600,000 in Harris County (Houston) will flood in Cat 4/5

[logybogy]

This is just in Harris County. It's a given in this analysis that Galveston County will basically cease to exist....so we got 600,000 in Harris and almost 300,000 in Galveston. Potentially, 900,000 people severely impacted just by flooding from storm surge! We aren't talking wind damage yet!

Feb. 20, 2005, 1:09AM

Models show 'massive devastation' in Houston
Damages could cost up to $50 billion -- 10 times Allison's cost
By ERIC BERGER
Copyright 2005 Houston Chronicl


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."

[BReb]

Houston will flood in a category 4/5? Then explain this:

http://www.wxresearch.com/houhur.htm

Within the Houston city limits, only a small portion of far southeast Houston could be expected to be affected by a storm surge from an intense hurricane. Therefore, most of Houston would not need to evacuate. If your property lies below the 25 elevation and has flooded before; if you live on the edge of bayous that are liable to overflow; or if you live in a mobile home, make plans to evacuate when a hurricane threatens the upper Texas Coast. Most of the damage in Houston will result from high winds, tornadoes and rainfall flooding. It would be reasonable for residents of Houston to expect winds, from a nominal hurricane, in excess of 75 mph, and the damage that would result from these type of winds.

- end quote -

Here is the official Houston storm surge map for a Category 5 hurricane, which there is very little chance of Rita being with the water temperatures and dry air near Texas:

http://www.houstontx.gov/oem/c5ss.jpg

[soonertwister]

Houston will flood in a category 4/5? Then explain this:

http://www.wxresearch.com/houhur.htm

Within the Houston city limits, only a small portion of far southeast Houston could be expected to be affected by a storm surge from an intense hurricane. Therefore, most of Houston would not need to evacuate. If your property lies below the 25 elevation and has flooded before; if you live on the edge of bayous that are liable to overflow; or if you live in a mobile home, make plans to evacuate when a hurricane threatens the upper Texas Coast. Most of the damage in Houston will result from high winds, tornadoes and rainfall flooding. It would be reasonable for residents of Houston to expect winds, from a nominal hurricane, in excess of 75 mph, and the damage that would result from these type of winds.

- end quote -

Here is the official Houston storm surge map for a Category 5 hurricane, which there is very little chance of Rita being with the water temperatures and dry air near Texas:

http://www.houstontx.gov/oem/c5ss.jpg

Right. So when you refer to Houston you mean Houston city limits only. It's funny, in Colorado, people don't consider just the city of Denver's half a million or so residents as Denver, but the metro area of over 2.2 million.

And your map leaves out about 3/4th of the entire area that would flood. Additionally, the category 5 flood zone is only about 10-15% larger in terms of flood surge than category 4.

So now it's only the people in Houston proper, and we'll ignore the massive number of people outside the city limits? How humane of you.

Don't worry about the fortress, even if your peasants beyond the wall are beset upon. Take a look at the latest official evacuation map for Brazoria/Harris/Galveston County if you want a better picture of what we are dealing with.

http://www.srh.noaa.gov/hgx/tropical/wo ... ionMap.pdf

[logybogy]

Houston is in Harris County. Like most major metros, the city limits are quite small in both size and population compared to the entire county where that city is in.

[wxman57]

Houston will flood in a category 4/5? Then explain this:

http://www.wxresearch.com/houhur.htm

Within the Houston city limits, only a small portion of far southeast Houston could be expected to be affected by a storm surge from an intense hurricane. Therefore, most of Houston would not need to evacuate. If your property lies below the 25 elevation and has flooded before; if you live on the edge of bayous that are liable to overflow; or if you live in a mobile home, make plans to evacuate when a hurricane threatens the upper Texas Coast. Most of the damage in Houston will result from high winds, tornadoes and rainfall flooding. It would be reasonable for residents of Houston to expect winds, from a nominal hurricane, in excess of 75 mph, and the damage that would result from these type of winds.

- end quote -

Here is the official Houston storm surge map for a Category 5 hurricane, which there is very little chance of Rita being with the water temperatures and dry air near Texas:

http://www.houstontx.gov/oem/c5ss.jpg

That "small part of southeast Houston" has over 1 MILLION people in a surge zone. I believe 1.5 million people would have to be evacuated surrounding Galveston Bay. That's a lot of homes at risk. Communities like League City, Webster, Dickinson, Kemah, Clear Lake, Friendswood, Texas City, La Porte, Baytown, Pasadena, Deer Park could all be flooded.