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

LABORATORY EXPERIMENTS ON THE
EFFECTS OF BACKWASH CURRENTS OFTSUNAMIS ON SAND TRANSPORT
Tran The Anh 1, Eiichi Furusato2 and Norio Tanaka3,4
1Research Institute for the Management of Seas and Islands - Vietnam Administration of Seas and Islands,
trantheanhv9@gmail.com
2Graduate School of Science and Engineering, Saitama University, furusato@mail.saitama-u.ac.jp
3Graduate School of Science and Engineering, Saitama University, tanaka01@mail.saitama-u.ac.jp
4Institute for Environmental Science & Technology, Saitama University

For estimating the effects of tsunami current on sand movements, a laboratory experiment wasconducted. The distribution of sand on the slop after run-up and backwash currents was measured.
Tsunami over land flow was modeled as a turbulent bore generated by quickly opening of a gate. Ninecases with different tsunami heights and sad diameters were conducted. On the land slope (1/20), themaximum length of sand deposit after the backwash is limited to the range close to the shorelinecompared to that after the run-up. In particular, this tendency is remarkable for the case of high water
depth in the tank (case H30D22-simulating a big tsunami): the length of sand deposit decreased by halfby the backwash compared to the case after the run-up. Based on results, it concluded that the effect ofback wash currents on the maximum sand deposit length varied with sand mobility index. Therefore thebackwash effect should be considered for risk assessments from historical tsunami deposits.
Key Words : Flume experiments, tank water level, sand diameter, sand mobility index, historical tsunami

1. INTRODUCTION
Tsunamis can bring a large amount of sea sandinto land and ports, and hindered the socioeconomic
functions. On the other hand, paleo tsunami-sanddeposits are useful to analyze the magnitude ofhistorical tsunamis (cf. Sawai et al. 2009, Uchida etal. 2010, Goto et al. 2011). After the 2011
Tohoku-oki tsunami, the importance of theinformation of historical sand deposition increased(Goto et al. 2011) in Japan. There has been someresearch related to sand transport by tsunamis, suchas field observations (Minoura et al. 1987,),numerical simulations (Goto and Imamura 2007),and laboratory experiments (Hasegawa et al. 2001,
Sugawara et al. 2003, Harada et al. 2011, Takahashi
et al. 2011). However there is insufficientknowledge of the process in details and thecharacteristics of sand transport. Because ofdifficulty of observation of sand transport due to areal tsunami, laboratory experiments can provide
important information. Harada et al. (2011) reported
that the distribution of tsunami deposit is influenced
by the grain size. Hasegawa et al. (2001) pointed outthat the effects of backwash current on sanddistribution. However, the effects of sand diameterand tsunami heights on sand transport were notaddressed in the previous research. From thequalitative point of view, flow velocity and flow
depth of a tsunami, grain size and land slope havecombined effects on the characteristics of sandtransport.
The estimation of maximum inundation lengthof paleo tsunamis using the characteristics of sanddeposits is important for disaster management infuture. For understanding the sand deposition inland
after a tsunami, we believe that an estimation of theeffects of not only the tsunami run-up but also thebackwash is needed. In this study, we conductedlaboratory experiments to simulate the sandtransport on land due to tsunamis. Particularly, theeffects of backwash were investigated using a rangeof tsunami heights and grain sizes. In this paper, we
describe mainly the experimental results forunderstanding the sand transport phenomenon itself
due to tsunamis.

2. MATERIALS AND METHODS- 26 -Fig.1 Experimental flume and measuring points

Table 1 List of notations(1) Flume
Laboratory experiments were done in a flume(Fig. 1（15.5m long, 0.4m width, and 0.7m height）
with a sand glued impermeable slope (1/20).
Tsunami over land flow was modeled as a turbulent
bore that was generated by rapid opening of a gate.
The storage tank is 3.5m long including thewave-making and draining gates. To generate awave, the wave-making gate was opened, and thedraining gate was opened when the wave front stops
on the slope (after arrive at the maximum distance)
to have an undisturbed backwash. (Fig. 1).
The source of sand was a sand bar at the shoreline.
The sand bar is modeled by a layer of sand with thethickness of 0.05 m, and 1.0 m length (Fig. 2). For
the preliminary experiments, two sand bar heights(Hb =0.03, 0.015m) were used.
(2) Measurements
The transported sand weight, flow velocity, andflow depth were measured. Ultrasonic displacement
meters (KEYENCE Co.,) and propeller type flow
meters (KENEK Co.,) were used to measure theflow depth and flow velocity respectively on theslope (Fig. 1). The distance between the measuring
points is 0.6m. To measure the sand transport of bedand suspended, a wooden trap with cells each 1.4 x
0.2 x 0.2 m was made (Fig. 2). Similar equipments
have been used in the previous studies (Hasegawa etal. 2001, Harada et al. 2011). The transported sandwas measured for both the after run up and afterbackwash. The timing of trapping were decided
based on velocity measurements of each cases. TheSymbols

