When most people think about hurricanes, they think about the wind.
But it’s the water that usually does the most damage. While torrential rain causes flash flooding, it’s the hurricane’s storm surge that can be particularly destructive.
We all know about the sea level rising and falling with the tides. That’s due to the moon’s gravitational impact as it orbits the Earth.
But a hurricane sends waves of water well above the normal tidal oscillation crashing ashore, and moving water imparts significant pressure on anything it encounters. Think about this: A cubic foot of water weighs 62 pounds.
Now, think about a wall of water pushing ashore. That’s a lot of force hitting your car, house, trees, etc.
There are many factors that impact how high a storm surge will be.
The most obvious is the strength of the hurricane’s wind and its direction relative to the shore.
If the wind is blowing in perpendicular to shore, then it’s just a matter of wind speed. The stronger the wind, the higher the push of water will be.
And by the way, some people think that the deep low pressure associated with hurricanes is also a cause, but keep in mind that this only contributes 5% to the overall storm surge height.
Obviously, if the wind is blowing at an angle toward shore and not directly at it, then that reduces some of the storm surge height.
Something else to consider is the continental shelf and how quickly it becomes shallow as you approach shore.
A great example provided by the National Hurricane Center is a comparison between the Louisiana and Miami coastlines.
A Category 4 hurricane heading into Louisiana encounters a very wide and shallow continental shelf and generates a 20-foot storm surge.
The same hurricane approaching Miami is dealing with a steep drop-off in the continental shelf, resulting in an 8 or 9-foot storm surge! It's all because of the difference in geography of the ocean floor at the two locations.
Something else usually referenced when talking about a landfalling hurricane, but not usually considered important by many people, is beach erosion.
A strong storm surge is a violent push of water that can move tremendous amounts of sand and destroy coastal roads. I’ve seen many photos after strong landfalling hurricanes showing the huge push of sand inland, and it is a slow, tedious process to remove this sand.
Probably the most famous storm surge event occurred with Hurricane Katrina in 2005. The surge was estimated to be 25 to 28 feet above normal tide level and caused $75 billion in damage to the Louisiana and Mississippi coasts. According to the National Hurricane Center, many of the 1,500 deaths in Hurricane Katrina were directly or indirectly caused by storm surge.
This is why it is so vitally important to evacuate if told to do so.
Once a storm hits, law enforcement agencies will generally NOT come rescue you if water is suddenly pounding your home and you don’t think it’ll withstand it.
You had your chance and decided to accept the odds. It is not acceptable to put a first responder’s life at significant risk of being lost. Listen to the instructions given by your emergency manager or local government, and leave if told to go.
As far as Hurricane Dorian is considered, if the current forecast track (offshore) holds true, then most of our coast will receive north or northeasterly winds, so storm surge won’t be nearly what it would be if the storm were to make landfall.
However, all hurricanes really churn up the sea and generate significant wave action well away from the center, so dangerous and damaging surf, waves and rip currents are likely as the storm moves northward.