Last year's Typhoon No. 19 In some areas including the inundated "old river channel" along the Tama River

Last year's Typhoon No. 19 The area including the inundation "old river channel" along the Tama River was also on October 19 at 9:40.

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Last year's Typhoon No. 19 caused a series of inundation damage in residential areas along the Tama River in Tokyo and Kanagawa, such as the inability to drain rainwater and the overflow of tributaries, but as a result of expert investigation, a wide range It was found that most of the flooded areas in the area contained a terrain called the "old river channel", which indicates that the river once flowed.

Experts point out that it is easy for water to collect due to a slight difference in elevation, so we would like you to pay attention to the formation of the land in order to help you evacuate quickly.

"Old river channel" in 13 of 15 areas where flood damage was confirmed

Last year's Typhoon No. 19 did not break the embankment in the Tama River, but the rainwater could not be drained completely and the tributaries overflowed, resulting in a series of inundation damage.

Associate Professor Jiro Komori of Teikyo Heisei University, who specializes in geography, explained the damage situation and the origin of the land in 15 areas such as Kawasaki City and Setagaya Ward, Tokyo, where flood damage was confirmed in a wide area along the lower reaches of the Tama River. I investigated the relationship between.

As a result, 13 areas contained a terrain called the "old river channel," which indicates that the river once flowed.

According to Associate Professor Komori, the "old river channel" is often lower than the surrounding area, so it is easy for water to collect and there is a high risk of early inundation.

Of these, in the residential area of ​​Nakahara Ward in Kawasaki City, inundation damage was concentrated in a specific area, but most of them coincided with the range of the "old river channel" and were about 1 to 2 meters lower than the surrounding area. is.

"Old river channel" in and around JR Musashi-Kosugi Station

In addition, the "old river channel" was also included in and around JR Musashi-Kosugi Station, where rainwater could not be drained completely.

Inundation has also been confirmed at a location about 800 meters inland from the Tamagawa River.

Associate Professor Komori said, "In areas where urbanization is progressing, it is difficult to see the slight difference in topography on the old river channel. The effects of inundation may start early, so it is necessary to consider evacuation routes. Yes. In addition to the hazard map, I want you to find out what kind of terrain you are in. "

Did the embankment deepen behind the old river channel?

A survey by Associate Professor Komori also found places where the damage caused by flooding could have been more serious because it was along the old river channel.

In Takatsu Ward, Kawasaki City, a tributary of the Tama River, the Hirase River overflowed due to the "backwater phenomenon" in which water could not flow and flowed back, and the area where it merged with the Tama River was flooded.

The condominium was flooded up to 2 meters, killing a man who lived on the ground floor.

According to Associate Professor Komori, the area around the condominium is an old river channel, and the inundation may have become more serious due to the soil embankment behind the building.

This embankment in a residential area is based on the accumulation of soil due to the flow of the river, and is still used as a "haze embankment" to stop the inundation of the lower reaches of the Tama River.

Associate Professor Komori points out that the embankment was behind the building, so there was no escape for the water that had flowed into the area of ​​the old river channel, and the inundation may have been deeper.

Search for old river channels with "hydraulic topography classification map"

According to Associate Professor Komori, since the Edo period, the Tama River has been under construction to straighten the flow of the river and to create an irrigation canal. Is believed to have been a river.

Hazard maps created by local governments assuming inundation are created based on land height difference data, and many of the old river channels are assumed to be inundated.

However, it is color-coded according to the maximum depth that can be flooded, so it is not possible to know the exact location of the old river channel or how low it is.

You can find out where the old river channel is by looking at the "Hydraulic Topography Classification Map".

It is made by the Geospatial Information Authority of Japan for first-class rivers managed by the government. After opening the website called "Geographical Survey Map", from the "Map" mark on the upper left, "Land Formation / Land Use" If you select "Hydraulic Topography Classification Map" (updated version) in, it will be displayed on the map.

The place where the blue line is on the white background is the "old river channel".

In this map, in addition to the "old river channel", ▽ "back swamp" where mud is easily contained due to the accumulated land and may be submerged for a long period of time, and ▽ from upstream due to past floods The plains created by the accumulation of earth and sand show various disaster-risk terrains such as the "flooding plains" where there is a risk of flooding again.

You can see the undulations of the land with the "shadow undulation map"

Also, to know the detailed undulations of the current land, you can refer to the "Shadow undulation map" that can be selected from the same Geographical Survey Institute map.

Since the undulations of the land are highlighted, you can visually understand how low it is.

It can also be displayed by superimposing it on a hydraulic topography classification map.

The Geospatial Information Authority of Japan is calling for the use of hazard maps as a stepping stone to learn about the characteristics of the surrounding topography and use it for disaster prevention.