FREEZE/THAW PROCESSES DRIVE SEASONAL MASS WASTING ON COLD-REGION COASTAL BLUFFS: EXAMPLE FROM A CASE STUDY IN SOUTHEASTERN WISCONSIN

Record-high water levels in the Great Lakes are causing widespread erosion of coastal bluffs and damaging shoreline infrastructure. Wave-driven toe erosion steepens the bluff face, increasing the likelihood of upslope failure via mass wasting and escalating associated inland recession of the bluff crest. Measurements of erosion along Wisconsin’s Lake Michigan shoreline are mostly limited to long-term toe and crest recession estimates from comparisons of historical orthophotos with little information about the short-term scales (months to years) that affect many management decisions. This research investigates the physical factors driving bluff response to toe erosion at seasonal timescales using a multi-method case study of a mixed lithology bluff in Southeastern Wisconsin.

Four drone surveys were conducted in December 2018 and June, November, and December 2019. Time lapse photography was collected from June through December 2019. The bluff face volumetric erosion rate between December 2018 and June 2019 was 9.3 m³/day. This is 240% larger than the erosion rate between June 2019 and November 2019. Qualitative interpretation of the time lapse photography indicates that high magnitude erosion events are correlated with precipitation, snowmelt, and freeze/thaw events. These observations suggest that upslope bluff erosion occurs episodically and is dominated by mass wasting during winter and spring freeze/thaw and snowmelt events.

To investigate the mechanisms driving this pattern, we use a heuristic approach of permeability reduction to model transient groundwater head response to a frozen bluff face. Model results show that increases in head up to one meter are possible at the face of the bluff over six days, which corresponds to an 8-12% reduction in slope stability. This supports the hypothesis that observed increases in upslope bluff erosion during the winter and spring are a result of increased pore pressures in combination with vadose zone sediment strength reduction due to high relative saturation. As elevated lake levels continue to steepen bluff toes, upslope impacts will likely be temporally clustered during times of freeze/thaw. Further research is needed to understand how climate change impacts on the frequency and magnitude of freeze/thaw events will influence coastal erosion in cold regions.