Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

STUDY ON DAMAGES OF LAVER
AQUACULTURE FACILITIES IN TOKYO BAYFROM 2011 TOHOKU EARTHQUAKE TSUNAMI
D.P.C. Laknath1, Takahide Honda2, Kazunori Ito3, Jun Sasaki4, Yuriko Takayama5
and Yukinobu Oda6
1Technology Center, Taisei Corporation, laknath@pub.taisei.co.jp
2Technology Center, Taisei Corporation, hndtkh01@pub.taisei.co.jp
3Technology Center, Taisei Corporation, kazunori.ito@sakura.taisei.co.jp
4Faculty of Urban Innovation, Yokohama National University, jsasaki@estuarine.jp
5Technology Center, Taisei Corporation, yuriko.takayama@sakura.taisei.co.jp
6Technology Center, Taisei Corporation, od-ykn00@pub.taisei.co.jp


In scientific literature, damage processes of laver aquaculture facilities (Nori-Hibi) under the tsunami
flow have not been dealt with in depth so far. Hence, as an initiative step to explore the possible failure
mechanisms and resultant drifting processes of the Nori-Hibi compartments, Nori-Hibi damages in Tokyo
Bay, caused by 2011 Tohoku Earthquake Tsunami was studied with numerical simulation. After
estimating possible damages in study area based on reproduced hydrodynamic conditions and resultant
external forces, different failure modes were assessed to understand the failure mechanism. Due to thecomplexity of the selected failure modes, driftage simulation was performed only for the "breakage of
supporting pole" failure mode. Field investigated damage intensities of the Nori-Hibi compartments in the
Futtsu cape and Banzu tidal flat areas can be qualitatively explained through this failure mode.
Key Words: Nori-Hibi, tsunami damages, failure modes, Delft3D, driftage simulation model

1. INTRODUCTION

2. STUDY AREA

2011 Tohoku Earthquake Tsunami caused
significant damages to the laver aquaculture farmingin Tokyo Bay. Since there is a possibility ofrecurrence of tsunamis in the same region, further
study about the intensity of past damages anddamage mechanisms of the laver aquaculture
facilities (here after Nori-Hibi sets) are important toestimate the future damages. In recent studies,
Sasaki et al. (2012) investigated damages due to thetsunami on Nori-Hibi farming in eastern part of thebay. Their study identified a positive correlation
between the scale of damages and the hydraulicresistance on laver nets. However, in the scientific
literature, damage mechanisms of Nori-Hibi sets
and driftage under the tsunami flow have not been
well revealed because of the shortage of field data.
Therefore, for the further understanding of thedamage processes, the objectives of the present
study are to consider possible failure modes andexplain the scale of damages and driftage ofNori-Hibi sets in the Futtsu cape and Banzu tidal
flat areas in Tokyo Bay (Fig. 1 (a)).

Nori-Hibi is an aquaculture facility for catching
the spores of laver ("nori"), and to grow the lavernursery. In early days, laver spores had beencollected by planting bamboo or brushwood ("hibi")
in shallow waters. In 1940s, a traditional farming
method was improved to the present one using"fixed nets" (or "pole system") consisting ofsynthetic material nets and bamboo poles. In thisFig.1 (a). Nori-Hibi sets damaged areas in Tokyo Bay(b). Typical arrangement of the Nori-Hibi compartment(c). Sectional view of Nori-Hibi compartment across XY.

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Table 1
Location
Banzu
tidal flat
Futtsucape

Damaged details of Nori-Hibi sets in Tokyo Bay.
(Source: Sasaki et al., 2012)
DamageDescription of damage
scale
High
x
Ushigome: 2 sets collapsed
damagesx
Kaneda: 7 sets destroyedx
Futtsu fishery cooperative
Farming bed: 33 sets destroyed.
Very
high
x
Shin-Futtsu farming bed: 170
sets were damaged and 116 sets damages
destroyed.

arrangement, laver aquaculture nets are stretchedbetween supporting poles (bamboos). Supporting
poles are fixed to the shallow water bed (Borgese,1980). Laver nets float on the water surface with thetidal motion (Fig. 1 (b & c)). During the ebb tideperiod, the nets are exposed periodically to the air.
According to the field survey results of Sasaki etal. (2012), identified tsunami damages of Nori-Hibi
sets in study area are summarized in Table 1, where
the details of damage are presented in terms of thenumber of damaged Nori-Hibi sets. Accordingly,
the scale of damage for Nori-Hibi sets in the Futtsu
cape area is much higher than that in the Banzu tidal
flat area.

