Seventh Symposium (Presentation of Kochi : Kochi Prefectural Government)

Countermeasures against Tsunami in Kochi Prefecture

Thank you, chairman, for your kind introduction. I’m Tokihiro Kadota, Director General of the Port, Harbour and Airport Bureau of Kochi Prefecture.

First of all, I would like to offer my heartfelt sympathy to the Sri Lankan victims who lost their lives and assets in the massive tsunami that hit the area in December 2004.

Today, I have the pleasure of having the opportunity to make a presentation.

The theme of the symposium is ‘Enhancement of Productivity to Face the Global Challenges through Sister Ports.’ For this purpose, Kochi Prefecture has been actively involved in various economic exchange activities with INAP member ports.

Let me show you an example of such activities with Sri Lanka. There have been a growing number of companies in Kochi which have trade relationship with companies in Sri Lanka. A company in Kochi, for example, imports COIR products as agricultural or civil engineering materials. Another company exports construction machines. Incidentally, last week, several Kochi companies visited here to have business meetings.

More lately, there was a party to commemorate establishment of a joint venture company in Sri Lanka yesterday. The joint venture is Daioh Lanka, which was established in September by Daioh Machinery, a company in Kochi and Pheonix Trading, a company in Sri Lanka. Daioh Lanka’s purpose is mainly to provide high-tech maintenance of construction machines. Currently, Sri Lanka is on its way of rehabilitation from damages caused by the massive tsunami, and is in a process of development of the country. Construction machines will be actively used for such purposes. I am sure that the company will greatly contribute to the rehabilitation of the country.

Kochi Prefecture is determined to further promote economic exchanges through INAP to develop and expand the mutual relationship among the member ports.

Now, I would like to show you some examples of provisions against tsunami in Kochi Prefecture.

Kochi Prefecture, is located on the Pacific rim earthquake belt and has been hit by a number of massive earthquakes on a scale equivalent to the Sumatra Earthquake.

This is because Kochi Prefecture is on the Eurasian plate, which is gradually being dragged into the Philippine Sea plate at the speed of approx. 4.6cm a year. The Philippine Sea plate is also slowly sliding under the Japan Trench at approx. 10,000m below sea level.

Such sliding accumulates stress in the plates. When the stress reaches a certain level, a massive earthquake occurs, emitting all the energy that has built up in the plate. This is called a trench type earthquake. Trench type earthquakes equivalent to magnitude 8 we said to occur at an interval of 100 to 150 years. Should an earthquake of such a scale occur in Kochi Prefecture, it would generate a vibration of 500 gal level for about 80 seconds. A massive tsunami of 4 to 12m in height would then follow, hitting the coastal side of the prefecture 5 to 30 minutes after the occurrence of the earthquakes. Estimated 10,000 lives would be lost in Kochi Prefecture.


In the near future, there is likely to be a massive earthquake off the coast of Kochi Prefecture. This is called the Nankai Earthquake. The size of the Nankai Earthquake will be slightly larger than the earthquake that hit the prefecture in 1946, which is the same scale as one with a magnitude of 8.4 or so, which hit the prefecture in 1854.

When an earthquake of such size hits the prefecture, the sea bed off the coast of Kochi Prefecture will be pushed up 2.2 m or so and a fault will be generated by the sliding of the sea bed to the southeast direction by 6m. As a result of this, central Kochi Prefecture will sink like a punctured balloon. The sea bed around the Port of Kochi will also sink by 1m. The sea water on the bed will also drop by 3m causing water to flow from the higher point to the lower point. This is the mechanism of a tsunami generation.

The word, tsunami, is a Japanese word but is now understood universally. ‘Tsunami’ consists of two words; ‘tsu’ meaning a bay and ‘nami’ referring to a wave. A tsunami will grow larger along the geographical feature ‘tsu.’ A tsu is a good natural harbor protected from waves generated by a typhoon and other natural disasters. Tsu is where people and industries gather. ‘Tsu’ is a place which has been repeatedly damaged by a large-scale ‘wave.’ This is the origin of the word ‘tsunami.’ This is not a happy fact, but Japan is the most vulnerable country to tsunamis in the world.

Now I will explain how the height of a dike is determined to protect against a possible tsunami. Dikes along Kochi Prefecture are well designed to protect against waves caused by typhoons. For this reason, walls along the outer sea tend to be high and others inside bays are designed to be lower. During an earthquake, the ground on the entire island of Shikoku deforms. The ground on the southern part of the east-west center of Tosa Bay, then, lifts up and the northern part of it sinks.

It is quite difficult to estimate where on the Nankai Trough an earthquake will strike. So, based on the data of hypocenters of past earthquakes, five possible hypocenters are estimated and the height of tsunamis reaching to the coastal area is assumed.

This is the result of the estimation. Even considering ground settling and deformation, no tsunamis will grow larger than the height of most dikes at the coastal areas except for those installed along the deeply-indented coastal zones of inside the bays. Tsunamis can go through the gap of bays, shipping ports and rivers.

A massive earthquake and a large tsunami hits Kochi Prefecture once every 100 to 150 years. This means after an occurrence of them, they will not occur again for another 100 to 150 years.

It is not an easy task to protect Kochi citizens just by the improvement of infrastructure, because of the durability and cost for it. This necessitates a change from the current policy of infrastructure-centered ‘disaster prevention’ measures to the policy for ‘reduction of damage against disasters,’ in order to protect Kochi citizens from such infrequent disasters occurring in such a long frequency.

