HIGH-RESOLUTION MONITORING OF SUSPENDED-SEDIMENT CONCENTRATION AND GRAIN SIZE IN THE COLORADO RIVER IN GRAND CANYON USING A LASER-ACOUSTIC SYSTEM

摘要

To monitor sediment transport in the Colorado River in Grand Canyon, Arizona, USA, we have designed and are evaluating a laser-acoustic system for measuring the concentration and grain size of suspended sediment every 15 min. This system consists of (1) a subaqueously deployed laser-diffraction instrument (either a LISST 100 or a LISST 25X) connected to an automatic pump sampler, and (2) an EZQ acoustic-doppler current meter. When laser transmission drops below a user-defined threshold (as a result of increased suspended-sediment concentrations), the LISST triggers the automatic pump sampler to collect samples at a user-defined rate. This allows samples to be collected when the suspended-sediment concentrations exceed the upper limit for the LISST and the EZQ acoustic-Doppler current meter (around 2,000 mg·l~(-1) -3,000 mg·l~(-1). Beginning in August 2002, we began testing this system on the Colorado River in Grand Canyon, and have developed stable coefficients relating the pump, laser-diffraction, and acoustic-backscatter measurements to cross-sectionally integrated measurements of suspended-sediment concentration and grain size. Variability between either sequential laser-diffraction or acoustic-backscatter measurements is substantially less than the variability between sequential cross-sectionally integrated measurements of concentration and grain size (collected with standard U.S. Geological Survey samplers and methods). Furthermore, the variability between either the laser-diffraction or acoustic-backscatter point measurements and the cross-sectionally integrated measurements is typically less than the variability between paired cross-sectionally integrated measurements of concentration and grain size. These observations suggest that more error may be introduced during the computation of suspended-sediment loads based on conventional sampling methods than is introduced during the computation of suspended-sediment loads using the laser-acoustic system.