PROPERTIES OF LONGSHORE BARS IN THE GREAT LAKES

James H. Saylor, Edward B. Hands

Abstract


Longshore bars are permanent features of nearshore bathymetry along the windward coasts of the Great Lakes The stability and permanency of these features have been noted by numerous investigators, but movements of the bars and troughs vn relation to varying lake levels and incident wave energies are not fully understood Studies of nearshore bathymetry and sediment properties were conducted during 1967 and 1969 along a forty-five kilometer reach of the eastern coast of Lake Michigan Results show that the offshore bars migrate significantly due to changes in lake level, a rise of one-half meter in the surface of Lake Michigan between 1967 and 1969 was accompanied by a shoreward movement of bar crests and troughs over a distance averaging SO meters Elevations of the crests and troughs are also built upward toward new equiblibrium levels during rising water levels, but elevating of the crests lags the increase in stage Extensive shore erosion occurs because of the reduced effectiveness of longshore bars in dissipating incident wave energy The average crest depth was found to increase linearly in the offshore direction Average distances between crests increase exponentially These relationships are preserved during the bar growth and shifting that accompanies long term changes in lake level Bar troughs are characteristically crescent shaped, with no abrupt changes in slope Fathograms from several ranges show atypical trough configurations consisting of flat bottoms with discontinuities in slope on ascent to adoacent crests This unusual trough shape is indicative of an immobile stratum exposed along the bottom of the trough.

Keywords


longshore bars; sand bar; Great Lakes

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