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

INVESTIGATION OF SHORELINE CHANGE
CAUSED BY THE GREAT EAST JAPAN TSUNAMI
2011 AROUND SAMEGAWA RIVER MOUTH
Mohammad Bagus Adityawan1, Hitoshi Tanaka2 and Hisao Nagabayashi3
1Dept. of Civil Engineering, Tohoku University, bagus@kasen1.civil.tohoku.ac.jp
2Dept. of Civil Engineering, Tohoku University, tanaka @tsunami2.civil.tohoku.ac.jp
3Dept. of Civil Engineering, Nihon University, nagabaya@civil.ce.nihon-u.ac.jp

The Great East Japan Tsunami 2011 had caused shoreline retreat due to severe erosion, especially insandy coast. The coastal morphology after the tsunami may not be the same to the pre-tsunami condition.
It may cause for the river mouth. Samegawa river mouth area is investigated in this study by analyzingsatellite images of pre-tsunami condition, immediately after the tsunami, and most recent condition. The
images were digitized and analyzed. It was found that sediment intrusion occurs in the river mouth which
may block the nearby drainage system.
Key Words : tsunami, shoreline changes, aerial photograph, river mouth

1. INTRODUCTION
The Great East Japan Tsunami 2011 hadsignificant impact to the affected coastal area. The
wave reached the northern coast of Japan withmassive force, causing immense damages. In someplaces, the run up height reached to about 40 meter(Mori et al, 2012).
Shoreline in Miyagi and Fukushima prefectures
were heavily affected by the tsunami. Adityawan etal. (2012) had analyzed the tsunami propagation onland and in river along Sendai plain. Tanaka et al.
(2011) had provided report on the tsunami impact inMiyagi coast based on aerial photos. Shoreline
retreat due to severe erosion occurred, especially insandy area. Udo et al. (2011) had shown land
subsidence and erosion occurred in the northern
Sendai coast area. The coastal morphology after thetsunami was highly unstable. After some period oftime, a new balance coastal condition will beachieved. The new coastal condition often different
from the pre tsunami condition.
There are cases where the new coastal conditionis causing problem for the river mouth, i.e. New
Kitakami River. The sandspit in New KitakamiRiver mouth was reformed in a more upstream
location that blocks drainage outlet. A more severe
problem occurred in Nanakita River where its rivermouth was completely closed and required dredging
to re-open (Tanaka et al., 2012). Therefore, it isimportant to investigate the shoreline change aroundthe river mouth.
Samegawa river mouth area is investigated inthis study by analyzing satellite images and aerial
photo of pre-tsunami condition, immediately after
the tsunami, and the most recent image.

- 31 -NFig.1 Study Area.

2. STUDY AREA
The study area is coastal area around Samegawa
river mouth, Fukushima, Japan as shown in Fig.1. It
is situated in Nakoso coast, covering approximately
4 km of shoreline facing the Pacific Ocean. Thecoastline is mostly sandy area. Joban Kyodo Power
Plant is located nearby. The discharge from thepowerplant is separated with the river discharge by ajetty in the left bank of the river mouth, constructedin 1982. More detail on long-term changes in sandmovement for this area can be found in previous
study (Sato et al., 2004).
Severe erosion occurred due to the The Great
East Japan Tsunami 2011. An outlet of nearby
drainage is located around the rivermouth. The
drainage system may face some problem in thefuture due to the sediment intrusion.

2005-04-27

NN

2011-03-12

A

2011-11-01

NAerial photo2012-02-28

NFig.2 Satellite images.

