
erio
dic
(coa
stal
)
Typ
ical
Fac
e P
rote
ctio
nN
atur
al &
syn
thet
ic (
exte
nsiv
e)N
atur
al &
syn
thet
ic (
exte
nsiv
e)N
atur
al &
syn
thet
ic (
limite
d)
Tab
le 2
Key
issu
es /
proc
esse
s re
lati
ng to
dif
fere
nt e
mba
nkm
ent s
truc
ture
s
MORRIS&HASSAN 5
In summary therefore, and considering each structure type in turn, in relation to the keyprocesses of breach location, initiation and formation:
2.1 Valley Embankment Dams
2.1.1 Breach Location
Identifying breach location is not such a significant issue for valley dams. The route ofany flood water from a breach is clearly defined. However, exact location may affect theformation process (see below)
2.1.2 Breach Initiation
The valley dam is a contained structure for which monitoring, instrumentation andinspection may be undertaken relatively easily. As such, identification of breachinitiation is likely to be made early in the process. However, given the potential for highheads and dambreak flood conditions, it is essential to identify initiation at the earliestpossible time.
2.1.3 Breach Formation
This is critical for predicting potential flood flows, emergency warning times andmechanisms for repairing or preventing catastrophic breach. The location of the breachacross the dam may influence the rate and mechanism of growth (e.g. formationadjacent to a rock abutment will limit the rate of lateral growth and hence rate of floodwater release).
2.2 Bunded Reservoirs
2.2.1 Breach Location
Bunded reservoirs potentially have long lengths of embankment with multiple routes forbreach flood water to flow. Identifying potential breach location is therefore importantwhen considering potential assets at risk.
2.2.2 Breach Initiation
Long lengths of embankment make it difficult to inspect, monitor and instrument thewhole structure. Identifying likely breach initiation areas is therefore important.
2.2.3 Breach Formation
Breach formation is always important for predicting rate of growth and hence release offlood water.
2.3 Linear Flood Defences
2.3.1 Breach Location
With thousands of kilometres of flood defence embankments to manage, a means foridentifying likely breach location is critical for prioritisation of limited resources inoperation and maintenance works.
MORRIS&HASSAN 6
2.3.2 Breach Initiation
Very long lengths of embankment make it difficult to inspect, monitor and instrumentthe whole structure. Identifying likely breach initiation areas is therefore very important.
2.3.3 Breach Formation
Breach formation is always important for predicting rate of growth and hence release offlood water. Coastal defences will suffer repeated flooding under tidal conditions,increasing the urgency for emergency repairs under flood conditions.
Considering all of these structures and conditions together, the objectives for predicting‘breach formation’ therefore include:
• Prediction of breach initiation mechanisms(Factors contributing, relative importance of factors, � breach location,rate of change of factors etc)
• Prediction of breach initiation times(Timing of various phases of breach development)
• Prediction of breach formation processes(Growth mechanisms, interaction with materials, composite structuresetc.)
• Prediction of breach formation rate and flow through a breach(Rate of growth, interaction with flow / water levels, flow through breachgeometry)
By no means a simple task!
3 WHAT HAS ALREADY BEEN DONE TO PREDICT BREACHCONDITIONS?
Considerable work has been done in the past (and continues today) to predict breachformation. Different approaches have been taken and may be classified (according toWahl, 1998) as non-physically based, semi-physically based and physically basedmethods
3.1 Non-physically Based MethodsTypically entail fitting of equations to a selection of past failure events. By selectingparameters such as embankment height, volume of water stored etc. these may then beused to predict failure conditions for similar structures, without a detailed assessment ofthe structures themselves.
Examples of these approaches include equations by Macdonald and Langridge-Monopolis (1984), Froehlich (1995) and Broich (1998). Wahl (2001) presented aquantitative analysis of the uncertainty associated with these various methods, andconcluded wide bands of uncertainty within the processes. His work does demonstratethe basis upon which many of the equations have been developed and offers guidanceon selection of the most appropriate.
