What the flood inundation area ratio means
The number Settle Map labels "flood" is the area expected to be inundated under a worst-case (L2) rainfall, divided by the total area of the municipality. It is computed by clipping the raster tiles of the Hazard Map Portal to the administrative boundaries of the National Land Numerical Information dataset. The unit is area — not damage, not water depth.
Across 1892 municipalities the median is 7.5%, and 43 municipalities score 90.0% or more, meaning nearly the whole municipality lies inside the expected inundation area. The highest is Saitama / 蕨市 (100.0%), while 178 municipalities sit at 0.0%. Towns whose built-up area fills a floodplain score high; towns that include large mountains score low.
| Municipality | Prefecture | Flood | Land price | Quake |
|---|---|---|---|---|
| 渋谷区 | Tokyo | 21.8% | 1,595,000 JPY/m² | 50.7% |
| 目黒区 | Tokyo | 23.5% | 1,095,000 JPY/m² | 48.5% |
| 文京区 | Tokyo | 24.6% | 1,210,000 JPY/m² | 64.7% |
| 豊島区 | Tokyo | 25.1% | 758,000 JPY/m² | 39.1% |
| 練馬区 | Tokyo | 26.4% | 451,500 JPY/m² | 44.0% |
| 港区 | Tokyo | 27.3% | 2,320,000 JPY/m² | 44.6% |
| 新宿区 | Tokyo | 28.4% | 954,000 JPY/m² | 49.5% |
| 杉並区 | Tokyo | 29.7% | 640,000 JPY/m² | 60.9% |
| 世田谷区 | Tokyo | 31.8% | 738,000 JPY/m² | 44.5% |
| 品川区 | Tokyo | 34.2% | 928,000 JPY/m² | 53.1% |
| 中野区 | Tokyo | 35.9% | 700,000 JPY/m² | 53.4% |
| 千代田区 | Tokyo | 41.8% | 3,680,000 JPY/m² | 54.4% |
| 板橋区 | Tokyo | 43.1% | 492,000 JPY/m² | 54.3% |
| 大田区 | Tokyo | 45.2% | 575,000 JPY/m² | 63.7% |
| 江東区 | Tokyo | 52.4% | 560,000 JPY/m² | 78.1% |
| 北区 | Tokyo | 58.8% | 566,000 JPY/m² | 63.2% |
| 中央区 | Tokyo | 65.2% | 1,685,000 JPY/m² | 50.5% |
| 江戸川区 | Tokyo | 78.1% | 419,000 JPY/m² | 89.5% |
| 台東区 | Tokyo | 80.2% | 996,500 JPY/m² | 48.7% |
| 葛飾区 | Tokyo | 90.3% | 368,000 JPY/m² | 77.1% |
| 足立区 | Tokyo | 92.6% | 328,000 JPY/m² | 84.0% |
| 墨田区 | Tokyo | 92.7% | 591,000 JPY/m² | 57.0% |
| 荒川区 | Tokyo | 94.5% | 546,500 JPY/m² | 80.4% |
How to read this table
The table lists the 23 wards of Tokyo from the lowest ratio. The lowest is Tokyo / 渋谷区 (21.8%) and the highest is Tokyo / 荒川区 (94.5%). Wards on the terrace and wards on the lowland between the Arakawa and the Sumida separate cleanly. For flooding, "the 23 wards of Tokyo" is not one place.
The ratio ignores where people actually live. A municipality can be mostly inundation area while most of its houses stand on high ground, and a municipality with a low ratio can have its entire station district inside the area. The ratio describes the shape of the land, not the risk at an address.
Once a municipality is on your shortlist, check the address itself on the Hazard Map Portal's overlay map, with the river flood, inland flood and landslide layers switched on. Each municipality page links to the portal centred on that municipality.
What the numbers do not tell you
Inundation areas are published only for rivers designated by their river administrator. Small undesignated rivers and sewer backflow are not counted, so a low ratio does not mean the ground stays dry.
The aggregation reads tiles at zoom 12 (about 31 m per pixel) and decides membership at the pixel centre, so thin areas along a boundary shift the result by a few per cent.
The worst-case (L2) scenario assumes rainfall far heavier than the planning-scale (L1) scenario. Being inside the L2 area does not mean flooding to that depth every year.