University of Twente Student Theses


Flow-Vegetation Interactions at Vegetated Riverbanks : Experimental Analysis of Flume Data and Measurements in the River Dinkel

Denkers, A.J. (2023) Flow-Vegetation Interactions at Vegetated Riverbanks : Experimental Analysis of Flume Data and Measurements in the River Dinkel.

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Abstract:Riverbank vegetation has numerous positive effects on water quality, biodiversity and riverbank stability. However, it could also obstruct natural water flow and thereby contribute to flooding. To make informed decisions on the management of riverbank vegetation in natural streams, it is crucial to understand the complex flow-vegetation interaction. Previous research into flow-vegetation interactions has mostly relied on simplified laboratory experiments, which may not fully represent real-world conditions. Therefore, further investigation is needed to accurately study and compare the flow dynamics within and around natural riverbank vegetation in order to inform better management practices. Furthermore, this study aims to investigate the representativeness of the idealised derived formulae from flume studies when applied under real-world conditions. In order to get insight into the effect of the spatial configuration of flexible vegetation patches in a flume on the longitudinal and cross-sectional development of the mean flow velocities, the flow in and around two submerged macrophytes species, Callitriche platycarpa (dense patch) and Groenlandia densa (sparse patch) were analysed to investigate the flow-vegetation interaction in aquatic habitats. The frontal area per canopy volume, which represents the ratio of the cross-sectional area of the vegetation patch perpendicular to the flow direction to the volume of the vegetation patch, is 7.70 and 1.30 for the dense and sparse patch, respectively. Two different spatial configurations were considered with two vegetation patches. The first one was the aligned configuration, representing riverbank vegetation with the two patches of vegetation positioned at the sides of the flume. The second configuration was the staggered configuration, representing the interaction of riverbank vegetation with instream vegetation patches. The findings of this flume study emphasize the importance of considering the interaction of different vegetation patches when studying the flow-vegetation dynamics in natural aquatic habitats, as the spatial configuration of two vegetation patches affects the wake development and the recovery downstream of a vegetation patch. Furthermore, the flume study underlines that two vegetation patches within 0.9 meters close to each other do not necessarily mean that they act hydrodynamically as one vegetation patch. A field study at the vegetated riverbanks of the River Dinkel was performed to investigate the flow-vegetation interaction under natural conditions and to test the representativeness of idealised derived formulae from flume studies. The flow was measured at three different riverbank vegetation patches. Patches of two different macrophyte species were found; two patches consisted of Carex sylvatica with a frontal area per canopy volume of 1.69 and 1.30, and one patch consisted of the Sparganium emersum with a frontal area per canopy volume of 2.94. The flow-vegetation interaction measurements performed in the Dinkel show that the density of the riverbank vegetation affects the flow velocity and turbulence within the vegetation patch and across the river cross-section. A shear layer formed between the slow flow within the riverbank vegetation and the faster flow in the open channel, where Kelvin-Helmholtz vortices could manifest when the shear layer has a strong enough velocity gradient. Normalized transverse flow velocity fluctuations and frequency analysis revealed the manifestation of coherent Kelvin-Helmholtz vortices. Large horizontal Kelvin-Helmholtz vortices were visible in the power density spectra of transverse flow velocity fluctuations. These Kelvin-Helmholtz vortices made a significant contribution to the transverse turbulent shear stress. The distribution of transverse flow velocity and Reynolds shear stresses revealed an inflection point, which may not align with the riverbank vegetation edge. Furthermore, the growth of the size of the vortices outside of the vegetation patch is not influenced by the characteristics of the vegetation patch itself. Quadrant analysis of Reynolds shear stress highlighted the dominance of sweeps and ejections in momentum exchange and turbulent shear stresses. Lastly, the applicability and representativeness of some idealised derived formulae were tested on the field data. This study evaluated the performance of the two layered vortex-based model of White and Nepf (2008) and the hybrid eddy viscosity model developed by Truong and Uijttewaal (2019) for predicting transverse momentum exchange, and the analytical exponential-based model proposed by Liu et al. (2022) for predicting the lateral streamwise flow velocity profile. This comparison showed that the vortex-based model of White and Nepf (2008) demonstrates relatively accurate results but tends to overestimate lateral momentum exchange near inflection points and within the riverbank vegetation patch. The hybrid eddy viscosity model by Truong and Uijttewaal (2019) effectively captures transverse momentum exchange. The existing analytical exponential-based model proposed by Liu et al. (2022) demonstrates good predictions when utilizing directly derived mixing layers widths from the flow velocity measurements in the field. However, the model performed inadequately when using the empirical estimates of the mixing layer widths proposed by White and Nepf (2008) and Liu et al. (2022). The limitation of the hybrid eddy viscosity model is that is relies on actual flow velocity measurements. To overcome this limitation, one potential approach is to utilize the exponential-based model by Liu et al. (2022) as input for the hybrid eddy viscosity model. This combination shows reasonable results but has a much more significant potential for a better understanding of flow-vegetation interactions without performing actual flow velocity measurements. However, the performance of the empirical relationships in both the exponential-based model and the hybrid eddy viscosity model highlights the need for further research to establish better empirical relationships for the mixing layer widths under natural circumstances and without performing actual flow measurements. Previous studies that derived the empirical formulae are, to a great extent, under the influence of many idealizations. For instance, in these studies, relatively dense rigid cylinders are used as mimics for vegetation patches on a flatbed without any bedforms and with relatively uniform transverse flow conditions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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