MORRIS&HASSAN 7
3.2 Semi-physically Based MethodsUse simplified assumptions for some of the processes – for example, assuming a predefined or time dependent breach growth process. Examples of this type of model arework by Singh et al (1989) and Walder et al (1997). Whilst these models allowprediction of an outflow hydrograph, rather than simply an estimate of peak discharge,the model user is still required to make modelling assumptions for which there islimited guidance. For example, the user is often required to define the rate and extent ofbreach growth – from which the model then predicts the outflow hydrograph. Given theuncertainty surrounding the whole breach formation process this does not provide aprediction of breach growth, but rather a likely hydrograph for different potentialscenarios. The user is still left without guidance on the likely rate of breach growth.
3.3 Physically Based MethodsTypically simulate the embankment failure mechanisms, trying to simulate the physicalprocesses observed. These approaches entail detailed computations combiningprinciples of hydraulics, sediment processes and soil mechanics. The advantage of thisapproach is that the model provides an estimation of the breach formation process andsubsequently the potential flood hydrograph.
The most famous of all of these models is probably the NWS BREACH modeldeveloped by Danny Fread in the 1980’s. (Fread, 1988). This model was developed anddistributed as part of the NWS DAMBRK model, which having been placed in thepublic domain has been widely used around the world. As with many other models,problems have been found with this model (Mohamed, 1998).
Both before and after the NWS BREACH model there have been a number of differentmodels developed in the attempt to produce a reliable tool for predicting breach growth.Table 3 below provides a summary of the main models developed between 1965 and2001, along with a summary of the perceived limitations and deficiencies of the models(Hassan, 2002).
It should be noted that the majority of the models listed under Table 3 considerbreaching through an embankment dam, rather than through a flood defenceembankment. The process of breach formation through a flood defence embankment isdifferent from that through an embankment dam, since the approaching flow conditionsare different (i.e. flow parallel or normal to flood embankment). The degree to whichthe formation process varies is unclear and further research in this area is required.However, given the general level of uncertainty associated with predicting the simplercase of breach formation through an embankment dam, it would be wise to first improveour ability to predict conditions under this simpler case, and later extend modellingprocesses to river embankments. In the meantime, models predicting breach growththrough embankment dams offer the best or closest tool for predicting breach growththrough river flood defences.
M
OR
RIS
& H
AS
SA
N 8
Mod
elB
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ach
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Lim
itat
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efic
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Chr
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hes
(198
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ith
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ris-
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(196
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r in
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row
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mec
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mec
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sms
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pipe
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idth
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xner
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ith
Mey
er-P
eter
-Mul
ler
Non
e•
No
slop
e st
abili
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echa
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s•
Use
of
the
regi
me
conc
ept
• N
o la
tera
l ero
sion
aft
er p
eak
flow
Lou
Rep
orte
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Sing
h (1
996)
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t ef
fect
ive
stab
lese
ctio
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osin
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rve
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l Sa
int
Ven
ant
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s1.
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hris
tofa
no’s
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rica
lfo
rmul
a2.
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uboy
&
Ein
stei
nfo
rmul
a3.
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ou’s
for
mul
a
Non
e•
No
slop
e st
abili
ty m
echa
nism
s•
No
late
ral e
rosi
on a
fter
pea
k fl
ow•
Em
piri
cal
form
ula
to
com
pute
th
eer
osio
n•
Inap
prop
riat
e m
etho
d to
mod
el t
hebr
each
gro
wth
Nog
ueir
aR
epor
ted
bySi
ngh
(199
6)
Eff
ecti
ve s
hear
str
ess
sect
ion
(Cos
ine
curv
esh
ape)
Ful
l Sa
int
Ven
ant
equa
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sE
xner
equ
atio
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ith
Mey
er-P
eter
-Mul
ler
Non
eL
imit
atio
ns a
s fo
r L
ou’s
mod
el
M
OR
RIS
& H
AS
SA
N 9
Mod
elB
reac
hM
orph
olog
yF
low
calc
ulat
ion
Sedi
men
ttr
ansp
ort
Geo
-mec
hani
csof
bre
ach
side
slop
es
Lim
itat
ions
& D
efic
ienc
ies
NW
S B
RE
AC
H(F
read
, 198
8)R
ecta
ngul
ar
and
trap
ezoi
dal
Bro
ad c
rest
ed w
eir
form
ula
for
over
topp
ing
Ori
fice
fl
ow
for
pipi
ng
Mey
er-P
eter
-Mul
ler
mod
ifie
d by
Sm
art
1.
