Vertical Exchange of Gravels Over a Flood Series

Sediment transfers through mountainous watersheds involve the vertical exchange of bedload and bed sediments during floods that vary in magnitude. Because the depth of sediment exchange increases with flow strength, the extent of vertical mixing caused by multiple floods should reflect the exchange depth of the largest ones. This study examines the cumulative result of vertical exchanges produced by a long series of floods.

Empirical observations come from Carnation Creek, a small gravel-bed river with large woody debris located on the west coast of Vancouver Island, British Columbia. Frequent floods and the relatively limited armor layer facilitate streambed activity and relatively high bedload transport rates, typically under partial sediment transport conditions. Over 2000 magnetically tagged stones, ranging in size from 16 to 180 mm, were deployed on the bed surface between 1989 and 1992. These tracers have been repeatedly recovered to quantify the vertical exchange of gravels over distances up to 2 km. Over 200 floods capable of moving bedload occurred during the study period, with five exceeding the estimated bankfull discharge.

Gravels are vertically mixed to a mean depth of about 20 cm after two flood seasons or less. The specific flood sequence determines how quickly this mean depth is achieved, with flood magnitude overriding total hydrologic forcing in determining exchange depths. Subsequent floods have little effect on the mean depth, which corresponds to 1.7 times the thickness of the armor layer. These results corroborate expectations of a partial sediment transport regime, where the exchange depth is limited to less than two times the thickness of the armor layer.