ANALYSIS OF ROCK FALL AND DEBRIS FLOW HAZARDS IN SMUGGLERS NOTCH, GREEN MOUNTAINS, NORTHERN VERMONT

Concern about the hazard posed by the fall of a large boulder from a cliff on the side of Smugglers Notch in Lamoille County led the Vermont Geological Survey to undertake this study for the Vt. Department of Forests, Parks and Recreation. A combination of historical research and geomorphological mapping based on field work and air photo interpretation was used to delineate the extent of the cliffs, prominent gullies or rock chutes that transect the cliffs, fallen boulders, talus slopes, debris flow cones and tracks, and alluvial fans. Most blocks detaching from the cliffs are delimited by three principal sets of joints (foliation-parallel, parallel to pegmatite-filled spaced cleavage, and a third set that is nearly orthogonal to the first two). Areas of high landslide hazard were identified and both the temporal distribution of events and the relationship between heavy rainfall and landslide events were documented.

Two types of slope failures occur in the study area: Rock falls and slides (RF) and debris flows (DF). Most of the boulders in the floor of the Notch are the result of RF. DF are slurries of water, mud, pebbles, cobbles, and boulders that flow within shifting channels on the slopes below the cliffs. In two locations the DF have been extensive enough to build out partial cones of material. The DF are activated by heavy rainstorms. Of 21 landslide events, all occurred between May and December, with 19 occurring between May and October. Peak monthly occurrence is in July. Landslides have been occurring in the Notch for as long as records have been kept. Future RF can be expected to involve individual blocks in excess of 500 or 1000 tons. The individual DF can be expected to range from a few cubic meters of mud, pebbles, cobbles, and boulders up to many thousands of cubic meters. They can be expected to sweep down to and across Vt. Rt. 108.

A draft slope instability hazard map shows a relative ranking of hazards due to RF and DF. This study indicates that DF cannot be expected to continue in their exact present locations as the paths are liable to shift over time as blockages occur. The history of landslides here shows a strong correlation between precipitation events and landslides. This could be used to predict future landslides and to mitigate their hazards.