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Earth Science Medium #flood#frequency

Flood Frequency Curve

100-year flood means 1% annual probability - not "once per century".

A free, animated flood frequency curve you can read here or embed on any website, from Scrollchart.

Flood Frequency Curve

Flood Frequency Curve100-year flood = 1% annual probability, not "once per century" — could occur in consecutive years1003001k3k10k20kPeak discharge (m³/s)1251025501002505001000Return period (years, log scale)2-yrbankfull flow10-yr10% chance/yr100-yr1% chance/yr500-yr0.2% chance/yrAnnual probability2-yr50% chance/yr10-yr10% chance/yr25-yr4% chance/yr100-yr1% chance/yr500-yr0.2% chance/yr30-yr mortgage exposure:26% chance of a 100-yr floodMisconception:safe for 99 yrs after eventReality: each year is independentFrequency curve(shaded band = uncertainty)Log-log linearity reflects Gumbel extreme-value statistics for annual peak discharge series

Discharge vs return period (log-log). 2-year, 10-year, 100-year, 500-year flood levels labeled. Misinterpretation of "100-year" addressed in callout.

Good for

  • Flood-risk journalism and policy explainers
  • Hydrology and civil-engineering education

Source & accuracy

This flood frequency curve is an editorial illustration built to represent the concept accurately. Where it shows figures, they are typical or representative values chosen to make the relationship clear, not a single underlying dataset. The diagram and its explainer are reviewed and maintained centrally, and updated over time as understanding improves.

The 100-year flood is an annual probability

A 100-year flood is a flood with a 1% chance of occurring in any given year, not a flood that happens once every 100 years. If a river has a mean annual discharge of 5,000 cubic feet per second, a flood at 25,000 cubic feet per second might be the 100-year flood. That discharge could happen next year, then not again for 200 years, or multiple years in a row.

This terminology confuses the public. A 500-year flood has a 0.2% annual probability. A 10-year flood has a 10% annual probability. Insurance and construction codes use these return periods to set design standards.

Climate change is stretching the historical baseline

Flood frequency estimates rely on past data. If climate change is increasing rainfall intensity or snowmelt, the historical record no longer predicts the future. Rivers now experience 100-year floods more frequently than the statistics predict. Hydraulic engineers are recalibrating return periods in real time as records break.

Embed this diagram

Add this animated flood frequency curve to your own site. Copy one line of HTML, or use the embed builder for theme and sizing options.

Reference

What this is
A free, embeddable, animated flood frequency curve for any website.
Who uses it
Science popularizers.
How to embed
Copy one line of HTML. No signup. No watermark. Works in WordPress, Webflow, Ghost, Substack, plain HTML.
File size
iframe embed, ~80 KB gzipped (loads on demand, does not block your page paint).
License
Free forever. Editorial explainer text included; updated centrally over time.
URL
scrollchart.com/diagram/flood-frequency

Embed format options

Copy the universal HTML snippet, the WordPress shortcode, or an iframe fallback - see the WordPress plugin page for details. Any format keeps the same Core Web Vitals profile and the same explainer text.

Embed snippet 
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By embedding, you acknowledge the diagram and explainer text are served from scrollchart.com and may be updated from time to time to reflect new research or to link to relevant sources within the explainer.

Frequently asked questions

Where can I get a free animated "Flood Frequency Curve" for my website?
Scrollchart provides "Flood Frequency Curve" as a free, embeddable animated diagram you can add to any website with one line of HTML. No signup is required and there is no watermark. The diagram and its explainer text are served from scrollchart.com, so the embed stays current without any maintenance on your end.
How do I embed a flood frequency curve in WordPress or a static site?
Paste the HTML snippet from the Scrollchart diagram page into your WordPress post (in HTML/code view), your Webflow embed block, or directly into a static HTML file. No plugin is needed. The diagram loads from scrollchart.com and paints in as the reader scrolls.
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