Frozen debris lobes (FDLs) are slow-moving landslides in permafrost, many of which are present along the Dalton Highway in the Brooks Range. While twenty-three FDLs are within a mile uphill of the highway, the closest, FDL-A at MP219, is less than 190 ft from the highway shoulder as of August 2013. In 2010, FDL-A was 65-ft high and 560-ft wide at the toe. Using some estimated values for cross-sectional shape, unit weight of the soil, and the historic rate of movement, in 2010 FDL-A was moving about 60 tons of debris towards the highway every day. Measurements indicate that FDL-A started moving faster in recent years, and is beginning to demonstrate characteristics of further instability. A few years ago, similar signs of instability were present in FDL-D, another frozen debris lobe three miles to the south. In essence, FDL-D “detached” from its catchment area and moved 380 ft in three years (between 2010 and 2013). Should FDL-A follow a similar trend, it could cover the distance to the highway in about a year, and be almost half way to the Trans Alaska Pipeline System (TAPS) in three years. While a Phase 1 study provided a preliminary characterization of the internal structure of FDL-A, additional study is needed to define further the movement mechanism/s of the FDL and to characterize better its internal makeup. This information is necessary to identify the best mitigation technique/s for implementation.
visit: fdlalaska.org for additional information.