DSpace at VNU: Effects of climate change on geo-disasters in coastal zones and their adaptation

DSpace at VNU: Effects of climate change on geo-disasters in coastal zones and their adaptation tài liệu, giáo án, bài g...

Effects of climate change on geo-disasters in coastal zones and their adaptation

K Yasuharaa,*, H Komineb, S Murakamib, G Chenc, Y Mitanic, D.M Ducd

a Institute for Global Change Adaptation Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan

b Department of Urban and Civil Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan

c Graduate School of Civil Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

d Department of Geotechnics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received 19 May 2010

Accepted 19 December 2010

Available online 9 February 2011

Keywords:

Global warming

Climate change

Compound disaster

Torrential rainfall

Great earthquake

Adaptation

a b s t r a c t

Results of recent investigations suggest that climate change tends to exacerbate geo-disasters Therefore,

it is understood clearly that adaptation to climate change has rapidly become the most important and urgent issue for the future existence of human beings on Earth These inferences form the background of this research In comparison to those examining water disasters, few studies have examined climate-change-induced geo-disasters This study aims at upgrading the methodology for estimating effects on geo-disasters of combined events, e.g., global warming with increased typhoon and rainfall severity or occurrence of great earthquakes Such a methodology is expected to contribute to progress in thefields of natural disaster mitigation and land preservation, particularly near seacoasts and rivers

Ó 2011 Elsevier Ltd All rights reserved

1 Introduction

Coastal zones and riversides are the residential areas that are

most likely to be influenced by global warming Especially,

seashores will be strongly influenced if global warming induces

increased frequency or intensity of typhoons Climate factors might

cause disasters that are more severe if smaller disasters were to

occur concurrently A disaster that is deemed important and which

occurs because of overlapping factors is designated herein as

a complex disaster.Fig 1 depicts such a complex disaster

sche-matically Thisfigure portrays that complex disasters are roughly

classifiable into disasters related to water (water disasters) and

disasters related to soil or ground conditions or behaviors

(geo-logical hazards)

Fig 2 portrays compound disasters as classified into disasters

that occur by overlapping of global warming factors, and those by

overlapping of factors related to and independent of global warming

Large cities in Japan are especially vulnerable to natural threats

because they are located in low-lying coastal areas It is therefore

important to predict these natural threats and to consider

appro-priate countermeasures Previous studies have revealed the

following (Suzuki et al., 2006; Suzuki, 2007;Suzuki, 2008;Komine,

2007a,b;Yasuhara 2008)

a) The risk of flooding by high tides is enhanced in long ago developed reclaimed lands and their periphery in the inner parts of Japan’s Three Major Bays (Tokyo Bay, Ise Bay, and Osaka Bay) when global warming progresses (Suzuki 2007, 2008) b) Sea level rise results from climate change and changes in the frequency and intensity of rains Those influences magnify high risks in areas prone to liquefaction, especially in coastal zones (Yasuhara et al., 2007; Yasuhara , 2008; Yasuhara et al., 2010) c) Great economic merits of sandy beaches and tidalflats will disappear because of a sea level rise (Suzuki, 2008)

d) River levees might be degraded and damaged if seawater invades into riverine areas because of a sea level rise This mechanism was clarified, and river levees’ vulnerability to rainfall because of global warming has been clarified on

a national level in Japan (Komine, 2007a,b)

e) Because of global warming, disaster risks on slopes adjacent to coastal zones are magnified by heavy rains It is therefore necessary to study slope recovery plans using a risk index (Yasuhara, 2008; Yasuhara 2010)

2 Supporting evidence of triggering natural disasters Hazards that have been predicted to result from global warming can be either certain or uncertain One highly certain occurrence is

a sea level rise, as reported by the IPCC A recent report by the IPCC (2007) describes that the sea level will rise to around 60 cm, on

* Corresponding author Tel.: þ81 294 38 5166; fax: þ81 294 38 5268.

E-mail address: yasuhara@mx.ibaraki.ac.jp (K Yasuhara).

Contents lists available atScienceDirect Geotextiles and Geomembranes

j o u rn a l h o m e p a g e : w w w e l s e v ie r c o m / l o c a t e / g e o t e x m e m

0266-1144/$ e see front matter Ó 2011 Elsevier Ltd All rights reserved.

