Dignitaries and longtime residents gathered under gray skies at the memorial to Katrina’s victims in a New Orleans cemetery where dozens who perished in the storm but were never identified or claimed are interred.

“We do everything to keep the memory of these people alive,” said Orrin Duncan, who worked for the coroner when Katrina hit. He comes to the memorial every year, opening the cemetery gate and making sure the grass is cut.

A Category 3 hurricane when it made landfall in Louisiana on August 29, 2005, Katrina inflicted staggering destruction.

The storm killed nearly 1400 people across five states and racked up an estimated $US200 billion ($267 billion) in damage, flattening homes on the coast and sending ruinous flooding into low-lying neighbourhoods.

Two decades later, it remains the costliest US hurricane on record, according to the National Oceanic and Atmospheric Administration.

The failure of New Orleans’ federal levee system inundated about 80 per cent of the city in floodwaters that took weeks to drain. Thousands of people clung to rooftops to survive or waited for evacuation in the sweltering, under-provisioned Superdome football stadium.

Mayor says New Orleans came back ‘better and stronger’

At the cemetery memorial, revered jazz clarinetist Michael White played When the Saints Go Marching In as a procession carried several wreaths to lay beside mausoleums of the storm victims.

Mayor LaToya Cantrell recalled the city’s sacrifices and projected optimism for its future.

“New Orleans is still here; New Orleans still stands,” Cantrell said. “New Orleans came back better and stronger than ever before.”

Former Vice President Al Gore was among attendees at an event in the city’s Lower Ninth Ward, a predominantly Black community where a levee breach led to devastating flooding that was exacerbated by a delayed government response.

As crowds gathered along the levee wall, construction workers laid bricks to finish a restoration on a Katrina memorial that had fallen into disrepair, frustrating residents.

Thousands were expected to join a brass band parade known as a second line. The beloved New Orleans tradition has its roots in African American jazz funerals, in which grieving family members march with the deceased alongside a band and trailed by a second line of dancing friends and bystanders.

A parade has been staged on every Katrina anniversary since local artists organised it in 2006 to help neighbours heal and unite the community.

“Second line allows everybody to come together,” said the Rev. Lennox Yearwood of Hip Hop Caucus, an organiser of the anniversary events.

“We’re still here, and despite the storm, people have been strong and very powerful and have come together each and every year to continue to be there for one another.”

City leaders are pushing for the anniversary to become a state holiday.

Katrina’s impact still felt

In Mississippi, where hundreds perished as Katrina’s storm surge demolished homes overlooking the Gulf, residents and officials gathered to mark the anniversary in Gulfport.

Haley Barbour, Mississippi’s governor when the hurricane struck, recalled the “utter obliteration” he witnessed from a helicopter after the storm passed.

“It looked like the hand of God had wiped away the coast,” Barbour said.

The population of New Orleans, nearly half a million before Katrina, is now 384,000 after displaced residents scattered across the nation. Many ended up in Atlanta, Dallas and Houston.

In the aftermath, the levee system was rebuilt, public schools were privatized, most public housing projects were demolished and a hospital was shuttered.

About 134,000 housing units were damaged by Katrina, according to The Data Centre, a nonprofit research agency.

The storm had a disproportionate impact on the city’s Black residents. While New Orleans remains a majority Black city, tens of thousands of Black residents were unable to return after Katrina.

A botched and racially biased federal loan program for home rebuilding, coupled with a shortage of affordable housing, have made it harder for former residents to come back.

New Orleans resident Gary Wainwright said never misses the cemetery memorial service on Katrina’s anniversary.

On Friday he wore a frayed red necktie, covered with the phrase “I love you.” He salvaged it from his battered home in the storm’s aftermath.

“It’s a little bit tattered, like the city,” Wainwright said. “But it’s still beautiful.” he said.