Fan‐Surface Evidence for Debris‐Flow Avulsion Controls and Probabilities, Saline Valley, California. Issue 5 (7th May 2019)

• Record Type: Journal Article
• Authors: de Haas, T.; Densmore, A. L.; den Hond, T.; Cox, N. J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 5(2019)
• Page Start: 1118
• Page End: 1138
• Publication Date: 2019-05-07
• Subjects: debris flow -- alluvial fan -- avulsion -- Mojave Desert; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2018JF004815
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract : Debris‐flow fans form by shifts of the active channel, termed avulsions. Field and experimental evidence suggest that debris‐flow avulsions may be induced by depositional lobes that locally plug a channel or superelevation of the channel bed above the surrounding fan surface, by analogy to fluvial fans. To understand debris‐flow avulsion processes, we differentiate between these controls by quantifying the spatial distribution of debris‐flow lobe and channel dimensions, along with channel‐bed superelevation, on nine debris‐flow fans in Saline Valley, California, USA. Channel beds are generally superelevated by 2–5 channel depths above the fan surface, and locally by more than 7 channel depths, thereby substantially exceeding superelevation on fluvial fans. Depositional‐lobe thickness and channel depth decrease with distance from the fan apex, although both are highly variable across the fans. Median channel depths roughly correspond to the 50th–75th percentiles of lobe thicknesses, while minimum channel depths roughly correspond to the 10th–25th percentiles. In contrast, the thicknesses of lobes that have triggered avulsions roughly equal local channel depths and are on average twice as thick as the local median lobe thickness. The spatial correspondence between avulsion locations and thick lobe deposits, and the lack of correlation with channel‐bed superelevation, leads us to infer that avulsions on these fans are mostly caused by thick lobes forming channel plugs. Although results may vary with climatic and tectonic setting, our findings indicate that avulsion hazard assessment on populated fans should include mapping and monitoring of channel depths relative to typical deposit thicknesses on a given fan. Key Points: Debris‐flow avulsions are mainly caused by thick lobes plugging the main channel Channel‐bed superelevation can be large on debris‐flow fans and is a weak indicator for avulsion A spatial framework of debris‐flow lobe dimensions and channel depths can be used to predict avulsion locations and probabilities