Monitoraggio dei bacini montani per la valutazione delle dinamiche del legname e dei sedimenti ed i pericoli associati

Mountain torrents are major sources of sediment and large wood for rivers. While they play key ecological roles, their transport during sediment-laden floods, debris-flows, and de-bris-floods can cause severe damage and fatalities. In recent decades, several “peculiar” events have shown how multiple factors can exacerbate such processes, leading to unexpected impacts. Current hazard assessments are often single hazard oriented, neglecting the interac-tion with multiple processes. The implementation of a multi-hazard approach in debris-flow, debris-flood and sediment-laden flood hazard assessment requires improved knowledge on the factors that can exacerbate it such as the initial sediment and large wood storage and the in-teraction between geomorphic changes and large wood recruitment. This thesis forms a small contribution towards this end. The first chapter reviews the current scientific knowledge on debris-flow and debris-flood hazard assessment focussing on the concept of residual hazard and analyse factors that can alter such phenomena presented in the literature. This state of the art is completed with a proposal of a new framework for a more informed and perspective hazard assessment. Chapter 2 and 3 analyses the geomorphic responses (and the fluctuations of large wood loads in the case of chapter 3) to assess the effects of ordinary events on the sediment and large wood availability inside a debris-flow prone channel and a mountain torrent. These two case studies highlighted how also ordinary events can affect the morphology of the channels, increase loose sediment availability and affect large wood loads. Chapter 4 analyse the geomorphic response and the consequent large wood recruitment during debris-flows and debris-floods triggered by a large, infrequent disturbance (Storm Alex, 2nd-3rd October 2020) affecting the Vésubie valley in south-eastern France. The results highlighted strong correlation between geomorphic changes and reduction in forest cover loss and three power law equations for the prediction of different forest cover loss scenarios were proposed. Chapter 5 propose a statistical and a probabilistic methodology to predict the large wood length distribution once recruited by debris-flows and debris-floods from the height distribu-tion of a Canopy Height Model and the catchment features (catchment size and associated channel width). Both approaches resulted appropriate to estimate the length of longer piec-es, whereas the catchment-specific probabilistic method was able to approximate the whole distribution. A general conclusion and some perspectives are then presented.