自然災害科学

特別号（
特別号

（
）

65

）

EXPERIMENTAL STUDY ON
RIVER DYKE BREACH
CHARACTERISTICS DUE TO
OVERTOPPING
Pawan Kumar BHATTARAI ＊・Hajime NAKAGAWA ＊＊・Kenji KAWAIKE ＊＊＊・
Hao ZHANG ＊＊＊＊

EXPERIMENTAL STUDY ON RIVER DYKE BREACH
CHARACTERISTICS DUE TO OVERTOPPING
Pawan Kumar BHATTARAI*, Hajime NAKAGAWA**,
Kenji KAWAIKE*** and Hao ZHANG****
Abstract
Every year flood discharge induced overtopping flow through river dykes causes large
damage to society, the environment and infrastructure. Although several researches havebeen conducted to understand embankment dam failure, there has been little focus onriver dyke breaches. The lateral widening of the breach process and the duration time
at failure are poorly understood. Therefore, preliminary laboratory tests were conducted
to investigate the dynamic lateral widening process and breach characteristics. The entire
process was recorded optically using video and three-dimensional cameras for detailed
analysis of the breaching phenomena. The result indicates the important effect ofparticle size on the dynamic lateral widening process, breach discharge and the time of
failure. A systematic variation of the particle size reveals important findings regarding
breach characteristics. Finally, the curve for the duration time at failure is also
proposed.
Key words： Dyke, Breach process, Lateral widening, Overtopping, Breach Discharge

Student Member of JSNDS, Doctoral Student, Department
of Civil and Earth Resources Engineering, Kyoto
University (Katsura Campus, Nishikyo-ku, Kyoto
615-8540, JAPAN)＊＊
Member of JSNDS, Dr. of Eng., Prof., DPRI, Kyoto
University (Yoko-oji, Fushimi, Kyoto 612-8235, JAPAN)

Member of JSNDS, Dr. of Eng., Assoc. Prof., DPRI,
Kyoto University (Yoko-oji, Fushimi, Kyoto 612-8235,
JAPAN)＊＊＊＊
Member of JSNDS, Dr. of Eng., Assist. Prof., DPRI,
Kyoto University (Yoko-oji, Fushimi, Kyoto 612-8235,
JAPAN)

＊

＊＊＊

65

66

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPING

1. INTRODUCTION

phenomena. The process of breaching is the result

A large number of embankment structures haveof the interaction between three elements, i.e.

been built by humans or formed naturally alongwater, soil and structure. Therefore, the breachingrivers, lakes, and coastal lines throughout theprocess differs according to hydraulic loading,world. A dyke is defined by geometric properties

material type and state as well as embankment(e.g. height and width of the crest, slopes etc.) andcondition (Naulin et al.2010).

by material characteristics (density, grain size

During breach initiation, the so-called residual

diameter, cohesive strength, erodability,

strength of the dyke is consumed by different

permeability etc.). River dykes are constructed tomechanisms and processes. For most authors,

prevent flooding of valleys and their population andbreach growth starts when the dyke crest lowers

confine the flow of the river for higher and fastersignificantly and erosion cuts through theflow. Flooding of river valleys is a longstanding

upstream face of the dyke. According to Morris etproblem and numerous events still occur yearlyal. (2009), breach formation is the stage of thecausing both monetary and human losses. Recent

breaching process when rapid and continued

major dyke failures include those of the Yangtze

vertical and lateral erosion of the embankment

flood in China 1998, the Elbe flood in Germany inoccurs. The erosion process itself is 3-dimensional

2002, the New Orleans flood in 2005, theand the hydraulic conditions vary constantly.

Mississippi flood in 2008, the Pakistan flood in 2010

Chinnarasri et al. (2003) observed four stages inand the Queensland flood in Australia inplane dyke erosion, namely: (1) small erosion on

2011(Schmocker et al. 2011). The lack of properdyke crest after initial overtopping, (2) slope

repair and maintenance and increased flood

sliding failure with ongoing erosion, (3) wavelike-discharges are important factors in the dyke

shaped dyke profile and (4) large sediment wedge

failures in recent years.

deposition with small slope at erosion end.

