Analisi di campo sulla larghezza a piene rive in un torrente delle Dolomiti

The analysis has the goal to study the bankfull conditions for a steep mountain torrent (the Rio Cordevole) in the Dolomites, northern Italy (Belluno). In the main course of the upper Rio Cordevole basin a revised Montgomery and Buffington classification of the morphological stream units was applied. Intensive field surveys were carried out on the main geometrical features of the cross sections at the bankfull stage. The results are focused on the bankfull width characteristics related to the different morphological typologies, as colluvial reaches, plane bed, step pools, and stepped or glide cascades. By way of a statistical analysis we shed on the light that the bankfull width is a significant variable to distinguish the morphological units, and that there is a strong similarity between step pool and stepped cascades. In addition, colluvial reaches are the unique units having the bankfull depth well correlated with the drainage area. This finding points out a different formative process of the colluvial units respect to the downstream alluvial units. The degree of correlation of the bankfull width to the respective drained area can be deemed as a sign susceptibility of each morphological type to be modelled by the water discharge. Colluvial reaches and pools show the best fitting with the drainage area, while steps and cascades have poorer correlations. The behavior depends on the absence of lateral constriction in colluvial reaches and pools, while the presence both of coarse elements in the bed, both of boulders on the banks can affect the bankfull width adjustments in reaches dominated by steps and rapids. For a given drained area of the basin, rapids are on average 30% larger than steps, and this difference remains approximately the same if we compare the pools downstream of rapids to pools downstream of steps. The features observed in the field among different morphological units are also helpful to restore mountain streams by means of works built with boulder, as artificial step pools and ramps, that mimic the natural morphological pattern.