NEARSHORE EROSION ON A COHESIVE SHORELINE

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 inf luence 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 u ndercut during periods of high waves. Till outcrops in the nearshore c lose to the shoreline and again beyond 100 m, but considerable quantit ies of sand and gravel exist in the zone between, forming a distinct n earshore bar located 40-60 m offshore. Elevation, surficial sediment t hickness and size, and characteristics of the till surface were determ ined along 7 lines spaced 20 m apart. Vertical erosion was measured wi th 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 cumulati ve erosion during the period May to October, 1992 being on the order o f 3-4 cm. Erosion in any time period increased with increasing cumulat ive wave energy. Softening of the till surface was observed in many lo calities and appears to be an important factor that increases the eros ion potential, particularly lakeward of the nearshore bar where the ti ll is always exposed. Erosion appears to be reduced where there is pro tection by surficial sediments, but it is necessary to determine that the cover is maintained during high wave conditions. The annual rate o f vertical lowering determined from these measurements is of the same order as predicted from long-term bluff recession rates, assuming main tenance of an equilibrium profile.