AUSTIN — How great is the risk of flood damages along the lower Colorado River, one of the most flood-prone regions of the United States? Has the risk changed over time? Are there effective ways to reduce the risk of property damages and loss of life during major floods?
The answers to these questions began to form this week with the release of preliminary results from the first comprehensive study of flooding in the lower Colorado River basin. The results show that the probability of flood damages determined in the 1970s for the Highland Lakes is generally accurate, except on Lake Travis and the section of Lake Marble Falls between the U.S. Highway 281 bridge and Starcke Dam, where the risks are slightly greater than previously thought.
The analysis of data for the rest of the river, downstream of Lake Travis, is not yet complete. Those results will be reported as soon as they are available, probably in a few weeks.
The study results are the foundation of a larger cooperative effort by the U.S. Army Corps of Engineers, LCRA and local communities to find solutions to flood problems in the river basin. The Corps of Engineers will use the new information as the basis for determining the damages that are likely to occur as a result of flooding and to identify alternatives for reducing damages.
One significant finding so far has to do with the peak levels the Highland Lakes would reach during a severe flood, referred to as a 100-year flood. The term 100-year flood has to do with the probability — 1 percent — of a flood of such severity, or greater, occurring in any year. Floods of this magnitude have occurred on all the Highland Lakes except Lake Travis. Mansfield Dam and Lake Travis were redesigned to manage floodwaters after a 100-year flood occurred in 1938, during their construction.
The study shows that the peak levels for a 100-year flood are close to those currently expected along most of the Highland Lakes, based on flood studies conducted in the 1970s. However, the study shows that the peak level on Lake Travis during a 100-year flood would be 722 feet above mean sea level (msl). (The level of the ocean halfway between high and low tides provides a point of reference for measuring lake and river levels). This is 6 feet higher than the level, 716 feet above msl, determined in the 1970s. On the section of Lake Marble Falls between the U.S. Highway 281 bridge and Starcke Dam, the peak level is 2 feet higher: 755 feet above msl. Additional results from the study indicate that floods previously thought to have a 1 percent chance per year of occurring on Lake Travis or Lake Marble Falls near Starcke Dam actually have a probability of 2 percent.
"This information will help us — LCRA, the Corps of Engineers and communities along the river — with our goal of protecting lives and property," said LCRA General Manager Joe Beal. "It?s a key piece of the puzzle that will help in finding the right solutions."
A number of factors contributed to the changes. The study and analysis tools — computer models and simulations, ground and aerial surveys, and geographic information systems (GIS) — are more sophisticated, powerful and accurate than those used in previous flood studies in the 1970s. This more comprehensive, basinwide study drew upon more information: 25 more years of flood and rainfall records. Also, the study reflects a clearer understanding of the volume and movement of floodwaters, and the limitations on using long-range forecasts as the basis for effective floodgate operations.
The Corps of Engineers expects to distribute a report this fall that includes a flood damage analysis and the results of an initial investigation into possible solutions for lakes Travis and Marble Falls. Specific solutions would be evaluated by 2005, according to the current schedule. Congressional approval and funding may be sought for the projects or programs that are economically and environmentally sound.
"This level of detail is essential for understanding how floods develop in the Colorado River basin and how best to manage them," said Michael Mocek, Deputy District Engineer for Project Management of the Corps of Engineers. "Together with LCRA and local communities, we are eager to move forward to the phase of study that will identify possible solutions."
The basinwide flood study is one among several efforts that communities in the basin, LCRA and other agencies began after devastating floods in the 1990s raised awareness of the need for greater flood protection efforts. LCRA has invested about $92 million over the past eight years on various projects, including the basinwide flood study, strengthening and improving the dams, and expanding the weather and streamflow monitoring system.
Elected and appointed officials from throughout the river basin met with LCRA in Austin in September 1999 to discuss flood issues and formed a steering committee to evaluate options for resolving them. Based on some of the issues raised, LCRA asked the Corps of Engineers to contribute its technical expertise to determine the feasibility of programs or projects that could help reduce flood damage. The Corps of Engineers and LCRA, as the local sponsor, hired a team of consultants in 2000 to gather the base information that was reported this month. In 2001, more than 30 cities and counties formed an alliance, the Texas Colorado River Floodplain Coalition, to make their floodplain and emergency management programs more effective and coordinated.
The coalition identified more accurate floodplain maps as a critical need for making better land use decisions. The flood level data developed as part of the basinwide study does not change the official floodplain designations used by communities and by the Federal Emergency Management Agency (FEMA) in its flood insurance program. However, communities could work with FEMA to update official floodplain maps, based on the new data. FEMA awarded a $447,000 grant to the coalition in 2001 to begin revising floodplain maps for Wharton and Bastrop counties and producing them in digital format.
"The coalition represents communities that have taken responsibility and initiative for protecting their citizens from the impact of natural disasters," said Domingo Montalvo, Jr., the coalition chair and a Wharton city councilmember. "We are doing everything we can to use and leverage our programs and resources. However, this tremendous challenge requires partnerships and assistance from state and federal agencies. It is encouraging to see progress from our combined efforts."