From the study of past dyke breach phenomena,

Recently, the Civil Engineering Research

the failure by overtopping constitutes the primary

Institute for cold regions, Hokkaido, Japan

failure modes of earth embankments besides

performed a real scale model study in the Chiyoda

internal erosion. The resulting breach flow isExperimental Flume regarding observation ofparticularly dangerous and damaging to thebreach phenomena. They observed that the breachsurrounding infrastructure and the environment asphenomena start with frontal shoulder erosion and

it usually occurs unpredictably both on a temporal

accelerate in the downstream direction.

and a spatial scale. In flood defence projects, dyke

In the past several years, many studies have beenbreach scenarios should be accounted for in theperformed regarding dam breaches. The mostassessment of the residual risk for the overload

commonly studied breaching parameter of dams

case, when determining the emergency action

includes depth and breach width together with theplans and the associated preparedness measures.

breach side slopes or shape at the final stage. The(Schmocker et al. 2011)

time at failure was reported in less than half of the

Although the risks of dyke breach processes arecases and often with great uncertainty. Ultimate

known from historical incident, little research hasbreach depth can be estimated with acceptablebeen conducted in this field by hydraulicaccuracy in terms of dam height. Several

researchers. The main reason behind this is theinvestigators have attempted to relate the elapsed

complexity associated with the dyke breachtime required for failure in terms of the hydraulic自然災害科学

特別号（

67

）

head above invert, breach height and volume oferoded materials but these are parameters that canbe determined following a breach, so they are lessuseful for the prediction of time at failure. Given theenormous damage potential regarding dyke
breaching, it is necessary to understand thedamage process of dykes in detail.
As the majority of dyke breaches are due toovertopping, this is investigated in detail in thecurrent research. Systematic breach tests were

Fig. 1 Plan view of laboratory setup

conducted at the Ujigawa Open Laborator y,
Disaster Prevention Research Institute, Kyoto
University, Kyoto, Japan. The main goals of thisstudy are to establish a simple model setup for
testing the pure overtopping failure mode, obtain
information such as the temporal and spatial lateralwidening process and resulting water and sedimenthydrograph, develop simple curves to determine

Fig. 2 Cross-section along X-X shown in Fig.1

the time at failure prediction and finally provide
laborator y data for the next stage of theexperiment and a numerical model.

2. Laboratory setup
2.1 Model flume
The main hydraulic test conditions assumed in
the current study include: 1) Trapezoidal shaped
dyke kept parallel to the river flow direction (Fig. 1
and Fig. 2), 2) homogenous sediment, 3) 5%
moisture content by weight was added initially to

Photo 1. Flume view

dyke materials and compacted using a similar
procedure throughout all the experiments and nochannel is also 0.3 m at a distance of 3.35 m fromcore layer was added, 4) steady inflow withthe upstream. The water flows out from the dyke

downstream gate control, 5) optical recording byinto the downstream reser voir through thevideo, high-speed and three-dimensional cameras.

horizontal plate. The horizontal plate acts like a

The breach test was performed in a Poly Vinyl

supporter for the side channel and as a working

Chloride (PVC) sided flume as the river and theplatform. The intake is equipped with a flow

dyke were placed parallel to the river flow alsostraightener to generate undisturbed flow from thehaving PVC walls on both sides (see Photo. 1). The

underground tank with a pump that circulates

maximum discharge provided is 7.89 liters/sec.

water continuously between upstream and

The horizontal channel is 0.3 m wide, 0.3 m highdownstream. The eroded sediment and theand 8 m long and dyke opening at the side ofresulting water flow were collected manually with a

68

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPING

plastic box at the end point of the dyke. The plan

overtopping, a temporary wooden plate was used toand section of the experimental setup are shown incover the dyke area until the required depth ofFig. 1 and Fig. 2, respectively.

water in front of the dyke was reached. Then, theThe parameters investigated include: Flume

wooden plate was slowly removed to generate puredischarge Q = 7.89 l/s, dyke height H = 0.15 m,

overtopping flow. The discharge was measured atdyke length L = 0.3 m, dyke crest width, b = 0.075m

the end of the dyke point with the help of plastic(0.5H), and particle size = No. 5, No.6, No. 7 andboxes at certain time intervals.

No. 8 (See Fig. 3). The upstream dyke slope is Su =1:2.5(V: H) and downstream slope is Sd= 1:2(V: H)

2.2 Experimental procedure

resulting in a total dyke width of B =2.5H+0.5H+2H

All experiments were conducted using a constant=5H. The pilot channel (2.5 cm x 2 cm) is made atflow discharge in a flume Qin, referred to as thethe center of the dike as a point for the initiation of

inflow, to obtain the simple boundary condition.

breach at the desired location.

