April 2002

Fact Sheet: Study of Flooding on the Lower Colorado River

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 or the magnitude of damages? A study team formed in July 2000 has published preliminary results that are a key step toward finding solutions to flood problems in the basin. This fact sheet reports on these preliminary findings.

Who is conducting the study and what is its purpose?

This study is a building block for a larger effort, called the U.S. Army Corps of Engineers Lower Colorado River Basin Feasibility Study. The Corps of Engineers, LCRA and communities along the lower Colorado River are cooperating on the feasibility study with the goal of improving their knowledge of flooding in the basin and to identify ways to reduce property damage and loss of life during major floods.

The basis of any good decision is accurate information. To gather and analyze the most accurate and up-to-date information possible about flooding in the basin, LCRA and the Corps of Engineers hired a team of consultants in July 2000. The team includes Halff Associates, Inc.; David Ford Consulting Engineers; Espey Consultants, Inc.; and Surveying and Mapping, Inc. The Fort Worth and Tulsa districts of the Corps of Engineers, LCRA, the U.S. Geological Survey and independent consultants reviewed the team's work. The preliminary results were published this month in a draft report called, “Hydrology-Hydraulics Appendix.”

What are hydrology and hydraulics and what information do they provide?

They are fields of science that examine the probability of flooding and the effects of flooding on the river and surrounding area. The consultants assessed current physical conditions of the river, lakes and surrounding areas. They also analyzed the patterns of past floods and used computer models to help understand and predict how similar floods would affect the river’s flow.

How will the hydrology-hydraulics information help in identifying solutions?

It provides the foundation for the Corps of Engineers to determine the damages that are likely to occur as a result of flooding, to identify alternatives for reducing damages, and to select the best alternatives to pursue.

What geographic areas did the study include?

In general, the consultants studied 473 miles of the lower Colorado River — from San Saba, along the Highland Lakes and downstream to the Gulf of Mexico — and the surrounding 18,000 square mile watershed and tributaries. However, they developed flood models for the Colorado River only, not for the streams and rivers that feed into the Colorado River. Models may be developed later for those tributaries.

What does the study show about flooding on the river and lakes?

One of the most significant findings has to do with the peak levels expected during a very severe flood, referred to as a 100-year flood. The term 100-year flood has to do with the probability of a flood of such severity occurring in the span of a year. The risk of such severe flooding is 1 out of 100 each year. A flood of that magnitude or greater has occurred on all the Highland Lakes, except Lake Travis. Mansfield Dam and Lake Travis were redesigned to manage floodwaters as a result of a 100-year flood that occurred in 1938, during their construction.

The study shows that peak levels during a 100-year flood would be about the same as calculated in prior studies in the 1970s for most of the Highland Lakes. They are higher on Lake Travis and the section of Lake Marble Falls between the U.S. Highway 281 bridge and Starcke Dam. On Lake Travis, the level would reach 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 peak level, 716 feet above msl, calculated in the 1970s. On the section of Lake Marble Falls between the bridge and Starcke Dam, the peak level is 2 feet higher: 755 feet above msl.

The analysis of data for 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.

Why did some of the 100-year flood levels change?

The new calculations are based on more accurate and complete data than was available when the levels currently in use were established in the 1970s. This is the first comprehensive study of the entire lower Colorado River basin. The study and analysis tools — such as computer models and simulations, aerial and ground surveys and geographic information systems (GIS) — are more sophisticated, powerful and accurate than those used in previous flood studies. More information was available: 25 more years of flood and rainfall records, including several major floods during the 1990s. 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.

What does this mean for property around Lake Travis and the section of Lake Marble Falls between the U.S. Highway 281 bridge and Starcke Dam?

It means the probability of flooding is greater than previously determined. For example, the probability is 2 percent (or 2 out of 100) per year, instead of 1 percent (1 out of 100) per year that floodwaters will reach 716 feet above msl on Lake Travis or, on the section of Lake Marble Falls between the bridge and Starcke Dam, 753 feet above msl. The probability decreases to 1 percent at 722 feet above msl on Lake Travis and at 755 feet above msl on the section of Lake Marble Falls between the bridge and Starcke Dam.

Keep in mind that 710 feet above msl, recorded on Dec. 25, 1991, is the highest level Lake Travis has reached since it was completed in 1941. The record level on Lake Marble Falls was 756 feet above msl on Sept. 11, 1952. Experts believe a 100-year flood will occur again on the Highland Lakes; they just can’t predict when it will happen.

Does this change the floodplain around Lake Travis and Lake Marble Falls? Do I have to buy flood insurance?

The results of this study do not change the official floodplain designations used in the Federal Emergency Management Agency (FEMA) flood insurance program. Communities have the option of working with FEMA to update official floodplain maps. Some, like Wharton and Bastrop counties, have chosen to do so. If you want information about flood insurance, contact Ross Richardson or Carl Watts, FEMA Region VI, at
(940) 898-5210.

Whom should I call to find out if my property is at the 100-year flood level?

Contact your local floodplain administrator. This city (or county if you live outside city limits) official can help you determine if your property is in the 100-year flood and what flood protection options are available. Keep in mind that this study does not change the official floodplain designations used in the FEMA flood insurance program.

What happens next?

The Corps of Engineers will review and verify the consultants' findings, assess the potential damages from flooding and issue a report, scheduled for release in October 2002. The second phase, developing alternatives for reducing the risk of flood damages and selecting the best solutions, is scheduled to begin in October 2002 and end in October 2005. Congressional approval and funding will be sought for the selected approach.

Whom should I contact for more information about the study?

Call LCRA and ask to speak to Mark Jordan about the Lower Colorado River Basin Feasibility Study. Inside Austin, call LCRA at 473-3200. Outside Austin, call toll-free at
1-800-776-5272.