THE STABILITY OF THE LAKE MICHIGAN BLUFFS IN ALLEGAN COUNTY, MICHIGAN, AND THE RELATIONSHIPS BETWEEN AIR TEMPERATURE, GROUNDWATER LEVELS, AND DOWNSLOPE DISPLACEMENT

Unstable bluffs composed of heterogeneous glacial sediments account for approximately 60 percent of the Great Lakes shoreline. This instability has been attributed to toe erosion, soil saturation, surface water flow, and/or misguided human intervention. The U.S. Army Corps of Engineers and Western Michigan University conducted a joint study between 1996 and 2008 that resulted in a temporal dataset allowing significant insight into the causes of bluff failure. Electronic instrumentation, installed in 2003 within the bluffs of Lake Michigan, north of South Haven, MI, has led to data that grants explanations for the mechanisms and causes of failure. Instruments include subsurface arrays of in-situ inclinometers, vibrating wire piezometers, thermistors, and weather stations. Over the 12-year acquisition of displacement, groundwater, wave, precipitation, and temperature data, the damaging effects of groundwater activity, especially during times of freeze/thaw cycles, have been adequately observed. Groundwater was also discharged from vertical pumping wells during the winter seasons of 2003 and 2005. The vertical pumping wells were located in a dewatering site next to a (non-dewatered) control site. Downslope displacements were reduced by as much as 400 percent when compared to the control zone displacements during the dewatering times.

Statistical evaluation of the data has emphasized the erosional effects during freeze/thaw cycles. Correlation and kernel density studies of: (1) air temperature versus downslope rotational displacement, (2) air temperature versus perched ground water potentiometric surface elevations, and (3) groundwater-level fluctuations verses downslope rotational displacements, have demonstrated instantaneous displacement activity when air temperatures cross 0° Celsius. The freezing of bluff surfaces produces a barrier to perched water discharge, which then raises pore pressures to produce a factor of safety of less than one; this results in simple shear displacement within the bluff. As the frozen bluffs thaw there is a rapid discharge of stored groundwater, which creates an additional lagged downslope displacement that accompanies the increase of flow pressure. The second pulse of displacements tends to be back rotations that accompany block movements of coherent soil.