During the weekend of October 17 and 18, 1998, torrential rains fell over south and southeast Texas. Up to 22 inches of rain fell, resulting in flash flooding from San Antonio to Austin followed by record breaking river floods along the Guadalupe and several other south Texas rivers the following week.

The Lower Colorado River Authority (LCRA) is responsible for the management of the flood control capabilities of Lake Travis so as to minimize the effects of floods on the Highland Lakes and the lower Colorado River. LCRA commissioned a study by a consulting hydrologist to estimate what the effect would have been on the Highland Lakes if the storm of October 1998 had occurred 85 miles north of its actual location.

The LCRA emergency management team used the study as a training exercise. By having the rainfall data fed to them in hourly increments, and based on the rainfall data and estimated lake levels, the team made decisions about opening floodgates at the various dams with the goal of minimizing the flood damage both in the lakes and downstream.

The October, 1998 storm in that hypothetical location would have created what LCRA considers a “worst case” scenario, that is a major storm starting in the north and slowly moving southeasterly, creating flooding on the Llano and Pedernales River tributaries as well as on the Colorado River itself.

The Actual Storm

All of the ingredients for extremely heavy rains came together over south Texas the morning of Saturday, October 17, 1998. A strong upper level trough and surface front were approaching from the west while a persistent low-level southeast flow of very moist air covered south Texas. In addition, a plume of moist air was streaming across the area from Hurricane Madelain off the west coast of Mexico.

Storm conditions began about 5 a.m. Saturday over San Antonio and in the words of the National Weather Service, “developed explosively.”  The storm spread north to Austin. A cold front arrived Saturday night, supporting and intensifying the already heavy weather. Several tornadoes were spawned by the arrival of the front, followed by torrential rains.

As the initial flood wave moved down the rivers Saturday night and Sunday, up to an additional foot of rain fell on the drainage areas. When the event ended, as much as 22 inches of rain had fallen over parts of south and southeast Texas, with many areas receiving over one foot of rain.

By Sunday, October 18, the heavy rains tapered off, and the event became a major river flood affecting seven river basins, draining approximately 10,000 square miles.

The Guadalupe River crested at 25 feet above flood stage in Cuero, spreading into a stream three miles wide. Record high water levels were recorded at 15 locations on these rivers, including Wharton, Cuero, Smithville and Columbus. Some 60 counties in Texas, representing 21,000 square miles and making up nearly one fourth of the state, experienced some degree of flooding from the storm.

An estimated 3,000 persons were in shelters throughout the area, including 700 people in New Braunfels and 600 in San Marcos. Seven thousand persons were evacuated, including 1,000 in Cuero where two-thirds of the town was under water. FEMA estimated that a total of 14,462 homes were destroyed or damaged by the flood.

More than one-third of the city of Victoria was flooded. Three drowned on the north side of San Antonio after 15 inches of rain fell. The San Antonio airport recorded 11.26 inches of rainfall, a new record for the day and the month.

Damage estimates for central Texas were $570 million in residential damage, $207 million damage to roads and bridges, and $53.7 million damage to businesses, totaling $831 million. Applications for federal assistance totaled over 25,000, and 4,800 flood insurance claims were filed.

In total, thirty-one persons lost their lives in the flood, including 26 drownings, 2 tornado deaths, 2 heart attacks and one electrocution/drowning.

Isolated Incident or Common Occurrence?

In May 1999, 125 hydrologists, emergency workers and government officials met at Southwest Texas State University in San Marcos to examine the October storm. The Austin American Statesman quoted Richard Earl of the Freshwater Research & Policy Center of S.W. Texas State as saying, “These huge, catastrophic floods are a very common factor in the flooding history of this region.” Each year in San Marcos, for example there is a five- percent chance of a storm dumping at least ten inches of rain on the San Marcos-New Braunfels area, according to historical data.

Several factors combine to make central Texas prone to severe rain events.   We live near a geologic feature called the Balcones Escarpment. The escarpment is the boundary between the lowlands of the Coastal Plains and the Texas hill country, where the elevation above sea level suddenly rises to about 700 feet.

From a point near Del Rio on the Rio Grande River, the escarpment and its accompanying fault zone run eastward to a point just north of San Antonio, and then swing northeast, crossing the Colorado River just above Austin. Although less well defined Northeast of the Colorado River, the escarpment runs north all the way to the Red River.

The Balcones Escarpment plays two roles in helping to foster severe rain events along its path. First, the sudden rise in elevation causes moisture laden air to rise and cool, then lose its moisture as rain. The steep slopes of the escarpment cause rainfall in the hill country to gain speed quickly as it finds its way down into the cities along I-35. The thin soils and rocky canyons associated with the Escarpment do not lend themselves to absorption of rainwater, but rather rapid runoff.

