SULLA PREVISIONE DELLA PENDENZA DI EQUILIBRIO IN ALCUNI CORSI D'ACQUA CALABRESI MEDIANTE UN APPROCCIO DI TIPO PROBABILISTICO

The equilibrium bed slope is that particular value of slope ie that characterizes a streambed reach of well known shape and discharge and that allows particles of fixed size to rest in equilibrium on the bed itself. The knowledge of this slope is crucial for hydraulic designers because height, distance, and number of the check dams that can be built to stabilise the bed depend on the value of this parameter. The evaluation of the equilibrium bed slope is related to the geometric characteristics of the stream, to the size of the bed material and to the hydraulic condition for that stream reach. Its prediction can also be focussed on a larger scenario that concerns the incipient motion problem in moving river beds. However, this approach requires some subjective choicesconcerning the use of a sediment transport equation, the calculation of a dominant discharge and the sampling criterion for collecting the bed material. In this context, the dynamic approach, i.e. the method that makes use of a criticaI value of the bottom shear stress to get to the equilibrium value of the slope, can be improved if the probabilistic aspect of the problem is accounted. The aim of this paper is to verify the suitability of a modified Gessler approach to estimate the criticaI shear stress for a mixture of sediments, and then the equilibrium bed slope, by considering a different theoretical law expressing the probability of movement for particles of fixed size. In particular, the originaI data set collected by Gessler was replotted and then fitted with a new probability distribution. Finally, the effect related to the particle size (by choosing a mean grain size distribution for each torrent or a different distribution for each stream reach) is also investigated