Regional Environmental Changes Around the 6.26

Deluge in the Minami-Aso Area

Naoto TANAKA1 , Keisuke MATSUDA2 and Keisuke IWATA3

1Associate Professor, Center for Policy Studies, Kumamoto University (Kurokami 2-39-1, chuou-ku, Kumamoto City, Kumamoto 860-8555, Japan)

E-mail:naotot@kumamoto-u.ac.jp 2Master course student, Dept. of Civil Eng., Kumamoto University

(Kurokami 2-39-1, chuou-ku, Kumamoto City, Kumamoto 860-8555, Japan) E-mail:keisuke02040@gmail.com

3Assistant, Center for Policy Studies, Kumamoto University (Kurokami 2-39-1, chuou-ku, Kumamoto City, Kumamoto 860-8555, Japan)

E-mail:iwatake@kumamoto-u.ac.jp

This study aims to clarify the local characteristics by analyzing the environmental changes in the set-tlements caused by the large-scale deluge in the Minami-Aso area on 26th of June 1953 (sobriquet: 6.26) in order to contribute to regional management and the planning of a natural disaster prevention plan for the community. Specifically, the history of natural disasters in the research area is summarized; the distinctive features that reflected the regional characteristics of the great natural disaster that was the 6.26 deluge are explained; and the records of the changes in the settlements before and after the natural disaster are visu-alized. After that, three regions that had distinctive changes were surveyed in detail, and with a chrono-logical analysis of the damage and recovery process in each region, the changes in the regions were cate-gorized and the characteristics of each region were clarified. Key Words : history of civil engineering, community,6.26 deluge, Minami-Aso area, reconstruction

1. INTRODUCTION (1) Background and Aims

The public has become extremely wary of large-scale natural disasters after experiencing the phenomenal disaster called the Tohoku Earthquake, and taking measures has become an urgent matter. Settlement reloca-tion is one solution cited for preventing large-scale disasters. Settlement relocation is a permanent evacuation including the buildings, and is a drastic disaster measure not only for human life but also for assets. On the other hand, in reality, there is much hesitancy about relocation. As regards settlement relocation, it seems difficult due to complicated relationships between economic and social matters.

So this study analyzes the relocation of settlements after the large-scale disaster in the Minami-Aso area (Fig.1) of Kumamoto Prefecture. By clarifying the process of how the villagers made up their minds to relo-cate, a basic knowledge of the factors affecting settlement relocation would be gained. (2) Previous studies

The study on the Tsunami disaster prevention plan and related problems of the expanding urban area of Taro-cho, Iwate Prefecture by Matsumura et al.1) is an example of a previous study on relocation after a large-scale disaster. The construction of a seawall designed for tsunami had only been evaluated for its effec-tiveness for flood management; however the fact was observed and clarified that it brought about an expanded city area along with a fading anti-tsunami disaster consciousness.

Moreover, the study by Shimazaki et al. 2) suggested what needs to be learnt measures from the previous large-scale tsunami disaster that hit Sanriku in 1933; the vast cost of constructing disaster prevention structures in reconstruction projects, and the effects on resi-

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ves and the environment, and the possibility of a huge tsunami exceeding the plans all lead to the conclusion for a need for new ideas among the other ideas that have already been realized in the recon-struction project. As described above, many of the previous studies on relocation of tsunami victims are ana-lyzed from the actions resulting from the reconstruction project.

This study, with regards to the victim relocation factor, was researched chronologically from the time of the accident occurrence, taking a close look at the relationship between accident occurrence and the real lives of the victims, and relocation of the village, and summarizes it as historical facts.

Fig.1 Research site; the Minami-Aso area Fig.2 Plan of Minami-Aso area before 6.26 deluge

Table 1 Chronological table of disasters in Minami-Aso area Fig.3 Plan of Minami-Aso area after 6.26 deluge Fig.4 The image of Section in the Minami-Aso area

Year Date Disasters

1796 2nd JuneFlood of Tatsunotoshi; The greatest deluge in history, extreme rainfallparticularly in Mt. Aso peak Nekodake direction caused extremeflooding in R. Shirakawa

1806 17th July Eruption of Mt. Aso1814 Eruption of At. Aso, burst into flames1874 7th February Eruption of Mt. Aso

1900 6~16th JulyDeluge of R. Shirakawa, serious damage on the banks of Aso andNangodani valley.

