Daily Stack 20250706 - Flood Risk Update

A Comment on the Tragic Kerr County Flooding over the July 4 Weekend

We, at Bits & Bytes, are heartbroken learning of the devastation and loss of life caused by the catastrophic flooding in Kerr County Texas over the July 4 weekend. It is especially painful when a kids’ summer camp has been utterly destroyed. What should have been a celebratory weekend turned into a hellscape.

ABC News provides a timeline of the tragedy:

https://abcnews.go.com/US/timeline-catastrophic-flooding-texas-claims-lives-27/story?id=123500139

It is with great sadness that as our climate changes, caused by the increasing greenhouse gases in our atmosphere, we will be seeing more human tragedies like this unfold with greater frequency and intensity in the coming years.

To illustrate the changing frequency and intensity of flooding over the years caused by climate change, we present an analysis of the extreme rainfalls recorded at Austin TX. Since Austin is just east of Kerr County, this data can be used as a proxy to get an idea of the extreme rainfall experienced by those affected in the County.

In the graph below, it shows the extreme rainfalls in millimetres (mm) per day, recorded over two 21-year periods: 1950 to 1971 (blue curve) and the more recent 2000 to 2021 (orange curve). There is a 29 year gap between the two periods. The x-axis is the Return Period in years. The return period can be thought of as the average time between events of a certain magnitude.

In the past 1950 to 1971, a 207 mm / day (8.2 inches per day) might happen at least once in 100 years. However, as our climate has changed, based on the recent data of 2000 to 2021, that amount of rainfall now can happen at least once in 13 years (see the short red line). This is nearly an 8 times increase in the frequency compared to the 1950 to 1971 period.

It was reported in the media that a catastrophic 254 mm (10 inches) of rain fell over a few hours in the county with daily totals likely much higher. This amount of rainfall would be equivalent to a 1 in 500 year event based on the 1950 to 1971 data, but right now it can happen at least once in 30 years (see the longer red line). This is nearly a 17 times increase in the frequency.

The analysis we present compares the change in rainfall intensity and their return periods over a 29 year gap. If the change remains the same, then that 254 mm (10 inches) of rainfall has a chance to increase in frequency by 17 times so that in 29 years time, it can happen once every 2-years. In the intervening years, we can expect rainfalls and therefore floods of that catastrophe to happen more and more frequently.

What Causes such Intense Rainfall?

As our planet warms with ever increasing greenhouse gases being emitted into the atmosphere from fossil fuels, the air has the ability to hold more and more water. And what goes up must come down. So the hotter our days get the worse rain we will have.

What’s Next?

In upcoming posts, we will explore more into thunderstorms and extreme rainfall, and heatwaves.

If you want a read of our previous post on flooding to get additional context:

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