Chapter 6
Coastal Science
Field of expertise: Coastal Engineering

Keiko Udo
Translated by Catherine Sachi Kikuchi

SummaryThe 2011 Great East Japan Earthquake and Tsunami caused widespread land subsidence andbreaches in coastal embankments. This caused inundation damage in a wide area and large-scale
beach erosion, which had a great impact on the coastal regions. This chapter describes thedamage to the coastal embankments and beach erosion at the time of the earthquake andtsunami, the subsequent restoration and recovery process, and outlines the impact of theearthquake and tsunami on coastal management since then.
Keywords: beach erosion, coastal embankment, land subsidence, beach restoration, coastalenvironment, coastal management

Introduction
The coast is a space that exists at the boundary between the sea and the shore. It fosters arich environment, and also exists as a place for recreation. At the time of the Great East Japan
Earthquake, beaches all over Japan, especially sandy beaches, had already been severely eroded
due to rapid development after the war, and not enough countermeasures had been taken. In thischapter, we describe the impact of the Great East Japan Earthquake and Tsunami on coastalmanagement and future prospects, illustrating the example of Yamamoto Coast in southern Sendai
Bay, Miyagi Prefecture.

1: Problems Revealed by the Great East Japan Earthquake and Tsunami
What happened?
After the 2011 earthquake and tsunami, the first thing I saw was the image of a tsunami
invading the Sendai Plain. The southern coast of Sendai Bay (about 60 kilometers of coast that
extends from Sendai City, Miyagi Prefecture, to the Fukushima Prefecture border) sufferedsignificant erosion damage, mainly in the southern part, as shown in the photos from March 2011,
Figure 6-1 (Udo et al., 2015). In some beaches, much of the coastal forest was washed away by
the tsunami. In particular, on the Yamamoto coast (see Figure 6-1(d), September 2006), where the1coast was already severely eroded, coastal levees were breached everywhere and there waserosion up to several hundred meters inland. Topographic analysis before and after the earthquake
and tsunami shows that the eroded sediment was deposited in the sea at a depth of about 5 to 10
meters by receding waves.
Comparing the aerial photographs of March and June 2011 in Figure 6-1(d), the topographythat had been gouged at a sharp angle changed to a rounded shape in the months after theearthquake and tsunami, indicating some recovery of the beach. According to the results of thestructure analysis by ground-penetrating radar, the recovery rate was the greatest immediately
after the earthquake, with a maximum of 18 meters/month, and decreased to 2-3 meters/month
one year later.
The reality of the damage
Due to the earthquake and tsunami, 426 coastal areas (about 190 kilometers) out of 515
coastal areas (about 300 kilometers of coastal embankments and seawalls) in Iwate, Miyagi, andFukushima prefectures were damaged, resulting in widespread devastation. Figure 6-2 (Tojo &Udo, 2018) shows the coastal erosion caused by the earthquake and tsunami, and subsequent
recovery in Iwate and Fukushima Prefectures. The size of the circle indicates the scale of erosion
by the earthquake and tsunami, and the blue color of the circle indicates the percentage ofshoreline recovery four years after the earthquake and tsunami. There are long sandy beaches inthe Sendai coast in Miyagi Prefecture, and erosion was limited in the beaches that maintained
sufficient beach width before the earthquake. On the other hand, the size of the circles in Figure 62 is larger on the beaches where beach erosion had already progressed at the time of theearthquake, and on the beaches along the Sanriku coast in southern Iwate and northern Miyagi
prefectures, where short, pocket beaches are scattered, indicating that significant erosion
occurred. In these areas, the percentage of red in the circles is large, and since sand supply fromthe surrounding areas could not be expected, there was little recovery afterwards.

Figure 6-1. Aerial photographs of the southern coast of Sendai Bay from September 2006, March2011, June 2011, and June 2013 (Udo et al., 2015)

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Figure 6-2. The amount of beach erosion before and after the earthquake and tsunami from Iwate
Prefecture to Fukushima Prefecture and the state of beach recovery up to 4 years after theearthquake and tsunami (Tojo & Udo, 2018)