1916 19th April Eruption of Mt. Aso, Rumbling, Ash fall1918 16th January Eruption of Mt. Aso, Rumbling, Ash fall (3rd crater)1932 7th December Eruption of Mt. Aso1933 23rd February Explosion of 1st crater of Mt. Aso

1947 26th MayExplosion of 1st crater of Mt. Aso becomes a large plume; an enormousvolcanic ash fall on Nangodani valley area led to the death of 200horses and cattle.

1953 27th April Eruption of 1st crater of Mt. Aso. 5 students on school trip fromHyogo Prefecture died and 90 injuries.

26th June

6.26 deluge; Severe torrential storm by seasonal rain front in the wholeprefecture; Kumamoto City becomes a sea of mud in one night, and theimmense damage in every area included people, livestock, houses,farmland, roads, and others.

1957 4th December Eruption of Mt. Aso

1958 24th June Eruption; Sudden major eruption at 1st crater of Mt. Aso with a largeblast at 22:15

1965 23rd OctoberLarge eruption at Mt. Aso and 136 minor eruptions were repeated onand off until year end.

1975 22nd JanuarySeries of earthquakes caused by the volcanic activity of Mt. Aso.Serious damage to roads, landslips, destruction/demolition of houses,and injury around the Teno hamlet in Ichinomiya-cho.

1977 20th July Eruption of Mt. Aso, 13:21. 3 light injuries1978 19th September Series of volcanic earthquakes from peak Nakadake of Mt. Aso1979 6th September Eruption of 1st crater, Mt. Aso, at 13:06

1980 26th June

Flood of Kawagoda; Affected by a typhoon, the seasonal rain frontwas hyper-activated, and caused locally torrential rainfall. 18 housesdestroyed/demolished, 3,540 inundated above floor level,3,245inundation below floor level.

1988 3~4th May Heavy rain. Many places were affected.1990 1~3rd July Torrential rainfall from a seasonal rain front

2005 10th SeptemberLocalized torrential downpour. Great landslip damages around theTochinoki hamlet, Minami-Aso-Mura village

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2. THE HISTORY OF DISASTER IN THE MINAMI-ASO AREA

In this chapter, the history of disasters in the Minami-Aso area will be organized, and the characteristics of the disasters will be analyzed. After that, examples from the disaster are specified and the previously existing reports on the target disaster are organized. (1) Chronological table of disasters in the Minami-Aso area3)

As seen in Table.1, definite activities are seen in a certain period around volcanic disasters and earthquakes. In past records, examples of multi-complex disasters are rare, and a case where several kinds of disaster oc-curred in one year which thus worsened the damage is the 6.26 deluge. This disaster was a complex disaster consisting of volcanic and flood origins. Focusing on the flood disaster, flood damage occurred in the rainy season of June and July and around the time when typhoons pass through.

Today, the R. Shirakawa has had 38 known floods4) which were recorded from as far back in history as 652, year 3) and up to 1946, a period of 1294 years, and that suggests roughly

once in a 34-year cycle; and in references after the Meiji era (1868-1912) the cycle became six years. Taking a close look at the scale of damage in those records, the greatest deluges in modern time are specified as the Tatsunotoshi deluge in 1796 and the 6.26 deluge in 1953. (2) Characteristics of the disaster in the Minami-Aso area

The characteristics of the disaster in Minami-Aso area are in how they reflected the climate, geographical features and topography. The following are a summary of the climate, topography and geographical features. 1) Climate5)

Despite the fact that Kumamoto Prefecture generally belongs to the warmer zone of southern Japan, this area is affected by its topography and the climate displays a highland and cold, high rainfall climate type. The characteristics are: 1wind strength is generally weak but the direction of the prevailing wind is westerly or north-westerly in spring, south-westerly in summer and north-westerly in autumn and winter. For the most part the weather is not good, daylight is weak and short in length (annual total sum of about 1900 hours) and hu-midity is high (84% on average) 3The annual average temperature is 10-and night temperatures is large. Frost falls from the end of October to the end of April, which is a long period for Kyushu, and affects various businesses and transport. Rainfall is 2,000-3,000mm and heavy rains and drought alternate, in particular, a heavily rained area is formed in the northern area during the rainy season, while typhoons often pass through this area and cause river flooding. 2) Topographic features

