Drag coeffcient and hydraulic roughness generated by an aquatic vegetation patch in a semi-arid alluvial channel
2019
Abstract Vegetation is one of the main factors that compose the alluvial dynamic of the semiarid environment. The determination of the hydraulic roughness generated by the plant in the alluvial environment is essential to understand the behavior of the forces that describe the flow, the processes associated with the loss of energy and the sediment transport rates. The objective of this study is the characterization of the flow resistance imposed by an aquatic vegetation patch in an intermittent stream, located in Pernambuco's hinterland, Brazil, in the emerged and submerged conditions, through the parameters of vegetal drag coefficient (CD) and vegetal drag stress (τveg) of the different intrinsic species of this alluvial environment. The sampling took place in the Cachoeira intermittent stream to monitor the channel geometry, vegetative parameters, and liquid and solid discharges. Based on the submergence ratio, the flux was divided into two distinct layers that allowed the understanding of hydro-sedimentological dynamics, the first one of direct contact with the vegetation and the second one the free flow zone. The average values of CD to the aquatic vegetation patch (composed by Senna obtusifolia, Ipomoea pes-caprae and Cynodon dactylon (L.) Pers.), according to the methodologies adopted in this study, were 10.97 and 3.73 m−1. The generated physical forces were in opposition to the flow and, consequently, in the condition of increase of the vegetal density, they promoted the higher values of vegetal drag stress, and retarded the flow velocity, reducing the transport rate of suspended sediment.
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