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

Abstract

The 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. Plain Language Summary Recent rising lake levels in the North American Great Lakes is causing bluff erosion. Since 2013, the level of Lake Michigan has increased about 1 m at a rate of 16 cm/year, a rate of water level rise that has been observed several times over the past 100 years. We used a drone-mounted camera to photograph three eroding bluffs along the southwestern shoreline of Lake Michigan. High-resolution digital elevation models of the bluff shape from the pictures were used in a three-dimensional slope failure model. This model includes the slope shape, ground water elevation, and material properties of the bluff to predict areas that are likely to fail. Our results indicate that failures beginning at the base of the bluff migrate upslope at a rate of~4.4 m per year, and the tallest bluffs will continue to erode at the top for at least a decade after the base stabilizes. This information will be relevant for people and organizations with infrastructure located near the coast.

Potential for Shoreline Recession to Accelerate Discharge of Groundwater Pollutants to Coastal Waters

“…While it is known that water levels in the Great Lakes vary over 20-30 year cycles (Gronewold & Rood, 2019), recent future projections estimate that by 2040-2049 average annual water levels of Lake Superior, Lake Michigan-Huron and Lake Erie are projected to increase by +0.19, +0.44, +0.28 m, respectively due to an increase in over-lake precipitation and basin runoff compared to relatively smaller increase in lake evaporation (Kayastha et al, 2022). While there are no studies predicting future erosion rates and shoreline recession in response to the projected water level changes for the Great Lakes, several studies show that high water levels correspond with accelerated shoreline erosion, including bluff erosion, on short time scales (e.g., Kaczmarek et al, 2016;Krueger et al, 2020). For instance, Castedo et al (2013) simulated erosion rates to range between 0.5 and 1.5 m/yr in response to lake water level variations of 0.5-1 m. A more recent study by Volpano et al (2020) on coastal bluff erosion along Western Lake Michigan's shoreline found that erosion rates increased from 0.18 to 0.43 m/yr between 2009 and 2014 (period with below long term average water levels) to 0.49-1.19 m/yr between 2014 and 2018 (period with above long-term average water levels).…”

“…While it is known that water levels in the Great Lakes vary over 20–30 year cycles (Gronewold & Rood, 2019), recent future projections estimate that by 2040–2049 average annual water levels of Lake Superior, Lake Michigan‐Huron and Lake Erie are projected to increase by +0.19, +0.44, +0.28 m, respectively due to an increase in over‐lake precipitation and basin runoff compared to relatively smaller increase in lake evaporation (Kayastha et al., 2022). While there are no studies predicting future erosion rates and shoreline recession in response to the projected water level changes for the Great Lakes, several studies show that high water levels correspond with accelerated shoreline erosion, including bluff erosion, on short time scales (e.g., Kaczmarek et al., 2016; Krueger et al., 2020). For instance, Castedo et al.…”