1.) Introduction

2.) Wave Action

3.) Lake Levels

4.) Surface Runoff

5.) Groundwater

6.) Freeze & Thaw

7.) Wind

1. Erosion Processes

Bluffs are a dynamic part of the landscape, continuously changing, and constantly moving toward a stable slope angle. For bluffs composed of glacial till such as those in Wisconsin, the stable angle is approximately 22-degrees. Bluff erosion is caused by many factors, as illustrated by the diagram below:

Each erosion factor is depicted in the above diagram. Click the button below or continue scrolling to learn more about the variables that influence erosion.

2. Wave Action

The constant pounding of waves against the bluff toe is the primary cause of erosion, leading to bluff instability. Wave power is determined primarily by wind speed, duration and the open water distance (fetch) over which the wind is in contact with the water surface. As wind speed, duration and fetch increase, so does wave power. Storms increase wave activity, accelerating erosion.

Wave power is lessened as the waves break near the shore due to a shallow lakebed. The lakebed is maintained by a continuous resupply of eroded sediment. If the sediment supply is restricted due to shoreline protection, the lakebed may erode, allowing waves to hit the shore with greater energy.

3. Lake Level Change

Lake Michigan water levels vary due to climatic conditions that affect the entire drainage basin. Levels can vary from season to season, over several years or over tens years or more.

The human influence on Lake Michigan water levels is limited to a few inches. Natural cycles, seasonal and long-term, affect water levels by several feet.

As mentioned above, waves contacting the shoreline are the primary cause of erosion. High lake levels allow waves to reach sections of beach or bluff they would otherwise not under lower lake levels.

4. Surface Runoff

Water flowing over the ground during intense storms causes soil erosion. The duration, intensity and amount of rainfall are important factors, but other factors such as surface type greatly impact the rate of erosion.

Rill erosion on Lake Michigan bluffs.

Vegetation shields the soil from the direct impact of rain drops and helps holds the soil in place with its root structure. Hard surfaces such as concrete and pavement concentrate flow. If flow is concentrated into a small area, it can break apart and move larger amounts of soil.

5. Groundwater Flow

The hidden activity of groundwater can be more dangerous than the visible effects of surface water. Groundwater can trigger large, deep landslides or slumps. Water in soil beneath a slope weakens the bluff by adding weight and reducing frictional forces that hold the soil particles together.

Natural causes such as rain or human influences like redirecting the flow of surface water. Slump problems develop primarily where there are zones of water saturation above the main water table. Off-site groundwater sources, septic systems and dry wells also contribute to groundwater flow and/or retention.

6. Freeze & Thaw

In Wisconsin, seasonal changes can be particularly destabilizing. Winter freezes water in soil pores, causing the ground to expand. When the soil thaws it becomes saturated from the water within the bluff and also from snow melting on the surface.

Results of freezing and thawing on a Lake Michigan bluff.

The winter freeze and spring thaw work together, breaking apart the soil and removing the frictional properties holding it together. A bluff is least stable during heavy precipitation or rapid thawing of snow cover. The weight of the saturated soil and the changes in composition prime the bluff for landslides or slumping.

7. Wind

Wind→

Wind impacts the shoreline in three ways: as the main cause of waves, by moving loose sediment on the bluff, by shoving ice against the shore.

As mentioned above, wind blowing across the water surface generates waves. Wave height and frequency increase as wind speed and distance increase.

Wind has a larger impact on sandy shorelines where it can move large amounts of sediment toward or away from the shore. On bluffs, wind moves sediment that has been weathered by freeze-thaw.

Winter ice that forms along the shoreline can be moved directly by the wind or indirectly due to wave action. Ice can scour the lakebed, causing downcut, or can collect sediment within the ice. This removes the natural protection of a shallow lakebed, allowing waves to impact the shore with greater energy.