It is a basin-like landform with the R. Shirakawa running through the center (floor) of the area, and is sur-rounded by the south rim that runs south to north and the central cone. Therefore, the rain that falls in this area pours into the R. Shirakawa in the central part, and rapidly increases the water flow. The R. Shirakawa flowing in the middle has tributaries which flow in a south-north direction (central cone south rim direction) leading to mountain collapse and landslide disasters. 3) Geology

The geology of this entire region is mainly formed by propylite and Aso lava, and these geological features have low water permeability. Besides, the headwater section has an accumulation of volcanic ash soil from the eruptions of Mt. Aso, and loosened ground caused by heavy rain leads to mountain collapse and landslide disasters amplifying the damage. (3) Determination of the object disaster in this study

Taking those conditions into account, this area is prone to flooding and volcanic disasters, as well as land-slide disasters like mud avalanches and debris flows caused by them conjoining and the potential for large-scale disasters is quite high. Previously, landslide disasters occurred along with the deluge in many cases. Since modern times, the 6.26 deluge has caused the greatest damage in the history of disaster in this area. Moreover, it is an example of the volcanic ash which fell in April of the same year increasing the damage, and the conjugated natural conditions caused a large-scale disaster. This is a characteristic of the disaster in Nangodani Valley, and also with respect to the residents, the 6.26 deluge seemed to have made a deep im-pression as it greatly reflected the local characteristics. So this study focuses on the 6.26 deluge and analyzes the settlement transition.

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3. VISUALIZING THE DAMAGE CAUSED BY THE 6.26 DELUGE

This study analyzes the changes in the settlements after the disaster. Therefore, the settlements at the time of the 6.26 deluge, as they are the base point of the settlement transition, are analyzed and the conditions of the flood damage are reorganized. After that, the relationship between the damaged areas and the village is ana-lyzed. (1) Reorganization of the previously existing 6.26 deluge documents.

The first crater of Mt. Aso erupted in April 1953. The casualties included the deaths of 5 students on a school trip from Kakogawa Nishi in Hyogo Prefecture and 90 injuries. Because of this eruption, about 1.2m of vol-canic ash accumulated in the mountainous areas and that volcanic ash later became one of the factors mag-nifying the damage of the 6.26 deluge.

showed abnormal development, and the rainy season started half a month earlier than usual with continuous rainfall recorded from May 27th. Then, in late June, a seasonal rain front that was spread east to west and had been moving in a narrow region south to north in southern Kyushu started moving north early in the morning of the 25th as warm air came in. By around noon on the 26th, very wet air blew in from the South China Sea on the strong south-westerly wind to the south side of the seasonal rain front. The seasonal rain front was hyper-activated causing phenomenally heavy rainfall from the stationary front in the Aso and Kumamoto areas.

Precipitation on June 26th was recorded at 411.9mm in Kumamoto City, and even higher, 500.2mm in Kurokawa in the Aso valley. Tateno had a phenomenal 503.0mm and Mt. Aso had intensive rain with a total of 423.3mm. The highest hourly precipitation was 59mm in Kumamoto and 63mm in Mt. Aso. It became the heaviest rain on record. Looking only at the Aso area, Asodani Valley, including Kurokawa and Uchinomaki, had greater precipitation and Takamori in the Nangodani valley had less at 316.9mm. As Takamori is situated on the south-east side of the caldera, the wind direction at that time may have had an influence. This heavy rain caused the most serious landslides and mud avalanche disasters in Hakusui Village in Aso-gun. Mt. Oka-madoyama (1,150m) had 400mm of precipitation on the 26th alone, and the surface of the southern foot of the mountain at about 800m in elevation collapsed in many places. (2) Analysis of the village location before and after the disaster occurrence

In this section, the locations of the settlements before and after the 6.26 deluge are analyzed using aerial photographs and a cross section of the Nangodani valley. The aerial photographs of before and after the 6.26 deluge are shown as Fig.2 and Fig.3, and the cross section of the Nangodani valley is Fig.4 below.

