ASBPA Coastal Voice: February 2013

by Charles W. Shabica, Ph.D., Great Lakes Chapter president

A universal phenomenon in the Great Lakes is cyclical lake levels. Since the late 1990s, water levels have been in a downward cycle and reached near record lows in early 2013. If low lake levels continue, shipping and recreational boating will suffer as harbor and channel dredging costs increase substantially. Additionally, accelerated lakebed erosion on many urban Great Lakes shores will result in deeper water nearshore, with consequences similar to rising sea level.

Anxiety about global warming has led some to hypothesize that we are in a permanent downtrend in lake levels. Evaporation and precipitation in the Great Lakes Basin are the primary controls of lake levels and winter ice-cover plays a key role in reducing evaporation. According to Jia Wang (2012), “There was a significant downward trend in ice coverage from 1973 to the present for all of the lakes, with Lake Ontario having the largest, and Lakes Erie and St. Clair having the smallest. The total loss for overall Great Lakes ice coverage is 71%.”

While it might be appealing to some to see this as evidence for global warming, many scientists view this phenomenon as a function of natural meteorological variations. Wang (2012) explains “…lake ice mainly responds to the combined Arctic Oscillation and El Niño–Southern Oscillation patterns.” Geologic history of the Great Lakes shows lake level cycles to be unpredictable over the short term but relatively stable within a narrow range over the last 2,000 years (Chrzastowski et al., 1991).

This year we may be witnessing the end of an 18-year period of low water. Heavy spring rains have resulted in a 20 inch rise in water levels in Lakes Huron and Michigan, and a fall region-wide cold snap with associated snow may continue the upward trend in lake levels. Regardless of the variations in lake levels, there is a message for coastal property and beach managers: Maintenance costs will rise. Many areas of the lower Great Lakes are currently suffering high levels of shore erosion, despite a long period of low lake levels.

In Lake Michigan we have learned that most cohesive sediment shores erode regardless of whether lake levels are high or low. According to Charles Shabica (2008): “For the last 2,000 years, Great Lakes water levels have varied within a narrow range, for example, 6.3 feet (1.9 m) in Lake Michigan, and are unpredictable and water-budget related. During lake highs, natural beaches are narrow and storm waves eroded the backshore dunes and bluffs. During periods of low water, beaches are wider and the locus of erosion is transferred to the shallow lakebed. The net result under natural conditions is a progressive recession of the shore with beaches in a dynamic equilibrium. Other than relatively stable rocky coasts located mainly in the northern Great Lakes, most Great Lakes shores, including those in Illinois, are cohesive glacial tills and sands with cobble and sandy beaches and erodible lakebeds.”

Over the last century, as urbanization progressed in the southern Great Lakes, shore protection projects proliferated. In some urban areas, extensive coastal armoring cut off the supply of sediments from bluff erosion, a primary source of new sand to the system. Today, the main sand supply on urban cohesive lakeshores is wave erosion of the nearshore glacial till lakebed that often contains as little as 10% sand (Shabica and Pranschke, 1994).

In Lake Michigan it is estimated that in coastal areas less than 10 feet (3 m) deep, where sand deposits are thin, the nearshore lakebed may erode at rates averaging 8 inches (0.2 m) per year (Nairn, 1997). Lakebed erosion, without concurrent shore recession, has resulted in deeper water nearshore and consequent larger waves impacting the coast. The result is rising costs of shore protection and beach nourishment.

Lakebed erosion in the Great Lakes has similar consequences to sea level rise, deeper water nearshore. But the Great Lakes message for most urban marine coastal managers is that even a several foot rise in sea level over the next century will not have the catastrophic consequences predicted by doomsayers. New construction and maintenance costs will go up as sea level rises and it would be unwise for coastal communities not to be prepared.

But we’ve learned from the Great Lakes experience and recent hurricanes that it is not a few inch rise in sea level that can cause catastrophic damage, but storm surges that can exceed 10 or 15 feet, and high storm waves and flooding. Hurricane Sandy would have been a disaster regardless of whether global sea level were a foot higher or lower.