- 32 -Projected Coordinate System (UTM-WGS`84 Zone
54N). On the other hand, aerial photo contains nogeo-reference information. Thus, rectification wasconducted prior to the analysis. The images wererectified using reference points to the same
coordinate system as the satellite images. Moredetail on rectification method can be found inprevious study (Pradjoko and Tanaka, 2010).
The projected images were digitized to obtain
shoreline position and protection/structures. A local
coordinate baseline was used to quantify theshoreline change. The shoreline change wasinvestigated further by analyzing several crosssections along the coastline.
The accuracy of the digitized map follows theresolution of the source map. However, the satellite
images do not contain information on the exact time(hour, minute) when the image was captured.
Therefore, it is not possible to provide correction
value based on tidal level (Fig.4). Nevertheless,maximum error that may occur can be estimated forthe particular day (Table 1).

2011-04-18

1

2012-06-07
Location: Fig.2-Box A

0Water Level (m)Fig.3 Shoreline retreat.

3. METHOD
Satellite images and aerial photos have been
extensively utilized in shoreline change detection.
Pradjoko and Tanaka (2010) had assessed shorelinechanges in Sendai coast based on a series ofconsecutive aerial photos.
Satellite images for the study area were collectedfrom Google Earth for 27 April 2005 (DigiGlobe,resolution 0.6 m), 12 March 2011 (DigiGlobe,
resolution 0.6 m), and 28 February 2012 (GeoEye,resolution 0.5 m), representing the condition before
the tsunami, immediately after the tsunami, andpresent. In addition, an aerial photo was acquired
showing the condition on 1 November 2011. The
satellite images and the aerial photo are shown inFig. 2. Field observation was conducted and photos
were taken. Fig. 3 shows the condition of theshoreline shortly after the tsunami and the mostrecent condition. It can be seen the retreating andthe breaching of the shore line.
The satellite images were acquired in
Geographical Coordinate System (GCS-WGS`84).
Therefore, rectification process is not required. The
coordinate system was directly changed to the-1-2-361218Hour

2005, April 27
2011, March 122012, February 28 (Astronomical)
2011, November 01 (Astronomical)

Fig.4 Tidal level.

Table 1 Data accuracy
Image2005, April 25
2011, March 122011, November 01
2012, February 28- 33 -SpatialResolution (m)
0.6
0.6
1.0
0.5

Possible Tidal
Error (Max) (m)
28.21
24.61
7.46
5.86

4085500

UTM WGS 84, Zone 54 N

CliffSamegawa RiverNDrainage

4085000

500

1000
4084500

2000
DrainageY (meter)

4084000

4085000

Y (meter)

2500

2012

40835002005
4083000

Protection

Binda RIver

4084500

20050427

3750

20110312
4082500

20111101

4000

4084000
48250020120228
Baseline (x`,y`)4082000
481500482000482500483000Box i

483500484000483000483500X (meter)

484500485000485500X (meter)Fig.5 Shoreline detection.

approximately 0.1. The tidal data were obtained
from Onahama port. Unfortunately the port was
heavily damaged by the tsunami. Water level data
right after the tsunami is not available.

800

4. RESULTS AND DISCCUSSION
400

14-Aug-13

01-Apr-12

18-Nov-10

06-Jul-09

22-Feb-08

10-Oct-06

28-May-05

014-Jan-04

Shorelline position (meter)

1200

Year
500

1000

2000

2500

3750

4000

Fig.6 Shoreline temporal variation.

.
The maximum error in this study for each images
were estimated by analyzing tidal level variation
during day time. The error is given by the maximum
deviation recorded, converted to horizontal distanceusing the beach slope. The beach slope isThe digitized map showing the shoreline
detection is given in Fig. 5. It is found that thenorthern part of Samegawa River was severely
eroded starting from the river mouth to the cliff due
to the tsunami. On the contrary, erosion in thesouthern part of the river mouth was milder than thenorthern part. The retreating distance of theshoreline varies as shown in Fig. 6. It is shown thatthe highest retreat occurred around cross section
1000 with the distance of approximately 400 meter.
This agrees well to the field observation as shown
previously in Fig. 3. The erosion in the southernpart of the river mouth occurs up to cross section
4000 where no significant changes of shoreline
position is found (Fig. 6).
A closer look of the Samegawa river mouth(Fig. 5 box i) shows that the sandspit in front of theriver mouth was completely eroded by the tsunami.

