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[Weather on the 25th] The weather is not stable, so be careful of sudden thunderstorms today.

 
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It is necessary to be careful about sediment-related disasters, lowland inundation, lightning strikes, and gusts.
 

Atmospheric instability will continue nationwide on Friday, 25th.Especially in the afternoon, it rains and thunderstorms here and there, so ... → Continue reading

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Sediment disaster

Sediment disasterWhat's the meaning of(I.e.,EarthquakeAccompanySlope failure(Landslide・Slides),Landslide,Earth and stone flowBecause oflife,propertyThreatened by[Note 1][1].

Definition

Configure the slope of the groundrock,Satsand The重力When it becomes unstable due to some reason, it moves downward. There are several types of styles such as rockfall, landslide, collapse, quicksand, debris flow. Collectively, all of these phenomenaMass movement(English: mass movement)''[2], Or “Slope movement (English: slope movement)” or “(in a broad sense) landslide (English: landslide)''[Note 2][3]The term is technically defined.

"Sediment disaster" is the aboveMass movement(Or slope movement, landslide in a broad sense) refers to general disasters[4].. However, three categories, slope failure, landslide, and debris flow, have been established, and we may explain that these three are landslide disasters. EspeciallySabo, I often make such classifications and explanations in disaster prevention situations.[5].. Note that this concept is not universal (cf.#Japanese peculiarities)[Note 3].

"Slope disaster" focusing on the fact that synonyms occur on slopes[6][3], "Mountain disaster" focusing on the fact that it occurs in the mountains[7]There are some cases, and in some cases they are synonymous with each other or used properly. In addition to this, it occurs in the terrain where the ground is artificially shaved or raisedSlopeDisasters such as collapse may be included in sediment disasters[8].

Uniqueness of Japan

Later (#Cause), landslides are more likely to occur on undulating land.Japan7% of the national landMountain-hillsThe land occupiesCrustal movementThe active fluctuation zone (Pacific Rim Zone),VolcanoSince there are many, sediment-related disasters are likely to occur. Moreover, there are few plains and there are restrictions on land use.Second World WarSubsequent economic growth and population growthplateauThe hills and hills have become urbanized, and sediment-related disasters have made it easier to reach residential areas.[8].

On the other hand, in countries with a large land area, such as the United States, Canada, and Russia, although they also have vast mountains, it is cheaper to relocate them even if roads and houses are damaged due to landslides. Therefore, it is rare to reconstruct it in the same place and to perform work to prevent further collapse.[9].. For example, in the case of the United States, public works carried out by the government/administration are mainly for the purpose of preventing the inflow of sediment onto roads and preventing the outflow (erosion) and inflow (accumulation) of sediment such as agricultural land. , Not for the purpose of protecting the house. This is to select the residence by incorporating the risk for disasters such as slope failures.Beneficiary burden principleIt seems that the idea of ​​looking at the weight is the underlying[10].

Also in the field of study, in Europe and the United States, soil mechanics dealing with the properties of clayey ground are widely accepted, and a detailed “landslide classification” based on the particle size of soil and the soil quality, movement form, movement speed, etc.[Note 2]Has developed, and there is little awareness of disaster prevention. On the other hand, in Japan, civil engineering is good at the theory of rocks, gravel ground, slope stability, etc., and the classification is also unique to the classification of "slides" and slopes "landslides".[9][11].

Cause

Generating mechanism

The soil blocks and bedrock that make up the slopes are usually重力,FrictionAs a result of such actions, it is stable in a state in which the "force to resist" is larger than the "force to move the slope". Here, if the former becomes larger or the latter becomes smaller, the balance is lost and deformation occurs.[12].. In terms of soil mechanics, this is called peak failure due to an external force that exceeds the rigidity of the soil mass, and the external force at the time of failure is called the peak strength.[12].. Also, when considering the stability of the slope, slide the mass on the assumed slip surface.Shear failureIs[13][14].

Sliding begins when the "shear stress" that attempts to move the slope exceeds the "shear resistance" that resists it. The ratio of the former to the lattersafety ratio It is called Fs and is an index of slope stability. actually,Coulomb destruction criteriaCalculation is performed using an analysis method that incorporates soil strength constants obtained by[15].

Corresponding to the small deformation that occurs just before the peak breakage, in the experiment of slope failure, etc.CreepIt has been confirmed that a deformation called[12].. This creep is also one of the causes of what is called the precursory phenomenon of landslides.

Occurrence of slope failures and landslides is often related to the action of water contained in the soil mass. This is because the water that penetratedPore ​​water pressureIncrease the effectiveness of soil particlesstressIs the mechanism by which the shear resistance decreases and the shear resistance decreases. It is effective as a countermeasure to reduce the pore water pressure by draining water from the soil or by construction that does not allow water to penetrate.[16].

Predisposition

The main causes of sediment-related disasters on the earth are crustal movements, volcanic activity, and cold climates. Crustal movement isFaultCrushing the ground by movement,Mountain buildingBy the groundUpheavalEtc. Volcano is easy to collapseVolcanic ash,Pyroclastic flowSuch as deposits (Volcanic debrisA) in large quantities at a time to create an uneven lava landform. Repeated freezing and thawing in cold regions such as high latitudes and high mountainsPeriglacial actionCreate a geology that is easy to collapse[17].

Huge collapse/landslide in the world (collapse volume 107 --109 m3) Occurrence area,Indonesia,ネ パ ー ル,Chugoku,Japan,Taiwan,フィリピン,New Zealand,America,カナダ,ペルーMost of them are located in the fluctuation zone (Rim Pacific Rim Zone or Alps-Himalayan Change Zone).[17].. Exceptionallyノルウェー,スウェーデンAnd so onGlacierRock landslide due to the receding ofQuick clayLandslide occurs[17].