3. METHODOLOGY
For the detailed understanding of damages on
Nori-Hibi sets, tsunami induced hydrodynamic
characteristics, including flow velocities and water
depths were reproduced using Delft3D–Flowhydrodynamic model (Deltares, 2010) for six hours
of model period. To evaluate the resistance ofNori-Hibi set due to the external forces induced bytsunami flow, effects on idealized Nori-Hibi
compartment consisting of 4 nets (40 m × 3 m)
and 40 supporting poles (32 mm diameter) wereconsidered (Fig. 2). As main horizontal forces on
Nori-Hibi compartment, hydraulic resistances onlaver net (FNet) and drag forces on supportive poles(FDrag) were considered. According to Yagi et al.
(2009), laver net resistance force (for single net) was
calculated using the following equation.
1
F =  2(1)
2
where ρ is the density of sea water, U is the depth
averaged velocity and A is the surface area of the net.
The resistance coefficient of the laver net (Cnet) was
taken as 0.02, assuming high quantity of laver at the2011 tsunami event (Yagi et al., 2009). Further, thetotal laver net force (FTNet) acting on a compartment
was calculated by multiplying FNet by the number oflaver nets in the compartment (Fig. 2 (a)). The totaldrag forces on supporting poles per compartment
were calculated based on the "drag law" using thefollowing equation (Schlichting, 1979).

Fig.2 Typical arrangement of Nori-Hibi compartment.



1

F=  ℎ2 (2)2where Cd, is the resistive coefficient of a circularcylinder (=1.1), D is the diameter of the supporting
pole, h is the water depth and N is the number ofpoles per compartment. In case of vertical direction,buoyancy forces (b) on submerged laver nets andself-weight (w) of the Nori-Hibi set were considered
as main forces. In this study, possible failure modes
for Nori-Hibi compartment were assumed because
the real failure mechanism is unknown. Thus, on thebasis of combined different external forces, threepossible failure modes namely, "breakage of
supporting poles", "breakage of ropes" and "pullingout of poles" were defined and tabulated in Table 2.
Considering the material properties (e.g. bending
strength/shear strength of bamboo, tensile strengthof rope) and maximum internal forces developed in
the structure (bending and shear stresses), damage
modes were further analyzed to find out thethreshold values of the parameters at the failure. As
shown in Fig. 3, computed hydrodynamic
parameters (flow depths and flow velocities) using
Delft3D-Flow and criteria developed to initiate thedriftage of the drifting objects were used as theFig.3 Sequence of processes and input/ output of each process.

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Table 2

Failure modes of Nori-Hibi set.
ForcesFailure Mode

FNet

FDrag

w

b

1. Breakage of supporting poles
2. Breakage of ropes

○
○
○

○
○

○

○

3. Pulling out of poles○ = Included, - = Not included

input parameters for the driftage simulation model.
This model has been successfully used by
Takayama et al. (2010) to simulate the driftage
paths of Scallop Larva in Lake Saroma.Finally, on
the basis of identified driftage pattern, damages on
Nori-Hibi sets in the study areas were explained.

4. RESULTS AND DISCUSSION(1) Hydrodynamic characteristics
Fig. 4 shows flow patterns and time varying
flow velocities, water depths and external forces
calculated for the representative locations for Banzu(a).

Velocity (m/s) | t = 150 min.

U (m/s)

B0

F0

(b).

