A multi-billion dollar system of seawalls, levees, pumps and other flood controls helped shield New Orleans from the surge of ocean water swept up by Hurricane Ida. But Ida also encountered natural barriers like wetlands that act as a “speed bump” and blunt the impact of storms.

Although natural defenses have their limits — especially with major events like Ida — experts say they can play an important part in protecting communities and the environment when storms strike.

“There is always a benefit by having these wetlands between you and the storm,” said Gerald Galloway, a recently retired University of Maryland engineering professor who previously worked for the U.S. Army Corps of Engineers.

Since the devastation of hurricanes Katrina and Rita in 2005, a key part of Louisiana’s efforts to bulk up its storm defenses has been restoring the wetlands and barrier islands the state has been losing for decades along its coast.

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HOW DOES NATURAL TERRAIN PROTECT AGAINST STORMS?

Wetlands and other natural terrain can act as buffers for wind and water, giving storm surges more to overcome before they reach people and buildings.

“They help with storm protection because they are dissipating energy,” said Hugh J. Roberts, chief operating officer at the Water Institute of the Gulf, an environmental research group.

Roberts compared it to a cyclist riding over rocky debris. Sparse wetlands are like riding over a few pebbles, he said — hardly noticeable. But robust wetlands with elevated terrains are like trying to ride over a long, bumpier stretch of gravel.

The Water Institute has received funding from the Walton Family Foundation, which also supports The Associated Press’ coverage of water and environmental policy.

WHAT ABOUT BIG STORMS LIKE IDA?

Wetlands are generally more effective at lessening the impact of small to moderate surges of ocean water, said Ed Link, a researcher at the University of Maryland who previously directed an analysis of New Orleans’ storm protections after Katrina.

With bigger storms like Ida, the effect may be more muted. Once wetlands are full of water, they lose their ability to slow down waves and the storm can travel over them, Link said.

Philip B. Bedient, who directs a center focused on severe storms and flooding at Rice University, agreed that wetlands are ideal for smaller events that might bring 100 mph (161 kph) winds but can be outmatched by major storms. Ida was a Category 4 storm and had winds that reached 150 mph (241 kph), but did not push as much storm surge ashore as Katrina.

Still, experts say natural barriers offer one more line of defense even with major storms and can make a difference. With Ida, officials say it will take time to assess their impact.

In the Northeast, areas with coastal wetlands had an average of 10% less property damage during Hurricane Sandy compared to those without, according a 2016 study by researchers at the University of California Santa Cruz, the Nature Conservancy and the Wildlife Conservation Society.

HOW IS THE STATE BULKING UP NATURAL DEFENSES?

Louisiana has been investing heavily in restoring its natural barriers to help protect against storms.

Since 2007, the state has created or restoredaround 76 square miles (197 square kilometers) of marshland and 60 miles (97 kilometers) of barrier island.

And the state’s most recent coastal protection and restoration planincludes dozens of projects that build or maintain more than 800 square miles (2,072 square kilometers) of land. The total cost is estimated to be $50 billion — but the projects are expected to prevent more than $150 billion in damage over the five decades. They include plans to use dredged material and diverted sediment to help build marshes.

“What we are trying to do is hold the line or do our best to ensure the wetlands are keeping the Gulf away from the cities,” Galloway said.

IS THE RESTORATION WORKING?

Despite the ongoing work, the state’s restoration work isn’t keeping up with its land losses.

Louisiana has been losing land since at least the 1930s, as flood control measures along the Mississippi River stopped the natural process that deposited sediments from the river into the delta. In 2012, state officials predicted their restoration work would eventually reverse the problem. Five years later, they stopped making that prediction.

The Louisiana Coastal Protection and Restoration Authority now says the restoration is merely slowing down the loss.

ARE THERE OTHER BENEFITS TO WETLANDS?

In addition to helping protect against flooding, wetlands can help curtail erosion and preserve the landscape. They also provide habitat for fish and migratory birds.

The scale of the environmental benefits can be significant. Louisiana provides winter habitat for 5 million migratory waterfowl and its fishing industry supplies a quarter of the country’s seafood, according to a state report. Louisiana wetlands also account for half of the country’s coastal marshland, the report said.

How that wildlife habitat fared through the storm isn’t yet clear. A spokesperson with the Louisiana Coastal Protection and Restoration Authority, Chuck Perrodin, said it could be weeks before the agency is able to assess the extent of any damage to the state’s coastal projects.

But between 2004 and 2008, Louisiana lost more than 300 square miles (777 square kilometers) of land due to hurricanes.

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