Analisi sperimentale delle scabrezze di superficie e di fondo per la modellazione dinamica dei flussi torrentizi e della caduta massi

This research project is aimed to study the mechanisms of motion resistance in mountain streams and slopes to better understand the explanation dynamics of some natural processes. In particular, the role-played by the local topography and the effects of the presence of vegetation. Scientific objectives are to analyse in detail the interactions between processes, sliding surface, and vegetation. The investigation was carry out through field observations and surveys of two real events of debris flows and rockfall event, and small-scale experiment of debris flow and full-scale experiment of rockfall. The project objectives have been structured on three main points: 1. providing a critical review of the literature procedures for estimating motion resistance for each investigated process; 2. exploring the possible relationship between process and roughness elements, in particular morphology, topography, and vegetation; 3. defining a new methodology for surface roughness quantification spatially distributed and implement it within a numerical model. The final goal of this study is to deepen knowledge about the kinematics of natural processes and to improve guidelines management for protection forest. Consequently, at first the issues are dealt with a scientific way and later providing practical guidance. The following study areas are chosen for the investigation: a) a portion of the slopes of Mount Antelao (Vodo di Cadore, Belluno Province) recently affected by a massive rockfall and the forest stand downstream; b) an avalanche channel in the Belledonne Massif (municipal forest of Vaujany, France) where full-scale tests of rockfalls were carried out; c) two debris-flow deposits and the flood-involved trees along the Somprade (Auronzo di Cadore, Belluno Province) and Senago (Bolzano, Bolzano) creeks; d) two reaches of a fish ladder built on the river bank of Vanoi torrent (Canal San Bovo, Trento Province) with a step-pool morphology, offering an equivalent to a full-scale physical model. In addition, to support the field work, the study of the relations between debris flows and forest stand was deepened by executing small-scale laboratory tests on a tilting plane rheometer of the faculty of agricultural and food sciences of Milan. The study of the mutual interactions between rockfall and debris flows and forest stand was carried out at different scales of analysis: through field post-event investigations, small-scale laboratory tests (for debris-flow process) and finally performing some numerical modelling. The field surveys have allowed to increase the knowledges of the damage produced by the boulders and by debris-flows impacts on trees, and how they can provide an effective mobility suppression. Laboratory tests have allowed to examine the effect of the forest management (high forest and coppice forest) on the final deposits. The numerical modelling have allowed to explore in more detail the way and the limitations of the implementation of trees and how the protection offered by an even-aged forest changes in the medium to long term. Field evidences and laboratory simulations confirm that shrubs and trees influence the deposition processes of rockfall and debris flows. In particular, the importance of coupling forest population growth model and process numerical model for the study of rockfall and comparing the scenario without forest with forested scenarios. The different debris-flow regimes affect the contribution of trees in the sediment deposition and mobility suppression. When, the collisional regime dominates, large trees with low-density produce higher deposit thicknesses, directly proportional to the diameter. Whereas, when frictional regime dominates, debris-flow suppression depends mainly on the surface resistance, so a dense forest of small trees, or shrubs, provides better retention capacity. In addition, coppice forest seems able to contribute to debris-flows deposition on shorter distances, which decrease with increasing of the solid concentration. The study of the hydraulic kinematic in step-pool morphology confirms previous researches: the standard deviation of the bed profile provides a more reliable measure of roughness over different channel morphologies, as opposed to grain size. Moreover, in these morphologies, the relative submergence decreasing and the slope increasing seem to collaborate in the dissipation of energy and the most of the energy loss, especially for low discharge, is due to the 'spill resistance’. A general overestimation of the variables analysed was produced by the implementation in RockyFOR-3D of the new automated method for defining the roughness parameter proposed in this work. Nevertheless, the good approximation of the maximum values indicates a potential for development of the proposed method