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Newsletter and Technical Publications Creating a Rainwater Utilization Based Society for
Sustainable Development 1 Re-thinking of water policy in cities Until now, many cities of the world have had a policy based on the idea of drawing city water from a great distance; 100km or indeed several 100km away. Tokyo is no exception. On the basis that its water was insufficient, Tokyo has sought to build dams in its upper hinterland area. It is now true to say that for most of its piped water, Tokyo depends on dams in the Tone River water system. This is about 150 km distant from the center of the city. According to a water account drawn up by Tokyo Metropolitan Government this year, water resources is equivalent to 56 % of Tokyo’s annual average rainfall of 1405mm, and the greater part of this comes from the Tone River water system. But this policy of increasing dependence on the upper streams of the water resource hinterland area is beginning to show signs of deadlock. In 1987 rain did not fall in the upper streams of the water supply area for a long time and the Kanto area was faced with a serious water shortage. Development of the upper streams of the Tone River water resource area had reached a limit, so Shunichi Suzuki, the governor of Tokyo, worked out the so called “Shinano River Watershed Concept”. This idea was to go outside the watershed area and draw water from the Shinano River into the Tone River. The governor of Niigata prefecture at the time, Takeo Kimi vehemently opposed this. He said, “Seeking water resources in another prefecture on the basis that ones own water is in short supply is too simplistic”. Building dams in the upper watershed means submerging many houses and fields and mountain woods beneath the water, and it means destroying the culture of hill country villagers that has been nurtured over a long period of time. It is not easy to win over people whose homes will disappear under the water. Because of this it takes a very long time to complete the construction of dam. Compensation that has to be paid to people from submerged areas, and compensation for the submerged land has a tendency to increase over time, and there is usually a resulting increase in the cost of construction. There is already no further opportunity for the development of large dams in the upper streams of the Tone River, Tokyo’s principal water resource, and development of dams already proposed is progressing slowly. In addition, dams that have already been constructed have been filling with silt over a long period of time, and there has been a tendency for the quantity of water effectively collected to be reduced. Tokyo must now change its approach and must no longer depend on the water resources of the upper watershed area. Tokyo must aim for independent water resources depending as much as possible on rainwater, ground water, and wastewater reclamation. It must also progress with water conservation. Up to now Tokyo has only thought of rainwater as a nuisance, and of how it can be disposed of quickly through drains. It has virtually ignored how rainwater can be used as a resource. But the piped water consumed in Tokyo during one year which is about 2 billion cubic meters, is exceeded by the 2.5 billion cubic meters of rain that falls. This rain should be harnessed as a resource, and should be actively used. Reclaiming the city’s rainwater as a resource, and the importance of securing water by changing from “off-site” to “on-site” sources is a significant exercise which must be based on learning from Tokyo’s water resource policy up till the present time. (2) Independent “life-points”, not dependent on a “life-line” for water supply When the city increases the degree of its dependence on a distant water resource, and there is a long period without rainfall in the upstream dam sites, there is an immediate paralysis of the effective functioning of the city. The same thing can be said about reliance on a pipeline drawing water from a water resource area to the city. A city, which is totally reliant on a pipeline drawing water from a distant water resource, is very vulnerable in the face of a large-scale natural disaster. The Hanshin Awaji great earthquake showed how this can happen in reality. Kobe City depends for most of its water resource on the distant Biwa Lake. Water is brought from there by a pipeline, but the earthquake destroyed the supply line. During the approximately one month until the pipeline was repaired, Kobe’s municipal functions were completely paralyzed. During the long period that water was cut off from pipes, well water from within the city served as an alternative source of water. The Kobe City building, where the Disaster Restoration Center was set up is an example. From the time when the building was constructed, the water supply system was divided into city water and well water, with toilets using well water. The huge earthquake did not damage the well water system, and the toilets continued to function. The move “from life-line to life-points” must be reinforced. The lesson learned from the Kobe disaster is about this change in thinking. Numerous small-scale scattered water resources must be secured as “life-points” within the city. These can draw on rainwater and ground water, making the city strong in the face of water shortages and earthquakes. (3) From the flow of rainwater to the storage of rainwater to restore regional water circulation Due to a speed up in urbanization, a lot of the world’s large cities are facing problems with urban floods. For Tokyo, the central point of this problem is that the Tokyo regional water cycle is being destroyed. The cycle is made up of rain falling and over time seeping underground to become groundwater. A rich supply of underground water produces springs and the submerged flow of rivers. In 1991 the proportion of areas where rain did not permeate reached 81.8 %. An increase in the proportion of areas where water does not penetrate speeds up the flow of rainwater. Water then accumulates in drains and streams within a short time. Every time there is concentrated heavy rain, there is a flow back of sewage from drains, and medium and small sized rivers repeatedly flood. Without an overflow, the heavy rain often causes an outpouring of sewer water into rivers and streams, from countless sewer outlets and sewer pumping stations, and the quality of the city’s river water becomes tainted. In Tokyo, New York and elsewhere prevention of water tainting has become an important aspect of conserving the river water quality. The concrete and asphalt structure of cities has tended to eliminate the regional water cycle, and reduce the amount of rainwater permeating underground. Of the 1405mm of rainwater falling on Tokyo annually, only about 360mm penetrates underground. As a result, the amount of ground water that flows into rivers and ponds via springs is thought to be no more than 30-100mm in the Tokyo central area. Because many city springs have dried up, the flow of city rivers, dependant on these springs as their water source has been reduced. City construction that destroys the regional water cycle has various effects on the city environment. One is drying of the city. This happens because the springs dry up, rivers and watercourses are covered, and greenery is cut down. The average humidity at the center of the Tokyo in January has fallen by close to 20 % in the last 100 years. Then there is the city’s heat pollution. Midsummer in Tokyo covered with concrete and asphalt is like a heated frying pan. In hot summer an asphalt road at midday reaches a temperature of over 60·C. The heat expelled from coolers further aggravates this. The number of tropical nights also rises with urbanization. The change in the city environment with the earth covered by concrete and asphalt has a profound impact on the ecology of the city. For example, the numbers of swallows in Tokyo has declined. The destruction of the regional water circulation brought about by urban concrete and asphalt structures is the basic cause of significant problem, including urban flooding, tainting of the water quality in rivers caused by the overflow from drains when it rains, drying up of spring water, turning of the city into a heat island and its desiccation, and destruction of the city’s ecological environment.
In order to achieve a comprehensive solution to this problem, a complete change in city structure must be brought about. The problem must be examined from the perspective of the regional water cycle. There is a need to look for a basic solution through the restoration of regional water circulation. According to generally accepted city planning practices, sewer facilities have been developed with the assumption that the coefficient of rainwater drained away will have to be increased. From the stand point of preserving the regional water circulation, it is better to retain rain water and to promote its permeation, by preserving natural ground and greenery, thereby limiting the increase in the coefficient of rainwater drained away.
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