Alexander S. Kolker is an associate professor at the Louisiana Universities Marine Consortium. He studies how water and mud move across coastal areas, and how climate change and people impact these movements. He lives in New Orleans.
Last Wednesday, I stared at the long thin, shovel that sat in the back of my car. I had just used it to collect cattail stalks that I planned to send to colleagues in California—as a coastal scientist in Louisiana, collecting those samples is a small part of my job. But with a tropical storm system developing, the shovel looked well suited for another purpose: cleaning out the catch basin along my New Orleans street.
With less than 48 hours to prepare for the storm, I had a dilemma. Should I spend my limited time cleaning out the catch basin, to prepare for Barry’s intense rains? Or, fearing the confluence of storm surge and high water on the Mississippi River, should I evacuate? Or should I trust that the drainage systems would function and use the time to work on analyses that I owe my collaborators?
There was no easy answer for me or many others in south Louisiana who stared down Barry. Although the storm was thankfully less severe than predicted, Barry still demonstrated that climate change can turn situations that are easy to manage into hard ones, setting up a series of tough choices.
To see how this played out this past weekend, it is helpful to understand that there are three major types of events that lead to a large flood in Louisiana: local rainfall, storm surge, and the rising Mississippi River. We use different pieces of infrastructure to deal with these different kinds of flood factors.
For a while, it looked like we would need to use them all for Barry, and at the same time. This “perfect storm” of floods did not emerge, but someday it might.
To state the obvious, it rains a lot along the Gulf Coast, a product of the region’s hot, subtropical environment. Last Wednesday, July 10, many places across the New Orleans area received between four and eight inches of rain. This flooded streets, cars, and some buildings in many (but not all) parts of the region.
Managing rainfall like this depends largely on a system of pipes, pumps, and canals that transport water from city streets into Lake Pontchartrain. The system’s capacity is limited, and made worse by years of neglect and the accumulation of trash and debris (hence, my shovel).
Storm surges are an entirely different cause of flooding in and around New Orleans. High winds from the south and east push ocean waters onshore, raising coastal water levels. The largest of these surges are caused by hurricanes and tropical storms, and they are held off by a mix of earthen levees and concrete and steel walls, often reinforced with sheet pilings underneath.
Natural systems, like marshes, barrier islands, and oyster reefs, can help keep surge at bay too. The state of Louisiana is engaged in a massive project to build both levees and natural systems to keep back hurricane storm surges. The New York region is engaged in similar work post-Sandy.
Floods in the Mississippi River have a different origin and require a different flood-control system. The Mississippi River is so large and has such a vast watershed that rainfall in Louisiana has little impact on it. Instead, Mississippi River floods are caused by rainfall in the central United States, typically in the Ohio River Valley. During the 19th and early 20th centuries, river floods were particularly challenging, and the federal government responded by constructing a system that today consists mostly of earthen levees and strategically placed outlet valves, extending from Cairo, Illinois, to near the river’s mouth at the tip of Louisiana’s boot.
This system protects against floods by holding water inside the channel, which also keeps the river flowing fast, kicking sediment out to sea and facilitating navigation and commerce.
There are plenty of other ingredients that go into the flooding recipe in Louisiana. While it would take a book to explain them all, the short story is that the ground is sinking, the coast is losing land, humans have altered the way water flows across the region, and the climate is changing. Climate change is likely leading to more intense rainstorms, fiercer hurricanes, and rising sea levels. This creates the kinds of conditions we faced in New Orleans last week, with intense street flooding, a high river, and an approaching tropical cyclone. The greenhouse effect is not the only reason Louisiana floods—the state is naturally flood-prone—but it exacerbates the state’s vulnerability.
As Barry bore down, people asked what more could be done to reduce flood levels. Could the U.S. Army Corps of Engineers open a relief valve on the Mississippi River, reducing the river’s height? One such valve, the Morganza Spillway (located slightly more than 30 miles northwest of Baton Rouge), was closed; another, the Bonnet Carré Spillway, about 20 miles west of New Orleans, was operating at two-thirds capacity. Should hurricane levees be built up? Should New Orleans run its pumps more aggressively?
Understanding how difficult these choices are show how tough it will be to cope with a warmer, wetter climate.
For example: Had the Army Corps opened the Morganza Spillway, that would have sent water down the Mississippi’s main distributary—the Atchafalaya River—raising water levels in the port city at the river’s mouth, Morgan City. It was good that this did not happen. The Atchafalaya was already swollen, and Barry made landfall just west of Morgan City. The river experienced a three-foot storm surge that brought water levels within inches of the all-time record.
As for the Bonnet Carré Spillway, its long opening this year was probably responsible for algae blooms along the Mississippi’s Gulf Coast, and possibly dolphin deaths in the region. Opening it even longer hardly seemed attractive.
Levees can be problematic, too. By reducing water levels for those on the inside, levees can push the water up for those on the outside. During Hurricane Isaac in 2012, a community called Braithwaithe east of New Orleans flooded, partly because it was adjacent to newly constructed levees that were designed to shelter New Orleans. During Barry, residents of Isle de Jean Charles, about 50 miles southwest of New Orleans, had to be evacuated; their flood risks were elevated by the construction of levees around more populated areas nearby.
Levees also make it harder for rainwater to escape. Yet taking water out of the soil in cities like New Orleans can cause the ground to sink. There are also concerns about toxins and contaminants that accumulate in floodwaters as they travel through the drainage system. As folks say in Louisiana, “the water’s gotta go somewhere.”
There are ways to reduce flood risks that rely less on hard infrastructure. Rebuilding land on Louisiana’s coast is an important part of pushing back the sea. Elevating buildings can allow water to flow underneath instead of inside. There are laws and regulations that can be changed to encourage people and businesses to locate in less flood-prone areas. But hard infrastructure will almost certainly remain part of the flood-defense recipe for years ahead, as it predictably lowers flood risks and often allows people to stay in place—at least in the near term.
The decision I faced last week—whether to flush out water from my neighborhood, leave the city, or continue as normal with my daily tasks—is reflective of a larger story. Climate change will not only put stress on our infrastructure, but will make our decisions about how to use that infrastructure more difficult. As other places build more flood defenses to keep out rising waters, the dilemmas we face in Louisiana could multiply around the country. Given these hard choices, another decision may be easier: to lower the concentration of greenhouse gases that ultimately cause so many waters to rise.