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

ABSTRACT In order to get realistic values of suspended sediment concentration (S.S.C.), 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, however in the Sillaro basin clays represent the dominant feature, in the Lavino basin sandstone and clay marl are widely present and main part of the Savena watershed is characterized by sandrock, pebbly sandstone and clay marls. These differences are recognizable in the particle size distribution of the suspended sediment samples collected in these basins. The aim of this work is to estimate S.S.C. of river water samples with two indirect methods, turbidity measure and free settleable solids in Imhoff cones, and the relations with suspended sediment concentration and particle size distribution. Along with this analysis, it has been performed a laboratory campaign of analysis in order to validate these methods under controlled conditions. The turbidity measurements resulted highly significant, but their reliability is influenced by the concentration and the particle size distribution. In order to estimate S.S.C., it has been verified the necessity of building a specific regression curves for each stream. The estimate of S.S.C. obtained through a free settleable solids analysis has resulted reliable, and, furthermore, less sensitive to sample characteristics. The measure of the settleable solids after 1 h 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 S.S.C.. This last method showed to be inexpensive, fast, reliable and applicable to the all the situations studied. D. Pavanelli1, A. Bigi2 1 Researcher, Department of Agricultural Economics and Engineering, University of Bologna, Italy; e-mail: donatella.pavanelli@unibo.it 2 PhD Student, Department of Agricultural Economics and Engineering, University of Bologna D. Pavanelli is scientific manager and the author of plan, she has processed the experimental data and has written the article. A. Bigi has collected and cooperated to process the experimental data and he has cooperated in the writing and traslating of the article.