LANDSLIDE AND ALLUVIAL FAN ACTIVITY DUE TO TROPICAL STORM IRENE: EXAMPLES FROM MONEY BROOK, BLACK RIVER WATERSHED, VERMONT

Heavy rains from Tropical Storm Irene (TSI) in August, 2011 reactivated landslides throughout the steep headwater streams of the central and southern Green Mountains in Vermont. Money Brook (3.1 km²) in the Town of Plymouth was the site of extensive mass wasting and sediment transport, with tens of thousands of cubic meters of coarse sediment (up to 2 m diameter) being transported eastward to an alluvial fan at the edge of the Black River valley. Along with this coarse sediment movement, a large quantity of fine sediment was transported downstream to the Black River and a chain of lakes including Amherst Lake, Echo Lake, Lake Rescue and Lake Pauline.

The watershed is forested and elevations range from 1,000 m on Salt Ash Mountain to 357 m at the Black River. The channel length is ~3 km with an average slope of 0.14 m/m. The predominant source for both the coarse and fine sediments is a landslide complex approximately 450 m upstream of Vt. Rt. 100 which has formed in dense, fine-sandy glacial till. The material contains abundant boulders, cobbles, and pebbles in a matrix that consists of 69% sand, 23% silt and 8% clay. Landslides have existed at this site since at least the 1930s. TSI initiated landslides over a greater surface area than was previously active. Based on a hydrological analysis, TSI flows were greater in magnitude than the floods of the 1970s or the 1930s, and similar in magnitude to the 1927 flood. Peak discharge from the watershed during TSI is estimated at 17.6 cubic meters per second.

Based on survey data from 1996 and 2012, ~71,100 m³ of material were eroded from the landslides and upper Money Brook channel during this time period. Nearby streamflow records suggest that TSI was the main erosion event during this time. About 56,600 m³ of material were deposited on the alluvial fan during TSI. An estimated 3,800 m³ of material are lodged within debris jams in the valley upstream of Rt. 100.

The predominant driver of landslide formation and production of sediment from the area of the landslides is erosion along the landslide toes during high-flow events. Between these high-flow events, sediment is delivered to the downstream channel by rilling/gullying along the open face of the slides and continued re-working of sediment stored in the channel. Freeze-thaw action and groundwater seepage are also contributing to erosion.