Gazette Article by: Charles W. Shabica, Ph.D.
Appeared in the Gazette: Fall 1999
Living on the Illinois shore of Lake Michigan in a community like Winnetka has great advantages tempered by a few – but important – problems. Since the mid-to-late 19th century, scientists and engineers have faced the challenge of balancing the need for safe access to the lake against damaging this powerful and evolving system. With 150 years of data, we have learned through study and trial and error what works and what doesn’t.
We now know that unlike the Cape Hatteras coast of North Carolina or the South shore of Martha’s Vineyard, Illinois lakeshore erosion can be stopped. Great Lakes storm waves are less damaging ocean waves and, more importantly, while sea level is rising, lake levels have varied little over the past 2,000 years. Many wooden seawalls built on the North Shore in the late 19th century, later rebuilt in new materials like concrete and steel, continue to occupy the same locations. No homes in Illinois have succumbed to lake storms.
The first inhabitants of the North Shore found steep eroding clay bluffs fronted by narrow beaches nourished by sand and gravel washed out of the bluffs by storm waves. By the 1800s stone-filled wooden piers and seawalls were built up and down the North Shore for storm wave protection. The piers trapped snad, functioning as shore-perpendicular groins. The rule of thumb was: no pier, no beach; and the longer the pier, the wider the beach. The bluffs were saved at the cost of losing the natural beaches.
Protecting the bluffs from erosion has created a new problem. In an already sediment-starved system, the meager supply of sand from bluff erosion has been cut off. By stabilizing the shoreline, we have caused the site of erosion to shift from the bluffs to the nearshore lakebed. With the main source of sand cut off, all North Shore beaches have thinned, and lakebed erosion has accelerated, causing groins and seawalls to fail prematurely as nearshore lake depths increase. Even a large beach like Tower Road Beach is irreversibly losing sand to the recent lakebed downcutting (Shabica & Pranschke, 1994).
Coastal research by scientists and engineers has demonstrated that the rate of lakebed erosion, while very active in the surf zone, decreases to nearly zero in depths of about ten feet. The bad news – that water depths next to seawalls or stone revetments will be approximately 10 feet within 30 years – is tempered by the good news that once nearshore water is 10 feet deep, the system will achieve long-term stability. This explains why the entire Chicago lakefront parkland has lasted so long; it was built in water 10 or more feet deep.
Chicago is protected by lakefill or narrow barrier islands called breakwaters. First proposed by Frederick Law Olmstead in the 1909 Plan of Chicago, breakwaters are a long-term shore protection solution that protect beaches nearshore, and if built offshore as wider islands can provide new parkland, wildlife refuge, safe beaches, passive recreational and fishing areas, and fish habitat. Northerly Island, now a peninsula used by Meigs Field, was the only one of Olmstead’s islands to be constructed. Mayor Daley proposes to complete it as Olmstead envisioned.
On the North Shore, breakwaters are replacing groins as the method of choice for protecting bluffs and beaches. Five years of state and federally mandated monitoring have confirmed their success. They are not inexpensive. It costs approximately $250,000 to protect 200 feet of lakefront with a breakwater and 5,000 tons of new sand. Thirty years from now, the nearshore water will be deeper and the construction cost substantially higher. At that time, offshore barrier islands may become the best solution to this regional problem. How to pay for them is another story.