1% AEP explained (NSW flood planning)
This is a definitional page that ranks well and feeds internal links to FPL/freeboard pages.
What 1% AEP means (often called the “1 in 100” flood) and why it’s used in NSW flood planning and Flood Planning Levels.
Quick answer
1% AEP (Annual Exceedance Probability) means there is a 1% chance of a flood of that size (or larger) happening in any given year. It does not mean floods happen only once every 100 years.
Why it’s used in planning
1% AEP is a common reference level used in flood studies and planning controls. Councils often build Flood Planning Level (FPL) from a flood level (like 1% AEP) plus freeboard.
Common myth to avoid
The “1 in 100” label can mislead people into thinking it won’t happen again soon. Multiple 1% AEP floods can occur in short periods.
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FAQs
Is 1% AEP the same as ‘1 in 100 year’?
They’re commonly used interchangeably in casual language, but the safer explanation is the probability statement: 1% chance in any year.
Do councils always use 1% AEP?
Often, but councils may use different levels for different purposes. Always check local controls.
Official references
These links help you verify details with primary sources. (We don’t control third‑party sites.)
| Source | Link |
|---|---|
| Hornsby YourSay — Flood definitions (AEP explanation example) | Open |
| Environment NSW — Flood risk management manual page | Open |