MODELLING SHORELINE BLUFF STABILITY IN OZAUKEE COUNTY, WI

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 accelerate the creep rate or 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 sediment budgets and bluff retreat.

One factor limiting the accuracy of bluff stability estimates is a lack of high-resolution digital elevation models (DEM) from which static force balances are estimated. Additionally, the DEMs that exist for the bluffs were acquired prior to the recent rise in lake level and subsequent base steepening. We used drone acquired photographs to create an 18 cm resolution DEM using Structure from Motion for a 300 m reach of shoreline bluff in Ozaukee County, WI. The high-resolution DEM, stratigraphic properties mapped from previous surveys, root cohesion, and a groundwater surface were used to estimate slope stability with Scoops3D, a three-dimensional moment equilibrium slope stability model. The slope stability results from the high-resolution DEM were compared with results from coarser DEMs collected in 2010 (2 m resolution) and 2012 (1 m resolution).

Factor of safety results from Scoops3D indicate that the shoreline bluffs are susceptible to failure, especially near the base where recent steepening has occurred. The spatial predictions generated from the different DEMs are comparable, but the higher resolution provides a more defined failure zone. In-situ measurements of bluff creep were recorded upslope from the modeled unstable region, supporting model predictions of local instability. The use of a 3D hillslope model with high-resolution DEM provides a significantly more precise prediction of hillslope stability for the coastal bluffs than previously possible. This method for estimating slope stability can be expanded to other regions of the Lake Michigan coastline, as well as other regions of unconsolidated glacial deposits.