- 34 -The newly formed sandspit is located around100-200 meter (Fig 6. cross section 2000 and 2500)
more upstream of the pre-tsunami condition. Thismay cause some problem since it may block thenearby drainage outlet. Moreover, the protection atcross section 2500 now becomes the shoreline
without any buffer from sand deposit. It was also
found that the Binda river mouth was eroded after
the tsunami (Fig. 6 cross section 3750), and the newriver mouth moved further north due to theelongated sandspit (Fig. 5).

No. 2301367), the River Environmental Fund (REF)from the Foundation of River and Watershed
Environmental Management (FOREM), Japan. The
authors would also like to gratefully appreciate
Assistance for Technological Development, Tohoku
Construction Association for their support. The firstauthor is a Postdoctoral Fellow granted by JSPS
(No. P11367).

5. CONCLUSSIONS

Adityawan, M., B., Roh, M., Tanaka, H., Mano, A., Udo, K.
(2012): Investigation of tsunami propagation characteristics
in river and on land induced by the great east japan
earthquake 2011, Journal of Earthquake and Tsunami. (inpress)
Mori, N., Takahashi, T. & The 2011 Tohoku Earthquake
Tsunami Joint Survey Group (2012): Nationwide post event
survey and analysis of the 2011 tohoku earthquake tsunami,Coastal Engineering Journal, Vol. 54, No.1.
Pradjoko, E. and Tanaka, H. (2010): Aerial photograph ofSendai Coast for shoreline behavior analysis, Proc. 32nd Int.
Conf. Coastal Engineering, p. 13.
Sato, S., Kajimura, T., Abe, M., and Isobe, M. (2004): Sand
movement and long-term beach evolution in a fluvial
system composed of the samegawa river and the nakoso
coast, Coastal Engineering Journal, Vol. 46, No.2, doi No:
10.1142/S057856340400104X.
Tanaka, H., Adityawan, M., B., Mano, A. (2012): River mouthmorphology changes at the nanakita river mouth after thegreat east japan tsunami, Annual Journal of Coastal
Engineering, B2-68. (in press) (in japanese)
Tanaka, H., Nguyen, X. T., Umeda, M., Hirao, R., Pradjoko, E.,
Mano, A. and Udo, K.(2011): Coastal and estuarine
morphology changes induced the 2011 Great East Japan
Earthquake Tsunami, Coastal Engineering Journal, Vol.54,
No.1. doi: 10.1142/S0578563412500106
Udo, K., Sugawara, D., Tanaka, H., Imai, K., and Mano, A.
(2011) Impact of the 2011 tohoku earthquake and tsunami
on beach morphology along the northern sendai coast,
Coastal Engineering Journal, Vol. 54, No.1.
doi:10.1142/S057856341250009X

This study provide valuable information which
will be very useful for restoration process as well as
future disaster prevention. Overall, it was found thatafter the tsunami until present, the southern partcontinues to erode while deposition occurs innorthern part.
The elongated sand spit in Binda river mouth
and the retreat of sand spit in Samegawa rivermouth occurs due to natural process in achieving a
new balance condition. The general sediment
movement in this area occurs from south to north.
Therefore, the northern part will draw sedimentfrom the southern part up to an equilibrium point,
which is approximately around cross section 4000.
As the sediment moves north, it will pass theSamegawa river mouth and fill the upstream partfirst since the river mouth bed was quiet deep due tothe tsunami. On the contrary, Binda river rivermouth depth is much lower than Sameagawa rivermouth, hence the elongated sandspit.
ACKNOWLEDGMENT: The authors would liketo thank the financial supports from Grant-in-Aidfor Scientific Research from Japan Society for
Promotion of Science (No.21360230, No.22360193,

REFERENCES- 35 -(Received March 18, 2011)