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Testing laser-based sensors for continuous in situ monitoring of suspended sediment 3 (a) -CALIBRATED LlSST100B POINT MEASUREMENT o CROSS-SECTIONAllY INTEGRATED WITH 0-77 SAMPlEr (b) -DISCHARGE OF WATER ;;:: 140 m E. t5 120 " g100 z w u Z 80 S " ~ 60 00 " ~ 40 Z w ~ 00 20 " 00 20 500 140 400 120 100 300 80 ~o \ 0 ERROR? 200 o 60 o 100 40 o o 0:00 o 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 7-19-2001 7-20-2001 7-19-2001 7-20-2001 Fig. 2 (a) Comparison of sand concentrations and (b) silt & clay concentrations measured at the Grand Canyon gage using LlSST.IOOB and a D-77 bag sampler during the one-day July 2001 test. Discharge data are from the Grand Canyon gauge. variable, ranging from about 50 to 60 mg rl during the diurnal peak (Fig. 2(b)). This is likely the result of a high degree of turbulence at the test site that keeps the fmes well mixed and sampled more consistently, relative to sand-sized particles. A second field test was conducted from 22 September, 2001 to 8 February, 2002, to explore performance characteristics of both the LISST -I OOB and a LISST -25 during louger, continuous deployments required for long term monitoring. Both the LISST- 100B and the LISST -25 measure only the volumetric concentration and grain size of suspended particles. However, mass concentration can be estimated by the user once a suitable density conversion is gravimetrically determined. The LISST -25 tested has a size range similar to the LISST-IOOB, however, the LISST-25 provides only a sauter mean diameter (the ratio of particle surface area to volume) rather than a size distribu- tion, as provided by the LISST-IOOB. During autumn 2001, the LISST-IOOB was fitted with a path-reduction module (PRM) to expand the instrument's concentration range by almost a factor of four (optical path of 5 cm reduced to I cm). Although path reduction does allow for higher-concentration measurements (by reducing the sample volume and related number of particles that attenuate laser transmission), the optical accessory used for testing turned out to be flawed and altered the raw data in ways that were not trivial to resolve. The PRM's influence was most pronounced on scattering related to the sand-sized particles (inner rings of the detector). It is therefore essential that each PRM used with LISST be tested and carefully evaluated prior to field deployment. In contrast to the LISST-IOOB with the PRM, the LISST-25 (fixed optical path of 2.5 cm) measured higher concentrations of suspended sediment with no discer- nable complications over the 4-month long test. Despite complications introduced by the faulty PRM, the 10-18 January, 2002, concentration data obtained from the LISST-IOOB also compared well with cableway samples for sand (Fig. 3(a)) and silt & clay (Fig. 3(b)) collected with the D-77 bag sampler. 500 400 " in () I ,. 300 '" " m "3 200 i 100 o 0:00 02351