Roots reinforcement by riparian trees in restored rivers

Riparian vegetation directly and indirectly affects many fluvial processes. In particular, the root system of riparian trees and shrubs may greatly influence river morphodynamics by imparting higher shear strength to riverbanks compared to unvegetated channel margins. Therefore, the bank-stabilizing role of riparian vegetation should be quantitatively considered in river restoration projects. This work aims to quantify the additional strength due to the root systems of naturally established riparian vegetation after two river restoration projects carried out in the Italian Alps (Ahr and Mareit rivers, South Tyrol). These projects represent the largest restoration interventions carried out so far in Italian rivers, and the two sites feature contrasting channel patterns along with varying river-floodplain dynamics. The mechanical properties of roots and their distribution in the floodplain soils in the two sites have been measured to estimate root cohesion distribution associated to gray alder (Alnus incana) and pioneering willows (Salix spp.). Results show that roots of plants colonizing the gravelly substrate of building islands in the Mareit River have less tensile strength than those belonging to trees growing in the floodplain of the Ahr River, rich in fine sediments. However, the higher root contribution to the overall soil strength occurs in the island site, due its lack of fine sediments. (C) 2020 Elsevier B.V. All rights reserved.