The dyke material was initially provided with 5%

To prevent seepage-induced dyke failure prior tomoisture content by weight and thoroughly mixed
and then compacted with the help of a tamping
plate and cylindrical roller by spraying water layer
by layer until about approx. 90% compaction wasachieved. The inflow was added rapidly to attain asteady state before over topping star ted byadjusting the gate at downstream and closing thedyke area by the wooden plate. The flow
overtopped the dyke from the pilot channel thus
resulting in the lateral widening of the breach fromthat point.

Fig. 3 Particle size distribution of materials for modeldyke
Table 1: Experimental Condition

A total of 12 experiments were conducted for
four sediment types. Each experiment was
repeated three times to observe the effect of
repeatability. The three experiments for each

Case Name

Particle
Size No.

Mediandiameter (d50)Remarks

particle size showed a similar trend and values. So,

CaseS5_1

50.327 mm

Main exp.

only the representative values here are taken for

CaseS5_2

50.327 mm

Repeated

the analysis i.e. the values from the one experiment

CaseS5_3

50.327 mm

Repeated

only. There was no tail water effect and the dyke

CaseS6_1

60.278 mm

Main exp.

overflow was always free. The test was stopped

CaseS6_2

60.278 mm

Repeatedwhen the entire dyke material was washed out and

CaseS6_3

60.278 mm

Repeated

the flow from the dyke reached its equilibrium

CaseS7_1

70.083 mm

Main exp.

state. The experiments were therefore limited to

CaseS7_2

70.083 mm

Repeated

dynamic change in the lateral widening process. To

CaseS7_3

70.083 mm

Repeated

represent an actual dyke breach scenario, the

CaseS8_1

80.049 mm

Main exp.

hydraulic model should consist of a main channel

CaseS8_2

80.049 mm

Repeated

representing the actual river and a perpendicular

CaseS8_3

80.049 mm

Repeatedside channel where the dyke is placed.

自然災害科学

特別号（

69

）

Furthermore, the basic upstream boundary

creates erosion of the channel surface by shear

condition is obtained by selecting the constantstress. Interestingly, on both sides of the channel

inflow scenario. In prototypes, a dyke is overtopped

there is the formation of a vertical slope.

once the river water level exceeds the dyke crest

The vertical slope seems to be sufficiently stable

elevation. The overtopping flow depends on thedue to the matric suction of the unsaturated soil.

breach progress and the river discharge

When the vertical undermining reaches the fixed(Schmocker, 2011). The breach and riverbed, then the erosion starts to expand laterally

discharges interact so that there is an almost

from the toe of the opposite slope of the dyke. In

constant inflow to the breach section.

the meantime, the seepage travels through thevertical slope developed on both sides of the

2.3 Experimental evaluation

channel. If the vertical slope is sufficiently

As the pilot channel was constructed at thesaturated to overcome the sectional resistancecenter of the dyke to clearly visualize the lateralstress, then those slopes collapse in the channel

widening patter n, the lateral view wasflow at the center. The relatively dry sand in the

representative for the breaching process analysis.

beginning has a minimal tensile strength. As the

The process was recorded with a Casio Exlim high-degree of saturation of the dyke progressively

speed camera, Go-Pro Black edition high-speed

increases when the overtopping starts, the tensile

Camera, Sony HD camera and Fuji film 3D camera

strength first increases to a maximum depending

continuously. The 3D camera was used to recordon the particle size and porosity, followed by athe 3D image at intervals of 5 sec and later Fujifilm

reduction to zero at near full saturation (Lu et al.

software was used to generate the spatial

2009)

coordinates. The lateral widening process at

Consequently, both the collapsing of verticalintervals of 5 sec was generated with the 3D

slopes as well as the increase of lateral width of thecamera while breach discharge, sediment

channel from the toe occur simultaneously anddischarge and seepage movement were analyzed

finally the entire dyke collapses. Figure 4 shows

using the recorded images of the opticalthe breach width change pattern with respect toinstrument.

time. The first two columns of Fig. 4 show the
widening patterns with absolute values while the3. General observations

third column shows the widening speed derived

3.1 Lateral widening processfrom the second column of the figure. The result

The effect of particle size on the lateral wideningshows that the breaching phenomenon was very

process is presented below in Fig 4. For particles

slow at the beginning at the top, middle and toe

No. 5, 6, 7 and 8 with all other parameters constant,section. Then as the vertical breaching reached thedifferent breach shapes resulted- but the process of

fixed bed, the toe section as well as the top section

the widening seems to follow similar procedures.

