‘100-year flood’: explained

Weather

After a heavy rain event and/or a significant tropical system, you often here the terms ‘100-year flood’ or ‘500 year-flood’ associated with significant flooding. But what does that mean? And do these floods really happen every 100 or 500 years?

What is a ‘100-year flood’?

When talking about weather (and most other sciences), statistics are often used to describe the frequency or significance of an event. For example, we use percentages when expressing rain chances.

Scientists talk about the probability of floods in ‘recurrence intervals’.

As defined by the USGS, a recurrence interval is “the probability that the given event will be equaled or exceeded in any given year”. When applying this to the term ‘100-year flood’, it simply means there is a 1% chance of a given-sized flood to occur in any given year. It does not mean that a flood of a set-size happens every 100 years. It’s actually possible that a location sees multiple 100-year floods in a single year.

Recurrence intervals are location-specific and based on at least 10 years of data. These intervals are analyzed and updated when there are changes related to average rainfall, data records, climate variations, etc.

What is considered a 100-year flood in Austin?

A ‘100-year flood’ in Austin is defined as a rain total of nearly 13″ in a 24-hour period. (This recurrence interval was recently adjusted in 2018.)

‘Atlas 14’ study: an adjustment in flood risks

In September of 2018, the National Weather Service conducted a study called ‘Atlas 14′ with the purpose of redefining flood risks in Austin using a longer historical record. They did this by adding in data up to the year 2017 (previous analysis: up to 1994). This resulted in new recurrence intervals and flood risk guidelines.

Flood risks before Atlas 14 study:
— ’25-year flood’ (4% chance of occurrence in a year): up to 7.6″ in 24 hours
— ‘100-year flood’ (1% chance of occurrence in a year): up to 10.2″ in 24 hours
— ‘500-year flood’ (0.2% chance of occurrence in a year): up to 13.5″ in 24 hours

Flood risks after Atlas 14 study:
— ’25-year flood’ (4% chance of occurrence in a year): up to 9″ in 24 hours
— ‘100-year flood’ (1% chance of occurrence in a year): up to 13″ in 24 hours
— ‘500-year flood’ (0.2% chance of occurrence in a year): up to 19.5″ in 24 hours

What does this mean?

The addition of the data from the last ~20 years shows an increase in frequency of high rainfall-producing events in the Austin area. It now takes up to 13.5″ of rain in 24 hours to define a 100-year flood, not 10.2″ as previously thought. And what was initially thought of as a 500-year flood is actually a 100-year flood, meaning there is a higher chance of occurrence in any given year.

The redefinition also puts a greater area of the city of Austin in a floodplain, an area susceptible or at a higher risk of flooding. This has further implications on future development, re-development or remodeling of properties.

This also can hit your wallet as those who live in designated floodplains pay more for flood insurance…. as there is a higher probability of a significant flood occurring in that area.

To find out if you live in a floodplain, click here.

Copyright 2019 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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