|I.Land and Climate of Japan
Japan's 4 main islands - Honshu, Hokkaido, Shikoku, and Kyushu - and more than 3,000 small islands cover a combined area of 377,727km2. These islands extend over 2,000km in total length but spread only about 300km in width.
Located in the Circum-Pacific "ring of fire", Japan is predominantly mountainous - about three-fourths of the national land is mountains - and long mountain ranges form the backbone of the archipelago. The dramatic Japan Alps, studded with 3,000-meter peaks, bisect the central portion of Honshu, the main island. Japan has around 200 volcanoes, about 60 of which are active. Consequently, earthquakes and volcanic eruptions are common.
This mountainous setting creates rivers that generally are short and have steep channel slopes. The rivers carry their sediment to the flatlands where they deposit it to form moderately sized alluvial plains.
The population is primarily concentrated in lowland urban areas, Particularly in those of the Kanto, Chubu, and Kinki districts along the Pacific coast of Honshu. The census of 1991 pegged the population of Japan at about 123.6 million, the seventh largest population in the world following China, India, the former USSR, the USA, Indonesia and Brazil. Japan's population density, 327 persons/km2, is one of the heaviest in the world.
According to a 1992 National Land Agency survey on land utilization, 252,100km2 (66.7% of the national land area) is forests, 52,600km2 (13.9%)is cultivated fields, and 16,500km2 (4.4%) is residential areas. The cultivated area is decreasing year by year, though gradually.
Japan lies in the northeast tip of the Asian Monsoon Zone that encompasses India, China, Korea, and the Southeast Asian countries. The weather is generally mild and humid with considerable variation from north to south, and between the Pacific Ocean side to the east of the central mountain ranges and the Japan Sea side to the west.
The country's four distinct seasons feature three periods of heavy precipitation: Heavy winter snowfalls blanket the Japan Sea side in deep layers of snow, Particularly in the north (although the Pacific Ocean side normally remains clear and dry); tsuyu (the rainy season) brings continuous heavy rains to most of the archipelago during the second annual wet period in June and July; and typhoons that originate in the southern Pacific, assault the country - especially the southern portions - during the third wet period in September and October. These three wet periods shove the nation's average annual precipitation which is almost double of the world average.
Generally, precipitation occurs mostly during the tsuyu and typhoon seasons on the Pacific Ocean side, and during the typhoon season and in winter (in the form of heavy snow) on the Japan Sea side.
Due to Japan's extreme topographical and meteorological conditions, the nation's rivers exhibit distinctive natural characteristics. Generally speaking, Japan's rivers can be characterized as follows. The rivers are prone to flooding because they flow rapidly, due to the steepness of slopes along their basins and the relative shortness. The ratio of peak flow discharge to basin area is relatively large, ranging from 10 times to as much as 100 times that of major rivers of other countries. The water level rises and falls very quickly. The river regime coefficient - the ratio of the maximum discharge to minimum discharge - is between 200 and 400,10 times larger than that of continental rivers.
The volume of sediment runoff is large.
Since early times the residents of Japan have engaged in rice cropping in the alluvial plains created by flooding rivers. Dependent on the rivers for irrigation and water but ever vulnerable to the inevitable floods, the residents regarded the rivers as both their mentors and their rivals. In spite of the constant danger of disasters, they warded off the rains and the flooding in order to carve out places for living and farming in the plains where river water was easily available for irrigation.
As this attitude towards land utilization evolved through the years, population and industry continued to accumulate in the lowland areas along rivers where danger of flood disasters remains a constant threat. In this way, major communities have developed mostly in flood-prone areas along rivers.
In particular, due to the remarkable shift of population and social assets into urban areas since the period of high economic growth began in the 1960's, urbanization has progressed in the areas with a high risk of disasters near lowland marshes, alluvial fans and cliffs. Today, 48.7% of the population and 75% of holdings are located within the flood-prone areas of rivers (1985).
The number of dead or missing as a result of natural disasters, from the end of World War K to late 1950's exceeded 1,000 almost every year because of successive big typhoons and earthquakes. Notable among the statistics makers was the Ise-wan typhoon of 1959, which caused the greatest flood damage recorded since the War, with over 5,000 dead or missing.
Flood mitigation measures over the years during the postwar rehabilitation period have decreased occurrence of major rivers overflowing their banks and of embankment failure, reducing both the severity of flood damage and the total area affected by it. In recent years, however, the inflation of property values due to rapid economic growth, and the continued concentration of urban property in floodplains have increased costs of flood damage in urban areas. Flood damage density (the ratio of damage to affected area) has risen sharply; and property damage due to river overflow and water collecting behind levees as a percentage of total damage has been on the increase, too.
Japan's rugged topogaphy is responsible for landslides nearly every year.
Disasters in Coastal Areas
Surrounded on all sides by the sea, Japan is vulnerable to storm surges, high waves, and tsunami on the coasts.
Japan's water resources are characteristically plentiful mainly during the tsuyu, typhoon, and spring thaw seasons.
Although annual precipitation far exceeds the world average, this does not mean abundance of water resources. Due to Japan's dense population the per capita precipitation in Japan is only about one-sixth of the world average. Furthermore, since the rivers have small basins and steep channels, rivers flow erratically and relatively little of their water is actually available for use.
Compared with cities in other countries, Japan's major cities store surprisingly little water reserves, and every year there are water shortage problems somewhere in the country.
