LAKE MICHIGAN BLUFF STABILITY AND SEDIMENT DELIVERY IN WISCONSIN

Recent elevated water levels have caused erosion at the base of Lake Michigan’s unconsolidated bluffs in Wisconsin. This erosion can lead to accelerated creep rates and/or sudden failure due to steepening of the bluff face. These processes cause the bluff crest to retreat inland and deliver sediment into the nearshore system, impacting the coastal sediment budget. Predictions of bluff recession and sediment budgets can be improved with better modeling of the bluff stability and description of stratigraphic variability in the coastal bluffs.

A typical bluff stability estimate is obtained by calculating a static force balance using digital elevation models (DEMs), the accuracy of which is a limiting factor. For Lake Michigan, existing DEMs were created before the recent water level rise and subsequent steepening at the base. In early summer 2018, we collected aerial photographs of a 500 m reach of shoreline in Sheboygan County, WI using a drone. The photographs were used to create a 10 cm resolution DEM using Structure from Motion. The high-resolution DEM was combined with stratigraphic properties obtained from previous surveys, estimated root cohesion, and a groundwater surface to estimate slope stability using Scoops3D, a three-dimensional moment equilibrium slope stability model.

Factor of safety results highlight the susceptibility of bluff failure, especially near the base. Model estimates of regions likely to fail and the continuous stratigraphic model created with Petrel assist in characterizing sediment budget inputs to the nearshore system. Repeat observations of the bluff face pre and post failure provided a time series of Lake Michigan shoreline bluff erosion that were used to verify model estimates. We have imaged 6 km of additional bluff failure along the Lake Michigan coastline and the slope stability analysis performed in Sheboygan County will be expanded for additional areas susceptible to failure.