GLEAM

Great Lakes Environmental Assessment and Mapping Project

Shoreline extensions

Breakwater near Mackinac Island
Breakwater near Mackinac Island. (Photo J. LeFevre)

Shoreline extensions have been constructed throughout the Great Lakes to protect shoreline features (harbors, marinas, lakefront properties) from erosion, but they are most common in areas of high population density.

Groins and breakwaters are two types of shoreline modifications that extend perpendicularly from the shoreline or are located offshore. Such structures intercept and/or divert the transport of sediments along the shore, potentially causing excess sediment accumulation in some areas and sediment starvation in others. These changes to sediment transport regimes can negatively impact nearshore biota.

  • Groins are shoreline modifications constructed at right angles to the shoreline in order to modify or control the movement of sediment. Groins are most often costructed of steel, rock, timber, or concrete and may be placed as single units, or as part of a groin field (multiple coordinated groins).
  • Breakwaters are protective structures usually built off-shore to protect beaches, bluffs or harbor areas from wave action. Some breakwaters are attached to shore by groins and are most often constructed as rubble mounds.

 

Mapping shoreline extensions as a Great Lakes stressor

We completed an inventory of shoreline structures along approximately 86% of the Great Lakes' 17,000 km shoreline using high resolution (< 1 meter) imagery (multiple sources, capture dates 2000-2010).1

 

Locations of Great Lakes shoreline extensions.

shoreline_extensions.jpg

To estimate stress, we summed the number of shoreline extensions in each water pixel adjacent to the shore. Some shoreline structures (e.g., groins) have been reported to affect downdrift nearshore bathymetry for a distance of four times the groin length for individual groins and six times the groin length for a groin field.2 Some structures in our dataset extended as far as 0.5 km into the lakes. We applied a 2-km buffer to limit the overall effect of shoreline extensions to a distance of 2 km from the shore.

Distribution of shoreline extensions as a stressor in the Laurentian Great Lakes (Inset: Western Lake Erie).

Web_07_ShorelineExtensions.jpg

Data Sources: 

1. The primary imagery source for U.S. shorelines was the 2008-2010 USDA NAIP imagery. The primary imagery source for Canadian shorelines was Google Earth supplemented with Bing Maps.
2. Meadows, G.A., L.A. Meadows, D.D. Carpenter, W.L. Wood, and B. Caufield. 1998. The Great Lakes grown performance experiement, phase II: effects of a groin field. Shore and Beach, 66(2):14-18.