BLUFF PROFILE CHANGES OVER 40 YEARS ON WISCONSIN'S LAKE MICHIGAN SHORELINE

Much of Wisconsin’s Lake Michigan shoreline has 10-m to almost 50-m high clay-till dominated bluffs that are susceptible to wave erosion at the base that eventually leads to bluff-top recession. Houses and other structures near the top of the bluff are put in danger when the bluff top recedes to a position close to the building. Over 200 bluff profiles were measured near the end of a high lake level period in a 1970s study and many more profiles were added in the 1980s. In 1995, 277 of these older profiles were re-measured and bluff stability evaluated by calculating a factor of safety for each of the 1976, 1980s, and 1995 profiles. In this study we used 2012 USACE LIDAR to re-measure profiles at all of the earlier profile sites that could be located with a reasonable level of confidence. A factor of safety analysis has been done on 283 of the 2012 LIDAR profiles.

Because the locations of these older profiles were marked on air photos, and not located using GPS, their locations cannot be verified to be exactly correct. Also, because there is no spatial control on the top or base of previous profiles, this comparison cannot be used to measure bluff top recession rates. It is possible, however, to conclude based on all of the profiles, that slopes are more stable, have more gentle slopes, and have more sediment accumulated at the toe of the bluff than was there in the mid-1970s or mid-1990s. Of the 190 1976 profiles that were “re-occupied” in 2012, 161 increased their factor of safety, 3 had safety factors that remained the same, and only 26 showed a decrease in factor of safety between 1976 and 2012.

Presumably this is mostly because of generally low lake levels over the past few decades. Data from our studies of erosion of Wisconsin’s Lake Michigan and Lake Superior shorelines since the 1970s, have been recently posted at: http://floodatlas.org/wcmp/obliqueviewer/. These include low-level oblique photos from the 1970s, 2007, 2008, 2012, 2016 and 2017, mapped evaluation of bluff conditions along the shore from 1970s and 2007-2008 based on interpretation from oblique air photos, shore structures in 1976 and 2007 and 2008, and bluff and bathymetric profiles from 2012. See also: http://floodatlas.org/wcmp/obliqueviewer/about.html