Along the Tama River Why is a new construction in the “flooded area”… Thorough analysis December 3, 19:51
Tokyo and Kanagawa along the Tama River have been flooded by Typhoon No. 19. A problem unique to the Tokyo metropolitan area where the population is concentrated, such as flooding of evacuation centers, occurred. I was curious about what was happening during the interview. “The hazard map is red, but there are many new houses and apartments…”. Why are houses built on risky land? I tried to find the cause by using the collected data as a clue.
(Social Department reporter Keijiro Saito Kiyoto Imamura)
Thorough analysis of population data along the Tama River
Tokyo and Kanagawa Prefecture, where the population continues to grow. Is the population increasing even in areas where there is a risk of inundation?
First, we collected open data for 23 cities and wards along the Tama River.
Collected are the population data of the Basic Resident Register for each “town chome” and “the largest possible flooding assumption” in the country that is the basis of the flood hazard map.
We visualized with computer GIS = geographic information system, and divided the “machichome” area into the flooded area and other areas, and examined the increase and decrease of the population from 5 years ago.
The data of “Machichome” that we were able to compare are 2354 districts! It is a steady work to check the number one by one while facing the map. “What kind of result will come out…” The analysis proceeded with half-trust.
Population increased even in high-risk areas
The result was more than I expected.
The 908 districts of 23 cities and wards include some of the “flooded areas” at risk of flooding. Of these, the population was increasing in 622 districts, 69% or 2/3.
In addition, we will analyze in detail the “regions where the risk of flooding is particularly high”. There are 464 districts that contain a “flooded area” of 3 meters or more that reaches the roof of the first floor of the house.
Among them, the population increased in 316 districts, which is 68%.
What was particularly surprising was that the “region including the flooded area” had a higher population growth rate than the “region not including it”.
“Excluded area” is 3%. On the other hand, the population growth rate in the “including area” for 5 years was 4.2%.
The following table is a list of areas where the population has increased by more than 10% in areas that contain “flooded areas” of 3 meters or more.
Not only in the 23 wards of Tokyo near the city center, but also in the Tama area of Tokyo and Kanagawa Prefecture.
If you look closely at the map and data, it seems that there are many convenient places to live in areas where the population has increased, such as railroads nearby and many parks.
However, data alone does not tell why the population is growing in high-risk locations. So I actually visited and walked around the site.
When visiting the area flooded by Typhoon No.19
We headed to Mitsuguchi 6-chome, Takatsu-ku, Kawasaki City. In this area, the heavy rain caused by Typhoon No. 19 flooded the Hirase River, a tributary of the Tama River, causing massive inundation and the death of a man on the first floor of the apartment.
This convenient area is close to Futakoshinchi Station on the Tokyu Denentoshi Line. The hazard map assumed a flood of up to 7 meters, but the population has increased by 17% over the past five years.
If you walk around the area, you will notice brand new houses and condominiums. In the immediate vicinity of the condominium where the man died, construction of a new house was still underway.
Why is the population increasing now?
When I talked to a man who lived near the district for more than 40 years, the district was flooded frequently and the fields spread before the rivers were improved.
However, it has been said that over the last 10 years, farmland has changed to residential land one after another, and many people have moved to live there.
(Local man)
“In the past, it was not only a place where people could live, but only fields and factories. However, recently there was no major flooding, and the memory of flooding has faded away. So even better, the new migrants will not know. ''
In an area where the population increased by 10% in 5 years
There are places that left a lasting impression while visiting the population growth area.
Ishida 1-chome, Hino City, Tokyo. The population has increased by 10% over the last five years. Close to the Tama Monorail and Keio Line stations, walking through the residential area lined up detached houses that seemed to be built for sale.
I was worried that there was a garbage incineration facility in the district. “I wonder if the population is increasing because the land is cheap if it is close to a garbage incineration facility?” I asked Mr. Hisao Utsuko, a real estate appraiser who is familiar with this area.
However, Mr. Utsuko says that the garbage incineration facility has nothing to do with population growth. In the past, it has been regarded as a “nuisance facility”, but now it is no longer an obstacle to choosing a house due to improvements in processing technology.
(Tsuko)
“The former“ nuisance facility ”is no longer a“ nuisance facility. ”The“ decision risk ”is more important than the“ nuisance facility ”. is"
“Aged people going down the mountain”
In addition, Mr. Utsuko taught me something interesting. The number of “elderly people going down the mountain” is increasing.
Mammoth housing complex such as Tama New Town developed in the mountains during the period of high economic growth. The elderly who lived there said that there was a movement to avoid the land with many slopes and move to a low land along the river, which is relatively flat and has little burden on the body.
Why is residential land development progressing in the flooded area?
As a result of interviews, it is said that even in areas where there was a high risk of inundation and was once avoided as residential land, development is progressing because it is convenient and the population is increasing.
I asked Yasunori Tsuji, an associate professor at the Graduate School of Yamanashi, who is studying the relationship between disaster prevention and urban development. Why is residential land development progressing?
(Associate Professor Yasunori Tsuji)
“Because the residents, the development side, and the local government have their merits. The residents can buy convenient land. The development side also makes money because there is a demand for housing. The local government increases tax revenue as the number of residents increases. Is a delicious story. ''
Measures for flooded areas What should I do?
Then what should I do? Associate professor listed two necessary measures in the future.
One is the regulation of residential land development and building construction.
“In areas where there is a high risk of inundation, for example, construction is limited to houses with two or more floors, and in condominiums, the first floor is used as a parking lot, etc. Is a combination of regulations. "
The second is to inform the residents.
In areas designated as “earth and sand disaster caution areas” such as along the mountain, there is no obligation for the risk of flooding, whereas legally there is an obligation to explain in real estate transactions. It is said that the risk of floods should be actively reported. .
“Development of residential land in flooded areas will bring future costs”
He pointed out that the development of land at risk of inundation leads not only to life risks but also to future costs.
(Associate Professor Yasunori Tsuji)
“Randomly developing residential land will require restoration work every time a disaster occurs. In times when disasters occur frequently, it may become a“ costly district ”that presses the local government's finances. `` I think it is necessary to rethink the way it is developed because the population will decline and tax revenues are expected to decline. ''
Is “disaster vulnerable” increasing in the risk area?
This time's interview that analyzed the area at risk of inundation from the data and the site. What impressed me was the appearance of a newly-built child-rearing household and the “elderly person going down the mountain” pointed out by real estate appraiser.
Perhaps the number of “disaster vulnerable” children and elderly people is increasing in lands that are ironically vulnerable to disasters. I would like to continue to report on the types of people who are increasing in areas with high risk of disaster.