"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Sensing Stream Bedload with a Linear Array of Reflective Sensors

By R.A. Schmidt

Abstract

This design idea uses optical sensors in place of the small chokes (coils) in the magnetic tracer system that provided useful bedload data at Squaw Creek, Montana, where about 70 % of the sediment is naturally magnetic (Bunte, 1996). Two rows of reflective sensors (like barcode sensors) produce two binary pulse-trains indicating presence or absence of light reflected from bedload elements (Fig. 1). On-line analysis of the pulse trains produces particle size estimates, and correlation of signals between the rows gives estimates of speed (Fig. 2).

figure 1
Figure 1.

figure 2
Figure 2.

Each sensor contains a light-emitting diode (LED) and a phototransistor, in package widths ranging from 6 mm down to 3 mm (surface-mount). Package width determines resolution of particle size. For example, an array with 5-mm resolution would contain 200 sensors in a 1-m section. Pulsing the sensor LEDís in sequence produces maximum light with minimum power. The rate of rereading a segment determines resolution of speed estimates. Pulse trains are stored in first-in, first-out (FIFO) memory modules on each array section, and transmitted to a streamside computer for processing and recording.

A major technical problem in this design is determining the optimum on-off threshold level in the sensor amplifier circuit. Other problems to be solved include shielding from ambient sunlight, and finding a window material that resists abrasion by sediment. If these obstacles can be overcome, calibration remains a challenge, since the array only samples particles within a few millimeters of the bed surface. The video system sketched in a separate abstract offers some hope in calibration. One other encouraging note: correlations between reflective sensors gave estimates of speed profiles in snow avalanches (Dent et al. In press).

References

Bunte, K., 1996. Analysis of the temporal variation of course bedload transport and its grain size distribution: Squaw Creek, Montana, USA. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, General Tech. Rep. RM-GTR-288, 123 pp.

Dent J., In press. Annals of Glaciology, May 1997 symposium.


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