Comparing the location of settlements before and after the 6.26 deluge, no remarkable changes are seen. As a characteristic of settlement location in the Nangodani Valley, the following three points can be cited. a) Settlements are seen clustered along the old National Route 325. b) Settlements are seen along the R. Shi-rakawa. c) Some settlements are seen on the edges of farmland and rice paddies. Each analysis result is shown below. 1) Concentration of settlements along the old National Route 325

At that time, the old National Route 325 used to be the main road connecting Choyo-son and Takamori-cho in the Minami-Aso area, and was built from the middle Meiji era to the Taisho era. There was no alternative route for this road until the Takamori railway (present Minami-Aso railway) was opened in Showa 3 (1928). Therefore the settlements seemed to gather along the road. 2) Settlements around the R. Shirakawa

The livelihood of the people in the Nangodani valley was very deeply related to farming and stock-raising. Most of the people were farmers. Considering the remarkable use of irrigation channels and agriculture in the Minami-Aso area, these irrigation channels were made using water from the various springs around the Nangodani valley and R. Shirakawa. For that reason, the farmers using the irrigation channels for growing crops are presumed to have gathered around the river. 3) Settlements on the edge of farmland and rice paddies

A north-south cross section of the Nangodani valley is illustrated in Fig.46). The Nangodani valley consists of the caldera wall, taluses by slope failures of the central cone, alluvial fan, and river terraces formed by R. Shirakawa erosion. In the alluvial fan formed by large-scale landslides from the central cone, numerous springs are dotted around along the end of the fan, and rice cultivation takes place in these places which have plenty of water. On the other hand, lands on the alluvial fan which are poor in surface water were mainly used for dry field cropping.

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River name Outline of the river Summary of the damage

R. Shikimigawa

R. Shikimigawa s river head is in the R. Marukawa that comes from thesoutheast side of the peak Takadake; the basin area is 13.1km2, riverlength is about 5km, and the average riverbed gradient is about 1/18.5.It is the top torrent stream among the 10 big and small tributaries of R.Shirakawa.

This mountain torrent, being one of the largest causes of the deluge has itsriverhead in Maruyama. Both collapse areas and soil volume are thelargest in the branches of the R. Shirakawa, and the collapses werecaused by vertical erosion and surface landslips; therefore the depth isshallow and only about 0.5m on average.

KurikaradaniValley

Kurikawa Valley s river head is in the Sunasenrihama. It runs acrossShirakawa village, and merges with the mainstream of R. Shirakawa inTakeochi. The river length is 6.6km, basin area is 6.5km2, and it is asteep rapid torrent stream with a riverbed gradient of 1/5 in its upperreach, 1/26 in the lower reach, and 1/9 on average. At an elevation of500m, a right fork merges 2.8km above the merging point with R.Shirakawa, and each mainstream and fork has about ten forks.

This deluge scoured 5m at most, and 3m on average, and the left andright riverbanks even lost about 2m. In this major collapse, especially thecollapses at Maruyama and the riverhead of Kurikaradani at 1303mwere marked as the top two major collapses, and both of them causedlandslips from the mountain peak but the depth was relatively shallow.

R. Takagigawa

The riverhead of the R. Takagigawa is in the southwest of Takagigawa-Sunasenrigahama, and runs through a part of the Ichinoseki hamlet inHakusui-son village, and merges with the R. Shirakawa in Mitera hamlet.River length is about 5km, basin area is 5.7km2, and the averageriverbed gradient is 1/14, and it has a right fork and a few more forks.

Mainly caused surface landslips by vertical erosion but the depth wasshallow.

R. Karigawa

R. Karigawa is in a pasture between the west area of Sunasenrigahamaand east of peak Eboshidake. The riverhead is in the east ofOkamadoyama, river length is about 6.5km, the basin area is 9.1km2,and the average riverbed gradient is 1/10.8, and is a steep rapid torrentstream, running to the south in parallel with the R. Takagigawa. It mergeswith the R. Shirakawa at a point passed the Mitera hamlet.

In the same way as the Kurikaradani valley and R. Takagigawa, thedeluge and strong rain caused serious damage to roads, especially theKiyomasa road, which is a typical local feature in that it is made on landlower than rice paddies. It then became a river itself and extreme verticaland horizontal erosion caused a considerable amount of rock andmudflow.

R.Mizukuchigawa

R. Mizuguchigawa s riverhead is in the Okamadoyama and it has tworight and left forks that merge to the northeast of Matsunoki hamlet. Itruns through the hamlet and flows west of Mizutagami until it merges withthe R. Shirakawa. It is a rapid torrent stream with a river length of about2.4km, basin area of 3.2km2, and an average riverbed gradient of 1/6.3.

The peak of Okamadoyama is mostly covered with green, and thecollapse was mainly limited to the area along the mountain torrent streambanks and previously collapsed areas. Also, the mudflow from theriverhead near the peak of Okamadoyama seemed to be a considerableamount. Including the layer of sediment of volcanic ash, the scoureddepth is supposed to have been about 0.5m.

