field survey of the 2011 tohoku earthquake and ...

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Coastal Engineering Journal, Vol. 54, No. 1 (2012) 1250011 (26 pages) c World Scientific Publishing Company and Japan Society of Civil Engineers

Coast. Eng. J. 2012.54. Downloaded from www.worldscientific.com by Mr. Takahito Mikami on 11/24/13. For personal use only.

DOI: 10.1142/S0578563412500118

FIELD SURVEY OF THE 2011 TOHOKU EARTHQUAKE AND TSUNAMI IN MIYAGI AND FUKUSHIMA PREFECTURES

TAKAHITO MIKAMI∗ , TOMOYA SHIBAYAMA† and MIGUEL ESTEBAN‡ Department of Civil and Environmental Engineering, Waseda University, 3-4-1 Okubo Shinjuku-ku, Tokyo 169-8555, Japan ∗ [email protected] † [email protected] ‡ [email protected] RYO MATSUMARU IRM Ltd., Kasuya Setagaya-ku, Tokyo 157-0063, Japan [email protected] Received 30 October 2011 Accepted 13 January 2012 Published 29 March 2012

At 14:46 on March 11, 2011 (local time), a large earthquake of magnitude Mw 9.0 took place, generating a tsunami that caused severe damage to the east coast of Japan. To comprehensively record tsunami trace heights and impacts along the coastal region, the Tohoku Earthquake Tsunami Joint Survey Group was organized immediately after the event. As part of this group, the authors conducted a field survey in Miyagi and Fukushima Prefectures. The surveyed area can be divided into 2 parts from the point of view of its geographical features: the northern part (a rias coastal area) and the southern part (a coastal plain area). In this paper, the characteristics of the damage due to the tsunami in each area are analyzed by using both the results of the authors’ own field survey and the Joint Survey Group. In the rias coastal area, inundation heights were more than 10 m, which resulted in the flooding of the low-lying grounds located at the inner part of the bays. The tsunami wave caused widespread destruction in this area, and coastal buildings (including reinforced concrete buildings) suffered severe damage. In the southern coastal plains, inundation heights were 5–10 m and the tsunami reached a few kilometers inland, though unfortunately there were not enough high locations or buildings for the residents 1250011-1

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T. Mikami et al. to evacuate. In addition, an extensive line of coastal dikes and forests, which had been placed to protect the wide plains behind them, also suffered extensive damage. From these geographically dependent inundation and destruction patterns, a number of important lessons on how to modify and improve future risk management strategies can be obtained. Keywords: Tsunami; field survey; tsunami trace height; coastal structure; 2011 off the Pacific coast of Tohoku earthquake.


1. Introduction At 14:46 on March 11, 2011 (local time), a large earthquake of magnitude Mw 9.0 took place, generating a tsunami that caused severe damage to the east coast of Japan. To comprehensively record tsunami trace heights and impacts along the coastal region, the Tohoku Earthquake Tsunami Joint Survey Group (hereinafter denoted as the Joint Survey Group) was organized immediately after this event [The 2011 Tohoku Earthquake Tsunami Joint Survey Group, 2011a]. Surveys in Tohoku region began on March 25 after the completion of major search and rescue operations [Mori et al., 2012]. Several teams conducted field surveys in Miyagi Prefecture as the first teams of the Joint Survey Group [Kakinuma et al., 2012; Suppasri et al., 2012]. The authors were also one of the first teams in this group to conduct field surveys in Miyagi and Fukushima Prefectures, and traveled along a wide area of the coastline recording and analyzing the type of damage that had taken place. The affected area was quite broad and the tsunami trace heights and impacts were different at each location. In this paper, the authors would like thus to analyze inundation and destruction patterns due to the tsunami by using not only the results of their own field survey but also those of the Joint Survey Group. 2. Tsunami Survey The field survey was conducted from the 25th to 28th of March, 2011, 2 weeks after the event, in Miyagi and Fukushima Prefectures. The aim of this survey was to understand the distribution of tsunami trace heights along the coast, as well as to understand the situation of the affected areas, such as the extent of the damage to coastal structures and buildings. At each survey point, the precise location of the points observed was first recorded by using GPS instruments. Then, the height of the tsunami traces (as measured on the sides of buildings, trees, etc., as shown in Fig. 1) were surveyed by using a laser ranging instrument (IMPULSE, Laser Technology Inc.), a prism and staffs. The tsunami inundation or run-up height at each location was established by using the sea water level at each location as a reference point, and hence each of the points surveyed were traced back to the edge of the sea. Inundation and run-up heights measured by the Joint Survey Group were corrected to the heights above the estimated tide level at the time of arrival of the tsunami. All the data used in this paper corresponds to this corrected dataset. Table 1 and Fig. 2 show the results of 1250011-2

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Field Survey of the 2011 Tohoku Tsunami in Miyagi and Fukushima

Fig. 1. Types of traces in the field survey: (a) broken branch, (b) debris, (c) mud line on a structure, (d) structure damage due to the tsunami.

