Coastal Bluff Evolution in Response to a Rapid Rise in Surface Water Level

Abstract

AbstractThe water level in Lake Michigan began rising in 2013. Historically, erosion of the bluffs along the shoreline increases when water levels rise and remain high, beginning at the toe and propagating upslope, potentially damaging infrastructure near the crest. Bluff retreat is well understood over decadal scales; however, transient perturbations of force instabilities from toe to crest during disequilibrium are difficult to document. This instability for three bluffs, distinguished by height, was investigated using Scoops3D, a three‐dimensional limit equilibrium stability model. For each study site, high‐resolution (10 cm) elevation models were created using Structure from Motion from photographs collected by small unmanned aerial vehicle (sUAV), 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. Photographs from all sites and a repeat sUAV flight at one site verified the model results. Slope angle and relative strength of the sediment are the most significant factors controlling stability, while pore water pressure below the water table acts as a destabilizing agent. At the time of the analysis, unstable surfaces had propagated to the top of the shortest bluffs while the propagation was still working upslope in the tallest bluffs. Unstable surfaces progressed up the bluff faces at an average rate of ~4.4 m/year, indicating the tallest bluff faces will not experience crest recession for a minimum of a decade following stabilization of the toe after water levels stabilize or begin to fall.