Finally, the geographic location of the Escarpment happens to be at a point of frequent collision of moisture laden air coming up from the Gulf, cold fronts from the north and storm systems originating in Mexico and the Pacific. The occasional Gulf hurricane adds to the complexity and violence of weather events along the Escarpment.

Some of the most notable rainstorms of the North American continent have occurred along the Balcones Escarpment. Examples include Thrall, Texas where 38.2 inches fell in 24 hours on September 9-10, 1921. During this storm, the greatest of all continental rainstorms, rainfall of 19.65 inches was recorded at Taylor in 12 hours. A peak discharge from the Thrall storm of 647,000 cubic feet per second is the second largest ever recorded in Texas.

On June 10-15, 1935, Uvalde recorded 12.5 inches of rainfall in a ten-hour period. At the center of the storm, 17.6 inches was recorded. Peak discharges of 616,000 cubic feet per second were recorded at the Nueces River at Uvalde and 580,000 cfs at Brackettville represent the third and fourth largest flood discharges ever recorded in Texas.

Another classic rainstorm occurred at D’Hanis between San Antonio and Uvalde on May 31, 1935. At the center of the storm, 22 inches fell in a 2 ¾ hour period. On the Pedernales River at Johnson City, on September 9-11, 1952, a rainstorm produced 28.8 inches of rain.

Some other notable rainstorms along the Escarpment include:

New Braunfels, May 11-12, 1972, 3.0 inches in 20 minutes; 12 inches in one hour; 16 inches total in four hours.
Little River at Cameron (1921) - peak discharge 700,000 cubic feet per second
Devils River near Del Rio (1932) - 400,000 cfs
Hondo Creek near Hondo (1919) - 90,000 cfs
Guadalupe River at New Braunfels (1972) - 90,000 cfs
Sink Creek near San Marcos (1972) - 200,000 cfs
Purgatory Creek near San Marcos (1972) - 40,000 cfs
Little Red Bluff Creek at Carta Valley (1948) - 30,000 cfs
Bunton Creek at Kyle (1936) - 11,000 cfs

Another way to illustrate the susceptibility of the area we live in to disastrous rainstorms is to compare the annual flow of a stream with its maximum peak flow. In other words, how does the extreme flow compare with the normal flow? Hydrologists have made this comparison for several hundred locations around the United States. Of all these locations shown in the hydrology textbook by Linsley, Kohler and Paulhaus, only 18 locations have a maximum flow as much as 100 times the normal flow. Texas has six of these, all located along the Balcones Escarpment. The values for the six Texas locations, moving from south Texas to north Texas, are 14000, 284, 205,172, 1890 and 2020.

Another graphic (Exhibit 1) from that text shows the “Maximum Flood Centers of the United States.” This map shows those locations in the United States, which have a history of extreme rainfall events. Two areas stand out as having the highest probability of extreme rain storms, the Appalachian Mountains and south central Texas. Of a total of 17 “hot spots” of heaviest maximum stream discharges in the U.S., south central Texas accounts for eight.

Austin Has Its Share of Floods

The City of Austin has experienced the effects of damaging floods on several occasions since the 1930’s. Exhibit 2 shows the estimated total discharge from a number of storms that flooded the Colorado River and its tributaries in Austin. Also shown on the graph is a bar on the right side representing the estimated discharge which Austin would have experienced had the October, 1998 storm on the Guadalupe have occurred 85 miles further north.

The 1998 “virtual” storm would not only have been the largest flood event among those recorded in Austin, it would have exceeded the discharge for the standard used by the federal government as the “100 year storm.” The 100-year storm represents a storm which, based on historical records, would have a 1% probability of occurring in any given year.

The Memorial Day storm of 1981 is not included in Exhibit 2 because of inadequate data. The storm produced two inches of rain in its first thirty minutes, measured at the airport, while some parts of the city received 11 inches in two hours. Shoal Creek, which has a normal flow of 90 gallons per minute, swelled to 6.5 million gpm at the peak of the storm. Six hundred homes were flooded, and 13 lives were lost. It was the worst loss of life since the Austin flood of 1935.

Virtual Storm

The transposition of the storm 85 miles north to the watershed of the Highland Lakes was intended to illustrate what could happen to the lakes and the river downstream in the even of a major rain event. The report was also used as a training exercise for the LCRA’s emergency response team that analyzes the data during actual storms and makes the decisions about releases from the dams.

The job of transposing the Guadalupe River storm of October, 1998 to the Highland Lakes consisted of (1) moving the precipitation data from its actual location to the area of the Highland Lakes tributaries, (2) calculating the volume of rainfall over each of the watersheds of the major tributaries of the lakes, including the Llano, Pedernales and Colorado Rivers, (3) predicting runoff from each of these watersheds and inflow into the lakes, and (4) routing flows through the lake system and predicting Lake Travis elevations on an hour-by-hour basis.

