5:15 PM - 6:45 PM
[HDS11-P11] Probabilistic Inundation Risk Assessment Incorporating Tide Level Variations
Keywords:probabilistic inundation risk assessment, tide level variation, cost-benefit analysis
1. Tide Level Variations in Inundation Risk Assessment
The higher the tide level, the larger the tsunami inundation area and the greater the damage caused by inundation. It is common to use the spring mean high tide level as the standard in tsunami damage estimation by local governments, since it is assumed to be on the hazardous side. In this study, we attempted a probabilistic inundation risk assessment that incorporates the variation of tidal levels to promote effective tsunami countermeasures against earthquakes that occur along the Japan Trench and the Kuril Trench.
2. Selection of Target Areas
To select a target area for inundation risk assessment, we evaluated the probability that present coastal embankment such as breakwaters, river levees and other structures (referred to here as "present embankments") can protect against inundation damage from tsunamis caused by earthquakes along the Japan Trench and the Kuril Trench, which is referred to “non-exceedance probability”. The non-exceedance probability was estimated using tsunami hazard curves along the coastline (Ohno, et al. (2022, JpGU)). Considering the evaluation of the present embankments, topography, population and urban area, Kushiro city was selected as the target area.
3. Preliminary Study on Probability Density Function of Tide Level
According to Baba et al. (2022), the variation of tide levels is estimated by convolving the probability density function obtained from the tide level data (2013-2022, Japan Meteorological Agency) of Kushiro city to inundation depth data. The shapes of the probability density functions of the tide level and the inundation depth need to match. To compare the shapes of the two functions, a tsunami inundation simulation was conducted for a characterized earthquake fault model (Mw 9.2) along the Kuril Trench. As a result, it was confirmed that the shapes of the probability density functions of the tide level and inundation depth do not match in areas where the elevation changes rapidly.
4.Tsunami Inundation Simulation incorporating the Variation of Tidal Levels
We attempted to correct the probability density function of tide level to match the shapes of the probability density functions of tide level and inundation depth. We selected 115 characterized earthquake fault models along the Japan Trench and the Kuril Trench and conducted inundation simulations under three tidal conditions: the lowest tide (-121 cm), the highest tide (+93 cm) and the mode (tide with the highest probability density) (+15 cm). Assuming that "the greatest extent of inundation occurs under the highest tide condition," the probability density function of inundation depth estimated from the probability density function of tide level was then convoluted downward to the inundation depth data obtained under the highest tide condition.
5. Cost-benefit analysis of embankment raising
Tsunami inundation simulations were conducted for two embankment conditions, the present embankment and the raised embankment, to verify the effectiveness in reducing inundation damage. The amount of embankment raising was set so that the height of the embankment would be equivalent to a 1,000-year return period calculated from the hazard curves along the coast. The total damage (property damage and human damage) caused by inundation was calculated for each case (per characterized earthquake fault model) under the two embankment conditions to obtain the average amount of mitigation damage. A cost-benefit analysis was then conducted using the construction cost of embankment raising and the mitigation damages to evaluate the effectiveness of raised embankment. In the presentation, the results of the cost-benefit analysis will also be presented.
This study is conducted as part of the research project "Research on Hazard and Risk Assessment for Natural Disasters" of the National Research Institute for Earth Science and Disaster Resilience (NIED).
The higher the tide level, the larger the tsunami inundation area and the greater the damage caused by inundation. It is common to use the spring mean high tide level as the standard in tsunami damage estimation by local governments, since it is assumed to be on the hazardous side. In this study, we attempted a probabilistic inundation risk assessment that incorporates the variation of tidal levels to promote effective tsunami countermeasures against earthquakes that occur along the Japan Trench and the Kuril Trench.
2. Selection of Target Areas
To select a target area for inundation risk assessment, we evaluated the probability that present coastal embankment such as breakwaters, river levees and other structures (referred to here as "present embankments") can protect against inundation damage from tsunamis caused by earthquakes along the Japan Trench and the Kuril Trench, which is referred to “non-exceedance probability”. The non-exceedance probability was estimated using tsunami hazard curves along the coastline (Ohno, et al. (2022, JpGU)). Considering the evaluation of the present embankments, topography, population and urban area, Kushiro city was selected as the target area.
3. Preliminary Study on Probability Density Function of Tide Level
According to Baba et al. (2022), the variation of tide levels is estimated by convolving the probability density function obtained from the tide level data (2013-2022, Japan Meteorological Agency) of Kushiro city to inundation depth data. The shapes of the probability density functions of the tide level and the inundation depth need to match. To compare the shapes of the two functions, a tsunami inundation simulation was conducted for a characterized earthquake fault model (Mw 9.2) along the Kuril Trench. As a result, it was confirmed that the shapes of the probability density functions of the tide level and inundation depth do not match in areas where the elevation changes rapidly.
4.Tsunami Inundation Simulation incorporating the Variation of Tidal Levels
We attempted to correct the probability density function of tide level to match the shapes of the probability density functions of tide level and inundation depth. We selected 115 characterized earthquake fault models along the Japan Trench and the Kuril Trench and conducted inundation simulations under three tidal conditions: the lowest tide (-121 cm), the highest tide (+93 cm) and the mode (tide with the highest probability density) (+15 cm). Assuming that "the greatest extent of inundation occurs under the highest tide condition," the probability density function of inundation depth estimated from the probability density function of tide level was then convoluted downward to the inundation depth data obtained under the highest tide condition.
5. Cost-benefit analysis of embankment raising
Tsunami inundation simulations were conducted for two embankment conditions, the present embankment and the raised embankment, to verify the effectiveness in reducing inundation damage. The amount of embankment raising was set so that the height of the embankment would be equivalent to a 1,000-year return period calculated from the hazard curves along the coast. The total damage (property damage and human damage) caused by inundation was calculated for each case (per characterized earthquake fault model) under the two embankment conditions to obtain the average amount of mitigation damage. A cost-benefit analysis was then conducted using the construction cost of embankment raising and the mitigation damages to evaluate the effectiveness of raised embankment. In the presentation, the results of the cost-benefit analysis will also be presented.
This study is conducted as part of the research project "Research on Hazard and Risk Assessment for Natural Disasters" of the National Research Institute for Earth Science and Disaster Resilience (NIED).