Edenville and Sanford Dam Failures: Field Reconnaissance Report

Four dams located north of the town of Midland, Michigan, as well as impounding lakes along the Tittabawassee and Tobacco rivers, were damaged in May 2020, following two days of heavy precipitation ranging from 9.7 cm to 20 cm, fueled by moisture from Tropical Storm Arthur, and resulting in a 500 yr flood. All four dams were damaged by the flooding, and the two lower dams, Edenville and Sanford, failed, releasing the impounded waters. Following the dam failures, a reconnaissance team was organized by the Geo-Institute Embankments, Dams, and Slopes (EDS) Committee. Details are given on the work of the EDS team in the weeks following the dam failures, when the affected dams were deployed and rapidly surveyed using on-ground assessments techniques and Unmanned Aerial Vehicles (UAVs).

Edenville Dam consists of two sections of earth embankment, which total approximately 1.86 km (6,100 ft) in length and are separated by Michigan Route M30. Route M30 generally parallels the Tittabawassee River north of Sanford, and then jogs west to pass through the farm community of Edenville, about 1.8 km (1.12 mi) south of the dam. The elevation difference between Secord Lake and the Tittabawassee River below Sanford Dam is approximately 44.2 m (145 ft). Hence, based on the existing topography it is estimated that the Tittabawassee River likely flowed at an average slope of 0.06% before construction of the dams. The lower peninsula of Michigan experienced at least five continental glacial advances during the Pleistocene, which began about 2.5 million years ago. Geologic deposits associated with earlier advances were erosionally removed by the last major advance, the Wisconsin Stage, which began about 75,000 years ago.

On May 18, 2020, 9.7 cm (3.83 in.) of precipitation was recorded over a 48 hr period south of the dams in the town of Midland, Michigan, and up to 20 cm (8 in.) was reported at gauges to the north and east of the dams, according to the National Weather Service (NWS) Detroit/Pontiac (DTX) and Gaylord (ATX) weather forecast offices. This heavy precipitation was driven by moisture from Tropical Storm Arthur and followed approximately 1 in. to 2 in. of precipitation across the Tittabawassee watershed earlier in May 2020. The observed 48 h rainfall at the Gladwin, MI rain gauge (~4.2 in.) is expected to recur at 25 yr to 50 yr intervals, based on statistical analysis of rainfall frequency in the Midwest. Evaluating the daily, month-to-date, and year-to-date precipitation data suggests that the precipitation of May 18 was significantly above normal. Runoff from this precipitation event was recorded at three USGS gaging stations in the Tittabawassee watershed.

Aerial reconnaissance was conducted with a variety of UAVs on several occasions following the dam failures at Edenville, Michigan. In addition to LiDAR, the UAVs were equipped with optical and thermal infrared cameras. A DJI Matrice 600 with Riegl miniVUX unit, which performs 100 scans (mirror rotations) per second with a measurement rate up to 100,000 points per second, was used to collect LiDAR data. The UAVs Digital Elevation Models (DEMs) were georeferenced against real-time kinematic (RTK) GPS ground control points but were subsequently matched to the LiDAR DEM using iterative closest point (ICP) registration. A DJI Mavic 2, with its built-in 20 mp optical camera, as well as a DJI Mavic Pro 1, with its built-in 12 mp optical camera, were also deployed at the Edenville Dam, at M30's Wixom Lake Bridge, and at the Sanford Dam, with the purpose of creating 3D models of these areas.

Edenville Dam was constructed ~1.5 km (0.9 mi) north of the confluence between the Tittabawassee and Tobacco rivers, where the natural topography is dominated by flights of river terraces at various elevations above the modern river. The failure of Edenville Dam was captured on video by a bystander, which provided a unique opportunity to better identify the sequence of dam failure. Digital image analysis was conducted to better understand the deformation of the failure. Field observations regarding historical lake elevations and the breach exposures near the left abutment are described. The impact on the M30 highway and upstream faces of the dam is discussed. Three processes, or combinations thereof, damaged the upstream face of the Edenville Dam following the breach: wave action, rapid drawdown, and scour.

Sanford Dam is located downstream of Edenville Dam; both dams were built between 1924 and 1925. Historical photographs show that similar construction techniques were used on both dams, including grading, excavations, and concrete works. After the May 19, 2020 breach, water from Wilcox Lake flowed into Sanford Lake and overwhelmed Sanford Dam, which was overtopped and consequently failed on the morning of May 20. The area mapped by Unmanned Aerial Vehicles (UAVs). as well as optical imagery for the Sanford Dam are presented. Pre- and postfailure images of Sanford Dam and the digital elevation model created with the UAV images and close-range photogrammetry are also included; these were used to estimate embankment height and scour depths.

Understanding the causes of the Edenville Dam failure is important because Michigan, Ohio, and neighboring states have hundreds of old earth dams that were similarly constructed, specifically., without benefit of modern geotechnical engineering practices pioneered by Karl Terzaghi or compaction quality control measures that were proposed for earth fills by Ralph Proctor in the 1930s. Although rehabilitations can extend their service life, many dams remain in condition that is similar to that at the time of construction. An investigation is described, revealing that the earth dam at Edenville was built with low-dry-density sandy fills that were contractive in nature with nominal shear strength. Prefailure LiDAR and dam inspection photographs are included, which show that the slopes in the vicinity of the recent failure were uneven, that is, they had bulges and oversteepened areas that are also consistent with soil movement.