[VOL.28] Unexpected Inundation Risks Due to Increasing Heavy Rain | 3 Preparations Taught by an Architect
1. Is a "once-in-50-years" event happening every year? An era where the unexpected becomes expected
1-1. 2024 Tohoku Case Study: 1,521 homes suffered flood damage
A "once-in-50-years" heavy rain event is now becoming a "once-in-10-years" event.
In July 2024, heavy rains centered in the Tohoku region caused 1,521 homes to suffer above-floor and below-floor flooding.
of the victims have said, "I never thought it would happen to us."
Climate change is no longer a story of the distant future.
Unexpected flooding has already become an "expected" risk.
1-2. The sense of crisis that families considering building a home in Osaka Prefecture should know now
Osaka Prefecture is no exception.
There are vast low-lying areas, and the water retention capacity is decreasing due to urbanization.
As heavy rains increase, the risk of flooding is definitely rising.
As a first-class architect who has been involved in home building in Osaka for over 20 years, I will explain the measures you can take starting now.
From land selection and design to insurance, I will provide specific information from a professional perspective.
2. Why does the risk of heavy rain continue to increase? Scientific evidence of climate change
2-1. A 1°C rise in temperature leads to a 7% increase in water vapor! The reality that heavy rain in the Kinki region will increase by 1.9 times
Due to climate change, the risk of heavy rain is definitely increasing.
Scientifically known as the "Clausius-Clapeyron relation (the scientific relationship between temperature rise and water vapor amount)," it can be explained simply as follows:
When the temperature rises by 1°C, the amount of water vapor contained in the atmosphere increases by about 7%.
The more water vapor there is, the more rain falls.
In fact, in the Kinki region, heavy rain with hourly precipitation of 50mm or more is predicted to increase by about
1.9 times compared to the 1990s.
(2°C rise scenario, Osaka District Meteorological Observatory).
2-2. Linear precipitation zones are increasing. It can no longer be dismissed as "luck"
Furthermore, what has been attracting attention in recent years is the "linear precipitation zone (a phenomenon where cumulonimbus clouds form in a line and continue to drop heavy rain in the same place)."
According to Japan Meteorological Agency data, the frequency of linear precipitation zones is on an upward trend, increasing by about 1.4 times.
to increase.
The terror of linear precipitation zones is that a large amount of rain falls locally in a short period of time.
Predictions are difficult, and it cannot be dismissed as "I just happened to be there."
2-3. Osaka's low-lying risk: The reality that flood risks exist over a wide area
Osaka Prefecture has vast areas that are at or below sea level.
In low-lying areas, the risk of not only river flooding but also backflow from sewers (inland flooding) increases.
Furthermore, due to urbanization, rainwater does not soak into the ground and continues to flow into the sewers all at once.
The decrease in water retention capacity is further increasing the risk of flooding.
3. The limits of hazard maps and 3 reasons for unexpected flood risks
Hazard maps are a necessary tool.
However, overconfidence is forbidden. As a professional, I will explain 3 limits you should know.
3-1. Reason 1: The update cycle is 5-10 years. Information freshness cannot keep up with abnormal weather
The update cycle for hazard maps is 5-10 years on average.
In other words, "safe" or "dangerous" is judged based on data from 10 years ago.
However, the speed of climate change is fast, and the frequency and scale of heavy rain have changed significantly in the last 10 years.
If the information is old, the risk assessment will also be lenient.
Even if it is marked as "safe" on a hazard map, it does not necessarily accurately reflect current risks.
3-2. Reason 2: Assumed rainfall is based on "past maximum" standards, underestimating future risks
The assumed rainfall for hazard maps is based on the "maximum rainfall observed in the past."
However, heavy rain is on an upward trend due to climate change.
"Past maximum" is no longer "future maximum."
Experts at the Japan Meteorological Agency also point out that "updates reflecting future maximum rainfall are essential."
Current hazard maps are highly likely to be underestimating future extreme weather.
3-3. Reason 3: Damage is actually occurring (Tohoku case study)
In the 2024 Tohoku heavy rain mentioned at the beginning, 1,521 homes were damaged.
Hazard maps are a "minimum guideline."
Whether inside or outside the range, the risk of flooding exists.
[Table 1: 3 Limits of Hazard Maps]
| Item | Current Status | Problems |
| Update Cycle | 5-10 years | Cannot keep up with the speed of climate change |
| Assumed Rainfall | Past maximum standard | Underestimates future extreme weather |
| Actual Damage | 2024 1,521 homes | No guarantee of "safety" |
4. 3 patterns where unexpected flooding occurs [From the actual experience of a first-class architect]
Even if it is "unexpected" by the hazard map, flooding occurs.
