Coastal Flooding Challenges Cities Around the U.S.

Coastal flooding has various causes and often has serious consequences. Above, a beach home damaged by a storm surge generated by Hurricane Sandy. Photo: Western Area Power Administration (CC BY 2.0).

Backgrounder: Coastal Flooding Challenges Cities Around the U.S.

By Joseph A. Davis

Coastal flooding has an inevitability that real estate developers and retirees may never understand. 

Long term, we are going to see significant sea level rise from climate heating. That will take years — decades — but will still come faster than many imagine. Federal scientists predict a rise of roughly one foot along U.S. coastlines by 2050. It will keep rising.

So in the long run, that beach house may be toast (although the new coastal wetland replacing it may be a utopia for wildlife and birders). Owners might want to sell while they can.

 

Washington Post journalists

often run videos of

beach houses on stilts

falling into the waves.

 

Meanwhile, environmental journalists can give the public a needed warning. Washington Post journalists, for instance, often run videos of beach houses on stilts falling into the waves. You get the idea.

But not all coastal flooding is the same. Storm surges from Atlantic and Gulf hurricanes can be monstrous. Inundation resulting from the sinking or subsidence of coastal land can be significant. “Nuisance” flooding from unusually high tides is another category. The constant erosion and shifting of barrier islands is another. And along the Great Lakes and St. Lawrence Seaway, entirely different dynamics are at work.

There are officially 95,471 miles of U.S. coastline. Doomscrollers need to understand that coastal flooding is many different things. Just what it is — and when it happens — will vary widely from place to place.

Here’s a basic review.

 

Global sea level rise

Yes, it will happen. It’s already happening. It will keep happening for centuries. Face it.

Some perspective: 13,000 years ago, when continental ice sheets covered the Earth, sea level was roughly 430 feet below where it is now. The Great Lakes didn't exist. The Chesapeake Bay was a river. Much of the Atlantic shoreline was what we now call the Continental Shelf.

Glacial melting largely stabilized a few thousand years ago. As a result of human-caused global heating, it is no longer stable.

Two big factors go into global sea level rise: the melting of glaciers and ice sheets, and the thermal expansion of the warming water in the oceans.

By the year 2100, scientists expect sea level rise from melting ice sheets in Greenland and Antarctic will total three to six feet. By 2300, their contributions may be as much as 16 feet.

 

Storm surges are different — and dangerous

Storm surges are coastal flooding events with an abrupt and abnormal increase in sea level, which can reach a height of nearly 30 feet in the rarest cases. They are typically caused by tropical cyclones or landfalling hurricanes.

 

Storm surges are

sudden and often fatal —

sometimes sweeping away

whole coastal populations.

 

They are sudden and often fatal — sometimes sweeping away whole coastal populations when people do not evacuate.

Even smaller storm surges, which are more common, can be enough to kill.

Storm surge hazard maps and real-time advisories are handy prediction tools for reporters trying to protect their audiences.

Journalists can also help their coastal audiences envision climate-driven sea level rise by comparing it to storm surges.

 

‘Nuisance’ flooding

If a house is in coastal Florida and the street in front shows an inch of water on a sunny day — that is what is called “nuisance” flooding. Think of it as water flowing out of the storm sewer grates instead of into them.

A lot of different mechanisms cause nuisance flooding, but one of the basic ones is unusually high tides. People often call them “king” tides, and they happen naturally every so often. Watch the tide charts.

Several factors cause the natural variability of tides. One is how close the moon is in its orbit around the Earth — exerting extra gravitational pull. The varying closeness of the sun is an additional factor. Tide charts take this into account.

Nuisance flooding will only get worse with sea level rise. Again, owners should consider selling the house.

 

The Great Lakes not always great

The Great Lakes are a whole different ball game. They are less subject to tides and ice melt, but their levels vary considerably over time.

 

Few people are aware that

the Great Lakes can

experience a tide-like

effect called a seiche.

 

Few people are aware that the Great Lakes can experience a tide-like effect called a seiche (pronounced “saysh”). It’s rare and its mechanics are different from a tide.

A seiche is a large standing wave or oscillation that can be caused by big changes in air pressure or winds. Relief of the pressure sets up an oscillation in water height if conditions are right. Think of it like water sloshing in a bathtub.

Water levels vary considerably from year to year in the Great Lakes. They are a complex hydrological system — further complicated by human-made locks and canals. The explanations are complicated and hard to predict, even when they are understood.

Rainfall and snowmelt variations are part of the cause. Seasonal ice-cover variations further affect evaporation and heating. They are easier to measure than to predict.

In any case, there are consequences. Shoreline erosion is the big one. High lake levels can erode bluffs, where they exist. Sometimes, that threatens houses and other structures built on the shoreline.

Armoring the shoreline rarely helps. High water can also amplify sand movement, making beloved beaches disappear.

 

Then there’s land subsidence

Land subsidence is another factor. And sometimes land actually rises. When the huge weight of glacial ice is lifted by melting, the land in some places may rebound.

But land subsidence can amplify coastal flooding.

Humans often make it worse. Especially when coastal communities meet their freshwater needs by pumping huge amounts of groundwater from coastal aquifers.

That can also cause saltwater intrusion into the aquifer, making it useless for drinking.

 

Where it matters most

Coastal cities have special (often worse) vulnerability. Savannah, Georgia, is one example. It has been struggling with the risk of flooding for a long time. Miami is another. The bottom floors of those highrises (not to mention the South Beach nightclubs) may be doomed.

But New Orleans is the city where it all converges. When you sit at the Cafe du Monde enjoying those sugary beignets, you are below sea level. The flooding from Hurricane Katrina was a warning.

[Editor’s Note: For more on covering coastal flooding, see our Topic on the Beat pages on Disasters and Water & Oceans.]

Joseph A. Davis is a freelance writer/editor in Washington, D.C. who has been writing about the environment since 1976. He writes SEJournal Online's TipSheet, Reporter's Toolbox and Issue Backgrounder, and curates SEJ's weekday news headlines service EJToday and @EJTodayNews. Davis also directs SEJ's Freedom of Information Project and writes the WatchDog opinion column.

---

* From the weekly news magazine SEJournal Online, Vol. 10, No. 28. Content from each new issue of SEJournal Online is available to the public via the SEJournal Online main page. Subscribe to the e-newsletter here. And see past issues of the SEJournal archived here.