enceladus
2005-09-22 17:19:06 UTC
Models show 'massive devastation' in Houston
Damages could cost up to $50 billion -- 10 times Allison's cost
By ERIC BERGER
Copyright 2005 Houston Chronicle
Houston's perfect storm would feed on late summer's warm waters as it
barreled northward across the Gulf of Mexico, slamming into the coast near
Freeport.
A landfall here would allow its powerful upper-right quadrant, where the
waves move in the same direction as the storm, to overflow Galveston Bay.
Within an hour or two, a storm surge, topping out at 20 feet or more, would
flood the homes of 600,000 people in Harris County. The surge also would
block the natural drainage of flooded inland bayous and streams for a day or
more.
Coastal residents who ignored warnings to flee would have no hope of escape
as waters swelled and winds roiled around their homes. Very likely,
hundreds, perhaps even thousands, would die.
Meanwhile, as the storm moved over western Harris County, its most dangerous
winds, well in excess of 120 mph even inland, would lash the Interstate 45
corridor, including Clear Lake, the Texas Medical Center and downtown.
Many older buildings could not withstand such winds.
Anything not tied down, from trees to mobile homes to light poles, would
become missiles, surreally tumbling and flying through the air, flattening
small houses, shattering skyscraper windows and puncturing roofs.
"Unfortunately, we're looking at massive devastation," said Roy Dodson,
president of the engineering firm Dodson & Associates, which Harris County
asked to model realistic "worst-case scenarios" for a major hurricane
hitting the area.
Dodson's firm modeled more than 100 storms of varying power, speed and
landfall. It concluded that a large Category 4 or Category 5 -- a storm only
moderately larger than the four that struck Florida last summer -- would
cause as much as $40 billion to $50 billion in damage. That's 10 times the
cost of Tropical Storm Allison and approximately the city of Houston's
entire budget for the next 15 years.
And this wasn't an academic exercise. Of the 17 Category 4 and Category 5
storms that have struck the United States since 1900, three, all Category 4
storms, have hit the Greater Houston area -- unnamed storms in 1900 and 1915
and Carla in 1961.
Coastal development
With considerable coastal development since then and lower elevations
because of groundwater pumping, no one knows what will happen when a major
storm hits. But what's clear is that models of a hurricane's three modes of
destruction -- winds, storm surge and inland flooding from heavy rainfall --
offer little comfort.
With sustained winds between 131 mph and 155 mph, the power of a Category 4
storm exceeds that of most building codes.
Houston's commercial building rules call for structures to withstand
three-second bursts of at least 110 mph, said Dennis Wittry, managing
director of Houston Structural Operations at Walter P. Moore, an engineering
firm.
Newer skyscrapers, including many built during Houston's downtown boom in
the '80s, were modeled in wind tunnels to determine their performance in
extreme weather events. Most should survive the storm, Wittry said. And the
downtown window loss like that experienced during Hurricane Alicia, a
Category 3 storm that struck in 1983, actually could be less in a bigger
storm.
That's because roofs that were then anchored by gravel -- which become
bullets in high winds -- are now held down by specialized concrete that
should not blow off, Wittry said.
Residential homes, built with less exacting standards and lesser materials,
would fare worse.
"You'll definitely see more significant damage in residential construction,"
he said. "Lower-end homes, or some homes in older areas, would probably be
completely destroyed."
Tie-downs, a structural device that prevents wind blowing over a structure,
creating a vortex and sucking off the roof, have been mandatory only since
the late 1980s, Wittry said.
Various studies of a large storm hitting the Houston area have estimated
that 100,000 to 125,000 homes would be destroyed.
20-foot wall of water
More devastation would be caused by winds blowing over the Gulf of Mexico
and pushing surface water inland -- creating up to a 20-foot storm surge.
Such a wall of water would swamp most development near Galveston Bay,
including Texas City, Kemah and Johnson Space Center. Varying levels of
water would flood much of the area between Sam Houston Parkway and the bay.
On Galveston Island, the seawall could hold back much of the storm surge,
but at some point the water would creep onto the island from the bay side.
The island's highest point is just 22 feet above sea level.
Much like a river becomes deeper and more turbulent when it narrows, a storm
surge also can increase in height and intensity when its source of water
narrows. Dodson said this has profound implications for the Port of Houston.
Some models ended with a 30-foot wall of water in the Ship Channel near the
port's turning basin, he said. "It would be huge," he said. "It could
overwhelm chemical storage facilities, water treatment plants and other
sensitive areas."
The port's severe-weather plan calls for most cargo ships to exit the
facility and weather the storm at sea in preparation for the possibility of
flooded buildings.
