THREE-DIMENSIONAL STABILITY ANALYSIS OF BLUFFS ALONG THE LAKE MICHIGAN SHORELINE IN EASTERN WISCONSIN

Abstract

The water level in Lake Michigan has been rising since 2013. Historically, erosion of the bluffs along the shoreline has increased during times of lake level increase. Erosion begins at the bluff toe and progresses upslope, potentially damaging infrastructure at the bluff crest. The factors controlling stability for the three bluff types, distinguished by height and retreat type, were investigated using Scoops3D, a three-dimensional limit equilibrium stability model. The bluff types can be summarized as follows: 25 m height with parallel retreat, 40 m height with complex retreat, and 15 m height with complex retreat. For each study site, high-resolution (10 cm) elevation models were created from photographs collected by drone using Structure from Motion, and steady state groundwater models were developed using MODFLOW. These were used as inputs for Scoops3D along with physical properties of the sediment reported in the literature. Unstable areas were verified through comparison to the photographs at all sites and a repeat drone flight at one study site. Slope angle and relative strength of the sediment appeared to be the most significant factors controlling stability, while the pore water pressure from the depth below the water table acted as a destabilizing agent. Increases in the water table, even for short periods, cause previously stable areas to become unstable. Scoops3D was able to capture the retreat type associated with each bluff type, and the relative location of unstable areas along the bluff face suggest that the upslope progression of erosion reaches the crest of shorter bluffs faster than taller bluffs.