Location B0

Location F0

tidal flat and Futtsu cape areas (B0 and F0
respectively). As shown in Fig. 4 (a), higher flow
velocities are visible in both areas. Since flow
velocities are positively correlated with induced
horizontal forces, much higher forces andconsequently higher damages can be expected inboth areas. As clearly shown in Fig. 4 (b), total
force exerted on a Nori-Hibi compartment in the F0
area is much higher than in the B0 area. Therefore,
the intensity of damage in the F0 area could behigher compared to that in B0 area. Thus, field
investigated damages are in agreement with thenumerical reproduced results.
(2) Assessment of various failure modesa) Breakage of supporting poles
Breakage of supporting poles was assessed forboth bending and shears failure modes. As a resultof exerted horizontal forces (FNet and FDrag),
maximum bending moments and shear forces along
a bamboo pole were analyzed (Fig. 5). To estimate
the horizontal laver net force acting on a single pole,
total laver net force on a compartment (FTNet) was
equally distributed among the number of supporting
poles. Since Delft3D simulation results are depth
averaged values, velocity distribution of flow
currents in vertical direction could not be obtained.
Due to the fact that tsunami wavelength is far longer
than normal waves, approximation under "veryshallow water wave condition" can be adopted.
Under this condition, horizontal velocity distribution
along the depth does not change much. Therefore,drag force along the pole can be assumed as
uniform. Considering horizontal loading conditions
on supporting pole, maximum bending forces(MMax) and maximum shear force (VMax) were
calculated. Further, internally developed forces suchas maximum bending stress (fmax) and maximum
shear stress (τmax) across the critical cross section of
the bamboo pole were calculated by using theory ofbending and shear. Since size and material
properties of bamboo pole are unknown values,
threshold velocities were calculated for three criteria(for bending failure: BF1, BF2, BF3 criteria andcorresponding SF1, SF2, SF3 criteria for shear
failure). For each criterion, selected outer and inner
Laver netforce
P= FTNet/N

Drag force(Idealized)

BMD

SFD

WD

L
Mnet

NB: Depth<1.5 mÆFNet = 0

Fig.4 (a). Depth averaged velocity at 150 min. (b).Variation of
external forces (F), flow velocity (U) and flow depth (h) at B0
and F0.



Mnett

Rnet M

D

RD

MD

MMax

Rnet

RD

VMax

Fig.5 External horizontal forces, bending moment (BMD) andshear force diagrams (SFD) for supporting pole.

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Table 3
Criteria
BF1
BF2
BF3
SF1
SF2
SF3

Details of bending and shear failure criteria.
Type

D0 mm

DI mm

fs N/mm2

Bending
Bending
Bending
Shear
Shear
Shear

32
50
100
32
50
100

20
40
90
20
40
90

25
84
84
-

τs N/mm2

7.5
7.5
7.5

diameters (DO and DI), maximum bending (fs) andshear (τs) strengths of the bamboo pole arepresented in Table 3. Dimensions and strength
properties of the bamboo were selected in order torepresent the weak and strong properties of thematerial. Further, to estimate the threshold velocity
against the shear failure, lower limit of the shear
strength of bamboo (7.5 N/mm2) was considered(Janssen, 1981). Accordingly, minimum flow
velocities required for the failure of bamboo forboth bending and shear modes were calculated foreach criterion at different depths. As highlighted inFig. 6, bending failure is always dominant forbamboo pole because minimum flow velocities
require for the bending failure are much less thansimilar size shear failure case (e.g. BF1 and SF1).
Hence, bending failure governs the damages ofbamboo poles. Further, calculated thresholdvelocities for the bending and shear failure werecompared with the flow velocities representing
normal (Case-I: Hs=1 m, Tp=12 sec) and extreme(Case-II: Hs=3 m, Tp=12 sec) wave conditions. As
illustrated in Fig. 6, minimum velocities require forthe bending failure (velocities between BF1 and
BF3) are lower than the extreme wave condition
velocities (Case-II). This result implies the higher
tendency of damages (due to bending failure) for the
supporting poles under the extreme wave
conditions. On the other hand, compared with theflow velocities corresponding to the normal wave
conditions, calculated threshold velocities for thebending failure (BF1~BF3) are expected to behigher than the normal wave condition (Case-I).
However, as shown in Fig. 6, BF1 values are lower
than the normal wave condition (Case-I).
According to this result, it is clear that,
estimated threshold velocities for the bending failure

Fig.6 Comparison of threshold flow velocities (for the bending
and shear failure of poles) and velocities for normal (Case I) andextreme (Case II) wave conditions.

Fig.7 (a). Tension force on a rope due to equally distributed
horizontal force on laver net (b). External effects on a
Nori-Hibi net from damaged neighboring compartments.