Based on my experience seeing many sites as a first-class architect, I will explain 3 patterns that require special attention.
4-1. Pattern 1: Inland flooding (backflow where sewage/drainage treatment cannot keep up)
**Inland flooding**
refers to backflow where sewage and drainage facilities cannot handle the volume, rather than river flooding.
This is a phenomenon where excess rainwater flows backward from manholes and toilets.
Osaka Prefecture has many low-lying areas and is a region with a high risk of inland flooding.
In the heavy rains that occurred in the Kyushu region in August 2023, drainage processing could not keep up, and about 800 buildings
were damaged. The same thing can happen in Osaka.
Especially in urbanized areas, rainwater does not soak into the ground due to road paving and an increase in buildings,
and it flows into the sewer system all at once.
When this exceeds the processing capacity, backflow occurs.
In low-lying areas within Osaka Prefecture, this risk may increase even further in the future.
4-2. Pattern 2: Changes in "Water Paths" Due to Surrounding Development (Risks in Emerging Residential Areas)
What is often overlooked is the increase in inundation risk due to "surrounding residential development."
Even if your own land is on high ground, if residential development progresses in the surrounding area, the flow of rainwater will change.
When places that were farmland or forests become residential areas, rainwater no longer soaks into the ground,
and it flows into low-lying areas all at once.
*Runoff volume increases 2-3 times due to conversion from farmland to residential land
According to data from the Ministry of Land, Infrastructure, Transport and Tourism, rainwater runoff is said to increase by 2 to 3 times due to residential development.
Even for land that had no problems at the time of purchase, there are cases where the risk of inundation increases due to surrounding development a few years later.
As a real estate notary, I recommend that you always check surrounding development plans when purchasing land.
4-3. Pattern 3: Localized Torrential Rain (Occurs Even Outside Hazard Map Ranges)
Localized torrential rains, such as linear precipitation zones, can exceed the assumptions of hazard maps.
The assumption that "this place is safe" can also lead to delays in evacuation.
From my experience as a first-class architect who has seen sites, I can say that "unexpected things happen."
That is why countermeasures are important.
5. [With Costs] 3-Step Countermeasures Taught by an Architect to "Prepare for the Unexpected"
Inundation risk is a problem that "can be prevented if you take measures." From the multifaceted perspective of a first-class architect, real estate notary, and FP2, I will propose the best measures for each stage of home building.
5-1. Step 1: Land Diagnosis Before Purchase (3 Types of Hazard Maps + Elevation Check)
**The land selection stage is the most important.**
I have conducted land diagnoses for over 100 properties as a real estate notary, but 80% of "cheap land"
has some kind of inundation risk.
Before purchasing, please be sure to check the following 5 items.
[Checklist 1: Inundation Risk Diagnosis Before Land Purchase (5 Items)]
□ Check 3 types of hazard maps (flood, storm surge, inland water)
□ Measure elevation/ground height (the registry alone is insufficient)
□ Check past topography with old maps/aerial photos (be careful with former rice fields/ponds)
□ Check surrounding development plans at the city hall (disclosure requests are possible)
□ Check the capacity of drainage facilities (inquire with the municipality's sewer department)
**Requesting a land diagnosis from an expert costs about 30,000 to 50,000 yen, but this will prevent a future 3 million yen
loss.
** As a real estate notary, I consider this investment essential.
5-2. Step 2: Water-Resistant Design for New Construction (Foundation Raising and Equipment Placement Ingenuity)
For new construction, measures at the design stage are the most cost-effective.
Foundation Raising (Usually 40cm → Recommended 60cm)
As a first-class architect, what I recommend most is "foundation raising."
The normal foundation height is about 40cm, but in areas with high inundation risk, I recommend raising it to 60cm or more.
**Cost**: 2 to 3 million yen (about 3 to 5 million yen including foundation work)
**Effect**: Can prevent inundation up to 60cm
You may feel it is "expensive," but compared to the damage repair costs mentioned later (3 to 5 million yen
scale), it is reasonable as an investment.
Ingenuity in Equipment Placement
Place water heaters and air conditioner outdoor units on the second floor or in a high place.
Also, install the electrical panel at a position 50cm or more above the assumed inundation height.
