Most of the world’s biggest cities and about half of the global population live within 100 kilometers (60 miles) from the ocean. That’s pretty important, especially given the huge amount of land that isn’t near a coastline. We humans just tend toward the ocean - it’s got food, it’s got ships, it’s got beaches and waves. It’s got unimaginable beauty. But not everything about the coast is great, especially when all that ocean water starts finding its way up the shore and into developed areas. We’ve talked about riverine flooding caused by intense precipitation . But, there’s another type of flooding that has almost nothing to do with rain and almost everything to do with air. Hey, I’m Grady and this is Practical Engineering. Today, we’re talking about storm surge and coastal flood protection.

Whether you call them hurricanes or typhoons, tropical cyclones are some of the most devastating phenomena that Mother Nature has to throw at us. They get their own names and their own elite aircrew reconnaissance squadrons and their own cult following of hardcore weather nerds (including me). These gigantic rotating storms coalesce over the ocean, fed by warm tropical waters. And, when they happen to make landfall, the results can be catastrophic. Hurricanes and typhoons are a study in extremes, producing some of the fastest sustained winds and the highest precipitation depths across the globe. But, the most damaging part of a tropical cyclone is an effect called storm surge which produces flooding and inundation of coastal areas that can be practically unimaginable. In fact, the vast majority of lost lives and dollars of damage caused by hurricanes every year can be attributed not to the wind, lightning, or rainfall but to the storm surge. So, what is it?

Storm surge is an increase in sea level that results mostly from all that wind. Still water is level - that means its surface is perpendicular to the local gravity vector - which at small scales is a just straight line. But, when you start introducing other forces, things change. For example, the gravity from the sun and the moon cause seas to bulge and contract, leading to high and low tides. The other force that can affect the level of the sea is wind. When air passes along the surface of a waterbody, it creates a shear force. The viscosity - or stickiness - of the wind transfers some of its momentum, carrying the water along with it. When that water encounters an obstacle, like a shoreline, it has nowhere to go but up. The effect is that the wind blows the water against the shore and it bulges up above the normal sea level. But, the ocean is always windy. This effect is so pronounced for tropical cyclones because of the intensity and consistency of the winds. If you’ve ever experienced one of these storms, you know how bizarre it is. Hurricanes aren’t just gusty, the wind is faster than highway speeds and it’s constant. That’s what allows so much of the sea to move toward the coast and up the shoreline.

Characterizing storm surge might seem pretty simple at first glance. Just measure how high the sea goes for various wind speeds. Connect the dots and you’ll know, for any hurricane, the height of the surge as long as you know the speed of the wind. But, like all real-world challenges (and especially those involving weather), things aren’t quite so simple. First off, just knowing the wind speed of a hurricane is pretty challenging on its own. It varies from the center of the storm to the outside and from the top to the bottom. Magnitude of storm surge also depends on how fast the storm itself is moving and where it makes landfall. Hurricane winds move in a circular motion rather than a straight line, so every part of the shore sees a different wind direction. In fact, one side of the storm can actually pull water away from the shore, creating a reverse storm surge. Tropical cyclone winds change over time as the storm itself changes intensity, direction, and size. Storm surge height is also sensitive to the air pressure and the ocean bathymetry, the depth and shape of the terrain below water. A steep coastal shelf keeps the water deeper for longer which leads to lower storm surge. A gradual, shallow shelf creates a higher surge. All this complexity combined together means it is not that easy to predict the height of storm surge we’ll see along the coast during a hurricane or typhoon. In the U.S., the National Weather Service uses a numerical model to try and capture all these details called Sea, Lake, and Overland Surge from Hurricanes, or just SLOSH. This model gets run to develop an approximate map of potential storm surge that can be used by emergency managers for purposes like coordinating evacuations.

It’s difficult to overstate how vulnerable our cities are to storm surge. We build so much stuff along coastlines: ports, houses, roads, railways, airports, and more. Around half of the world’s economic activity happens in coastal areas. Storm surge can raise the ocean’s level by upwards of 8 meters or 26 feet in the worst cases, like when the storm landfalls during the normal high tide. Once that water’s there, waves crash against buildings and other things that weren’t meant to withstand the enormous force of the sea. The damage can be incredible. Storm surge also makes freshwater flooding worse, something that can be particularly bad during a hurricane because there’s so much rain. Urban drainage systems mostly work by gravity, so storm sewers are sloped downward to carry floodwaters away. If storm surge has pushed seawater inland, you have a smaller difference in elevation between where the water is and where you need it to be, slowing down the drainage of rainwater and making the flooding even worse.

Coastal flooding is a difficult challenge to address, but we do have ways of managing it. The first is detection and monitoring. Real-time sensors track sea levels and relay the data online so we can see the timing, extent, and magnitude of storm surge as it happens. This can help us make critical public safety decisions like where to evacuate. These instruments also help us evaluate coastal flooding after it occurs so we can be better able to predict what will happen the next time. Another way we deal with storm surge is just to build stuff further up. The higher you go in elevation, the lower your vulnerability to storm surge. That can be tough to do in shallow coastal plains where the ground isn’t much higher than sea level. We use elevated foundations like pilings to try and keep water from damaging structures. We can also put more resilient parts of buildings, like parking structures, on the lower floors to minimize flood damage. Another major way we deal with coastal flooding is with barrier infrastructure: walls or dams that hold back the sea when it rises too high. These can be as simple as earthen coastal levees or as sophisticated as the Delta Works in the Netherlands, a complex series of dams, locks, and levees which protect the vulnerable Dutch coastline from storm surge. Many of these structures, like the Maeslantkering are normally opened to reduce impacts on the environment and allow for the passage of ships. They only close when a storm threatens to raise the sea level and flood the coast.

Take a look at a generalized hazard map and you can truly get a sense of how exposed coastlines can be to this type of flooding. And, it’s not getting any better. The Intergovernmental Panel on Climate Change synthesized a huge body of research on tropical storms and specifically how their frequency and intensity might change in the future. Their conclusion was that the frequency of tropical cyclones may stay the same or even go down over time, but their intensity - that is the wind speeds and rainfall amount - is likely to increase due to greenhouse warming. Combine that with expected rise in sea level and it has some pretty important implications to coastal areas. Those risks of flood damage are baked into the cost of development in hurricane-prone regions. Most importantly, it means we need to continue to innovate solutions to not only reduce the likelihood of storm surge with protective infrastructure but also to reduce the consequences of it by making cities more resilient to flooding so that when the next storm comes, they can recover more easily and quickly than ever.