Effects of Bar Morphology and Vegetation on Flow Resistance 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 andpool sequences, presence of riparian vegetation in the floodplain, etc. The complexity and interaction of these features can produce spatial variation of flow resistance and sediment transport. The recent work by Francalanci et al. (2012) has shown that alternate bars in a gravel-bed river can strongly enhance bedload transport at low Shields stress. Conversely, they do not seem to alter much the flow resistance. The present work investigates the additional effect of the vegetation on the development of alternate bars, until the equilibrium conditions of the bed are reached: at the river-reach scale, floodplain vegetation increases water levels and flow resistance, and it could alter the topographic evolution of bed morphology.