How Flood Protection Can Increase Consequences of Large Floods

Academic research into flooding suggests that “Flood protection tends to reduce small floods, but increases the consequences of rare, large floods.” This is one of the central ideas in modern flood-risk management.

It does not mean that flood-control projects are bad. Rather, it describes an unintended sociological feedback loop that can emerge over the long term, if structural protection is not paired with prudent land-use planning.

The concept traces back to the work of Gilbert F. White in 1945 and has been expanded by researchers such as Raymond J. Burby, Gilberto Di Baldassarre, and others. It is commonly referred to as the levee effect or the safe-development paradox. Simply stated, flood-risk-reduction projects can lull people into a false sense of security.

The Basic Mechanism

The process unfolds over decades rather than years. It follows these steps:

1. Flood protection is built…
2. Frequent flooding declines…
3. Confidence increases…
4. More homes, businesses and infrastructure are built…
5. Property values rise…
6. A flood larger than the “design event” (i.e., the 100-year flood) occurs…
7. Communities experience much greater losses than would have occurred decades earlier.

Structural flood protection often reduces flood frequency. However, it encourages much more development behind the protection, so exposure increases dramatically. Then, when an event exceeds the design capacity, total losses can be much larger than before.

Why This Happens

Here’s an example. Imagine an area that floods every five years.

Initially, only 100 homes might occupy the area because of flood frequency.

• A moderate flood might damage 50 homes.
• Then, a levee is built to withstand the 100-year flood.

For the next 30 years:

• No floods occur.
• Confidence grows.
• The area now contains:
  • 5,000 homes
  • Schools
  • Hospitals
  • Shopping centers
  • Utilities.

Then a 500-year flood overtops the levee.

The levee did exactly what it was designed to do for decades. But because so much development accumulated behind it, the rare failure produces vastly greater losses than would have occurred before the levee existed.

That is the paradox.

Example: New Orleans and Katrina

Perhaps the world’s best-known example is New Orleans during Katrina.

For decades, extensive levees reduced frequent flooding. Large areas below sea level became heavily urbanized. And population and infrastructure expanded behind the levee system.

When Hurricane Katrina exceeded parts of the system’s design, multiple levees failed. Approximately 80% of New Orleans flooded. More than 1,800 people died. Economic losses exceeded $100 billion.

Researchers argue that the levees enabled much greater development in areas that remained vulnerable to catastrophic flooding if protection was exceeded.

The Implication for Houston and the San Jacinto Basin

This idea has direct relevance to the Lake Houston Area and the San Jacinto River Basin.

Suppose all the projects proposed after Harvey (additional upstream detention, optimized gate operations on Lake Conroe, channel conveyance improvements, more floodgates for Lake Houston, etc.) are implemented and reduce the probability of flooding from moderate storms.

However, they could create a widespread perception that “This land is now safe.” And that perception could lead to substantial new development in flood-prone portions of the basin over several decades such as the 5,300 acre Scarborough property at the confluence of the West Fork and Spring Creek.

Exposure increases. Property values increase. Infrastructure becomes more concentrated.

Then, when an event exceeds the protection system — just as Harvey exceeded many historical design assumptions — the total losses may be larger than they otherwise would have been.

This is precisely why many modern flood-risk experts argue that structural protection must be paired with land-use management rather than viewed as a substitute for it. But “land use management” is fightin’ words in Houston.

Breaking the Cycle

The encouraging news is that the research also points to ways to avoid the paradox. The most commonly recommended strategies are:

• Preserve portions of the natural floodplain instead of developing every protected acre.
• Use regional detention and natural storage to reduce flood peaks.
• Educate home buyers about the limits of flood protection.
• Require resilient construction (elevated structures, floodproofing) even behind levees or reservoirs.
• Manage development at the watershed scale, not just parcel by parcel.

Why This is Especially Relevant to the San Jacinto

This concept is particularly important for the San Jacinto basin because it reframes the objective of the SJRA’s Joint Reservoir Operations Study.

The study’s success should not be measured solely by whether it lowers flood stages by a few inches. It should also include:

• Strong warnings about the safe development paradox
• Education about design assumptions for flooding, i.e., the height of foundations above the 100year flood level.

Said another way, the study should discourage development that eventually erases any gains – one of the central themes in contemporary flood-risk research.

Posted by Bob Rehak on 7/5/2026

3232 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.