INDEPTH: FORCES OF NATURE|
CBC News Online | March 17, 2005
What is a storm surge?
The United States National Hurricane Center defines a storm surge as "the abnormal rise in water level caused by wind and pressure forces of a hurricane. It can be extremely devastating, and is in fact a major cause of damage from hurricanes. The storm surge itself is caused by the wind and pressure 'pushing' the water into the continental shelf and onto the coastline. The height of a surge is basically measured as a deviation from the mean sea level in the area, and in some historical storms, this value has reached over 20 feet (six metres)."
Two views of a storm surge. The first picture shows a storm surge of six to eight feet (1.8 to 2.4 metres) plus a normal high tide of two feet (0.6 metres). The second shows a storm surge of 10 feet (three metres), plus a storm tide of 12 feet (3.6 metres), eroding dunes and damaging property. Image courtesy Escambia County Florida Department of Public Safety Emergency Management.
What causes a storm surge?
As the Environment Canada weather warning indicates, a storm surge is caused by a combination of factors.
How dangerous is a storm surge?
- The intensity and size of the surge depends on the strength of the winds. Most storm surges occur during hurricanes and usually the stronger the hurricane, the higher the storm surge.
- Low pressure acts like a vacuum, creating taller waves and raising the sea level.
- The direction of the wind is important, wind blowing on to the coastline pushes water inland, wind blowing along the coast (as is sometimes the case in a hurricane) pushes the water along the shore.
- A lower elevation (as in Bangladesh, which is often threatened by storm surges) means the storm can move water further inland.
- The slope of the sea bottom can also magnify a storm surge under certain conditions.
- Higher tides at the time of a storm surge sometimes called storm tides produce surges that can be 50 per cent higher than normal high tides.
The U.S. National Ocean and Atmospheric Administration says the storm surge "is unquestionably the most dangerous part of a hurricane," adding the surge can "act like a giant bulldozer sweeping everything in its path." In tropical areas, a storm surge spawned by a hurricane can sometimes cause nine of every 10 fatalities.
The danger doesn't just come from wind and water. The surge smashes everything in its path boats, trees, utility poles and buildings.
During Hurricane Camille in 1969, one of the highest storm surges on record 25 feet or 7.6 metres smashed into the town of Pass Christian, Miss., killing 30 people who had been attending a "hurricane party" at an ocean-view apartment complex.