GLACIAL BLUFF LANDSLIDES, LAKE MICHIGAN: A RESPONSE TO HIGH WATER LEVELS, PRECIPITATION, AND WIND STORMS

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 are subject to erosion, including shallow to intermediate depth landslides. The Lake Michigan average-water levels have surpassed the 100-year high record for January and have triggered increased wave-induced bluff erosion. The interaction of bluff morphology with ground and surface water has also contributed to bluff failure. Abrupt landslides in the unconsolidated sediments typically start with surface bluff erosion, creeping, and slumping due to groundwater saturation, seepage and water runoff that may affect top / toe recession.

During 2019, repeat unmanned aerial system flights were completed in three communities on the southeastern shores of Lake Michigan: 1. St. Joseph along Lakeshore Drive, a residential area; 2. north of South Haven, (Miami Park), a rural vacation home area; and 3. a relatively unpopulated park area north of Ludington storage reservoir. The St. Joseph and Miami Park locations have generally 4-13 meters of diamicton over 10-20 meters of glaciolacustrine deposits. The Ludington site contains 50 meters of glaciolacustrine deposits deformed by numerous diapirs of diamicton. All sites are capped by 1-3 meters of eolian sand. The St. Joseph site has dense vegetation on the bluff face, but recent linear displacement scarps are evident in the UAS photography. The Miami Park site is moderately vegetated with ongoing attempted remediation of a recent landslide. In summer 2019 other small to medium landslides impacted the Miami Park site, some of which displayed continuous movement for months. The Ludington area has minimal vegetation on the bluff face with evidence of small recent landslides and surface erosion due to rainfall and concentrated groundwater seeps due to low permeability lacustrine clays deformed by glacial tectonics.

During this phase of high lake level, bluff erosion has been accelerated by oversteepening due to a possible combination of saturated bluff sand and the removal of bluff toe. To date multiple counties in Michigan have applied for additional mapping and aviation support, including Muskegon, Van Buren, Lake, Mason, Ottawa, Oceana and Manistee counties.