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