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Transcript of River Embankment Failure due to Overtopping - In Case of ... 4/Sessi… · River Embankment Failure...
Nov 4, 2014
2014 International Workshop on
Typhoon and Flood
River Embankment Failure due to Overtopping
- In Case of Non-cohesive Sediment -
Prof. Hajime NAKAGAWA
Prof. of Disaster Prevention Research Institute,
Kyoto University, Japan
Contents
1. About river embankment failure
2. An example of recent flood disasters
due to river embankment breach
3. Flood risk in Japan
4. Research on bank breach due to overtopping
5. Conclusions
3
1. About river embankment failure
Characteristics of river EMB
Principally made of sediments from the several viewpoints (economic,
easy to get the material, easy to repair, easy to stick to foundation
ground, easy to modify, good for environment, etc.)
Cause of river EMB failure
Phenomenon Cause of REB failure Form of failure
Overtopping ・Abnormal flood
・Water level rise due to
bridge pier, weir, etc.
・Short of river EMB height
Erosion at the top
and toe of the
slope
Sliding ・Seepage of rainwater into
river EMB body
・Seepage of river water due
to water level rise
Slope siding
Erosion ・Water collide portion
・Steep slope river
Erosion at the toe
of front and back
side slope
Leakage of
water
・Insufficient protection of
river EMB against seepage
・Existence of water channel
in the river EMB body
Piping failure
boiling
http://www.zina-studio.com/p489212137/
h1834C228#h1834c228
Illustration by Zina Deretsky/National Science
Foundation
Damaged form overtopping not
overtopping
total
Breach 183 62 245
Partial collapsed
(riverside)
26 184 210
Partially collapsed
(residential side)
19 37 56
Not damaged 78 0 78
total 306 283 589
Classification of failure form of REB
(Muramoto,1986)
(75%) (25%)
6
2. An example of recent flood disasters due to river embankment breach
EMB breach at the Maruyama River in 2004
Breach at Tachino, Maruyama River
River Bank Breach
Torii Bridge
11
8
3. Flood risk in Japan (Related to river EMB)
Cities are locating on low-lying area in Japan
Low lying
areas are
Protected
by REB
Landuse in Japan
Population Property
Low lying area
River & lakes
Out of low lying area
Forest
In Japan, 50% of population and 75% of property exist in the flood prone area
that is 10% of the Japan territory. Therefore, once big flood and bank breach
happen, damage becomes very severe. Low lying areas
are Protected by
REB
Teritory
(380,000km2)
Holland
(storm surge)
UK
(Thames R.)
USA
(Mississippi R. )
France
(Seine R. )
Japan
1/100
1/Return Period
1/500
1/1,000
1/10,000
Completed in 1985
Completed 89% in 2002
Completed in 1983
Completed in 1988
Completed 60%
in 2006
1/30-1/40 years (Big rivers)
1/5 – 1/10 (Small rivers)
(Current target)
Delay of river improvement works
No./
ye
ar
No./
year
1977 ~ 2004(year)
1977 ~ 2004(year)
1976 ~ 2004(year)
1987 ~ 1996 1997 ~ 2006
1987 ~ 1996
1997 ~ 2006
6. Extraordinary heavy rainfall
14
230
186
110
157
103
188
251
190
295
156
112
256
132159
95
178
331
275245
207
174182
360
196
243
194
254
169
209
275
0
50
100
150
200
250
300
350
400
57 58 59 60 61 62 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
1982-1991
187 events
16
10
5
8 9
15
29
15
11 12
6
11
6
10 10 10
28
31
10
22
11
16
24
8
23
14
18
12
16
21
0
5
10
15
20
25
30
35
57 58 59 60 61 62 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
2002-2011
16.3 events 1982-1991
13 events
1992-2001
14.4 events
1.2 times
1.3 times
Precipitation in one hour > 50mm (1,000 observatories)
Global warming effect, vulnerability of the society, rainfall prediction, runoff process, etc.
1992-2001
199 events
2002-2011
226 events
Precipitation in one hour > 80mm (1,000 observatories)
In such an environment and situation, basic
research on the mechanism of river embankment
and its countermeasures are essential.
Necessity of Research on River
Embankments
13
4. Research on bank breach due to overtopping 4.1 Objectives
Objectives
The main objective of this research is to develop a new
numerical model that can compute the river EMB erosion
process in accordance with the infiltration calculation at the
EMB surface.
River EMB Research:Approach from the
Hydroscience and Hydraulic Engg.