Fig. 2. Distribution of tsunami inundation heights along the surveyed coastline. 1250011-3

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T. Mikami et al. Table 1. Results of the measurement survey. No. 1 2 3 4 5 6 7


8

Location

11

14

17 18 19

Watermarkb

E 141◦ 35.0300

7.79

DB

0

E 141◦ 34.7750

11.84

BB

Motoyoshichonakajima, Kesennuma City

N 38◦ 46.3410

E 141◦ 30.7260

9.23

DB

0

E 141◦ 30.7110

10.88

BB

Shizugawa, Minamisanriku Town

N 38◦ 40.4260

Onagawa Port, Onagawa Town

◦

N 38 54.429

◦

N 38 46.334

E 141◦ 26.7040

14.30

DB

0

E 141◦ 26.7300

15.41

SD

N 38◦ 26.7340

E 141◦ 26.6860

13.52

MO

E 141◦ 26.6400

13.27

DB

17.39

MI

E 141◦ 29.3060

8.73

DB

E 141◦ 29.2560

11.91

DB

0

11.05

DB

E 140◦ 59.1030

9.32

BB

◦

N 38 40.416

◦

0

◦

0

N 38 26.761 N 38 26.612

Takenoura, Onagawa Town

N 38◦ 26.5960 ◦

0

◦

0

N 38 26.572 N 38 26.614

Wakabayashi, Sendai City

15 16

Height [m]a

N 38◦ 54.5940

12 13

Longitude

Kesennuma Port, Kesennuma City

9 10

Latitude

Yuriage, Natori City Arahama, Watari Town Isobe, Soma City

N 38◦ 13.1020 ◦

E 141 26.742

◦

E 141 29.440

8.43

SD

N 38◦ 10.8420

E 140◦ 57.4250

5.44

SD

N 38◦ 10.4720

E 140◦ 57.4010

8.81

DB

N 38◦ 02.2190

E 140◦ 55.1560

◦

N 38 02.234

0

N 37◦ 46.7020

◦

0

0

N 38 13.130

0

◦

E 140 59.078

4.82

DB

0

7.56

BB

E 140◦ 58.8760

6.78

SD

◦

E 140 55.241

a All

data are inundation heights as corrected by the 2011 Tohoku Earthquake Tsunami Joint Survey Group [2011b] to include tidal level at the time of the tsunami. b Explanatory notes on watermarks: BB broken branch, DB debris, MI mud line inside building, MO mud line outside building, SD structure damage.

the authors’ own measurement survey. Inundation heights were more than 10 m in the northern part of Miyagi Prefecture (more than 15 m in some places) and 5–10 m along the coast of the southern part of Miyagi Prefecture and the northern part of Fukushima Prefecture, known as the Sendai Plain. This distribution of heights mainly resulted from geographical differences at each location, as will be explained later. The Pacific coast of Tohoku has a variety of different geographic features, which have to be carefully considered in order to understand the characteristics of the tsunami disaster. A large portion of the northern part of the Pacific coast of Tohoku, known as the Sanriku Coast, is made up of rias, while the southern part is generally characterized by the presence of sandy beaches. 1250011-4

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Rias are distinctive geographical features, somehow similar to the Norwegian fjords, but that originate from the flooding (due to rising sea levels in the past) of deep river valleys (as opposed to fjords, which originate from the flooding of glacier valleys). In the same way as fjords, they create an indented coastline, where these “indentations” are made up of very deep sea bays that allow the tsunami to concentrate its power as it charges inland. The water in these bays is generally quite deep, which together with the refraction and reflection effects due to the indented nature of the coastline can enhance the tsunami damage in many ways. These areas are mountainous, with the land rising rapidly from the sea and villages and towns located at a very narrow fringe of land directly under the mountains. In these areas the tsunami inundated the narrow low-lying areas next to the coast and then ran-up the sides of the mountains, resulting in heavy damage to human settlements, which were generally located next to the water or slightly above it on the sides of the mountains. On the other hand, the southern part of the area surveyed is made up of sandy beaches that give way to very flat plains which are mostly used for rice growing. These plains only gradually increase in height and are (with the exception of the massive coastal dikes) often devoid of large scale constructions, and hence allow the tsunami to flood extensive areas with ease. Due to this intrinsic difference, the location of protective structures was also different in each area, and this also influenced damage patterns. In the northern rias area breakwaters had been constructed at the entrance of some of the bays that had suffered high damage in previous tsunami events, though some of these breakwaters had been constructed with tsunami waves in mind and others were designed against storm waves. Generally, fewer breakwaters were installed along the south coast of this region, where sea defenses consisted mainly of costal dikes constructed with the purpose of protecting against storm waves. This area, however, did possess a number of small ports that were indeed protected by breakwaters, forming a third distinctive type of damage pattern. In this way, the surveyed area can be divided into 2 areas, the northern part (a rias coastal area) and the southern part (a coastal plain area), which are different in terms of land usage and types of coastal structures. The southern plain can in turn be divided into sandy beaches (forming the majority of the coastline) and small ports. At each location, the tsunami trace heights measured by both the authors and the 2011 Tohoku Earthquake Tsunami Joint Survey Group [2011b] (Fig. 3) are presented and the characteristics of the damage due to the tsunami will also be described and analyzed.

