QUANTIFYING AND MODELING BLUFF EROSION ALONG WISCONSIN’S LAKE MICHIGAN COASTLINE

Elevated water levels in Lake Michigan are causing widespread toe erosion of Wisconsin’s coastal bluffs. This toe erosion steepens the bluff face, increasing the likelihood of slope failure via shallow to intermediate-depth rotational and translational slides and escalating associated inland recession of the bluff crest. The supply of sand-sized sediment to Lake Michigan has historically been dominated by the erosion of coastal bluffs, but anthropogenic armoring of significant portions of the coastline has likely decreased the average bluff erosion rate and reduced the amount of sand-sized sediment available to sustain beaches and dune complexes.

A subset of ‘feeder’ coastal bluffs are hypothesized to contribute an outsized proportion of sediment to the nearshore system due to differences in lithology and hydrogeologic conditions. To assess this hypothesis, we evaluate erosion rates at two bluffs with different lithologies and morphologies using a time series of high-resolution (10 cm) digital elevation models (DEMs) generated via drone-based structure-from-motion (SFM) photogrammetric techniques. To better understand the processes leading to upslope bluff erosion, we input the high-resolution DEMs along with sediment geotechnical properties and pore pressure models based on groundwater level observations into a three-dimensional moment balance slope stability model (Scoops3D). This yields regions of modeled instability, which are compared to observed regions of erosion to evaluate whether sliding (along with wave-caused toe erosion) adequately explains bluff erosion patterns.

We find that the site with a higher percentage of coarse-grained sediments is contributing a larger volume of sand to the nearshore system than the site comprised of more cohesive sediments. Additionally, erosion rates and modeled instability are highest in regions of the bluff proximal to active groundwater seeps. This indicates that the feedback between sediment characteristics and groundwater flow plays a role in controlling upslope bluff erosion processes. Drone-based SFM provides an inexpensive, repeatable method of assessing heterogeneous landscape evolution and improving predictions of bluff erosion rates.