A LIDAR-BASED LANDSLIDE INVENTORY OF WASHINGTON COUNTY, CENTRAL VERMONT

The Vermont Geological Survey (VGS) completed an inventory of landslides (LS), gullies, and related features in Washington County using lidar topographic data, high-resolution orthophotos, bedrock outcrop locations, and field visits to selected sites. Pre-existing bedrock outcrop locations were incorporated in the analysis since slope failures in surficial materials are more likely in areas where bedrock is sparse. Field-based stream geomorphic assessment data on mass failures plus bedrock grade control from the Vermont Rivers Program were also used.

The data is summarized on a 1:100,000 scale map and in a GIS database. Most LS are located close to streams at sites of active streambank toe erosion. Available long-term data indicates that these LS are generally at sites that have been failing for a long time. Where a river meander is migrating downstream the adjacent LS show a similar pattern of downstream migration. Many of the active gullies are receiving stormwater runoff from developed areas, suggesting that development has led to or at least worsened the instability.

Several high-elevation LS have been identified on the upper slopes of the Green Mountains in the western part of the county. These consist of source areas dominated by rock falls from steep bedrock slopes and transport and deposition zones dominated by rock slides and debris slides. The most recent of these occurred in August, 2017. At least one probable debris flow site has been active in historic time (Slide Brook in Fayston, 1897). Talus deposits have been identified at the bases of large cliffs, but there is little sign of activity at present.

The principal causes of slope failures appear to be over-steepening of slopes due to fluvial erosion of banks and stream beds during flash floods and decreases in shear strength of soils due to increases in soil water pore pressures due to heavy rainfall.

Lack of signal return from areas of heavy conifer coverage remains a problem with some of the lidar data. This study shows that despite such difficulties it is feasible to make accurate maps of existing landslides over extensive areas in an accelerated time frame, meeting community needs for hazard planning. New data on landslide locations can be added via the VGS Landslides Inventory Geoform at http://dec.vermont.gov/geological-survey/hazards.