3. Results of the Survey Table 2 shows casualties and population estimates for each surveyed city and town. All areas suffered heavy damage, which explains the high casualty rates, especially 1250011-5

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T. Mikami et al. Table 2. Casualties and population estimates at surveyed cities and towns. Prefecture

City/Town

Miyagi Miyagi Miyagi Miyagi Miyagi Miyagi Fukushima

Kesennuma City Minamisanriku Town Onagawa Town Sendai City Natori City Watari Town Soma City

Death tolla

Missinga

Populationb

1,027 561 571 704 911 257 456

377 341 409 26 70 13 3

73,154 17,378 9,932 1,046,737 73,603 34,795 37,721

a Casualties


are correct as of October 11, 2011 according to the Fire and Disaster Management Agency [2011]. b Population estimates are correct as of March 1, 2011 according to the websites of Miyagi and Fukushima Prefectures.

for the case of Onagawa Town, where about 10% people of its inhabitants lost their lives. Also, the fishing industry and aquaculture, which are the main industries for many of these coastal communities, were also heavily damaged by the tsunami. One interesting characteristic of this tsunami was the quantity of damaged cars, which were displaced by the tsunami and ended stranded in random locations, such as the top of buildings, or in piles against buildings that managed to survive the tsunami. This can be of course attributed to the large number of vehicles now owned in Japan, compared to the relatively reduced number in previous tsunami events in Japan or other countries. 3.1. Damage in rias coastal area In this area, the authors conducted surveys around 3 main locations, and in the following subsections the inundation patterns, coastal structures, and buildings present at each location will be detailed. The authors will also present some other findings obtained from talking to local residents and civil servants in various locations. 3.1.1. Kesennuma City Kesennuma City is located on the northeastern edge of Miyagi Prefecture. In this area the rias coastline forms Kesennuma Bay, which opens to the Pacific Ocean at its southern part and has Kesennuma Port located at its innermost part. The mouth of the bay is 2.6 km wide and the bay is 15.4 km2 in area [International EMECS Center, 2009]. The overall results of the Joint Survey Group generally show that tsunami inundation heights were measured to be more than 10 m at many points outside of Kesennuma Bay, while inside Kesennuma Bay they were generally less than 10 m (Fig. 3(a)). 1250011-6

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Fig. 3. Regional distribution of tsunami trace heights. From left to right: results of the authors’ survey, distribution of inundation heights measured by the Joint Survey Group, distribution of run-up heights measured by the Joint Survey Group. (Note that in the leftmost map, height shows the maximum height measured in the survey at each location.) 1250011-7


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Fig. 3 (Continued )

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The field survey was conducted at 2 places in this region, Kesennnuma Port and Motoyoshicho-nakajima. At Kesennuma Port, the tsunami carried with it many large and small fishing vessels, which were left stranded inland (Fig. 4). These vessels, which drifted during the tsunami attack, contributed to and exacerbated the damage to houses and other structures. Sludge, with a thickness of around 10 cm or more in some places, was also carried from the sea and deposited inland, which hindered relief operations. Measured inundation heights around the port were 7.79 m and 11.84 m. At Motoyoshicho-nakajima, which is located about 15 km south of Kesennuma Port, around 300 m of coastal erosion could be observed (Fig. 5). In this area, the lowlying ground extended around the mouth of Tsuya River, and hence a wide area was inundated. Inundation heights near the eroded area were measured to be 9.23 m and 10.88 m.

3.1.2. Minamisanriku Town Minamisanriku Town is located on the northeastern part of Miyagi Prefecture. It is located inside Shizugawa Bay and the main area of the town (which is known as the Shizugawa district), is located on the northwestern part of the bay. The bay opens to the Pacific Ocean at its east side. The size of the bay is characterized by having a 6.6 km bay mouth and being 46.8 km2 in area [International EMECS Center, 2009]. The overall results of the Joint Survey Group show that there was no clear difference between the inundation levels in the inside and outside part of Shizugawa Bay and that a wide area was inundated on the northwestern and the southwestern part of the bay (Fig. 3(b)). In this case no breakwater had been constructed at the entrance of the bay, and hence it is not surprising that the inundation levels were similar as the tsunami wave could progress unhindered towards the town. The field survey was conducted at Shizugawa. The area where it is located consists of low-lying ground that surrounds 3 rivers. The town itself was protected by a multitude of protective structures, including coastal dikes and river mouth water gates, which were severely damaged or totally destroyed by the tsunami. This area also suffered severe damage due to the 1960 Chile Tsunami, and hence some monuments were also built to teach and remind residents of the danger of tsunami, though these were also destroyed by the tsunami. Because the waterfront area is located too far from the surrounding hills and there were not enough tall building in this area, a 4-story building near the shoreline was constructed specifically to act as a tsunami evacuation building (Fig. 6). For its construction the Guidelines for Tsunami Evacuation Buildings [Cabinet Office, Government of Japan, 2005] was used, one of the latest documents that provides design recommendations for tsunami evacuation buildings in Japan (though it should be noted that this is not a mandatory engineering code). This building survived the event with almost no structural damage with the exception of some scour on the sides of the building. 1250011-9

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Fig. 4. Fishing vessel left stranded inland at Kesennuma Port.

