In late August of 2005, one of the most dangerous tropical storms in history began brewing. The waters of the Gulf of Mexico were unusually warm that month, and the high temperatures transformed the ocean basin into a giant cauldron with the optimal conditions for growth.
As the tropical storm cut across the tip of Florida and entered the Gulf, it immediately began to swell. In less than 24 hours, the storm doubled in size. And as it grew into a full-blown hurricane, the weather experts gave it a name: Hurricane Katrina.
Katrina churned through the tropical waters of the Gulf and quickly escalated to peak intensity. It ripped through the atmosphere with remarkable force, registering gusts of wind that exceeded 175 mph (280 km/h) and lasted for more than a minute. By the time the storm hit the southeastern coast of Louisiana on August 29th, Hurricane Katrina was nearly 120 miles wide.
A storm of Katrina’s size is expected to cause flooding and damage, but coastal cities and neighborhoods use a variety of flood walls and levees to prevent total catastrophe.
These walls are built along rivers and waterways and act as a barrier to hold back usually high waters and prevent flooding.
Shortly after Hurricane Katrina made landfall, it became clear that the levees of New Orleans might not be able to hold back the rising waters. A few hours in, the director of the National Hurricane Center said, “I do not think anyone can tell you with confidence right now whether the levees will be topped or not, but that’s obviously a very, very great concern.”
Minutes later, the levees began to fail. The waters breached the levees and floodwalls of New Orleans in more than 50 different places. Entire districts became submerged in more than 10 feet of water. Evacuation routes were destroyed as bridges and roads collapsed. At Memorial Medical Center in the heart of New Orleans, the surging water killed the backup generators. Without power, temperatures inside the hospital rose to over 100 degrees as doctors and nurses took turns manually pumping each breath into dying patients in a desperate attempt to keep people alive.
Water flooded more than 80 percent of the city. And in the days that followed, the death toll began to rise. Bodies were found floating down the streets. Rescue and recovery efforts failed to track down missing people. At least 1,200 people died, and hundreds more were unaccounted for—the total number of dead is still unknown to this day.
So many residents were displaced by Hurricane Katrina that the population of New Orleans dropped by 50 percent from 484,000 before the storm to 230,000 one year later. In total, the damages from Hurricane Katrina surpassed $100 billion. It was the costliest natural disaster in the history of the United States.
The Margin of Safety
The great mistake of Hurricane Katrina was that the levees and flood walls were not built with a proper “margin of safety.” The engineers miscalculated the strength of the soil the walls were built upon. As a result, the walls buckled and the surging waters poured over the top, eroding the soft soil and magnifying the problem. Within a few minutes, the entire system collapsed.
This term, margin of safety, is an engineering concept used to describe the ability of a system to withstand loads that are greater than expected.
Imagine you are building a bridge. The maximum weight for a fully loaded commercial truck is around 80,000 pounds (36,000 kg), but any decent engineer will build a bridge that can safely carry vehicles weighing far more. You don’t want to drive an 80,000-pound truck across a bridge that can only hold 80,001 pounds. Just to be safe, the engineer might build the bridge to handle 5x the expected weight, say 400,000 pounds. This additional capacity is known as the margin of safety.
Of course, maintaining a proper margin of safety is crucial not only in construction and engineering but also in many areas of daily life.