Our recent drought in central Texas ended in late 2000 when Lake Travis
rose 36’ in two days. How
much rain did it take to do this? Just
over 5” in four days.
What would happen if a tropical storm such as Allison
(2001) parked itself over the Hill Country west of Austin? Allison reportedly dropped 37” of rain in the Houston area
over six days causing $5 billion in property damage. Granted, Houston’s proximity to the Gulf of Mexico enabled
the tropical storm to hit with full force.
But, don’t be fooled, central Texas can be equally hard hit.
It has happened before and it will surely happen again.
Yes, central Texas is prone to frequent, intense
storms that can produce tremendous amounts of rain.
In 1981, the “Memorial Day Flood” killed 13 after 10” of rain
fell in four hours. In
November 1974, a cold front dropped between four and ten inches of rain in
central Texas resulting in the death of another 13.
In June 1935, 22” of rain fell in three hours leaving 3000 Austin
residents homeless. The
1921 “Great Thrall/Taylor Storm” still holds the record as the
greatest of all continental U.S. rainstorms during 18 consecutive hours.
23.11” fell within 24 hours at Taylor; 36” fell in Thrall
within 18 hours . . . 40” of rain in total.
224 people perished during this storm.
Other great storms in 1915, 1900, and 1869 produced tremendous
property damage and loss of life.
I have to admit that my concerns about Lake Travis
are generally limited to how much water is available for me to enjoy
sailing and other recreational activities.
Seldom, if ever, have I worried about losing my boat to a flood.
More importantly, I had the mistaken impression that our system of
modern dams is iron-clad protection against flooding of biblical
proportions. Make no
mistake, flooding in the Colorado River watershed remains a lethal
possibility. You can read
below about the probabilities and consequences of an “Allison”-type
storm in our area.
I am thankful that the LCRA, the Texas Floodplain Management Association, local counties and cities, and other organizations are working to prevent loss of life and to minimize property damage should a catastrophic storm visit central Texas in the future.
Here are a few photos that show the 1935 flood as well as the construction of Mansfield Dam circa 1938. Click on the image to see a larger version.
For more informations, refer to the following resources:
Courtesy
of the Austin-American Statesman
-- August 23, 2001
If Tropical Storm Allison had moved 250 miles northwest -- a distinct if remote possibility in the crapshoot of weather -- Central Texas would still be cleaning up, six weeks later. Thousands of homes, dozens of businesses and miles of road would be devastated in a storm like Allison, which dropped up to 37 inches of rain during six days, according to a Lower Colorado River Authority analysis.
Unless miracles were plentiful, lives would also be lost.
Town Lake would surge 40 feet, submerging the bridges at Lamar Boulevard and South First Street and sending water down South Congress Avenue.
Lake Austin, suddenly 21 feet higher, would sweep through dozens of million-dollar homes as Tom Miller Dam's nine floodgates struggled in vain to keep up.
Lake Travis would rise 60 feet, snapping marina cables, cracking docks and fouling hundreds of homes. Mansfield Dam would roar like Niagara Falls with a 26-foot-thick wall of water dropping more than 100 feet from the upper spillway.
The computer analysis, developed by the LCRA and obtained Wednesday by the Austin American-Statesman, placed Tropical Storm Allison in the worst possible area -- the flash-flood-prone heights between Junction and Mason in the Hill Country.
It was not just an academic exercise for the LCRA, which manages the dams that form the Highland Lakes northwest of Austin. By using data from an actual storm, the agency can test how the rivers, lakes and dams would handle a serious flood.
More important, the exercise is a stark reminder that the Colorado River dams are no guarantee against large floods that periodically hit Texas, particularly when tropical systems move inland and stall.
"Those lakes aren't big enough to protect us from the level of flooding our neighbor river basins have experienced in the last couple of years," said Wes Birdwell, river operations manager for the LCRA.
"It's easy to build computer models like this. The hard thing is to convince people it could actually happen -- and has happened in the past," Birdwell said.
Sixteen major floods have crashed down the Colorado River or its tributaries in the past 100 years, including six in the 1990s. Most recently, a November 2000 deluge added 36 feet to Lake Travis in two days -- breaking a drought that had drained the lake to a 16-year low.
Some of the most devastating Texas floods have followed the arrival of tropical systems -- Allison in Houston (22 dead), Tropical Storm Charlie in Del Rio (nine dead in 1998), Tropical Storm Amelia in Gillespie County (15 dead in 1978).
"These storms are being steered by the Bermuda high. They push inland about 100 miles or so, they lose that steering current and can sometimes linger here for days," said Bob Rose, an LCRA meteorologist.
The worst local example was the Thrall flood of 1921. Thirty- eight inches fell in 24 hours, drowning 93 people in Williamson County. The confluence of the San Gabriel River and Brushy Creek, typically minor waterways, ran 10 miles wide.
Although Tropical Storm Allison would have behaved differently in the Hill Country -- it would not have been continuously fed by moisture from the Gulf of Mexico -- similar rainfalls have occurred in the past. That makes Allison fair game for LCRA hydrologists and planners, and the study, which will be released this week, is sobering.
