COMPLEX EARTH SLIDE-FLOWS IN GLACIOLACUSTRINE DEPOSITS OVER THE PAST 100 YEARS IN JEFFERSONVILLE, VERMONT

At least six landslide events occurred on the west side of a 46-meter-high terrace along the east bank of the Brewster River adjacent to the Village of Jeffersonville, VT. Slides occurred prior to 1922, one in 1954, three in 1999, and one in 2006. Toe deposits from the 1954 and 1999 slides crossed the river and reached the edge of the village. Future slides south of the 1999 slide would threaten several shops, residences, and school property.

The terrace is capped by several meters of early Holocene fluvial sand and gravel. Below, are late Pleistocene glaciolacustrine deposits consisting of an upper unit of thick sand to clayey silt varves, and a lower unit of thin silt, silty clay, and clay varves. These are underlain by dense sand-silt till.

Small events are characterized by landslide-gully complexes located north and south of the 1999 slides. They are responsible for ridge cutback and loss of land area. Larger events are complex earth slide-flows. Although there are shallow rotational components to these larger slope failures, much of the movement can be explained by translational sliding of earth blocks and earth flows.

The 1999 slides had a runout length of 233 meters (crown to toe), extending ≈110 meters across of the river, and travel angle of 11.2°; original slope angle was 34°. The causes of failure may include toe erosion by the river and long-delayed increases in pore-pressure due to rain and snowmelt. Piezometers set in the lower silt-clay unit show a slow response to precipitation and inflow from a losing stream to the east.

The present pattern of activity, and downstream migration of a river bend, suggest future failures are most likely to occur north of the 1999 slide. This location puts two houses at the top at risk and threatens houses and businesses in the village below. The short-term potential for toe erosion has been reduced by armoring the toe of the slope with rip-rap. Monitoring water levels and slope stability continue.