The Great Lakes are also subject to regular changes in lake levels. Astronomical tides on the lakes
are small and not pertinent to the practical problems of shore protection design. However, records of
lake levels dating from 1836 reveal seasonal and annual changes due to variations in precipitation (rain
or snow) each year and from year to year. Lake levels (particularly Ontario and Superior) are also
partially controlled by regulatory works operated jointly by Canadian and United States authorities,
resulting in minimizing lake level changes. Average monthly lake level elevations, showing data for the
past calendar year and the present year to date, and a forecast for the next 6 months are published
monthly by the U. S. Army Corps of Engineers, Detroit District (see Other Help Section).
THE EROSION PROBLEM
The Importance of Shoreform
Shoreforms are those distinct shapes or configurations, which mark the transition between land
and sea. Throughout the United States, the basic shoreforms that predominate are bluffs (and cliffs),
gently sloping plains or sand beaches (with or without dunes), and wetlands or marshes. All shorelines
share some predominant feature with at least one of these shoreforms. Of course, a shoreline may
combine two or even all three of these forms. For instance, a shoreline may be a high bluff with a sand
beach at the base, or a gently sloping plain fronted by a marsh. In that case, one must consider the
interaction of these features with the erosive forces and then single out the most important for further
study.
Bluff and Cliff Shorelines. Cliff shorelines consist primarily of relatively resistant rock. On the
other hand, bluff shorelines are composed of such sediments as clays, sands, and gravels, or erodible
rock. Cliffs rarely suffer severe or sudden erosion but undergo slow, steady retreat under wave action
over a long period. Such shorelines cannot be protected at a low cost because available alternatives
would not be as durable as the rock forming the cliff.
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