High concentration suspended sediment measurements using a continuous fiber optic in-stream transmissometer

Suspended sediment loads mobilized during high flow periods in rivers and streams are largely uncharacterized. In smaller and intermittent streams, a large storm may transport a majority of the annual sediment budget. Therefore, monitoring techniques that can measure high suspended sediment concentrations at semi-continuous time intervals are needed. A fiber optic in-stream transmissometer (FIT) is presented and laboratory tested for continuous measurement of high concentration suspended sediment in flowing water. FIT performance and precision were demonstrated to be reasonably good for suspended sediment concentrations up to 10 g/L. The FIT was compared in laboratory tests to two commercially available turbidity devices and provided better precision and accuracy at both high and low concentrations. Both turbidity devices were unable to collect measurements at concentrations greater than 4 g/L. The FIT and turbidity measurements were sensitive to sediment particle size. Particle size dependence of transmittance and turbidity measurement poses the greatest problem for calibration to suspended sediment concentration. While the FIT was demonstrated to provide acceptable measurements of high suspended sediment concentrations in these studies, approaches to real-time suspended sediment detection need to address the particle size dependence in concentration measurements.