Bre
ach
side
slop
e st
abili
ty2.
T
op
wed
gefa
ilure
du
ring
pipi
ng
orov
erto
ppin
g
• U
nifo
rm e
rosi
on o
f th
e br
each
• In
com
pati
ble
com
puta
tion
met
hod
• In
accu
rate
slo
pe s
tabi
lity
anal
ysis
• Si
mpl
ifie
d m
odel
ling
of t
he f
ailu
re o
fco
mpo
site
em
bank
men
ts
BE
ED
Sing
h &
Q
uiro
ga(1
987)
, Si
ngh
(199
6)
Tra
pezo
idal
Bro
ad c
rest
ed w
eir
form
ula
Ein
stei
n -
Bro
wn
Bre
ach
side
sl
ope
stab
ility
• U
nifo
rm e
rosi
on o
f th
e br
each
• In
com
pati
ble
com
puta
tion
met
hod
• In
accu
rate
slo
pe s
tabi
lity
anal
ysis
SIT
ES
(199
8)3
stag
e fa
ilure
:1.
C
over
fai
lure
2.
Hea
dcut
form
atio
n3.
H
eadc
ut e
rosi
on
Pri
ncip
les
ofhy
drol
ogy
and
hydr
aulic
s to
prod
uce
spill
way
flow
– s
tage
cur
ve
For
St
ages
1,
2
ade
tach
men
t m
odel
was
use
d. F
or S
tage
3 an
en
ergy
diss
ipat
ion
equa
tion
was
use
d.
Spill
way
ex
itch
anne
l sta
bilit
y•
Doe
s no
t m
odel
co
mpl
ete
emba
nkm
ent
failu
re p
roce
ss•
Em
piri
cal
coef
fici
ent
to
com
pute
eros
ion
NC
P B
RE
AC
H(A
ndre
ws
&C
olem
an, 1
998)
Par
abol
icE
mpi
rica
l for
mul
aE
mpi
rica
l for
mul
aN
one
• E
mpi
rica
l fo
rmul
a ba
sed
on
smal
lsc
ale
mod
els
• N
o sl
ope
stab
ility
mec
hani
sm
ED
BR
EA
CH
(Lou
kola
&
Hou
kuna
, 199
8)
Tra
pezo
idal
Bro
ad c
rest
ed w
eir
form
ula
Mey
er-P
eter
-Mul
ler
Top
wed
ge f
ailu
redu
ring
pip
ing
• U
nifo
rm e
rosi
on o
f th
e br
each
• In
accu
rate
slo
pe s
tabi
lity
anal
ysis
BR
ES
(Vis
ser,
1998
)5
stag
es o
f fai
lure
Bro
ad c
rest
ed w
eir
form
ula
Seve
ral
tran
spor
tfo
rmul
aeN
one
• N
o sl
ope
stab
ility
mec
hani
sm•
Inco
mpl
ete
com
puta
tion
met
hod
M
OR
RIS
& H
ASS
AN
10
Mod
elB
reac
hM
orph
olog
yF
low
calc
ulat
ion
Sedi
men
ttr
ansp
ort
Geo
-mec
hani
csof
bre
ach
side
slop
es
Lim
itat
ions
& D
efic
ienc
ies
DE
ICH
_N1/
N2
(Bro
ich,
199
8)D
iffu
sion
app
roac
hSh
allo
w
wat
ereq
uati
ons
Seve
ral
tran
spor
tfo
rmul
aeN
one
• P
arab
olic
bre
ach
shap
e•
No
slop
e st
abili
ty m
echa
nism
• U
nrea
listi
c m
odel
ling
of t
he v
erti
cal
and
late
ral e
rosi
on
Ren
ard
&R
upro
(Rep
orte
d by
Paq
uier
, 199
8)
Uni
form
er
osio
n of
the
pipe
Ori
fice
equ
atio
nM
eyer
-Pet
er-M
ulle
rF
ailu
re o
f m
ater
ial
abov
e th
e pi
pe•
Unr
ealis
tic
mod
ellin
g of
fa
ilure
of
mat
eria
l abo
ve t
he p
ipe
• N
o sl
ope
stab
ility
mec
hani
sm
Floo
d L
evee
Bre
ache
s(F
ujit
a &
Tam
ura,
198
7)
Rec
tang
ular
br
each
shap
e ab
ove
wat
erle
vel
– tr
apez
oida
lbe
low
Cri
tica
l fl
oweq
uati
onSe
dim
ent
tran
spor
tra
te
esti
mat
edas
sum
ing
ener
gysl
ope
cons
um
ed o
nly
in
sedi
men
ttr
ansp
ort
Non
e•
Uni
form
ero
sion
of
the
brea
ch•
No
slop
e st
abili
ty m
echa
nism
Tab
le 3
A c
ompa
riso
n an
d as
sess
men
t of
phys
ical
ly b
ased
bre
ach
pred
ictio
n m
odel
s
MORRIS & HASSAN 11
