The Great Lakes as a Test Model for Profile Responses to Sea Level Changes

Abstract

Abstract : The average annual water level on Lake Michigan rose 0.8 m between 1967 and 1973 causing adjustments in nearshore processes and topography. Assuming 2 mm/yr as a typical rate for sea level rise, it would take 400 years to observe similar adjustments on an ocean beach. The rapid rise in average water level is one of several features that makes the Great Lakes useful for testing concepts of shore response to sea level changes. The key assumption in the Bruun concept of response to sea level rise is that the same average beach profile is reestablished relative to the higher water elevation. This assumption was confirmed by detailed measurements over a 9-year period of 25 beach and offshore profile transects along 50 km of Lake Michigan shoreline. Complete profile adjustment lagged 3 years behind the water level change. A simple equation was developed expanding Brunn's concept to account for (a) gains and losses of sediment from causes other than the water level variation, (b) erosion of different size sediments in the receding shoreface, not all of which would be stable in the shore zone, and (c) accretion of beach material during falling water levels. The expanded Bruun sediment balance approach reduces the problem of estimating long-term shore response to sea level rise to the simpler problem of determining an appropriate closure depth for the responding profile. In the Great Lakes, this closure depth can be estimated as about twice the 5-year-return-period wave height for the site under consideration.