INVESTIGATING A FAILING LAKE MICHIGAN BLUFF USING UAS IMAGERY AND GROUND BASED GEOPHYSICAL TECHNIQUES,

In areas along the eastern shoreline of Lake Michigan, rapid bluff erosion has been documented in recent years (2019-present), coinciding with a period of increased water levels in the lake. These bluffs composed of unconsolidated glacial tills interbedded with and/or overlain by glaciodeltaic sand and lacustrine silt and clay are experiencing toe erosion, surface erosion, creeping, and slumping due to groundwater sapping, runoff and seepage. Lake level rise from 2014 - 2020 (over 1 m increase from minimum recorded in 2012), contributed to accelerated bluff toe erosion, and longshore currents rapidly removed the sediment and redistributed it along the coast. This study is focused on a 2 km stretch of subdivisions centered on Miami Park (MP), MI, and expands on work done by the USGS and Michigan Geological Survey (MGS). From 2016-2022 we conducted uncrewed aerial systems (UAS) flights documenting the locations and patterns of failure. Many locations have active groundwater seepage at multiple clay layer contacts on the bluff faces. The mosaicked images show the thickness of these bedded sands and clay layers varies notably from South to North along the bluff. To investigate subsurface variability in the spatial extent of these features, electrical resistivity imaging (ERI) and ground penetrating radar (GPR) surveys were carried out on two-dimensional transects along the bluff parallel to the face both in the Miami Park subdivision and in the Casco township Nature preserve in July of 2022. ERI data were acquired with a Lippmann 4-point light 10 W resistivity meter with 200 electrodes spaced every 1 m using a dipole-dipole electrode array while reflection GPR measurements were done using Pulse Ekko bistatic antennas with frequencies of 100, 200 and 250 MHz to map the changes. The MP area experienced significant bluff retreat, despite toe armoring with large boulders along several sections. At the Nature Preserve, the bluff top retreated as much as 5-10m. Resistivity and GPR results showed contrast in the silt - clay contents in the sand with a varying high resistivity zone (200 - 1400 𝜴m) and discontinuous high amplitude reflectors between 4 and 15 m. This geophysical mapping complements the UAS mapping of the bluff face, and on-the-ground geological mapping transects to provide a more complete view of the internal bluff structure.