Hurricanes: Their Nature and Impacts on Society - Climate Science ...

120 HURRICANES: THEIR NATURE AND IMPACT ON SOCIETY
.Rainfall
.Tornadoes.
Of these weather features, the stonn surge has accounted for over 900/0 of the
deaths in a hurricane. In recent years, <strong>and</strong> particularly in the aftennath of
Hurricane Andrew, more attention has been paid to the effects of hurricane
winds.
5.2.1 Stonn surge
"Storm surge" refers to a rapid rise of sea level that occurs as a storm
approaches a coastline. This is in addition to changes in variations in sea level
due to tides. Thus, a storm surge causes greatest inundation at high tide. A
very strong hurricane may produce a storm surge of 20 feet (6 m) of which
about 3 feet (1 m) is due to the lower atmospheric pressure at the center of a
hurricane. The remaining storm surge is due to the following two factors: (i)
the piling up of water at the coast, generated by the strong onshore winds; <strong>and</strong>
(ii) a decreased ocean depth near the coast, which steepens the surge. A
common misconception is that the lower pressure at the center of a storm is
the primary cause of the storm surge.
At l<strong>and</strong>fall, storm surge is highest in the front right quadrant (with respect
to the storm's motion) of a Northern Hemisphere tropical cyclone, where the
onshore winds are the strongest. It is also large where ocean bottom bathymetry
focuses the wave energy (e.g. as in a narrowing embayment). Peak
storm surge from a l<strong>and</strong>falling cyclone increases with greater wind speeds <strong>and</strong>
the areal extent of the storm's maximum winds, out to about 30 miles (48
kilometers).
Storm surge also occurs when a storm parallels the coast without making
l<strong>and</strong>fall (Xie <strong>and</strong> Pietrafesa 1995). The storm surge will precede the passage of
the storm's center when winds blow onshore preceding passage of the eye.
Similarly, the surge will lag the storm's center when the hurricane is moving
such that onshore winds follow passage of the eye. Offshore winds which are
associated with a storm can produce a negative surge as the sea level is
lowered by the strong winds blowing out from the coast.
Storm surge is generally estimated to diminish in depth by 1-2 feet (0.3-0.6
meters) for every mile (1.6 kilometers) that it moves inl<strong>and</strong>. Even if the inl<strong>and</strong>
elevation were only 4-6 feet (1.2-1.8 meters) above mean sea level, a storm
surge of 20 feet (6 meters) might typically reach no more than 7-10 miles (11-
16 kilometers) inl<strong>and</strong>. Thus, the most destructive effect of the storm surge
hazard is on beaches <strong>and</strong> offshore isl<strong>and</strong>s. Stone et al. (1996) provide a recent
evaluation of extensive beach profile changes near Pensacola, Florida as a result
of the storm surge associated with the l<strong>and</strong>fall of Hurricane Opal in 1995.