The transposed storm produced a “worst case” scenario for LCRA. That is, at the same time that runoff from the tributaries to the lakes is reaching the flood control storage at Lake Travis, local runoff is increasing in the Colorado River downstream of Austin. Normal operational rules call for reducing Lake Travis releases to avoid causing damage at Austin and downstream. But excessive runoff upstream has now reached Travis and filled it, so that water must be released at Mansfield Dam to protect the safety of the dam.

The virtual storm produced an average of nine inches of rain over the part of the watershed involved in the storm, or about one-third of a normal year's rainfall. The storm runoff produced peak flows into Lake Buchanan of 300,000 acre-ft., Lake LBJ of 1,112,198 cfs and for Lake Travis, 1,408,254 cfs. For Lake Travis, this flow is about ten times the average annual flow into the lower river.

The peak release from Mansfield Dam during the virtual storm was 430,000 cfs. In other words, the ability of Lake Travis to store about one million acre-ft. of flood water reduced the flow rate of the water reaching the City of Austin (and its impact on the City) by more than two thirds. Austin's worst recorded storm occurred on June 16, 1935, when peak flows at the Congress Avenue bridge reached 481,000 cfs. The two photographs show the Congress Avenue bridge during that flood, with the river spreading to a width of a mile near downtown and three miles at Webberville.

The total volume of water which would have passed through Lake Travis during the 171 hours of the virtual storm is 2,690,000 acre-ft. Lake Travis contains about 1,000,000 acre-ft. at pool stage, so during the virtual storm, it passed the equivalent of 2.7 times its entire volume.

Effect on the Lakes

The simulation simply passed the runoff from the virtual storm through the upper lakes and into Lake Travis, the only flood control lake in the system. While there certainly would have been temporary peaks in lake levels and flooding at Lake LBJ and Marble Falls, no data was included in the study for lake levels other than at Travis.

The simulation estimated that the lake level at Lake Travis would peak at 734 feet, and remain at or above 714 feet (spillway height) for about 72 hours. At the peak level for Travis, all 24 flood gates would be open, and 20 feet of water would be coming over the spillway. At that level, the roadway on top of the dam would be about sixteen feet above the water level in the lake.

The maximum water level ever attained for Lake Travis is 710.44 ft., reached on December 25, 1991. No more than 8 floodgates have ever been opened at the Mansfield Dam at one time. The design criteria for the dam modernization project is a “Probable Maximum Flood” which would produce 40 inches of rain in three days. The virtual storm of 1998 would have produced about 20 inches of rain in two days.

The integrity of the Mansfield Dam would have remained intact during the virtual storm, although the new Hwy. 620 bridge at the dam would have been underwater at its low end.   Wirtz Dam (Lake LBJ) would have been topped by some four feet and probably damaged, the spillway at Inks Dam washed away, and the floodgates at Starcke Dam (Marble Falls) damaged. The flood modernization program now underway on the LCRA dams, if it had been completed, would likely have prevented any damage to the smaller dams from this rain event. The project is currently about half finished.

Lake Travis Flooding

Virtually all of the waterfront streets in the cities around Lake Travis would be flooded during the virtual storm. Exhibit 3 is a partial listing of these streets which would be partially or completely flooded. There are residential areas around Lake Travis, such as Graveyard Point, which are built in the 100 year floodplain, but the streets listed are located in cities in which construction in the flood plain is prohibited.

A lake level of 734 feet is well above both the levels designated by FEMA as  the 100 year flood plain and the 500 year flood plain. That would imply that the probability of floods of the magnitude of the virtual storm would be less than one tenth of one percent in any given year. Yet when we see the record of similar or worse storms which have hit areas within 100 miles of Austin on a fairly regular basis since records have been kept, we wonder if the 100 year and 500 year storm flood plains may understate the flooding danger.

Summary

In summary, those of us who live along the Balcones Escarpment (I-35), reside in one of the most likely places in the United States for the occurrence of disastrous rainstorms. The October, 1998 storm on the Guadalupe River, which resulted in the loss of  31 lives and hundreds of millions of dollars of property, could just as easily have happened 85 miles north of its actual location.

If it had occurred 85 miles to the north, it would have caused a record-breaking storm on the Llano, Pedernales and Colorado River watersheds above the Highland Lakes and at the same time, heavy flooding on the Colorado River and its downstream tributaries. This combination of events is considered by LCRA to be the “worst case” scenario.

