The role of in-channel vegetation in driving and controlling the geomorphic changes along a gravel-bed river

Gravel-bed rivers are fluvial systems featuring highly complex morphological and evolutionary patterns because of their configuration characterized by the contemporary presence of many distinct morphological units. Complexity can also be observed in the response of gravel-bed rivers to flow conditions, which can be highly varied. This work aims to analyze the evolution of gravel and vegetated areas in a gravel-bed river during a period of persistent low flow conditions, focusing also on the responses exhibited by the different types of riparian vegetation. To this end, a 2200 m-long segment of the Piave River (NE Italy) was considered. It was surveyed twice by LiDAR (August 2003 and August 2010), permitting the analysis of the spatial-temporal evolution of gravel and vegetated areas through both cover type analysis and the DoD technique. During the study period (August 2003 – August 2010), the flow conditions were constantly ordinary, i.e., under-bankfull. The cover type analysis stressed an increase of +115 % in vegetated areas, which have diffusely colonized the river corridor. However, the low flow conditions also induced geomorphic changes with 54 % of the study area affected by variations in surface elevation. The DoD computed a diffuse deposition over the main and secondary channels, while erosion was observed in specific areas of the main fluvial island and along the right floodplain. In terms of vegetation changes, five eroded patches were identified, which contributed to 60 % of the vegetated area entirely eroded in the study reach. These patches were mainly characterized by mature vegetation taller than the mean vegetation height observed over the study reach. Therefore, the analysis of the response of the Piave River to persistent under-bankfull flows stressed a complex evolution of the gravel and vegetated areas, which can be appreciated also because of low flow conditions, emphasizing the composite dynamics typical of a gravel-bed river. These results could improve riverine management considering vegetation structure and dimension, as well as the geomorphic settings of gravel-bed rivers in view of future low flow conditions.