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 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 conf igurations 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 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.
Erosion, problems are most common along bluff shorelines where a variety of forces and processes
act together (Figure 5). The most prevalent causes of bluff erosion and recession are scour at the toe
(base) by waves and instability of the bluff materials themselves. Slope stability problems are highly
technical and can only be analyzed correctly using methods of geotechnical engineering. Therefore, they
are beyond the scope of this report. A brief discussion of factors affecting slope stability and how to
recognize potential problems is presented below. It is suggested that if a property is endangered by an
apparent slope stability problem, a registered professional geotechnical engineer should be contacted.
As Figure 5 illustrates, a typical bluff often consists of different soils deposited in distinct layers,
such as clay, sand, silt, or glacial till. (Glacial till contains a mixture of particle sizes and is common
throughout the Great Lakes region.) Soils are not generally stable at a vertical face, but form a slope that
varies with the soil and groundwater conditions. This slope forms as a result of a series of failures whose
nature depends on whether the soil is cohesive (clay) or granular (sand, silt, gravel, etc.). Cohesive soils
generally slide along a circular or curved arc, the soil moving downward as it rotates along the failure
surface. Granular soils, on the other hand, fail when vertical-sided blocks drop to the bottom or when the
soil suddenly flows down an inclined plane. Height is a factor because high bluffs (over 20 feet) impose
greater stresses and are likely to suffer more severe stability problems than low bluffs.
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