Fig. 5. Eroded land area at Motoyoshicho-nakajima, Kesennuma City. 1250011-10

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Table 3. The actions and thoughts of a resident at the tsunami evacuation building at Shizugawa. Stage

Actions and thoughts of the resident

Important lessons

Soon after the earthquake occurred

• After the long ground shaking, the resident decided to evacuate to the rooftop of the building with his daughter. • At first he thought about using a car to evacuate; however finally he decided to go to the rooftop because he expected the streets to be congested with people trying to escape the tsunami.

• In case streets are congested or roads are broken due to an earthquake it is necessary to place tsunami evacuation buildings near the shoreline.

Before the tsunami reached

• Usually the rooftop was locked; however the resident could get there because a caretaker who owns the key was in the building.

• It is important that any area that is considered to be a tsunami shelter should have its highest point unlocked and free of any obstructions that could slow the flow of residents in their escape.

During the tsunami inundation

• The tsunami reached the building just after they arrived at the rooftop. • The water level reached around his thigh (71 cm above the floor of the rooftop), and then he took his daughter up in his arms to avoid her getting wet.

• Because even the rooftop was not high enough in this event, it is necessary to reconsider the height of tsunami evacuation buildings in each area. • In order to avoid evacuees becoming weak due to getting wet in cold weather, some protection against the cold is needed in evacuation areas.

One resident on the first floor of this building explained to the authors the events of the earthquake and his thoughts regarding evacuation. The 5 main points that guided the thought processes of this individual as he attempted to escape the tsunami threat are shown in Table 3. These points highlighted how the tsunami evacuation building was actually used in this event and problems related to this construction and to the evacuation process in general. Particularly important is how residents think of the possibility of evacuating using vehicles, and how the roads are generally congested due to everybody attempting to escape in this way. According to this resident’s testimony, the ornament on the seaside edge of the rooftop was broken by the tsunami and this would mean an inundation height of 15.41 m. Debris were also found on a window screen on the fourth floor, which would thus represent an inundation height of at least 14.30 m. 1250011-11

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Fig. 6. Tsunami evacuation building at Shizugawa, Minamisanriku Town.

Fig. 7. Overturned building at Onagawa Town. 1250011-12

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3.1.3. Onagawa Town Onagawa Town is located in the middle part of Miyagi Prefecture, inside Onagawa Bay. This bay has Onagawa Port at the innermost part, with numerous small fishing villages being situated along the rest of its coastline. The bay opens to the Pacific Ocean to the east, and is characterized by having a 2.5 km bay mouth and a total area of 12.1 km2 [International EMECS Center, 2009]. The results of the Joint Survey Group show that the tsunami trace heights were more than 10 m high along the coastline of the bay and that at the innermost portion of the bay the tsunami propagated inland for over 1 km (along the narrow valleys carved into the coastal hills that surround Onagawa Town) (Fig. 3(c)). The field survey was conducted at 2 locations in this region, Onagawa Port and Takenoura. The main area of Onagawa Town is located around the port, though the town also has a more elevated part that was not so affected by the tsunami. Takenoura village is located on the northeast part of the bay, and is mainly a smallscale fishing village. Around Onagawa Port, many buildings, including reinforced concrete structures, were partially or completely destroyed and 4 overturned buildings were found (Fig. 7). Some of these buildings were built on pile foundations and, as a consequence of the earthquake, the upper end of the piles had sheared and, subsequently, lateral loading combined with buoyancy forces overturned the structures. Liquefaction of the foundation due to the earthquake is likely to have also played a part in this. The main area of the town had the town hall, a hospital, and a station, all of which suffered serious damage. At the town hall, the tsunami reached the third floor, representing an inundation height of 13.52 m. The hospital, which stood on top of a hill, was flooded up to the first floor, which in this case meant an inundation height of 17.39 m. Onagawa Port was protected against high wind waves by a breakwater constructed at the mouth of the bay, though this structure was totally destroyed except for a few caissons located next to the land on each side of the bay (Fig. 8). It is interesting to note how, according to a civil engineer working as civil servant in Onagawa Town, the caissons on the northeastern side of the bay failed towards the town, while the caissons on the southwestern side failed towards the sea, probably due to the force of the water as it went back into the sea. This breakwater was not actually designed with tsunami forces in mind, but rather against wind waves, and it is not surprising that it failed, though it is of some concern the catastrophic manner in which it failed (as opposed to a partial failure). At Takenoura, although there was a bay mouth breakwater and a seawall (1.75 m high above the ground) in front of the village, the whole area was inundated by the tsunami. Inundation heights were measured to be 11.05 m and 8.73 m inside the seawall and 11.91 m outside the seawall. The breakwater suffered partial damages, with the top part of the breakwater being broken in many locations (Fig. 9). 1250011-13

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Fig. 8. Bay mouth breakwater at Onagawa Town (a) before and (b) after the tsunami. (Note that almost no trace of the breakwater was left.)

Fig. 9. Failure of breakwater at Takenoura, Onagawa Town.

