When you think of a Hurricane, you probably picture powerful winds and maybe heavy rainfall pummeling a coastline. While you aren't wrong about those impacts, there is one hazard that a good chunk of people don't immediately bring up when they think of a Major Hurricane. And that hazard is storm surge. But what exactly is it, and why is it the leading cause of death in tropical systems? Let's break it down.
Storm Surge is a drastic rise in sea level from a storm system, which occurs due to winds pushing water up against the shoreline and preventing it from spilling back out into the ocean. Most storm surge occurs from tropical systems like hurricanes and cyclones, but they can also be a result of subtropical and non tropical systems, like Nor'Easters and coastal winter storms.
Graphic Courtesy of NOAA
The graphic above from NOAA gives a good explanation as to how it surge works visually. When a storm system nears the shallower waters of the coastline, that water build up is unable to be pushed farther down, like what happens out in the deeper ocean. This causes all that water needing to build upwards and outwards, over the already tidally influenced waters at the coast.
Storm Surge is not a universal constant for evert system that develops. This is thanks to many factors that can affect the longevity and even the intensity. Some common factors that affect the strength and duration of surge are listed below:
It seems quite obvious at first, but the larger and stronger a storm is, the more potent it's surge can be. Stronger winds allow for more water to be piled up along the coastline. Now if you also have a larger storm as well (by larger storm, we mean that the tropical wind field radius extends farther out from the center), it also covers a much bigger surface area of water, leading to an even higher surge.
The speed of a storm doesn't necessarily affect whether or not you see high surge, but rather it's more of an influence on the distance and longevity of the surge. Sure, a faster storm does pile up more water at the shores, but the duration of such is shorter and it also isn't able to push water farther inland. Now take a slower storm: the persistent onshore winds stay around for much longer. This alows for water to stick around at the immediate coast, but also helps carry surge farther inland than what normally occurs. Both slow and fast storms have their pros and cons, but neither is ideal.
So whether or not your storm is strong or slow, the way it interacts with the coastline is crucial, and some coastlines are more at risk than others. As you can see with the graphic above, a storm that moves parallel to the coastline will have lesser impacts in terms of storm surge vs one that approaches from more of a perpendicular angle. The shape of the coastline is also super important to the severity of surge. More concave shaped coastlines, like Apalachee Bay in Florida, allow the water to get trapped easier. Convex coastlines, like the Outer banks of North Carolina, are not as conducive for higher surges.
Whether it's surge from a tropical storm or a major Hurricane, all that water coming into the coastline can be quite destructive and deadly. Crashing waves and water funneling into streets/neighborhoods lead to some serious structural issues. The force of water is so great, that in the worst storm surge cases, weaker structures can be knocked of their foundations and float away. Something you see with raging flash flood events.
The graphic below gives you a quick idea of scale when looking at storm surge heights. While 1-3 feet of water may not sound like a lot on paper, it's enough to knock you off your feet, even carry you away if you're not agile enough. This amount of water is also enough to get into someone's home or place of business, causing significant flood damage. Still, the higher the surge heights, the more dangerous and life threatening it ends up being to those living along coastal communities.
If you're in the path or close to the path of an impending tropical system, knowing how high the potential surge will be is your first bet to figuring out a plan. For surges under 3 feet, sandbagging can be an option, though the force of surge may still be able to get through. If you're planning on ridding it out, you also will want to avoid driving and walking through flooded out areas, just like in any flash flooding situation. However, if the forecast is for greater than 3 feet, you really can't stand a chance from the sheer power of the water. If you haven't already been asked to evacuate, you should do so. Water levels can rise quick, sometimes in as little as a day before the storm arrives. This does not giving you much time to act and get out if you wait till the last minute, especially since escape routes may be cut off and underwater. Always have an emergency plan in case you do need to evacuate!