Geotextiles and Geomembranes 30 (2012) 24e34

average, by the end of the 21st century, although several scenarios

have been proposed According to early research byMimura (2006),

70e80% of sandy beaches will disappear if the sea level rise (SLR)

reaches 60 cm A sea level rise would also force groundwater levels

(GWL) upward, thereby engendering infrastructural instability

along coastal zones

Low atmospheric pressure, including intensified typhoons,

ge-nerally brings about locally heavy rainfall, so-called torrential

rainfall, as it did in 2004; sometimes it produces“Guerilla” heavy

rains, as in 2008 in Japan.Fig 3presents variations of the

occur-rence frequency of torrential rainfall during the last 30 years

Torrents exceeding 50 mm/h are shown there according to data

obtained from the Ministry of Land, Infrastructure and

Trans-portation, who summarized data provided by the Japan

Meteoro-logical Agency (JMA) Results presented inFig 3show the average

frequency of torrential rainfall over 100 mm/h as well as over

50 mm/h These strong rains might increase geotechnical damage

such as slope failure, and might even magnify it, particularly when

great earthquakes strike soon before or after severe local rainfalls

Regarding earthquakes, which cannot be related directly to

global warming, it is unfortunate that both earthquakes with large

and small seismic intensity have been increasing, as portrayed in

Fig 4 Special attention must be devoted to the possibility of severe

damage to slopes, earth structures, and grounds that must

be induced by the combination of torrential rainfall with great

earthquakes

The climate and weather variations described above show that

geo-hazards tend to increase over time

3 River levees 3.1 Methodology

A sea level rise attributable to global warming is expected to cause seawater to move up-river Consequently, brackish water regions in river downstream areas might extend upstream Such an outcome might affect river levees, which are a necessary disaster prevention facility Quantitative assessment of the vulnerability of infrastructure facilities by global warming is a pressing issue Therefore, in addition

to investigations directed at securing of river levees, suitable adap-tation measures are demanded to mitigate this vulnerability For evaluation of global warming’s impact to river levees, river levees soils were collected nationwide: experiments were con-ducted to investigate the soil consistency (deformability according

to water content) and compressibility (ability of volume change by force) Changes of deformability and compressibility will induce settlement and instability of river levees, engendering the overflow

of river water (Komine, 2007b) Results of these experiments

clar-ified mechanisms of river levee degradation and damage when the seawater moves up-river because of the sea level rise Details of the experimental results described above are available for reference in previous reports (Komine, 2007a, 2007b)

Moreover, a test was conducted to investigate the water-holding capacity of the soil to evaluate the vulnerability of river levees to rainfall A water-holding property database of earth materials assumed to constitute the river levee of every region was created Results indicated vulnerability of river levees to rainfall, in rough terms, on a Japanese national level Details of the experimental results presented above have already been reported in the litera-ture (Uchida et al., 2007)

3.2 Future estimate of river levee vulnerability

An important concern is that the sea level rise and frequent heavy rain occurring because of global warming will magnify the vulnerability of river levees Sea level rises are expected to expand brackish water areas near rivers

Moreover, it is expected that frequent heavy rains and increases

in river levels will encourage soaking into river levees Experiments were conducted from such a viewpoint assuming the incursion of seawater into a river levee or increasedflood volume River levee vulnerability was evaluated (Komine, 2007a, 2007b)

Nine soil materials assumed to be used for river levees were collected from various locations in Japan, as presented inFig 5 Several experiments using the “test method for liquid limit and plastic limit of soils (JIS A 1205:1999)”, the “test method for one-dimensional consolidation properties of soils using constant rate of strain loading (JIS A 1227:2000)” and the “The Japanese Geotech-nical Society Standard the Method of water retentivity test of soil (JGS 0151-2000)” were conducted to elucidate physical properties that are useful for estimating river embankment vulnerability to erosion byfloods

Effects on levee embankment materials of the extension of river brackish water regions attributable to sea level rise are estimated as presented inTable 1 The future cannot be designated as a definite period in this estimate

Fig 6portrays maps of vulnerability assessment produced based

on the results presented inTable 1 Estimates and measures used to assess river levee embankment materials’ vulnerability to rainfall are shown inTable 2 (Uchida

et al., 2007) In relation to these estimates, the future cannot be designated as a definite period

Fig 7displays a map of the vulnerability assessment obtained according to the results inTable 2and respective measures River

Fig 1 Disasters caused by overlapping multiple phenomena caused by global

warming.