4 WHAT ARE OUR CURRENT CAPABILITIES FOR BREACHMODELLING?
The CADAM Project (Concerted Action on Dambreak Modelling) provided a goodopportunity to determine current breach modelling capabilities. CADAM was funded bythe European Commission as a Concerted Action Programme that ran for a period oftwo years ending in January 2000. The project promoted the comparison of dambreakand breach modelling performance and practice across Europe.
4.1 Key Outputs from the CADAM Project
• Proceedings from 4 workshops covering a variety of dambreak topics• Test case data for various dambreak laboratory tests as well as field data• Guidance document (complementing ICOLD Bulletin 111 (ICOLD, 1998)) entitled
“Dambreak Modelling Guidelines and Best Practice” (Morris & Galland, 2000)• Project report document providing an overview of the project work and
summarising key findings, including recommendations for future R&D
All publications from the CADAM project may be accessed via the project web site atwww.hrwallingford.co.uk/projects/cadam. Paper and CD-ROM copies are alsoavailable.
4.2 Conclusions from the CADAM Project
A total of 31 specific conclusions are identified within the CADAM project report.Selected conclusions from this report relating to breach formation are presented below:
Conclusion 12: Uncertainties within the breach modelling process may be the greatestcontribution to uncertainty within the whole dambreak analysis process.
Conclusions 13 & 15: Our current ability to predict the rate and location of breachgrowth is quite limited. Breach model accuracy is very limited. An estimate of ±50% forpredicting peak discharge is suggested, with the accuracy of predicting the time offormation considerably being worse.
Conclusion 17: Currently, there is no single recommended breach model. Whilst theNWS BREACH model is widely used it has significant limitations. A number ofresearchers are currently working on the provision of improved breach models. There isa clear need to integrate knowledge from both the hydraulics and soil mechanicsdisciplines in order to advance expertise in this field.
One of the findings that perhaps best demonstrates our current ability for predictingbreach growth is a comparison of CADAM breach modeller results against observedfield / lab data. Figure 1 below shows a typical scatter of modelling results found for the
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CADAM test cases. Models comprised a range of university and commercial models,including the NWS BREACH model. When you consider that data set No. 7 wascalibrated to the test data (set 0) and should therefore be discounted from thiscomparison, this plot shows that none of the models were able to ‘reliably’ predictbreach formation.
