WATER LEVEL AND LITHOLOGY IMPACTS ON COASTAL BLUFF EROSION ON THE EAST SHORE OF LAKE MICHIGAN FROM 2012-2021

Along the east coast of Lake Michigan (LM), shallow to intermediate depth landslides are influenced by lake water levels, bluff ground water saturation, and lithology. The bluffs are composed of unconsolidated glacial tills interbedded and overlain by glaciodeltaic sand, lacustrine silt, and clay. These bluffs are experiencing toe erosion due to lake level rise, surface erosion, creeping, and slumping due to groundwater infiltration, surface water runoff and bluff seepage.

LM water levels rose after 2013 following a below average period (~1999-2013), peaking at record levels in 2019-2020 before falling slightly in 2021. This water level rise accelerated bluff toe erosion, and longshore currents rapidly removed the sediment and redistributed it along the shoreline. Recent bluff failures have brought media attention due to real estate and roadway losses. Many property owners have chosen to armor the shoreline to prevent further erosion. Following lake level drop, continued erosion has been noted in areas showing high bluff seepage at sand/clay contacts.

This study is focused on: (1) a 1.9 km stretch along Lakeshore Dr. in St. Joseph (SJo), MI; (2) a 2 km stretch of subdivisions centered on Miami Park (MP), MI; and {3) a 1 km stretch of natural vegetated area north of a water reservoir near Ludington (LU), MI. Bluffs have an average failure of 1 foot per year.

Nadir and obliques photos obtained in July 2017, 2019 and 2021 using Unmanned Aerial Systems (UAS), combined with a 2012 lidar survey show that all sites experienced erosion, landslides, and bluff top retreat at varying rates. SJo site has dense vegetation on the bluff face but recent failures have removed vegetation and sediment along the slope face. MP area experienced significant bluff retreat, despite toe armoring with large boulders along several sections. At a Nature Preserve (MP), the bluff top retreated as much as 5-10m, with loss of vegetation on the bluff face and multiple shallow landslides. Examination of bluff profile changes at LU shows accelerated toe erosion and increased bluff steepness during high water level periods, with continued erosion higher up the bluff face towards reaching a more stable equilibrium status as water levels drop. Erosion rates during highest water levels (from 2019 to2021) were three times the erosion rate calculated for 2012-2019.