R. Tarutamagawa

The riverhead of the R. Tarutama is in the southwest of peak Eboshi-dake and the northwest part of Okamadoyama. It runs through theTarutama spa and Hakamano hamlet, merging with the R. Shirakawa tothe south of Choyo-son village, and the lowest reach of the Nangodanivalley tributaries. The river length is about 4km, basin area is about4km2, and it is a rapid torrent stream with an average riverbed gradient is1/10.

The majority of the collapses in the Eboshidake peak are enlargements ofprevious collapses.

Fig. 5

Table 2

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(3) Illustration of the damage situation The 6.26 deluge caused the greatest damage in Nangodani Valley and there are many photographic and

descriptive references on local damage, but illustration of the spatial extent of damage is scarce. For the analysis of the transition of settlements, disaster damage distribution is shown below (Fig.5, Table.2). The survey methods were as follows. 1) Bibliographical survey

Aso sankei houkai chousasho7) (Investigation report on the collapse of the Aso mountain system) and 2) (Erosion control facility register)8) owned by the Sediment Control Division, Kumamoto Prefecture, the collapsed mountainous area in Nangodani valley was summarized.

Aso sankei houkai chousashoDrawn up in August 1953 by the then Sediment Control Division, River Bureau, Ministry of Construction after the 6.26 deluge. The landslide location and number are indicated on a 1:20000 scale map, as well as expla-nations in report format and photographs corresponding to the numbers. The reference specializes in the source of the landslides. (b) (Erosion control facility register), (Forest conserva-tion facility register) : Although it is not an exclusive reference of the 6.26 deluge, it recorded facilities like weirs in table format: by old city, town, village units (former names), stream units, date of completion, con-struction cost, scale, location (text) and so on. There also is a register with a 1:25000 scale map that indicates the location of the weirs. 2) Site survey

Because no reference indicating distribution of damage by the flood was found, the local residents were interviewed to complete the research on the damage situation in this area. (a) Methods: Residents of the Kawagota area of the former Choyo-son, Ichizeki area of the former Ha-kusui-mura, and Yoshida-Sanku area of the former Hakusui-mura were interviewed. Those areas were all damaged by the 6.26 deluge, but also, in order to understand the damage situation in the whole Nangodani valley area, the survey areas were carefully selected from the whole area avoiding any distribution bias. (b) Survey method: All the residents who cooperated in the interview lived in the settlements at the time of the 6.26 deluge, and actually experienced the disaster. The following two were studied by site survey: i) 6.26 deluge damaged places, and ii) the damage situation. 3) Survey result: The areas damaged by 6.26 and details of the damage are indicated in the topographical map (Fig.5). Table.2 indicates a summary of the collapses along each river and valley. (a) On the mountains: Collapse of the mountain surface facing the central cone occurred as the precipitation of the Aso area was great in the 6.26 deluge. Earth and mud carried by those landslides was carried by the branches of the R. Shirakawa in a south-north direction and caused serious damage along the river. This ten-dency is remarkable in the Rivers Tagaki-gawa, Mizuguchi-gawa, and Tarutama-gawa, and in the Sannou-dani Valley as well, and the details are shown in Table.2. (b) Along the R. Shirakawa and on the lowland area

The low-lying rice cultivating areas along the R. Shirakawa in the Minami-Aso area were completely cov-ered by the earth, mud and rocks from the landslides and mud avalanches. When the R. Shirakawa overflowed with muddy water, houses in the settlements on lowland such as Yoshida-Sanku were swallowed up, and it resulted in significant damage. The areas with comparatively lighter damage among the severely damaged areas tend to be the residential areas tracing the outer borders of the damaged area. 4. DESCRIPTION OF THE SETTLEMENT TRANSITION IN THE RECONSTRUCTION PROCESS

During the changes in the post-disaster settlements, the reconstruction project is presumed to have greatly affected the village relocation. So in this section, firstly, the history of the main part of the reconstruction project - river improvements and the construction of sediment control weirs will be organized. After that, the settlement transition showing distinctive changes will be described. (1) River improvement

After the 6.26 deluge, the shape of the R. Shirakawa was dramatically changed. In the reconstruction plan to be described later, the improvement plan for the upper reaches of the R. Shirakawa (present Nangodani Valley)

