Outdated Rainfall Assumptions: A Systemic Design-Lag Problem

3/31/26 – Since Hurricane Harvey, one of the dominant themes of more than 3,000 posts on this website has been outdated rainfall assumptions and how flood-infrastructure design lags those assumptions. That design lag contributes to flooding nationwide.

How Infrastructure Design Lags Rainfall Estimates

Almost nine years after Harvey, Harris County finally has new “draft” flood maps issued by FEMA based on Atlas-14 rainfall assumptions. But they won’t take effect for another two to three years. Meanwhile, parts of the region have built and will continue to build infrastructure, homes and businesses based on long-outdated rainfall predictions.

Before Atlas-14-based flood maps become effective in Harris County, FEMA will issue Atlas 15 predictions throughout the U.S. in 2026.

Different regulatory agencies and jurisdictions update at different speeds.

Imagine members of a choir each singing from different songbooks.

These rainfall predictions affect many aspects of flood control and building codes, including:

Stormwater detention basin sizing
Floodplain mapping
Subdivision drainage criteria, i.e., size of storm sewers
Elevation of homes and businesses above the 100-year floodplain and street level
Acceptable foundation types in flood- hazard areas
Building in floodways including width, depth, bracing and other requirements for piers
Where fill can and cannot be used

In 2009, Harris County revised its floodplain development standards. After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.

Subdivisions built with the updated standards experienced one twentieth the amount of damage.

EVA and the Difficulty of Developing Rainfall Estimates

So getting the rainfall right is important. But why do we get it wrong so often? Rainfall predictions stem from an obscure branch of mathematics, known as EVA (Extreme Value Analysis). With EVA, forecasters try to predict the probability of unobserved future events based on the frequency of somewhat smaller past events.

But there’s a problem. All predictions (500-year storms, etc.) are based on extremely small data sets. EVA may produce the best numbers possible, but predicting 500-years into the future based on 100 years of data takes a lot of guess-work. And we design entire cities based on these probabilities!

And that’s what I mean by the “design lag” problem based on outdated rainfall assumptions. Assumptions change with major new storms, such as Harvey. But we can’t just wave a major wand and change trillions of dollars of infrastructure already in the ground. We are systematically under-designed for current rainfall statistics.

This isn’t just a Houston problem. And it’s not just “in the ground” infrastructure. For example…

Mississippi Levees

Engineers typically design levees to contain a 100-year (1% annual chance) storm. Even small increases in in rainfall when levees constrain a river can produce huge increases in water surface elevation and push crests above levees. We’ve seen this happen recently in 2008 and 2019.

Design lag manifested itself as systemic underestimation of river stage, leading to overtopping of levee defenses.

Street Flooding in New York City

In 2023, extreme rainfall (up to 2.5 inches per hour) snarled the New York metropolitan area. It knocked out subways, commuter rail lines, flooded basements and closed a terminal at LaGuardia. It was the City’s wettest day since 1960 and came just two years after Hurricane Ida killed at least 13 people.

Engineers from previous generations designed storm sewers for much lower peak intensities. Usually, they design urban systems around peak intensities for short time periods (minutes to a few hours). But pre-Atlas 14 standards underestimated those short duration bursts.

Storm sewers were simply designed for much lower peak intensities. Systems failed almost instantly. Subway entrances became inlets for street flooding.

The Lesson Learned

Flooding results not just from rainfall. It results from under-designed infrastructure for rain that falls. For example:

Homes not high enough.
Channels not wide enough.
Storm sewers not big enough.

As a result, NOAA hopes to update rainfall frequency estimates much more frequently in the future. NOAA developed Atlas 14 region-by-region from approximately 2000–2018.

Texas lagged other regions by years. Before Atlas 14, the prior standard (TP-40) dated to 1961. And during that time, Houston grew exponentially. So most of our infrastructure is built to older standards.

NOAA’s new Atlas 15 is designed to be the nation’s first unified data set. And it is built from the ground up to incorporate climate trends and be updatable as new data becomes available.

NOAA is trying to eliminate the “outdated by the time it’s adopted” issue. But:

Local adoption will still likely lag availability
Regulatory inertia will remain the bottleneck.

See Lessons Page For More Information

For more information about other causes of flooding, see the Lessons page of this website.

Posted by Bob Rehak on 3/31/26

3136 Days since Harvey