Creeping Before the Fall: Glacial Bluff Landslides, Lake Michigan

The impacts of bluff failures and their contribution to the nearshore sediment budget in the Great Lakes have been documented since the mid-19^th century. Bluffs composed of unconsolidated glacial deposits exhibit erosion, including significant failures. The predominant bluff retreat mechanism in this area is shallow to intermediate depth landslides. Recent Lake Michigan high-water levels have triggered wave erosion, but the interaction of bluff morphology with ground and surface water has also induced bluff failure. Rapid bluff failures attract public attention and command the deployment of resources to mitigate the local and regional impacts, but abrupt landslides in the unconsolidated sediments typically start with surface bluff erosion, creeping and slumping due to water runoff and seepage that may not affect top / toe recession.

To test this hypothesis three sites were flown in July 2019 using a DJI Phantom 3 Professional unmanned aerial system (UAS): St. Joseph, along Lakeshore Drive, a residential area; north of South Haven, (Miami Park), a rural vacation home area; and a natural vegetated area north of Ludington storage reservoir. All 3 sites experienced small to medium landslide failures during the summer of 2019 and are composed of glacial till, clay and silty sands. The St. Joseph site has dense vegetation on the bluff face, but recent displacement scarps are evident in the UAS photography. The Miami Park site is moderately vegetated and experienced a significant landslide. The Ludington area has minimal vegetation on the bluff face showing evidence of small recent landslides and surface erosion due to rainfall and water seepage.

Comparing the Ludington UAS data with 2012 lidar data, the cumulative erosion change measured on the bluff surface normals is circa 190,000 m³ (over a length of ~1km). The same site exhibits an active water seepage that may have caused sand slumping and creeping of an area ~ 100 m in width. Older creeping / slumping scars and water seepage areas are present as well. The Miami Park area had an obvious landslide that is undergoing attempted remediation by dumping dirt on the slope of the bluff and adding large riprap boulders at the toe of the bluff. If the rainfall and water seepage continue, surface application of riprap and soil alone is unlikely to fully mitigate the erosion that is occurring.