Kasumitei for skillfully dealing with flood risks

Kasumi-tei, discontinuous open levees, are one example of using traditional knowledge to reduce river floodings. Kasumi-tei reduce the impacts of floodings by releasing part of the flow smoothly through breaks between the sub-levees. Kasumi-tei form a flood control mechanism that have historically been developed to suit the topography and climate of Japan. The method of dispersing the flow to normally unused land or farmland when the flow rate increases is an NbS that has been nurtured over a long period of time along with the development of rice farming in Japan.

Since ancient times, Japan has experienced many river floods due to geographical factors. In addition, due to the development of farming, rice fields were formed along the rivers in many areas, and residential areas spread outside the rivers. Embankments were constructed in an unplanned manner to protect only important areas. As a result, discontinuous levees and topographically prone areas existed along the river, and during floods, inundation was naturally controlled by the surrounding rice fields. With modernization, however, many of these Kasumi-tei have disappeared from major rivers and were replaced by grey infrastructure, which made it difficult to carry out reconstruction.

Re-evaluation of traditional flood control system

As the population increased due to modernization and land use plans were developed, embankments began to be systematically built so that people could live close to the rivers. The law created a system in which more lives and property could be protected by continuous levees. However, due to unpredictable amounts of precipitation, localized heavy rains, and increased inflow due to development in upstream areas, there have been situations where levees have broken and caused extensive damage. Other issues have also come to light, such as the destruction of river basin ecosystems and agricultural landscapes due to the establishment of levees. As such, the historical flood control mechanism of Kasumi-tei has been re-evaluated and is expected to be used as an effective NbS measure for flood control throughout the basin, in combination with ordinary embankments.

©Misato Uehara

 

Comparison with gray infrastructure: technological and legal aspects

Kasumi-tei do not entail difficult technology, but rather are the simple mechanism that makes effective use of the topography of the land. With grey infrastructure, it is necessary to pump the excess water back to the river, but with the Kasumi-tei, there is a natural original gap, and any overflow of water should return to the main river naturally.

As densely populated areas have increased in river basins along with economic development, the national government has taken responsibility to create a series of levees by law with the aim of protecting human life. As a result, when larger floods occur, the levees are raised, which can lead to a vicious cycle of further damage. While Kasumi-tei can reduce huge flood damage, they might cause frequent small inundations to the area, and therefore require a legal interpretation, cultural background, etc. that allows for this.

Tips for users

• It should be recognized that continuous levees (gray infrastructures) can possibly be developed along with a denser population as a result of economic development.
• Proper assessment must be carried out to check if the levees should be connected before the construction.
• New technology is not necessarily superior in terms of cost and efficiency. It is essential to use it in combination with historical methods of control. Topography and land-use information for residential areas can support the decision on how to create a hybrid of green and grey infrastructure.
• It is technically possible to leave gaps in a continuous levee, but it would be difficult in reality.
• A system must be created that can respond flexibly when designing social mechanisms such as laws.

 

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