Meet Urgent Need for Infrastructure to Fight ‘Floods from Inland Waters’

Torrential rains that strike in a short period of time have caused large amounts of water to overflow and flood local urban areas one after another. The central and local governments must urgently take measures to increase rainwater treatment capacity.

In central Akita City, which was hit by torrential rains this month, houses were flooded over a wide range of areas. The damage was reportedly exacerbated by a combination of the Taihei River overflowing in the city and what is termed a “flood from inland waters,” meaning that rainwater flowing into sewer systems exceeds their drainage capacity, backs up and gushes out of the ground.

Floods from inland waters are more likely to occur in urban areas, where the amount of forests and farmland with natural drainage functions has decreased due to the development of residential land and the paving of roads. Because of the high concentration of populations there, the impact on the lives of citizens is significant.

Typhoon No. 19 in 2019 caused floods from inland waters in Tokyo and 14 other prefectures. In recent years, due partly to the occurrence of linear precipitation bands that bring heavy rainfall in a short period of time, significant damage has been caused even in local cities with relatively small populations.

To mitigate the damage, it is essential to enhance the treatment capacity of rainwater. Along with sewerage facilities, it is necessary to develop facilities for temporary storage of rainwater and prepare drainage pumps, among other steps.

The central government has asked municipalities nationwide to develop such facilities based on the assumption of a heavy rainfall of the size that occurs once every five years. However, due to the time and expense required, only 60% of these plans have been achieved.

Three years ago, a facility capable of storing 4,000 tons of rainwater was completed underground at JR Shibuya Station in Tokyo. But, it cannot be said to be realistic to expect such large-scale projects in local cities with shrinking populations and weak fiscal foundations.

Local governments need to consider measures to minimize damage according to the size of the urban areas.

One idea would be for the central and local governments to support private companies when constructing commercial buildings to help them take measures such as the construction of small-scale underground rainwater storage facilities.

It will also be effective to lower parts of school grounds below the surrounding land surface so that a certain amount of rainwater can be stored there. Efforts should be made to make effective use of local resources, through such measures as promoting the use of “rice paddy dams” to temporarily store rainwater in rice paddies.

Flooding from inland waters can cause water levels to rise suddenly, even in areas far from rivers. Residents need to confirm the expected inundation zones and evacuation sites in advance.

There is fear that people in flooded areas could fall down manholes whose covers have been pushed off by a backflow of sewage. For those who go out in cars, there is a possibility that water may enter the engine or exhaust port, causing the vehicle to stall, after which the entire car may be swept away.

If your surroundings are flooded, it is absolutely necessary to thoroughly protect your life by such steps as refraining from going out for nonessential purposes.

(From The Yomiuri Shimbun, July 28, 2023)