Observations of Barred Coastal Profiles under the Influence of Rising Water Levels, Eastern Lake, Michigan, 1976-71.

摘要

Descriptions of lakeshore bathymetry and its temporal variations over a 4-year period are based on 125 shore-normal profiles from 35 stations and aerial photos covering 50 kilometers of shore near Pentwater Harbor on the eastern shore of Lake Michigan. A sequence of four, well-developed, straight longshore bars paralleled the shore. These bars were observed to merge with one another only twice along the entire 50 kilometers, and were persistent year-round features. There was also a temporary coastal bar which displayed at least four distinct aerial patterns. In wavelength, lacustrine longshore bars overlap with the sand waves from continental shelves; in relief, bars are somewhat smaller (0.1 to 2.2 meters). Bar relief equals approximately one-half crest depth or one-third trough depth. The upper limit on the cross-sectional area of a given bar is numerically about the same (in square meters) as the spacing (in meters) between it and the next bar shoreward. As depth decreases toward shore, the cross-sectional dimensions, including bar height and spacing, decrease in regular progressions. Waves were observed to break almost exclusively by spilling, indicating that contrary to previous reports, plunging-type breakers are not necessary for bar development. The frequency of waves breaking on the outer bar was calculated to be about 18 hours per decade, suggesting that the breakers at this depth (averaging 5.3 meters) may occur too infrequently to account for observed bar mobility and maintenance. From 1967 to 1971, the annual mean elevation of Lake Michigan rose 0.5 meter. Inner bars north of Pentwater Harbor rose 0.5 meter and migrated an average of 26 meters landward.