Abstract
In‐stream and riparian vegetation significantly affect flow discharge and sediment transport in channels with rigid and flexible vegetation under emergent and submerged conditions. Riparian tree cover increases flow resistance inducing sediment deposition and root systems stabilize deposited sediments. So acting vegetation affects stream morphology and sediment transport through soil cohesion exerted by plant root system. In order to estimate the effects of the vegetation root system on sediment retention and stream morphology roots density and strength are measured through the method of trench combined with image analysis in 5 trenches dug in Mareiter River in South Tyrol, hosting one of the largest river restoration interventions carried out so far in Italy. Samples of roots are brought to laboratory for tensile strength testing and root reinforcement arecalculated. Moreover, an analysis of vegetation patch difference between pre- and post-restoration is performed in order to extend the effect of root reinforcement at a reach scale. Increased surface occupancy of riparian vegetation and vertical growth is estimated by calculating difference of DEM (DoD) produced with airborne LiDAR and SfM photogrammetry. Results show a decrease in number of patches as result of merging process, while the total covered area by vegetation increases by almost 300%. The minimum size of the patches decreases between pre- and post-restoration attesting a recruitment of new small patches located in little islands in the middle of the channel, whereas the maximum patch surface increases in post-restoration conditions as a result of merging patches. Results from root reinforcement analysis show that roots can exert an additional cohesion from 5 to 30 kPa that underlines the importance to consider the effect of the vegetation role in stabilizing the bed and this consequence on morphology dynamics in gravel‐bed rivers.