Wave modeling
Another, perhaps even-now-unanticipated effect is large waves accompanying
the storm surge.
A waves expert at Texas A&M University at Galveston, Vijay Panchang, said he
and colleagues were surprised when they observed wave data associated with
Hurricane Ivan shortly before it slammed into Alabama last September.
A wave-measuring buoy about 60 miles south of Dauphin Island, before it
snapped, registered an average wave height of about 50 feet, Panchang said.
That means the biggest waves were a staggering 100 feet tall. Such wave
heights, according to his modeling, should only occur every 300 years or so.
Either Ivan's waves were a freak event, or hurricane forecasters may need to
adjust their wave expectations for large storms in the warm Gulf waters.
"This is from a storm that hit only a few hundred miles to the east of us,"
he said. "There's nothing to say that another storm won't create really big
waves for us."
These large waves caused by Ivan may have been as responsible, if not more
so, than the storm surge for severely damaging the I-10 bridge bear
Pensacola, Fla., Panchang said.
Surprises after landfall
Engineers and forecasters say the most unpredictable element of a storm
comes after landfall, when it either dumps rain and floods creeks and bayous
or moves quickly enough that relatively little rain falls.
Tropical Storm Allison probably isn't a good model for what to expect. The
system was so poorly organized and slow moving that some hurricane
forecasters say it wasn't a tropical storm. In some areas of the city,
enough rain fell to classify Allison as a 10,000-year rainfall event. Still,
because a large hurricane's storm surge likely would block the flow of bayou
waters into Galveston Bay, any significant rainfall could back up into
inland streets and homes quickly, Dodson said.
The last major hurricane most Houston residents remember was Alicia, which
made landfall on the west end of Galveston Island in August 1983.
Unfortunately, planners say, as devastating as that storm was, it's a poor
predictor of what to expect from a larger, Category 4 or bigger storm.
Alicia's highest sustained winds on land were measured at 96 mph. Most of
the Greater Houston area received just 5 inches of rain. Storm surges across
much of the area were less than 10 feet, although Seabrook measured 12 feet.
The storm spawned 23 tornadoes, killed 21 people and destroyed 2,300 homes.
"Alicia was a marginal Category 3," Dodson said. "Its rainfall doesn't come
close to this area's top 20 historical floods.
"I guess what I'm saying is that I hope people don't ignore evacuation
warnings because they remember that things weren't apocalyptic during
Alicia."
***@chron.com
Damages could cost up to $50 billion -- 10 times Allison's cost
By ERIC BERGER
Copyright 2005 Houston Chronicle
Houston's perfect storm would feed on late summer's warm waters as it
barreled northward across the Gulf of Mexico, slamming into the coast near
Freeport.
A landfall here would allow its powerful upper-right quadrant, where the
waves move in the same direction as the storm, to overflow Galveston Bay.
Within an hour or two, a storm surge, topping out at 20 feet or more, would
flood the homes of 600,000 people in Harris County. The surge also would
block the natural drainage of flooded inland bayous and streams for a day or
more.
Coastal residents who ignored warnings to flee would have no hope of escape
as waters swelled and winds roiled around their homes. Very likely,
hundreds, perhaps even thousands, would die.
Meanwhile, as the storm moved over western Harris County, its most dangerous
winds, well in excess of 120 mph even inland, would lash the Interstate 45
corridor, including Clear Lake, the Texas Medical Center and downtown.
Many older buildings could not withstand such winds.
Anything not tied down, from trees to mobile homes to light poles, would
become missiles, surreally tumbling and flying through the air, flattening
small houses, shattering skyscraper windows and puncturing roofs.
"Unfortunately, we're looking at massive devastation," said Roy Dodson,
president of the engineering firm Dodson & Associates, which Harris County
asked to model realistic "worst-case scenarios" for a major hurricane
hitting the area.
Dodson's firm modeled more than 100 storms of varying power, speed and
landfall. It concluded that a large Category 4 or Category 5 -- a storm only
moderately larger than the four that struck Florida last summer -- would
cause as much as $40 billion to $50 billion in damage. That's 10 times the
cost of Tropical Storm Allison and approximately the city of Houston's
entire budget for the next 15 years.
And this wasn't an academic exercise. Of the 17 Category 4 and Category 5
storms that have struck the United States since 1900, three, all Category 4
storms, have hit the Greater Houston area -- unnamed storms in 1900 and 1915
and Carla in 1961.
Coastal development
With considerable coastal development since then and lower elevations
because of groundwater pumping, no one knows what will happen when a major
storm hits. But what's clear is that models of a hurricane's three modes of
destruction -- winds, storm surge and inland flooding from heavy rainfall --
offer little comfort.
