LANDSLIDES IN THE GREAT BROOK WATERSHED, WASHINGTON COUNTY, CENTRAL VERMONT

Great Brook (GB) is a 37.5 km² tributary of the Winooski River. GB is ~13 km long with an average slope of 3%. The area has had 13+ damaging floods since 1857 and has been the site of numerous landslides (LS) and road and bridge washouts. Two houses were destroyed by flooding (1984 and 1989) and one was recently removed due to an imminent slope failure. An LS study funded by the Central Vermont Regional Planning Commission identified 47 active LS, 3 inactive LS, 7 sites of streambank erosion, 15 gully sites (11 are landslide-gully complexes), and 2 alluvial fans. Most of the active LS are on steep slopes adjacent to GB. Most LS >1000 m² are found along the lower 4.5 km.

The principal causes of LS are over-steepening of slopes due to fluvial erosion during flash floods and decreases in soil shear strength due to increased soil pore pressure from heavy rain. Fluvial erosion in GB is dominated by channel planform migration and vertical incision (headcutting). Bedrock grade control may be the major factor determining the locations of LS. Slopes underlain by dense till and lacustrine diamict typically fail by detachment of irregular blocks. Loose materials typically are detached as single grains. The initial failures on wooded slopes are as translational slides, which commonly carry blocks of soil and trees, with depths of 0.5 - 1.5 m. Sliding blocks may break up into flows. In rare cases, a rotational slump may occur in place of or following a shallow translational slide. Rotational LS are more common in lacustrine, ice-contact, or stream terrace deposits than in till, but some have been observed in dense till deposits that were severely undercut by catastrophic flooding, such as the thunderstorm-related flash floods of May 2011 and those from Tropical Storm Irene in August, 2011.

Mass wasting processes slowly modify the fresh landslide surfaces. After the first winter the outer 1 - 2.5 cm of even the densest till begins to soften. Rills dissect both landslide faces and toe deposits. Although pioneer vegetation appears after ~1 yr, headcuts and planform migration continue to destabilize the banks.

Observations over the last 20 yrs show that although LS tend to recur at roughly the same locations, there are slow downstream shifts due to stream meander migration and rapid, catastrophic shifts due to large floods.