22 May 2019

Storm surge: the deadliest part of hurricanes

Storm surge resulting in severe coastal flooding during Sandy (2012). Photo: Master Sgt. Mark C. Olsen/U.S. Air Force
Hurricanes are categorized only by the peak sustained wind found somewhere in the storm... that is the metric used in the Saffir-Simpson scale.  That category rating unfortunately tells you nothing about how large the storm is, how much rain it could produce, or how much storm surge it could generate*.
(* The original scale did link the wind scale with a surge depth, but that was abandoned in 2010 after numerous storms demonstrated that the correlation between the two isn't very good.)

Perhaps surprisingly, about 3/4 of hurricane-related fatalities in the United States are due to storm surge (1/2) and rain (1/4), while the wind accounts for just 1/12!

The definition of storm surge from the National Hurricane Center is:
An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

In other words, storm surge is the water that gets bulldozed onto land by a hurricane's wind.  It can cause water levels to rise gradually, abruptly, by a little bit, or by over 20 feet... it all depends on the size, speed, direction, and intensity of the storm, the topography of the land, and the bathymetry of the nearby ocean. Because of all of these variables, it is challenging to predict the extent and depth of storm surge far in advance.  Furthermore, storm surge is not just limited to the immediate coast; it can travel into bays, rivers, and canals.

But when the storm surge arrives can make a big difference in its flooding potential. Will the peak storm surge arrive at low tide or high tide? Will it last long enough to span multiple high tides? The combination of the storm surge and the regular astronomical tide is called the STORM TIDE, and that's the water level you experience.

I have three examples here to illustrate this: Wilma (2005), Irma (2017), and Sandy (2012).

The Wilma case is taken from Virginia Key (Miami).  At its closest, Wilma's center passed about 60 miles to the northwest of this location, but the wind field was large. In this tide chart spanning one day, the regular astronomical tides are shown by the blue line, the observed water level is shown by the purple line, and the difference between them (the "residual") is therefore the storm's contribution, or storm surge.  The storm surge was about four feet, but fortunately, it arrived abruptly right at low tide!  A few hours later, there was an "anti-surge", where winds essentially blew the water away from land.

The Irma case is again from Virginia Key (Miami).  Like the Wilma tide chart, this one spans one day, but you quickly notice that it looks quite different. Irma's storm surge didn't come or go abruptly, but rather very gradually.  At its closest approach, Irma's center passed 95 miles to the west, and it too was a large storm.  In this situation, the storm surge was also about four feet, but it hovered in the 3-4-foot range for hours during high tide, producing a peak storm tide that was 1.1 feet higher than Wilma's. 

Finally, the Sandy case is taken from Bergen Point (New York).  This one is a little trickier, but illustrates the timing issue. First thing: this chart spans five days rather than one... Sandy's wind field was enormous. The regular astronomical tides are shown by the dark blue line, the observed water level is shown by the red line, and the storm surge is the green line (ignore the dashed ones for now). At its closest approach, Sandy's center passed 80 miles south of this location. The first thing you notice is that the peak 9.4-foot storm surge arrived at high tide on the evening of October 29.  This unfortunate timing maximized the amount of flooding that would occur. 

But with the exact same storm and track, I shifted the landfall time to be 5.3 hours earlier than what really happened to minimize the peak storm tide... the result is 3.3 feet lower!  This hypothetical scenario is illustrated by the dashed lines.

Clearly, storm surge must be taken seriously.  Storm surge, not wind, is the aspect of hurricanes that often defines evacuation zones and priorities. Sandy had Category 1 winds when it created the 9.4-foot storm surge and inundated parts of New York City, so the "it's only a Category 1" mentality needs to change in favor of thinking about all of the hazards -- not just the wind.  There's more to the story than the category!

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