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For the case of this breakwater, it is interesting to note that the caissons that had failed did so at a poor construction joint, which happened during construction due to the contractor running out of concrete during the pour of the top concrete cap.a Of course, even if the breakwaters had not had its top portion removed, it is unlikely that it would have offered any additional protection, due to its relative low crest and the high inundation heights of the tsunami.


3.2. Damage in the plains area The coastal plains area starts in the southern part of Sendai City (the biggest city in the Tohoku region, with a population of over 1 million people) and extends southwards for around 20 km. The authors started their survey there and then slowly progressed south towards Natori River, Natori City, Watari Town, and then towards Soma City. 3.2.1. Wakabayashi, Sendai City Wakabayashi is one of the wards that constitute Sendai City, being located in its southeastern part and close to Natori River to the south. This coastal area, which is part of the Sendai Plain, suffered severe damage, especially around the Arahamab district. Arahama used to be a residential area of Sendai City, starting right next to the coastline and extending around 1 km inland. In this area of the coastline the Teizan Canal runs parallel to the shoreline at a distance of about 450 m from it. Coastal forests generally spread between Teizan Canal and the shoreline, providing some degree of protection, though at Arahama the houses extended over the canal and arrived right up to the shoreline. The overall results of the Joint Survey Group show that the tsunami propagated inland for more than 3 km from the shoreline, with the tsunami trace heights gradually decreasing as the wave progressed inland (upper part of Fig. 3(d)). Natori City (lower part of Fig. 3(d)) and Watari Town (Fig. 3(e)), which will be mentioned later, also compose the Sendai Plain and similar inundation patterns are found in these area. The field survey in this area was conducted at Arahama and around Natori River. At Arahama, most of the buildings, except for a few houses and an elementary school building, were destroyed (Fig. 10). The power of the wave was such that at this school building the tsunami reached up to the second floor, even though it was located about 700 m from the shoreline. It should be noted how school buildings in Japan are built to a high standard, and this is likely to have contributed to the resilience of the structure. Inundation heights near the shoreline were 9.32 m and a Private

communication between an Onagawa Town civil servant responsible for the design of the breakwater and the authors. b The name “Arahama” will appear again in Sec. 3.2.3 Watari Town. Although both of these locations bear the same name, they are not the same place.

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Fig. 10. Tsunami damage at Arahama, Wakabayashi, Sendai City.

Fig. 11. Accumulated debris around the mouth of Natori River. 1250011-16

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8.43 m. At the south of Arahama, many driftwoods were found, with one of them being about 70 cm in diameter. These woods were originally from the coastal forests spreading between the canal and the shoreline. Around the mouth of Natori River, a great amount of debris accumulated on the river bank (Fig. 11), with the inundation height being measured as 5.44 m.


3.2.2. Natori City Natori City is located in the middle part of Miyagi Prefecture and close to Natori River to the north. The right side of the river mouth is referred to as the Yuriage district, which has a port and residential area. Sendai Airport is located at the southern part of the city, around 1 km away from the shoreline. The field survey was conducted at 2 places in this region, Yuriage and Sendai Airport. At Yuriage, river embankments were partially damaged with cracks recorded at intervals of about 50 m. These cracks were found at joints between different structural elements in the embankments (Fig. 12). Discontinuities in these elements and other structures are often believed to play a role in the damage that can be caused by either a tsunami or an earthquake. It is thus important that careful attention is paid to joints during the design and execution of coastal and fluvial structures, especially when they will form the first line of defense against a tsunami. Many buildings were destroyed around the port and even a small hill, which was the highest place in this area, was overtopped by the tsunami wave (there was a tsunami trace 2.10 m above the top of the hill). Thus, there was no place high enough around the area that could have been used by residents to evacuate, highlighting some of the reasons for the high casualty rate recorded in this area. The inundation height near the port was 8.81 m. At Sendai Airport, mud was left inside the terminal building and a lot of cars were damaged and displaced by the tsunami. The tsunami inundated the entire area of the airport facility and the measured depths of inundation were 2.82 m inside and 2.98 m outside the terminal building. 3.2.3. Watari Town Watari Town is located on the southern area of Miyagi Prefecture, close to Abukuma River to the north. The right side of the river mouth is referred to as the Arahama district, which has a swimming and surfing beach. This area has a brackish water lake, called Torinoumi, and a port is located on the northern part of the lake. The residential area of Arahama is located between the river and the lake. For this location the authors conducted their field survey at Arahama. The river, as is usual in Japan, was protected by river embankments, which suffered no visible damage. However, the tsunami was higher than the level of the embankments and thus overtopped them, causing damage to the area behind it. At other locations along the coastline, coastal dikes had been constructed to protect against storm surges (caused by typhoons) and these were completely destroyed at many locations 1250011-17

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Fig. 12. Damage of river embankments on the right side of Natori River: (a) upward view, (b) downward view. (Note that a crack started from a joint.)

Fig. 13. Failure of coastal dikes at Arahama, Watari Town. 1250011-18

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(Fig. 13). The overtopping tsunami caused scouring at the back of the dikes, which undermined the structure and lead to the exposure of the central sand and gravel core, which was easily eroded by the tsunami and this caused its subsequent collapse. Inundation heights were 7.56 m near the coastal dikes and 4.82 m at inland area.


