Determinazione delle caratteristiche reologiche di un debris flow

Debris flows are natural, highly concentrated water-debris mixtures, flowing in mountain streams after long or intense rainy periods, that may cause considerable damage if overflow on the alluvial fan or on other vulnerable deposition zones. In order to predict flow characteristics it is necessary to determine the rheological properties of these mixtures. It is here suggested that the mixture composing a debris flow (which often contains boulders surprisingly large) can be obtained by adding to clear water successively coarser particles derived from a sample of the debris flow material. At each addition different suspensions are obtained that can be considered as progressively coarser interstitial fluids of the complete mixture. These suspensions are easier to be tested and their rheological properties can thus be determined. The behaviour of the complete material can then be inferred from these results. This procedure has been adopted for a debris flow that occurred in Italy in 1995; several suspensions have been generated using a sample of the debris flow and a wide range of rheometrical techniques have been used to determine their rheological characteristics: laboratory rheometer, inclined plane test, large-scale rheometer, field tests. The rheological parameters and behaviour type of the complete material have been inferred from the yield stress-solid fraction curve and from the flow curve evolution. The behaviour has been found to be that of a pseudoplastic fluid with a yield stress representable by a Herschel-Bulkley model.