Il materiale legnoso in corsi d'acqua a canali intrecciati: volumi, mobilità , degradazione ed influenza morfologica

Natural wood in rivers has received wider attention in the scientific community only over the past two decades from different perspectives (biological, geomorphological and engineering). Wood has been studied mostly in mountain basins whereas large rivers have been less investigated. The present research deals with different aspects related to in-channel wood in gravel bed rivers, including physical wood characteristics, wood storage and transport, and link between wood abundance and river morphology. The analysis of physical characteristics of wood was carried out in three rivers: the Piave River (Veneto, Italy), the Magra River (Toscana, Italy) and the Ain River (Rhône- Alpe, France). Different variables were compared for a specific species (Populus nigra). Mean wood density range from 0.25 to 0.35 g/cm3, with Ain’s values differing significantly from the other rivers. The age of sampled wood elements is similar in the Piave and in the Magra (16 and 17 yr, respectively), whereas Ain’s samples are much older (31 yr). The growth coefficient si instead similar for the three rivers, but resistance to penetration is significantly higher in the Piave and presents a positive correlation with sample’s age. As to wood transport, 10 logs were tagged with metal tags and radiofrequency transmitters to monitor their mobility. Unfortunately, the 2005 fall season was very dry and no significant events occurred, also the 2006 spring snowmelt runoff did not reach a sufficient stage to mobilize the tagged logs. In September 2006, the largest event of the year (below bankfull stage) did not reach the logs, but the bar erosion taking place near one tagged log caused its recruitment into the flow and its transport downstream. Transport distance was about 4 km, i.e. about 18 times the mean channel width. The wood storage survey has regarded a 30 km reach in the Piave river between Ponte nelle Alpi and Busche (Belluno). Here the rivers features braided and wandering morphologies. Two integrated methods were implemented: field measurements in selected plots and interpretation of high-resolution (10 cm pixel) aerial photos at the reach scale.. Errors associated to wood measurements by using aerial photographs were evaluated the field plots. Results are encouraging, showing a good potential and efficiency of such approach for rivers where a total field survey would be unfeasible (ED < 50% per D < 15 cm; 40 < ED < 50% per 15 cm < D < 20 cm; ED < 10% per D > 20 cm). The analysis based on aerial photos led to estimate average wood load to be within the range 3.7 – 9.1 m3/ha, depending on how wood jam volume is computed. Single wood elements present a mean diameter of 28 cm and a mean length of 7.5 m. Tree species indicate most of the wood derives from the riparian corridor but other species are also present due to lateral erosion of recent low terraces. The analysis of wood storage at different reaches showed that wood is more abundant in braided channel more than in wandering reaches. Wood is mostly located on bars (rather equally in high and low ones), but a considerable fraction of wood is associated to islands in the wider braided reaches. A strong negative correlation is present between single wood elements’ volume and relative area occupied by low flow channels, whereas wood jam volume is strongly correlated with the relative area of islands.