By Karen Gleason
The 830 Times
International Boundary and Water Commission (IBWC) officials in mid-November unveiled a plan for a permanent solution to deal with identified problems at Amistad Dam.
Civil engineer Karla Benitez at an IBWC Lower Rio Grande Citizens Forum presented the “Amistad Dam Safety Mitigation Plan” on Nov. 16. A newspaper story about the presentation and plan focused on Amistad Dam’s Class II rating – “urgent/potentially unsafe” – by the U.S. Corps of Engineers, a rating the dam has held since 2007.
The Corps of Engineers gave the Amistad Dam a Class II rating, the second-to-worst of five classes, after a 2007 evaluation of the dam and its surrounding support structures and the geology of the immediate area.
“The (Corps of Engineers) report concluded that the entire dam foundation is in need of further evaluation and study due to potential impact of seepage from naturally-occurring sinkholes, and the high risk in terms of the combination of potential loss of life and economic damages,” according to a fact sheet on the IBWC web site.
Sinkholes near the dam were inspected regularly and monitored over the years, and efforts moved forward to develop a mitigation plan, Sally Spener, United States Secretary of the International Boundary and Water Commission, told the 830 Times in an interview Friday.
Spener spoke about the mitigation plan and addressed a recent news story from the Rio Grande Valley that seemed to suggest the problems at Amistad Dam had recently worsened.
“One of the things that I wanted to clarify is that there is a headline floating around, from a south Texas newspaper, that suggested that the dam safety classification had recently been changed for Amistad Dam, and I want to make sure you understand that is not the case. It has not changed since 2007, so I wanted to make sure you understood that,” Spener said.
“We’ve done all these studies, and we want to make sure that when we invest in a mitigation for this sinkhole problem that it’s effective and cost-effective, and so there were a couple of different alternatives. The main ones were narrowed down to an upstream overbuild, a downstream overbuild and a cut-off wall.
“The upstream overbuild and the downstream overbuild are basically to put a bunch more rock on the dam, make it a bigger structure, and if you did have issues, the rock would help fill these voids that might exist because of the sinkholes. That didn’t quite meet the risk reduction standards the Corps of Engineers has identified, so we had identified that as an interim measure. (The overbuilds) are also much cheaper than the permanent solution, which is the cut-off wall, and several years ago, the Mexican government did not believe that it would be able to provide funding support for the cut-off wall (solution),” Spener said.
In the past year-and-a-half to two years, Spener said, the Mexican government has taken another look at the cut-off wall solution, which is detailed in the mitigation plan presentation made in November.
“We had some additional meetings and some additional discussions and now have consensus that the appropriate approach is for the permanent solution, which is the cut-off wall, even though it’s more expensive,” Spener said.
“We’re going to be doing some additional work because we think the cost of implementing the cut-off wall, based on some additional assessments that we’re undertaking, may not be as much as we had thought previously, and we now have the Mexican government’s support for the cut-off wall, so I would say the big news is that we’re moving forward with a permanent solution. With the information that we have right now, we intend to move forward with a permanent solution, which is the cut-off wall,” Spener said.
According to the presentation on the mitigation plan, work on the cut-off wall is expected to begin in the summer of 2024.
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For further information about Amistad Dam, its safety classification and dam safety, click on the following links:
- IBWC Amistad Dam Safety Mitigation Plan: https://ibwc.gov/Files/CF_LRG_Amistad_Dam_Safety_Mitigation_Plan_111622.pdf
- 2007 Engineers’ Report on Amistad Dam conditions: https://www.ibwc.gov/Files/SOD_Report_Amistad.pdf
- IBWC overview on Amistad Dam conditions: https://www.ibwc.gov/mission_operations/SoD_Amistad.html
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