1. No category

International Workshop on the Application
of Fluid Mechanics to Disaster Reduction
February 21, 2014, IRIDeS, Tohoku University
Large scale
L
l scouring
i
by the 2011 Tohoku Tsunami
Akira Mano
IRIDeS, Tohoku University
1
Introduction
Background:
On the Pacific Coast of Tohoku, Japan, the lands are protected by coastal
levees, inlet gates, and breakwaters from the hazards of tsunamis, storm
surges, and
d wind
i d waves. Th
The 2011 T
Tohoku
h k T
Tsunamii overtopped
t
d almost
l
t allll
these structures and brought huge amount sea water onto the hinterlands.
The water soon started to return to the sea and scoured the lands on a large
scale especially near the structures
structures. Such scouring retards the
2km
reconstruction of the structures and recovery from the disaster.
Purposes are
 to know the scouring process and mechanism
 to develop the tool to predict the scouring
 to find better way to control the scouring
*
2km
2
Location Map
2km
*
Sendai Bay area
3
Sankiku Coast
Yamamoto Coast
2km
Before the tsunami （Photo March 2010）
Coastal erosion, T-shape jetties, Nourishment
After the tsunami（Photo 19 March 2011)
4
2km
Dense breach, tsunami bay, tsunami channel
Minami-Snariku-Cho, Shizugawa Bay
From left,
left Mizushiri R.,
R Hachiman R.,
R and Nitta R
R.
2km
Before the
tsunami
（25 July
2010)
After the
t
tsunami
i
（14 March
2011）
*
2km
5
Minami-Sanriku-Cho, Sizugawa Bay
Hachiman River
津波前
（2010年6
月25日）
2km
After the tsunami
（20 March 2011)
Before the tsunami
（25 July 2010）
*
2km
6
Sakamoto River, Yamamoto Coast
津波前
（2010年6
月25日）
2km
After the tsunami
（14 M
March
h 2011）
Before the tsunami
（10 D
December
b 2009）
*
2km
7
Ootsuchi R. and Kotsuchi R., Ootsuchi Bay
Railway bridge at left bank
2km2000）
（Photo,
After the tsunami
（24 Marchi 2011）
Railwayy bridge
g at left bank
（Photo 2007）
8
2km
Scouring process, Yamamoto Coast
Sea
Land
Capture of tsunami video
Breaking of the coastal levee
Tsunami profile at Abukuma gate and the phase of the video shooting
9
Scouring process, Abukuma River mouth
Return flow from the breach point
河川堤防の被災状況
Large scale scouring behind the levee
海岸堤防の被災状況
10
Scouring process, Fujitsula
Return flow trace from the breach point
Scouring by the tsunami
3m scouring behind the levee
Scatter of the armor blocks
11
Scouring at ditches, Yamamoto Coast
formation of tsunami bay
12
Mechanisms of large scale scouring
First step：impact
p
p
of the first wave surge
g
Break of the levee parapet or at the joint
Huge amount of seawater inflow to the land
Second step: Scouring by the flow concentration
 at the break points
 beyond the ditches
13
West
Prediction of large scale scouring
Review 1: The report of the 2004 Indian Ocean Tsunami
Tsunami Inundation Scour of Roadways, Bridges, and Foundations,
Observations Technical Guidance from Great Sumatra Andaman Tsunami
EERI/FEMA NEHRP2006 Professional Fellowship Report
Principal Advisor: Harry Yeh, Oregon State University
Excessive pore pressure in sand layers
leads liquification and then deeper
scouring. The report concludes that this
mechanism is extremely
y high
g to cause the
scouring.
Marina Beach, Chennai
Arugam Bay Bridge, Sri Lanka
Kamala Beach, Phuket
Prediction of large scale scouring
Review ２：Tonkin et al
al.(2003)
(2003)
Tsunami scour around a cylinder
cylinder,
Journal of Fluid Mechanics, vol. 496, pp. 165-192, 2003
S. Tonkin, H. Yeh, F. Kato, and S. Sato
Wave surface, velocity, pore pressure in the bottom sand were measured for
solitary wave. Since the maximum scouring occurred for smaller velocity,
liquification due to excessive pore pressure is concluded to be the major cause of
the scouring.
scouring
15
Prediction of large scale scouring
Review 3：Pan et al
al.(2012)
(2012)
Numerical modeling of tsunami wave run
run-up
up and effects on sediment scour
Around a cylindrical pier, Journal of Engineering Mechanics,2012.138.1224-1235
C. Pan and W. Huang, Zhejiang Institute of Hydraulics and Estuary, China.
Pan et al. conducted numerical simulation of the Tonkin’s experiment(2003）by the
shallow water equations, suspended sediment transport equation with erosion by the
bed shear stress. They can reproduce 8cm scouring out of the maximum 15cm.
Two mechanisms of scouring are
・excessive pore pressure
・bottom shear stress
16
Our model on the scouring
Iid (2014)
Iida
The shallow water equations
Quasi-3D sediment transport equation
C 1 q sx 1 q sy C  q x q y 



