Rapid urbanization and changes in water circulation

Urbanization affects water circulation. The spread of sewerage systems, as described previously, caused a deterioration in river and canal environments as a result of water flow loss. Other changes in water circulation are caused by the paving of roads and the conversion of farmland into housing.

The urbanization of Tokyo started in the 1950s. Tokyo's population increased rapidly from the latter part of the 1950s into the 1960s, and is now some 12 million, warranting the name "mega-city." Urbanization at such a pace has changed the water circulation of Tokyo drastically, and has become the cause of new urban flood hazards. The Kano River typhoon, on 26 September 1958, produced the highest recorded rainfall per day (392 mm) since 1875 in Tokyo, and caused great flood damage in the newly developed housing areas of the western part of Tokyo. Since then, damage caused by rainfall has increased in parallel with new housing developments. Changes in water circulation during heavy rains are the main cause.

The urbanization of Tokyo has been accompanied by the populace's desire for a higher standard of living, resulting in a heavy burden on rivers and water circulation. These burdens have involved the control of water, the utilization of water, the environment, and the landscape. Embankments became taller as a result of river improvement works undertaken to protect against water hazards, thus spoiling views along rivers and streams. The construction of highways to provide easy access to the areas alongside these rivers has also spoiled riverside scenery.

In today's Tokyo, projects that aim to restore the rivers and water circulation of the city are finally under way. They include advanced water treatment systems in purification plants, the discharge of treated sewage into rivers and streams, the utilization of treated sewage in high-rise buildings, the encouragement of urban renewal work, and river improvement work in the development of the Super Embankment along the Sumida River - a river that is emblematic of Tokyo.