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NEW 100-YEAR FLOODPLAIN

Click aerial map to see detail of how the changes affect a portion of Lake Travis; this image shows the area near the Austin Yacht Club. New maps from the Federal Emergency Management Agency are expected to change Lake Travis' 100-year floodplain elevation from 716 feet above mean sea level (msl) to 722 feet msl.

PHOTOS FROM 1991 FLOOD:

TOP: Photo of house submerged at Lake Travis during Christmas 1991 flood. BOTTOM: Aerial photo of houses at Graveyard Point during Christmas 1991 flood. Lake Travis reached its historic high of 710 feet msl during this flood.

n unusual weather pattern sets Lake Travis on its record-breaking rise. Wave after wave of Pacific storm systems pass slowly over the Hill Country. Over a period of 10 days, 10 inches of rain shower down on the James River, a tributary of the Llano River in Mason County. This is the center of a storm covering 10,000 square miles of the Hill Country.

Creeks, streams and rivers swollen by heavy rainfall pour into the Highland Lakes, causing high water, fast currents and dangerous conditions. LCRA, which is responsible for managing the Highland Lakes and dams, has opened floodgates at the dams along the upper Highland Lakes to move the floodwaters as quickly and safely as possible to Lake Travis. This is causing swift, high water in three “pass-through” lakes: Inks, LBJ and Marble Falls. Historic highs – 903 feet above mean sea level (msl) on Inks Lake and 760 feet msl on Lake Marble Falls – are recorded. Inks Lake remains above normal operating level for nine days.

Floodwaters spill over Starcke Dam, which forms Lake Marble Falls, and into Lake Travis at almost 400,000 cubic feet per second (cfs). To illustrate the power of the river at this stage, the volume of water approaching Niagara Falls during peak flow season is 202,000 cfs.

Mansfield Dam, which forms Lake Travis, does its job by holding back most of the floodwaters in its flood pool, the area above 681 feet msl. Holding the floodwaters in Lake Travis helps protect Austin and downstream towns from severe river flooding. Before the storm, Lake Travis was full at 681 feet msl. Now, floodwaters begin to fill up its flood pool above 681 msl.

In accordance with U.S. Army Corps of Engineers regulations, LCRA begins to make flood releases from Mansfield Dam in measured amounts to limit flood damage below Mansfield Dam. Flood releases are initially made through the three hydroelectric-generation units. However, because of the magnitude of the storm, Lake Travis continues to rise faster than water can be released through the hydro units.

As the lake continues to rise, LCRA begins to open floodgates at Mansfield Dam. Before the floodwaters reach the spillway on the dam, at 714 msl, 10 of its 24 floodgates are opened. As flood waters rise above the level of the spillway, water begins to cascade through the spillway down the front of the dam. Some floodgates can now be closed since water is flowing through the spillway. Finally, the level of Lake Travis crests at the historic high of 722 msl, eight feet above the spillway.

For Lake Travis to reach flood stage is not unusual. Nevertheless, this flood is different, seriously different. It’s a 100-year flood, which has not occurred on the Colorado River since 1938, before Mansfield Dam and Lake Travis were built. Spectators are stunned to see water pouring over the spillway at 50,000 cfs, and through the seven gates that remain open and all three hydrogenation units at 40,000 cfs – a total of 90,000 cfs.

Flood releases from Mansfield Dam also create a rise in Lake Austin above its normal elevation. Water levels rise as much as 10 feet immediately below the dam, but not quite as high further downstream on the lake. These flood releases are matched at Tom Miller Dam, which forms Lake Austin, with hydroelectric generation and floodgate openings. Below Tom Miller Dam, Town Lake in Austin peaks at 10 feet above its normal level and remains above normal for 12 days. Water from Town Lake flows over the hike and bike trail and Cesar Chavez Street almost to the First Street bridge, stopping traffic in both directions. It takes one and a half to two weeks for the downstream flooding to subside. Over the next several days, river levels below Austin crest above flood stage all the way to the Gulf of Mexico.

Not just size makes this flood different. The lake stays above flood level for at least two weeks. If it rains downstream, several weeks or months would pass before Lake Travis drains to its normal operating level. Damage during this flooding has been widespread.

Extent of the immediate damage
The flood has completely upset life on Lake Travis. Large numbers of homes and businesses have been built on the lake over the years. Now the swollen lake floods any structure built at or below 722 feet above mean sea level (msl).

Along Lake Travis, 1,200 single-family homes, 15 multifamily dwellings and 471 other structures were touched or totally covered by floodwaters. A sea of rolling muddy water floods Graveyard Point and Pace Bend Park and reaches into parts of Lago Vista, Briarcliff, Bullick Hollow, and hundreds of other points scattered about Lake Travis. Property damage from the flood on Lake Travis has been calculated to be at least $153 million.

What is reassuring is that the flood waters did not cause more damage than they did. Why is this?

Why damage was not greater
The Colorado River flood-control system constructed by LCRA functioned as it was designed to do. The damage without Mansfield Dam would have been much greater if the floodwaters had flowed unchecked downstream, as they did in the great floods of the 1930s.

Two pieces of the flood control system also helped keep damages lower on the Highland Lakes. Unlike the levees in New Orleans during Hurricane Katrina, the flood control structures on the Highland Lakes remained intact.

The six Highland Lakes dams all can withstand what is known as the probable maximum flood, a flood greater than a 500-year flood – one that has one chance in 500 of happening in any year. LCRA spent more than $50 million over a 10-year period to upgrade the dams to be able to withstand the probable maximum flood. That investment paid off well during the 100-year flood.

Also, LCRA has expanded the Hydromet system of more than 200 monitoring gauges at key locations throughout the basin. This system serves as LCRA’s eyes and ears for monitoring rainfall, streamflow and lake levels. It contributes to LCRA’s new flood models that provide better forecasts. These forecasts give longer lead times to open floodgates and to pass along information to emergency management officials and the public.

Lesson from Hurricane Katrina Part of the tragedy in the aftermath of Hurricane Katrina is that many of the hurricane flood victims did not know they lived in a floodplain and did not carry flood insurance. On the Gulf Coast, as in many communities, floodplain maps are outdated and inaccurate or do not exist.

That need not be the case for Travis County residents. Because the Federal Emergency Management Agency (FEMA) is remapping the Travis County floodplain with the best available information, residents soon will have a more accurate picture of the flood hazard area.

Roy Sedwick, executive director of the Texas Floodplain Management Association, advises Lake Travis residents to buy flood insurance whether or not you live in the 100-year floodplain.

“Floods don’t read the maps,” Sedwick said. “When folks hear 100-year floods have a 1 percent chance to happen in any year, they lull themselves into a false sense of security. The question is not if a catastrophic flood will occur. The question is when it will occur.”

The concept of a 100-year floodplain is based on a statistical probability needed by the insurance industry as a standard on which to base policies. The 100-year floodplain designation is not arbitrary, but it is limited to the best information at the time and it is not a determination of where and how frequently actual flood damage will occur.

For example, any house built within the Lake Travis flood pool – at or below about 753 msl – is at some risk of flooding. The top of Mansfield Dam is at 750 msl. The affected landowner would need to rely on an insurance policy or hope that limited federal relief funds would be available to help with any damages or losses, Sedwick said.

“In any event, you should have flood insurance, a home disaster preparedness plan, a NOAA weather radio, and an offsite location for your records, heirlooms, anything you can’t replace,” Sedwick said.

The cost of flood insurance through the National Flood Insurance Program is less for properties above the 100-year floodplain. “It’s a good bargain considering what a property owner may lose in a flood,” Sedwick said.

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