PHYSICAL MODELING OF EROSIONAL PROCESSES LEADING TO SLOPE FAILURE ALONG LAKE MICHIGAN SHORELINE

Increased precipitation due to climate change has globally increased the likelihood of slope failure hazards. On a local level, the bluffs along the Lake Michigan shoreline face increased erosion due to increased lake level, surface water runoffs, and groundwater movement during the freeze thaw cycle where groundwater moves towards the cold front during winter, freezes and expands in cracks creating planes of weakness, and pore pressure from the excess water leading to slope failure in spring. In order to monitor, predict, and mitigate slope failure hazards, we need to understand the rate of groundwater movement in response to a temperature gradient, initial water content in the saturated and unsaturated zones, as well as the rate of lake erosion processes impacting the toe of the bluffs.

Our study focuses on the North Beach Park of Port Washington, WI, on the Lake Michigan shore. Potentially unstable bluffs in this location can impact new and existing infrastructure as well as the tourist-driven economy of the town. Previous studies conducted in this location show that the bluffs are mostly composed of relatively impermeable silty clay with interbedded layers of more permeable clayey silt and fine sand. The exposed silty clay layer is eroded by surface water runoff, while groundwater movement along the more permeable sandy/clayey layers create groundwater seeps along the bluff and weakens the slope. These, added to constant lake erosion along the bluff toe, can lead to bluff failure in the near future.

For our project we are replicating these erosional processes in sandbox models using layers of sand with grain sizes ranging from >2mm to <0.5mm, and both standing and flowing water to represent the lake and surface water respectively. We are using liquid nitrogen to simulate winter freeze and a fan to create “lake waves” at the base of the bluffs. Our goal is to experimentally determine the rate of water movement from saturated to unsaturated zones due to a temperature gradient and its role in making slopes unstable. This information can provide important insights in the mechanisms causing bluff failure and help us determine appropriate mitigating measures. Here we present our experimental setup, preliminary data, and broader implications of our findings.