ANALYSIS OF FREQUENT MASS MOVEMENT USING DENDROGEOMORPHOLOGY: ALASKA HIGHWAY, NORTHWAY JUNCTION, INTERIOR ALASKA

The more than 2,700-km-long Alaska Highway (AH) was built in less than one year (1942) through austere, wild environments, with inadequate design and maintenance. Aggravating this is the underlying permafrost that, with projected warming, will likely decrease the overall stability of portions of the roadway. In preparation for construction of a proposed natural-gas pipeline, a 4.8-hectare site was selected between Alaska Highway mileposts 1265 and 1267 near Northway Junction for dendrogeomorphic analysis. This section of the AH is affected by three large, retrogressive mass movements expanding headward from the bluffs of the Chisana River. We used a Swedish increment borer to sample 30 tilted Picea mariana (black spruce) trees in order to maximize the record of reaction wood growth as a result of tilting. After sample preparation, reaction wood events were characterized using microscopic analysis and recorded as modified skeleton plots of event-response phenomena, which resulted in a replication-summary plot. Of the 29 trees analyzed (56 cores), the chronology for these tilted specimens was 175 years (1840-2014). Visual and statistical analyses from 1900-2014 show that before 1989 (a period of 88 years), reaction wood accounted for only 5% of recorded tree growth; however, from 1989 onward (a period of 24 years), reaction wood accounted for 35% of recorded tree growth. Spatial evaluation of select reaction-wood years (1966, 1989, 1995, 2006 and 2011) reveals site-wide disturbance during each event. We propose that this section of the AH was stable from at least 1899 to 1987, with no recognized failure associated with the 1942 emplacement of the highway. In the late-growth or post-growth season of 1988-1989, a mass-movement event destabilized the site with continued, significant (re)mobilizations proposed in 1994, 2005, 2010 and, by extension, 2013. With climate impacting higher latitudes at greater rates, Alaskans can expect climate-induced effects on infrastructure; depending on the climate model and implemented adaptations, these could add up to 20% ($6.1 billion) to normal wear and tear costs from now until 2030 (Larsen et al., 2008).