LANDSLIDES FROM TROPICAL STORM IRENE IN THE DEERFIELD WATERSHED, WESTERN MASSACHUSETTS

Four landslides (3 translational debris flows and 1 rotational slide) occurred along the Cold River within the Deerfield River watershed (1440 km²) in northwestern Massachusetts closing a six mile section of Route 2, a major east-west transportation corridor, for 3.5 months. These are among the largest landslides to occur in Massachusetts since 1901. Tropical storm Irene dropped 180-250+ mm of rain in a 12 to 15-hour period on the Deerfield watershed preceded by 130-180 mm of rain in the 1.5 weeks leading up to Irene. Soils were saturated, an unusual condition for the month of August, and probably contributed significantly to slope failure.

The three translational slides occurred at approximately 10 am on August 28, 2011, involved 765 m of slope at an average angle of 28-33°, covered an area of 1.2 ha and moved about 7645 m³of material. Bedrock sheeting joints oriented parallel to the slope (284°, 38-40° dip) provided the slip surface upon which the overlying 0.6-1.2 m of colluvium and glacial till slid.

The rotational slide occurred along an unarmored section of the Cold River. The slip surface was a 4-8 foot thick layer of laminated lake-bottom sediments overlain by 12-19 feet of stream terrace and debris flow/alluvial fan deposits transported by Trout Brook, a smaller tributary to the Cold River. This section of Route 2 has experienced chronic failures beginning with the storm of 1938. The cost to repair this six-mile section of Route 2 was $22.5 million.

Flooding within the Deerfield watershed was extreme with a record-breaking peak flow of 3100 m³/s (72 year record) where the Deerfield enters the Connecticut River. Approximately 1.6x10⁸ m³ of water was discharged through the Deerfield during the event indicating that ~112 mm of Irene’s rainfall was converted directly to runoff, a yield of between 45% and 62%. Clays and silts locked in storage in the glacial sediments within the watershed were mobilized resulting in record-breaking sediment loads 5-times greater than predicted from the pre-existing rating curve. Approximately 1.2 Mtonnes of sediment was discharged by the river during Irene. Where the Deerfield and Connecticut Rivers meet, the Deerfield watershed area is one tenth the size of the Connecticut River, yet the Deerfield produced as much as 40% of the total sediment observed on the lower Connecticut.