The most plausible cause of the Edenville Dam failure, which spurred catastrophic flooding in the Midland, Michigan, area in May 2020, is saturated, loose sand in an embankment that rapidly lost strength, according to a new interim report from an independent forensic team.
The 42-page document was released Monday nearly 16 months after the dam failures — the Edenville Dam failed first on May 19, leading the Sanford Dam to fail — forced thousands of people to temporarily evacuate as the waters ravaged properties in Midland and Gladwin counties.
The Federal Energy Regulatory Commission tapped a five-member team of independent forensic experts to investigate the physical and human factors that contributed to the failures. The new report focused on the "physical mechanisms of the dam failures." The evaluation of the human factors is still in progress, the document said.
A final report from the team is months away.
The forensic team identified a phenomenon called "static liquefaction" instability of sands in the downstream section of the embankment as the most plausible reason for the failure of the Edenville Dam. Static liquefaction occurs when the strength of saturated and loose sand decreases quickly to values significantly less than the stress against it, according to the report.
"This failure mechanism has been rare, but not unprecedented, for water storage dams, and water storage dam engineers have not typically considered it," the report said.
Dam safety staff at the Michigan Department of Environment, Great Lakes and Energy was not surprised by the panel's findings, since they also "were reasonably certain the failure started with that part of the embankment," department spokesman Hugh McDiarmid Jr. said Monday.
The department is reviewing the panel's findings and will use them to strengthen Michigan's dam safety program, he said.
McDiarmid also pointed to the panel's finding that such failures are not unheard of in water storage dams and that "significant rainfall" preceded the failures.
The federal report's conclusion was supported by accelerations of the failing soil in a video of the dam failure, evidence of loose sand in the embankment and strength loss exhibited in laboratory tests of specimen collected from remnants of the breach, according to the findings.
The team said there was "uncertainty concerning the exact trigger or triggers that led to the static liquefaction failure." However, the record lake level at the time of the failure "almost certainly contributed."
Heavy rainfall over a period of days in May 2020 preceded the dam failures. On May 19 — the day they occurred — Wixom Lake, the lake impounded by the Edenville Dam, eventually reached about 5.5 feet above normal pool level, about 3 feet higher than the previous pool of record, according to the report.
"This represents a hydraulic load on the embankment about 10% higher than had been previously experienced, which could have produced historically high shear stresses," the report said.
The federal document focused on a video taken of the dam failure. A downstream section of the Edenville Dam failed at about 5:35 p.m. May 19. The failure section was 40 feet to 80 feet wide, the report said. The section failed in less than 10 seconds.
The breach enlarged over the next hours, releasing the water stored in Wixom Lake. The Sanford Dam failed because water began spilling over the top of its embankment after the Edenville Dam failure.
"Given the failure of Edenville Dam, the failure of Sanford Dam was not unexpected," the report said. "Regulators and engineers understood that should a breach occur at Edenville, Sanford would almost certainly be overtopped and fail."
The forensic team said it closely examined three potential primary failure causes for the Edenville Dam. The other two studied were embankment overtopping and erosion. The team said it was confident the embankment didn't overtop, and internal erosion was judged to not be plausible.