3.2.4. Soma City Soma City is located at the northeastern part of Fukushima Prefecture. The city has a port (Soma Port) situated at its northern part, and there is also a lagoon that is separated by a long sandbar from the Pacific Ocean (referred to as Matsukawaura). Isobe district is situated to the south side of Matsukawaura, being composed mainly of low-lying paddy fields. The results of the Joint Survey Group show that inundation heights near the shoreline were more than 10 m and the tsunami reached a few kilometers inland (Fig. 3(f)). For the case of this area, the authors conducted their own field survey at 2 locations within this region, Soma Port and Isobe. At Soma Port, part of a wharf was broken and carried about 30 m away from its original position, with a road attached to the wharf also collapsing (Fig. 14). At Isobe, the tsunami came over the coastal dikes and through the forest behind the dikes. As in Watari, the dikes also suffered damage, though the extent and pattern of the damage was different. The parts of the dikes which had been reinforced by having placing tetrapods placed in front of them survived, though other parts were completely destroyed due to scouring of the inner core of the dikes and subsequent collapse of the structure on top of them (Fig. 15). Many houses were washed away, with roads and houses left covered with a sediment sand layer. The inundation height in this area was 6.78 m. In the coastal plain area, made up of low-lying paddy fields, the tsunami reached around 1.8 km from the shoreline.

4. Discussion From the results of the authors’ field survey and the Joint Survey Group dataset, a number of inundation and destruction patterns could be observed. These inundation patterns give valuable information on how to improve evacuation preparedness in each area, while the destruction patterns highlight lessons on how the designs of the different infrastructure elements located in coastal areas should be modified during the rebuilding process. These lessons are important for policy makers, civil servants, engineers, and academicians to understand how coastal structures, tsunami evacuation buildings, and coastal forests should be improved, as every disaster should be followed by a careful reflection to improve the resilience of coastal communities against future events. 1250011-19

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Fig. 14. Damage of wharf at Soma Port.

Fig. 15. Failure of coastal dikes at Isobe, Soma City. 1250011-20

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4.1. Inundation patterns In the rias coastal area, inundation heights were more than 10 m at all places surveyed by the authors. According to the Joint Survey Group dataset (Figs. 3(a)–3(c)), the low-lying grounds located at the inner part of the bays were flooded, though the inundation heights did not largely change from the shorelines to the maximum runup points. However, because the direction, size, and shape of the bays vary from place to place, the distribution of tsunami trace heights was different from place to place. For example, around Kesennuma Bay tsunami trace heights inside of the bay were smaller than those outside of the bay, while around Shizugawa Bay there was no clear difference between the inside and the outside of the bay. In addition, it should be noted that a tsunami does not always behave in a similar way as to that observed in this event. For example, Nakamura and Emura [1961] compared the distribution of tsunami trace heights in the 1933 Sanriku Tsunami and those in the 1960 Chile Tsunami in Ofunato Bay and Hirota Bay, which are located about 10 km and 20 km north of Kesennuma Bay, respectively. These authors found that the inundation height at the bay head was 2–3 times larger than that at the bay mouth in the 1960 tsunami, while the height decreased with distance from the bay mouth in the 1933 tsunami. In the plains area, inundation heights were 5–10 m at all the locations that were surveyed by the authors. According to the Joint Survey Group dataset (Figs. 3(d)– 3(f)), the tsunami reached a few kilometers inland and the edge of the tsunami run-up was parallel to the shoreline. Unlike in the case of the rias coastal area, the inundation heights gradually decreased as the tsunami progressed inland. The difference between the inundation patterns observed requires evacuation strategies to be different in each area. In the rias coastal area, the effects of the tsunami were felt on the low-lying areas and although the inundation heights recorded were large, these low-lying areas were surrounded by high hills which served as adequate evacuation areas. Thus, for the case of these areas it is important that adequate access routes to the hills should be prepared and maintained for the people living near to the shoreline. It is important also to note that many residents will attempt to evacuate using motor vehicles, and that while this cannot be recommended (as it would probably result in congestion and slower evacuation times for the general population, as opposed to evacuating by foot), it is something that policy makers and planners should consider. Thus, it would be important that these evacuation routes are as wide as possible to avoid possible congestion during the evacuation procedure, wherever possible and feasible. Also it is important that tsunami evacuation buildings, such as the one at Shizugawa (Fig. 6), should be prepared as temporary evacuation places in case it is felt that residents would not have enough time to reach a hill. In the plain area, the ground consisted mainly of paddy fields (or other agricultural uses) and buildings were mainly low in height. It was clear that there were 1250011-21

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no suitable places to evacuate, though inundation heights gradually decreased from the shoreline as the tsunami progressed inland. Thus, in many of these locations evacuation buildings (where the population density is a bit higher) or tsunami shelters (in those areas that are very sparsely populated) should be prepared so that all individuals can easily evacuate in the event of a disaster.


