Factors Controlling Rates of Bluff Recession at Two Sites on Lake Michigan

Abstract

Empirical relationships between recession rate of bluffs and precipitation, storm frequency, lake level, deep water wave power, and wave impact height are derived for two Lake Michigan shoreline reaches in Wisconsin. Recession rates are determined from digital orthophotos constructed using historical aerial photographs at least once every decade from the 1940s to present. The recession measurements represent spatial averages of rates measured at increments of 10-20 m along the shoreline over a distance of about 500–700 m. The temporal variations in recession rates over intervals ranging between 6 and 17 years were determined for the toe and crest at the sites with high (30–45 m) bluffs and for the crest, but not the toe at the site with low (9–11 m) bluffs. Trends in precipitation, storm frequency, and deep water wave power show weak relationships with changes in bluff recession rate. Both the toe of the high bluffs and the crest of the low bluffs show temporal recession-rate patterns that closely match the changes in the average lake-water level. The crest of the high bluffs recedes at a rate that is relatively insensitive to lake level changes. The annual average of the peak monthly wave-impact height appears to be the best predictor of bluff recession rate over the intervals studied.