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is recorded as, light of the damage and outcome of this flood, referring to the river bank after the great erosion, the width of the river is to be widened, river walls as well as short-cut-channels built to accompany the steep curves and prevent river erosion. ngodani Valley where the majorities are farmers, the effects of the R. Shirakawa improvements on the residents and settlement transition cannot be ignored. The history of the R. Shirakawa improvements are summarized and indicated in Fig.6 and Fig.7 and Table.3 below. Taking the opportunity of the June 1953 great disaster in Kumamoto Prefecture, the

(The Basic Improvement Plan for the R. Shirakawa) was drawn up in 1954. As improvements pro-gressed based on this, a new river law was enforced in 1965 and the R. Shirakawa was specified as a class A river in April 1966. In the same year, -Suikei Kouji-Jisshi Kihon (The Basic Construction Plan for the R. Shirakawa System) was confirmed. Moreover, in order to correspond immediately to the needs of local society and valley Shirakawa-Suikei Kouji-Jisshi Keikaku Construction Plan for the R. Shirakawa System) was revised in 1980.

Fig.6 Plan of Minami-Aso area before the river improvement Fig.7 Plan of Minami-Aso area after the river improvement (2) Construction of the sediment control walls Measures against the landslides on the mountain surface that worsened the 6.26 damage were taken starting right after the disaster by building sediment control walls. The history of the construction of the sediment control walls is summarized here based on the records of sediment control management in 1955-65, and the locations are indicated in Fig.5.

Fig.5 is based on the Shisetsu D (Erosion Control Facility Register) and Sisetsu (Forest Conservation Facility Register) owned by the Sediment Control Division, Kumamoto Pre-

fecture, and were made to compare the area damaged by the 6.26 deluge and locations where sediment control walls were built. Having said that, as the completion dates of the sediment control walls on the map were unknown, sediment control walls that are on the records were plotted. The illustration shows that sediment control walls have been built in most of the areas where landslides occurred. Taking a close look at the

Basic Improvement Plan1954 1967 1980

Period 1954~1966 1967~1979 1980~Plan making factors Flood of June 1953 By Article 26 of the new river law Revision of the water dischargeScale of plan (Excess rate per year) About 1/80 About 1/80 1/150Basic amount of rainfall 283mm/day rainfall in Miyaji (reference) 283mm/day rainfall in Miyaji (reference)Design flood before regulation 2,500m3/sec (Kokai bridge) 2,500m3/sec (Kokai bridge) 3,400m3/sec (Yotsugi bridge)Design high-water discharge 2,500m3/sec (Kokai bridge) 2,500m3/sec (Kokai bridge) 3,000m3/sec (Yotsugi bridge)Design flood water discharge - - 3,000m3/sec (Dawn) TatenoMain point of design high-waterdischarge at the Kokai Bridge 2,500m3/sec 2,500m3/sec 3,000m3/sec

Main plans

1. Prevention of collapses in Mt. Aso and erosioncontrol. 2. Flood and soil erosion control forAsodani valley and Nangodani valley and riverimprovement in harmony. 3. Reduce the flooddamage between Tateno and Ozeki bridge,prevent bank erosion. 4. Widen the river anddeepen the riverbed from the estuary to a point16.2km from there, coordinating with otherprojects.

1. As regards the city area of Kumamoto City,excavation and building special embankments aim toincrease the sectional area of the river. 2. The areabetween Rendaiji to the estuary to be thoroughlyexcavated as well as widened and embankmentsimproved to increase river volume. 3. Where themeander of the river is steep like Kojima area (mainriver 2.5 ~ 3.5km), build a diversion route for floodtimes. 4. Build a high embankment at the estuary.

1. Flood control by the Tateno dam plan. 400m3/sec.2. Change the reference point from Kokai bridge toYotsugi bridge. 3. Design flood before regulation of2,500m3/sec to be changed to 3,000m3/sec. 4. Thedesign high-water discharge of 2,500m3 to be changedto 3,000m3/sec. 5. The increased design high-waterdischarge to be dealt with by excavation of theriverbed.

