Seasonality in cold coast bluff erosion processes

Abstract

High water levels in the Laurentian Great Lakes are causing widespread erosion of unlithified coastal bluffs and damaging infrastructure. Measurements of bluff recession along Wisconsin's Lake Michigan coast are mostly limited to estimates generated from comparisons of historical orthophotos with little information about the short-term scales (months to years) that affect many management and hazard-based decisions. Here we investigate the physical factors driving coastal bluff response to toe erosion at seasonal timescales using a multi-method case study of a mixed lithology bluff in Southeastern Wisconsin, an area that undergoes annual freezing. Five drone photogrammetry surveys were conducted from the winter of 2018 through the winter of 2020, and time lapse photography was collected from March 2019 through February 2020. The bluff face volumetric erosion rate between December 2018 and June 2019 was 3800 m3 a−1, which is 300% larger than the erosion rate between June 2019 and November 2019. Statistical analysis of the time lapse photography indicates that high magnitude erosion events are significantly correlated with environmental factors, including freeze-thaw events. These observations suggest that bluff erosion occurs episodically and is dominated by freeze-thaw related mass wasting events during the winter and spring. To investigate the physical processes driving this pattern, we model transient pore pressure response to a frozen bluff face as an input to a 3-D moment-balance slope stability model. Results show that increases in pore pressures up to one meter are possible at the face of the bluff over 5 days, which corresponds to 4–7% reduction in slope stability, indicating that observed increases in upslope bluff erosion during the winter and spring are a result of increased pore pressures, in combination with sediment strength reduction and high vadose zone saturation. As elevated lake levels continue to steepen bluff toes, upslope impacts will likely be temporally clustered during times of freeze-thaw. To accurately predict the seasonality of bluff recession on cold coasts, landscape change models must incorporate transient conditions caused by freeze-thaw events as they appear to be a dominant process.