Interaction among vegetation and morphology in channelized rivers

The presence of aquatic vegetation on riverbed and embankments influences flow structure and consequently flow resistance, sediment transport, morphology, and ecology. These influences would lead to a hydraulic diversity, which is a key ingredient of physical habitat in streams. According to this fact, vegetation is commonly incorporated within stream restoration. Although significance of vegetation as an inseparable part of riverine systems is recognized, but yet it is still difficult to predict how the associated influences will respond to the introduction of vegetation and how advantages of vegetation can be optimized to a multitude of different processes. The primary impact of vegetation is slower flow velocity and thus, reduction in conveyance capacity. In addition to affecting the velocity profile over the full depth, vegetation affects turbulence intensity and diffusion. When mean kinetic energy converts to turbulent kinetic energy within the area planted with stems, turbulence intensity will begin to intensify. As a result of velocity and turbulence changes, aquatic vegetation can affect sediment movement and consequently bed form shapes could be stabilized with new patterns. Bed form characteristics (length, shape, structure, dimensions, stability, etc.) also depend on flow structure and can be divided into different categories according to the bed load materials. Locations and extension of vegetation in river channels is a fundamental factor should be considered besides the general impacts of vegetation in rivers. Isolated patches of vegetation are more common in practical applications rather than uniform vegetated channels in which the mean properties of the vegetation canopy are independent of the location. The present study considered the changes in bed forms through semi-circular patches of emergent vegetation, which are located at the banks of the channel. One of the goals is to find out how an island of vegetation modifies the morphology of rivers and mass transport. In particular, this research focuses on the physics of sediment transport and its effect on bed forms, and flow resistance in the presence of a patch of vegetation, by using experimental data and numerical modeling. Providing a physically based model for estimating the effects of vegetation on flow parameters, turbulence dispersion, and sediment transportation, the results of the present study contribute to extending the knowledge of morphology and mass transport in vegetated streams