Basic plan of work execution

Table 3

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Nakamatsu, Ichinose and Yoshida-Sanku areas which were severely stricken by the 6.26 deluge, the upper reaches of the R. Mizuguchi does not have so many, but many sediment control walls were built on the upper reaches of the Takagigawa and Shikimigawa rivers, which had played a major role in the damage. (3) The settlement transition model

The settlement transition is analyzed focusing on the three settlements of Yoshida-Sanku, Nakamatsu and Ichizeki and which were the most severely stricken by the 6.26 deluge in the Minami-Aso area, and where distinctive changes could be seen. The following Fig.8 to Fig.10, are aerial photographs of the three areas in 1948, 1964 and 1985, and analyze the settlement transition. 1) Yoshida-Sanku

In 1948, this area was situated on a meandering part of the R. Shirakawa and houses were packed densely together. In 1953, houses in the lowland areas were swept away in the 6.26 deluge, and much damage was done by above or below floor inundation. The total death toll was 11. This is 25% of the total loss of life as the death toll from the 6.26 deluge was 44 in Minami-Aso village. The meanders of the river have changed greatly in this area, and local people as well as civil engineers from outside the prefecture have worked to improve it.

The following three have been cited as characteristics of this area s transition. 1) Residential area is low in density and spread out over a large region. 2) Number of households has not changed very much from before the disaster (about 140 households). 3) Many people still live along the river bank.

Fig. 8

Fig. 9

Fig. 10

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The results of a survey based on those factors described above are as follows. (a) Wide and thin spread of residential area (b) The change in the total number of households is small. (c) Still live along the river after the disaster

2) Nakamatsu area Nakamatsu is situated on the right bank of the R. Shirakawa and in the west of the former Hakusui-mura. It

is adjacent to Ichizeki, Hakusui-mura on the east, Choyo-son on the west, and Kugino-son on the south. To the north is the southern skirt of Mt. Aso and it extends to Aso City. National Route 325 runs east to west. One characteristic of this area is that the settlements are situated slightly off the former National Route 325 towards the central cone, along the R. Mizuguchigawa, a tributary of the R. Shirakawa. The area lost houses mainly along the R. Mizuguchigawa and others were flooded by the 6.26 deluge. Details of the disaster damage are described below.

As for the details of the damage in the area, mud avalanches and floods hit the left bank of the R. Mi-zuguchigawa upriver from the fork in the river, and damage is seen on the right bank downriver from the fork. This stems from the topography of the area, and the height difference between the right and left banks was the cause. At the fork, the depth and width of the river became slightly greater, and damage was limited to the very riverbank. After the disaster, the s nearby where the damage was relatively small, or to Disaster Residences by the locals. In this area also, most of the settlement along the river was flooded, and in the severely affected areas, the houses were lifted up and washed away.

The following three points are characteristics of the transition in this area. 1) The settlement around the upper reaches of the R. Mizuguchi-gwa has relocated. 2) The number of houses has increased. 3) The resi-dential area has not changed.

The results of a survey based on those factors described above are as follows. (a) About the settlement relocation around the upper reaches of the R. Mizuguchi-gawa. (b) About the increase in house numbers (c) About the residential area

3) Ichizeki area Ichizeki is situated on the right bank of the upper reaches of the R. Shirakawa, and in the center of the former

Hakusui-mura. It is adjacent to Yoshida, Hakusui-mura on the east, Nakamatsu, Hakusui-mura on the west, and Kugino-son on the south and the north stretches to the top of Mt. Aso and Aso-cho. Since 1948 the set-tlement areas were along the former National Route 325 east to west, and the R. Shirakawa tributaries, R. Takagigawa and R. Hodegayagawa south to north. Seven or 8 houses were demolished in the Shimoseki area, and almost all houses had above floor inundation or were partly destroyed. Details of the disaster damage are described below.

The loss of life in the area is concentrated in the Shimoseki area along the R. Takagigawa. As seen in il-lustration 5, the valley along the R. Takagigawa caused mountain surface landslides from the direction of the central cone, and that became a mud avalanche which then hit this area. Therefore the damage got greater as it got closer to the settlements along the R. Takagigawa. Actually, all the 7 or 8 houses demolished were situated nearby or along the R. Takagigawa. Most of the houses that escaped being destroyed had above floor inun-dation and many houses had to be rebuilt.

