The Wisconsin shoreline of Lake Superior is highly spatially variable. Bluff materials present range from Precambrian bedrock to glacial outwash sand and gravel to clay tills. Beach materials also vary widely including beaches of sand, beaches of cobble, and beaches that are a mix of the two sand and cobbles.
Wave runup measurements showed the Nielsen and Hanslow (1991) wave runup equations were appropriate for predicting wave runup at the study sites. Reduction factors needed to be applied to the predicted wave runup to account for diminished runup on cobble beaches and mix beaches.
Wave impact height (WIH) was tested as an wave-bluff interaction index to predict the spatial variability of bluff recession rates. Positive correlations exist between cumulative wave impact height (CWIH) and bluff recession rates. A general trend exists when considering all sites together, but there is much scatter in the data. Adjustments to the correlation to account for variability in beach width, bluff height, and bluff material type remove much of the scatter in the CWIH versus recession rate correlation. The correlations indicate the recession of the sand bluffs has a stronger dependence on CWIH than the cohesive bluffs. However, relatively few data points were available for sand bluffs and considerable scatter still existed for the cohesive bluff correlation after all adjustments to the correlation data were made.
Improvements to the CWIH versus recession rate correlation could be made by obtaining higher quality recession data and obtaining more wave runup measurements to better characterize wave runup on a site by site or reach basis.
The CWIH analysis quantitatively showed the dependence of certain bluff types on erosion, and WIH has shown promise as an index for indicating where recession rates will be high and low in a coastal area. While the following study was performed on Lake Superior, it is expected similar relationships will exist for other shorelines, though the correlations would need to be calibrated for the specific coastal area.