COARSE WOOD INHIBITS DEBRIS FLOW RUNOUT IN FORESTED SOUTHEAST ALASKA

Due to their long run-out, debris flows are a major geologic hazard and the dominant geomorphic process that shapes steep channels in mountainous environments. Understanding runout is therefore essential to minimize risk to communities in the near-term and interpret the pace and pattern of debris flow erosion and deposition over longer geomorphic time scales. Many debris flows, especially in the North American Cordillera, occur in forested landscapes where they mobilize large volumes of coarse woody debris (CWD) in addition to sediment, which likely influences runout behavior and affects the global carbon cycle. To isolate the effects of CWD on debris flow runout, we analyzed recent debris flows in SE Alaska, where vast swaths of the terrain are vegetated with mature temperate rainforests that have been minimally altered by humans. We first analyzed debris flows that occurred on 18 August 2015 in the Sitka region by measuring volumes, inundation areas, and runout lengths of 12 representative debris flows ranging in size from 80 to 25,000 cubic meters. Compared to a global compilation of debris flow data, the Sitka debris flows inundated channel cross sections that were 59% larger, and their deposits covered just 39% as much planimetric area. Similarly, maximum runout lengths averaged 35% of those predicted by the global data set for a given volume and elevation change. At all sites, abundant CWD was clustered at the deposit’s distal end and often interlocked with standing trees. We therefore interpret that CWD in the Sitka debris flows provided additional frictional resistance to flow, compared to debris flows elsewhere, thereby causing thicker, less extensive deposits and shorter runout lengths. To further elucidate the role of CWD in controlling runout, we compared runout characteristics of hundreds of historical landslides in the surrounding Tongass National Forest to spatial variations in forest composition. Debris flows that occurred entirely in old growth stands were significantly less mobile than those that occurred in harvested units, and debris flow mobility decreased significantly with average forest stand age. These results imply that large trees are important for constraining the export of sediment as well as organic carbon from steep mountain channels to lower elevation settings.