Lake Travis would have risen to 734 feet, 24 feet higher than the record level set in 1991, and 20 feet higher than the spillway of  Mansfield Dam. Most of the waterfront homes on Lake Travis would have been flooded, in spite of being built above the level of the “500 year” flood.

Mansfield Dam would have caught a large part of the runoff from the “virtual” storm but the runoff below Mansfield, coupled with the spillage from Mansfield Dam, would still have caused a record-breaking flood in Austin and in the smaller cities downstream from Austin.

LCRA has a dam modernization project underway. Under the extreme circumstances of the virtual storm of 1998, at least two of the smaller dams above Lake Travis would have probably undergone severe damage or failed outright. Mansfield Dam would have maintained its integrity. Upon completion of the dam modernization project, LCRA officials believe that all of the Highland Lakes dams would have survived the extreme water levels and discharge rates of the virtual storm without severe damage.

Briarcliff
Briarcliff Sleat, Dunkel & Cat Hollow Club

Hudson Bend
Hornsby Hill. McCormick Mtn, Rainbows End, Lands End, Webb, Forest Way, Feather Rock Canyon,
Hudson Bend & Hopkins.

Jonestown
Lakeside, Cross, North Lake Hills Dr. and Harbor Drive.

Lago Vista
Highland Lake, Mockingbird, Azure, Blue Lake, Crystal, Cardinal, Red Bird, Truman, Patriot, Point,
Continental, Cody, Concord, lakefront, Parliament, Trace & Green Shores.

Lakeway
Sailfish, Corinthian, Cutlass, Star, Bermuda,
Challenger, Sailmaster, Ladin Lane, Robin Dale, Edgewater Cove, Squires, and Palos Verdes.

Volente
Posada, Arrowhead, Reed, Booth, Ginger & Geronimo.

Graveyard Point
All under approximately 30 feet of water.

State Senator Kenneth Armbrister gave the opening remarks at a regular meeting of the Lower Colorado Regional Water Planning Group in Bay City on August 12, 1999.
He said,   "the State has provided us with regions under Senate Bill 1, and it was done for a very particular purpose, ... and that was to break the hold that a single bureaucracy in Austin had over water issues, water planning."

Sen. Armbrister serves as Vice-Chair of the Senate Natural Resources Committee.

Readers are invited to guess which "single bureaucracy in Austin" Senator Armbrister was referring to. (Clue: it's initials are TADB).

Considering that the Texas Water Development Board is currently undergoing a review by the Sunset Advisory Commission, Senator Armbrister's remarks may have caused some sweaty palms among TWDB members.

Water Matters' advice - don't worry!

Bob's cartoon this month was inspired by a meeting of the Regional Water Planning Group for Region L (San Antonio) on July 13, 1999. At least 60 people were in the audience at that meeting, including ranchers from Cuero, Catholic nuns from Goliad and residents of Bastrop County. All had one thing in common; the fear that San Antonio water planners would build reservoirs on their property or import water from their local water supply. San Antonio is still considering several dozen alternative water sources to supplement its supply from the Edwards Aquifer, including 12 from the Colorado River basin. So far as we can determine, no single alternative being considered has any supporters but each of the alternatives appears to have many opponents.

Inquires as to membership in Highland Lakes Group or a subscription to this newsletter should be directed to the above address.

This web version of Water Matters is reproduced with the permission of HLG. It is produced in html format by Lonnie Moore of Volente Neighborhood Association (VNA). VNA is a full member of Highland Lakes Group and recommends all visitors to this site support Highland Lakes Group.

Highland Lakes  Group has six directors, three area directors representing each of the three counties of Burnet, Llano and Travis, and three at-large directors. Directors serve three-year staggered terms, and two directors are elected each year.

In 1999, the Llano County area director's seat, which was open, and the at-large seat held by incumbent Dan Roark were up for election. Dan Roark was unopposed and was reelected. Two candidates filed for the Llano County seat, Leon Seidl and Bob Benton, both from Kingsland. Seidl won the election in a close vote.

In September, Director Dan Roark resigned from the HLG board for health reasons.

The remaining board members voted to ask Bob Benton to fill Roark's seat for  the remainder of Roark's term (1999 - 2002). Benton accepted and will fill the seat vacated by the resignation of Dan Roark.

Bob Benton is retired from the airline industry, and is a board member of the Kingsland Municipal Utility District. We welcome Bob as a volunteer director of Highland Lakes Group.

Dan Roark, General Manager of the Hurst Creek M.U.D., served HLG as a director from 1996 through 1999. Dan filled the seat vacated by the death of former Lakeway Mayor Burton Barnes, and was elected to that seat earlier this year. Dan Roark made a very strong contribution of his talents and experience to Highland Lakes Group, and he will be missed. God speed, Dan.

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