Lake Houston Dam Comprehensive Evaluation of an Ambursen Dam Interim Report

The Lake Houston Dam, owned by the City of Houston and operated by the Costal Water Authority, was constructed in 1954 on the San Jacinto River, near Houston, Texas. The Ambursen dam has a maximum height of 66 feet and a length of 12,100 feet. In November of 2006, the Authority authorized a team led by Freese and Nichols to perform a comprehensive evaluation of the dam. Phase IA identified 32 issues that could affect the safety of the dam and resolved 15 of them with a recommended list of maintenance and minor improvements. Phase IB included the resolution of the remaining 17 issues through an extensive field exploration program and a detailed stability analysis of the spillway structures. Because of the lack of original instrumentation and difficulties in access, previous stability analyses had not identified actual uplift pressures under the 3,077 foot long spillway structure. They also did not sufficiently describe the strength parameters and were found to be using strength values that were significantly too high. The results showed that the spillway structure does not have sufficient resistance to sliding with factors of safety for normal lake levels ranging from 1.03 to 1.26, depending on uplift loading assumptions. Ongoing analyses are designed to refine the relationship between the uplift pressures and tailwater conditions. This information will be used in the design of remedial measures, currently scheduled for the fall of 2008. Freese and Nichols has led the analysis and design of two other large Ambursen dam rehabilitation projects, the Morris Sheppard and Wesley Seale Dams. Some of the concerns and problems addressed at those two structures have also been discovered at Lake Houston Dam. This paper will discuss the details of the field exploration program, the findings of the stability analysis, and the proposed solutions to the concerns that have been discovered. 15 pp., 2 tables, 7 references.