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Why Do Bluffs Erode?
A steep bank
is not as stable as one with a more gradual slope. Picture a sandcastle on the
beach. When left alone, wind, rain, and waves will gradually wear down the
structure until it eventually becomes part of the smooth sloping beach again.
Like the walls of a sand-castle, all slopes will eventually reach a stable angle
— an angle with a more gradual slope that is not as likely to erode.
A Stable Angle
Eventually a bluff top recedes until it reaches a more gradual slope that is no longer subject to significant recession. This is the stable angle.
Stable angles vary from bluff to bluff depending on the type of material in the bluff, the groundwater level, and other conditions. A bank composed primarily of clay, for example, becomes stable when it reaches a 14 degree angle. A bank composed of bedrock, on the other hand, can stabilize at a much steeper angle.
Most slopes
that are undisturbed have reached that stable angle. However, on the Lake
Superior shoreline, waves undercut the base making it steeper and causing
significant erosion. Several other factors impact the erosion rate.
Storm water runoff
Groundwater seepage
Changing lake levels
Wind and other forces of nature
When the base of the bluff is undercut, soil or rock above is no longer stable,
and the slope begins to fail as shown in the figure above. This continual
inland movement of shoreline is known as recession. Failure can take
place gradually or suddenly.
Slides or slumps – These occur suddenly. Twenty to thirty feet of bluff top can collapse at once. Property owners may find that their land has literally changed overnight.
Creep – This slow, down-slope movement commonly occurs on clay. This change is not as obvious and property owners may find that they’ve lost several feet of shoreline without ever noticing it. Although this process is slow, it is difficult to stop and will eventually cause the bluff top to recede.
For more information contact:
Bayfield County Planning & Zoning Department 117 E. 5th Street P.O. Box 58 Washburn, WI 54891 715-373-6138