Fig 2 Disasters caused by overlapping of the sea level rise and local heavy rains

brought about by global warming and a phenomenon unrelated to global warming,

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 25

levee vulnerability is known to be influenced by boundary

condi-tions such as the levee size, and by external condicondi-tions such as the

rainfall intensity However, these conditions vary widely, making it

difficult to evaluate river levees’ vulnerability from the perspective

of levees’ boundary and external conditions In this study, levee

vulnerability is evaluated according to mechanical properties of the

soil materials used for river levees

4 Variation of liquefaction risk

4.1 Methodology

The earth is in an“age of ground disturbance” now; presumably,

events resulting from abnormal weather caused by global warming

will overlap with other events and trends

However, short-term and long-term factors can engender

different variations of groundwater levels For example, the

groundwater level rises temporarily according to local heavy rain

Alternatively, groundwater levels can rise slowly over many hours,

as in a recent case in which underground structures of Ueno and Tokyo Stations were lifted When local heavy rains increase because

of global warming, the groundwater level rises abruptly

Accordingly, the possibility of liquefaction might increase not only in coastal zones, but in inland areas also Consequently, global warming presents the risk of exacerbating disasters

The following issues are pointed out for foundations and earth structures of coastal land areas that are influenced by a sea level rise attributable to global warming Increased risk to foundations of liquefaction caused by earthquakes is likely Coastal structures’ instability will affect levees, shore protection, and breakwaters Problems resulting from a rising groundwater level, such as diffu-sion by submerdiffu-sion of soil pollutants that had existed above the groundwater level, and groundwater salinationeegroundwater of high salinity extending inlandeeis likely to occur For that reason, thefluctuation of groundwater levels in coastal zones caused by

a sea level rise or climate-change-induced rainfall should be predicted A diagnostic technique of local disaster prevention capabilities must be established considering the fluctuation of groundwater levels Such an impact evaluation of climate change to liquefaction risks in the case of an earthquake has been done for the Tokyo Bay coastal zone

Fig 3 Recent situation of rainfall in Japan (data from JMA, 2006 ).

Fig 4 Frequency of recent earthquakes in Japan (data from JMA, 2006 ).

Fig 5 Map showing expansion of river brackish water regions on levee embankment

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 26

Impact evaluation was performed as follows: the ground

structure was modeled using the soil information database in the

object region Then the rise of the groundwater level in coastal zone

foundations caused by sea level rise or climate change was

pre-dicted using the 2-D unconfined groundwater flow analysis method

(Murakami et al., 2005) with finite element method (FEM)

Vulnerability assessment was performed using this method in the

event of an earthquake for a coastal zone ground affected by the sea

level rise Variation of liquefaction risk was computed before and

after the sea level rise and climate change A climate unification

scenario produced by the Meteorological Agency, RCM20, was used

for the assumed rainfall

4.2 Evaluation of variation of liquefaction risk

A rise in ocean water levels raises the groundwater level not only

at coastlines but at riversides as well Consequently, the liquefaction

risk attributable to the sea level rise increases concomitantly with

a site’s proximity to coastal zones and riversides Variation of rainfall

resulting from climate change raises groundwater levels in inland regions, where rainfall increases Consequently, liquefaction risk posed by rising groundwater levels increases in inland regions where local heavy rains increase Ground liquefaction caused by earthquakes is a factor increasing the building collapse risk and damage, such as a collapse of building foundations or bridges and ground subsidence, accompanied by the lifting of underground structures such as sewer pipes or a lateralflow of soil Assessment of the grade of effects of climate change on this liquefaction risk is important to elucidate the geological hazards posed by earthquakes

in an area

Among areas where increased rainfall is expected because of climate change, the areas of Kawasaki and Yokohama between the Tsurumi and Tama Rivers on the Tokyo Bay coastal zone were

Table 1

Effects on levee embankment materials of the extension of river brackish water

regions attributable to sea level rise.

Region and district Predicted impact on levee embankment materials

Hokkaido Strength reduction, increased compression, and enhanced

permeability of levee embankment materials are anticipated The main dyke break pattern is expected to include seepage and overtopping failure.

