A hydrochemical study of urban landslides caused by heavy rain: Scarborough Bluffs, Ontario, Canada

Abstract

Scarborough Bluffs is a 15 km long stretch of the Lake Ontario shoreline east of downtown Toronto. This heavily urbanized area currently represents Canada's most serious erosion problem. The worst affected zone is 1.5 km long and lies along South Marine Drive where 50 m high bluffs are failing by shallow retrogressive failures of jointed glacial clays over underlying deltaic sands and clays. The erosion rate is about four times that for the coastline as a whole.Heavy rains in the Toronto area in August and September 1986 produced a spate of slope failures and mud flows. Particularly heavy storms on September 10 and 29 triggered extensive retrogressive slope failures at South Marine Drive. Hydrochemical investigations of discharge waters suggest that slope failure was caused by surface runoff on the bluff top recharging lower slope areas by infiltration through joints in the upper clay capping. Discharge of water from the lower slope is impeded by less permeable barriers in the deltaic stratigraphy at the site and by a cover of clayey slope debris. Data suggest that provision for adequate drainage of the bluff top, by interceptor drains, is a prerequisite for controlling slope behaviour in the area. Key words: slopes, erosion, groundwater, hydrochemical, recharge, drainage.