are directly depended on the strength and sectional
properties of the bamboo poleb) Breakage of ropes
To assess the possibility of breaking the ropes
which tie laver nets and supporting poles, maximum
tension force acting on a rope was analyzed. For our
calculation, total horizontal force (FTNet) on a
Nori-Hibi compartment was equally distributed
among all ropes such that to estimate the averagetension force developed on a single rope (Fig. 7(a)). Assuming type of rope as Nylon and diameteras 5 mm, its minimum breaking strength was taken
as 3.91 kN (Engineeringtoolbox, 2012). Thus,
minimum flow velocity required for the tension
failure of the ropes was estimated as 5. 64 m/s.
Compared with the maximum flow velocities of
1.58 m/s in the very highly damaged Futtsu capearea, calculated threshold value for the rope
breaking is much higher. However, according to
Sasaki et al. (2012), some damages due to the rope
breaking was recorded in Kaneda in the Banzu tidal
flat. Therefore, in addition to the horizontal force
that has been exerted by laver nets in thecompartment itself, there would be sequential
external forces on laver nets due to the flock of
Nori-Hibi from the outside damaged compartments(Fig. 7 (b)). Thus, this convergent force might besufficient to break the rope under the lower flow
velocities, despite the minimum velocity required
for a single compartment.
c) Pulling out of poles
As illustrated in Fig. 8, supporting poles could
be pulled out either in inclined direction or verticaldirection. The prior process is more common andcould be resulted due to the failure of bed soil andsuccessively due to the effects of the FY (componentof FNet along the pole). In case of vertical pulling out
process, vertical forces such as weight of wet lavernet (w), buoyancy force (b) of submerged laver nets
and resistance force such as skin friction on thepoles are governing forces. Since laver seeding
period starts in December and growing period is up
to April, larger quantity of laver might have beencollected on laver net at the 2011 tsunami event.

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could be expected in both areas if simulation could
include three failure modes reasonably.
However, on the basis of selected failure criteria
and availability of the field information, intensities
of damages in the study area are qualitatively
explained with driftage simulation model.

5. CONCLUSIONS

Fig.8 Poles pulling out possibilities.

Therefore, resultant higher laver net (FNet) andbuoyancy forces (b) could have increased thepulling out possibility in both situations.
(3) Driftage simulation
In this study, initiation of drifting was assumed
to occur at the same time at which the Nori-Hibi sets
were damaged. In failure mode analysis, minimum
velocities required for the "breakage of supporting
poles" for bending criteria (BF1, BF2 and BF3)
were computed for different depth ranges. Thus,
threshold parameters corresponding to "breakage of
supporting poles" were used as the only failuremode. The failure modes of "breakage of ropes" and"pulling out of poles" which had been discussed
based on number of assumptions was not considered
for the drifting simulation due to the complexity ofthe processes. Fig. 9 illustrates the simulated
drifting locations for the damaged Nori-Hibi sets at t
= 0 and 4 hours respectively. According to thesimulated results under each criterion, Nori-Hibi
sets in F area have been shifted up to a significant
distance compared to the Nori-Hibi sets in B area. It
agrees with the damage scales summarized in Table
1. Further, according to the field records, BF1
results are closer to the real damages shown in
Table 1. In case of BF2 and BF3 criteria, intensityof damage has been reduced with the stronger
bamboo poles. Since these results reflects only thedamage effects of supporting poles, higher damages

Fig.9 Driftage simulation model results for BF1~BF3 criteria.



The magnitude of the calculated total external
forces on the Nori-Hibi sets, exerted by tsunami
flow are in agreement with the damage intensities in
the Futtsu cape and Banzu tidal flat areas.
Accordingly, a positive correlation was identified
between the scales of damages and both external
forces (laver net force and drag force along the
supporting poles) exerted on Nori-Hibi sets. In order
to initiate the driftage of the damaged Nori-Hibi indriftage simulation model, failure mechanisms wereassessed under different failure modes. However,Due to the complexity of other modes, only
"breakage of supporting poles" mode was selected
for the simulation. In case of breakage of supportive
poles, failure was governed by the bending. Further,diameters of bamboo pole and bending strength are
identified as important parameters which govern theaccuracy of the results.
According to this study, the driftage simulation
model has given reasonable results to assess theNori-Hibi sets damages patterns qualitatively.
However, it is plausible that a number of limitations
and assumptions could have influenced the results
obtained.
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Borgese, E.M. (1980): Sea farm: The tory of aquaculture, Harry
N. Abrams, Incorporated, New York.
Deltaers. (2010): Delft3d-Flow user manual, version 3.14.
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Janssen, J. J. A. (1981): Bamboo in Building Structures, Ph.D.
Thesis, The Eindhoven University of Technology, The
Netherland.
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Takayama, Y., Yokota, K., Furuta, A. and Takagi, H. (2012):
Behavior of the 2011 Tohoku earthquake tsunami andresultant damage in Tokyo Bay, Coastal Engineering
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(Received June 15, 2012)- 50 -