Indirect analysis methods to estimate suspended sediment concentration: reliability and relationship of turbidity and settleable solids

A key element in a stream-monitoring programme for sediment transport is the choice of the measuring technique for suspended sediment concentration: this can highly affect both project costs and data reliability. The gravimetric method represents the standard analysis to directly measure suspended sediment concentration in a water sample. Indirect techniques are often employed for their inexpensiveness, although they need to be calibrated on gravimetric analysis results. In this study, the reliability of settleable solids in Imhoff cones was addressed as an alternative indirect method to estimate suspended sediment concentration (SSC), verifying the results with the turbidity measures of a laboratory nephelometer. The results show a high correlation of settled solids with suspended sediment concentration. Also, the results from the turbidimetric analysis showed a good correlation with SSC, but limited to water samples that did not need dilution. Measurements were repeated after storing the samples for 1 month: results exhibit an increase in turbidity and settleable solids, probably due to algae growth and anaerobic processes with the production of gases, causing an increase in matter. From our study, the Imhoff cone was shown to be a useful instrument to estimate suspended sediment concentration for the simplicity, the reliability and the low cost of their results. Our results prove how Imhoff cones are preferable to turbidimetric analysis to estimate SSC, specifically for highly turbid samples.