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Sediment in forested and logged gullies, coastal British Columbia Millard, Thomas H.


This study examines sediment storage and transfers in gullies of coastal British Columbia, and how logging affects sediment storage and transfers. Both fluvial and debris flow transport of sediment occur in gullies, and the amount of fluvial transport of sediment which occurs will affect the magnitude of a subsequent debris flow. Coarse woody debris (CWD) may affect the storage and transfer of sediment in the gully channel, and logging can affect the supply and type of CWD. To determine whether logging affects storage and transfer of sediment in gullies, sediment budgets were constructed for gullies in four treatment classes: A. Logged, slash full, no recent debris flows : "slash-full (SF)." B. Logged, slash removed, no recent debris flows : "slash-clear (SC)." C. Logged, naturally scoured by debris flows : "torrented (T)." D. Unlogged, naturally loaded with CWD : "unlogged (U)." Each sediment budget had input to the channel, storage in the channel, and output from the channel estimated. Significant differences between treatment types occurred, summarized below. Treatment classes grouped together (in brackets) did not have significant differences. Budget term : Input, greatest→least, Torrented→Slash-full→(Unlogged and Slash-clear). Budget term : Storage, greatest→least, Torrented→Unlogged→Slash-clear. Budget term : Output, greatest→least, Slash-clear→(Torrented and Unlogged)→Slash-full. One objective of the study was to assess the effectiveness and feasibility of cleaning slash from the gully channels. To be effective, cleaning slash must either reduce the magnitude of a debris flow in a treated gully, or else reduce the likelihood of initiation of a debris flow in the treated gully. Removal of slash will reduce the volume of a subsequent debris flow by about 15 percent, simply from the reduction in the amount of CWD. Reduction in sediment stored in the treated channel may reduce the volume of a debris flow by a further 4 percent. There is no evidence that removing slash will decrease the likelihood of initiation of a debris flow.

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