Definitions

Srt

Total sand weight on slope after run-up

Sbt

total sand weight after backwash (remained aftertsunami)

SrSand weight of each point after run-up

SbSand weight of each point after backwash

Hb

Initial sand bar height

HtWater level of tank

LHorizontal distance from SWL to measuring point

LW

Maximum horizontal run-up distance from SWL

LSr

Maximum horizontal sand transported distancefrom SWL after run-up

LSb

Maximum horizontal sand deposit distance fromSWL after backwash⊿LS

Difference between Lsr and Lsb

Vrm

Maximum velocity of run-up of each measured
point

Vbm

Maximum velocity of backwash of each measured
point

positions of sand trapping covered inundation
length. Each test was conducted three times for theaccuracy. Data obtained above methods wererepresented as notations (Table 1). Maximum
horizontal sand transported distance is defined byN.D. value (0.025g/cm2)- 27 -1

H30D11

Sb /Sbt

0.80.6Hb=5.0cm
Hb=3.0cm
Hb=1.5cm

0.4
0.2
00

Sb /Sbt

Sand diameter[cm](d50)[mm]H30D11*230

0.110.2

H30D22300.22

0H30D32

300.32

H25D11250.11

H25D22250.22

H25D32250.32

H20D11200.11

H20D22200.22

0.4 0.6
L /LW0.8

10.400.2Fig. 4 The effects of initial sand bar height on sanddistribution

initial sand amount of the source on experimental
results, preliminary experiments were conducted. Asfor representative conditions, two cases: the large
tsunami height with small sand diameter (H30D11),and low tsunami height with large sand diameter(H20D32) were selected (Table 2). The initial sandvolumes were changed by changing the sand barheight (Hb=0.05, 0.03, 0.015m).

0.32

*1

Maximum inundation lengthLWLS rLS b

3. RESULTS AND DISCUSSIONS
Sand distribution after run up

Slope (S )

10.6

Ht*1

Gate

0.8

H20D32

0.8

Table 2 Experimental conditions

H20D32*2
20
Tank water level*2
Used for preliminary experiments

0.4 0.6
L /LW

1Fig.2 Sand trap equipments

Case No.

0.2

Sand distribution after backwash

Fig. 3 Schematic figure of notations of experimental results(3) Experimental conditions
Table 1 shows the experimental cases with theinitial water depth of the tank (Ht) and sanddiameter. Qualitatively, sand transport is controlled
by three factors, tsunami height as the externalforce, sand size, and land slope. We focused on thetwo parameters, tsunami height and sand diameter.
(4) Preliminary experiment
Harada et al. (2011) reported that the location ofsand source affects the characteristics of sanddeposit. In our study, the boundary of the source isat the edge of the slope. For estimating the effect of(1) Effects of initial sand bar heightFig. 4 shows the results of the preliminaryexperiments. With different heights of the sand bar
the sand distribution pattern on the slope after
backwash did not change greatly. The effects of theinitial sand bar height on the sand distribution after
the backwash were relatively small. Furthermore,
although it is not shown, change in the run-up length
by the different sand bar heights was less than 5%.
Therefore, the effect of sand bar height can beneglected to discuss the results of this study.
(2) Current
The tsunami currents play an important role inthe sand transport and deposit on ground.
Fig. 5 shows the maximum velocity of both
run-up (Vrm) and backwash (Vbm) of each point.
Except for the low water level case, the maximum- 28 -Vrm-H30D11
Vrm-H20D11100500234L (m)567(Srt -Sbt ) / Srt

1.00.80.60.2

1.00.0d=0.11d=0.22d=0.320.6

d50=0.12mm backwash
d50=0.32mm backwash

0.4

Fig. 5 Maximum velocity of each case0.8

d50=0.11mm backwash

1.2

Vbm-H20D11

LSr /LW
LSb /LW

Vrm , Vbm (cm/s)

Vbm-H25D111d50=0.22mm run up
d50=0.32mm run up

1.4

Vbm-H30D11150

d50=0.11mm run up

1.6

Vrm-H25D11200

050100(Ht S )/d 50

150Fig. 7 Relationship between index of sand transport force bytsunami current (Ht S/d50) and maximum sand transportdistance

0.4
0.2
0.0
00.1

0.2
Ht (m)

0.3

Fig. 6 Relationship between tank water level and ratio of totalsand volume removed by backwash to total sand volume on
slope after run-up

velocity of each point of the backwash currents was
greater than that of the run-up current.

