Hurricane Katrina was a Category 5 hurricane that made landfall on Florida and Louisiana in August 2005, causing catastrophic damage, particularly in the city of New Orleans and the surrounding areas. Subsequent flooding, caused largely as a result of fatal engineering flaws in the flood protection system known as levees around the city of New Orleans, precipitated most of the loss of lives. The storm was the third major hurricane of the record-breaking 2005 Atlantic hurricane season, as well as the fourth-most intense Atlantic hurricane on record to make landfall in the contiguous United States, behind only the 1935 Labor Day hurricane, Hurricane Camille in 1969, and Hurricane Michael in 2018.
The storm originated over the Bahamas on August 23, 2005, from the merger of a tropical wave and the remnants of Tropical Depression Ten. Early on the following day, the tropical depression then intensified into a tropical storm as it headed generally westward toward Florida, strengthening into a hurricane only two hours before making landfall at Hallandale Beach and Aventura on August 25. After very briefly weakening again to a tropical storm, Katrina emerged into the Gulf of Mexico on August 26 and began to rapidly intensify. The storm strengthened into a Category 5 hurricane over the warm waters of the Gulf of Mexico but weakened before making its second landfall as a Category 3 hurricane on August 29, over southeast Louisiana and Mississippi. As Katrina made landfall, its front right quadrant, which held the strongest winds, slammed into Gulfport, Mississippi, devastating it.
Katrina caused extensive destruction and casualties: overall, at least 1,836 people died in the hurricane and subsequent floods, making Katrina the deadliest United States hurricane since the 1928 Okeechobee hurricane. Severe property damage occurred in numerous coastal areas, such as Mississippi beachfront towns where boats and casino barges rammed buildings, pushing cars and houses inland; water reached 6–12 miles (10–19 km) from the beach. The total property damage was estimated at $125 billion (2005 USD), roughly four times the damage wrought by Hurricane Andrew in 1992, tying Katrina with Hurricane Harvey of 2017 as the costliest Atlantic tropical cyclone on record.
Over fifty breaches in surge protection levees surrounding the city of New Orleans, Louisiana was the cause of the majority of the death and destruction during Katrina. Eventually, 80% of the city, as well as large tracts of neighboring parishes, became flooded, and the floodwaters lingered for weeks. Most of the transportation and communication networks servicing New Orleans were damaged or disabled by the flooding, and tens of thousands of people who had not evacuated the city prior to landfall became stranded with little access to food, shelter or basic necessities. The scale of the disaster in New Orleans provoked massive national and international response efforts; federal, local and private rescue operations evacuated displaced persons out of the city over the following weeks. Multiple investigations in the aftermath of the storm concluded that the U.S. Army Corps of Engineers, which had designed and built the region's levees decades earlier, was responsible for the failure of the flood-control systems, though federal courts later ruled that the Corps could not be held financially liable because of sovereign immunity in the Flood Control Act of 1928.
There were also widespread criticisms and investigations of the emergency responses from federal, state and local governments, which resulted in the resignations of Federal Emergency Management Agency (FEMA) director Michael D. Brown and New Orleans Police Department (NOPD) Superintendent Eddie Compass. Many other government officials were criticized for their responses, especially New Orleans Mayor Ray Nagin, Louisiana Governor Kathleen Blanco, and President George W. Bush. Several agencies including the United States Coast Guard (USCG), National Hurricane Center (NHC) and National Weather Service (NWS) were commended for their actions. The NHC was found to have provided accurate hurricane forecasts with sufficient lead time.