The Significance of Lakebed Erosion and How to Detect It
What is Lakebed Erosion?
Lakebed erosion is the abrasion of exposed, erodible near shore lakebeds containing clay, by the motion of small amounts of sand and gravel over the lakebed. This motion is caused by waves and currents. Over the centuries, glacial tills submerged beneath the water, soften and become more susceptible to erosion. Lakebed erosion seems to be a continuous process, even though major storms probably cause a lot of the erosion. Even during low water level years and times of small wave conditions, the abrasion goes on.
Significant lakebed erosion occurs from the shoreline out to water depths of roughly 34 feet (10 meters). Lakebeds are probably protected from erosion where they have layers of cobbles (baseball size) or boulders (bowling ball size and bigger). Stationary sand deposits more than a few inches (five - 10 centimeters) thick probably protect otherwise erodible lakebeds from erosion.
Significance of Lakebed Erosion for Shore Property Owners
Lakebed erosion has been measured at a few Great Lakes locations as diverse as the wave-exposed coasts of Lakes Michigan and Ontario, and the sheltered, shallow waters of Maumee Bay on Lake Erie. Average erosion rates ranging from 0.5 to 6 inches (1 – 15 centimeters) per year have been measured.
A hypothetical example of significance is an average erosion rate of 2.6 inches (6.6 centimeters) per year at a lakeshore site on Lake Michigan, from October 1986 (the month of record high water level) to October 2001: 15 years. The lakebed in front of a shore protection structure on this property is 39 inches (one meter) lower in 2001 than in 1986.
Breaking wave heights in front of the structure are limited by water depth. Assume that the structure had a maximum of two feet (0.6 meters) water depth at its base, and a maximum
wave height of 1.2 feet (0.4 meters) breaking on it in 1986. If the same water level were present in October 2001, there would be a maximum water depth of 5.3 feet (1.6 meters) at the base of the structure and a maximum wave height of 3.2 feet (0.9 meters) breaking on it in October 2001. The maximum wave energy striking this structure in 2001 is seven times the maximum wave energy striking this structure in 1986.
Fortunately for the stability of the hypothetical structure, water levels in October 2001 were nearly five feet lower than in October 1986. A return of record high lake levels and storms might overwhelm such a structure and cause it to fail.
How to Detect Lakebed Erosion
Circumstantial evidence for the existence of lakebed erosion includes the following features:
- The beach width is virtually non-existant, or narrow over a broad range of lake levels.
- The lakebed is glacial till or clay with or without thin sand deposits less than a few inches (less than five to 10 centimeters) thick.
- The lakebed has a steeper slope nearest the shoreline than slopes further from shore.
The lakebed has a soft, watery clay texture, easily penetrated with a small diameter probing rod that has a tell tale coating of clay when withdrawn from the lakebed. A rod may encounter some initial resistance from surficial gravel, but persistent probing may penetrate an inch to more than a foot (several centimeters to more than one third of a meter) into the softened till or clay.