The 100 year flood

Hydrologists know that the “100 year flood” is a shorthand term for a flood with an annual exceedance probability of 1% and an average recurrence interval of 100 years.   Talking about a “100 year flood” flood can be confusing for people because, not unreasonably, they imagine we are saying that floods like this can only occur about every 100-years.  Check out these community comments following a flood in Christchurch, New Zealand.   And, it’s not just New Zealanders; following flooding in Dec 2015 the Mayor of Keswick, UK was reported as saying :”The flood defences were designed for a one in 100 year event and since it’s six years since we had the last one, we were sort of surprised that we got one so soon”.

The current recommendation from Engineers Australia is to refer to floods in terms of annual exceedance probability e.g. 1% flood, or for smaller events as a number of exceedances per year e.g. 4 exceedances per year and not the ‘3 month flood’ was we might once of said.  The Australian Committee on Large Dams (ANCOLD) prefers using ‘1 in Y years’ when referring to annual exceedance probability e.g. the ‘1 in 100 year’ flood.

Acceptable risk

The water level associated with the 1% flood, along with 300 mm of freeboard, is commonly used to determine minimum floor heights and is assumed, by planners and engineers, to represent an acceptable risk to residents.  See for example,  section 802 7(b) of the Victorian Building Regulations and the Victorian Water Act (Section 204).

When people are actually asked what they regard as an acceptable flood risk, the answer is much more conservative than a flood with an average recurrence interval of 100 years.  Molino Stewart undertook an Internet survey to gauge perceptions of acceptable risk and found that most people considered their houses should never flood.  Less than 1% of people thought it was acceptable if their house should flood more than once in a lifetime.

Acceptable flood risk

Figure 1. Molion, S. and Karwaju (2012). Do we understand what is an acceptable flood risk? The people of Australia have their say. Floodplain Management Association 2012 Conference.

I have converted some of the probability statements from Figure 1 to equivalent average recurrence intervals in the following table.  These imply that people expect greater levels of protection than is afforded by the use of the 1% (100-year) flood as a design event.

Probability statement Interpretation Average recurrence interval
50/50 chance of it occurring in my lifetime 50% probability of at least one flood in 80-years 115
1 in 6 chance of it occurring in my lifetime 1/6 chance of at least one flood in 80- years 439
1 in 100 chance of it occurring in my lifetime 1% probability of at least one flood in 80-years 7960

Floods v Earthquakes

Earthquake engineers need to consider rare events but use different terminology.  According to a recent article in Engineers, Australia magazine, the design earthquake used in Australia has a 10% chance of being exceeded in 50 years while in the US the standard is to consider an earthquake with a 2% chance of exceedance in 50 years.

We can calculate the equivalent annual exceedance probability.  For example, in the Australian case:

1-(1-P)50 = 0.1

Where P is the annual exceedance probability.

This implies P = 0.002105, or an average recurrence interval of 475 years.

For the US case, P = 0.000404, or an average recurrence interval of 2,475 years


Preventing, preparing, responding, recovering; emergency management requires all of these. Using the 1% flood level for planning should help prevent problems but there is evidence that our emergency management is out of balance.  For example, the Australian government spent $180 million on disaster mitigation over the past 6 years and at least $6.7 billion on disaster recovery (PC, 2012, p21).

Seems we are not spending enough on prevention and part of the problem is the routine use of the 1% flood to indicate an acceptable risk.

Further reading

  • Molino, S., Roso, S. and Hadzilacos, G. (2010) How much risk should we take? Developing a framework for holistic risk based floodplain planning.  Paper
  • Molino, S. Morrison, T. and Roso, S. (2012).  Application of the holistic risk based framework in floodplain planning. Paper
  • Reducing flood losses: is the 1% chance (100-year) flood standard sufficient?  Background reading for the Gilbert F. White National Flood Policy Forum 2004 Assembly.  National Academies Keck Centre. Washington D. C. September 21-22, 2004. Report
  • Wenger, C., Hussey, K. and Pittock (2013) Living with floods: key lessons from Australia and abroad, National Climate Change Adaptation Research Facility, Gold Coast. Report.
  • Recommendations for a national levee safety program. A report to congress from the national committee on levee safety.  There is a good summary of the history of the use of 100-year flood on pages 9-11. Report.
  • Risk Analysis and uncertainty in flood damage reduction studies.  The National Academies Press. The pdf can be downloaded for free.
  • Henrich, L. et al. (2015) Effects of risk framing on earthquake risk perception: life-time frequencies enhance recognition of the risk.  International Journal of Disaster Risk Reduction 13:145-150. (abstract)


PC (2012) Barriers to effective climate change adaptation.  Productivity Commission Inquiry Report No. 59, 19 September 2012 (report)

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