Fig. 1 Typical scatter of model results trying to predict breach formation
5 WHAT NEEDS TO BE DONE? - WHAT IS BEING UNDERTAKEN?Conclusions that may be drawn from the review of past breach modelling work andfindings from the CADAM work include:
There are no existing breach models that can reliably predict breach formation throughembankments. Discharge prediction may be within an order of magnitude, whilst thetime of breach formation is even worse. Prediction of breach formation time due to apiping failure is not yet possible.
Whilst the NWS BREACH model is used widely in some countries, it is only calibratedagainst a very limited data set. The author (Danny Fread) confirmed that it is based onapproximately 5 data sets. In addition, research (Mohamed, 1998) has shown that themodel can produce inconsistent results under some loading conditions.
Existing breach models should be used with caution and as an indicative tool only. Arange of parameters and conditions should be modelled to assess model performanceand results generated.
There is a clear need to develop more reliable predictive tools that are based on acombination of soil mechanics and hydraulic theory.
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During the last year there have been a number of workshops (at least 2) held in the USlooking at dam safety and research needs (USACE, Nov 2001 and USDA, June 2001).The most relevant of these for breach formation work is the workshop held by USDA atOklahoma, 26-28 June 2001. As part of this workshop participants (drawn from a rangeof authorities from across the US, and internationally) were asked to identify andprioritise research needs. Whilst the results from this survey are presented in a variety offormats within the workshop proceedings, the Editor’s prioritised list of leadingresearch and development needs (top 8 only) comprised (After USDA 2001):
1. Develop forensic guidelines and standards for dam safety representatives and
experts to use when reporting dam failures and incidents. Create a forensic team
that would be able to collect and disseminate valuable forensic data.
2. Using physical research data, develop guidance for the selection of breach
parameters used during breach modelling.
3. Perform basic physical research to model different dam parameters such as soil
properties, scaling effects, etc. with the intent to verify the ability to model actual
dam failure characteristics and extend dam failure knowledge using scale models.
4. Update, revise and disseminate information in the historic data set / database. The
data set should include failure information, flood information and embankment
properties.
5. Develop better computer-based predictive models. Preferably these models would
build upon existing technology rather than developing new software.
6. Make available hands-on end-user training for breach and flood routing modelling
which would be available to government agencies and regulators, public entities
(such as dam owners) and private consultants.
7. Record an expert level video of Danny Fread along the lines of previous ICODS
videos from Jim Mitchell, Don Deere etc.
8. Send US representatives to co-operate with EU dam failure analysis activities.
It is not clear whether any of these US recommendations have yet been acted upon –other than No. 8!
Other initiatives within the last and next couple of years that the authors are aware ofinclude the following:
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5.1 Government / Commercial
5.1.1 HR Wallingford
HR Wallingford has undertaken a 3-year research programme combining hydraulics andsoil mechanics to develop a new breach model (HR BREACH). This modelincorporates soil mechanics theory for slope stability assessment, and allows bothcontinual erosion of material and the mass failure of breach sides and embankmentfaces. The model also allows a probabilistic approach to be taken for embankment slopefailure mechanisms, relating to uncertainty in soil parameters, construction quality,condition etc. The model has not been calibrated against any particular data set but hasperformed well in comparison with 5 case study data sets. The model has been appliedto a number of real case studies and is currently being tested on case studies along theRhine in Germany. The model will be applied and further developed throughout theIMPACT project (see below) and is likely to form part of commercial software withinthe next 24 months (www.wallingfordsoftware.com).
