On the 10th anniversary of Hurricane Katrina, New Orleans is far better protected now than it was then. The same can't be said about Miami.
As Tropical Storm Erika stalks the Western Atlantic Ocean and threatens to morph into a hurricane, Miami's multibillion dollar coastline has little protection from storms.
Erika could change course or intensity at any time, meaning that it could weaken by the time it reaches the coast as easily as it could strengthen. Emergency preparations are already underway in some Florida communities, even though the storm is not expected to reach the state until early next week.
It brings to mind a daunting realization: America's coasts are packed with people and property, and yet they are highly vulnerable to extreme storms.
Read MoreTropical Storm Erika strengthens, heads toward Florida
"Of the 20 large (global) cities that people talk about as being highly at risk for coast storms, about eight of them are on the American coast: the East Coast and the South coast," said Greg Baecher, a professor of engineering at the University of Maryland.
"If a Katrina directly hit Miami, you are talking about damages that could be several multiples of what happened in New Orleans," Baecher told CNBC. "First, you have the size of the city, and the fact that there is nothing between the coast and the ocean."
Miami has become more committed to storm preparedness since Hurricane Andrew hit in 1992, causing $26 billion in damage. At the time, Andrew was the most expensive natural disaster in U.S. history.
South Florida does have levees, canals and other systems for reducing the impacts of flooding. For example, there are levees that prevent the Everglades from flooding nearby dry land. But there is still little shielding the city from the ocean and the path of ocean hurricanes.
The rock beneath Miami—and the ocean water immediately around it—is a kind of porous limestone that presents challenges as the population of South Florida grows, as its coastline becomes developed, and as nature changes. The limestone bedrock makes rising sea levels a concern—saltwater from the ocean can penetrate the rock and seep into Florida's freshwater supply, said Ben Kirtman, a professor of atmospheric science at the University of Miami.
The limestone is so porous, "that to build a seawall, you would actually have to drill way deep to get past the limestone in order to build an effective wall," Kirtman said.
Building levees or seawalls on that kind of rock may protect against the force of some storm surge, but water can still seep up through the pores, bypassing the walls entirely.
"I have seen that even during hurricanes, where houses are flooded not because of overland flooding, but because the water is coming up through the rock from underneath," said Harold Wanless, a professor of geological sciences at the University of Miami.
But even above the ground and water level, Miami isn't well protected. The area has a few seawalls in select places, but nothing resembling a complete levee system. The city also is low lying.
"Only 9 percent of Miami-Dade County is more than 10 feet above sea level at high tide," Wanless said. "It's crazy how low we are, and how vulnerable we are to storm surges, to flooding."
Miami relies heavily on a pumping system to push water back out faster than it can flood the city.
"That sounds crazy, but for the near term, that is probably one of the most viable solutions," Kirtman said.
Sea levels around South Florida are expected to rise between 6 and 9 inches by 2030, and Kirtman's own guess is toward the high end of that.
Rising sea levels will cause average yearly global flood losses to increase to $52 billion by 2050, up from about $6 billion a decade ago, according to the Organization for Economic Co-operation and Development.
"Due to their high wealth and low protection level, three American cities"—Miami, New York and New Orleans—"are responsible for 31 percent of the losses across the 136 cities" by 2050 around the world, the report said. New Orleans and New York have already been hit in recent years by Katrina and Sandy, respectively.
Some of that flooding will be chronic, or seasonal. But a severe storm with a large surge of ocean water is also a danger.