Side-Scan Sonar Investigation of Breakwaters at Calumet and Burns Harbors in Southern Lake Michigan

Side-scan sonar surveys revealed the following features: 1. Many of the wood cribs in the Calumet Harbor breakwater have tilted or have been displaced, and reflections from behind the breakwater indicate gaps and voids. Fan-shaped deposits of coarse rubble on the harbor floor may be debris from the damaged cribs. 2. Both breakwaters are obviously exposed to a dynamic environment, as shown by fields of sand waves and ripples on the lakeward sides. 3. Dramatic sand waves occur in a gap between sections of the Calumet Harbor breakwater and may indicate the movement of a veneer of sand over an essentially clay harbor floor. 4. The Burns Harbor breakwater is purported to have been built on a thick sand mat, yet our data indicates that a featureless clay floor extends to the very base of the structure.