(3) Total sand weight transport
Fig. 6 shows the ratio of the total sand removedby backwash to the total sand weight on the slope
after the run-up.
The Total sand volume removed by backwash
current was as high as 90% (H30D11) in the small
sand cases (d50=0.11mm). On the other hand, forthe large sand and small water level case (H20D11),
the effects of backwash currents were very small.
Therefore, to estimate total sand deposit weight after
tsunami, the effects of backwash current isimportant depending on tsunami height and sanddiameter.
(4) Maximum sand transported distanceFigs. 7 and 8 show the maximum sandtransported distance for each case. We named Ht
S/d50 as sand mobility index. We used only one land

slope condition (1/20) for the experiment. Thus, Ht
S/d50 varies with only tank water level and sanddiameter.
As shown in Fig. 7, the ratio of maximum sanddistance after run up (LSr) to maximum run up
distance (LW) increased with an increase of Ht
S/d50 whereas Ht S/d50 was greater than 70, therelationship was not clear. On the other hand, thisdistance ratio (LSr/LW) decreased by one-half afterbackwash. The range of this ratio varied between
0.3 and 0.6. In the cases of backwash currents, theeffect of Ht S/d50 on the sand transport distance ratio
was ambiguous.
Fig. 8 shows the relationship between Ht S/d50
and the ratio of maximum horizontal sand transportdistance after backwash to that of after run up(LSb/LSr). With an increase in Ht S/d50, LSb/LSr
decreased. The effect of backwash currents on sandtransport was significant with an increase in sandmobility index (Ht S/d50). In particular, when Ht
S/d50 was 140, sand transport distance decreased
more than 60% during backwash. In ourexperimental results, the maximum velocity ofbackwash currents is same or greater than that ofrun up currents (Fig. 6). According to thecharacteristics of velocity, the effects of backwash
current not only on the total transported sand weight(Fig. 6) but also on the maximum deposit distance(Fig. 7) were significant. This effect becomesprominent in large Ht S/d50 cases.

- 29 -0.8

LSb /LSr

ACKNOWLEDGMENT: The authors wish tothank N.A.K. Nandasena for valuable discussions
on the manuscript.

d50=11d50=22
d50=32

1.0

REFERENCES

0.60.40.20.0050100(Ht S )/d 50150Fig. 8 The relationship between Ht S/d50 and decrease rate ofsand deposit distance by backwash current

4. CONCLUSIONS
We conducted a laboratory experiment for
estimating the effects of tsunami currents on sandmovement.
The maximum velocity of back wash currents
was same or greater than that of run up currents inall cases.
The ratio of total sand weight removed by backwash currents reached over 90 % as maximum. The
ratio of sand deposit length to inundation distance
was from 0.6 to 0.8 after run up. On the other hand,back wash currents make this length short. Thiseffect was maximum at large Ht S/d50 case.
Research on the effects of backwash currents onsand transport distance with different land slope
conditions is needed to analysis historical tsunamis
from sand deposits in future.

Goto, K., F. Imamura (2007): (Review) Numerical models for
sediment transport by tsunamis, The Quaternary Research,
Vol. 46, 463-475.
Goto, K. et al. (2011): New insights of tsunami hazard from the2011 Tohoku-oki event, Marine Geology, Vol. 290, pp.
46–50.
Harada, K.., K. Imai, Tran The Anh and Y. Fujiki (2011):
Hydraulic experiment on sand deposit by tsunami run-up
with land slope，Jour. of JSCE, Ser. B2 (Coastal Eng.)，Vol.
67，No. 2，pp. I_251-I_255. (In Japanese).
Hasegawa, F., Takahasi, T. and Uehata, Y. (2001) Hydraulicexperiment on sediment due to tsunami run-up. Proceedingsof Coastal Engineering, JSCE, Vol. 48, pp. 311-315 ( inJapanese).
Minoura, K., Nakaya, S. and Sato, H. (1987): Traces oftsunamis recorded In lake deposits - An example from Jusan,
Shiura mura, Aomori -, Journal of Seismological Society ofJapan, Vol. 40, pp. 183-196. (J+E).
Sugawara, M., Ohkubo, S., Sugawada, D., Minoura, K. andImamura, F. (2003): Hydraulic experiment of sand andstone movements by a tsunami on a uniform slope.
Proeedeings of Coastal Engineering, JSCE, Vol. 50. pp.
266-270.
Sawai, Y., Jankaew, K., Martin, M.E., Prendergast, A.,
Choowong, M. and Charoentitirat, T., (2009): Diatom
assemblages in tsunami deposits associated with the 2004
Indian Ocean tsunami at Phra Thong Island, Thailand.
Marine Micropaleontology, Vol. 73, 70–79.
Uchida, J.-I., Fujiwara, O., Hasegawa, S. and Kamataki, T.
(2010): Sources and depositional processes of tsunami
deposits: analysis using foraminiferal tests andhydrodynamic verification. Island Arc, Vol. 19, pp.
427–442.
(Received March 18, 2011)- 30 -