Other factors are strong weathergranite(Masago soil) Or volcanic soil (ShirasuEtc.), thick sediment (responseLocal geology, such asOvergrazing,Air pollutionbyVegetationArtificial and social factors such as the destruction of forests, excessive mining, urbanization of mountains and plateaus.[17].

Incentive

Incentive (trigger) of earth and sand disaster,Torrential rain,typhoonHeavy rain due to[18],Earthquake, Volcanic activity,Natural damOut of order, artificialExcavationand so on[17].

Status of sediment disasters in Japan

Since modern times

Historically, in JapanEdo PeriodThroughMountain forestIt seems that the devastation of No. XNUMX continued to progress, and it was in the most devastated state from the late Edo period to the early Meiji era. In this era, most of the materials, building materials, ships and carts, fuels, fertilizers, etc. necessary for the Japanese to live in this erawood,草It was because of that, and depended heavily on the forest. The common people used to use it as fuel by digging up strains and roots from their daily lives. It was said that the slash-and-burn field was banned, tree planting, logging and mountain climbing were banned in various places, but the effect was considered to be poor. Tree-free forests that have been cut down mainly in satoyama close to the village spread out over a wide area of ​​Honshu, especially in western Japan.Bald mountainIs presumed to have been widely distributed.As a result, severe soil erosion and severe soil erosion in forestsSurface collapseOccurred frequently[19].

MeijiWhen you enterIndustrial revolutionDue to this, alternatives to timber began to be developed and spread, but there was a high demand for timber due to industrialization and wars, and there were areas where forests were laid down to the depths of the mountains and felling was promoted. On the other hand, for disaster prevention, the Meiji government[Note 4]As a nationControl"Or"SaboStarted the business of. With the introduction of technology from Europe, the technology of hillside construction and sabo dams to stabilize the sediment of the bald mountain has been developed.[20].

After World War IIAt the same time, the demand for timber for reconstruction will result in a rapid expansion of logging to the interior. On the other hand, due to the afforestation policy, the felled areas will be rapidly transformed into artificial forests, and bald mountains will be almost eliminated. Also, during the high economic growth period,Imported timberReplace domestically produced trees, resulting in a rapid decline in logging. However, surface collapse and debris flow occurred frequently in young forests (young plantations) where the collapse prevention function is weak. Furthermore, the urbanization has caused the collapse of urban areas. From the latter half of the 1960s to the first half of the 1970s, more than 100 people fell victim to sediment-related disasters each year, accounting for more than half of the natural disaster victims. In response, hardware measures such as landslide prevention and erosion control dams will be promoted. After that, a large number of forests were planted after the war, the function of preventing collapse increased, and the number of victims gradually decreased due to the progress of hard measures.[21][22].

modern

During the 1979 years from 2008 to 30, an average of about 1,000 landslide disasters occurred annually[23].. Also, in the five years from 2009 to 2013, there are about 5 cases per year.[24]. However,weatherDepending on conditions etc., there is a large change from about 200 cases to over 2,000 cases.[23].. For example, in 2004Niigata/Fukushima heavy rain,Fukui heavy rain, Landslides occurred frequently due to numerous typhoon landings, etc.[24].

The number of deaths also fluctuates from year to year. For example, during the 2003 years from 2013 to 11, the number was 2007 in 0 and 2010 in 11. Man,Typhoon 12In 2011, when there were many casualties due to the flood damage caused by the Kii Peninsula in 85, the number reached XNUMX.[24].

The ratio of landslide disaster damage is not low, even when looking at all natural disasters.1980 eraから2000 eraThe annual fluctuation is large, but it is 1% to 4%, and it reaches 6% depending on the year.[22].

