Buchanan Dam on Lake Buchanan. Photo courtesy of the LCRA.
Floodplain boundaries in the Highland Lakes could be changing according to new information from the Lower Colorado River Authority. Officials is Burnet and Llano counties are sending certified letters to residents and property owners between Buchanan Dam and Inks Dam with information about the changes and possible flooding dangers. The new flood plain analysis is based on what the LCRA says is more accurate information than has been available in the past.
“We want to provide people who live between these dams with a clear and accurate understanding of their risks related to flooding,” said John Hofmann, LCRA executive director of water. “For example, there are neighborhoods built in the shadow of Buchanan Dam’s floodgates, and we want to make sure property owners know the hazards they may face when we open those floodgates.”
The LCRA commissioned the new floodplain study to provide updated and detailed information about flood hazards between the two dams.
Past Federal Emergency Management Agency studies of some of the affected areas were not based on detailed analysis and showed only approximate floodplain boundaries, according to the LCRA. Flow patterns from Buchanan Dam have historically been difficult to analyze because of the design of the dam, which is about 2 miles long and has 37 floodgates arranged in three separate sections.
The dam also has a spillway. Water from the gates and spillway flows overland into Inks Lake. The new floodplain study uses the latest digital terrain data and flood modeling methods. LCRA officials said the model results were confirmed by observations during flooding in 2016 and 2018.
The LCRA, with approval from officials in Burnet and Llano counties, submitted the updated floodplain information to FEMA. Upon final approval, FEMA will use the information to update its Flood Insurance Rate Maps that identify a community’s flood hazard areas, including floodplains and floodways.
This new information will likely result in changes to the floodplain boundaries, moving some areas into the floodplain for the first time and moving some areas out of it.
“It’s been more than 80 years since the biggest flood in Buchanan Dam’s history,” Hofmann said. “In July 1938 — less than a year after the dam was completed — a flood prompted LCRA to open 22 of the dam’s 37 floodgates at once. As operators of a series of dams in Flash Flood Alley, we are well aware we could see that kind of flooding again one day. We’re sharing this information to help educate residents, emergency first responders and others so they can stay prepared.”
Once the counties notify affected property owners, FEMA will also publish notices in local newspapers. Property owners and the public will have 90 days to appeal the maps on a technical basis. The revised maps will become effective 30 days after the appeal period ends and all technical appeals are resolved.