Concurrent Session 01A: Demopolis Lock Upper Miter Sill Breach

Abstract Description: Demopolis Lock and Dam is sited below the confluence of the Black Warrior and Tombigbee rivers at Demopolis, Alabama. The lock provides navigation service for commercial navigation and recreation interests in Alabama's Black Warrior river basin and, through the Tombigbee River's Tenn-Tom Waterway, serves users from multiple states and federal navigation systems. The project was designed and constructed by the Mobile District, US Army Corps of Engineers (USACE) and was put into service in 1954.

At 5:45 a.m. January 16, 2024, a large block of the lock's concrete upstream miter sill broke away, leading to an uncontrolled release through the lock and its open downstream miter gates, which are not designed to be closed under flowing conditions. This type of breach was unprecedented in USACE, as were the response and repair measures required to arrest breach flow and return the lock to service. Breach flow was arrested by tug-assisted closure of the lower miter gates. The lock was repaired, during the flood season, by rebuilding and enlarging the lock miter sill using underwater concrete forming and placement methods. Navigation service was disrupted through many inland waterways for four months. Causal mechanisms were postulated by examination of design, operation, and construction records; concrete testing and examination; and Potential Failure Modes Analysis. Contributory factors identified included the age and use of the lock; accidents and operational events resulting in adverse loading conditions on the sill; design factors, most notably the upper miter gates' vertically-framed arrangement and the absence of steel in the sill to resist tensile loading and crack propagation; and construction factors owing to the arrangement of unbonded block joints. Advance intervention in this case was not likely, as the presence of cracking was hidden and not expressed visually. Lock owners with vertically-framed miter gates should assess the design, construction, and condition of their miter sills to identify miter sill breach risk. This paper summarizes the incident, response, damage assessment, repair, and incident investigation, concluding with recommendations.