We used lidar differencing and field observations to map volumes, and interpret the origins of, sediment mobilized from mountain canyons by large post‐wildfire debris flows near Montecito, CA, USA in 2018. The debris flows progressively entrained and partially redeposited 550,000 m3of previously stored channel sediments throughout the canyon networks. The observations that scour depths and volumes were highest where the largest volumes of bouldery colluvium and debris‐flow deposits had accumulated, and that scour persisted beyond the mountain front, indicates that debris‐flow volumes in this extreme event were ultimately controlled by the coarse sediment reservoir available for scour. Because the volumes of available stored sediment result from the stochastic interaction of colluvial mass wasting, the magnitude and frequency of previous debris flows, and the accommodation space provided by valley morphology, the study reinforces the importance of estimating stored sediment volumes when developing debris‐flow hazard assessments.
Due to their potentially long runout, debris flows are a major hazard and an important geomorphic process in mountainous environments. Understanding runout is therefore essential to minimize risk in the near‐term and interpret the pace and pattern of debris flow erosion and deposition over geomorphic timescales. Many debris flows occur in forested landscapes where they mobilize large volumes of large woody debris (LWD) in addition to sediment, but few studies have quantitatively documented the effects of LWD on runout. Here, we analyze recent and historic debris flows in southeast Alaska, a mountainous, forested system with minimal human alteration. Sixteen debris flows near Sitka triggered on August 18, 2015 or more recently had volumes of 80 to 25 000 m3and limited mobility compared to a global compilation of similarly‐sized debris flows. Their deposits inundated 31% of the planimetric area, and their runout lengths were 48% of that predicted by the global dataset. Depositional slopes were 6°–26°, and mobility index, defined as the ratio of horizontal runout to vertical elevation change, ranged from 1.2 to 3, further indicating low mobility. In the broader southeast Alaskan region consisting of Chichagof and Baranof Islands, remote sensing‐based analysis of 1061 historic debris flows showed that mobility index decreased from 2.3–2.5 to 1.4–1.8 as average forest age increased from 0 to 416 years. We therefore interpret that the presence of LWD within a debris flow and standing trees, stumps, and logs in the deposition zone inhibit runout, primarily through granular phenomena such as jamming due to force chains. Calibration of debris flow runout models should therefore incorporate the ecologic as well as geologic setting, and feedbacks between debris flows and vegetation likely control the transport of sediment and organic material through steep, forested catchments over geomorphic time. © 2020 John Wiley & Sons, Ltd.
more » « less- NSF-PAR ID:
- 10457089
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 45
- Issue:
- 7
- ISSN:
- 0197-9337
- Page Range / eLocation ID:
- p. 1555-1568
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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