Form and damage of slope failure, landslide, debris flow

Types
/ (Main alias)
FeaturesAspect of damage
Slope failure[13]
/(Landslide[13][25], Landslide[13][25], Landslide[13], Rock fall[26], Steep slope collapse)
  • On a steep slope with an inclination of 20 degrees or more, the soil mass suddenly collapses and is pushed downward, and moves at a speed of 1 mm or more per day.[26].
  • Those that collapse about 2-3 m on the surface of the slope (surface collapse) are common, and can occur anywhere on a weathered slope. Especially likely to occur in areas where aquatic sedimentary rocks, volcanic ash, and weathered soil (mass sand) of granite are distributed or where there is a large amount of rainfall.[26].
  • Those that collapse to the bedrock deep below the slope (deep layer collapse) are less frequent. It is considered that the water pressure in the groundwater vein rises due to the infiltration of a large amount of rainwater, and is triggered by heavy rain, earthquakes, etc.[26].
  • The time to collapse is very short, so if it occurs near a house, it often delays running and causes death.[27].
  • The heavier the clod and the steeper the slope, the greater the collapse force.[13].
  • Many disasters are smaller than landslides and debris flows. However, large-scale disasters such as deep-seated landslides are large in scale, such as inducing debris flows or forming natural dams, and their collapse affects a wide area downstream.[26].
Landslide[28]
  • Inclination 5 degrees-20 degrees on a gentle slope, underground specific layer (sliding surface) as a boundary, the soil mass shifts while maintaining its shape. Move at a slow speed of less than 1 mm per day and continue to move for several days[29].
  • Deeper slip surface compared to slope failure[30], Tend to be large[29].
  • The cohesive soil (landslide clay) is the main slip surface. Occurs frequently in areas where landslide clay is distributed[29].
  • Three types of landslide areas are known in Japan. 3. Nagasaki Prefecture, where weathered clay of the Tertiary layer is distributed in hilly areasKitamatsu area,Hokuriku region, 2. by fault motionShatter zoneAnd the past orogenyCrystal schistAre distributedCentral structure lineAlongTokushima-EhimeEtc., 3.SpaOf extra hot spring soil that has been weathered due to the chemical composition ofHakone,Kirishima,Beppu,NarukoHot spring area[31] .
  • In cold regions, year-round landslides that occur with the melting of snow[30], There are areas where landslides are likely to occur due to periglacial action[17].
  • It is persistent and recurrent. Things that were moving slowly at the beginning may turn faster and then move again after stopping.[29].. Most of the places where the landslide occurred in the vicinity in the past (often the "secondary slip" of the past landslide)[30].
  • Characteristic "landslide topography" such as horseshoe-shaped cliffs, stair-like slopes, and ridges below the slopes often appear in past landslide sites, and by identifying these, we can identify the places where landslides are likely to occur. it can[30].
  • Since the progress is slow and the precursors are easy to catch, human casualties are not often caused. However, in 1985 Nagano City, Nagano PrefectureMounted ground26 people were killed and 55 buildings were completely destroyed in some cases, such as a landslide disaster that caused great human damage[32].
  • Large scale movement of clods causes large physical damage[27].
Earth and stone flow
/ (Flash flood[33], Tsunami[33], Mud flow[33])
  • On the slopesRiver bedSediments fluidize and flow out along the valley[34].. It refers to a mixture of sediment and water of the same degree, and if the proportion of sediment is small, it will be a river flood flow.[35].. Often includes large rocks and driftwood, which can be swept over and over again[36].
  • There are three forms. 3. Unstable sediment in the mountain stream (river bed) is fluidized by heavy rain, 1. Slope failure due to heavy rain, landslide sediment is fluidized with a large amount of spring water and surface water, 2. Slope failure・Landslides block riversNatural damThat form a waterfall, then the water level rises, collapses, and flows down[34] .
  • It is often found in the mountains and volcanic areas, where land production is active. Also, it tends to occur near mountain streams with a slope of about 15 degrees or more.[35].. In many cases, it flows out on a flat ground, becomes gentle, and stops and accumulates near an inclination of 2 degrees.[34].. However, it has a large momentum and tends to go straight without turning the curve.[36].
  • Since it occurs suddenly (like a guerrilla) regardless of geological conditions, it is difficult to predict when and where it will occur. In soil mechanics, it is classified as a "sand flow" with extremely high fluidity.[33].. The flow is usually as fast as 20-40 km/h, sometimes exceeding 60 km/h, making it difficult to escape[27][36].. Compared to landslides and landslides, when it occurs in a residential area, it is more likely to cause death or death, resulting in life-threatening damage.[33].
  • Evacuation can be estimated by obtaining the lower limit of cumulative rainfall (tens of minutes to several days), which is derived from past rainfall and debris flow occurrence in that area.[37].

Other sediment disasters

Rock fall/fall
A piece of rock that separates and falls from a steep cliff. Not only natural factors such as ground loosening and earthquake motion due to rain but also artificial factors such as vibration due to traffic act. In the past cases, the bus and trains are often damaged because vibrations are larger than passenger cars. It should be noted that it is easy to narrow down the danger points, but it is difficult to predict because there are few signs[38].
Collapse/landslide due to earthquake
The force of the seismic motion has the effect of making the slope steeper and increasing the weight of the slope. As a result, collapse due to rain does not occur much, and even collapses occur on slopes with a slope of 10 to 20 degrees, causing a wider range of collapse than during heavy rainfall. It collapses even on steep cliffs without topsoil. The seismic motion is easily amplified in the areas that are higher than the surrounding area, and mountain ridges and ridges that would not normally collapse due to heavy rain also collapse. In addition, the scale of collapse tends to be large, and the movement distance of earth and sand is likely to be long. It is difficult to evacuate because it is sudden[39].
Damage from driftwood
In the case of debris flow and flood, driftwood generated along with the collapse of the upstream of the river may flow down together and spread the damage. In particular, there is a high risk of damage caused by direct impact of driftwood on houses and bridges, and on the way that debris flows down, bridges and waterways are clogged with driftwood and flooded with dirt and muddy water to the surrounding houses. .. In terms of tree species, coniferous forests tend to have a larger volume of driftwood per area than hardwood forests.[40].
Landslide disaster
Disaster such as slope failure. The artificially grounded ground such as cut soil, embankment, valley filling, and belly filling is in a geologically young and loose state, so that it is more likely to collapse or fluidize than the naturally formed ground. In addition, since it is often used in residential areas and the like, and if a disaster occurs, it can have a great impact on human life and property, so it is necessary to carry out construction assuming phenomena that may occur after construction. in Japan,Building land development regulation law(Established in 1961) establishes standards for the safety of the ground in residential areas and establishes retaining walls on cliffs and drainage facilities.[41].
Mountain collapse, debris avalanche
A large-scale collapse of a mountain area due to an earthquake or eruption. Due to the large head, the collapsed soil flows down a long distance at high speed (Debris avalanche). Larger ones have a thickness of 100 m or more and flow down a valley at a speed of 100 km or more, and the horizontal movement distance is about 10 times the height difference. Volcanic areas, especiallyStratovolcanoIt is easy to happen in. When it flows down into the sea(I.e.Generate[42].

(I.e.

In order to prevent damage, avoid using dangerous land in the beginning, and if it is unavoidable, install civil engineering equipment to prevent collapse and refer to the precursory phenomenon and how it rains at appropriate times. Evacuation is effective.

Hazardous areas are especially based on the lawEarth and sand disaster caution areaPlaces designated as, places where basic surveys are being conducted, and places included in sediment-related disaster risk areas surveyed by prefectures (cf.#Dangerous zone announced by the government). However, even if it does not correspond to these, caution is required in mountainous areas and land with slopes and cliffs around it.[43][44].