River EMB of the Jamuna River,
Bangladesh
This study focuses on
• erosion due to overtopping flow
• non-cohesive homogeneous
sediments
15
Embankment
dam body Seepage
Overtopping flow
Erosion
Water level raising
*Unsaturated soil
*steep slope
Deposition Slope failure
Embankment
dam body
The process of embankment breach by overtopping is very complex because it
involves mutually dependent interactions between fluvial hydraulics, sediment
transport and embankment stability.
16 Fig. Scheme of Unsaturated Soils
Bulk water
Meniscus water
r1
r2 ua
uw
T
Meniscus water
F F
Particle Particle
Surface tension and interface curvature give rise to a difference
between pore-water pressure (uw) and pore-air pressure (ua) that is
generally equated to matric suction (=ua – uw).
The internal stress is induced by the capillary tension of meniscus
water clinging to the contact point of soil particles and acts so as to
connect the soil particles tightly.
In some circumstances, the water in the soil will form menisci
between particles.
12
11
rrTuu wa
• Suction:ua-uw
au : pore-air pressure
wu : pore-water pressure
21, rr : radius of curvature of meniscus
T : surface tension of meniscus water
Unsaturated Soils
Resistance shear stress due to suction is …
Moisture characteristic curve
Suction
Appare
nt
sh
ear
str
ength
mH
2O
No.8(d=0.1mm)
No.7(d=0.174mm)
No.6(d=0.334mm)
Suction
W
ate
r conte
nt
In the unsaturated condition,
sediment of smaller sized
diameter is more difficult to be
eroded due to suction.
No.8(d=0.1mm)
No.7(d=0.174mm)
No.6(d=0.334mm)
Critical shear velocity of the
sediment particles(u*c )
Density of sediment particle: 2.65
Kinematic viscosity of water: 0.01cm2/s
Sediment diameter : d(cm)
Iwagaki
Formula (cm/s)2
Shear stress is almost proportional to sediment diameter.
In the saturated condition, smaller the sediment diameter,
easier to move resulting in more erodible.
19
4.2 Numerical Model of
Embankment Failure
20
1. Seepage flow model • Richards’ equation
• van Genuchten equation(1980)
2. Flow model • Depth-averaged shallow water flow
equations (momentum and continuity
equations)
3. Erosion and Deposition Using framework of Non-Equilibrium
sediment transport model:
• pick-up rate equation (Erosion rate)
considering the effect of shear strength due to suction
• sediment movement is calculated using
motion of equation
• sediment deposition (probability density
function )
4. Slope failure Slope stability analysis:
• Simplified Janbu’s Method considering
the effect of shear strength due to suction
2-D (vertical slice)
2-D (vertical slice)
Horizontal 2-D
Numerical Model of Embankment Failure
21
Seepage analysis model (1/2)
tC
zK
zxK
xzx
1
Richards’ equation
: water pressure head,
zx KK , : hydraulic conductivity in the x and y directions,
C : specific moisture capacity, wC
w : volumetric water content of the soil
※continuity equation in the unsaturated EMB
Seepage analysis in the EMB
22
Flow model Depth averaged model
: depth averaged flow velocities
xyyyxx ,, : Reynolds stresses
vu,
g Gravitational acceleration H : water level
bybx , : bottom shear stresses
: density of water
Momentum equations
)()(
xyxxbx
yxhx
Hg
y
uv
x
uu
t
u
)()(
yyxyby
yxhy
Hg
y
vv
x
vu
t
v
x
v
y
u
y
v
x
u xyyyxx
,2,2
6*hu
31222 hvuugnbx
31222 hvuvgnby
0)()(
y
vh
x
uh
t
h
Continuity equation
h : flow depth
n : Manning’s roughness coefficient
Overtopping flow analysis
pm
succp
s
kGF
g
dp
*
0 11
sL
sL
k
kG
1
cos
s
sL
sL
bbs k
k
1
cos
cossincos*
Pick-up rate model(Nakagawa・Tsujimoto, 1985)+ introduction of effect of suction
c* : non-dimensional critical shear stress
,*G
: coefficients of pick up rate and non-dimensional critical shear stress considering the local slope and flow direction
sp :pick-up rate
:non-dimensional shear stress
:specific weight of sediment particle (=2.65) :angle with near bed flow direction and
direction of sediment particle motion
:angle with direction of sediment particle motion and max. slope of bed
: experimental coefficients
pS :mesh area projected on the horizontal plane
rE :erosion rate
6,4 32 AA
:ratio of drag force to lift force (=0.85) Lk
:friction coefficient of sediment(=0.7) s
pp mkF ,,0
: shape coefficients of sediment particle
32 , AA
suc* : increse of non-dimensional critical shear stress due to suction
In saturated condition,
0.0* suc
Effect of suction is considered as a increase of non-dimensional critical shear stress
psp SpA
dAV
2
3
pV :sediment volume picked up in a unit time
pp
p
rS
VE
1
:porosity p
24
2
2
2
2dAuCR bDT
2
2
3
3 dAgfdAF suc
FRT
gdCCgd
u
D
suc
D
b
1
2tan
3
4
1
2
m
b
d
ad
u
u 2.30log75.5 10
*
210
2
10
2
**
19log75.51
2
19log75.5
1
3
4
1
mD
suc
mD
cc
ddgdC
ddCgd
u
210
*19log75.51
2
gdCD
sucsuc
ddm
succc ***
tantan
rs
r
rs
rwasuc guu
• Drag force • Friction force
Log low
63.0a
4.0,0.1tan DC cuu **
63 A
42 A
Shear strength due to suction, Vanapalli et al. (1996)
wa uu : matric suction
wa uu , : pore-air and pore-water pressure
: Water pressure head of surface mesh
w : moisture content : internal frictional angle
In order to take into account the role of shear strength due to suction, we consider
critical condition for sediment movement. (Egiazaroff, 1965) tanf
In the critical shear stress condition,
bu
By addition of the shear strength to friction force, Egiazaroff formula can be express by
following equation.
