Floodplains: A Shifting Target and a Political Swamp

10/26/26 – Few people understand that floodplains are a shifting target. They are determined by LIDAR accurate to within a centimeter, certified by professional engineers, and enshrined in government regulations. At the end of that process, residents borrow billions of dollars for mortgages, often based on where a professional surveyor determines a line should appear on a map.

How could they all be wrong? They aren’t. They’re right for a period of time. But those floodplains shift based on:

• New data collected after large storms that improves our understanding of “how much rainfall is theoretically possible“
• Upstream development and new roads that add impervious cover, speed up runoff, and change the timing of how floodwater aggregates
• A patchwork quilt of ongoing mitigation efforts designed to shrink floodplains
• Hundreds of jurisdictions within the San Jacinto River Basin that may not share Harris County’s flood-prevention priorities
• Who wins the political battle for flood-mitigation dollars at the state, county and local levels.

Shifting-Target Horror Story

How fast can things change? In the early eighties, I bought a house in the Dallas area on Spring Creek downstream from Plano. The engineer, survey, and developer certified that it was two feet above the hundred year floodplain. But Plano was the fastest growing city in America at the time. Within two years, floodwater entered our house after a minor rain.

So, the Army Corps resurveyed the creek. They determined that we were ten feet below the 100-year floodplain, not two feet above it. That was an expensive lesson for young homeowners. After making repairs, we sold the house at a huge loss.

We then moved to Houston, but on much higher ground relative to the floodplain at the time. We bought a home a mile beyond the 500-year floodplain. Now, however, new flood maps show the 500-floodplain reaches within 50 feet of my house.

Do Maps Really Reflect Everything Going on Around You?

As a result, I have learned all over again that floodplains are a shifting target.

If you came across a bear in the woods, you would immediately sense a threat and turn around. But when searching for a new home, you may not feel an immediate threat from flooding. So, many people let their guards down.

Even if you wanted to learn what a new home’s flood risk is, how would you?

The new flood maps posted on MAAPnext.org are not yet official. And even if they were, Harris County Flood Control based them on data it started acquiring in 2019. Since then, Montgomery County has become the third-fastest growing county in America – a game changer.

FEMA is now reviewing protests by home and business owners re: the extent of floodplains who fear their property values could plummet.

Plus, nine years after Harvey:

• The SJRA still hasn’t finalized its Joint Reservoir Operations Study.
• The Lake Conroe Association has commissioned its own study that attacks the one of the main premises of the SJRA study – the value of pre-release.
• Engineering for additional gates for the Lake Houston Dam has not yet finished. But we know we will need more funding that could potentially torpedo the benefit/cost requirement.
• No additional upstream detention projects have yet started.
• The Lake Houston Dredging and Maintenance District is scrambling to put together a business plan.
• The 2018 Flood Bond is still only half spent.
• Only 14% of the money designated for the San Jacinto River Basin in the bond has been spent.

Yep. You would think widening a channel or digging a detention basin would be a relatively simple task. But we seem to have designed a flood-control system inspired by Rube Goldberg. Read this ironic post I wrote 3000 days ago.

Posted by Bob Rehak on 10/26/26

3345 Days since Hurricane Harvey