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Oct 29
2012
Steve Marr

Storm Surge: Reality Check

Posted by: Steve Marr

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As Hurricane Sandy comes ashore a Reality Check is helpful in understanding the characteristics and effects of a storm surge. We have all seen damage, heard harrowing stories, and seen the number of deaths that follow.  Hurricane Sandy is predicted to create a surge between 6 and 12 feet in many places. What does this actually mean?

 Atlantic City

Above: flooding in Atlantic City

 A hurricane’s storm surge is the most dangerous and destructive part of the storm.  The surging water kills the most number of people who die in hurricanes. A storm surge is the water that is “pushed up on shore” during a major storm.  When a storm surge hits, there is literally no defense if you are in the path, except high ground.

 

Many people think of a storm surge like a tsunami - a wall of water coming inland. This is not accurate.  In a storm surge the water moves in at the same speed of the forward movement of the storm, usually between 8 and 15 miles an hour. The water can rise several feet in a few moments coming as a steady push rather than a solid wall.  Often, as demonstrated by Hurricane Sandy, the storm surge starts before the worst of the storm comes ashore, cutting off escape routes. For example, in a surge the road you would take to exit your home may be two or three feet lower in places than your house. As the water approaches your home your escape may be cut off before you consider exiting.

 

Above: person caught in storm surge

Those who survive the force of a storm surge have been able to grab onto something that floats, like a large timber, pole, tree or other object. However, the fast current often causes you to loose grip.

 

In addition, a storm can change direction while moving inland, sparing some places while creating a massive storm surge in others. If you are in the path of a storm that changes direction you likely cannot change your mind to evacuate due to the weather conditions.

 

Above: USGS picture before, and after storm surge

Escaping a storm surge

 Many believe they can ride out a storm and then evacuate if the storm surge gets too bad. This is a false premise. The water will rise very quickly, perhaps 6 to 10 feet in a few minutes. By the time you experience rising water you are trapped. Even 12” of surge water is enough to push your car around or off the road.

Others believe they can walk or swim out of harms way - another misconception. A surge creates a very strong current similar to “major rapids” which even a skilled kayaker would struggle to navigate. And, without skill you will likely do far worse on your own. In addition, a storm surge brings in a large quantity of debris and each piece will act like a battering ram.    

Predicting a storm surge

Predicting the exact size of a surge and where the worst impact will occur is difficult.  For example, Hurricane Sandy is a very large slow-moving storm. Every storm will be different. Keep in mind:

  • The greatest storm surge is on the right side of the storm near the center.
  • The counter-clockwise wind will push water ashore.
  • The larger the storm’s eye means the greater potential for a surge to develop.
  • The larger the storm, the more water the wind will move on shore. 
  • The surge will be greater where the water is shallower off the coast.
  • The slow moving surge will tend to push water up rivers into protected bays creating the worst flooding in these places.
  • The fast moving storm tends to inundate the coastal areas the most. 

Total Water Height  

A storm surge forecast does not reflect the full extent of the storm’s threat. A storm surge is defined as the height of the water pushed onshore above mean sea level. However, in forecasting the effects of a storm surge a more important factor to consider is what is known as the “high water mark”.

Tide is also a factor. A surge of 10 feet will push water to 12 feet at high tide or only 8 feet at low tide.

And, wave action is another factor. Storms will have large battering waves, in addition to the storm surge. The wave height will depend on the location. Waves may be 15-20 feet high by the open ocean. While a more protected inlet or harbor may only have 4-5 foot waves. Rivers further inland may experience no wave action, just the rising water.

For example, a 10-foot storm surge with a 10-foot wave can create a “high water mark” of 20-feet.  When in harms way you need to calculate the storm surge height, tide level, and the wave height to insure you are on higher ground.  The image below from NOAA illustrates this principle.

 

 Summary

  • There is no effective defense against storm surge except get out of the way.
  • The extent of a surge is hard to predict, small factors at the last moment may change dynamics for the better, or worse.
  • You can’t drive or walk away when the surge comes because the water action will be too strong.
  • King Solomon wrote long ago, “The wise are cautious and avoid danger; fools plunge ahead with reckless confidence.” (Proverbs 14:16 NLT). When a storm surge is coming, better to be wise, than be the fool.


 


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