Sulle dinamiche del materiale legnoso nei torrenti montani

The management of large wood (LW) in Alpine torrents is a complex and ambiguous task. On one hand the presence of LW contributes to in-channel and floodplain morphological processes, playing an important role in the ecological equilibrium and biodiversity of the river system. On the other hand LW markedly increases flood hazard particularly in the case some river cross-sections result critical for the human interface (e.g. culverts, bridges, artificial channels). In spite of the importance of the topic, only few studies deal with LW in channel dynamics. Research is needed both at basin scale (LW recruiting from hillslopes and banks) and at channel scale (movement and entrapment dynamics). This study focuses on full scale field tests in order to evaluate the LW dynamics in a mountain river. Data and information collected during the tests can contribute to the implementation of a network model of LW transport, and to the improvement of LW management and of procedures for the assessme nt of the hydraulic hazard. The study site is a river reach of the high-relief basin of the Cordevole torrent (Belluno, Central Alps, Italy), whose outlet is located at the Saviner village (basin area of 109 square kilometers). The reach is about 700 m long and the average thalweg slope is equal to 2.4%; it includes different sub -reaches: cascade, riffle pool, and a straight sub-reach stabilized by check dams. The water depth and the flow rate are provided by a monitoring station - placed on a check dam - of the Regional Agency for Environmental Prevention and Protection (ARPAV). During the snow melting season, LW transport tests have been conducted by using logs (same diameter of 0.2 m; three length classes of 0.5, 1 and 1.5 m). The experiments have been carried out introducing in the channel groups of logs with the same length, observing their passage through several control sections, and surveying their displacement velocity and the dynamics of entrapment. First results highlight: i) the importance of the element lengths on the LW mobility; ii) the mechanism of the entrapment processes driven by some morphological features like pools, loops and roughest reaches (cascades), where the ratio between log diameter and water depth suddenly decreases.