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In recent years the "global warming trend" has, while contributing to the sea-level rise, not only enabled the growth to inordinate size of hurricanes/typhoons and tornadoes but also led to expansive flooding. During the great flood which occurred in Thailand during October of 2011, prolonged drenching by floodwater meant many production facilities were adversely impacted along with the region's economy. Also fresh in one's memory is the horrible disaster wrought by Hurricane Katrina, in August of 2005. About 80% of New Orleans was flooded due to her destructive power, with more than 1,600 people perishing at that time. The catastrophic storm surge produced by this Category 5 hurricane destroyed seawalls, which together with the subsequent repair problems related to the old seawalls made this one of the worst disasters ever to be experienced in North America. Such or even larger-scale natural disasters will occur worldwide with experts warning that human existence itself may be endangered by future disasters. However, many big cities such as Tokyo, New Orleans and so on are built on large areas of low ground below sea level known as "zero-meter sites at or below sea level". These sites are potentially hazardous in that they carry a high risk of natural disasters due to their proximity to seas and/or rivers. Yet recently, due to the global warming, especially lower-lying land areas in cities have seen flooded streets many times as well due to precipitation amounts that surpass expected levels as exemplified by sudden torrential showers, beyond amounts that can be handled by today's sewerage system.

On the other hand, water shortage issue has become serious recently due to increasing population all over the world. This issue is especially serious in big cities since people gather in cities and an extraordinary amount of water for life is required at these heavily populated areas. Preservation of water in cities is necessary not only for daily life but also for the situation in disaster. In recent years, recognition has been renewed as to the fact that increased demand for water is leading to shortages in the drinking water supply as well as the fact that emergency water supply is vital for firefighting, as underscored after the experiences of the great earthquake in Kobe. Since urban water supply is for the most part delivered from distant locations using longer water pipes, outages for a prolonged period of time oftentimes occur during disasters like storms and earthquakes. As a result, there are deliberations concerning the use of rainwater within urban areas as a new source of water supply.

Also, because city areas including buildings, roads and so on are completely covered by asphalt and concrete, not only rain seepage into the underlying soil is prevented but also rainwater goes quickly and directly into rivers and goes to sea by way of subterranean sewer pipes and/or streams; these pipes and streams have been turned into drains, spilling into the sea without being soaked into the ground. However, during years with limited precipitation such a set-up results in water shortage, a situation which lends itself to reassessment during urban redevelopment. Therefore reservoirs are important for cities as is insisted by Prof. Norihito Tambo. A reservoir can also be employed in conditions of drought during times where there is little or no rain. In addition, maintaining a freshwater reservoir is a useful way to safeguard against any future water shortage in the wider local area.

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