rapidly eroded in comparison with other sections

For all the processes, initially, as the overtopping

causing most of the breaches of the dyke. The

begin in the dyke, the channel is eroded vertically

middle section acts like a hinge section for both thelayer by layer through the pilot channel until theerosion and finally the middle section collapse

water surface touches the fixed bed. The reason for

suddenly. Thus, finally the collapse of the entire

this is that the top layer beneath the water in thedyke occurs.

pilot channel is saturated quickly with the flow that

Similarly, the widening speed curve (Fig. 4) also

70

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPING

Fig. 4 Lateral widening process and average widening speed for different particle sizes自然災害科学

特別号（

71

）

shows that for all the particle sizes, toe widening

3.2 Particle and water discharge

velocity increases rapidly in comparison with the

Figure 6 shows the flow hydrograph generated

top and middle point;-as the time passes the top anddue to river dyke breaches due to overtopping for

middle widening velocity increases together but

different particle sizes. The breach discharge

finally when the toe widening speed becomesincreases rapidly after initial overtopping until thenearly zero, the top widening speed overpasses the

maximum breach discharge Qmax > Qin is attained.

middle and finally collapse occurs.

Here Qmax is the discharge of the hydrograph andFrom all the results obtained, it can be

Qin is the inflow discharge in the flume Afterconcluded that Particle No. 8 seems to be morereaching the maximum outflow, the breach

resistive to the erosion in the first few seconds

discharge decreases slowly to Q = Qin and thenthan the other particle types due to particle size

remains constant. Given the constant inflow

and slow saturation rate i.e. the suction pressure

scenario, Qmax depends on Qin in contrast to dam

acts more in the case of Particle No. 8 and breachbreak analysis considering reservoir where Qmax

took a long time to develop and finally collapse.

constantly increases. In Fig. 6, the dimensionless

Consequently, Particle No. 7, Particle No. 6 andmaximum breach discharge Qmax/Qin is discussed

Particle No. 5 follow the Particle No. 8 in thefor dif ferent par ticle types. The maximum

breaching time and collapse process.

discharge obtained is around 1.6 times thedischarge in the flume. The graph clearly showsthat the time lag of peak of the hydrographFig. 5 Lateral widening process (sample example for Particle No. 8)

72

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPING

Fig. 6 Breach hydrographFig. 7 Particle discharge hydrographincreases with the increase in sediment number i.e.

overtopping depth, d50 is particle size 50% finer bydecreasing d50 (median diameter or the medium

weight (median diameter), H height of the dyke Su

value of the particle size distribution). Figure 7

is upstream slope, Sd is downstream slope and θishows the obtained particle discharge hydrograph.

initial moisture content of the dyke materials

The sediment graphs have more than one peak

respectively. Parameters like y, θi , Su and Sd werewhich reveals that the vertical stable wall at thenot systematically varied and are therefore

side of the channel collapses into the center ofdropped. As only variation with particle sizes werebreach at different times. The figure also clearlytested, dyke height and specific discharge isdemonstrates the particle size effect on the erosion

considered as impor tant parameters. Byof the sediments that determines the breach width.

Buckingham-π theorem, we get,As previously stated, Particle No. 8 seems moreresistive than other sediment sizes.

(1)

3.3 Prediction of time of failure

Out of the three dimensionless quantities, theThe time of failure, here, is defined as the timefirst one on the left side is named dimensionlessthe second one first on

taken for the breach discharge to be maximum

time of failure (A) i.e.

from the time that overtopping starts. The time of

the right side is named dimensionless specificfailure is therefore directly observed from the flow

discharge (B) i.e.

hydrograph from the breach. To generalize thefrom the right is named dimensionless height of

results obtained with the current test, thedyke (C) i.e.

governing parameters considered are described as

are assumed to be constant for all tests anda functional relationship i.e. Time of failure,

therefore not taken into account here, given thatand the third one second

The viscosity and surface tension

scale effects are excluded.
where, q is discharge per unit width at flume, Ks

Figure 8 shows a graph of the relations between

hydraulic conductivity of the dyke materials, y isdifferent dimensional parameters obtained through自然災害科学

