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Vulnerability assessment to liquefaction hazard induced by rising sea-levels due to global warming.

This paper describes a procedure for liquefaction hazard mapping that incorporates rising sea levels caused by global warming, along with an application of that mapping procedure to an objective coastal region. Vulnerability to liquefaction hazard in the objective region was assessed by comparison of liquefaction hazard maps before and after sea-level rising caused by global warming.

INTRODUCTION

Ground damage caused by liquefaction is a serious problem. Attention should be given to groundwater level (GWL) changes because liquefaction potential is sensitive to rising GWL. In this century, sea-level rises (SLR) caused by global warming and global climate change have engendered rising GWL in coastal regions. The capabilities of prevention and mitigation against geo-disasters is reduced with rising GWL (Yasuhara et al., 2004). Therefore, it is necessary for sustainable development to consider the influence of global climate change on future situations of GWL. This study shows a procedure for liquefaction hazard mapping considering SRL and an application of the mapping procedure to Yokohama and Kawasaki cities in Japan. Vulnerability to liquefaction hazard in the objective region has been assessed through comparison of liquefaction hazard maps before and after SLR caused by global warming.

LIQUEFACTION HAZARD MAPPING IN CONSIDERATION OF GLOBAL CLIMATE CHANGE

Liquefaction potential depends upon ground properties, seismic magnitude, and GWL. Spatial distribution of GWL in an objective region depends on characteristics of rainfall, groundwater abstraction, and hydraulic boundary conditions. Where the objective area is a coastal zone, one hydraulic boundary condition is the sea level. Global climate change will influence the balance of water circulation and global warming will cause SLR. The liquefaction potential increases with rising GWL. Therefore, for disaster prevention and mitigation of earthquakes, it is important for coastal regions to consider future situations of GWL subjected to SLR. Such measures should take into account the GWL, not only of the present situation, but also that resulting from future trends.

Figure 1 shows a procedure for liquefaction hazard mapping in consideration of global climate change. Calculating the liquefaction hazard at a location in an objective region usually requires information on soil properties through depth, GWL and seismic shear stress. In recent years, geo-information digital databases, including boring logs, N-values, etc. have been made from site investigations in some regions in Japan. The geo-information database is available to predict geo-disasters such as earthquake-induced liquefaction. The database supplies necessary information of soil properties for calculating the liquefaction potential. Seismic shear stress in the ground is predicted considering assumed earthquake events or uses the same value ordained in a design code. Using groundwater flow analyses, the GWL is predicted on the assumption that global climate change will cause a different water circulation scenario from that of the present situation.

[FIGURE 1 OMITTED]

The present study calculates the liquefaction hazard at each location using a method proposed in The Japanese Highway Bridge Code (JHBC; so called "Dorokyoshihousho"), which was established in 1996 by the Ministry of Construction (currently the Ministry of Land and Transportation). This judgment procedure is based on the liquefaction resistance factor, [F.sub.L], which is defined as

[F.sub.L] = R/L, (1)

where R is the dynamic strength ratio determined by cyclic shear strength of soil and the correction coefficient concerning earthquake movement. Cyclic shear strength is estimated using unit weight and fine content of soil, N-value, and GWL. In addition, L is the seismic shear stress ratio generated during earthquakes; L is determined by seismic intensity, which depends on the ground classification and type of earthquake, just as R does. Liquefaction occurs when [F.sub.L], given by Eq. (1), is below 1.0.

Evaluation of possible liquefaction through depth in an objective location can be performed by integration of the liquefaction potential, [P.sub.L], through depth using

[P.sub.L] = [[integral].sup.20.sub.0] (F(z)w(z)dz, (2)

where F(z) is a function that is F(z) = 1 - [F.sub.L] when [F.sub.L] [less than or equal to] 1.0 and F(z) = 0 when [F.auv.L] >1.0. Here, w(z) is a weighting parameter defined as w(z) = 10.0 - 0.5z (z: GL-m) (Iwasaki et al., 1980).

The GWL is predictable using groundwater flow analyses. The sea level, which is a hydraulic boundary condition, is set as 0.8 m for obtaining simulated results considering SLR resulting from global warming. According to a report of the Intergovernmental Panel on Climate Change (2000), 0.8 m is the worst-case scenario of SLR in 2100.

The objective regions in this study are Yokohama and Kawasaki cities in Japan, as shown in Fig. 2. The figure also shows approximately 700 locations of site investigations in the geo-information database. These data have been used for groundwater flow and liquefaction hazard analyses.

[FIGURE 2 OMITTED]

GROUNDWATER LEVEL SIMULATION CONSIDERING RISING SEA-LEVELS CAUSED BY GLOBAL WARMING

Groundwater flow analyses based on finite element method were performed to obtain the GWL for calculating the liquefaction potential. The solving equation of the analysis is a 2D model for unconfined groundwater flow in a steady condition:

[partial derivative] / [partial derivative]x { k(h - [z.sub.0]) [partial derivative]h /[partial derivative]x } + [partial derivative] / [partial derivative]y { k(h - [z.sub.0]) [partial derivative]h /[partial derivative]y } = 0 (3)

where h is the GWL, k is the coefficient of permeability of the permeable soil layer, [z.sub.0] is the depth of the non-permeable surface and x, y are coordinates of the horizontal plain. Using Eq. (3), distribution of steady GWL subjected to a certain amount of SLR was simulated.

