Floodplain Encroachment: Another Consistent Driver of Flooding Worldwide

4/1/26 – In my quest to summarize the most important “lessons learned” since Hurricane Harvey, here’s Lesson #3: Floodplain Encroachment. Floodplain encroachment is consistently rated one of the most important drivers of flooding worldwide. Think about it. If people didn’t build in floodplains, no one would flood. But that’s only part of the story. Floodplain development also changes flood assumptions for communities downstream.

Why People Build in Floodplains

Despite the risks, people worldwide build in floodplains. The land usually costs less. And it can yield extraordinary profits to developers lucky or persistent enough to obtain building permits.

After all, people pay premiums to live near water. Water views are prestigious, beautiful and soothing. Plus, historically, living near water translated to “security.” Water sustains life. The need for water is hard-wired into our DNA, our culture, and even our economy.

One in five Texans lives in a floodplain. And the World Meteorological Society estimates that 40% of the world’s population lives within 100 kilometers (62 miles) of a coastline.

Yet during Hurricane Ike in 2008, storm surge reached 30 miles inland in places. And despite being leveled, within 10 years, homes on the vulnerable Bolivar Peninsula had built back.

While I have focused primarily on the Houston Area, floodplain encroachment is a global problem. Nearly all growing metro areas encroach on floodplains – coastal or riverine.

Loss of Natural Storage Can Increase Downstream Flood Elevations

The problem isn’t just “putting people in harm’s way.” It’s also about the loss of natural floodplain storage. In riverine systems, floodplains function as temporary storage reservoirs during overbank flows.

Insufficiently mitigated development can remove that storage volume or prevent it from being accessed.

That’s why after Harvey, Houston and Harris County changed floodplain regulations so developers couldn’t bring fill into floodplains. Fill displaces water. Nature compensates for the fill by increasing water surface elevations elsewhere.

Floodplain development can also reduce the duration of floodplain storage, resulting in faster, higher peaks downstream.

See examples below.

Mississippi Floodplain Development

Historically, the Mississippi River occupied a broad alluvial valley tens of miles across in places. It inundated seasonally . Floodplains functioned as massive, temporary storage reservoirs.

Encroachment occurred primarily through federal levee construction under the U.S. Army Corps of Engineers and urban/industrial development that occurred later behind the levees.

As a result, floodplain width narrowed dramatically. The levees confined the river’s flow into a narrower channel where floods moved higher and faster, often breaking through levees with devastating consequences.

Levee failure also played a major role in the inundation of New Orleans during Hurricane Katrina in 2005.

Floodplain encroachment can turn into a vicious feedback loop. Levees reduce frequent flooding. That attracts more investment and development behind the levees. But as the consequences of levee failure increase, there’s more pressure to build higher, stronger levees.

West Fork San Jacinto/Spring Creek Confluence

Closer to home for most of my readers, developers have recently been trying to figure out ways to develop 5,300 acres between the West Fork San Jacinto River and Spring Creek. Virtually the entire area is in floodplain or floodway. Though current flood maps don’t fully reflect the danger, FEMA’s new draft flood maps for the area show part of the property. See below.

A leading hydrologist in the area told me that developing this area would be like “aiming a fire hose at the Humble/Kingwood Area.”

It’s not clear yet what the developer has planned for the site. Both the Texas General Land Office (GLO) and the Texas Attorney General have denied FOIA requests for the plans.

According to State Rep. Steve Toth, the GLO invested $140 million in the development of the property. Ironically, the GLO also administers billions of flood-mitigation dollars in Texas. That creates not only a conflict of interest but a classic externality problem.

What is an Externality Problem?

An “externality problem” occurs when the production or consumption of a good, such as housing, imposes unintended costs or benefits on third parties not involved in the transaction. In economic terms, this leads to market inefficiencies. It is a form of market failure. Private costs/benefits differ from social costs/benefits.

For instance, sand mines help produce a raw material needed for concrete. It generates profit for producers. But in their zeal to maximize their profit, they mine too close to rivers and in a manner that exacerbates erosion and sedimentation.

Remediating that excess sedimentation has cost taxpayers more than $200 million for dredging. Miners have literally externalized their cleanup and safety costs.

It’s the same way with flooding. Developers profit from building in floodplains. And the vast majority try to do it safely.

Regardless, in 2023, the Joint Economic Committee of Congress estimates that each year flooding costs Americans between $179.8 and $496.0 billion. The total depends on the types of damage included, i.e., structural, lost economic output, infrastructure repairs, insurance losses, decreased tax revenues, transit, deaths, etc.

Assuming the higher flood-damage estimate of nearly half of a trillion dollars, that represents 7% of last year’s entire federal budget. I’d sure like a 7% tax cut. April 15th is two weeks away!

For More Information

For more “lessons learned” about flooding since Harvey, see this website’s Lessons Page.

Posted by Bob Rehak on 4/1/2026

3137 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.