特別号（

73

）

4. Conclusion
The present experimental work investigated thelateral widening breach characteristics withdifferent particle sizes due to overtopping. A
simple dyke model was used consisting of
homogenous, uniform materials of various particle
sizes. A steady inflow scenario was tested and all
the breaching processes were recorded using highspeed camera and a 3D camera.
Within the range of test parameters, the dyke
breach process was accelerated with the increase
in median diameter of particle d50. For coarse
Fig. 8: Time of failure curve

sediment, due to quick saturation of the dyke body,the collapse time decreased. The width of thethe dimensional analysis using experimental

breach increased at the top and toe section

results. The curve is obtained by joining thecompared to the middle section which acted as a

calculated value from the most suitable experiment

hinge point. In the final stage, the middle section

out of all the experiments. The curve shows that it

also failed leading to the entire dyke collapse. The

has a minimum value when dimensionless time ofbreach discharge and particle discharge showed= 33.5 which can be considered as

the time lag and peak according to the particle size.

the critical dimensionless failure time. For the

Finally, the time of failure curve was proposed foruniform particle case as in the current experiment,

the pre-determination of occurrence of peak breachthis value determines the minimum value of timedischarge based on the pre-disaster parameters

reaching the peak breach discharge. From

which, if later, validated, will be an important tool

Particles No. 5 to 8, the dimensionless time of thefor the natural disaster mitigation.

failure value

failure value decreases at first when bothdimensionlessspecific

discharge

Once the hydraulic and physical mechanisms ofandthe simple model dyke are understood, the test set-

dimensionless dyke height increase up to a criticalup can be adapted to account for further

value then increase from the critical value as theparameters affecting the dyke-breach such asother parameters increase.

surface protection, soil layers, core, compaction orValidation of the curve is still not complete due tosoil moisture content. The current findings have tounavailability of such data but this curve can be

be expanded in the next experiment consistingused later to check the validity of the next stage

larger dykes with a more realistic experimental

experiment of lateral dyke failure cases of ourarrangement. The next stage experiment isresearch. However, the result from the real scalecurrently in progress.

model study in the Chiyoda Experimental Flume of
the Civil Engineering Research Institute for cold
regions, Hokkaido, Japan shows good validation of
the curve.

ACKNOWLEDGMENT:The research is suppor ted by JST/JICA
SATREPS Program on disaster prevention/
mitigation measures against floods and storm
surges in Bangladesh (PI: Dr. Hajime Nakagawa).

74

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPINGThe authors are also grateful to the GCOE-ARS
program and research assistant grant of thebreaching due to overtopping J. Hydraulic Res.
47(5), 585–597.

Disaster Prevention Research Institute, Kyoto
University, for providing financial support.
This research was also partially funded by theJapan Society of Promotion of Science, Grant-in-Aid
for Scientific Research (B) (Head: Dr. Hajime
Nakagawa, Kyoto University, Grant No. 22360197).

REFERENCES
Chinnarasri, C., Tingsanchali, T., Weesakul, S.,
Wongwises, S.(2003). Flow patterns and damageof dike overtopping.” Int. J. Sediment Res. 18(4),301–309.
Coleman, S.E., Andrews, D.P. and Webby, G. (2002)“Over topping breaching of non-cohesive
homogenous embankments”, J. Hydraulic Eng. 128(9): 829-838
Froehlich, D.C. (1995). “Peak outflow from breachedembankment dam”, Journal Water Resource
Planning Management 121(1), 90-97.
Kakinuma T.Tobita D., Yokoyama H. and Takeda A.
(2013). “Levee Breach Observation at Chiyoda
Experimental Flume” Advances in River SedimentResearch, Taylor and Francis Group. Pg. No. 105.
MacDonald, T. C., and Langridge-Monopolis, J. (1984).
ʻʻBreaching characteristics of dam failures.ʼʼ Journalof Hydraulic Engineering, 110(5), 567–586
Morris, M.W., Hassan, M., Kortenhaus, A. and Visser,
P.J. (2009) “Breaching Processes, A State of the Art
Review” FLOODSITE Report T06-06-03
Nakagawa H., Mizutani H., Kawaike K., Zhang H.,
Yoden Y., Shrestha B.(2013) “Numerical
Modelling of Erosion of Unsaturated River
Embankment due to Overtopping Flow” Advancesin River Sediment Research, Taylor and Francis
Group, London.
Nakagawa H., Utsumi T., Kawaike K., Baba Y. andZhang H.(2012) “Erosion of unsaturated
embankment due to overtopping water” J. Japan
Society of Civil Engineers. Ser. B1 (HydraulicEngineering) 67 (4): II_1-II_4.
Schmocker, L. (2011). “Hydraulics of Dike Breaching.
PhD- Dissertation , VAW-Mitteilung 218, R.M.
Boes, ed. ETH Zurich, Zurich
Schmocker, L., Hager, W.H. (2009).”Modeling dike

（投稿受理：平成 26 年 4 月 18 日）