Kanto and Shin-etsu

districts

Deterioration of water permeability of embankment materials is expected The main dyke break pattern is expected to include levee body breakage caused by residual water pressure after water seepage into the levee.

Chugoku district Strength reduction, increased compression, and enhanced

permeability of levee embankment materials are expected Seepage and overtopping failure will cause dyke failure.

Kyushu district Permeability rise and decline and compression increase

and decrease might be observed depending on embankment materials Major dyke break patterns are expected to include seepage and overtopping failure and levee body breakage by residual water pressure after water seepage into the levee.

Fig 6 Assessment of river levee embankment materials’ vulnerability to rainfall

Table 2 Estimates and measures used to assess river levee embankment materials’ vulner-ability to rainfall.

Region and district Predicted impact on levee embankment materials Hokkaido After penetrating into a levee body by rainfall, water

drains, creating the possibility of abruptly decreased levee embankment material strength, possibly causing sudden slope failure Devices in drained areas can be regarded as countermeasures.

Kanto and Shin-etsu districts

After penetrating into a levee body by rainfall, water drains The volume considerable shrinkage of river levee embankment materials might decrease the freeboard Additional banking is among the candidate countermeasures.

Chugoku district The water retention capacity of levee embankment

materials is low This low holding capacity renders levees vulnerable to rainfall, thereby engendering their sudden collapse and volumetric shrinkage of slope faces Possible measures include sealing Deterioration of water-holding capacity and strength are especially rapid in the Kagoshima area, where remarkable volumetric shrinkage might occur; it is expected that synthetic measures are especially necessary there.

Kyushu district The water-holding capacity of levee embankment

materials is low This low holding capacity renders levees vulnerable to rainfall, thereby engendering their sudden collapse and volumetric shrinkage of slope faces Possible measures include sealing.

Fig 7 Assessment of river levee embankment materials’ vulnerability to rainfall

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 27

selected as our study object Geological structure modeling was

performed for the object region using a ground information

data-base Then, the groundwater level rise in coastal zone areas

accompanying a sea level rise or climate change was analyzed using

the 2-D unconfined groundwater flow analysis method with FEM

The applicability of numerical analysis to the objective region was

confirmed in a previous study (Suzuki et al., 2006) The effect of

climate change was evaluated (Murakami et al., 2005; Yasuhara

et al., 2007) The changes of liquefaction risk before and after the

sea level rise and climate change were computed using this

procedure Vulnerability assessment was conducted in the event of

an earthquake in coastal zone grounds because of the sea level rise

The liquefaction hazard at each location was calculated using

a method proposed by“The Japanese Highway Bridge Code (JHBC;

“Dorokyo-shihousho”)” established by theJapan Road Association

in 1980 This judgment procedure is based on the liquefaction

resistance factor, FL, defined as

FL ¼ R

where R is the cyclic strength ratio determined by the cyclic shear

strength of soil and the correction coefficient related to earthquake

movement The cyclic shear strength is estimated using the unit

weight andfine content of soil, the N-value and GWL Furthermore,

L, the seismic shear stress ratio generated during earthquakes, is determined by the seismic intensity, which depends on the ground classification and the earthquake type, and the same information as

R Liquefaction occurs when FL, given by Eq (1), is less than 1.0 Evaluation of possible liquefaction through depth in an objective location can be performed by integration of the liquefaction potential, PL, with depth as follows:

PL ¼

Z20 0

Therein, F(z) is a function that is F(z)¼ 1  FLwhen FL< 1.0 and F(z)¼ 0 when FL> 1.0 In addition, w(z) is the weighting parameter

defined as w(z) ¼ 10.0  0.5  z (z: GL  m) (Iwasaki et al., 1980) The liquefaction hazard increases concomitantly with the increasing PLvalue Using the PLvalue, the liquefaction hazard is categorizable into four ranks as presented inTable 3

The PL value computed using the soil condition including groundwater levels was used for estimating the liquefaction risk The climate unified scenario produced by the Meteorological Agency, RCM20, was used for the assumed rainfall The sea level rise was predicted to be 88 cm by 2100

Fig 8(a) portrays a liquefaction hazard map for the present day;

Fig 8(b) shows a liquefaction hazard map incorporating the sea level rise The maps collectively reveal that the liquefaction risk near the coastline is enhanced by the accompanying sea level rise, and that the liquefaction risk is exacerbated in areas near rivers This fact suggests that areas in which the coastal zone ground is

influenced by the sea level rise are influenced not only near the coastline, but in river downstream areas along the shore, which are

Table 3

Rank of liquefaction hazard PL.

