ADAPTATION MEASURE 021
Even if the levees of rivers are built higher, flooding cannot be prevented if the water overflows somewhere. This risk is increasing as climate changes.
Directing water to detention basins that utilize urban parks, abandoned villages, and farmland can contribute to flood reduction.
Many of Japan's rivers flow near urban areas and farmlands, and even if one tries to protect all areas with levees, they will break somewhere when the water level rises, resulting in a major disaster.
As climate change increases the frequency of flooding, which is said to occur once every few decades, it is more important to "release" water out of the rivers when floodwaters rise by creating multi-purpose flood control areas than to build levees that "prevent" large-scale heavy rainfall disasters. In Japan, there is a long-established tradition of "water control.
In Japan, there is an ancient saying that "controlling water (chisui) is like controlling the nation (chikoku)," and flood control measures have been taken to reduce disasters. The goal is to achieve coexistence between people and nature through flood control, which is an old and new method of disaster mitigation.
Multipurpose Flood Control Basin in Shin-Yokohama Park
The Tsurumi River flows through one of the most densely populated areas in Japan, and there are many houses near its levees, making it difficult to take measures such as widening the river. For this reason, Shin-Yokohama Park, including a popular stadium for soccer and other sports, was designed as a reservoir.
The reservoir, with a total storage capacity of 3.9 million m3, has been effectively flooded 16 times during the 13 years from 2003 to 2015. It is efficiently managed with observation equipment, and as a nationally famous and much-loved multipurpose park, it is highly exposed on SNS and other media when the floodwaters rise.
Watarase Yusuichi (Reservoir): a detention basin which overcame mining damage and became the key to flood control along the Tone River.
The Watarase River, which became a branch of the Tone River during the Edo period (4 centuries ago), has been suffering from the Ashio Copper Mine Mineral Pollution Incident and flood damage. Therefore, a plan to create a flood control area on the lower reaches of the Watarase River was proposed, and measures to mitigate mining damage caused by flooding were promoted by building a huge flood control pond in Yanaka Village. The area surrounding the flood control area was a low marshy area where several rivers intersected, and even after the completion of the flood control area, the surrounding area suffered from severe flood damage. Therefore, in order to make more efficient use of the reservoir, the area was converted into a control basin in 1963. Currently, there are three flood control facilities in the flood control basin, and together with the dam upstream, they prevent flood damage caused by heavy rains. The heart-shaped Yanaka Lake also functions as a reservoir for the Tokyo metropolitan area, where native reeds purify the water.
Open levee (Kasumi-tei) and side weir (Araizeki), which control flooding with interrupted levees and low embankments
In the Takatoki River, a branch of the Anegawa River in Shiga Prefecture, disaster damage in urban areas is controlled by creating side weirs (Arai-zeki) that are partially lower than normal levees, to prevent flooding downstream. In addition, open levees (Kasumi-tei), which are discontinuous levees, prevent the spread of flood damage by allowing water to partially overflow during flooding and return to the river when the water recedes. This type of disaster mitigation has been practiced since Japan’s Warring States Period in the 15th and 16th centuries.
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