Interaction of Bar Morphology and Riparian Vegetation in Gravel-Bed Rivers

Gravel-bed rivers are often characterized by complex bed topography, including single- and multiple-row alternate bars, bed undulations associated with channel curvature, riffle and pool sequences, presence of riparian vegetation in the floodplain, etc. The interaction of these features results in different morphologies with complex patterns and dynamics. The present work investigates the effect of the riparian vegetation on the bar dynamics, in particular it is investigated how the vegetation, which grows during the dry season on the bars, can alter the topographic patterns evolution during flood conditions. Performing two-dimensional numerical simulations we try to answer to the following research questions: which is the interaction of vegetation with bar morphology? which are the changes in sediment discharge and flow resistance, at cross-sectional and reach scale? Which are the changes in distribution of emerged and submerged areas, and potential feedbacks for vegetation growth? Which is the effect of vegetation on bar wave-length? The code BASEMENT (Faeh et al., 2010) has been used for performing the numerical runs. It has been properly modified in order to deal with the numerical description of the vegetation. The vegetation was allowed to grow during the dry season on the top of dry emergent areas, and the vertical distribution of vegetation in equilibrium condition was modeled as a function of the bed elevation using a simple analytical formulation, following Marani et al (2013). Then, during the flood events we assume that the vegetation distribution does not change, and that it can only be uprooted if the bed is eroded.The flow resistance was divided into a resistance exerted by the soil and a resistance exerted by the plants (Crosato and Saleh, 2010; Li and Millar, 2011); in this way it was possible to reproduce both the decrease in bed shear stress, reducing the sediment transport capacity of the flow within the plants, and the increase in hydraulic resistance, reducing the flow velocities. The model has been applied to different test configurations, which referred to the field conditions observed in the Magra River, close to Aulla (North Tuscany), which was characterized by several vegetated islands: starting from a flat bottom, bars developed under formative conditions. Over the resulting morphology, we allowed the vegetation to grow and interact with morphodynamic evolution simulating a sequence of floods and different scenarios to simulate both increased resistance and increased density of vegetation. Preliminary results confirmed that the riparian vegetation has a stabilizing effect on river morphology. As the density and strength of vegetation increases the vegetated bars became stable; emerged areas became wider and higher, leading to potential feedback for vegetation growth and establishment, and they are occasionally eroded during the peak flow discharge.Simulations concerning the configuration in which only one single bar is vegetated showed that this unit morphology has some feedbacks and interact actively with river morphology, i.e. reduction of bar wavelength downstream of the vegetated bar.