In order to avoid sediment-related disasters in the danger zone, pay attention to how it rains and various precursory phenomena, and when you notice a precursor, promptly notify the municipalities and neighboring residents, and also take the initiative to evacuate. Is valid[43][45].

The time to be careful is when there is a lot of rain, when it has been raining for a long time, and for a while after it has stopped.[43].. Also, be careful for a while even after a big earthquake[43].. in Japan,Japanese Meteorological AgencyBased on the amount of rainfall so far and the expected amount of rainfall up to several hours aheadHeavy rain warning,Earth and sand disaster caution informationEtc. have been announced, and this is a guide (cf.#Information that landslide disaster is likely to occur)[46].

Key points of measures

Japan is,AboveThere are many areas where housing and public facilities may be damaged due to the characteristics of the land.[27], The number is over 2020 as of 60[47]. JapaneseGovernment public relationsPage lists the following three basic methods to protect yourself from sediment-related disasters.[27].

  1. Where I usually liveEarth and sand disaster caution areaTo check[27].
  2. When it starts to rainEarth and sand disaster caution informationBe careful[27].
  3. Evacuate as soon as earth and sand disaster warning information is announced[27].

またTokyoDisaster prevention handbook published in 2015 byTokyo disaster prevention"Lists three points as "ordinary preparations" to protect yourself from sediment-related disasters.[48].

  1. Check the dangerous points (1. above)[48].
  2. Check the evacuation site[48].
  3. Prepare an emergency carry-out bag[48].

In addition, the book also lists the following:[48].

  • If there is a sign of a landslide disaster around the cliff and in the mountains, even if no warning is issued, ensure safety before evacuating.[44].
  • If you feel the danger of a sediment-related disaster, you should change your clothes quickly so that you can evacuate at any time.[48].
  • When evacuating, keep your belongings to a minimum and keep both hands[48].

Relationship with forest

Forests control soil erosion and surface collapse
The presence of trees has the effect of stabilizing the slope against surface collapse at depths of a few meters. Particularly effective on steep slopes[49].. The roots of trees that have spread in the soil resist the shearing forces that try to slide on slopes by deforming by receiving the displacement of the soil layer and resisting pulling out by frictional force[50], Or the root acts as a reinforcing bar to prevent deformation of the soil layer, maintaining an arch structure that resists soil slippage[51]It is believed that.
In addition, it has a high effect to prevent soil surface erosion.[52][53].
However, there is a limit to the deterrent effect, and even if there is a forest, surface collapse may occur if there is an abnormal heavy rainfall. If forests are lost in this way, restoring them as soon as possible will prevent further collapse.[54].
Deterioration effect of deep collapse is poor
On the other hand, deep landslides are less affected by surface vegetation because of the deep slip surface, and even in well-managed forests. The effect of deterrent is considered to be poor[55][53].
Logging accelerates collapse
Slopes from felled forest trees are prone to surface collapse because the roots that support the soil layers are weakened. However, several years after the felling-more vulnerable to collapse after a dozen years. This is because the root decay gradually progresses after logging. However, the degree varies depending on the tree species and environment[56].
Also, in forestry, in order to maintain a strong root system of trees, it is necessary to make the logging intervals longer and to grow them sufficiently, and to expose the forest floor when logging is underdeveloped and young or mature forests. It is desirable not to let it (keep it covered with fallen leaves and undergrowth)[57].
The longer the tree is, the higher the deterrent effect
The older the forest, the less likely the surface collapse will occur. Young forests (young forests) are more likely to collapse than forests over 40-50 years old[58].
Impact of thinning
In planted forests, thinning has the effect of preventing pests and wind damage and promoting good growth. In Japan, the number of forests with delayed thinning tends to increase due to the deterioration of the forestry management environment, and there is concern that the function of forests to prevent collapse will decline. However, some studies have shown that thinning forests increase resistance to landslides due to increased root volume, while others have reduced resistance, and the effect is not clear.[59].

Preventive measures against construction, etc.

There are various construction methods depending on the geology of the land and the purpose of land use.

In areas where there is a risk of slope failure or bedrock collapse, planting trees on slopes including slope spraying (slope protection), concrete spraying on slopes, installation of precast concrete frames (slope protection), and embedding of slope anchors. Vegetation works such as (sowing) and lawn upholstery are effective.[60].

In areas where there is a risk of landslides or landslides that can be caused by the action of water, water must be removed in order to eliminate water.culvertConversion, lateralボ ー リ ン グ, Water collectionwellGroundwater drainage work such as installation of water, surface drainage installation, rainwater infiltration prevention, etc. are effective.[16].

In landslide areas, there is also a method of using an upper earth removal work that removes the soil above the landslide surface and a restraining embankment work that embanks at the end and suppresses it with a retaining wall. The embankment is often used as a park[60].

In mountain streams where there is a high risk of debris flow, construct structures to deposit sediment.Sabo dam-Oyama DamInstallation of is also effective. However, its volume is limited and it fills up over time, so its effect is limited.[61].

For small-scale and sudden collapses/falls, rockfall along roads and railways in dangerous areasShedThere is also a method of installing a rockfall prevention net and a rockfall prevention wall,1989ToFukuiEchizen coastOccurred inCollapse accidentIn rare cases, a collapse of a larger scale than expected may occur and damage may occur.[62].. To compensate for this, rockfall detectors and landslide detectors that detect shocks and movements,Debris flow sensorThere is also a system that installs such equipment and links it with the alarm device of the road management office.[62].

Other than Japan, it has steep land areas and is often damaged by sediment-related disasters.Indonesia,ネ パ ー ルThere are places where Japanese erosion control technology has been introduced, mainlyJICAThrough technical support through[63].