With sustained winds between 131 mph and 155 mph, the power of a Category 4
storm exceeds that of most building codes.
Houston's commercial building rules call for structures to withstand
three-second bursts of at least 110 mph, said Dennis Wittry, managing
director of Houston Structural Operations at Walter P. Moore, an engineering
firm.
Newer skyscrapers, including many built during Houston's downtown boom in
the '80s, were modeled in wind tunnels to determine their performance in
extreme weather events. Most should survive the storm, Wittry said. And the
downtown window loss like that experienced during Hurricane Alicia, a
Category 3 storm that struck in 1983, actually could be less in a bigger
storm.
That's because roofs that were then anchored by gravel -- which become
bullets in high winds -- are now held down by specialized concrete that
should not blow off, Wittry said.
Residential homes, built with less exacting standards and lesser materials,
would fare worse.
"You'll definitely see more significant damage in residential construction,"
he said. "Lower-end homes, or some homes in older areas, would probably be
completely destroyed."
Tie-downs, a structural device that prevents wind blowing over a structure,
creating a vortex and sucking off the roof, have been mandatory only since
the late 1980s, Wittry said.
Various studies of a large storm hitting the Houston area have estimated
that 100,000 to 125,000 homes would be destroyed.
20-foot wall of water
More devastation would be caused by winds blowing over the Gulf of Mexico
and pushing surface water inland -- creating up to a 20-foot storm surge.
Such a wall of water would swamp most development near Galveston Bay,
including Texas City, Kemah and Johnson Space Center. Varying levels of
water would flood much of the area between Sam Houston Parkway and the bay.
On Galveston Island, the seawall could hold back much of the storm surge,
but at some point the water would creep onto the island from the bay side.
The island's highest point is just 22 feet above sea level.
Much like a river becomes deeper and more turbulent when it narrows, a storm
surge also can increase in height and intensity when its source of water
narrows. Dodson said this has profound implications for the Port of Houston.
Some models ended with a 30-foot wall of water in the Ship Channel near the
port's turning basin, he said. "It would be huge," he said. "It could
overwhelm chemical storage facilities, water treatment plants and other
sensitive areas."
The port's severe-weather plan calls for most cargo ships to exit the
facility and weather the storm at sea in preparation for the possibility of
flooded buildings.
Wave modeling
Another, perhaps even-now-unanticipated effect is large waves accompanying
the storm surge.
A waves expert at Texas A&M University at Galveston, Vijay Panchang, said he
and colleagues were surprised when they observed wave data associated with
Hurricane Ivan shortly before it slammed into Alabama last September.
A wave-measuring buoy about 60 miles south of Dauphin Island, before it
snapped, registered an average wave height of about 50 feet, Panchang said.
That means the biggest waves were a staggering 100 feet tall. Such wave
heights, according to his modeling, should only occur every 300 years or so.
Either Ivan's waves were a freak event, or hurricane forecasters may need to
adjust their wave expectations for large storms in the warm Gulf waters.
"This is from a storm that hit only a few hundred miles to the east of us,"
he said. "There's nothing to say that another storm won't create really big
waves for us."
These large waves caused by Ivan may have been as responsible, if not more
so, than the storm surge for severely damaging the I-10 bridge bear
Pensacola, Fla., Panchang said.
Surprises after landfall
Engineers and forecasters say the most unpredictable element of a storm
comes after landfall, when it either dumps rain and floods creeks and bayous
or moves quickly enough that relatively little rain falls.
Tropical Storm Allison probably isn't a good model for what to expect. The
system was so poorly organized and slow moving that some hurricane
forecasters say it wasn't a tropical storm. In some areas of the city,
enough rain fell to classify Allison as a 10,000-year rainfall event. Still,
because a large hurricane's storm surge likely would block the flow of bayou
waters into Galveston Bay, any significant rainfall could back up into
inland streets and homes quickly, Dodson said.
The last major hurricane most Houston residents remember was Alicia, which
made landfall on the west end of Galveston Island in August 1983.
Unfortunately, planners say, as devastating as that storm was, it's a poor
predictor of what to expect from a larger, Category 4 or bigger storm.
Alicia's highest sustained winds on land were measured at 96 mph. Most of
the Greater Houston area received just 5 inches of rain. Storm surges across
much of the area were less than 10 feet, although Seabrook measured 12 feet.
The storm spawned 23 tornadoes, killed 21 people and destroyed 2,300 homes.
"Alicia was a marginal Category 3," Dodson said. "Its rainfall doesn't come
close to this area's top 20 historical floods.
"I guess what I'm saying is that I hope people don't ignore evacuation
warnings because they remember that things weren't apocalyptic during
Alicia."
***@chron.com