The following two points are characteristics of the settlement in the area gained from the aerial photographs. 1) Even in the damaged area, no settlement relocation is to be seen. 2) Number of houses has not decreased much. The results of interviews based on the factors described above are as follows. (a) No village relocation is seen (b) A decrease in number of houses is not seen (4) Categorizing the settlement transitions

1) Diffusion within the area pattern (Yoshida-Sanku) - The scale of damage varies by location, relocation to the less affected higher plateau - Depopulation was stemmed by the effects of locally rooted civil engineering works - Houses are dotted around the higher plateau area 2) Merging settlements pattern (Nakamatsu area) - Interchanging farmland and house building site style - No change in population originating in the disaster

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- No settlements in new places 3) No change pattern (Ichizeki area) - Recognised as topical damage, and no relocation is seen - Farming took priority in recovery as the local infrastructure, and no depopulation - Houses rebuilt in practically the same place after the disaster As a result of analyzing the settlement transitions, these 4 points are considered to be the influencing factors:

past history of disasters geographical conditions of the the main regional businesses and oc-cupations changes in the ex 5. CONCLUSION

This study, with regards to the victim relocation factor, was researched chronologically from the time of the accident occurrence, taking a close look at the relationship between accident occurrence and the real lives of the victims in Minami-Aso area. The main results of this study are the following points. I) Damage situation of the 6.26 deluge was visualized by reorganizing the history of disaster in the Mina-

mi-Aso area. II) Comparing the settlement transition process in which were the most severely stricken by the 6.26 deluge in

the Minami-Aso area, the characteristics of each settlement recovery transition were summarized. III) Focusing on the 6.26 deluge in the Minami-Aso area, the primary factors of the settlement transitions were

considered from both before and after the disaster as well as the recovery process. The primary factors of the settlement transitions were three; (a) Geographic factor and Disaster history, (b)

Improvement of the external disaster factors and (c) Continuation of internal maintenance factor. (a) Geographic factor and Disaster history: The suffering situation is greatly caused by a local geographic factor. Ichizeki and Nakamatsu settlements are located on the hillside, so their damages centered on the rivers. On the other hand, Yoshida-Sanku settrement are located on the low land along the R. Shirakawa, a lot of places were flooded. But, considering the relocated before the 6.26 deluge, most houses relocated in Ichizeki and Yoshida-Sanku. So the damages were reduced in these districts. In Nakamatsu, there were difference about damage between the right bank and the left bank, the recovery process were different. (b) Improvement of the external disaster factors: A recovery process and the improvement of the external disaster factor are deeply related. In Yoshida-Sanku, the contracture of the R.Shirakawa was improved im-mediately, and some houses expanded. In Ichiseki and Nakamatsu, some river improvement and erosion control works were carried out, but there were some relocation where it took a lot of time for recovery projects. (c) Continuation of internal maintenance factor.: The agriculture which has been the main industry in these three settlements influenced the recovery process. In Yoshida-Sanku, most farmers hold their paddies and

Nakamatsu, some farmers exchanged their residential land and agricultural land and stayed where the im-provement works carried out rapidly.

The further subjects are following three; Investigation about the voluntary disaster prevention activity, Measurement and Making index of the disaster consciousness and Development of the install technique about moderate disaster consciousness. ACKNOWLEDGEMENT: Support from many people made writing this essay possible. Mr. Toshifumi HOASHI of the Aso World Cultural Heritage Promotion office, belongs to the Cultural World Heritage Promotion office of the Cultural Division, Kumamoto Prefecture to start with, everybody in Aso City Hall, Minami-Aso Village Office for the supply of reference materials and cooperation in the survey. This study is also an outcome of basic research for the conservation of the cultural landscape of Aso. Here, I express my gratitude. REFERENCES 1) Hirohisa MURAMATSU, Akira ANDO, Hideo IGARASHI and Ryuichi AKATANI; Study on influence of the tsunami seawall

construction for urban development after the tsunami disaster, Proceedings of history of civil engineering research, Vol.11, 1991. 2) Takeo SHIMAZAKI, Shigeru TAMAKI and Nobuo Shudo; Study on the reconstruction contract and its contemporary significance

of Sanriku big tsunami in Showa 8, Proceedings of history of civil engineering research, Vol.3, 1983. 3) Firefighting school in Kumamoto prefecture; The chronological table of disaster in Kumamoto prefecture, 1999. 4) Sadao OGIWARA and Susumu OBATA; Report of Kyushu flood disasters about Aso area Part-1 in June Showa 28,pp.3-4, 1954. 5) Yoshio MATSUSHIMA; Report of Kyushu flood disasters about Aso area Part-3 in June Showa 28,pp.200-203, 1954. 6) PREC institute Inc., Plan of preservation for Cultural landscape in Aso, p.96, 2011. 7) Erosion control section in the river bureau, ministry of construction; Investigation Report of Collapse, 1954. 8) Erosion control section in Dept. civil engineering, Kumamoto prefecture; Erosion control facilities account book

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