Jeffrey W.
Gartnera, Ralph T. Chenga, Pei-Fang Wangb,
and Kenneth Richterb
aU.S.
Geological Survey, Water Resources Division,
Menlo
Park, CA, USA
bMarine
Environmental Quality Branch, Space and Naval Warfare Systems Center,
San Diego, CA, USA
Abstract
Advances
in technology have resulted in a new instrument that is designed for in-situ
determination of particle size spectra.
Such an instrument that can measure undisturbed particle size
distributions is much needed for sediment transport studies. The LISST-100 (Laser In-Situ
Scattering and Transmissometry) uses the principle of laser diffraction to
obtain the size distribution and volume concentration of suspended material in
32 size classes logarithmically spaced between 1.25 and 250 micrometers. This paper describes a laboratory evaluation
of the ability of LISST-100 to determine particle sizes using suspensions of
single size, artificial particles. Findings
show the instrument is able to determine particle size to within about 10
percent with increasing error as particle size increases. The instrument determines volume (or mass)
concentration using a volume conversion factor Cv. This volume conversion factor is
theoretically a constant. In the
laboratory evaluation Cv is found to vary by a factor of about three
over the particle size range between 5-200 micrometers. Results from field studies in South San
Francisco Bay show that values of mass concentration of suspended marine
sediments estimated by LISST-100 agree favorably with estimates from optical
backscatterance sensors if an appropriate value of Cv, according to
mean size, is used and the assumed average particle (aggregate) density is
carefully chosen. Analyses of size
distribution of suspended materials in South San Francisco Bay over multiple
tide cycles suggest the likelihood of different sources of sediment because of
different size characteristics during flood and ebb cycles.
Keywords:
laser in-situ scattering and transmissometry (LISST), particle size
distribution, total suspended solids concentration, San Francisco Bay,
California