ABSTRACT: Nearshore erosion on a cohesive shoreline

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Abstract

A study to measure vertical erosion of till in the inner nearshore was carried out along a stretch of shoreline just west of Port Dalhousie, on the south shore of Lake Ontario, Canada. In addition to measuring the rate of vertical erosion, an attempt was made to determine the influence of thickness and mobility of the surficial sediment cover, and of softening of the till, on erosion. Bluffs at the site are 4-5 m in height and developed almost entirely in till. Beach width at the time of the study ranged from 0-3 m and the toe of the bluff was actively undercut during periods of high waves. Till outcrops in the nearshore close to the shoreline and again beyond 100 m, but considerable quantities of sand and gravel exist in the zone between, forming a distinct nearshore bar located 40-60 m offshore. Elevation, surficial sediment thickness and size, and characteristics of the till surface were determined along 7 lines spaced 20 m apart. Vertical erosion was measured with a micro erosion meter at stations spaced 10 m apart. A total of 16 stations was established along 3 lines to a maximum distance offshore of 120 m. Some erosion was measured in each time period, with cumulative erosion during the period May to October, 1992 being on the order of 3-4 cm. Erosion in any time period increased with increasing cumulative wave energy. Softening of the till surface was observed in many localities and appears to be an important factor that increases the erosion potential, particularly lakeward of the nearshore bar where the till is always exposed. Erosion appears to be reduced where there is protection by surficial sediments, but it is necessary to determine that the cover is maintained during high wave conditions. The annual rate of vertical lowering determined from these measurements is of the same order as predicted from longterm bluff recession rates, assuming maintenance of an equilibrium profile.

Bibliography of Canadian Geomorphology