Abstract

The Lake Erie shoreline at Painesville-on-the-Lake has receded ∼180 m since 1876, yet during the past decade rates have varied locally by 0–5 m/yr. The study area is a portion (0.5 km in length) of a continuous section of coastal bluffs ∼17 m tall. The section was selected to minimize variation in physical parameters, including shoreline orientation and exposure, offshore bathymetry and sediment, beach width, minimal shoreline protection structures, and generally “homogeneous” bluff materials. In spite of these similarities, some portions of the bluff recede episodically by block-fall and other portions continuously by slumping.

The coastal bluffs consist of glacial diamicts and glacial-lacustrine sediment. These studies indicate that heterogeneities within the diamicts exert subtle controls over mass wasting types and rates. Slumping is facilitated by the local occurrence of impermeable silt-clay lenses in the diamictons which, when they daylight in the upper portions of the bluff, facilitate groundwater discharge and slumping. In contrast, block-fall is facilitated by undercutting at the base of the bluff and appears related to the local presence of silt-clay lenses or layers above the basal lodgement till, within the wave impact zone. These differences are not readily apparent, yet have significant effects. For example, portions of the bluff dominated by slumping receded relatively continuously, while sections dominated by block-fall suffer episodic loss of blocks up to 50 m long by 5–20 m wide.

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