Distributions of the permeable soil layer, impermeable rock surface and ground level in the objective region were estimated using spatial interpolation method with a Geographical Information System (GIS). The coefficient of permeability was calculated using a linearly weighted combination of all soil layers up to the impermeable soil surface in each location. The weighting parameter was determined as the ratio of thickness of the soil layer to the depth of the impermeable rock surface. The coefficient of permeability of soil was assumed by referring to representative values of soils in Japan. Boundary conditions around the analytical area were determined considering not only the sea level, but also the river level affected by SLR.

Groundwater simulations before and after 0.8 m SLR were performed using the numerical method described above. Distributions of the GWL rise subjected to 0.8 m SLR are represented in Fig. 3. The GWL rise in reclaimed islands is regarded as the same value as SLR because the sea surrounds the islands. The rise in GWL is close to 0.8 m when approaching the coastline. However, the GWL rise is not always large near the coastline because the figure shows a larger value of the GWL rise along the Tsurumi River. The GWL rise on each side of the river is different. For that reason and others, it is necessary for investigation of groundwater-related geo-disasters to consider regional characteristics of the ground.

[FIGURE 3 OMITTED]

VULNERABILITY ASSESSMENT OF LIQUEFACTION POTENTIAL IN A COASTAL REGION

Distributions of liquefaction hazard in the objective region before and after SLR were compared to investigate vulnerability to liquefaction disaster induced by SLR caused by global warming. Figure 4 shows liquefaction hazard maps before and after 0.8 m SLR. Estimations of liquefaction potential at all site-investigation locations were calculated by Eq. (2). The geo-information database and simulated results of GWL before and after SLR were used as analytical parameters that are soil properties through the depth and GWL at the locations. For determining the seismic shear stress of the ground, seismic movements were assumed to be TYPE II, with earthquake motion corresponding to the Great Hanshin Earthquake in 1995, as specified in the JHB design code. Calculating the liquefaction potential at all locations, distributions in the region were represented as liquefaction hazard maps using spatial interpolation method with GIS. Comparing the map after SLR with others at the present situation of GWL, it is recognized that a severe liquefied area expands slightly around the coastline and riverside.

[FIGURE 4 OMITTED]

Using the liquefaction-hazard-map procedure considering SLR caused by global warming, the distributions of liquefaction potential were estimated for cases where the SLR is 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 m. Based on these results, Fig. 5 shows the relationship between SLR and area of PL-values over 5 in the objective regions. The area in which liquefaction potential is high increases with the rising sea level. The increment ratio of the area to SLR increases rapidly from 1.0 m. In regions around the coastline and riverside affected by SLR, disaster prevention against earthquake liquefaction is vulnerable to SLR resulting from global warming.

[FIGURE 5 OMITTED]

CONCLUSIONS

A procedure for liquefaction hazard mapping considering global climate change was proposed and applied to a coastal region in this study. Comparison of liquefaction hazard maps before and after SLR caused by global warming illustrates the changing vulnerability to liquefaction hazard. Those results show that the area of high potential liquefaction increases with rising GWL caused by SLR. Regions in which the liquefaction potential is high are not only those around coastlines and riversides affected by SLR. Therefore, for prevention and mitigation of liquefaction by earthquakes, it is necessary to consider regional variations of GWL caused by SLR attributable to global warming.

ACKNOWLEDGMENTS

This study was partly supported by Grants-in-Aid from the Saneyoshi Fellowship Foundation, the Ministry of Education, Culture, Sports, Science and Technology, and the Ministry of Environment of the Japanese Government. We wish to express our sincere thanks for financial support from these organizations. Geotechnical information, and support in Yokohama and Kawasaki cities were provided by the Kanagawa Environmental Science Research Institute in Yokohama and the Department of Environmental Protection, Environmental Directorate, Kawasaki City.

REFERENCES

IPCC WGI (2001): "Technical Summary, Climate Change 2001: The Scientific Basis", Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 83p.

Iwasaki, T., Tatsuoaka, F., Tokita, K. and Yasuda, S. (1980): "Prediction of liquefaction potential during earthquakes", Tsuchi-to-Kiso, J. Japanese Geotechnical Society, Vol. 28, No. 4, pp. 23-29 (in Japanese).

Japan Road Association (1996) "Specification for Road Bridge V. Earthquake-resistance design" (in Japanese).

Yasuhara, K., Murakami, S. and Fukuda, T. (2004) "GIS Application for Prediction of Liquefaction Potential Caused by Rising Groundwater Level", Proc. Int. Symp. Engineering Practice and Performance of Soft Deposits, pp.433-438.

MURAKAMI, S., YASUHARA, K., SUZUKI, N., NI, WEI AND KOMINE, H.

Department of Urban and Civil Engineering, Ibaraki University, 4-12-1 Hitachi, Ibaraki 316-8511, Japan
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Author:Murakami, S.; Yasuhara, K.; Suzuki, N.; Ni, Wei; Komine, H.
Publication:Geotechnical Engineering for Disaster Mitigation and Rehabilitation
Article Type:Conference news
Geographic Code:9JAPA
Date:Jan 1, 2005
Words:1777
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