In actual cases, even if floor flooding occurred, there were cases where "electricity and hot water supply were safe," shortening restoration by 3 days.
5-3. Step 3: Strengthening Existing Housing and Reviewing Insurance
(Water stop boards, backflow prevention valves, subsidies)
For those who already live there, measures can be taken retroactively.
Retrofit-able measures
**Water stop board installation (entrance/service entrance)**
Cost: 100,000 to 300,000 yen
Construction period: 1 to 3 days
Effect: Prevents inundation from the entrance/service entrance
**Backflow prevention valve installation (toilet/drain)**
Cost: 50,000 to 200,000 yen
Construction period: 1 to 2 days
Effect: Prevents backflow from sewage
Reviewing fire insurance flood coverage (FP2 perspective)
In Kansai, the attachment rate of fire insurance flood coverage tends to be lower than the national average of about 63.0%.
As an FP2, I strongly recommend reviewing your insurance.
**Checkpoints:**
- Is flood coverage included?
- Is it "building only" or "including household goods"?
- Is the coverage amount sufficient?
Insurance premiums vary by region and building age, but in areas with high inundation risk
It is essential.
Utilizing Osaka Prefecture Subsidies
In Osaka Prefecture, there are subsidy systems available through municipalities with caps of approximately 300,000 to 500,000 yen (e.g., Hirakata City: 500,000 yen).
If you are considering installing water stop boards or backflow prevention valves, please make use of these.
For details, please inquire with the disaster prevention department of your respective municipality.
5-4. Comparison of Countermeasure Costs: Buying "Peace of Mind" for 2 to 3 Million Yen
[Table 2: Comparison of Inundation Countermeasure Costs]
| Countermeasure | Estimated Cost | Construction Period | Intended Effect |
| Raising Foundation | 2-3 Million Yen | 10-20 Days | Prevents Inundation up to 60cm |
| Installing Water Stop Boards | 100,000-300,000 Yen | 1-3 Days | Prevents Inundation at Entrances/Service Entrances |
| Backflow Prevention Valve | 50,000-200,000 Yen | 1-2 Days | Prevents Sewage Backflow |
**On the other hand, repair costs for flood damage often reach the 3 to 5 million yen scale.**
Costs for replacing floors and walls, exchanging electrical equipment, replacing water heaters, and even temporary housing are also necessary.
Expenses are also required.
As a 2nd Grade Financial Planner, I assert: these measures are not an "expense" but an "investment."
6. Conclusion: Building a Home You Can Feel Secure in 20 Years from Now
6-1. Three Actions You Can Take Now and Utilizing Osaka Prefecture Subsidies
Inundation risk can be prevented with the right knowledge and countermeasures.
River improvement in Osaka Prefecture is still insufficient. That is precisely why self-help efforts become
important.
[Checklist 2: 3 Inundation Countermeasures You Can Take Now]
□ Check the 3 types of hazard maps (flood, storm surge, inland water)
□ Check and review your fire insurance flood coverage
□ Request a land and insurance diagnosis from a professional
In Osaka Prefecture, there are subsidy systems available through municipalities with caps of approximately 300,000 to 500,000 yen.
If you are considering installing water stop boards or backflow prevention valves, please make use of these.
6-2. Information on Free Land and Insurance Diagnosis by a First-Class Architect, Real Estate Notary, and 2nd Grade Financial Planner
"Is my land safe?" "Which measure should I start with?"
For those who have such questions, we offer**free land risk diagnosis and insurance diagnosis**services.
As a first-class architect, I provide support from the perspective of structure and design; as a real estate notary, from the legal perspective of land transactions;
and as a 2nd Grade Financial Planner, from the perspective of financial planning and insurance, providing**total support**.
Let's think together about building a home where you can live with peace of mind even 20 years from now.
**[References and Data Sources]**1. Japan Meteorological Agency "Climate Change in Japan 2025"
2. Osaka District Meteorological Observatory "Materials on Climate Change in Osaka Prefecture"
3. Ministry of Land, Infrastructure, Transport and Tourism "Flood Damage Report 2024"
4. Ministry of Land, Infrastructure, Transport and Tourism "Building Inundation Countermeasure Guidelines"
5. Ministry of Land, Infrastructure, Transport and Tourism "Hazard Map Portal Site"
6. General Insurance Rating Organization of Japan "Fire Insurance Flood Coverage Attachment Rate (2023)"
7. Osaka Prefecture "Medical Institution Inundation Countermeasure Project Subsidy" Related Materials
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