Storm surges
Heavy winds produced by hurricanes push the ocean in front of them. As this water gets pushed into the shallow zones along the coastline sea level rises. Since the storm surge is driven by the winds, the height of the rise in sea level is related to the velocity of the wind. For a moving storm the greater winds occur on the right side of the storm (in the northern hemisphere). Sea level also rises beneath the eye of the storm due to the low pressure in the eye. But, the surge generated by this low pressure is usually much less than the wind-driven surge. The height of the storm surge depends on wind speed, the shape of the coastline, and variations in the water depth along the coast line. Height also depends on tidal cycles. If a storm approaches the coast during high tide, the storm surge will be higher than if it approaches during low tide. Category 5 tropical cyclones can produce storm surges in excess of 6m (20 feet). The highest storm surge measured, 12.8 m (42 feet) occurred in 1899 in Australia.Because the storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the hurricane makes landfall. This is an important point to remember because flooding caused by the surge can destroy roads and bridges making evacuation before the storm impossible.
Since
thunderstorms accompany hurricanes, and these storms
can strike inland areas long before the hurricane arrives,
water draining from the land in streams and estuaries
may be impeded by the storm surge that has pushed water
up the streams and estuaries.
It is also important to remember that water that is pushed
onto the land by the approaching storm (the flood surge)
will have to drain off after the storm has passed. Furthermore
after passage of the storm the winds typically change
direction and push the water in the opposite direction.
Damage can also be caused by the retreating surge, called
the ebb surge.
Along coastal areas with barrier islands offshore, the surge may first destroy any bridges leading to the islands, and then cause water to overflow the islands. Barrier islands are not very safe places to be during an approaching hurricane!
Hurricane damage
Hurricanes cause damage as a result of the high winds, the storm surge, heavy rain, and tornadoes that are often generated from the thunderstorms as they cross land areas. Strong winds can cause damage to structures, vegetation, and crops, as described in the Saffir-Simpson scale discussed previously. The collapse of structures can cause death. The storm surge and associated flooding, however, is what is most responsible for casualties. Extreme cases of storm surge casualties have occurred as recently as 1970 and 1990 in Bangladesh.Bangladesh is an area with high population density and with over 30% of the land surface less than 6 m above sea level. In 1970 a cyclone struck Bangladesh during the highest high tides (full moon). The storm surge was 7 m (23 ft.) high and resulted in about 400,000 deaths. Another cyclone in 1990 created a storm surge 6 m high and resulted in 148,000 deaths.
The amount of damage caused by a tropical cyclone is directly related to the intensity of the storm, the duration of the storm (related to its storm-center velocity, as discussed above), the angle at which it approaches the land, and the population density along the coastline. The table below shows how damages are expected to increase with increasing tropical storm category. Like the Richter scale for earthquakes, damage does not increase linearly with increasing hurricane category.
| Category | Relative Damage | Median Damage (1990 Dollars) |
|---|---|---|
| 1 | 1 | $24 million |
| 2 | 4 | $218 million |
| 3 | 40 | $1.1 billion |
| 4 | 120 | $2.3 billion |
| 5 | 240 | $6.9 billion |
