Bank erosion and large wood recruitment along a gravel bed river

Riverine environments can be very dynamic and complex systems, particularly because of the interaction between active channel and riparian land during flood events of different magnitude. In recent years increasing attention has been paid to large wood (LW), focusing on its role and impact along riverine systems and fluvial landscapes. This research aims to analyze the characteristics of LW recruitment as a consequence of a flood event along a reach of a gravel-bed river. The study was conducted on a 3 km-long reach located in the middle course of the Piave River (north-eastern Italian Alps). A 20 m-wide buffer zone was considered along the floodplains and islands. Every standing tree in this buffer with diameter ≥0.10 m was measured manually (diameter breast height; height), whereas shrubs were not considered. The most common species in the study area are: Populus sp., Salix sp., Alnus sp., Carpinus sp., Fraxinus sp., Pinus sylvestris and Robinia pseudoacacia. An over bankfull flood (Q=1329 m³s^–1; recurrence interval=6 years) in November 2014 caused erosions along the floodplain (15,565.5 m²), pioneer islands (25.2 m²) and building islands (2085.6 m²), recruiting 690 trees. Four of these trees were recruited from the pioneer islands (0.16 tree m^–2), 79 from building islands (0.04 tree m^–2) and 607 from floodplains (0.04 tree m^–2). Accurate dendrometric measurements were used to define the input volume of LW from the floodplains (86.25 m³), pioneer islands (0.14 m³) and building islands (6.62 m³). The maximum distance traveled by LW recruited from the floodplain, pioneer and building islands was 8927, 1021 and 3727 m, respectively. Statistical analysis showed no significant relationship between the displacement and LW characteristics considered (diameter, length, volume, density). These results demonstrate that the recruitment and subsequent transport of LW is a complex mechanism that requires further study. To better characterize these mechanisms, it appears clear that it is important to consider not only the LW characteristics but also the connection between LW, morphological settings, and flood event characteristics.