Ida made landfall near New Orleans Sunday as a Category 4 hurricane, bringing fierce winds, heavy rains and dangerous storm surges. Ida is presenting the biggest test yet of the storm risk-reduction system that was installed in the New Orleans metro area after Hurricane Katrina. 

Since 2005, the U.S. Army Corps of Engineers has worked with other local, state and federal agencies to build a roughly $14.5 billion system of levees and other protections. Here’s what we know about the system. 

Why does New Orleans need a special storm risk-reduction system?

Louisiana, along with other Gulf Coast states, is often hit by numerous tropical cyclones off the Atlantic Ocean each hurricane season, which runs from June through November. Since New Orleans is bowl-shaped and much of it lies below sea level, the area is dependent on a complex drainage system. 

What is the hurricane risk reduction system?

The system consists of flood walls and levees to protect the area from storm surge (or abnormal sea level rise) and a pump system to siphon out excess water. A flood wall is usually a concrete structure, while a levee is made from earthen material. The Army Corps raised levees and flood walls all around the metro area and fortified them with vegetation to lower the likelihood of a “breach,” where a wall breaks down and water can flow into the city. 

The height of the levee walls is based on topography for the area, with some as high as 30 feet and others only 12 to 15 feet, said Rene Poche, public affairs specialist for the Army Corps New Orleans. When Hurricane Katrina struck the area in 2005, some flood walls were only 5 feet high.  

“There’s no comparison with what we have now and what we had 16 years ago,” said Mr. Poche. 

The Army Corps made the system more resilient by building pump stations where outfall canals drain into Lake Pontchartrain and ensuring existing pump stations can withstand roughly 205 mph winds. They also added concrete-walled safe rooms for the pump operators that allow them to work continuously, and redundant power systems so pump stations can operate when commercial power is lost. 

What area is protected by the system?

The system covers a 133-mile perimeter around the New Orleans metro area, which encompasses Saint Charles, Jefferson, Orleans, St. Bernard and Upper Plaquemines parishes.

What happened to spur Congress to pay for this system?

During Hurricane Katrina in 2005, there were more than 50 failures of the levees and flood walls, which caused flooding in 80% of New Orleans and all of St. Bernard Parish.

What is the concern? 

A storm surge warning was in effect from Morgan City, La., to the Alabama-Florida border, according to the National Hurricane Center. Forecasters said Ida could bring up to 15 feet of storm surge to some areas. It could be the first time the levees have been overtopped since the upgrades were made. 

Write to Rachel Wolfe at rachel.wolfe@wsj.com