Bluff Recession Rates and Wave Impact Along the Wisconsin Coast of Lake Superior

Abstract

Coastal bluff recession continually threatens homes, roads, and land. In areas where coastal development is increasing, an understanding of variability in bluff recession is critical to coastal management. In this study bluff recession rates and beach and bluff lithology and morphology are characterized at 28 sites along the Wisconsin coastline of Lake Superior. Bluffs are composed of clay, sandy clay, clayey sand, sand, and sandstone, and range from 1.1 to 37.3 m in height. Beach composition at the sites varies from sand to a mix of sand and cobbles, to cobbles and boulders, and beach slopes are between 3 and 14̊. Bluff-crest recession rates between 1966 and 1998, measured from aerial photographs, range from 0.07 to 0.57 m/yr. Field measurements of wave runup at the study sites were conducted to verify wave runup estimated from available methods in the literature. An index, cumulative wave impact height (CWIH), which accounts for the frequency, magnitude, and duration of waves impacting the bluffs, is used to assess the degree of correlation between this measure and bluff recession rates. The average yearly CWIH ( CWIH ¯ ) for the 1966-1998 epoch is correlated with the recession rates from the same period. Reasonable correlations between CWIH ¯ and recession rates at sites throughout the study area are found when comparing bluffs of similar lithology and height. These results suggest that bluff recession rates in this area are not only linked to wave impact at the bluff toe but also lithology, which affects a bluff's response to wave attack at the toe as well as other processes (e.g., gully erosion) that promote recession.