A new indirect method to estimate suspended sediment concentration in a river monitoring programme

In order to obtain realistic values of suspended sediment concentration (SSC), an ultrasonic flow meter and an automatic water sampler were installed at the basin outlet for three Reno tributaries (Bologna Apennine, Italy). The basins have a similar geological setting: in the Sillaro basin, however, clays represent the dominant feature; in the Lavino basin, sandstone and clay marl are widely present; and the main part of the Savena watershed is characterised by sandrock, pebbly sandstone and clay marls. These differences are recognisable in the particle size distribution of the suspended sediment samples collected in these basins. The aim of this work is to estimate the SSCs of river water samples with two indirect methods, turbidity measurement and free settleable solids in Imhoff cones, and the relations with suspended sediment concentration and particle size distribution. Alongside these investigations, laboratory analyses were performed in order to validate these methods under controlled conditions. The turbidity measurements were highly significant, but their reliability was influenced by the concentration and the particle size distribution. In order to estimate the SSCs, it was necessary to establish specific regression curves for each stream. The estimate of the SSC obtained through a free settleable solids analysis was not only reliable but also less sensitive to sample characteristics. The measure of the settleable solids after 1 and 24 h presented a constant relationship in all the samples from the three streams, due to the compaction of the sediment. The measure of the settleable solids after 24 h gave a better estimate of the SSC. This last method was shown to be inexpensive, fast, reliable and applicable to all the situations studied.