Predicting the path and the impacts of tropical cyclones is never easy, but Hurricane Florence, a powerful storm currently bearing down on the United States’s southeastern seaboard, has proved more mercurial than most. Florence has thrived hundreds of kilometres north of typical hurricanes, and is now turning south when the average storm would normally turn north.
But forecasting the hurricane’s trajectory is just the first step. Scientists are also racing to project the flooding that will occur as the storm pushes ocean water into the coast and drops rain inland.
That’s where Rick Luettich comes in. He is a civil engineer who studies storm behaviour at the University of North Carolina in Morehead City, and heads the university’s Coastal Resilience Center, with funding from the US Department of Homeland Security. Luettich and his colleagues specialize in modelling storm surges, which occur as atmospheric pressure drops and powerful winds drive ocean waves into the land. Government agencies use the centre’s projections to prepare communities as storms approach, launch recovery efforts in the aftermath and map out the risk of future floods.
In recent days, Luettich has been thinking about parallels between Florence and other storms — including Hurricane Harvey, which inundated Houston, Texas, in 2017; Hazel, a record storm that hammered the US east coast in 1954; and Matthew, which ravaged Haiti before wreaking havoc across the US southeast in 2016. Luettich says that Florence might well represent a new kind of threat. We asked him to tell us more.
What makes Florence so unusual?
This storm has a lot of things that are unusual about it. First off, it formed at a fairly high latitude. And the storm has become much more erratic and much slower than past storms. As a result, the impact zone has broadened considerably. We could be looking at a storm that comes to the coast and then stalls, very much like Harvey did.
Does climate change have anything to do with it?
Interestingly enough, one of the predictions of the past few years is that climate change expands the region over which storms will be initialized. And once they get started, the warmer ocean water can sustain these storms. I’m sure there will be studies post-season, but this may turn out to be one that fits very well into that category of what we may be looking for in the future.
What kind of impacts are your models projecting?
At this point, what we are looking at is storm surges of 10–12 feet [3–3.6 metres]. That’s about the height of the dunes. We’re expecting major beach erosion. We’re expecting dunes to be eroded. We’re expecting overwash in some areas, and in areas where there is overwash, there will be flooding behind the dunes. It’s a violent process.
Much of the region’s coastline is lined by natural strips of sand called barrier islands. How much protection do they provide?
The barrier islands are the first line of defence. But we’re expecting water to push up into estuaries that sit behind these barrier islands. Oftentimes they are funnel shaped, so the water can actually hit higher elevations than one sees along the coast. Those estuaries — and creeks and small riverlets — don’t have dunes along the edges of them, so when the water gets in, it spills over. Facilities, buildings and people who are in those areas will see flooding.
Why does the speed of the storm matter?
It’s a combination of storm surge and flooding due to rainfall. Rather than cross land and continue, this one looks like it’s going to approach land and then perhaps parallel it for a while. Along the way, you’ll see multiple high-tide cycles. A tide of plus or minus 2.5–3 feet [75–90 centimetres] can make a substantial difference.
This slow storm paralleling the shore guarantees it’s going to hit at high tide, and probably, several high tides. And then the rainfall associated with a slow moving storm brings water from the back side. I don’t know that we have a good analogue in history.
How do you define coastal resilience?
Coastal resilience is all about being able to deal with these events better and recover more efficiently. A lot of that has to do with planning, and with understanding people and how they respond. Once one of these events goes by, then you are faced with some sort of recovery process. There’s a lot of social science that goes to trying to manage that process. The immediate reaction is, ‘Boy, I’m going to build this back just the way I had it before.” That is often the worst reaction, but oftentimes it’s the human reaction.
People like to live on the coast, and the federal government still subsidizes the practice through flood insurance. Do you think the United States is getting any wiser about coastal development?
I’m fundamentally an optimist. There’s evidence that we are making progress. A substantial amount of money from Hurricane Matthew is going to buy outs [of properties in flood-prone areas], so that we can move people who are in the worst locations to areas that are not at risk from these coastal events.
This is going to require a culture change. We are seeing signs of that, although it’s flat out not going to happen over night.
What happens the day after the storm passes at your lab?
We will do our very best recreation of what has happened. We’ll have the benefit of knowing where the storm went, how strong it was, how big it was, how fast it was moving, how much precipitation it actually had. There is a lot of data out there, but with our models, we can really fill in the gaps. FEMA [the Federal Emergency Management Agency, which is in charge of the government response] will use that to do initial damage estimates. After that, we’ll go through a very careful analysis of how well our model performed.
But to be very honest, there’s another storm that looks like it may be popping up in the Gulf [of Mexico], and so we may just switch gears from this storm to that one.