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Vegetation present in the water streams, on the banks and in the floodplain areas largely affects the river hydraulics. Indeed, river vegetation significantly influences hydrodynamics, sediment transport, bedforms, and pollutant transport. Environmental management of rivers requires an understanding of the various processes and predictive capabilities of models. In the past, many studies were conducted, especially in laboratory settings, in order to quantify flow resistance due to vegetation. It is only recently that the effects of vegetation on sediment transport came to the attention of researchers. In particular, both suspended and bedload transport were considered. This paper reviews recent works conducted on the effect of vegetation on incipient sediment motion and bedload transport. With regard to the incipient sediment motion, methods based on critical velocity, turbulence, vegetation drag, and velocity in the bed roughness boundary layer have been discussed. For bedload transport, methods based on bed shear stress, turbulent kinetic energy, a revisiting of classical formulas for estimating bedload transport in non-vegetated channels, and estimation from erosion around a single vegetation stem are analyzed. Finally, indications on further research and new development are provided. Article Highlights: Incipient sediment motion in vegetated channels is different from that in bare channels New models for the estimation of sediment transport in vegetated channels consider turbulence effects The presence of vegetation in rivers contributes in promoting the stability of the streambed [ABSTRACT FROM AUTHOR]

Local scour represents the erosion process that occurs at the base of hydraulic structures overlying sediment beds. Horseshoe vortices forming at the bed-structure junction are the main responsible for sediment removal and dictate erosion rates as well as the maximum erosion depth resulting from a significant flow event. In steady-flow conditions this is often referred to as the equilibrium scour depth, which, for many hydraulic structures, represents a key parameter for foundation-design and risk-assessment purposes. The equilibrium scour depth has been investigated for decades and many predictive formulae have been developed following the classical empirical approach, whereby numerous experimental datasets are used to isolate and identify the influence of non-dimensional groups emerging from dimensional analysis. Within this context, the influence of obstacle Reynolds numbers, and consequently of viscous forces, has always been neglected because of the large Re values normally encountered in engineering and laboratory conditions. The present paper demonstrates that this assumption is largely incorrect especially for beds made of sand or finer material. The theoretical analysis presented in Manes and Brocchini ([1]) is herein extended to include viscosity effects and investigate their importance on equilibrium scour depths forming around obstacles resembling bridge piers.

Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging methodology (DAM). The flow measurements were carried out by the particle image velocimetry (PIV) technique. The double-averaged (DA) turbulent characteristics (DA Turbulent kinetic energy (TKE) components, form-induced TKE components, DA TKE fluxes, form-induced TKE fluxes, DA TKE budget) were analyzed for the WGB. To understand the effect of changed bed topography on the turbulent characteristics, the flow measurements were carried out over a screeded gravel bed (SGB), keeping the flow Froude number same as in case of WGB. Owing to water work, the bed topography of WGB was dissimilar to that of SGB, resulting in higher roughness size for the former than that for the latter. Comparative study of the DA turbulent characteristics of both the beds infers that especially in the near-bed flow zone, the flow parameters of the WGB are attaining higher values than those of the SGB. However, they are almost alike for both the beds in the flow outer layer.

Results are presented and discussed from two laboratory experimental campaigns specifically designed to investigate the behavior of a slot as countermeasure against local scouring at a smooth circular bridge pier, close to threshold flow conditions of initiation of uniform sediment motion. The investigation was aimed at evaluating the effectiveness of the slot by changing its sinking depth into a sand bed, and assessing the dependence of the scour depth on different dimensionless groups. The results showed that the slot reduces the local scour at pier. The maximum reduction of the scour depth was about 30% (about 70% for both scour area and volume) in the best configurations. A combined countermeasure is also proposed and tested, consisting of a slot and a bed sill placed downstream of the pier and adjacent to it: in the best configuration, the scour reduction in front of the pier reached about 45% on average (with about 80 and 90% for scour area and volume, respectively). This last result shows that a combination of slot and bed sill may be a very effective countermeasure against local scouring at bridge piers. DOI: 10.1061/(ASCE)HY.1943-7900.0000035 CE Database subject headings: Scour; Bridges, piers; Measurement.

Results are presented from laboratory experiments to investigate the effectiveness of bed sills as countermeasures against local scouring at a smooth circular bridge pier, for flow conditions near the threshold of uniform sediment motion. The bed sill was located downstream of the pier, and its effectiveness with the distance between pier and sill was evaluated. The dependence of the scour depth on different dimensionless groups was defined. The results showed that a bed sill placed at a short distance downstream of the pier reduces the scour depth, area, and volume. In particular, the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. The bed sill seems to take effect some time after the beginning of the test, as the scour hole downstream of the bridge pier develops sufficiently and interacts with the countermeasure. DOI: 10.1061/(ASCE)0733-9429(2009)135:1(13) CE Database subject headings: Scour; Bridges; Piers; Experimentation.

Even though both fluid mechanics and numerical studies have considerably progressed in the past decades, experimental knowledge remains an important tool for studying the resistance to flow in fluid media where a complex environment dominates the flow pattern. After a comprehensive review of the recent literature on the drag coefficient in open channels with emergent rigid vegetation, this paper presents the results related to 29 experimental accelerated subcritical flow profiles (i.e., M2 type) that were observed in flume experiments with emergent stems in a square arrangement at the University of Calabria (Italy). First of all, we used some of the literature formulas for the drag coefficient, concluding that they were unsatisfactory, probably because of their derivation for uniform or quasi-uniform flow conditions. Then, we tested a recently proposed approach, but when we plotted the drag coefficient versus the stem Reynolds number, the calculated drag coefficients showed an inconclusive behavior to interpret. Thus, we proposed a new approach that considers the calibration of the Manning coefficient for the simulation of the free surface profile, and then the evaluation of the drag coefficients based on the fundamental fluid mechanics equations. With the help of classical dimensional analysis, a regression equation was found to estimate the drag coefficients by means of non-dimensional parameters, which include vegetation density, stem Reynolds number and flow Reynolds number computed using the flow depth as characteristic length. This equation was used to simulate all the 26 observed profiles and, also, 4 experimental literature profiles, and the results were good. The regression equation could be used to estimate the drag coefficient for the M2 profiles in channels with squared stem arrangements, within the range of vegetation densities, flow Reynolds numbers and stem Reynolds numbers of the present study. However, in the case of the three profiles observed by the authors for staggered arrangement, the regression equation gives significantly underestimated flow depths. [ABSTRACT FROM AUTHOR]