DEFINITION OF STORM SURGES

Water that is pushed ahead of storms and

typically moves on land as exceptionally high tides in front of

severe ocean storms such as hurricanes. Storms and storm

surges can cause some of the most dramatic and rapid

changes to the coastal zones, and they represent one of the

major, most unpredictable hazards to people living along

coastlines. Storms that produce surges include hurricanes

(which form in the late summer and fall) and extratropical

lows (which form in the late fall through spring). Hurricanes

originate in the tropics and (for North America) migrate

westward and northwestward before turning back to the

northeast to return to the cold North Atlantic, weakening the

storm. North Atlantic hurricanes are driven to the west by

the trade winds and bend to the right because the Coriolis

force makes objects moving above Earth’s surface appear to

curve to the right in the Northern Hemisphere. Hurricane

paths are further modified by other weather conditions, such

as the location of high and low pressure systems, and their

interaction with weather fronts. Extratropical lows (also

known as coastal storms and nor’easters) move eastward

across North America and typically intensify when they hit

the Atlantic and move up the coast. Both types of storms

rotate counterclockwise, and the low pressure at the centers

of the storms raises the water several to several tens of feet.

This extra water moves ahead of the storms as a storm surge

that represents an additional height of water above the

normal tidal range. The wind from the storms adds further

height to the storm surge, with the total height of the storm

surge being determined by the length, duration, and direction

of wind, plus how low the pressure gets in the center of the

storm. The most destructive storm surges are those that strike

low-lying communities at high tide, as the effects of the storm

surge and the regular astronomical tides are cumulative. Add

high winds and large waves on top of the storm surge, and

coastal storms and hurricanes are seen as very powerful

agents of erosion. They are capable of removing entire beaches,

rows of homes, causing great amounts of cliff erosion, and

significantly redistributing sands in dunes and the back beach

environment. Very precise prediction of the height and timing

of the approach of the storm surge is necessary to warn

coastal residents of when they need to evacuate and when it

is not necessary to leave their homes.

Like many natural catastrophic events, the heights of

storm surges to strike a coastline are statistically predictable.

If the height of the storm surges is plotted on a semilogarithmic

plot, with the height plotted in a linear interval and the

frequency (in years) plotted on a logarithmic scale, then a linear

slope results. This means that communities can plan for

storm surges of certain height to occur once every 50, 100,

300, or 500 years, although there is no way to predict when

the actual storm surges will occur. It must be remembered

that this is long-term statistical average, and that one, two,

three, or more 500-year events may occur over a relatively

short period, but averaged over a long time, the events average

out to once every 500 years.

Storms are known to open new tidal inlets where none

were previously (without regard to whether or not any homes

were present in the path of the new tidal inlet), and to close

inlets previously in existence. Storms also tend to remove

large amounts of sand from the beach face and redeposit it in

the deeper water offshore (below wave base), but this sand

tends to gradually move back onto the beach in the intervals

between storms when the waves are smaller. In short, storms

are extremely effective modifiers of the beach environment,

although they are unpredictable and dangerous.

See also BEACH; HURRICANE.

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