COARSE-PARTICLE TRANSPORT IN A GRAVEL-BED RIVER

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COARSE-PARTICLE TRANSPORT IN A GRAVEL-BED RIVER

Movement of bed material in the Toklat River,Alaska,was monitored during 1988 and 1989 by measuring transport rates with a Helley-Smith bedload sampler and by tracking and locating coarse sediment using radio transmitters implanted in natural sediment particles.Median bedload size was about 8 millimetres and transport rates ranged from less than 10 to nearly 3000 megagrams per day.Transport rate related to about the 1.6 power of water discharge in excess of discharge required to initiate tractive sediment motion.As transport rate increased,mean and maximum sizes of bedload tended to increase.Radio-tagged particles,mostly about 90 millimetres in diameter,moved distances between about 500 and 2,000 metres during the 6-to 8-week periods of high flow.In the braided Toklat River,most particle movement occurred during the first few days of expoSure to flow and before particles tended to get abandoned on the flood plain or buried in the channel.Mean particle speed during the approximate periods of principal motion was about 50 metres per day or about 0.05 percent of water speed. Limited data suggest that for moving rocks,large particles are likely to move as far as small particles(and at about the same speed).In contrast to the Toklat River,data just collected from a single-thread channel in which abandonment and/or burial was less likely showed that mean particle speed was greater than 5,000 metres per day,t least for the first few hours after initial rock placement.In this stream, mean particle speed was about 3.3 percent of water speed.Motion-sensor data indicated that rocks were moving about one-seventh of the time;accordingly,when rocks were moving, they were moving at about 20 to 25 percent of the water speed.