5.1.2 Reducing the Risk of Embankment Failure Under Extreme Conditions UKDEFRA / Environment Agency
The UK Environment Agency and Department for Food and Rural Affairs (DEFRA) arecurrently funding a study looking at “Reducing the Risk of Embankment Failure UnderExtreme Conditions”. In addition to identifying key factors / issues relating to allaspects of flood defence embankment performance, the project will identify areas whereresearch should be focussed and opportunities exist to aid such research. Breachformation combined with managed retreat of coastal defences is one area likely to ratehighly within this review. In 2003 there are a number of coastal sites in the UK wheredefences comprising cohesive embankments some hundreds of years old will bedeliberately breached. These offer valuable sites for test monitoring and failure of realflood embankments through both piping and overtopping.
5.1.3 IMPACT Project – European Commission
The IMPACT project is a three-year research project (starting Dec 2001) funded by theEuropean Commission within which an extensive programme of work looking at breachformation has been scheduled. Further information may be found at the project web sitewww.impact-project.net
The programme of work combines field modelling (failure of 5 No. 6m highembankments), with laboratory modelling (failure of 25 No. 0.7m high embankments)and numerical model development / application. Field and laboratory data will bereleased to modelling partners within a test programme designed to maximiseobjectivity and to demonstrate model performance and uncertainty. Whilst the IMPACTproject team (for breach modelling) officially comprises 5 partners, the team will beexpanded to include as many international partners as possible. Electricte de France(EDF) and US Department of Agriculture (USDA) has already agreed to participate inthis way. Others such as TU Delft, US National Weather Service, US Bureau of
MORRIS & HASSAN 15
Reclamation have yet to confirm. However, the more partners that participate the widerthe comparison of models and the more effective the assessment of performance shouldbe. The following team members of the IMPACT project plan to investigate, developnew or extend existing breach modelling capabilities under the IMPACT project:
1. HR Wallingford (UK)2. Statkraft Groner (Norway)3. University Catholique de Louvain (Belgium)4. German Armed Forces University, Munich (Germany)5. CEMAGREF (France)6. Instituto Superior Technico (Portugal)
A separate module of work will also be undertaken looking specifically at factorscontributing to breach location, with the aim of developing a tool or methodology foridentifying the (relative) risk of a breach occurring at a specific location.
5.1.4 US Department of Agriculture – ARS (Darrel Temple / Greg Hanson)
Considerable work has been undertaken by USDA-ARS looking at erosion of cohesivematerials, and resulted in development of the SITES model. Whilst this model currentlypredicts erosion of the downstream face back as far as the upstream face, it isunderstood that this model may be extended in the future to allow full prediction of thebreach formation process. This work has focussed in considerable detail on themechanisms associated with erosion of cohesive material and should offer a moredetailed assessment of the breach initiation phases for cohesive embankments. USDA-ARS will participate where possible within the IMPACT project, offering anopportunity to compare model performance in this zone.
5.1.5 USBR / University of Quebec
It is understood that research is currently underway looking at the development of a newbreach model in the US that links with the GStars model. The extent of progress isunclear, however an approach of combined hydraulics, sediment transport and soilmechanics may be anticipated. The GStars model simulates flow behaviour through thedefinition of stream tubes. Application of this concept to the prediction of breachgrowth is an interesting pseudo 2D approach.
5.1.6 TU Delft (P Visser)
It is understood that Dr P Visser will be undertaking research into breach formationthrough cohesive materials and composite structures in the coming months / years.Further details are yet to be determined (P Visser will be a guest speaker at the 1st
IMPACT workshop at HR Wallingford, 16/17th May 2002).
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5.2 Universities
5.2.1 University of Birmingham (UK) Dr Gurmel Ghataora
It is understood that a 3-year research programme has been initiated to investigate andfurther develop a numerical model for the prediction of breach formation throughpiping. This is an area that is currently particularly poorly understood and poorlymodelled.
