Rapidan Hydropower Facility, MN FERC – Project-3071 After-Action Investigation and Independent Forensic Team Report

In May and June of 2024, the Rapidan Dam drainage basin in south-central Minnesota experienced significant precipitation resulting in the fifth wettest month on record and the second largest flooding event in the hydropower project’s 114 years of service. During the flooding, the left abutment of Rapidan Dam was breached following an accumulation of two interlocked woody debris masses that broke loose at different times from the upstream boater safety cable and two highway bridge piers subsequently blocking the five overflow spillway bays and restricting discharge capacity. The blockage resulted in raising the reservoir elevation, which then overtopped the dam crest and flowed around low areas at the left abutment. These flanking flows concentrated around obstructions and knick-points, increasing the hydraulic energy and turbulence, which caused significant scour of the weak fill and geologic materials in the abutment and led to uncontrolled release of the reservoir. The breach enlarged vertically and laterally, fully capturing the Blue Earth River, bypassing around the left side of the still standing and intact concrete buttress dam.

Following the failure, the Federal Energy Regulatory Commission (FERC), which has dam safety jurisdiction over the Rapidan Dam Hydropower Project, assembled a technical Independent Forensic Team (IFT) composed of experienced dam safety professionals from the U.S. Army Corps of Engineers (USACE) and the U.S. Bureau of Reclamation (USBR). The mission of the IFT was to evaluate the project background, site conditions, operations, performance, and past dam safety evaluations and modifications; evaluate the sequence, physics, and root cause of
the failure; assess the decisions made and actions taken by responsible parties; and develop lessons learned from the failure to inform the dam safety community and improve the FERC dam safety program as applicable. The results of this study are documented in this After-Action IFT Report and summarized in the following sections.