However, it is dangerous to assume that it is safe because countermeasure work has been carried out, since there are many cases where debris flow damage has occurred at the foot of the sand control dam.[64].

Main precursor phenomenon

  • Earth and stone flow
    • There is a rustling noise (sounds of stones hitting each other) in the river, or you can see sparks.[43][48].
    • The whole mountain makes a roaring sound (mountain noise) or trembles like an earthquake. Has an unusual odor (odor of rotten soil)[43][48].
    • The water of the river becomes cloudy, and the fallen trees (raw trees) flow with the water.[43].
    • Rain continues, but river water decreases[43].
  • Landslide
    • Water gushes from cliffs and slopes. Spring water increases[43].
    • Even though there is no wind, the trees in the mountains are awful. There is a cracking sound of a tree and a root cutting sound. Rushing and humming[43].
    • Water in ponds, ponds, and ponds becomes cloudy and suddenly increases and decreases.[43][48].
    • Cracks, steps, or depressions on the ground[43][48].
  • Slope collapse (landslide)
    • New water springs from the cliff. Also, the spring water may become cloudy, the amount may increase, or it may suddenly stop.[43][48].
    • Cracks on the cliff. Or the cliff swells[43].
    • Trees on the cliff sway and tilt. Rumbling[43].
    • Pebbles fall down from the cliff[43].. *A landslide may occur without warning[43].

It should be noted that these precursory phenomena do not always appear before they occur. In addition, it is difficult to detect at night, when there are many people sleeping in the dark, and during times of heavy rain, even if there is a precursory phenomenon. It is effective to avoid sediment-related disasters if you feel something is a little strange and to deal with it and to evacuate early.[65]

Dangerous area announced by the government

  • Sediment disaster warning area/special warning area- Sediment disaster prevention lawPrefectural governments conduct surveys, specify, and publish based on Encourage the evacuation of residents in areas where there is a risk of sediment-related disasters, and limit development especially in dangerous areas.[66].. As of the end of 2020, about 64 designated areas, of which about 52 (about 81%) are special caution areas, and about 3 areas are expected to be designated in the future.[47].
    • Earth and sand disaster caution area -MunicipalRegional disaster prevention planEstablish a warning system and evacuation system for each area[66].
    • Earth and sand disaster special caution area -Establishing building regulations and restrictions on development activities in addition to measures in the restricted area[66].
    • An area equivalent to the sediment-related disaster warning area announced after the basic investigation-Designation of the caution area will take a period of time such as an investigation. To prevent damage during that period, the prefecture is required to announce the area promptly after the basic survey.[66].
  • Sediment disaster danger point -OldMinistry of Construction・ Prefectural governments investigate and publish based on the investigation guidelines established by the Ministry of Land, Infrastructure, Transport and Tourism.Areas where there is a risk of damage to people's houses and public facilities due to sediment-related disasters[66].. It was conducted from 1966 to the 2000s, and it has been increasing year by year due to the progress of research and development, and as of March 2003, it has counted about 3 locations.[Note 5][67][68][69].
    • Sudden slope collapse danger point-A steep slope with a slope of 30 degrees or more and a height of 5 m or more that may damage people's houses and public facilities, and adjacent areas.[66].
    • Debris flow danger area -An area with a slope of a mountain stream of 3 degrees or more (2 degrees or more in a volcanic erosion control area) and a house or public facility in a dangerous area where damage is expected if a debris flow occurs[66].
    • Landslide danger area-Range of areas that may damage rivers, roads, public facilities, houses, etc., in areas judged to be likely to cause landslides by reading aerial photographs, investigating disaster records, and conducting field surveys.[66].
  • Mountain disaster area- Forestry Agency(Ministry of AgricultureThe department in charge of forests in the prefectures conducts an investigation based on the survey guidelines established by the Ministry of Public Affairs and transmits it to the municipalities. Areas where there is a risk of damage to houses and public facilities due to mountain disasters (hillside collapse, collapsed sediment discharge, landslides)[70].. forest(National forestOr) targeting. As of 2012, about 18 locations in total[71][69].
    • Three types of areas: hillside collapse risk area where there is a risk of slope failure and rockfall, collapse sediment outflow risk area where there is a risk of debris flow, and landslide risk area where there is a risk of landslide.[70].
  • Designated area by the Sabo Law-Drainage works, retaining wall works, and installation work for erosion control dams (project erosion control) is premised on performing a landslide disaster and designating an area with a high need for construction. The number of cases is smaller than that of landslide disaster areas.
    • Sabo designated area- Sabo LawIs designated by the Minister of Land, Infrastructure, Transport and Tourism. Development such as excavation, embankment, logging of bamboo trees and collection of debris is restricted.
    • Landslide prevention area- Landslide Prevention LawIs designated by the Minister of Land, Infrastructure, Transport and Tourism. Development such as excavation, embankment and debris collection is restricted.
    • Areas of steep slope collapse-designated by prefectures based on. Development such as excavation, embankment and debris collection is restricted.

Information to inform that sediment-related disasters are likely to occur

Heavy rain warnings and sediment-related disaster warning information inform the Japan Meteorological Agency that the risk of disasters is increasing, and evacuation orders strongly urge residents in dangerous areas to evacuate.Sediment-related disasters are difficult to escape after they occur, and they have the destructive power to wash away or completely destroy wooden houses, and human damage is likely to occur.On the other hand, dangerous areas can be narrowed down by examining them in advance, and human damage can be reduced by moving away from the dangerous areas as much as possible. Therefore, it is effective to evacuate early using various information as clues.