The annual volume of water for municipal and agricultural use taken from rivers is estimate at about 78.21 billion cubic meters, and that from groundwater about 13.15 billion cubic meters. It is apparent from these figures that river water is the important water resource in Japan and that its efficient use is essential.
1) In ancient times people lived on and cultivated hilly areas or small flat areas in Valleys where no floods occurred. Gradually they moved to more spacious lowland areas where the land was more fertile and more productive. Land along bigger rivers was rich with the natural fertilizer transported and deposited by the rivers, and was more convenient to the rivers for drawing water for irrigation.
Although the people knew these lowland areas were vulnerable to flood disaster, they were willing to brave the danger in order to make their lives more productive. They began to build levees and to dig diversion drainage by hand to prevent flood disasters. Floods frequently overflowed the levees and destroyed them, inundating farmland and houses.
2) Until the Nara Era (710 - 794) most farmland was located in the small valleys where water was drawn from small streams and floods were not a problem. History shows that in the later Nara Era people began to move near the big rivers and to build levees.
In the year 742, the government issued a decree that inhabitants who had settled land could own it as private property.
The law encouraged the people to expand their land holdings, and eventually a system of shoen (manors governed by aristocrat landowners) evolved.
3) In the Shoen Era (9th - 15th century) property did not increase much because shoen were relatively small communities with too little manpower to expand the cultivated land on a large scale. Most of the water for irrigation was drawn from small ponds or catchment reservoirs.
4) From the Sengoku Era (16th century) to the Edo Era (17th - 19th century) ancient feudal lords were much more interested in expanding their farmland in order to become more powerful by increasing their economic strength.
During these eras people moved to the vast flatland areas near the mouths of the big rivers where the river channels fanned out haphazardly. They began to try to improve and control the rivers utilizing methods such as constructing dikes or levees, and digging channels.
Shingen Takeda, an ancient feudal lord in Kofu - what is today Yamanashi Prefecture - began works to control the Kamanashi River in order to protect the Kofu Area after the flood of 1542.
Hideyoshi Toyotomi also executed remarkable river improvement works, relocating the channel of the Kiso River in the Inuyama area, and constructing levees along the Yodo River.
The family of Hojo constructed levees at Kumagaya and Minotani along the Ara River in the Kanto region.
Kiyomasa Kato, a lord in the Kyushu region, improved the Shirakawa, Kikuchi, and Midori Rivers by installing retarding basins to mitigate flood damage.
5) The Edo Era (17th - 19th century) was a prosperous and peaceful age when economic and cultural development was actively promoted, and the population increased. In this age the amount of cultivated land was also remarkably increased by powerful feudal lords under the supervision of the Tokugawa Shogunate. Ring dikes were first constructed in the lowlands to protect the relatively higher areas, and then afterwards the ring dikes were connected, creating continual levees along the large main rivers.
Ieyasu Tokugawa, the first Shogun had commenced work to divert the Tone River to protect Edo area - what is now Tokyo - as soon as he moved to Edo in the early 17th century.
Until it was diverted, the Tone River had flowed into Tokyo bay via the channels of what are now the Ara River and the Edo River. In 1621 Ieyasu began the excavation of a diversion channel which led the Tone River to the Watarase River. At this stage the Tone River still flowed into Tokyo Bay via the Edo River. Then, another diversion work was carried out to divert the Tone River to the Kinu River, through which the Tone River finally pours into the Pacific Ocean.
In addition to the flood prevention works done in the Edo era, channel excavation works were also done on the Tone River, Kiso River, Edo River, and others for the purpose of creating inland navigation routes for transporting rice as tax payments to the lords or the shogunate.
6) Dutch engineers were brought in at the beginning of the Modern or Meiji Era (from 1868). They provided guidance for the carrying out of channel dredging and sand control works to improve navigation. Work started on the Kiso River in 1887.
In the 26th and 29th year of Meiji (1893 and 1896), Japan was ravaged by heavy floods which became the motivation for formulating the River Law. Drafted in 1896, the River Law was to become the mainstay in modern Japan for administration and improvement of rivers to alleviate flood disasters. Work was begun on the Edo, Yodo and Chikugo Rivers in 1896, and work on other rivers followed in turn.
Big floods also occurred in 1902, 1907 and 1910. The flood in 1910, which wreaked disaster all over the country, precipitated a new epoch in the promotion of anti-flood measures, and an extraordinary flood control investigation council was set up to discuss measures to overcome flood disasters. In 1911 the "First Flood Control Plan" was authorized.
7) From the Taisho Era (1912 - 1926) to the beginning of the Showa Era (1926 - 1989), Japan had no big floods. Meanwhile, flood control works has proceeded steadily, based on the "Second Flood Control Plan", formulated in 1921, and the "Third Flood Control Plan", instituted in 1933. Soon afterwards, World War áU (1941 - 1945) broke out, removing flood control works from the list of national priority projects.
Ironically, big floods continuously attacked the devastated land after the War ended.
The Ise-wan Typhoon (1960), which took 5000 lives, battered the Chubu area causing a high tidal flood on the shore of Ise Bay. Soon after the disaster the "Erosion and Flood Control Emergency Measures Law" (1960) was drafted and the "First Five Year Plan for Flood Control" began securing continual national budget allocation for flood control. The "River Law" was finally drafted in 1964.