An Allison-type storm would produce a 500-year flood, a hugely destructive event that has a 1-in-500 chance of occurring in a given year.
Here's how the LCRA's projection plays out:
The storm stalls near Junction and Mason, an area known as flash- flood alley for its steep slopes and thin soils, and the destruction begins its cascade down the Llano River.
A chocolate-brown surge uproots trees and tosses boulders. The wall of water hits Lake LBJ like a sledgehammer, flooding hundreds of homes and adding boats and docks to the debris. The lake rises to record levels of 841 feet above sea level as water tops the Wirtz Dam embankment by three feet.
Next in the chain is Lake Marble Falls, which crests 18 feet above record heights reached in June 1997.
Lake Travis rises to 740 feet, well past its record height of 710.4 in 1991. The water settles within 10 feet of Mansfield Dam's top, and all 24 of the floodgates are open (six were open in 1957, the most at any time).
Lake Travis captures one-third of the floodwater. The rest of the barrage adds 21 feet to Lake Austin at Tom Miller Dam, although the floodwater would be higher the closer you get to Mansfield Dam. Then it's on to downtown Austin, where normally placid Town Lake resembles a swift river.
A day later, the Colorado River in Bastrop crests at 59 feet, 27 feet higher than the October 1998 flood. That record-setting flood also is surpassed in Smithville (by 12 feet), La Grange (10 feet), Columbus (9 feet), Wharton (3 feet) and Bay City (16 feet).
And when the water recedes, there will be the usual toll -- bacteria warnings, overrun sewage plants, uprooted power lines, mud and debris everywhere.
Still, the LCRA points to several bright points.
Emergency notices would range from several hours in Llano -- where extra rain and flood gauges have been added to improve warning times - - to several days downstream of Austin.
Also, all six Highland Lake dams would survive a 500-year flood, the LCRA says. The agency is spending $50 million to make sure all the dams survive a 1,000-year flood, known as the possible maximum flood. The work is done on Buchanan, Inks and Wirtz dams, with about a year of work remaining on Starcke Dam's floodgates on Lake Marble Falls. Tom Miller Dam on Lake Austin should be finished in 2003, while Mansfield Dam needed no work to survive a maximum flood.
Press
Release from the Lower Colorado River Authority [LCRA]
-- August 23,
2001
AUSTIN - Heavy rains comparable to those that occurred on the Texas Coast from Tropical Storm Allison this summer would have devastating effects if they occurred in the lower Colorado River basin, according to a computer study by the Lower Colorado River Authority.
Runoff from rains of up to 37 inches in the Hill Country would raise Lake Travis 26 feet above the Mansfield Dam spillway, forcing the LCRA for the first time to open all 24 of the dam’s floodgates, the study shows. The rains would result in record or near-record floods downstream, with the Colorado River rising as much as 24 feet above its flood stage at some locations, inundating homes and businesses in the floodplain.
Mansfield Dam would reduce the storm’s impact on downstream communities by holding back as much as a third of the floodwaters, the study indicates. Even so, floodwaters would inundate many downstream communities, as did floods in 1869 and 1913 long before the dam was built. Mansfield Dam forms Lake Travis, which is designed to store floodwaters.
"This degree of flooding would be much worse than anything the basin has experienced since the dams were built," said LCRA General Manager Joe Beal. "We experienced catastrophic floods early in the last century, and more recently major storm events have devastated adjacent river basins, but narrowly missed us. It’s not a question of if we will experience a catastrophic flood, but when.
"Although the chance of this type of storm event is small for our area, perhaps 1 chance in 500 each year, folks should realize that they are taking this risk by locating anywhere in the floodplain, even above the 100-year floodplain," Beal said. "The best protection is not to live in a floodplain, but if you do, you need to prepare for the worst. Buy flood insurance, buy and use a weather radio, and determine an escape route."
The LCRA conducted the study to determine the effects of a storm system that produced rains similar to those of Tropical Storm Allison, which pounded the Texas and Louisiana coasts in June with rains of up to 37 inches. Allison itself probably would not have dropped 37 inches of rain if it had moved to Central Texas due to the increased distance from the Gulf of Mexico. But LCRA hydrologists wanted to determine what impact a storm of similar size and strength could have on the Colorado basin. The study modeled the storm over an eight-day period similar to the time frame when Allison occurred. The study centered the storm between Mason and Junction in the watershed of the Llano River, which feeds into the Highland Lakes, to see what would happen if the lower Colorado River basin caught the full effect.
The storm would produce these results:
The Llano River at Llano would set a record crest of 56 feet – 17 feet higher than during the June 1997 flood – and a flow of more than half a million cubic feet per second (cfs). The current record of 41½ feet, 388,000 cfs, was set in June 1935. Flood stage is 12 feet.
Flows from the Llano, coupled with other storm runoff, would cause Lake LBJ to crest at a record elevation of 841 feet above mean sea level, overtopping the Wirtz Dam embankment by three feet, even with all 10 floodgates open at Wirtz.