 

D y
D  x
t D x
y 
  C    C  w0

C a  Cbottom 
   s H11号
   s
x  x  y  y  D
where C is vertically averaged concentration of the suspended sediment with
assuming the equilibrium vertical profile of Rouse.
Reference concentration by Van Rijn(1993) is adopted,
C a  0.015
d 0.3     c
D* 
a
 c
1.5




with Shields parameter 
17
Finite Volume Meshing
at Fujitsuka area
H11号
18
Surface elevation and velocity
30 minutes from the start
H11号
Surface elevation
Velocity and stream lines
19
Scouring depth
30 minutes from the start
H11号
Computed. Maximum depth, 10m
Observed on March 12, 2011. The
maximum depth is about 6 m
obtained from the trench survey.
20
Conclusions
 We find the mechanisms of the large scale scouring.
 We develop the quasi-3D sediment transport model which
reproduces the scouring qualitatively.
 Further studies are
 to quantify the scouring by shear stress and pore
pressure
H11号
 to reproduce tsunami channels and bays by the
simulation
 to find better way to control the scouring
21
最後に
Thank you
流れのモデル
•
非線形浅水流方程式



  qx  q y  0
y
x
t
•
  2q
 2qx



  bx
q x  uq x  uq y   gD

  t  2x 
 x

t
x
y
x
y 2





  2q y  2q y



  by

 t 

q y  vq x  vq y   gD
 x 2
t
x
y
y

y 2





底面せん断力
2
 bx  
•
n g
D
73
q x q x2  q 2y ,  by  
渦動粘性係数
12
 t  0.11
ng
D
16
n 2 g
D
73
q x q x2  q 2y
x，y：直交座標
η：水位
水位
qx，qy：x，y方向それぞれ
の単位幅流量
u，v：x，y方向それぞれの
水深平均流速
g：重力加速度
D：水深
ρ：流体の密度
流体の密度
（=1000kg/m3）
τbx，τby：x，y方向それぞれ
の底面せん断応力
νt：水深平均渦動粘性係数
n：Manningの粗度係数
q x2  q 2y
23
2013/10/18 Mゼミ
河床変動モデル
•
河床変位計算
Z b
1

Cbottom  C a w0
t
1 
•
Rouseの濃度分布式
C
1

Cbottom 1  z a*
•
1

z a*
 1  z
 *
 z
*


*

 1， z a
z a*




Ro 
Ro
ddz *
w0
u*
土砂輸送方程式
C 1 q sx 1 q sy C  q x q y 



 

t D x
D y
D  x
y 
  C    C  w0

Ca  Cbottom 
  s
   s
x  x  y  y  D
•
巻き上げ計算（van Rijn, 1993）
C a  0.015
d 0.3     c
D* 
a
 c
1.5




2013/10/18 Mゼミ
Zb：河床高
λ：空隙率（=0.4）
Cbottom：基準点沈降濃度
Ca：基準点巻き上げ濃度
w0：土粒子の沈降速度
土粒子の沈降速度
C：水深平均土砂濃度
za*：無次元基準点高さ
Ro：Rouse数
κ：カルマン定数（=0.4）
qsx，qsy：x，y方向それぞれの
単位幅土砂流量
εs：水深平均拡散係数
水深平均拡散係数
d：土砂粒径
D*：無次元土砂粒径
θ：無次元せん断力（
無次元せん断力（=ρu
ρu*2/(ρs-ρ)gd
ρ)gd）
ρs：土粒子の密度
θc：限界無次元せん断力
a：係数（=Δ/2=50d）
Δ：河床波の高さ
河床波の高さ
24