Analysis of a sudden bluff failure along the southwest Lake Michigan shoreline

Abstract

Elevated lake levels in the Lake Michigan-Huron basin have resulted in erosion at the bases of coastal bluffs composed of unconsolidated glacial deposits. Base erosion leads to a general steepening of the bluff face that can cause the bluffs to suddenly fail, transporting sediment into the nearshore hydrodynamic system. Sudden bluff failure impacts infrastructure built near the bluffs and the sediment budget of the coastal region. A better understanding of the mechanics that govern bluff failure will improve predictions of bluff retreat and sediment budgets. Here we present findings collected from a set of newly developed in-situ monitoring devices that record and measure several properties of the bluff leading up to and during a sudden failure. The devices are capable of accurately measuring, amongst other properties, displacement of the failing bluff at the millisecond sampling rate needed to capture sudden changes. A bluff failure was recorded on December 13, 2016, which prior to failure underwent an elevated creep rate for approximately 1.5 days and a highly-accelerated creep rate for the final 2.5 h. A creep to failure technique was used to analyze the displacement record and substantiated that the bluff failure was likely caused by the formation of segregation ice within the bluff as temperatures dropped from − 1 to − 12 °C over a 40-hour period prior to failure. Finally, a time to failure prediction estimate provided an upper bound for the time until ultimate failure from any one instance of time depending on the instantaneous creep rate.