However, since such information is uniformly distributed to a wide range of cities, towns and villages, "safety" means that "no warnings or evacuation orders have been issued" without the residents recognizing the magnitude of the danger of each place. It may be regarded as.For example, even within one mountain village, a building above the terrace has a lower risk of debris flow than a building below, while on the other hand, even if it is on the terrace, if the slope of the mountain is approaching nearby. There is a high risk of slope failure.Therefore, it is not preferable to regard "no warnings or evacuation orders have been issued" as safe, and it is necessary to judge whether or not to evacuate according to the degree of danger of each place.

If there is a sign of a landslide, it is necessary to evacuate and contact the municipalities, etc. even if no warning is given.[72].

However, the collapse due to an earthquake is sudden, and its location cannot be specified, and it tends to be large-scale. Therefore, there is almost no room for evacuation, and the only effective countermeasure is to avoid the use of dangerous land in advance.[73].

Heavy rain warnings, etc. indicate the risk of sediment-related disasters in stages. Municipal unit. Calculated by cumulative rainfall and expected rainfall, and announced by the Japan Meteorological Agency[46][74].

  • (I.e.Warning(Sediment disaster) -  Level 2-Heavy rain can cause disasters[75][74].
  • (I.e.alarm(Sediment disaster) -  Level 3 equivalent-Heavy rain can cause serious disasters.One of the factors for municipalities to determine the issuance of evacuation preparation information[75][74].
  • Earth and sand disaster caution information -  Equivalent to Level 4-After the announcement of the heavy rain warning, the risk of sediment-related disasters has become even higher.One of the factors that a municipality decides to issue an evacuation advisory[76][74].
  • Record short-time heavy rain information -It rained a few times every few years. Rainfall is rapidly increasing, and the risk of disaster is increasing[77][74].
  • (I.e.Special alarm(Sediment disaster) -  Level 5 equivalent-Heavy rain is extremely likely to cause a serious disaster[75][74].
  • Risk distribution of sediment-related disasters"Earth and sand kikikuru"[Note 6] -  Note, Be wary, Very dangerous, Extremely dangerous-The risk of sediment-related disasters is shown on the map in 1km square units, updated every 10 minutes.It is open to the public by the Japan Meteorological Agency, provided to the information system for disaster prevention organizations, and can be viewed on the Japan Meteorological Agency website.Shows a detailed degree of risk that reflects regional differences.It also includes forecasting factors based on the expected rainfall up to 2 hours later, but there are cases where rapid local heavy rainfall cannot be predicted.[76][74][79].
  • Sediment disaster emergency information-Information that the risk of disaster due to landslides with relatively slow speed, volcanic mudflow caused by ash fall after volcanic eruption, natural dam collapse, etc. has increased. Prefecture announces[80].

Evacuation orders issued by municipalities strongly urge residents in the target area to evacuate, and are issued by the mayor of the municipality with reference to warnings and rainfall.This means "evacuation evacuation" = evacuation to an evacuation site (movement to a designated emergency evacuation site) or evacuation to a safe relative / friend's house, etc. In that case, "emergency evacuation" = moving to nearby tall buildings, strong buildings, parks, etc., or "indoor safety measures" = staying in a safer place in the building.According to the Cabinet Office's "Guidelines for Evacuation Information" (2021), in the event of a flood, residents other than those requiring special attention should first prepare for evacuation at the stage of evacuation of the elderly, etc., pay attention to the information, and give evacuation instructions. On the other hand, in the case of sediment-related disasters, it is recommended that all residents in the target area "start evacuation at the stage of evacuation of the elderly, etc."This takes into account the nature of sudden and unpredictable sediment-related disasters in 2019.First Year of Reiwa East TyphoonHas been revised from the lessons of[81].

The following points should be noted when evacuating from a sediment disaster.

  • Even if no evacuation order has been issued, if you feel that you are in danger, you should evacuate voluntarily without waiting for the evacuation order.[84].
  • If you discover a precursory phenomenon of a sediment-related disaster, you should take the initiative to evacuate voluntarily and immediately contact the municipalities (reporting the precursor is one of the standards for issuing evacuation advisories and also for the safety of other residents. To contribute)[84].
  • In particular, wooden houses are at high risk of collapsing or being buried due to sediment-related disasters, and it is recommended that the residents move to the evacuation site early at the start of evacuation such as the elderly.[85].
  • Evacuation orders are issued even at night or during storms and heavy rains, where there is a high risk of outdoor activities.In this case, it is necessary to determine the danger of moving to a remote evacuation site and the surrounding conditions, and consideration should be given to moving to a nearby shelter and ensuring indoor safety.Furthermore, if such a situation is expected, it is desirable to evacuate to an evacuation site while it is bright and the wind and rain are weak.[86].
  • If it is dangerous to move to the evacuation site, such as when a flood or landslide has already occurred around the time you tried to evacuate, place as far as possible from the debris flow expected reach area or steep slope, as high as possible, or Consider moving to an emergency shelter, such as the upper floors of a sturdy building. For example, the upper floors of a concrete building nearby, a small elevated place away from the mountains, etc.[86].
  • In areas where small landslides are expected, if it is dangerous to move to an evacuation site, move to the second floor or above of your house, and make sure to consider urgent indoor safety. However, a normal wooden house may be completely destroyed by debris flow, so in areas where debris flow is expected, it is desirable to move to an emergency shelter outside your home. Evacuation to the second floor of your home is an unavoidable option, and it is desirable to evacuate to an evacuation site from an early stage so that it does not happen.[86].

Warnings and evacuation information are issued so that disasters are not "missed".Therefore, the so-called "missing swing", in which a disaster does not occur despite the announcement, is inevitable.Residents' consciousness is to avoid their own damage by thinking that they were vacant but "I'm glad there was no damage" and that they evacuated but "I was lucky because there was nothing" and try not to neglect warnings and evacuation information. In addition, it is thought that it will lead to deterrence of the situation where the administration side hesitates to issue an evacuation order.[87][88].

