The effects of softening on nearshore erosion of a cohesive shoreline

A study designed to measure spatial and temporal variation in the shear str ength of the surface of a till substrate in the nearshore was carried out o n the south shore of Lake Ontario, Canada. The primary goal was to determin e whether softening, or reduction in shear strength, occurred during the st udy period, and the significance of this as a control on the overall rate o f vertical lowering of the cohesive profile. Characteristics of the nearsho re environment and the substrate were determined through the collection of till shear strength measurements, substrate cores, and surficial sediment t hickness along a 300 m long underwater profile. Changes in shear strength w ere monitored over a four-month period in experimental plots in water depth s of 1.6, 3.5 and 4.75 m, together with measurements of bed erosion. Softening of the till was evident from cores which showed a rapid increase in strength and corresponding decrease in moisture content to a depth of ab out 0.1 m below the till surface after which they became constant. Shear st rength of the exposed till surface decreased during periods of low wave act ivity, and similar results were found in laboratory experiments. Periods of high wave activity resulted in the removal of a layer of softened material , thus exposing harder underlying till. The results suggest that erosion du ring a storm is related to the thickness of the softened layer that develop s during non-storm periods. Because of the steep increase in strength with depth below the till surface, maximum erosion during a storm is generally < 1 cm. The spatial and temporal variation in softening of the till surface i s complicated by consolidation that appears to occur when the surface is pr otected from low wave action by the accumulation of surficial sand and grav el in the form of a nearshore bar. (C) 2000 Elsevier Science B.V. All right s reserved.