The Levee Effect: How Flood Damage Can Rise as Flood Probability Decreases
3/8/26 – The “Levee Effect” is a term coined decades ago in flood mitigation. Some call it the “Safe Development Paradox.” It explains how total flood damage can rise even though flood probability decreases.
The Basic Mechanism
Think of the word “levee” in this case as synonymous with several forms of flood-control infrastructure including levees, reservoirs, and channelization.
After they are built, flood risk appears to be reduced. Land behind the protection becomes more attractive for development. And property values rise.
As population and infrastructure grow in the protected area, exposure to flooding increases. Then, when an extreme flood exceeds the design capacity, damage is far larger than before the protection existed.
Classic Examples
Mississippi River Levee System
The federal government established the Mississippi River Commission in 1879 to deepen the river channel, improve navigation, prevent major flooding, and increase river-based commerce. Against the advice of experts, the commission recommended raising extensive levees along its channels to contain the flow.
After levee construction, large areas of historic floodplain became urbanized. Agricultural and urban investment skyrocketed.
Then came the Great Mississippi Flood of 1927, the most destructive river flood in the history of the United States. It inundated 27,000 square miles up to 30 feet over the course of several months in early 1927. About 500 people died and more than 630,000 people were affected.
The river below Memphis reached a width of 80 miles. But Arkansas, Mississippi and Louisiana were hardest hit. The flooding triggered a great migration from the south to large cities in the north and midwest.
Sacramento-San Joaquin Valley, California
Levees enabled urbanization of flood basins. Today, millions live below river level. Some areas have only a 100-year level of flood protection. Economic exposure reportedly exceeds $100 billion dollars.
New Orleans and Hurricane Katrina
Levees let large areas below sea level urbanize. When the levees failed in 2005, so much population and infrastructure had accumulated in the protected basin that the consequences quickly became catastrophic.
Eighty percent of New Orleans, as well as large areas in neighboring parishes, flooded. An estimated 100,000 to 150,000 people remained in the City, despite mandatory evacuation orders.
1,392 people died and damage reached $125 billion. That ties it with Harvey for the costliest tropical cyclone ever in the Atlantic basin.
Video on Levee Effect
This 6-minute, 47-second YouTube video illustrates the levee effect and shows how it can make floods worse by opening up floodplains and wetlands to development. It also shows how levees provide protection…up to a point. But when a storm exceeds that level, the consequences can become catastrophic quickly. It contains a mixture of footage from real-life flood events and table-top models.
Transforming Risk
Levees and other flood-mitigation infrastructure shift risk from frequent, low-impact “nuisance” floods to rare, catastrophic, high-impact events.
They hide the underlying risk by preventing small, regular floods that would otherwise remind people of the danger. And that can make the consequences of a major event far greater.
For More Information
See these scholarly articles:
- Flood risk reduction through natural retention and physical protection: The problem with residual riskand the ‘levee effect’
- Reversal of the levee effect towards sustainable floodplain management
- Mitigating the levee effect – Spatial planning approaches to address residual risk
- Unraveling the Levee Effect: Understanding the Interplay between Levee Construction and Household Behavior in Coastal Cities
Posted by Bob Rehak on 3/8/2026
3113 Days since Hurricane Harvey