Case study

Category: Sediment disasters in each countrySee also
Ancash earthquakeLandslide due to earthquake (1970) [Earthquake, mountain collapse, debris avalanche, burial in urban area]
ペルーA large earthquake of M7.7 occurred in the northern part of the 6,000m classHuascaranA steep cliff near the summit of the mountain collapsed on a large scale. The rock mass was under the cliffMoraine[Note 8]It was scraped off to become huge and became a debris avalanche and debris flow. A debris avalanche is a city at the foot of a mountain about 230 km away, over a 10 m high ridge.YungaiStruck. The city suffered a catastrophic loss of 25,000 out of a population of 18,000. Collapse scale is 0.5-1.0×108m3Was estimated to be 280km/h[89][90].
Shimabara very Higo trouble(1792) [Mountain collapse, debris avalanche, tsunami]
Shimabara PeninsulaIt is inUnzendakeOne of the mountains, Bizan, collapsed and became a debris avalanche. After the villages and fields at the bottom were washed away and buried, the still vigorous sediment flowed into the Ariake Sea, causing a tsunami and hitting the coast. The height of the tsunami reached a maximum of 1 m along the opposite coast of Kumamoto Prefecture. 23 dead on the Shimabara Peninsula,KumamotoとAmakusaReached 5,000 people. Estimated collapse scale is 2.2×108m3[91][92].
Mount Nevado del LewisMudflow due to eruption (1985) [Eruption, glacier melting, volcanic mudflow]
Eruption of Mount Nevado del Luis in Colombia, followed by a volcanic mudflow caused by melting of a glacier at the summit of the mountain. A city 45km from the craterAlmeloKilled 30,000 people in a population of 21,000 and 2,000 people elsewhere. The mud flow reached up to 80 km from the crater[93][94].
Nagano prefecture west earthquakeLandslide due to earthquake (1984) [earthquake, slope failure, mountain area]
Near the epicenter due to M6.8 earthquake in western Nagano prefectureMt. OntakeA large landslide occurred on the southern hillside. The size of the collapse is 1,300 m in length × 400 m in width × 140 m in depth and 3.4 × 10 in volume.7m3The speed is estimated to be 70-80km/h on average. About 12 km down the riverside valley, 15 people in the vicinity were caught and died.[95].
Disaster(1972) [Heavy rain, slope failure, secondary disaster, mountainous area]
Early in the morning of July 1972, 47 (Showa 7), due to heavy rain, Tosayamada-cho, Kochi Prefecture (currently Kami City) A landslide occurred in a village along the valley of Shigeru. A second collapse occurred during this restoration, leaving one fire brigade alive. A large volume of 2 m was added to the place where many other members and residents were working to rescue the members.3, A large-scale collapse of 130 m wide attacked a village andShigetou StationI washed away the train that was stopped at. Sixty people were killed in a second disaster that killed many of the victims. In addition, the main line connecting Kochi prefecture and Kagawa prefectureNational Route 32And JNRDosan LineBoth were buried, and it was cut off for more than 20 days, and the road was forced to make a large detour.[96][97].
Haribara debris flow disaster(1997) [Heavy rain, slope failure, debris flow (collapsed debris flow), mountainous area]
June 1997, 6, Kagoshima PrefectureIzumi CityHeavy rain caused landslides in the upstream of the Harihara River in the Harihara area, and the sediment that entered the river became a debris flow that flowed down along the river. The landslide was a deep-seated landslide on a highly weathered slope, which was a debris flow that collapsed and then fluidized. At the top of the fan (close to the mountain of the fan), most of the houses were washed away and destroyed, 21 people were killed and 13 were injured. Houses and mandarin orange fields are buried in mud and flooded in the central and fan ends, and in the middle and lower reaches of the plains, driftwood and damaged houses are caught in bridges and water overflowing from the river causes flood damage. It was The time of occurrence was a few hours after the peak of heavy rain and before 1 am when many people were sleeping, and many people had no experience of sediment disaster in this area and many people were paying attention to floods The fact that there was a witness to the low water level of the river, but no one knew it was a sign of a debris flow, is analyzed as the cause of the greater damage.[98][99].
Slope failure caused by the earthquake in the Republic of Tajik (1989) [Earthquake, combined disaster of liquefaction and slope failure, uncut waterway]
Former Soviet Republic of Tajik (currentlyEur-lex.europa.eu eur-lex.europa.eu) Occurred in M5.5. In the town of Gissol near the epicenter, the height is several tens of meters and the slope is gentle.LoessThe slopes of the area were transformed into fluidized mud, which was collapsing everywhere at the same time. The topsoil of this area is loess that easily becomes brittle when it contains water.Waterway[Note 9]It was considered that the liquefaction of the ground occurred due to the action of the water. In the Sharala district, where the most casualties occur, 100 houses were buried and 220 people were killed. The largest collapse was over 800m x 3,600m in width, but the slope from the apex to the end was a gentle slope of only 1 degree.[100].
The collapse disaster caused by the earthquake in the Loess LandKilnThere are many cave-type houses called (Yao Tong) in ChinaLoess PlateauIn 1920Umihara earthquake(Chinese)But it is believed to have occurred[101].
Nagasaki floodDisasters (1982) [Intense rainfall, slope failure, debris flow (collapsed debris flow), urban areas, residential areas on steep slopes]
Record heavy rainNagasakiMany slope failures and debris flows occurred around the area. As for the types of landslides, there were many surface landslides and slightly large-scale landslides of weathered ground, but most of the 299 victims of sediment-related disasters were due to slightly larger landslides. Surrounded by mountains, there was not enough land on the flat land with a limited valley bottom, and the land was spread over the steep slopes, which was a factor. It is revealed that the land where the city is formed on the valley floor and the steep slope is vulnerable to sediment disaster.[102][103].
39 7 Mt. Gassan Yin Hokuriku Slope failure in Shimane Prefecture (1964) [Intense rainfall, slope failure, debris flow (collapsed debris flow)]
Heavy rainShimane Kamo-cho and Daito-cho (currently bothYunnan) Caused many slope failures and debris flows. The total number of collapses exceeded 10,000, and 109 people died due to multiple simultaneous collapses and the debris flow that expanded due to the collapse of the riverbed that started from the collapse.[104].
Mounted groundLandslide (1985) [Landslide, speeding up, and disaster victims]
Nagano City, Nagano PrefectureMounted groundThen, the slow landslide, which had been going on for several years, suddenly accelerated. Since the cracks on the hillside were discovered and monitoring was continued for several years, the cracks became more prominent than a few days ago, and suddenly began to slip at high speed on the evening of July 7. I buried the building at the bottom.Nursing home26 bedridden elderly people were killed and 55 houses were completely destroyed[32][105][97].
Byint damLandslide (1963) [Groundwater level rise due to landslide, mountainous area, tsunami, dam construction]
イタリアLandslide occurred at Byint Dam in the north. Was completed in 1960damAlthough the rocks of the bank were hard, there was a gentle slope on the left bank of the reservoir, which is considered to be landslide topography. Shortly after the water was stored, cracks and creeps were found on the gentle slope indicating the start of landslides, so monitoring was continued and a drainage tunnel for draining water was also established. Due to trial and error, the suspended water storage was restarted using this method because the speed of the landslide slowed down as the water level was raised and raised in small steps. However, in the fall of 1963, the speed of the landslide increased again on the way, so when I was trying to reduce the water level, it continued to rain and the water level rose further. A high-speed landslide occurred at midnight on October 10, causing the sand and sand to rush into the reservoir and generate a high tsunami, which attacked the opposite bank (right bank) and the villages upstream. In addition, the tsunami ran over the dam bank and attacked downstream villages. The victims amounted to 9. The tsunami reached a point 2,600 m above the surface of the lake, and the volume of water pushed out by the sediment was 240 x 1.28m3Estimated to be[106][97].
ElmLandslide (1881) [rock landslide, mountain area, mining]
Elm in the Swiss mountains(German)Landslide occurred in a village called. It is a material for slate roofs on the slopesSlateWas mined to a depth of about 50 m, which slipped down on a large scale and hit a village 2 km below, killing 115 people. Collapse scale is 1×107m3[107].

