High resolution measurement of bedload transport

A bedload movement detector of novel design was installed in a gravel-bed s tream as a component of the ongoing research on sediment transport dynamics in the Stuart-Takla Experimental Watersheds in north-central British Colum bia, Canada, The device is designed to collect information on the patterns and timing of bedload transport during a flood event. The device is based o n a passive magnetic sensor that produces signals of 10(-2) to 10(-5) V as its magnetic held is disturbed by passing clasts. A series of 82 sensors is housed in an aluminum beam placed across the stream, inserted such that it s surface is flush with the gravel bed. The device can be raised or lowered to compensate for bed aggradation and scour. A data acquisition system gat hers voltage signals from the sensors at rates of 30 to 100 Hz. This device is sensitive enough to record the movement of most volcanic, metamorphic, granitic and ultramafic clasts larger than a few millimetres. O'Ne-ell Creek watershed is a 68 km(2) tributary basin of the Middle River drainage system in the northern headwaters of the Fraser River. Bedload tra nsport generally occurs twice a year in the Stuart-Takla streams: once duri ng spring floods and again during salmon spawning activity. Bedload moved o nly during two days in 1998, at the peak of the relatively small nival food in May. Nevertheless, the device detected at least 3 x 10(6) passing clast s. A continuous record of bedload transport was obtained, showing. a pulsat ing pattern of activity seemingly independent of stage, lateral movement of the transport zone, and a sudden onset of bed movement with a tapered cess ation. We anticipate that more sophisticated calibration of the sensors and accele rated sampling rates will provide detailed information on the size and/or v elocity and magnetic permeability of particles moving over the device, and will contribute to a better understanding of bedload transport. Copyright ( C), 2000 John Wiley & Sons, Ltd.