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. Because slope stability problems are
difficult to analyze correctly without expertise in geotechnical engineering, 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. If you believe your property is endangered by a slope stability problem,
you should contact a registered professional geotechnical engineer.
As the figure 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.) These soils do not permanently stand at a vertical face, but form an
angled slope that varies with the soil and groundwater conditions. This slope forms following 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, and the soil moves downward and rotates
along the failure surface. With granular soils, on the other hand, vertical sided blocks will drop or the
soil will suddenly flow 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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