Processes Contributing to Bluff Erosion
There are many factors that contribute to shoreline erosion. The following is a short discussion on some of the major factors that affect erosion on the Lake Superior shoreline.
Water waves are one of the most significant factors leading to erosion. The energy that waves transport results in reshaping shorelines. Waves carry material away from the base of the bluff, and then remove material from the base of the bluff, creating undercutting. When the undercutting reaches a certain extent, the bluff will fail and the process will continue until the waves no longer reach the bluff.
Often water currents add energy to the nearshore system and to waves, resulting in increased erosion and sediment transport (Pincus, 1962).
Wind creates the energy needed for water waves. The increasing distance of water that the wind covers before meeting the shoreline (fetch) positively affects the energy created and the wave impact.
Runoff of Surface Water
Water running over the surface of the bluff can carry sediment and increase erosion (Chieruzzi and Baker, 1958 in Pincus, 1962).
Groundwater flow exiting the bluff will increase the instability of the bluff by increasing the pore water between grains in the bluff, and thus increasing the impact of gravity (Pincus, 1962).
Freeze / Thaw
During the spring, the pore water within the bluff material will freeze and thaw according to temperature. This process breaks down the cohesion of the material, and adds to downward movement of material (Chieruzzi and Baker, 1958 in Pincus, 1962).
Gouging of Shore Material by Lake Ice
Lake ice pushed by wind or wave action into the shoreline can cause major, irreversible damage to bluffs and reversible changes to beaches (Pincus, 1962).
Composition of Shoreline Material
The effect of all of the above factors will be dependent on type and strength of shoreline material (Pincus, 1962).
Vegetation on Shoreline
Either natural or planted vegetation can slow erosion due to the root network trapping sediment, and transpiration removing water from the soil.
The level of the lake can determine the effect and power of waves upon the shoreline.
Shoreline Protection Structures
Shoreline protection structures can decrease wave power and waves' effect upon the shoreline. However, caution should be used because shoreline protection structures can cause erosion / accretion far from the intended area.
Bluff erosion along the Lake Superior shoreline is caused by the combination of the above factors, causing the slope to over-steepen and fail. There are many types of failures that result in erosion, but in this area there are three main failure types that have been observed including translational slides, rotational slides, and creep.
Translational slides, rotational slides, and creep occur when a slope becomes steep enough that the force of gravity becomes greater than the strength of bluff material. In natural stable slopes, these forces reach a balance between slope angle and material strength, and the slope is stable. In our climate, these slopes are completely vegetated.
Slopes become unstable and begin to fail when downward forces in the bluff exceed the strength of sediment. This can happen when weight is added to the bluff top, water is added, or the slope is undercut or steepened. On Great Lakes shorelines, the later cause is generally what initiates instability, but individual failures are often triggered by rainfall or snow melt events.
In translational slides, soil moves down a straight surface and generally slides beyond the base of the slope to the beach. This type of slide is shallower than a rotational slide, and it is common to have a thickness to length ratio of less than 0.1 (Cruden and Varnes, 1996). Common thicknesses are about 1-2 feet (0.5 meters).
Rotational slides occur when material moves down a concave (vs. straight) slope. Often the top surface of the slumped material (the original ground surface) is tilted back towards the bluff. This deeper slide generally has a thickness to length ratio of 0.15 to 0.33 (Cruden and Varnes, 1996).
Creep is very slow moving and refers to movement of grains down a slope. While it can cause significant sediment movement, creep occurs at much smaller rates over longer time periods than slides, so that it is virtually unnoticeable.
Failures in Bayfield County
Along this shoreline, the failures are generally translational slides with a few rotational slides. Slow-moving creep is also most likely present in the entire area, but has the most noticeable effect on nearly stable slopes where translational slides and slumps do not occur.
The composition of the southeastern Lake Superior bluff was studied in detail during the summer of 1979. During this study, three till (glacial deposit) units, deposited by a minimum of three glacial advances and retreats, were identified and named (Need, et al., 1980).
Jardine Creek Till is the oldest of the three tills. It is a dark reddish-brown, sandy clay till. It can be distinguished from the other two tills by its more sandy appearance and the presence of a few pebbles. The next youngest material is the Hanson Creek Till. It is reddish brown with very little sand and no pebbles. In places, Hanson Creek Till incorporates thin veins of light gray clay. The youngest unit in the area is called Douglas Till. It is reddish brown with little sand and no pebbles. The upper two tills are very similar in appearance. The main visual difference between Douglas and Hanson Creek tills is that Hanson Creek till is composed of less sand, and often has gray clay veins present.
In addition to the till units, the bluff is composed in places of unconsolidated sand and silts, and sandstone bedrock.
Chieruzzi, R, and Baker, R.F., A study of Lake Erie bluff recession. Ohio State University Eng. Expt., Sta., XXVII, 6, 100 pp.
Cruden, D, and Varnes, D. 1996. "Landslide Types and Processes" in Landslides: Investigation and Mitigation. ed. Turner, A. and Schuster, R. Transportation Research Board, National Research Council; 247. p. 36-75.
Need, E. et al. 1980. Shoreline Erosion and Bluff Stability along Lake Michigan and Lake Superior Shorelines of Wisconsin, Appendix 9. Wisconsin Coastal Management, 272 p.
Pincus, H. J., 1962, Recession of Great Lakes shorelines, Great Lakes Basin--A symposium, Chicago, 1959.: AAAS Publication: Washington, DC, United States, American Association for the Advancement of Science, p. 123-137.