Coastal Bluff Failure
Details of Coastal Bluff Failure Mechanisms
Wave erosion, lakebed erosion and rainfall are the most significant mechanisms that cause slope instability on the cohesive coastal slopes of the Great Lakes. Rainfall and snowmelt cause surface runoff and erosion on slope faces and raise groundwater levels within bluffs; weakening the soil strength and increasing the soil load.
Banks fail by the same mechanisms, but their typically less-complex structure results in simpler failures that tend to be easier to anticipate and avert, than the failures that occur in bluffs.
Coastal bluffs are defined here as coastal slopes with an elevation of 20 feet or more above the back of the beach. Bluffs have more complex slopes and tend to have more variable soils than the slopes that are lower in height (defined here as "banks").
Many bluffs (and banks) are formed of glacial till deposits that are predominantly fine-grained soils; silt and clay. They are called "cohesive soils." Cohesive bluffs may contain numerous layers of different soils including sand layers and sand lenses. Some cohesive bluffs have sand dunes perched on top of them. Bluff heights range from 20 to more than 130 feet (six to more than 40 meters) along the shores of the Great Lakes.
The erosion of bluffs can be quite unpredictable. A bluff may not have eroded significantly, yet may lose five to 50 feet of bluff top in one failure event. Mechanisms that trigger landslides on bluff slopes include:
- intense rainfall
and major snowmelt
- wave or current erosion of the base of the bluff
- lakebed erosion
- rapid drop in water level (for submerged slopes)
- earthquake or other land movement