As Hurricane Ike races toward Texas, it is pushing a mound of water in front of it that could inundate parts of the Gulf coast with up to 25 feet of water. The surge involves some incredible feats of physics, and in many hurricanes it’s the leading cause of death.
Galveston Island, Texas, destroyed at least once before by a major hurricane in 1900, began to see the Ike-related water creeping up along its beaches Thursday and by Friday, parts of the city of Galveston were flooded by the surge coming in from Galveston Bay.
And, "it's only going to get worse," said Lance Wood, the Science and Operations Officer for the Houston/Galveston National Weather Service (NWS) office.
How it works
As a hurricane travels over the ocean, its strong winds push against the water's surface, causing it to pile up higher than the sea's ordinary level. As the storm approaches land, this pile of water, called storm surge, is pushed ashore.
The growth of a surge depends on wind strength, plus how long the wind blows in one direction. As has been the case with Ike, the surge can build for hours and hours while a storm is still hundreds of miles away.
Surges build gradually as a storm approaches, but when a hurricane makes landfall, the surge can suddenly grow into a towering monster. Storm surge is the main cause of the death and destruction associated with hurricanes; the raging waters can swiftly move in and flood roads and houses and wash away cars, trees and other debris.
Surge along the beach fronts in the Houston/Galveston area had already mounted to 8 feet high early Friday afternoon, Wood said. Projections right now have that surge hitting up to 14 to 16 feet on the Gulf side of the island and 15 to 20 feet on the Bay side.
"There's going to be a lot of water coming at them," Wood said.
Strength and size
Ike is piling up so much ocean water partly because of its strength — it is on the upper end of Category 2 storms (on the Saffir-Simpson scale, which rates hurricanes by their wind speeds) and could become a Category 3 storm sometimes before it makes landfall late Friday or early Saturday morning.
As far as storm surge is concerned though, Ike is effectively a Category 3, NWS storm surge specialist Will Schaffer told LiveScience. To estimate storm surge, forecasters look at the difference in pressure between the outside of the storm and the low at the storm's core; the greater the difference, the higher the surge.
Also a factor in Ike's substantial surge is its massive size. When a hurricane reaches Ike's size, its energy is spread out over a larger area, which in turn spreads the ravaging winds.
A hurricane rotates counterclockwise, and so ever since Ike entered the Gulf of Mexico, the winds to the north and east of the storm have been blowing toward the Gulf Coast states.
Ike's hurricane-force winds currently extend out 120 miles (195 kilometers), and its tropical storm-force winds are reaching out to 275 miles (445 km), which means that the surge associated with those winds can extend along the same range.
Reports already show surge flooding in Louisiana and as far away as Mississippi.
Forecasters expect the Houston/Galveston area and adjacent portions of the Texas coast to bear the brunt of the storm surge threat. Of particular concern is the side of Galveston Island facing Galveston Bay, which does not have the sea wall that protects the Gulf-facing portion of the island. With the amount of flooding that could overtake the unprotected areas, "it's going to be devastating," Wood said.
Even with the protective sea wall on the other side of the island, "it's going to be close to the top of the sea wall," Wood said.
Those storm surge estimates were made with the current predicted track of Ike. If the storm shifts so that Galveston is hit by the northwest quadrant of the storm, the island could be washed over by the highest storm surge levels predicted.
"We could overtop that sea wall by several feet," Schaffer said.
However, if the storm jogs and hits north of Galveston, the storms winds will be coming of the land and storm surge likely won't be as bad.
The level of surge has slowly and steadily climbed since yesterday, Wood said, but "it's not the abrupt rise that we're going to get eventually" as Ike makes landfall.
As the storm's eyewall comes closer and closer to land and the winds intensify, the surge levels will rise rapidly.
Galveston Island began a mandatory evacuation yesterday to get residents out of harms way, but the houses and other historic structures are staring down a threat that they haven't seen since the 1900 hurricane that completely destroyed the town.