4.2. Destruction patterns In the rias coastal area, it is important to analyze the effect that bay mouth breakwaters and tsunami evacuation buildings play in mitigating the tsunami damage. Because the rias coastal area is dotted with relatively small villages along various types of bays, most of these villages were protected from waves by breakwaters and other structures. The breakwaters observed in the areas surveyed by the authors (i.e. the ones mentioned in this paper) were constructed mainly to protect against wind waves, and hence the response against a tsunami attack had not been fully investigated. In Onagawa Town, 2 different patterns of damage were observed. One was the breakwater at Onagawa Port, which consisted of a northern section 300 m long and a southern section 300 m long, with a 150 m wide opening in between them. As a consequence of the tsunami almost all the caissons in each side were washed away, though those on one side of the bay were washed towards the town by the incoming wave, while the ones on the other side were carried towards the sea by the receding wave. This highlights how careful consideration must be given to the design of breakwaters not only against the incoming tsunami wave (which is the effect considered most frequently in the design of tsunami counter-measures) but also the forces and currents produced by the receding wave. As a tsunami wave is made of a number of waves, it is important that the structures have a certain amount of resilience to also cope with the second and subsequent waves, which can often be higher than the first wave. The other breakwater inspected by the authors was that at Takenoura, which consists of an eastern section (60 m long) and a western section (80 m long), with a central opening 50 m wide in between them. The breakwater suffered only partial damage, which as attributed to a poor construction “cold” joint that resulted from an interruption in the concrete supply during the pour. Both of these examples show that the damage to breakwaters requires careful re-examination in the light of the recent tsunami event, so that appropriate construction and safety standards can be achieved. Some buildings were selected for tsunami evacuation buildings in the rias coastal area. Figure 16 shows the contour maps (10 m interval) with the locations of tsunami evacuation buildings and tsunami trace heights at Shizugawa and Onagawa. At both places, there are narrow low-lying areas less than 10 m above sea level next to the coastline, which were completely inundated, as explained previously. Tsunami evacuation buildings were located close to the shoreline at points which were situated 1250011-22

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Fig. 16. The contour maps (10 m interval) with the locations of tsunami evacuation buildings and tsunami trace heights at (a) Shizugawa and (b) Onagawa. (The contour map was generated using the Fundamental Geospatial Data (10 m grid) from the Geophysical Information Authority of Japan.) 1250011-23

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at a relatively long distance from hills. At Shizugawa, the tsunami evacuation building (building A shown in Fig. 16(a)) was inundated up to its rooftop, though the lives of people who took refuge at its rooftop were saved. This shows that this tsunami evacuation building could function as a temporary evacuation place even in this high-order tsunami event (often described now by Japanese coastal engineers as a level 2 tsunami event [e.g. Suppasri et al., 2012]). However, two problems with tsunami evacuation buildings were exposed. One problem is that people can be isolated for many hours at the top of these buildings until there is no danger of subsequent tsunami waves. Tsunami evacuation buildings should have some measures to get information from the authorities and cold-related problems at their rooftop (it is important to remember that in the northern areas of Japan, such as Tohoku, temperatures are routinely below freezing during a large part of the year, and it is unlikely and unadvisable that fleeing residents will stop to take adequate winter clothing, especially if a tsunami was to happen at night. Hypothermia at the top of the evacuation areas could thus become a problem and should require some consideration). The other problem is about the resistance of the buildings against the forces exerted by the tsunami. The tsunami evacuation building surveyed by the authors at Shizugawa withstood the tsunami attack with only limited damage, mostly scour at the sides of the building. However, at Onagawa Town 4 reinforced concrete buildings were completely overturned and in some cases transported dozens of meters from their original location. Guidelines for Tsunami Evacuation Buildings [Cabinet Office, Government of Japan, 2005] say that reinforced concrete or steel-reinforced concrete buildings are required, though it is important that adequate requirements for withstanding high order tsunami forces should be included in future guidelines. An extensive system of coastal dikes and forests, which had protected the plains against wave attacks and storm surge before the tsunami, were lost in many areas. The shape of the shoreline at this plain area is simpler than that in the rias coastal area, thus making it easier to protect, which explains why before the tsunami most of the shoreline was actually covered by coastal dikes (with detached tetrapods placed in front of the dikes in some areas). Large-scale destruction of coastal dikes was observed. The causes of this destruction can be found in the large wave pressures acting on them for a prolonged period of time and the scouring induced by the tsunami wave as it overtopped the structures. The effect of tetrapods in preventing structural damage is something that is not well understood at present. At Isobe (Soma City), the sections of the dikes which had had tetrapods placed in front of them for extra protection survived, though other sections were completely destroyed. This shows that it is also important to investigate the relation between the damage to coastal dikes and surrounding structural and topographical features. In addition, the tsunami also caused serious damages to coastal forests, with many trees being knocked down and/or washed away by the violent nature of the waves. So far the role that forests have in mitigating the effects of tsunami has been 1250011-24

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investigated to some extent [e.g. Shuto, 1987; Danielsen et al., 2005], though there is still much discussion as to exactly how much protection they can provide, especially against a high order tsunami event. However, what has not been fully discussed is how to recover from the damage that these coastal defense structures (dikes and forests) can suffer after a high-order tsunami event. Thus, in the future it will be important to understand not only the destruction mechanisms of coastal dikes and forests, but also what parts of the system will be expected to fail (with the possibility of creating sacrificial elements or buffer zones in the path of the tsunami wave), and how the reconstruction of these elements will then be undertaken. For areas such as Tohoku, which have experienced and will continue to experience tsunami attacks in the future, it is important to evolve from the current disaster management strategy to a new one which encompasses coastal risk management in its entirety, and hence also includes the rebuilding process.

