HAS ANOTHER PERIOD OF BLUFF INSTABILITY BEGUN ON WISCONSIN’S LAKE MICHIGAN BLUFFS?

Wave erosion at the base of coastal bluffs is a major driver of slope instability. It is likely that over the long term, lake level is a major control of erosion rate. As the base of the bluff is steepened, failures take place on the lower part of the slope and through time, this instability migrates up slope, eventually causing bluff-top retreat. Over 200 bluff profiles were measured near the end of a high lake-level period in the 1970s when bluff-top recession was a serious problem for many bluff-top land owners. In 1995, 277 of these and other older profiles were re-measured and increasing bluff stability was evaluated by calculating a factor of safety for each of the 1976 and 1995 profiles. We used 2012 USACE LIDAR to re-measure profiles at all of the earlier profile sites that could be located. By 2012, after a 13-year period of below-average lake levels, slopes were less steep, more sediment had accumulated at the toe of the bluff, and slopes were more stable than they had been in the mid-1970s or mid-1990s. Of the 190 1976 profiles that were “re-occupied” in 2012, 161 had become more stable.

Between 2012 and 2014, Lake Michigan and Lake Superior both continued to have levels below the long-term average. Thus, between 1999 and 2014 more of the sediment shed from the bluff face accumulated at the base of the bluff because less sediment was removed by wave erosion. The bluffs continued to become more stable. Levels in both Lake Michigan and Lake Superior then rose above the long term mean in summer of 2014 and have remained high since. Summer of 2017 saw the highest levels in both lakes of this recent high-water period. Low-altitude oblique photos of the bluff taken in June 2017 have been examined qualitatively to assess the impact of this recent lake level rise. Higher lake level has resulted in erosion and steepening at the base of many unprotected bluffs. Data from our studies of erosion of Wisconsin’s Great Lakes shorelines since the 1970s, including the photos used in this talk, have been recently posted at: http://floodatlas.org/wcmp/obliqueviewer/.