0 < P L < 5 Rank 1

5 < P L < 15 Rank 2

15 < P L < 25 Rank 3

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 28

affected by tidalfloating.Fig 8(c) shows a liquefaction hazard map

incorporating rainfall: compared with the case of sea level rise only,

liquefaction risk has increased more inland than in riverside areas

Climate change exerts various effects Furthermore, the sea level

rise influences seashore and riversides, as does the variation of

inland precipitation Variations of rain intensity and frequency

increase in areas of high liquefaction risk

In practice, climate change simultaneously alters the sea level

rise and rain intensity and frequency Accordingly,Fig 8(d), which

shows both effects, is considered to represent the situation of the

future best.Fig 9 portrays regions in which the liquefaction risk

rank changes In the object region, variation of the sea level rise and

the increase in rain intensity and frequency caused by climate

change will expand the areas facing high liquefaction risk

5 Risk of slope disaster

5.1 Background

Climate change exacerbates geological hazards (Yasuhara, 2008)

A worst-case scenario of the effect of global warming shows that,

when phenomena attributable to global warming and other

phenomena, such as earthquakes, occur concurrently, they might

cause unprecedented complex disasters Moreover, they will take

place more frequently than at present For instance, the Niigata

Chuetsu earthquake in 2004, in which a huge earthquake occurred

after a long rain that continued for about one month, is a typical

example of a complex disaster (Yasuhara et al., 2007) Unusually

strong rainfall continued until immediately before the earthquake

Fig 9 Regions in which the liquefaction risks class changed in Fig 8 (a)e(d)

( Murakami et al., 2005 ).

Fig 11 Slope disaster risk map with global warming in Fukuoka ( Chen et al., 2007 ).

Fig 12 Model of assumed landslide mode ( Chen et al., 2007 ).

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 29

For that reason, water had accumulated on the slopes of hills and

mountains, causing them to collapse easily because of the

earth-quake’s vibration Then the large earthquake struck, causing

land-slides in about 4000 locations.Fig 10portrays the increase in the

frequency of landslide disasters

5.2 Methodology

Abnormal weather resulting from global warming might

inc-lude increased intensity of typhoons and frequent heavy rains

Typhoons and heavy rains engender severe damage to humans and

social property (direct economic loss) Heavy rains also cause landslides and economic loss through the effects of slope disasters (indirect economic loss) Predicting these economic losses and proposing measures to investigate the effects of global warming are important Correlation between the magnitude (intensity) of

a typhoon and the economic loss ratio (loss/assets) were analyzed

An inference-based model of economic loss with respect to the magnitude of a typhoon was established using past data to evaluate the risk of increased economic loss through intensification of typhoons as a result of global warming Moreover, a typhoon-risk-curve preparation method using a Monte Carlo simulation was established based on statistical characteristics of typhoon magni-tude The typhoon-risk-curve was inferred, assuming typhoon intensification resulting from global warming It was then com-pared with the present risk curve An increased risk of economic loss was demonstrated

Assessment of the risk of slope disaster by heavy rains because

of global warming was done methodically using a process devel-oped by the authors The proposed assessment method of the risk

of slope disaster includes a local-range method and a wide-range method according to the range of the assessment object (Chen

et al., 2006, 2007, Chen, 2008; Misumi et al., 2008) The local-range method determines the risk of slope disaster considering the compound effect of heavy rains and earthquakes on the object slope

For assessment of the risks of landslide disaster nationwide, the wide-range method, the risk of a landslide disaster over a wide area was estimated using a geographic information system (GIS) according to the following procedures

a) A hazard map was created to show the landslide probability using a probabilistic landslide estimation method based on rainfall in the fourth mesh of digital national land information b) The economic loss of property because of landslides was eval-uated using the estimation method of the asset distribution in the fourth mesh

c) The slope disaster risk was computed as the product of the landslide probability and economic loss attributable to the landslide

d) Slope disaster risk maps were created in the present climatic conditions and in the assumed climatic conditions of global warming in 2050, 2100, and thereafter The effects of global warming on slope disasters were inferred from a comparison of risk maps before and after global warming The risk of slope disaster and its increase by global warming in Fukuoka in 2050 was inferred based on this method

Fig 14 Modified geosynthetic revetments used against rising sea levels, presently used as permeable “wet” structures.