As floodwaters continued downstream, Lake Marble Falls would crest at a record elevation of 770 feet, more than 18 feet higher than the June 1997 peak.
Lake Travis would crest at a record elevation of 740, 10 feet below the top of Mansfield Dam and 26 feet higher than the dam’s spillway. The volume of floodwaters would force the LCRA to open all 24 floodgates for the first time in the dam’s 60-year history. Lake Travis is in its "flood pool" when it rises above an elevation of 681. The lake’s all-time high, 710.4, was set Dec. 25, 1991. The most floodgates that have been opened at Mansfield were six during a flood in 1957.
Lake Austin would crest at a record elevation of 513, which is 21 feet above its typical elevation and six feet below the top of Tom Miller Dam, even with all nine floodgates open. The floodwaters would send Lake Austin and Town Lake out of their banks, inundating homes along Lake Austin as well as downtown sections of Austin along Town Lake.
Downstream, the river at Bastrop would crest at a stage of 59 feet, almost 27 feet higher than the October 1998 flood and 34 feet above flood stage.
At Smithville, the river would crest at a stage of about 47 feet, more than 12 feet higher than the October 1998 flood, and 22 feet above flood stage.
At La Grange, the river would crest at a stage of about 56 feet, about 10.5 feet higher than the October 1998 flood, and 24 feet above flood stage.
At Columbus, the river would crest at a stage of 53 feet, more than 9 feet higher than the October 1998 flood and 19 feet above flood stage.
At Wharton, the river would crest at 52 feet, more than three feet higher than the October 1998 flood – which inundated the town – and 13 feet above flood stage.
At Bay City, the river would crest at a stage of 57 feet, 16 feet higher than the October 1998 flood, and 13 feet above flood stage.
LCRA hydrologists agree that a flood of this magnitude is possible in the lower Colorado River basin. In fact, such storms have struck adjacent basins in recent years. The October 1998 flood devastated communities along the Guadalupe River, and Tropical Storm Charlie in 1998 resulted in massive flooding in Del Rio, noted Wes Birdwell, LCRA manager of river operations. Also, Tropical Storm Amelia resulted in massive floods in the Guadalupe basin in 1978.
A small change in weather currents easily could have shifted any of these storms to hit the lower Colorado River basin head on, Birdwell said. The impact of such storms can be magnified if they are centered in the Highland Lakes watershed, which encompasses about a 15,000-square-mile area, Birdwell noted. In addition, the region’s steep slopes and thin soils make it one of the most flood-prone areas of the United States, according to the National Weather Service.
"LCRA’s construction of the Highland Lakes dams, and its success in managing floods smaller in magnitude than the one in the scenario, may have lulled some basin residents – especially floodplain residents along the lakes – into a false sense of security," Birdwell said. "It’s only a matter of time before we experience a storm of this magnitude. We all need to be ready for that eventuality."
The LCRA is nearing completion of a $50 million dam modernization program to upgrade the dams to withstand the effects of a "probable maximum flood," which would be greater than the one in the LCRA study. The LCRA has completed upgrades at Buchanan, Inks and Wirtz dams, is installing new floodgates at Starcke Dam, and is in the design phase of an improvement project for Tom Miller Dam. Mansfield Dam does not need upgrading because engineers have determined that it already can withstand the probable maximum flood.
Allison Teaches a Lesson
By Roy Sedwick, Exec. Dir. of the Texas Floodplain Management Assn.
Is Allison the largest flood to hit Texas? In a word, no. However, Allison has claimed another record--the costliest tropical storm in Texas history. The estimated damages are nearly $5 billion in Texas alone. The Federal Emergency Management Agency (FEMA) has received 111,174 disaster assistance registrations. FEMA's Individual Assistance Program has provided $679.5 million in payments to disaster victims. This includes $161 million for disaster housing, $324 million for SBA loans, and $184 million for individual and family grants.More staggering is the number of damaged structures: 48,439 single-family homes, mobile homes and multi-family homes. Of these, 3,602 were destroyed, 10,735 sustained major damage, and 34,102 sustained minor damage.
A Houston floodplain official has said that only about 7,000 of these flooded structures are within the 100-year floodplain defined on FEMA flood insurance rate maps. What went wrong?
One lesson we can learn from Allison is that Mother Nature doesn't read a FEMA flood map before she decides to flood an area. Floodwaters will not magically stop at the 100-year floodplain boundary during these extreme rainfall events. Inadequate drainage and rapid rainfall on near-level terrain can cause flooding.
Any property owner with a structure situated in the first two tiers of counties along Texas' Gulf Coast should forget the FEMA maps and purchase flood insurance. The insurance is affordable, especially if you are located outside the 100-year floodplain.
Some Texans are learning the lesson. When the next flood occurs--and it will--perhaps most of the damaged structures will be covered by flood insurance. A worthy goal!
© 1998-2003 by
Mark T. Melchior -- All Rights Reserved - Revised:
20 Dec 2007 16:28:41 -0500
"In a powerboat,
you get there in a hurry . . . in a sailboat, you are already there."