WSP06885

• File Number: 8220.101.10.A
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-AMWG
• State: AZ
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/3/2004
• Author: Melis-Topping
• Title: AMWG Meeting Attachments-March 3-4 2004-Testing Laser Based Sensors for Continuous In Situ Monitoring of Suspended Sediment in the Colorado River Arizona
• Water Supply Pro - Doc Type: Report/Study

Testing laser-based sensors for continuous in situ monitoring of suspended sediment 5

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- DISCHARGE OF WATER

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Fig. 5 Comparison of total suspended-sediment concentrations (1.3-250 Jlm)
measured at the Grand Canyon gauge using LISST -25 and D-77 bag sampler during
the multi-week fall 2001 test. Discharge data are from the Grand Canyon gauge.

LISST-25 provided the most compelling results as to how well these optical
instruments perform during continuous deployments (Fig. 5), Even during September
2001, when the LISST-25 was technically out-of-range (laser transmission <20%),
these non-calibrated data generally tracked the 0-77 samples.
Although the non-calibrated test results from the LISST-25 compare well with
cross-section measurements for total concentration (once the volume to mass
conversion was made, Fig. 5), the LISST-25 provides no possibility for separating
measurements of sand from fmer particles (the sand split is made at 63 ~m). This
LISST -25 limitation presents a serious shortcoming for monitoring deployments where
sand transport is of primary concern, such as the Colorado River. During the 2002 field
tests, the manufacturer developed a LISST -25 firmware upgrade to segregate sand
from fmer-particle data and made it available for testing (L25X). The July 2002 sand
concentrations derived from the L25X compared very well with cableway sample data
(R2= 0.86).
During relatively brief periods when the river below the dam becomes greatly
enriched with fme sediment from tributary inputs, LISST overestimates the
concentration of fme particles (Fig. 5). Such errors occur owing to multiple scattering
associated with abundant fines, a phenomenon that tends to bias counts on the outer
rings of the detector array. Multiple scattering has been identified as a significant
source of concentration error when laser transmission falls below 20% (Agrawal &
Pottsmith, 2001). To monitor such periods, a method was devised that allowed the
LISST to electronically control a programmable automated pumping sampler. The
protocol is as follows: when LISST measures laser transmission below the 20%

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