2: Paradigms Destroyed by the Earthquake and Tsunami
Conventional wisdom and necessary responses
Of the approximately 1,700 kilometers of coastline in the three prefectures of Iwate, Miyagi,and Fukushima, there is approximately 300 kilometers of coastline with embankments or seawalls,
approximately 190 kilometers was completely destroyed, making it necessary to hasten
preparations for coastal disasters such as storm surges and waves during typhoon season in theaffected coastal areas (https://www.thr.mlit.go.jp/sendai/kasen_kaigan/fukkou/kouzishi.html). By
the end of September 2011, emergency restoration corresponding to the height of storm surgesand waves was almost completed for a section about 50 kilometers long, behind which facilities
essential for recovery and reconstruction are located. After this emergency restoration, discussions
on coastal restoration and reconstruction proceeded at a rapid pace, but in the initial stage, thediscussions were mainly focused on disaster prevention that took the landscape and environment
into consideration.
In addition to widespread land subsidence and tsunami inundation, the southern coast of
Sendai suffered extensive damage to embankments, and emergency restoration was undertaken
along approximately 20 kilometers of coastline. The first step (protection against the recent
maximum storm surge) was completed by June 2011, and the second step (additional protection
against the recent maximum storm wave in critical sections) was completed by August of the sameyear. Even on the coast where rapid topographical recovery was observed immediately after theearthquake and tsunami, the recovery gradually slowed. Construction of a levee that can cope withstorm surges, which have a higher risk than the tsunami, proceeded while the topography had not
yet recovered. On the Yamamoto Coast, erosion was evident even before the earthquake andtsunami, and the Ministry of Land, Infrastructure, Transport and Tourism had been implementing
coastal improvement projects under its direct jurisdiction since 2000. In the overall plan of theproject, 4 million m3 of beach nourishment was planned before the earthquake and tsunami, and138,000 m3 of beach nourishment had been completed by the end of 2012. Due to the severe3

erosion by the tsunami, the beach nourishment project needs further nourishment and is expected
to take several more decades to complete.

3: A New Approach
As time passed since the disaster, the importance of not only disaster prevention, but alsoof the landscape and environment, such as the positioning of sandy beaches in the livingenvironment, had been widely recognized, and several cases made great efforts for reconstruction
planning by holding study sessions among residents about the future state of the coast. It is also
worth mentioning that there has been a high demand for the environment, such as theconservation of sandy beaches, on the affected coasts.
The long-term coastal erosion problems experienced after the end of World War II havemade it known that it is extremely difficult to restore eroded beaches, but it seems that until thedisaster, it was not expected that a temporary event such as a tsunami could cause such largescale erosion that would require a considerable revision of coastal maintenance plan. Climatechange also threatens to increase future coastal disaster risks, and according to the results of theauthors' beach loss prediction, up to 90% or more of the beaches in Japan are expected todisappear by sea level rise under climate change (Udo & Takeda, 2017). Since it is difficult topredict the scale and timing of tsunamis, it is impossible to incorporate the amount of beach loss
due to tsunamis into future projections, but when considering long-term countermeasures, thediscussion is becoming a premise for the impact of relatively low frequency sudden disasters suchas tsunamis.

4: Achievements and the Future
A new approach to disaster science
In terms of emergency response to the widespread damage of the Great East Japan
Earthquake and Tsunami, the post-disaster response can be evaluated as swift and accurate. On
the other hand, in thinking about the restoration and reconstruction of the coastal areas, it wasrecognized again that the coastal environment and its usage are equally important as coastal
disaster reduction, and residents seek to balance these functions.
In the formulation of coastal project plans, cost-benefit analysis is generally conducted toexamine the investment effects of the project. In the calculation of the benefits, it is preferable toconsider all the benefits of disaster reduction, environment, and utilization, which are the legal
objectives of the Coastal Act, but the evaluation of environmental benefits is particularly difficult,and further development of the evaluation methods are required. We hope that more research will
be conducted to properly assess the value of the coasts in order to leave a better coastal
environment for future generations, while also coping with various disaster risks including climatechange.

Conclusion - from the authors
Japan has the sixth longest coastline in the world and is constantly exposed not only totsunami risks, but also to storm surges caused by typhoons and others. It is also concerned thatthe coastal environment in Japan will become more severe in the future due to the rise in sea level
and the increase in intensity of typhoons caused by climate change. However, if we only pay4

attention to disaster reduction, the precious natural environment may be lost. While preparing forvarious coastal disaster risks, it is important to carry out coastal management that balances
disaster reduction, environment, and utilization.

References
Tojo, K. & Udo, K. (2018). Analysis of beach recovery after the 2011 Tohoku Earthquake Tsunami
based on shoreline extraction by ISODATA. Journal of Coastal Research, 85(sp1), 171–175.
https://doi.org/10.2112/SI85-035.1
Udo, K., Takeda, Y., Takamura, M., Mano, A. (2015). Serious erosion of the southern Sendai
Coast due to the 2011 Tohoku earthquake tsunami and its recovery process. In: Santiago-Fandiño
V., Kontar Y., Kaneda Y. (Eds.), Advances in Natural and Technological Hazards Research (pp.
225-236). (Vol. 44). Springer. https://doi.org/10.1007/978-3-319-10202-3_15
Udo, K. & Takeda, Y. (2017). Projections of Future Beach Loss in Japan Due to Sea-Level Riseand Uncertainties in Projected Beach Loss. Coastal Engineering Journal, 59(2), 1740006-11740006-16. https://doi.org/10.1142/S057856341740006X

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