An optical (photocell) sensing device was used to measure the relative transparency characteristics of sediment suspensions with a view toward improving our ability to measure the temporal variations of suspended-sediment concentration moving in streams. The instrument used was a commercial "falling stream" turbidimeter that measured the relative transparency of a gravity induced smooth flat sample stream formed as the flow drops vertically from an orifice.
Output results were recorded on a strip-chart recorder as a function of concentration, up to about 100,000 mg/2, for a range of particle sizes and for three kinds of sediment--a fine sand, a silty alluvial soil, and a clayey residual soil. The sensitivity range was improved by use of both the 1/8- and 1/16-inch thick orifices. The use of the two orifice or nozzle sizes provided data useful in evaluation of the effect of particle size and type of sediment at different concentrations. The unique relative transparency-concentration relationships were more linear for coarse than for fine sediments of a given kind. It was concluded that curvature in this relationship resulted from light reflections from the surface of the particles and (or) the fact that some particles moved through the system while in the "shadow" of other particles closer to the light source.
The ratio of suspended-sediment concentration to the square root of the median particle size of sand-sized sediments was found to be a useful parameter for elimination of the effect of particle size in the relative transparency-concentration relationships. Thus it was possible to evaluate the effect of the different type of sediment on the relative transparency independent of particle size. The use of this parameter to eliminate the effect of particle size was unsuccessful for finer sediments. -
