Purpose: Optical turbidity sensors have been successfully used to determine suspended sediment flux in rivers, assuming the relation between the turbidity signal and suspended sediment concentration (SSC) has been appropriately calibrated. Sediment size, shape and colour affect turbidity and are important to incorporate into the calibration process. Materials and methods: This study evaluates the effect of SSC and particle size (i.e. medium sand, fine sand, very fine sand, and fines (silt + clay)) on the sensitivity of the turbidity signal. Three different turbidity sensors were used, with photo detectors positioned at 90 and 180 degrees relative to the axis of incident light. Five different sediment ratios of sand:fines (0:100, 25:75, 50:50, 75:25 and 100:0) were also evaluated for a single SSC (1000 mg l-1). Results and discussion: The photo detectors positioned at 90 degrees were more sensitive than sensor positioned at 180 degrees in reading a wide variety of grain size particles. On average for the three turbidity sensors, the sensitivity for fines were 170, 40, and 4 times greater than sensitivities for medium sand, fine sand, and very fine sand, respectively. For an SSC of 1000 mg l-1 with the treatments composed of different proportions of sand and fines, the presence of sand in the mixture linearly reduced the turbidity signal. Conclusions: The results indicate that calibration of the turbidity signal should be carried out in situ and that the attenuation of the turbidity signal due to sand can be corrected, as long as the proportion of sand in the SSC can be estimated.
Bibliographical noteFunding Information:
Acknowledgements The authors would like to thank Casey J. Lee of the USGS Kansas Water Science Center, Lawrence, KS, and Christopher A. Ellison and Jason L. Roth of the USGS Minnesota Water Science Center, Mounds View, MN, on the valuable contribution to improving this article. The authors would also to thank Jorge Rosso of Solar SA, Florianópolis, Brazil, for the generous loan of the optical turbidity sensors used in this study and Kimberlie Perkins, USGS, National Research Program, Menlo Park, CA, for the particle size analysis. Financial support was provided by CNPq – Conselho Nacional de Desenvolvimento Cien-tífico e Tecnológico Programa Ciência Sem Fronteiras. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
- Optical sensor
- Scattered light
- Sediment flux