Bluff erosion along the east shore of Lake Michigan: Synergy between
water levels and lithology
Abstract
Along the east coast of Lake Michigan, 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 with / overlain by glaciodeltaic sand and
lacustrine silt and clay. These bluffs are experiencing toe erosion due
to lake level rise, surface erosion, creeping, and slumping due to water
runoff and seepage. Lake Michigan 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 rise
accelerated bluff toe erosion, and longshore currents rapidly removed
the sediment and redistributed it along the coast. 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. This study is focused on 3 sites: (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. All sites have active groundwater seepage at clay layer
contacts on the bluff faces. Nadir and obliques photos obtained in July
2019 and 2021 using Unmanned Aerial Systems (UAS) have shown that all
sites experienced erosion, landslides, and bluff top retreat. SJo site
has dense vegetation on the bluff face but recent failures have removed
vegetation and sediment along the face slope. 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 landslides. LU area had several landslides resulting in
both vegetation and land surface loss. From 2019 to2021, the LU area
experienced 177,000 ± 2300 m3 of erosion, which
indicates a rate of erosion three times the erosion rate calculated for
2012-2019 (190,000 ± 14,000 m3). More than 5m of
glaciodeltaic 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.