Storm Surge

 

The term “Storm Surge” is often used to describe the height of the water caused by an approaching hurricane.  While related to this, it is not actually correct.  We will look at what the storm surge is, what causes it, and what is the actual extent of the inland water rise.  First what metrological features cause it are simply 1) the “sucking up” of the water by the lower pressure associated with the center of a hurricane. This effect however, is much smaller that the “piling up” of the water being pushed ahead by the blowing wind.  So, 2)  the wind speed blowing around a hurricane,  is critical, but to this we must also consider the additional contribution (positive or negative, depending on the side of the storm you are on) of 3) the forward motion of the storm.  On the right side of the storm, it will add to the speed of the circulating storm. This will create larger waves.  The surge is not the peak or the trough of the wave, but average height.  In addition, 4) the angle of approach and 5)  the radius of maximum winds is also a factor. However, how far inland and how high the storm surge is critically depends on two geographical factors, 6) bathymetry, and 7) coastal conformation or shape. Bathymetry is the slope and width of the underwater land or continental shelf.  The smaller the slope, the greater the surge. This makes a great difference.  If the coast line is such that it would tend to funnel the water to a smaller area, then the surge will also be greater, often much greater.  So, the same storm will have a widely varying surge depending on where it make landfall. The same factors have the same effect for tsunamis, by the way.

 But that is not all.  All this must be added to 8) the height of the astronomical tide. The total effect is the combination of the storm surge (which can be pre-calculated for all the coast line for a variety of storm strengths)  and the astronomical tide.  But that is still not all (does this begin to sound like an “infomercial”)?  If you want to know if water will reach you or how high up the beach the water will reach, the high water mark, you have to add 9) the height of the breaking waves (remember the surge is the average water rise). Wind driven waves on top of the surge can add as much as another 40% to the height of the water reaching the shore, thus extending the inland high water make further. Usually this will add additional feet to the height of the water coming ashore.  Along the Gulf coast, storm surges have approached 30 feet for some historical storms. The record was in Australia and was 43 feet.  Often surges will go (funnel) up rivers with disastrous effects that extend far inland.  This is common along the Gulf coast.  Historically, there has been a tendency to significantly underestimate surge height in key locations.

NHC/NOAA has promised improved surge and maximum height plots for particular storms this year.  These maps, if accurate, will be a great help to shore and maritime interests.