Dear Tom,
Why do hurricanes cause coastal flooding?
Charles Kesman, Chicago
Dear Charles,
It's complicated. First, there is wave setup. A hurricane's huge breaking waves transport
so much water to shore so rapidly that it cannot all flow back before the next wave
arrives. Water literally piles up and the sea level rises as much as 3 feet along the coast.
Then there is the soda-straw effect. Lower air pressure at a hurricane's center causes
the sea there to rise about 1 foot for every 1-inch difference in air pressure. The rise
can be three feet. Finally, there is storm surge. A hurricane's counterclockwise winds
spiral inward toward the eye, pushing water with it. At the storm's center, the water
sinks and flows outward. But as a hurricane approaches land and shallow water, the
water cannot sink; the sea mounds up as much as 20 feet.
Additional complicating factors: storm speed and strength, coastline configuration and
near-shore water depth.