Table 4

Possible adaptation to natural disasters occurring in coastal zones.

Hazard Adaptation

Protection Accommodation Evacuation

River flood Additional banking

Water protection work

Early warning system and

evacuation system

Construction of shelter

Hazard map Appropriate land use

Regulation of land use in hazardous area Insurance

Restriction of development Evacuation from dangerous area Public support for evacuation Liquefaction Monitoring of GWL

Lowering of GWL

Additional banking

Soil improvement and

reinforcement

Hazard map Appropriate land use

Regulation of land use in hazardous area Insurance

Restriction of land use Evacuation from dangerous area Public support for evacuation Slope

failure

Protective pile

Geosynthetics reinforcement

Early warning system and

evacuation system

Hazard map Risk map Regulation of land use in hazardous area Insurance

Restriction of land use Evacuation from dangerous area Public support for evacuation Coastal

erosion

Seawater protection work

Conservation and replanting

mangrove forest

Early warning system of

extreme weather events and

evacuation system

Hazard map Risk map Regulation of land use in hazardous area Integrated coastal zone

management Insurance

Restriction of wetland use Evacuation from dangerous area Public support for evacuation

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 30

5.3 Analytical results for slope disaster risk

Analytical results presented inFig 11from the above-described

example for compound slope disasters in Fukuoka, Japan indicate

the following:

a)Fig 11presents the slope disaster risk (product of the collapse

probability of a slope and economic loss by collapse)

attribut-able to global warming for Fukuoka Trial calculations were

performed for Fukuoka based on these risk data The following

results were obtained

1) Slope disaster risk in the present rainfall conditione loss of

36 billion yen/year

- Slope disaster risk in the rainfall condition attributable to global warming in 2050 loss of 61.43 billion yen/year

- Consequently, the increase in slope disaster risk in Fukuoka posed by global warming is 70.6%

- This case study shows that global warming enhances the risk

of slope disaster, which underscores the importance of producing a slope recovery plan with a risk index

b) An assessment method of slope disaster risk incorporating the compound effect of heavy rains and earthquakes was devel-oped It was then applied to risk evaluation of a collapsed slope

in Shikanoshima by the Fukuoka Seihou-oki earthquake in

2005 This earthquake produced a cave-in geography with large deformation in the upper part of a slope The possibility of

Fig 15 Geotextile Wrap around Revetment in Sylt Island, Germany (Courtesy Dr Dette LWI).

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 31

large-scale landslides occurring with future earthquakes and

heavy rains is of great concern The magnitudes of potential

collapsed slopes were classified for the three cases presented in

Fig 12 Expenditures for measures vary widelyeefrom

hun-dreds of millions of yen to billions of yeneebecause of the

assumed collapse magnitude Therefore, risk analyses were

conducted for the three cases Results are depicted inFig 13

- The results suggest the following:

1) In the event of an earthquake, rainfall, and an earthquake after

rainfall, the risks of surface sliding and mid-depth

middle-scale sliding become high, but the risk of deep large-middle-scale

sliding is slight

2) When rainfall and an earthquake are combined, the risk of

mid-depth middle-scale sliding and surface sliding becomes

high too, but that of depths of large-scale sliding does not

become so high

- Therefore, the following were inferred

1) The risk of deep large-scale sliding in Case C is small

2) The risks of surface sliding in Case A and mid-depth

middle-scale sliding of Case B are comparable

3) If countermeasure costs are considered, measures against

surface sliding, considering a partial mid-depth slide, are

the most cost-beneficial This proposal has been adopted

for the recovery plan of Fukuoka city

6 Adaptation against natural disasters in coastal zones 6.1 General comments

What should be done to reduce the geo-disasters described above? Or what can we do? Can we protect ourselves completely from natural disasters? These questions have been addressed to us, engineers, academia and policymakers, since the Great Hanshin earthquake in 1995 severely altered so many human lives and the functions of the infrastructure, particularly in Kobe The concrete answers to them are difficult because we must consider many aspects of the problem to answer these questions Therefore, in this paper, we present only the following general comments:

1) Along with the influences of global environmental issues, including global warming, adaptation strategies against climate change-related natural disasters are classifiable, mainly from the geotechnical point of view, into three categories: protection, accommodation, and evacuation Their contents are presented inTable 4

2) We must also develop geotechnical engineering techniques that simultaneously satisfy aspects of mitigation of environmental impacts and reduction of geo-disaster severity (Yasuhara, 2007)

6.2 Possibility of application of geosynthetics Although the severity of the threat varies regionally, rising sea levels’ currently projected magnitude is expected to have great effects on the economic and social development of many countries (IPCC, 2006)

Geosynthetic structures are used primarily as shore protection, indicating that the structures will be useful only in the event of

a storm or related phenomena Because of their high permeability (geosynthetic and sand), they are not the most convenient option for use in permanent“wet” situations However, modification of geosynthetic structures to produce impermeable structures can be performed In such cases, geosynthetic revetments can provide

a countermeasure against rising sea levels

Notwithstanding, some modified structures have been used as permanent“wet” structures One example is the “Blue Water Retail and Leisure destination” (Dixon, 1998) (Fig 14) where the GWR was constructed normally, but behind it an impermeable geomembrane was installed to create an impermeable structure Similar struc-tures have been constructed near Shanghai, China (Yan, 1988) Considering the information above, it can be stated that nor-mally used geosynthetic revetments do not constitute the best countermeasure against rising sea levels However, if modifications

to their structure to transform them into impermeable structures (without reducing their stability) were accomplished, geosynthetic revetments would be a feasible countermeasure against rising sea levels Detailed study and research on this subject is urgently necessary, although fundamental research into the availability of wrap around revetments has already been conducted by Yasuhara and Recio (Yasuhara and Recio-Molina, 2007)

Revetments made with geosynthetics offer proven resistance capability according to expected wave conditions that are associ-ated with storms On the island of Sylt, a GWR structure has resisted strong wave conditions and has reduced cliff erosion by more than

10 m compared to coasts that are not protected by GWR installations (Nickels and Heerten, 1996) (Fig 15) Other GWRs constructed in Fiji have performed according to expectations, as presented inFig 16

As presented in Fig 17 by Lawson (2006), another possible adaptation using geosynthetics is to adopt geotubes for use in protection of coastal zones As presented inFig 18byNickels and

Soil to be protected

Water forces

causing erosion

Geotextile tube Local soil fill

Scour apron (optional)

Ocean waves

Waves break across breakwater

Seabed

Calm water Geotextile tube

breakwater

Scour apron

b Offshore breakwaters

Flood level or storm

Geotextile tube

covering

Scour apron

c Protection dykes

Fig 17 Example of the use of geotubes for protecting the coastal zones ( Lawson, 2006 ).

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 32

Heerten (1996), geotubes are available for protecting coastal cliffs

from erosion Geotubes are also effective for use against erosion of

sand beaches in Vietnam(Fig 19)

7 Conclusions

Based on the data presented and the subsequent discussions,

the following conclusions can be made:

1) Sea level rise caused by global warming expands brackish river

water regions, thereby degrading the levee strength

2) Sea level rise and anomalous rainfall raise the groundwater level and expand areas that suffer geotechnical hazards through liquefaction in the event of an earthquake

3) When rainfall and an earthquake are combined, the risk of mid-depth middle-scale sliding and surface sliding increase too, but the risk of deep large-scale slides does not become so high If countermeasure costs are considered, measures against surface sliding, considering a partial mid-depth slide, are the most cost-beneficial This proposal has been adopted in the recovery plan of Fukuoka city

4) Among possible adaptation measures against climate-change-induced geo-disasters, geosynthetics are and will be more

Fig 18 Geotube application against coastal cliffs ( Nickels and Heerten, 1996 ).

Fig 19 Geotube used against sandy erosion (Phu Thuan, Thua Thien-Hue, Vietnam).

K Yasuhara et al / Geotextiles and Geomembranes 30 (2012) 24e34 33