5. Conclusion The tsunami damage in the Pacific coast of Tohoku was analyzed by using both the results of the authors’ field survey, conducted 2 weeks after the event, and the Joint Survey Group dataset. The dataset compiled by the Joint Survey Group is so dense that it can serve as an effective tool to analyze the regional tsunami responses, and has already been used by Mori et al. [2011] to analyze regional tsunami characteristics. In the future, research that combines the use of this dataset, numerical simulations, and field surveys will be necessary to understand precisely what happened at each location within the affected area. Though much work will be needed before the event can be fully understood, it is already possible to derive a number of preliminary conclusions and lessons that can help to guide future research, as highlighted in this paper. The area surveyed by the authors can be divided into 2 parts from the point of view of its geographical features: the northern part (a rias coastal area) and the southern part (a coastal plain area). The inundation and destruction patterns due to the tsunami in each area were clarified, from which a number of important lessons on how to modify and improve future risk management strategies can be obtained. Because the damage due to a tsunami heavily depends on geographical features and local conditions, this kind of research is important for mitigating future tsunami damage. Also, in order to contribute to the restoration of the affected area and increase the awareness of people living in other tsunami-prone areas (both in Japan and abroad), further analysis of the potential damage at different locations should be carried out. This constitutes an area of research that should be prioritized in the future. It is important for the reader to understand that following this event there has been an intense debate in Japan regarding risk management, and this is likely to continue in the future. In the present paper, the authors have offered some of their 1250011-25

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thoughts on these issues, highlighted some failings, and pointed out areas of possible improvement. However, it is hoped that as a consequence of this event and the ongoing debate, disaster risk management theory and practice will improve in the future, leading to much lower levels of vulnerability in Japan and other countries. Acknowledgments


The present work was supported by the Grants-in-Aid for Scientific Research (B) No. 22404011 from the Japan Society for the Promotion of Science (JSPS) and the Disaster Analysis and Proposal for Rehabilitation Process for the Tohoku Earthquake and Tsunami from Waseda University Research Initiatives. Most of the figures were generated using Generic Mapping Tools [Wessel & Smith, 1998]. References Cabinet Office, Government of Japan [2005] “Guidelines for tsunami evacuation buildings,” http://www.bousai.go.jp/oshirase/h17/tsunami hinan.html (in Japanese). Danielsen, F., Sorensen, M. K., Olwig, M. F., Selvam, V., Parish, F., Burgess, N. D., Hiraishi, T., Karunagaran, V. M., Rasmussen, M. S., Hansen, L. B., Quarto, A. & Suryadiputra, N. [2005] “The Asian tsunami: A protective role for coastal vegetation,” Science 310, 643. Fire and Disaster Management Agency [2011] “Disaster information for 2011 off the Pacific coast of Tohoku earthquake (No. 140),” http://www.fdma.go.jp/bn/higaihou.html (in Japanese). International EMECS Center [2009] “Enclosed coastal seas in Japan,” http://www.emecs.or.jp/ closedsea-jp/closedsea-jp.htm (in Japanese). Kakinuma, T., Tsujimoto, G., Yasuda, T. & Tamada, T. [2012] “Trace survey of the 2011 Tohoku tsunami in the north of Miyagi Prefecture and numerical simulation of bidirectional tsunamis in Utatsusaki Peninsula,” Coast. Eng. J. 54(1), 1250007. 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,” Coast. Eng. J. 54(1), 1250001. Mori, N., Takahashi, T., Yasuda, T. & Yanagisawa, H. [2011] “Survey of 2011 Tohoku earthquake tsunami inundation and run-up,” Geophys. Res. Lett. 38, L00G14. Nakamura, K. & Emura, K. [1961] “Maximum water height at bay head in case of tsunami invasion,” Sci. Rep. Tohoku Univ., Ser. 5 : Geophys. 13(1), 32–42. Shuto, N. [1987] “The effectiveness and limit of tsunami control forests,” Coast. Eng. Jpn. 30(1), 143–153. Suppasri, A., Koshimura, S., Imai, K., Mas, E., Gokon, H., Muhari, A. & Imamura, F. [2012] “Field survey and damage characteristic of the 2011 east Japan Tsunami in Miyagi Prefecture,” Coast. Eng. J. 54(1), 1250005. The 2011 Tohoku Earthquake Tsunami Joint Survey Group [2011a] “Nationwide field survey of the 2011 off the Pacific coast of Tohoku earthquake tsunami,” J. Jpn. Soc. Civil Eng., Ser. B2 (Coast. Eng.) 67(1), 63–66. The 2011 Tohoku Earthquake Tsunami Joint Survey Group [2011b] “Survey data set (17-Oct2011),” http://www.coastal.jp/ttjt/. Wessel, P. & Smith, W. H. F. [1998] “New, improved version of generic mapping tools released,” Eos Trans., AGU 79(47), 579.

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