NEWS — IBWC engineer discusses proposed Amistad Dam fix

By Karen Gleason

The 830 Times

Engineers for the International Boundary and Water Commission say they have come up with a fix for sinkholes near the Amistad Dam that led to the dam’s rating as “potentially unsafe,” but the fix has yet to be fully funded.

The IBWC held its Middle Rio Grande Stakeholders Meeting in Del Rio Feb. 20, and that meeting included a presentation on Amistad’s sinkhole problems.

Karla Benitez, the IBWC engineer in charge of the Amistad Dam project, gave a detailed overview of the problems with sinkholes at the structure and the “safety mitigation plan” being proposed by the agency. The presentation was made in council chambers at City Hall.

Earlier in the day, local elected officials and city and county administrators toured the dam with IBWC officials, including IBWC U.S. Commissioner Dr. Maria-Elena Giner.

Only handful of Del Rioans attended the meeting at City Hall,

including County Commissioner Pct. 3 Fernando Garcia, City Manager Shawna Burkhart, Amanda Aldaco, the city’s emergency management coordinator; Karen Gonzalez, representing the Southwest Water Coalition; and Kurt Dove and Richard Phipps, Lake Amistad fishing guides.

Giner welcomed those attending the meeting and said, “We want you to walk away knowing that correcting the issues at Amistad Dam, protecting this community, is really an important element of our work. It is a priority.”

Benitez began her presentation with an overview of Amistad Dam. She told the group the dam is located on the Rio Grande about 11.8 miles north of the cities of Del Rio, Texas, and Ciudad Acuña, Coah., Mexico.

She said the dam is the second international storage dam built on the Rio Grande pursuant to the 1944 Water Treaty between the United States and Mexico.

Construction of the dam was started in 1963 and completed in 1969. The dam includes ports of entry for both Mexico and the United States, and the structure is co-owned and co-operated by the two nations.

Benitez said the primary purpose of the dam is water storage and its reservoir capacity is 5.3 million acre-feet of water, the 11th largest in the U.S. She said the dam is also used for flood control on the Rio Grande, as well as for the generation of electricity, fishing and recreational activities.

Benitez said, “It is important to note that the dam is founded on limestone bedrock that is karstic material. And what is karstic material? Karstic material is a type of rock that tends to dissolve with the action of water, and it creates geological features such as sinkholes or caverns. Please keep that in mind.”

She said before speaking about the dam and its safety issues, she wanted to go through some of the events in the life of the structure.

Benitez said between 1994 and 2004, there was a severe drought in the region, “and several sinkholes were identified” as water in the reservoir receded.

“(These) were on the upstream side of the dam, on the Mexican side. There was dye testing conducted, and it was confirmed that the upstream and downstream were connected, and these (tests) led to a series of grout holes being drilled on the upstream side between the toe and the centerline of the dam on the Mexican side,” Benitez said.
She said the IBWC continued conducting geological studies, and a binational joint technical advisory group, consisting of experts from the U.S. and Mexico, was formed in 2008.

“This group conducted an abbreviated risk assessment and concluded that the dam posed ‘significant risk,’ and (Amistad Dam) was assigned with a Dam Safety Action Classification (DSAC) Rating 2, which indicates that the dam is potentially unsafe,” Benitez said.

“The findings of the panel led to further studies, including the dam safety modification study and the dam safety modification report. These helped to identify and recommend a risk management plan, and we were able to identify that these risks were in a 2,000-meter portion of the Mexican side,” she added.

Benitez said more than 50 sinkholes were identified by 2024, and she showed the sinkholes in relation to the dam structure on a map.

“So let’s talk about sinkholes. What are sinkholes? Sinkholes are cavities in the ground, particularly found in limestone bedrock that is caused by the solution of water, and then it creates routes where the water basically disappears underground.

“This slide shows sinkholes from 1994 to 2023. I have pictures on the left so you can have an idea of the size of the sinkholes, the depth; they were pretty big. The map shows you that most of the sinkholes are concentrated on the upper side of the Mexican side (of the dam),” Benitez said.

Benitez also showed a graph listing the year and the number of sinkholes found in each year.

Benitez said technical advisors inspected the sinkholes and “concluded they do not pose an immediate risk to the dam.”

She said the advisors also recommended a procedure for the IBWC to treat sinkholes.

She noted IBWC first “cleans out” and excavates each sinkhole to determine its depth and extent. The sinkhole is then filled with granular material, and a marker placed on to identify it.

“We have this procedure just in case there is activity in the sinkhole, the material will go downward into the sinkhole, and that’s the way that we monitor any activity,” she said.

Benitez summarized, “So, sinkholes pose a potential risk to the structural stability of the dam. We still have the same DSAC rating, classification 2, ‘potentially unsafe,’ that has not changed.

“The classification is due to potential economic damage and population at risk. The dam has the potential to cause flood damage downstream, putting (an estimated) 400,000 people at risk of flooding. Additionally, it would also affect the water supply, recreation, hydropower, wildlife, in case of a failure,” she added.

Benitez said in October 2022, during a binational meeting, a joint expert panel agreed “a composite cutoff wall as the recommended risk management plan for Amistad.”

The composite cutoff wall consists of a two-line grout curtain and a concrete cutoff wall.

Benitez then gave a detailed explanation of how the grout curtains and concrete cutoff wall will be constructed. She said the composite cutoff wall “works by cutting off flow under the dam” and “reduces the risk of failure to a tolerable level.”

Benitez said in December 2024, the U.S. and Mexico signed an agreement for the composite cutoff wall project, with the U.S. agreeing to fund 56.2 percent of the cost and Mexico agreeing to fund 43.8 percent of the project.

She said in January 2024, a contract was awarded “to assist the commission in developing the bidding package and the scope of work for the design of the composite cutoff wall.”

Benitez said the solicitation was posted in May 2024 and awarded in September 2024.

“The tentative completion date of the design is March 2025, in a month, and we will follow with a solicitation and award of the grout curtain construction and the cutoff wall design, having a target date to start construction in July of 2025,” Benitez said.

“Then the cutoff wall construction would follow in January of 2027. Now, please note that these project dates depend on the completion of the design and the availability of the funding, meaning that if we don’t receive the complete funding, the cutoff wall construction might be (delayed),” she added.

“To finalize, we have a path forward and some recommendations. We definitely have to finalize the design to determine those specifics for the construction details to refine our costs, our estimated costs. We are to use existing funding to start the construction of the grout curtain. We need to obtain necessary funding for the rest of the work, such as the cutoff wall section and to start that work, so that we can ensure that the complete composite cutoff wall is built and then follow a few recommendations to continue routine monitoring programs of the dam’s automated and conventional instrumentation, ensure continuous information sharing between the two countries and to maintain an updated list of officials identified in the Amistad Dam Emergency Action Plan,” Benitez finished.

The writer can be reached at delriomagnoliafan@gmail.com.