Hydraulic roughness due to submerged, emergent and flexible natural vegetation in a semiarid alluvial channel

In the semiarid area of Brazil, vegetation in natural streams generates hydraulic resistance and becomes critically important in the design of soil conservation practices. However, most researchers have studied vegetation roughness under laboratory conditions. The present work has been studied under the rare phenomenon of flow conditions and roughness due to Beach Morning Glory (Ipomoea pes-caprae) in field conditions of submerged and emergent flexible vegetation on the Jacu River, an intermittent stream in the semiarid region of Brazil. The hydraulic roughness of the Jacu River was characterized by the vegetal drag coefficient (CD′) of 5.93 m−1 under submerged condition and 2.702 m−1 underemergent condition, which is sensitive to the amount of turbulence, especially when I. pes-caprae is submerged. Two zones of the velocity profile were witnessed under submerged and emergent conditions: a shear zone and a fast free flow zone. In the shear zone turbulent mixing and a linear increase in velocity were observed at a submergence ratio between 0.6 and 1, followed by the velocity-defect law. The fast free flow zone occurred on the top of vegetation where there was a decreasing shear dispersion and the velocity followed the logarithmic law for a submergence ratio greater than unity.