TOE EROSION OF GLACIAL TILL BLUFFS - LAKE ERIE SOUTH SHORE

Toe erosion and rates of recession of the toe were measured at four si tes along a 3.5 km long stretch of shoreline on the south shore of Lak e Erie from April to December 1986. The shoreline consists of bluffs r anging from 5 to 12 m in height and developed in overconsolidated till . Toe erosion was measured at peg lines consisting of pins driven hori zontally into the face of the bluff at 0.25 m intervals to a height of 1.75 m above the beach, and steel rods driven vertically into the bea ch with a spacing of 1.5 m. At each site three lines were established and monitored at 1-2 week intervals. Because of record high lake level s, beaches in front of the bluffs were generally <5 m wide, and some e rosion was measured on all but three occasions. Recession of the toe d uring any measurement period was generally 2-6 cm, with the maximum re corded being about 12 cm. Wave action during high-magnitude storms res ulted in erosion occurring much higher up the bluff face than for low- magnitude events, but the actual recession of the toe was not substant ially greater. A multiple regression model shows that there is a signi ficant relationship between toe recession and several variables that i ndirectly control wave energy at the bluff toe. However, factors such as beach width and thickness of beach sediment did not have a signific ant influence on recession rates in this study, likely because the hig h lake levels resulted in very narrow beaches at all four sites throug hout the study period.