footnote

[How to use footnotes]

注 釈

  1. ^ Accompanied by a volcanic eruptionLava flow-Pyroclastic flow-MudflowMay be included.
  2. ^ a b The World Landslide Catalog Committee, established by three international societies and UNESCO, is a collective term for movements that include sliding, falling, forward rotation, extension, and flow."landslideIt is defined as "(Japanese translation: landslide). Detailed landslide classification based on three elements: motion type, shear type, and material (landslide classification) It is carried out. There are three types of shear types: slip, liquefaction and creep, and three types of materials: rock, sandy soil and cohesive soil.
  3. ^ For example, even in Japanese law, the purpose of disaster prevention is to
    • Collapse of a steep slope (a natural phenomenon in which a land with a slope of XNUMX degrees or more collapses)
    • Debris flow (means a natural phenomenon in which debris, etc. resulting from the collapse of a hillside, or debris, etc. in a mountain stream flows down together with water.)
    • Landslide (refers to a natural phenomenon in which part of the land slips due to groundwater, or moves with it)
    It is defined as the damage caused to the life or body of the people with any of the three causes (Act on Promotion of Sediment Disaster Prevention Measures in Sediment Disaster Areas Article 2).
  4. ^ Three methods of flood control: River law(Established in 1896),Sabo Law(Established in 1897),Forest law(Established in 1897).
  5. ^ Approximately 1972 locations in 4.7, approximately 1993 locations around 17 (* steep slopes are 1992 figures), and approximately 2002 locations around 53 (* landslides are 1998). The rapid increase in 2002 was due to the fact that the survey target was expanded to include less than 5 houses, which was about 21 by the previous standard.Around this time, a basic survey based on the Sediment-related Disaster Prevention Law was started, so no new surveys have been conducted since then.
  6. ^ The name until 2018 is `` Sediment disaster warning judgment mesh information''[78].. Nicknamed in 2021.
  7. ^ A person who needs help in grasping the information necessary for evacuation, taking evacuation action, and protecting himself. Elderly people, disabled people, foreigners, infants, pregnant women, etc. Persons requiring assistance during a disaster.
  8. ^ Sediments and rocks deposited by the action of glaciers.
  9. ^ A plain irrigation canal is a simple form of irrigation that does not have stones laid on it. Water easily penetrates.

Source

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  2. ^ Geotechnical Society (2006), P.196
  3. ^ a b Toki/Kawada (2002), P.171 Jiro Doi "Landslide"
  4. ^ Geotechnical Society (2006), P.459
  5. ^ Fuchida et al. (2014), P.67
  6. ^ Fuchida et al. (2014), P.149
  7. ^ Takaya (2008), P.1
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  9. ^ a b Mizutani (2012), Pp.13-15
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  81. ^ | Revision of guidelines for evacuation information (May 3rd year of Reiwa)
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  101. ^ Hagiwara (1992), P.190
  102. ^ Hagiwara (1992), Pp.191-192
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References

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Related item

外部 リンク

Information to protect yourself from sediment-related disasters (Japan)
Basic knowledge/disaster prevention publicity
Example of sediment disaster
Sediment disaster and erosion control organization

 

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