Physical drivers of seasonal mass wasting on cold-region coastal bluffs

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 the Lake Michigan coast are mostly limited to estimates generated from comparisons of historical orthophotos with little information about the short timescales (months to years) that affect many management and hazard-based decisions. 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. 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 ~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 is dominated by thaw-related mass wasting during the winter and spring. To investigate the physical processes driving this pattern, we model the transient pore pressure response to a frozen bluff face for input to a 3-D moment-balance slope stability model. The groundwater flow model is constrained by in-situ seep discharge and hydraulic head measurements. Modeling results show that increases in pore pressures corresponding to 4-7% reductions in slope stability are possible. This minimum in stability was both observed and modeled for the winter and spring and was found to result from transiently high pore pressures caused by ice-dammed groundwater flow at the bluff face, sediment preconditioning via freeze-driven mechanical strength degradation, and high vadose zone saturation. These factors combine to produce a stability minimum during a thaw that immediately follows a sustained freezing event. 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 dominate the signal.