What a flood potential map is, and is not

"Potential" is often read as "will flood". It actually means: under an assumed rainfall, where and how deep the model says water could reach. Change the assumption and the answer changes.

How it is made

The Water Resources Agency produces the maps under the Flood Potential Data Disclosure Regulations; the third generation was commissioned from universities by each river bureau. The method:

• Design rainfall: not a real day's rain but a total such as "350 mm in 24 hours" distributed hour by hour with a hyetograph (e.g. Horner's formula).
• Terrain and land: digital elevation from 2010–2012, land-use survey from 2008, surveyed river and drain cross-sections, estimates from imagery and DEM where unsurveyed.
• Hydraulic model: e.g. SOBEK, simulating drains, rivers and overland flow.
• Assumptions: all flood-control and drainage works function normally, gates follow their rules, buildings do not block water, coastal boundary uses average July–October tide levels.
• Output: the maximum simulated depth per cell, in five classes: 0.3–0.5, 0.5–1, 1–2, 2–3, >3 m. Below 0.3 m is not drawn.

What it can answer

• Under the same 350 mm, which places flood first and how deep — relative elevation and drainage bottlenecks.
• How the depth at one spot jumps as rainfall rises from 200 to 650 mm — the point of this site's "tap once, see ten scenarios".
• For civil defence: where to evacuate first, where to put pumps.

What it cannot answer

• Will it flood tomorrow — that is weather forecasting plus live water levels; see the WRA disaster site or county flood warnings.
• Will this house flood — cells are 40 m, buildings, basements, thresholds and the blocked drain at your lane are not modelled.
• Did it really flood in some year — potential is a simulation; real records are the WRA historical flooding data and county disaster reports, not included here.
• Prices, insurance, whether to buy — the regulation says the data may not be a basis for land-use control or development restriction; this site does not extrapolate.

Why sources disagree for the same place

Second-generation, third-generation and county-made maps use different terrain years, models and assumptions. This site uses only the WRA's national third-generation release of August 2022 and states the version on every page. The WRA reviews the maps every five years; a new edition will replace this one.

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