What Is Storm Surge?

Storm surge is a buildup of water in the center of a hurricane.  It has several contributing factors. 

The air flows inward at an angle, and is strong enough to make huge waves.  These waves collide in the center of a hurricane and tend to build water up.  Generally, waves do little to move water, but under the extreme conditions water is transported.  Even though the eye of the storm may be calm, the waves become swells and outrun their wind field.  The difference in waves and swells is that after a wave leaves its wind field it is called a swell.

Normally, the only place water normally is moved significantly by a wave is at the coast, where it runs aground.  But, these waves crash against each other in the hurricane’s eye, and water cannot flow back for more water is pushing from the next waves and the wind, hence there is a buildup.

The other part of the storm surge is that the rotation is counter-clockwise, so on the right side of the storm there are onshore waves from a storm directly approaching the coast.  How much onshore water is pushed against the coast depends, in part, on the direction of approach.

So, after on onshore buildup of waves unable to flow back against a relentless wind, the bulge of water in the center of the storm superimposes on the buildup.

Contributing to the buildup is the wind strength, for stronger winds can push more water against the coast, and build a higher bulge in the center of the storm.

And, the width of the storm can be a factor.  If the buildup of water is over a small amount of coast, it can relieve itself by spreading, but in a hurricane the length of coast impacted often impedes this spreading.

Like with a tsunami, the water rises as it reaches a barrier such as land, so there in an increase in water height as waves pile it up at a barrier, the coast.  Bends in the coast can cause a higher piling of water that has more difficulty in escaping.  So, the contours of the coast are important in how high a storm surge will rise.  

A barometer can be constructed of a column of water just over thirty feet tall.  At one time it was speculated that a hurricane becomes a natural barometer with a bulge of water in the center due to reduced pressure.  But, this would account for only about three feet of water in a strong storm, not really enough to build even in the presence of land barriers.  The reduction of pressure certainly contributes, albeit in a minor way, to the overall piling pf water in the center of a hurricane.  It is not the main player, nor a significant contributor to storm surge. 

Other storms do not build up the bulge in the center because they lack the strong winds that last for such a duration, and do not normally swirl inward while driving large waves.  Onshore winds can increase the height of the water at the coast, but not as dramatically as a hurricane.  And, the onshore wind is usually less intense in other storms, and certainly of shorter duration.  

Waves are powerful.  A pounding by a massive wave can do more damage than a high wind, simply because water is much denser than air.  There is more mass providing the push. 

So, let’s consider waves.  Air flowing over the surface of an ocean transfers energy to the water.  This energy moves forward in the form of compressions of water molecules.  Since water is a liquid, it experiences pressure about equally is all directions, so there is an added upward push where pressure from more molecules is greater.  The air cannot match this push, so a wave bulges up.  After the energy moves forward the molecules separate and settle back. 

The energy at the surface of a water wave is apparent, but there is more under the surface that we do not see.  Waves have wave roots, energy unseen below the surface.  It is the wave root that causes breakers.  When the root encounters the bottom of the ocean, it drags due to friction.  The rest of the wave outruns the root, and curls over as a breaker.  So, breakers occur near a shore because the ocean is shallow enough to provide friction to the wave root.

Waves over storm surge are often large enough to have frictional drag on their roots, so churning water results.  This can cause added damage.

I have seen first-hand the damage of several hurricanes.  I remember Hurricane Betsy in 1965, Hurricane Camille in 1969, and Hurricane Katrina in 2005

After one of these hurricanes, and I am saying weeks after, my family went to the Mississippi Gulf Coast.  We could not go the normal way.  When we got to Bay Saint Louis there were concrete sections of the bridge at strange angles.  The hurricane had lifted them from below breaking their bolts in the process and dropped then other than flat on their supports.  They were tilted.  It seemed that most of them had been picked up and dropped.  This was the result to the power of waves from below.  I was impressed.  If water could life a section of a bridge it surly could do major damage.

I still have images of the damage from Hurricane Camille.  The storm surge beached three ocean going ships, large ships.  They had to be removed, and the only way to do it was to cut them up for scrap.  A tug boat came to rest across the highway from the gulf, and a barge washed in and back out several times, destroying buildings.

Heavy damage was done to buildings three blocks from the Gulf of Mexico.  It was apparent that small offshore islands could mitigate the damage, but where there were gaps in the barrier islands the waves did their worst.

Hurricane Katrina did a similar thing to the twin spans that connect New Orleans to the north shore of Lake Ponchatrain.  The bridges had to be replace, since one span was too badly damaged to reuse.  The other was used temporarily until a new bridge could be constructed that is much higher, with the intent that wave action will not reach it.  And, I had to pass a similar bridge replacement due to washing out in Florida, just past Pensacola.  It too was replaced with a higher bridge.

After Hurricane Katrina, and it persists today, there are expanses of vacant land on the Mississippi Gulf Coast.  Homes once stood there, but for about three city blocks from the gulf everything was flattened by storm surge.  All that remained was slabs and driveways.  It has never rebuilt, probably because insurance is either impossible or too expensive to obtain.  One notable structure that remains, even after Hurricane Camille and Hurricane Katrina, is the Biloxi lighthouse. 

The destruction done to the Mississippi Gulf Coast by Hurricane Katrina’s storm surge runs about forty miles. 

I also read an account of the hurricane that struck Galveston, Texas, in the early 1900s.  According to the article I read, which was printed in Waetherwise many years back, homes were built on pilings many feet up.  The waves lifted the homes, then used them as battering rams to take the next homes down.  Thousands perished.  There is no exact count, but between 6,000 and 8,000 is what is usually accepted.

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