ABSTRACT
The aquatic vegetation often grows in natural rivers and plays an important role in the river corridor system. The vegetation is often growing in areas where velocity and turbulent kinetic energy TKE are very low. Within those areas, the vegetation is extending in both lateral and longitudinal directions and traps sediments often combined with organic nutrition. Finally, the vegetation occupies large areas and presents a continuous pattern that locally changes the bed morphology. In wetlands and tidal rivers the vegetation often exhibits a patch with finite width and length, which contributes to a totally different morphology around and within the patch. This is worth to be further discussed because different flow patterns may result in a different distribution of sediment deposition. Besides, in recent years the studies regarding the vegetation patch are rare. Follett and Nepf (2012) have extensively investigated the pattern of sediment deposition inside the patch and also in the patch wake. Follett and Nepf also reported that the depressed sedimentation within a model patch was attributed to the presence of turbulent kinetic energy (TKE). Chen et al. (2012) proposed a method for estimating the wake length behind sparse and dense patches. Ortiz et al. linked the wake sediment deposition to both velocity and TKE, and found that TKE is the dominant factor that contributes to the depressed deposition in the wake. Within the model patch, Liu and Nepf (2016) experimentally determined the threshold of stem turbulence that is expressed using the stem Reynolds number, i.e. Red = 120, and presented two criteria for determining enhanced deposition inside a vegetation patch in laboratory experiments as well as in a field study. These investigations broaden our understanding for the interaction of vegetation patches and sediment deposition. Meanwhile, we know the flow characteristics upstream and downstream are linked to the patch configuration. In these studies, only one feed condition of upstream sediment was considered, i.e. a recycle sediment feeding. This may not be applicable for some natural rivers where the upstream sediment input is deficient. In a natural scenario, both the enhanced deposition and the depressed deposition behind a patch can be observed. Thus, we are interested in the scenario of reduced or even no sediment input from upstream, which may result in a totally different distribution of sedimentation. Based on the recorded velocity data, the longitudinal velocity profiles in both non-vegetated and vegetated cases are depicted in Figure 1. The model patch (D = 6 cm) blocked 20% of channel width (30 cm), resulted in the depressed velocity (≈ 40% less) directly behind the model patch. In this study, we designed a flume experiment (16 m-long, 30 cm-wide and 40 cm-high) to run detailed investigations on different distributions of sediment depositions in the wake of a vegetation patch. The velocity pattern is also discussed and linked to the change of the channel bed morphology. Longitudinal profile of normalized mean velocity at channel centerline in both non-vegetated and vegetated cases. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig118_1.tif"/>