For a uniform sediment,
d : diameter of sediment
: flow velocity affects the sediment
25
Sediment transport analysis
(transportation and deposition)
2,1,,,
,3
3
jkjkjkj
kjsed
km FWDtd
uddAC
Motion of equation of sediment particle
Sediment transport process after detachment is calculated by the
following motion of equation,
※direction
:drag force kjD ,
:weight in the water kjW ,
:friction force kjF ,
:additional mass ksedm
3
3 kmksed dACm
:additional mass coefficient mC
:ratio of sediment particle
density to that of water (=
2.65)
63 A
:velocity of sediment ksedu
tEDif rss
tEDif rss
Unsaturated soil
Saturated layer
Flow
26
infiltration rate:
Depth of saturated layer in one-step erosion
: Hydraulic conductivity in z
direction
: water pressure head
Erosion process of embankment surface
Erosion rate:
Unsaturated soil
Saturated layer
Flow
Unsaturated soil
Saturated layer
Flow
sD
stt
tUD ss :
zs KU zK
rE
tst
ut
tEtE rsr
urusr tEDtE
rsss EDt
0 usu tttt
Erosion within Saturated layer
27
Safety
factor
Slope stability analysis Flow
W
Δx
Ei
Ei+1
Xi Xi+1
Slip
surface
a
b
Ti Ni
As a slip surface calculation, simple
Junbu method is adopted.
n
i
ii
n
i
i
rs
rwiaiiiwii
s
W
luuluNlc
F
1
1
sin
tantan
sii
iirsrwiaiiw
s
i
iF
luuucF
W
N
tantan1cos
sintantan1
Safety factor considering the
effect of suction is
Modeling of local sliding
Dynamic programming method is used for the calculation of slip surface with minimum
safety factor
28
4.3 Numerical simulation for experiments (comparisons with experimental results)
500cm
Pump
Water supply 50cm
Drop
Water tank
Fixed bed
Video Camera
Experimental flume
Ujigawa Open Laboratory, DPRI, Kyoto Univ.
