In 2012, McMillen began investigating potential new hydropower and pumped storage projects, tidal and battery options, and capital improvement plans for SEAPA’s existing hydro facilities. Prior to field visits and exploration for new sites, we first determined the power demands for these remote areas of Southeast Alaska. Then, we set up an efficient and effective screening and evaluation system, providing SEAPA with consistent and meaningful comparisons of each opportunity. This allowed SEAPA to prioritize locations deserving of further research.
Analytical tools known as Site Definition Documents (SDDs) and alternative configurations were developed and compared several items such as:
• Potential power generation
• Existing hydrology
• Construction cost estimates
• Project risks
• Regulatory and environmental constraints
Our in-house regulatory specialists, multidisciplinary engineering teams, cost estimators, biologists, hydroelectric specialists, and environmental support staff efficiently produced comprehensive assessments for SEAPA. Additional support services included Probable Maximum Flood Analysis (PMFA), geotechnical evaluations, concept designs, and stream gauge monitoring.
Swan Lake Dam:
During our analysis, we discovered an opportunity for the most significant impact in the shortest amount of time—creating additional active storage to the existing Swan Lake concrete arch dam. SEAPA agreed, and we began the design to raise Swan Lake Dam by 15 feet to reach 174 feet high. McMillen served as the design lead and provided integrated civil, geotechnical, hydraulic, electrical, mechanical, and structural engineering.
Our design included a new 30-foot-high reinforced concrete pier, modifications to the existing parapet walls, and changes to the existing dam crest to accommodate the dam raise. We also provided mechanical and electrical updates, a new debris boom, and modifications to the intake gatehouse. In addition, we provided FERC regulatory licensing assistance that shortened the license amendment filing by approximately one year.
A truly unique aspect of this project was a new 78-foot-long emergency flashboard gate—the world’s only system of its kind. The steel flashboard provides an innovative way to release flood flows without overtopping the dam. The structure is so lightweight it has minimal impact structurally on the dam. In 2019, the project received two ACEC awards for engineering excellence.
Our responsibility included Finite Element Modeling (FEM), a .-scale model of the new flashboard spillway bays, a full-scale model of the flashboard gate system, and the addition of a 23-foot-wide vertical gate. Throughout the construction, we also provided engineering support, an onsite resident engineer who managed the FERC-approved Quality Control Inspection Program (QCIP), and weekly inspections. Challenges included the remote area in Southeast Alaska with access limited by air or water and the small footprint of the laydown area at the dam abutments.
This project has resulted in the potential for 25% more storage and a longer generation period—thereby reducing dependence on diesel generation.
