Department of Ecosystem Engineering, University of Tokushima

Tokushima, JAPAN

sugimotot@nita.co.jp

　

　

In September 2001, the Japanese government evaluated the probability of occurrence of the next Nankaido earthquake in the Pacific Ocean to be 80% within the next 50 years. It is therefore necessary that countermeasures against this disaster should be improved as soon as possible. In this study, a tsunami numerical simulation taking into account the effect of tide and flood gates on inundation was first developed. The effect of tsunami inundation from these gates was estimated for Usa town in Kochi prefecture, Shikoku Island. The gates were investigated so as to ascertain if they would be able to be closed when a tsunami comes. It was found that tsunami inundation decreased when the gaps of the gates decreased, in Usa town. In particular, these structures, which shut out the first and second tsunami waves, can serve to delay inundation time. Therefore, it is necessary to close such gates so as to allow longer evacuation time for inhabitants as a preventative measure against tsunamis. However, it was confirmed from investigation of gate control, that some gates could not be closed because of either poor maintenance or taking too long to close. In some cases, the manager who closed the gate would not be able to seek refuge in time, for gates located far from the evacuation place for gates, which take a long time to close. To be effectively useful as a regional tsunami countermeasure, some of these gates need improvement of the closing method and evacuation routes need regular maintenance so as to be kept in good condition.

　

　

A number of earthquakes have occurred at intervals of 100-150 years along the Nankai Trough. In the past, the coastal area of Shikoku Island in Japan has suffered great tsunami damage caused by these earthquakes.

　

In September 2001, the Japanese government evaluated the probability of occurrence of the next Nankai earthquake to be 80% Within the following 50 years. With this in mind, it is therefore necessary that countermeasures against such a disaster be improved urgently. Shimada et al. (1999) showed the change in distribution of tsunami inundation with time, clarified the process of human damage occurrence with evacuation activities of inhabitants, and proposed detailed tsunami countermeasures. Their disaster mitigation measures software is useful for predicting areas at risk of inundation due to a tsunami.

　

Large-scale hardware measures, such as the construction of tsunami breakwaters and rising of embankments and seawalls, require much time and money. But with a probable Nankai earthquake tsunami looming and the Japanese economy being somewhat depressed, the above-mentioned measures are unrealistic. Thus, both cost effective and substantial measures are required. More effective measures using existing structures will reduce the cost and can be completed in a relatively short time period.

　

In many existing structures, tide and flood gates are the focus of our attention. In Japan, coastal areas are more often struck by typhoons than by tsunamis. So, most gates have been constructed to prevent inflow of seawater to protect against storm surges. Tide gates are located as a part of seawalls to protect traffic and to prevent inflow of seawater. Flood gates are located along rivers, and prevent seawater inflow to the river. But, Kawata (2001) pointed out existing problems with the present tide and flood gates for tsunami countermeasures as follows: 1) It takes much time to close a gate and 2) Many gates are closed manually.

　

In this study, the purpose is to clarify the effect of tide and flood gates related to tsunami inundation. Also, countermeasures necessary for the effective use of these gates are proposed for the case of tsunamis.

　

　

Tsunami numerical calculation is applied to an existing method, which provides substantiation of repeatability of tsunami numerical calculation around an object area (Murakami et al., 1996(a), 1997). Usa town in Kochi prefecture was selected as the study area (Fig.1). This town has been damaged periodically by Nankai earthquake tsunamis. The 1854 Ansei Nankai earthquake tsunami (Magnitude 8.4) was used as a case study.

　

　

This tsunami was estimated to be the highest risk tsunami for this town in previous studies (Murakami et al., 1996(b)). Even the width of seawalls and gates, which are smaller than a grid interval, are considered in the calculation.

　

　

In the tsunami numerical calculation presented here, breakwaters, sea embankments and seawalls that are in calculation areas, are given in the boundary condition. When water surface elevation is higher than their crest elevations, flood volume is calculated by Honma's function (1940). If the widths of flood and tide gates are smaller than the grid interval, a traditional difference method can be used to calculate gates, walls and embankments within grid interval units, but this method cannot be used to calculate actual width. Therefore, in this study, gate open situations, which are smaller than the grid interval, are dealt with as follows.

　

a) When gate is closed

When tide and flood gates are closed and when tsunami height is greater than crest elevation, flood volume (Q) is calculated by Honma's function, for breakwaters, sea embankments and seawalls that are in calculation areas.

　

b) When gate is open

Flood volume (Q) is calculated using formula (1) or (2) depending on the relationship between water surface elevation in front of and behind gate and crest elevation.

When water surface is below crest elevation (H <BY ).

　

　

When water surface is higher than crest elevation (H >=BY ).

　

　

where, BX represents gate width, BY is gate height, H is inundation depth, ΔS is grid interval, and Q₀ is the flow volume which is not a given boundary condition. When flood gates are open, flow volume is given by forward and backward ground levels.

　

　

　

The effect of closed tide gates on tsunami inundation was evaluated by numerical calculation. The northern area of Usa town was focused upon, because there were many tide gates in this area.

　

Figure 2 shows the distribution of tsunami inundation for a 30-minute period after an earthquake. In this figure, the unfilled circles indicate the location of opened gates. If the gates are open, inundation flows in through the gates, and spreads out into the outside area of tide embankment, which are gray areas in this figure. On the other side of the figure, it can be seen that if all tide gates are closed, tide wall and closed gates are able to block tsunami inundation.

　

Figure 3 shows the relationship between water elevation and time at inland points of tide gates. The gray line shows water elevation when the tide gate is open, and the black line shows it when the gates are closed. When the 1st wave arrives, a closed tide gate reduces inundation significantly. But, 76 minutes after an earthquake when a tsunami flows over the top of the gate, the inundated seawater does not drain away.

　

　

　

　

The effect of closed flood gates on tsunami inundation was evaluated from numerical calculations. Figure 4 shows the distribution of tsunami inundation for 87 minutes after an earthquake. 87 minutes after an earthquake is the time that the third wave arrives at Usa. If the floodgate is open, tsunami runs up along the river, and the inundated area spreads out over land. On the other hand, if the floodgate is closed, the gate stops the inflow along the river and protects from inundation on land.

　

　

Figure 5 shows water elevation with time at an inland point of a floodgate. The gray line shows the water elevation when the floodgate is open, and the black line shows it when the gate is closed. When the floodgate is open, a tsunami runs up to the river since the 1st wave came. On the other hand, when the floodgate is closed, the gate stops inundation until the third wave came. But, inflow water has not drained away completely and remains pooled inside of gates and walls.

　

　

Here, the results of tsunami numerical calculations are integrated. Most tide and flood gates are always open. As a result of closing these gates, tsunami run-up and inundation are delayed and reduced in Usa town. In other words, these gates are effective against blocking tsunamis. They extend inhabitants evacuation time and decrease damage due to inundation.