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ABSTRACT

The Great Lakes coast contains numerous unstable bluffs underlain by heterogeneous glacial materials consisting of till, sand, and gravel layers, and lacustrine clays. Many of the bluffs are steeper than their equilibrium angles and typically move as slow earth slides or occasional rapid slumps. Such movements develop largely where interlayered sand and clay contain perched groundwater that acts to reduce effective stress during winter months when perched potentiometric surface elevations rise because water cannot discharge through frozen soil. Aerial photograph records dating back to 1938 show that bluffs recede in amphitheater-like depressions followed by "catch up" where headlands between amphitheaters are attacked by other forms of erosion. This bluff recession is particularly pronounced during stages of high lake levels.

The erosion control experiment described herein has been designed to determine the manner in which groundwater activity influences the causes and mechanisms of mass wasting on the Great Lakes coasts. Three dewatering demonstration sites were selected, monitored electronically for virtually all movement and cause relationships, and dewatered to demonstrate a potential mitigation strategy other than construction of wave barriers.

Erosion activity and dewatering effects were carefully monitored for three seasonal cycles. Results show that (1) dewatering greatly reduces ground displacements during winter months, and (2) bluff movements are almost perfectly timed to, or lag slightly after, the hours when potentiometric surfaces near the bluff face reach their highest elevations during freezing (greatest soil pore pressure) or their greatest rates of surficial discharge (soon after thaw).

This field guide project was supported by grants from the U.S. Army Research Office, Terrestrial Sciences Program (Grant 3467-GS) from 1996 to 1999 and the U.S. Army Engineer Research and Development Center (ERDC) from 2000 to 2007, and 2012, through U.S. Senate Bill 227 (National Shoreline Erosion Control Development and Demonstration Program), with support from Western Michigan University (WMU). Additional personnel involved were Alan E. Kehew, Co-PIand, WMU graduate students William Montgomery, Rennie Kaunda, Mark Worrall, Gregory Young, William Bush, and Amanda Brotz. Well and monitoring instrument positions were chosen by R. Chase and designed by Ronald L. Erickson and James P. Selegean, U.S. Army Engineer District, Detroit, Michigan. Well constructions and instrument installations were done by STS Consultants, Chicago, Illinois. This huge project was very smoothly administered by M. Eileen Glynn and William R. Curtis, ERDC, Vicksburg, Mississippi.

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