STRUCTURE-FROM-MOTION TO MONITOR BLUFF EROSION IN DIFFERENT SETTINGS ALONG EASTERN SHORE OF 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-19th century. Shallow to intermediate depth landslides are influenced by lake water levels, bluff ground water saturation, and lithology. The bluffs are composed of glacial tills interbedded with / overlain by glaciodeltaic sand and lacustrine silt and clay. Lake Michigan water levels rose after 2013 following a below average period (176.0 m during 2000-2013), peaking at record levels in 2019-2020 (above 177.3 m) before starting to fall again in 2021. This resulted in accelerated bluff toe erosion, that combined with surface erosion, creeping, and slumping due to water runoff and seepage, enabled more frequent landslides.

This study is focused on 3 sites: (1) a 1.9 km stretch along Lakeshore Dr. in St. Joseph (SJo), an urban area, MI; (2) a 2 km stretch of subdivisions centered on Miami Park (MP), a suburban area, MI; and (3) a 1 km stretch of natural vegetated area north of a water reservoir near Ludington (LU), MI, a natural area. All sites have active groundwater seepage at clay layer contacts on the bluff faces.

Nadir and obliques photos obtained in July to September 2019 and July 2021 and 2022 using uncrewed aerial systems (UAS) have shown that all sites experienced erosion, landslides, and bluff top retreat. SJo site was densely vegetated on the bluff face, but recent displacement scarps and small landslides are evident in the 2021 UAS data.

The MP area experienced significant bluff retreat due to several and repeat landslides that caused 3 to 5 m top recession, despite toe armoring with large boulders along several sections. At a Nature Preserve within the MP area, the bluff top retreated as much as 5-10m, with loss of vegetation on the bluff face, and collapse of a boardwalk and access road.

LU area had several landslides resulting in both vegetation and land surface loss. More than 5m of glacio-deltaic sand were lost during 2019-2021 around a water seep above lacustrine clay with marginal accumulation at the bluff toe, whereas during 2012-2019 toe erosion removed ~10m underneath that location, with ~5m loss normal to the face. Both MP and LU locations experienced an erosion rate approximately 3 times larger during the 2019 – 2021 period versus 2012 – 2019 period.