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New engineering report blames loss of soil strength for Edenville Dam disaster

Engineers for FERC noted there were no drains identified under the section that failed
Published: Sep. 14, 2021 at 7:56 AM EDT
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MID-MICHIGAN, Mich. (WJRT) - A new engineering report compiled for the Federal Energy Regulatory Commission blames saturated soil within the Edenville Dam for causing its failure 17 months ago.

The Independent Forensic Team Report used photos and videos taken minutes before, during and after the dam’s collapse on May 19, 2020, to determine whether it was caused by overtopping, internal erosion or saturated soil.

The interim report was compiled for federal regulators, who had oversight of the dam and its owner until months before the failure. FERC had revoked Boyce Hydro’s permit to generate power with the dam in 2018 and transferred oversight to Michigan regulators.

The FERC engineers concluded that sand on the downstream side of the 6,000-foot Edenville Dam became saturated with water and lost its strength to hold back rising water levels on Wixom Lake, causing a 40 to 80-foot wide section of the structure to wash out.

The break occurred on the east end about halfway between the hydroelectric power station and Tittabawassee River spillway.

The report notes that no drains were identified beneath that section of the dam during a 2012 inspection, which likely placed additional stresses on that area that the rest of the dam likely didn’t encounter. A final report on the disaster will be issued later.

The Edenville Dam was holding back water just below its normal level on May 16, 2020. Over the next three days, the watershed from Secord Lake south to Sanford received 5.9 to 2.95 inches of rain -- mostly on May 18, 2020.

Gates opened at the Secord, Smallwood, Edenville and Sanford dams that day to allow higher flows of water through each structure. By noon on May 19, 2020, the water levels behind all four dams continued rising above their normal water levels.

Precise water levels on Wixom Lake were not available for the afternoon of the Edenville Dam failure, but the engineers’ report says it likely rose about 6 feet over two days and exceeded the previous record high water level by 3 feet. That level was barely a foot below the maximum holding capacity for the dam.

Boyce Hydro employees first noticed potential trouble on the morning of May 19, 2020, when they saw signs of erosion and sloughing on the east side. They quickly contacted state regulators and contractors to begin an immediate inspection around 10:30 or 11 a.m.

Contractors were planning to shore up the dam with large sandbags and other measures later that day. No active water leaking was observed while Boyce Hydro staff, regulators and contractors walked along the face of the dam that morning.

Crews later considered a controlled breach of the dam on the Tobacco River side during the afternoon in an area where the dam height is among the lowest to relieve the rising water level. The situation worsened around 5 p.m., when witnesses began reporting a depression forming on the Tittabawassee River side of the dam.

The engineers’ report includes photos of the Edenville Dam about 35 minutes before the failure, showing an area of sagging along the rim. The report notes that stresses on the dam were about 10% higher than it ever experienced at that time and high areas of the structure were exposed to standing water for the first time.

The engineers report includes a time lapse of still images taken from a video at 5:35 p.m. showing a section of the dam fail and wash out over a few minutes. The failure started with a small rush of water near the top and the depression getting deeper.

Water began gushing uncontrolled through the dam within 40 seconds of the first stream visible on the downstream slope. The failed section quickly widened and deepened, sending the contents of Wixom Lake rushing down the Tittabawassee River toward Midland and Saginaw Township.

The Sanford Dam downstream failed about two hours later. Engineers concluded that was due to a simple overtopping after the torrent of water from Wixom Lake overwhelmed the Sanford Dam’s ability to hold it back or control it through spillways.

Water began rushing over the top of the Sanford Dam at 7:46 p.m. -- about two hours after the Edenville Dam failure -- and eroded the downstream face steadily until the structure failed.

The new engineering report says regulators understood well in advance of the disaster that failure of the Edenville Dam upstream almost certainly would overwhelm and overtop the Sanford Dam.

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