5.2.2 Asian Institute of Technology Dr Chaiyuth Chinnarasri
This is a good example of individual research projects being undertaken world-wide inan attempt to improve upon breach formation modelling. This particular site presentswork undertaken to develop a new model, combining both hydraulics and soilmechanics. See http://www.sce.ait.ac.th/research/dissertation/wem/2000/~chaiyuth/
5.2.3 Climate Change Research
The impact of climate change upon embankment performance is an issue that has beenidentified in a number of countries / projects. The author is aware of discussion aroundthis topic in both Belgium and the UK and it is considered likely that research into theseeffects might be initiated within the next few years. Such research is typicallygovernment funded at this time.
5.3 Risk Based Flood ManagementA growing area of research that is strongly linked with the prediction of breachformation through embankment dams and flood defences is that of flood riskmanagement systems. Developing tools to identify flood risk requires an assessment ofthe probability of failure of the embankment. This may be estimated as a single value oras a probability distribution related to, say, hydraulic loading, condition assessment etc.Whilst values or distributions may be attributed, the reliability of these values dependsupon knowledge of the basic processes. Some of the current initiatives include:
5.3.1 Modelling Decision Support System (MDSF) and Risk Assessment for Flood &Coastal Defence Systems for Strategic Planning (RASP) (UK)
These two initiatives funded by the UK Environment Agency, combine to form thebasis for the next generation of flood risk management tools in the UK. RASP offers therisk based system approach whilst MDSF offers the actual tools, which will be GISbased tools for determining flood risk at a given location. By combining economics,defence type descriptors, land use etc. the tools will allow assessment of existingconditions, identification of key (high risk) defences and assessment / development ofcost beneficial new defences. As with any risk based management tool, the accuracy ofbreach prediction depends upon the (failure) probability distribution entered (fragilitycurve) which again depends upon knowledge of embankment performance and failuremechanisms. These projects will take a further 2 years before completion. Seewww.rasp-project.net and www.mdsf.co.uk for more information.
MORRIS & HASSAN 17
5.3.2 Dam Risk Management (UK)
Following from the earlier development of a non probabilistic approach to riskmanagement for UK reservoirs (CIRIA, 2000), the UK government has funded a furtherstudy to identify a probabilistic approach to total catchment / reservoir systemmanagement. This work is not due for completion until later this summer (2002),however such an approach would need to quantify the probability of embankment dambreaching for different types of structure.
5.3.3 PC-RING (Netherlands)
PC-RING is a software tool developed in the Netherlands by Prof. ir. A.C.W.M.Vrouwenvelde of TNO (Netherlands Organisation for Applied Research) in conjunctionwith Prof. dr. ir.J.K. Vrijling of TU Delft. It is owned by the DWW of theRijkswaterstaat and may be used to determine the probability of failure of dike sectionswithin a ring dike system. The tool offers a system for risk identification / managementrather than breach formation prediction. It has currently been tested on 4 ring dikesystems and is about to be applied to 53 others. Future development is likely to includeuncertainty estimates within the probabilistic assessment process and consideration ofadditional defences such as walls, composite structures etc.
5.4 ConclusionsWhilst it is clear that there have been many attempts to develop breach models in thepast, it seems that many of these models have been based upon limited data andsimplified assumptions. It has been quite typical for the modeller to develop work fromeither a hydraulics perspective, or perhaps a soils perspective, but there seems to havebeen a lack of combined working. Breach formation combines hydraulics, sedimentmovement and soil mechanics – it is only logical to assume that the most appropriateapproach to modelling this process is by applying a combination of these processes.Within the last few years researchers have started to take this approach, withconsideration being given to embankment failure mechanisms and stability as well ashydraulic theory.
Whilst our inability to reliably predict breach conditions is perhaps disturbing, we atleast appear to appreciate what our limitations and needs are! A comparison of researchneeds and conclusions from European work and US workshops shows considerableagreement and the initiatives summarised earlier all go someway towards addressingthese various needs. There is a clear need to share / integrate the various researchprojects as much as possible to maximise value of the work to industry. Proactivedissemination of research work and modelling tools is also essential if consistentstandards and practice are to be achieved internationally.