Experiments on river EMB erosion by over topping
EMB
Channel width = 30cm
Embankment Shape
30
Flow
Fixed bed
15cm
15cm
30cm 30cm 30cm 20cm 10cm 10cm
Dam body
Erodible bed
Flat bed (bottom of the flume)
Flow
40cm
80cm 35cm
Dam body
Flat bed (bottom of the flume)
Fig. – Side view of embankment shape
80cm
Type-B
Type-A
Fixed bed
31
Case Dam Type
Discharge (cm3/sec)
Sediment type
Initial water
content (%)
Porosity of dam body
1
A 7840
Sediment-No.6
13.0 0.51
2 Sediment
-No.7 11.5 0.51
3 Sediment
-No.8 12.0 0.55
4 B
1172 Sediment-No.7
6.79 0.55 5 -
Table – Experimental cases (Non cohesive sediment)
Experimental Conditions for Type-A, B
Parameters Sediment-
No.6 Sediment-
No.7 Sediment-
No.8
Ks(m/s) 2.15*10-4 8.75*10-5 1.56*10-5
dm(mm) 0.334 0.174 0.100
d50(mm) 0.285 0.146 0.078
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
10.00 100.00 1000.00
Per
cen
t fi
ner
by w
eigh
t, %
Diameter (μm)
Sediment-8
Sediment-7
Sediment-6
Grain-side distribution of EMB
No.6
No.7 No.8
Each sediment size (Type-A,B)
32
Simulated Results
(Sediment-No.7)
80s
60s
40s 20s
Failure of River embankment due to overtopping
Simulated failure process of embankment due to
overtopping with volumetric water content variation
Experimental results
(Video data)
Comparisons between simulated and
experimental results of failure process
(Sediment-No.7)
33
Simulated Results
(Sediment-No.7)
(Sediment-No.6)
(Sediment-No.8)
dm=0.334mm
dm=0.174mm
dm=0.100mm
80s
60s
40s
20s
40s 20s
10s 30s
80s
60s
40s 20s 100s
Embankment shape (Dam type-A)
Simulated and
experimental results of
embankment shape
Simulated Results
Embankment shape
(Sediment-No.8)
7.1* c※Trial calculation 60s
40s 20s
100s
dm=0.100mm
Shields Diagram
Simulated and experimental results of
embankment shape (Dam type-A)
Comparisons of simulated results with and without suction
Simulated Results
(Sediment-No.8)
Sensitivity of hydraulic conductivity (Case-3: sediment-No.8) (K1; Ks=7.8×10-6, K2; Ks=1.56×10-5, K3; Ks=3.12×10-5).
(Sediment-No.8) Sensitivity of hydraulic conductivity
K1= K2 * 0.5
K3= K2 * 2.0
40 sec. 100 sec.
Comparisons of with and without suction
with
suction
without
suction }}
big erosion
rate
K bigger
then
E faster
36 Fig. – Comparisons of simulated and experimental results of temporal moisture variations
inside the embankment
Simulated Results
Fig. – Positions of WCRs (m1 to m9) in Case-5 experiment
Seepage process inside the EMB (Type-B)
0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800
Volu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m1)
Sim (m1)0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800
Volu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m2)
Sim (m2)
0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800
Volu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m4)Sim (m4)
0
0.1
0.2
0.3
0.4
0.5
0 200 400 600 800
Volu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m5)Sim (m5)
0
0.1
0.2
0.3
0.4
0.5
1000 1200 1400 1600 1800 2000 2200
Volu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m6)Sim (m6)
0
0.1
0.2
0.3
0.4
0.5
1000 1400 1800 2200V
olu
met
ric
Wat
er C
onte
nt
Time (sec)
Exp (m8)Sim (m8)
15cm
15cm
30cm 30cm 30cm 20cm 10cm 10cm
WCR
15cm 15cm 25cm 30cm 5cm
5cm
5cm
5cm
m1 m2
m3 m4
m5
m6
m7
m8
m9
5.0
10.0
15.0
20.0
25.0
30.0
35.0
390.0 410.0 430.0 450.0 470.0 490.0
Ele
vat
ion
(cm
)
Distance (cm)
Initial
sim-10s
sim-20s
sim-30s
sim-40s
exp-10s
exp-20s
exp-30s
exp-40s
37
Simulated Results
dm=0.174mm
Embankment shape
(Dam type-B)
Simulated and experimental results of
embankment shape (Dam type-B)
(Case-4: sediment-No.7)
38
5. Conclusions
5. Conclusions
39
• In this study, a new expression for resisting shear strength due to
suction was derived in pick-up formula to compute the erosion of
unsaturated river embankment by flow overtopping.
• The proposed model was tested for erosion of embankment under
different sediment sizes (Sediment No.6, 7, and 8) and embankment
shapes (Type-A and B).
An embankment erosion model is developed using combined depth
average flow, seepage flow, sediment transport model based on non-
equilibrium model, and slope stability model.
• Generally, the proposed numerical model well reproduced the
experimental results on processes of river embankment surface erosion
due to overtopping flow.
• We used pick-up formulas based on both saturated and unsaturated
conditions to simulate the erosion process of embankment surface that
occurs simultaneously with infiltration.
(1/2)
Conclusions
40
• In small-sized sediments condition, the great shear strength and the low
infiltration rate cause a lag of erosion and delay the failure time, and
these phenomena were able to be reproduced using the proposed
model.
• A numerical model that can reproduce the temporal change of
embankment shape during the erosion process due to overtopping flow
is important and useful to consider a countermeasure against
embankment failure.
(2/2)
• There remains many problems to be solved, such as effects of
cohesiveness, mixture of sediment size, applicability of the model to the
real river EMB, etc. The model refinement is necessary in very near
future.
Thank you very much for your kind attention!