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Rainfall intensity limitation and sediment supply independence of post-wildfire debris flows in the western U.S.
Santi, Paul M. MacAulay, Blaire
Santi, Paul M.
MacAulay, Blaire
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2019
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This work explores two hypotheses related to post-wildfire debris flows: first, that they are rainstorm-intensity limited and not rainstorm-volume limited, and second, that they are not sediment-supply limited. The first hypothesis suggests that it is common to generate more than enough water to account for the volume in the debris flow, but to actually produce a debris flow the water must be delivered in sufficiently large doses. This is demonstrated by a dataset of 44 debris flows from eight burned areas in California, Colorado, and Utah. Assuming that a debris flow is composed of 30% water and 70% solids, these events were generated during rainstorms that produced an average of 17 times as much water as necessary to develop a debris flow. Even when infiltration is accounted for, the rainstorms still generated an overabundance of water. Intensity-dependence is also shown by a number of cases where the exact timing of debris flows can be pinpointed and are contemporaneous with high intensity bursts of rainfall. The hypothesis is also supported by rainfall intensity-duration thresholds where high volume storms without high intensity bursts do not generate debris flows. The second hypothesis, that of sediment-supply independence, is supported by data indicating the dramatic increase in volume of flows that occur directly after wildfire, as opposed to flows in unburned terrain. Also, repeated flows within short time intervals are only possible with an abundance of channel sediment, dry ravel, and bank failure material that can be mobilized, and field observations confirm these sediment sources, even directly after a debris- flow event.
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