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6 WHAT DOES THE FUTURE HOLD?
Recent years have seen a clear trend towards more formal risk management of dams andflood defences. Whilst risk management frameworks and procedures may be developedand applied, the accuracy and reliability of these procedures will not improve withoutimproved knowledge of the basic breach formation processes.
The logical route for improving our knowledge in this area is to start with the simpleand progress towards the more complex. For breach formation this means starting withunderstanding the basic formation process for the simplest case - non-cohesivehomogeneous embankments, and progressing from there. The following stages may beenvisaged:
1. Breach formation through non cohesive homogeneous embankments � leading tothe more reliable prediction of a flood hydrograph
2. Breach formation through cohesive homogeneous embankments � leading to themore reliable prediction of a flood hydrograph
3. Breach formation through composite embankments� leading to the more reliableprediction of a flood hydrograph for more realistic, or real, embankments
4. Refinement of understanding of the breach formation process � leading to a morereliable estimate of the time of breach initiation
5. Refinement of understanding of the breach formation process � leading to a morereliable estimate of the location of breach initiation
These steps may be considered in relation to both piping and overtopping modes offailure, although analysis of overtopping failure offers the simplest conditions for initialanalysis.
It is difficult to estimate the time that it will take before we are able to achieve all 5stages above. Many researchers have tried to achieve this in the past, but currently westill struggle to achieve Steps 1 & 2 to a reasonable degree of accuracy and reliability.Whilst we can calibrate models to past events, it is proving far more difficult to developa model that can perform similarly well for new and unknown events.
When undertaking this programme of research work, there are a number of problemsthat need to be addressed. These may be solved through laboratory work or throughchanged practice in the field. Specifically:
Case Study DataFailure of embankment dams or flood defence embankments is a relatively rareoccurrence. Consequently there is a shortage of data against which models may bedeveloped or calibrated. Efforts need to be made to collate real event data as and whenfailures occur. This requires forward planning since, during extreme flood events, staff
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is typically occupied on other flood alleviation tasks. However, without such forwardplanning it is unlikely that we shall ever collate large amounts of field data.
Scaling of Laboratory TestsThe scaling of breach formation processes from field to laboratory scale is by no meanssimple, but essential if we are to undertake extensive analysis at a reasonable cost!
Instrumentation & MonitoringLinked both to the collation of case study data and the day to day management of assets,the development of simple, non intrusive and cost effective measures for assessing andmonitoring the condition of large lengths of embankment remains a key goal.
Many of these issues will be considered within the IMPACT project, but it would beunrealistic to assume that these problems can be solved overnight. However, withincreased awareness of flood risk, promotion of international links / exchange ofinformation / liaison for research and increased government support for research in theareas outlined within this paper, we should make progress and start to reduce the bandsof uncertainty surrounding the prediction of breach formation.
7 ACKNOWLEDGEMENTS
“The authors wish to acknowledge the financial support offered by the EuropeanCommission for the IMPACT project under the fifth framework programme (1998-2002), Environment and Sustainable Development thematic programme, for whichKaren Fabbri was the EC Project Officer. In addition, the author wishes toacknowledge the financial support offered by the UK Government (EnvironmentAgency) for whom the project manager was Dr Mervyn Bramley.
The contents of this paper are based mainly upon data and information produced withinWP2 of the project by HR Wallingford Ltd and Statkraft Groner AS. The overallcontribution made by the IMPACT project team is also recognised. IMPACT teammember organisations comprise:
HR Wallingford Ltd (UK)�������������� ��������� ����� (Germany)Université Catholique de Louvain (Belgium)CEMAGREF (France)Università di Trento (Italy)University of Zaragoza (Spain)ENEL.HYDRO (Italy)Stakraft Grøner AS (Norway)Instituto Superior Technico (Portugal)”
MORRIS & HASSAN 20
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