Alabama Power Company FERC Part 12 Dam Safety Inspections

Gomez and Sullivan continues to cultivate our dam safety capabilities, including advancing many of our staff toward qualification as Independent Consultants, keeping current with changes in dam safety regulations and practices, and expanding the depth and geographic range of our dam safety services. Among the Federal Energy Regulatory Commission (FERC) Part 12D dam safety inspections and reports performed by Gomez and Sullivan in 2021 were those for Alabama Power Company’s Yates, Thurlow, and Mitchell Developments in Alabama. Yates and Thurlow Dams are located on the Tallapoosa River and Mitchell Dam is located on the Coosa River.

Damian Gomez, PE is the FERC-approved Independent Consultant for these projects and performed dam safety site inspections of the three facilities in July 2021. In addition to walking inspections of the project facilities, both Thurlow and Mitchell dams were observed by boat inspection from the upstream reservoir. All three developments include left and right non-overflow sections, intake headworks, powerhouse structures, and concrete gravity overflow spillways. Yates has an ungated spillway, while Thurlow Dam has new, five-foot-high Obermeyer crest gates across the crest of the spillway, and the Mitchell spillway is equipped with 23 timber-faced Tainter gates and three steel-faced Tainter gates. The Thurlow and Yates spillways are both concrete dams overlain on circa 1840 stone masonry dams, originally constructed to provide hydropower for local mills.

We reviewed each project’s surveillance and monitoring plan (including instrument readings for deformation monuments, trammel points, piezometers, and weirs), operation procedures, and analyses of record regarding hydrology and hydraulics, geology and seismicity, and structural stability in accordance with current FERC guidelines. Relevant input from our water resources, civil, structural, and geotechnical engineers was incorporated in the Part 12 documents as appropriate for a comprehensive assessment of each development. Potential Failure Modes Analysis (PFMA) review sessions were also performed in conjunction with the onsite inspections, and findings were summarized in 2021 PFMA supplement reports.

Our services at the Yates, Thurlow, and Mitchell Developments are part of our ongoing relationship with Alabama Power (a part of Southern Company). We were initially retained by Alabama Power to perform hydraulic analyses of Mitchell Dam (including a spillway discharge capacity analysis, a dambreak analysis to determine the inflow design flood, and development of a tailwater rating curve), and were later engaged in an analysis of a proposed design by the owner’s design/build contractor to extend the east spillway apron, and subsequent hydraulic design of the modified apron extension and baffle block concept. These analyses included developing a three-dimensional computational fluid dynamics (CFD) model to evaluate pressures on the spillway under the probable maximum flood (PMF) and analyzing impacts on bottom velocities and scour potential for the proposed spillway apron extension and baffle block addition.

Our staff have been performing dam safety services in accordance with Part 12D regulations since the 1980s. Senior personnel have mentored and instilled their legacy dam safety knowledge in our junior staff, building a team with a solid understanding of the interconnections of the working components—e.g., how observations during field inspections, coupled with engineering analyses, can help identify the complete dam safety picture. Our staff stay current with industry standards, and our dam safety group has undergone Semi-Quantitative Risk Analysis (SQRA) training for PFMA development in preparation for FERC’s changes to Part 12D of the code of federal regulations.