The damage reduction measures in Kochi Prefecture consist of two factors. One is ‘to safely evacuate’ the citizens to protect them from a massive earthquake and a large-scale tsunami that are expected to occur with the possibility of 50% in the next 30 years, and the other is ‘to improve infrastructure to support the evacuation.’ At the same time, Kochi Prefecture is promoting the setting up of restoration centers that take a role as a base to promote restoration of damaged areas.

For ‘evacuation,’ Kochi has set three basic principles: (1) self-support, (2) mutual support and (3) public support.

The Japanese Government and Kochi Prefectural Government simulate seismic vibrations and the size of a tsunami at various measuring points in the prefecture, to measure the size of vibrations in various regions, to identify the possible path of a tsunami, and to estimate the depth of flooding caused by it. In addition, more detailed data is calculated; how many minutes it will take a tsunami to reach 10cm in height and then 50cm in height as well as to peak at each datum point in the municipalities in Kochi. Kochi Prefecture plans to plot a hazard map based on such data, including the evacuation path to the nearest evacuation area designated by each municipality. Such maps have already been distributed to each household in some municipalities.

Next are the tsunami prevention measures at ports and coastal areas.
The role of a port is not limited to provide logistics. It is an amenity area for the citizens as well. In Kochi, there are some beaches which a tsunami’s peak reaches within 10 minutes after the occurrence of an earthquake and where a flooding starts on the beaches evacuation. For example, Kannoura Shirahama Beach located on the eastern end of Kochi is facing the sea with its backside surrounded by rivers. Being a shallow beach of fine granitic white sand, the Kannoura Shirahama Beach receives more than 1,000 visitors a day in summer.

When the beach is hit by a massive tsunami, escape routes are cut off and a flood occurs at all the areas including the regions behind the beach. So, an artificial ground is constructed at the corner of the Park to secure a place of evacuation for resort visitors and residents. The artificial ground is of a concrete rigid frame structure, so that the park can be used as a park facility and is available for access in normal times.

Next is the Port of Kochi, a member of INAP.
The Port of Kochi was hit by abnormally high tidal waves due to the Typhoon No. 10, in 1970, and one third of Kochi City was inundated. The damage was the worst of its kind in the history of the city. A tsunami caused by an earthquake that hit the area in 1946 was also devastating; the ground sunk and the water retaining dikes were destroyed. The tsunami lasted for only 6 hours, but the city was inundated in water for 12 days. The former paddy field areas in those days are urbanized now. The dikes constructed after the Typhoon No.10 in 1970 have effectively protected the city area from Tsunami. However, a tsunami can still enter the city and cause a flooding. It breaks through the water gates and navigation locks installed in rivers. Moreover, a settling of the ground by 1.5m can make the city center sink to sea level. Should this happen, the city area will be flooded by 1m even at normal full tides.

In these areas, flooding caused by a tsunami can start about 20 minutes after the occurrence of an earthquake. It is too late if the water gate is closed after sensing an earthquake. Therefore, the water gates installed at major rivers are constructed so that they will automatically close when they sense the vibration of more than 250gal.

In addition to this is the reinforcement of pumping facilities to withstand an earthquake by forcefully discharging water to outside areas. As the technology to make pumping facilities able to withstand vibrations has not yet been established, the operation equipment in these facilities are reinforced to prevent them from being soaked in water and to withstand collapse and vibration damage.

The third is the Port of Susaki.
The Port of Susaki is located at the inner most area of the deeply-indented coastal line. It may be the fate of such geographical features, but the Port of Susaki was the only port in the prefecture that was damaged by a flood due to a tsunami caused by the massive earthquake that hit Chile in 1961. The hypocenter was far away from Japan on the other side of the Pacific Ocean.

In order to avoid tsunami entering into the bay area, fro the last 23 years coastal dikes have been under construction and the height of the dams along the coastal lines have been in the process of being raised. At present, 70% of the construction has been completed, aiming at full completion of the construction by the 2010s. The size of a tsunami that these dikes and dams are designed to withstand is equivalent to the one, that hit the area in 1946, caused by an earthquake measuring a magnitude of 8.0. However, the design size has recently been modified to the size of the tsunami caused by the earthquake that hit the area in 1854 measuring 8.4. Installing tidal dikes and dams of the original size is also meaningful, however, because they can decrease the depth of the flooding in the city area and delay the start of flooding.

The last topic is about the improvement of restoration points. Civil engineering structures in Japan in general are designed to withstand vibrations of approx. 250 gal. When an earthquake of magnitude 8.4 level occurs, the coastal area will be subject to vibrations of more than 500 gal. Seismic force twice as large as that for which they were designed will be applied to roads, rivers, potable and sewage water lines, and buildings in the prefecture. Devastating damage to structures on land is expected: roads will be damaged and severed. Given that aftershocks of magnitude 7 level are expected to continue for 6 months, considerable time will be necessary to restore the damaged roads. There is a limit to the utilization air transportation.

Thus, Kochi Prefecture promotes the improvement of seismic-proof reinforced water-retaining walls along the coastal line that can withstand vibrations of 500 gal. Construction is still ongoing, but is expected to be completed during the later half of 2010s.

Ports are fundamental infrastructures to support the movement of goods, but can be used as rehabilitation points in the case of massive disasters. I sincerely hope that the exchange of this sort of information will deepen friendship among INAP members. Thank you very much for your attention.