Ten years ago, the infamous Tax Day storm swamped the Houston area with off-the-charts rainfall. Nearly 2 feet of rain fell in less than 15 hours in parts of the region, starting on April 17, 2016. The rain flooded thousands of homes and exceeded a 10,000-year event at some gauges.

But the storm’s damage could have been much worse.

The brunt of the deluge hit Waller County, west of Houston, where the impact was largely on farms and ranches. Had the same volume of water fallen just a few miles to the east, over Houston’s dense urban core, the tragedy would have been far worse.

What made the Tax Day flood so devastating was its speed. It was a flash event that struck overnight, without warning.

At Rice University’s Center for Coastal Futures and Adaptive Resilience, we used state-of-the-art hydrological modeling to see what would happen if a similar storm struck more populated parts of the city today.

The results suggest that current flood planning strategies in Houston – and similar strategies used in communities across the U.S. – are dangerously narrow in how they consider what’s at risk. In today’s world of increasingly extreme downpours, preparing for flood disasters means preparing for more than just what’s probable – it means also preparing for extreme situations that are less likely but could be far more dangerous.

The perils of relying on probability

In the United States, flood risk is publicly defined by maps produced by the Federal Emergency Management Agency. These maps, suggesting which properties face flood risks, guide everything from emergency planning to decisions related to the National Flood Insurance Program.

However, FEMA’s risk maps are based on probabilistic modeling that typically stops at the 500-year flood risk level, meaning a property has 0.2% odds – a 1 in 500 chance – of being flooded in........

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