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Bruce MacVicar, Department of Civil and Environmental Engineering
Introduction
Flow is the governing factor in rivers, influencing the redistribution of energy, materials and living organisms within river networks. Flow patterns evolving around natural obstructions, such as boulder clusters, wood jams or patches of riparian vegetation, diversify fluvial environments. Understanding the mechanisms behind formation of these wakes will help address fundamental questions related to river morphology, sediment dynamics, potential impacts on biota and, consequently, the assessment and restoration of rivers.
Although there has been recognition of the importance of wakes for natural fluvial environments, there is a general lack of detailed field data. Most previous numerical and experimental studies were carried out for idealized geometries with defined characteristics of the obstruction, bed material and channel setup to minimize artefacts related to the scale effect. This paper presents fundamental hydrodynamic research to: (1) describe flow characteristics and visualize wake patterns behind models of solid and porous obstructions and (2) examine the effects of bed friction on wake patterns and their dynamics. It resulted in a detailed dataset. The design of field experiments was upscaled to correspond to field conditions, thereby allowing direct evaluation of scale effects in the corresponding laboratory datasets.
Methodology
Field experiments were carried out on a side branch of the Tagliamento River in northeast Italy which, during low flow periods, is fed only by perennial springs making the regime extremely stable during these periods. A 50 m long, 10 m wide rectangular in-stream flume was built with incoming flow velocity regulated by adding or reducing a number of plastic plates, which allowed control of the degree of flow obstruction (Figure 1). The obstructions used in the field experiments were porous circular objects composed of circular dowels arranged in a staggered formation.
Figure 1: In-stream flume. (a) Aerial view of the flume (I is the wall of the flume, II is a model obstruction and III is a needle weir; the yellow dashed line is the longitudinal transect and white dashed lines are locations of lateral profiles in a typical layout of experimental run). (b) Construction of the in-stream flume.
All experimental runs were identical in their setup and instrumentation, which were designed for visualizing the flow and measuring 3-D flow velocities and the topography of the free surface. For measurements of flow velocities and free surface topography, a custom-built, floating lateral platform with velocimeter sensors at fixed positions on the deployment bars was designed and deployed. Measurements of 3-D velocities were performed with an array of seven acoustic Doppler velocimeters (Figure 2).
Figure 2: Measuring frame and setup: (a) floatable platform (1 is a lateral platform, 2 is a float, 3 is a deployment mount, 4 is a porous emerged obstruction and the white arrow shows flow direction) and (b) instrumental setup (1 is a velocimeter sensor, 2 is a deployment bar and 3 is an orientation cable).
In each experimental run, measurements were performed along a longitudinal transect through the centreline of the circular obstruction and at eight or 10 lateral transects for emerging and submerged obstructions, respectively (Figure 1a). Each longitudinal transect was composed of 25 to 30 sampling locations unevenly spaced from locations situated upstream of the obstruction to locations situated far downstream of the obstruction. Except for the runs with emerged solid obstructions, each lateral transect included 14 sampling locations evenly spaced across the flow from the centreline towards the left wall of the flume.
Free surface topography was surveyed at seven locations for each of the lateral transects with the density of transects higher in the vicinity of the obstruction. The surveys of free surface were spatially mapped to obtain the differences in elevations relative to the elevation in the upstream section of the setup. For visualization of the flow patterns, green and red dyes were continuously injected at both sides of the obstruction. Visualizations were recorded 40 m above the free surface using a drone, near the middle of the in-stream flume.
Outcomes
The experimental programme consisted of 30 runs that systematically varied the diameter of the surface-mounted obstruction, its solid volume fraction, its porosity at the leading edge, the object's submergence and the approach velocity and measured the system's responses. The time series measured in the wake clearly displayed large fluctuations of the lateral component of velocity with a period of around 20 s (Figure 3a). Calculations of the variance of the velocity components for cumulatively increased sampling durations showed that estimates become generally stable for durations exceeding 60 to 120 s for both the approach flow and the wake flow, which shows that the velocity sampling duration of 200 s was sufficient (Figure 3b). The sampling duration and frequency were also sufficient for resolving turbulence spectra at low frequencies (Figure 3c).
Figure 3: Examples of measured velocity data: (a) time series of 3-D velocities, (b) cumulative variances as a function of the band period and (c) turbulence spectra.
Post-processing of video recordings included visual inspection and selection of representative time intervals. By using bench marks in the digitalization procedure, exact coordinates from the fields' surveys were identified (Figure 4).
Figure 4: Example of JPEG image recorded in run 13 (numbers on the picture mark the distances in metres for the bench marks on the walls).
Conclusions
This paper presents a unique and detailed dataset containing hydrodynamic data, bathymetric and water free surface data, and visualizations of the flow patterns obtained via controlled field experiments completed in a gravel-bed side arm of the Tagliamento River in the northeast of Italy. The dataset will help researchers to better understand the dynamics of shallow wake flows forming in natural fluvial environments, including the effect of drift wood accumulation, which can significantly alter flow dynamics and morphodynamic processes on floodplains.
In addition, because of the simplified and idealized configuration of the experimental channel and porous obstruction, the experimental data can be used for further development and verification of advanced numerical models by direct and independent assessment of controlling factors. Aspects that require further investigation include the stabilization of large-scale coherent structures by bed friction and the impact of the wake dynamics on sediment transport and bar formation.
The dataset described in the study is publicly accessible via https://doi.org/10.5281/zenodo.3968748
Shumilova, O.O., Sukhodolov, A.N., Constantinescu, G.S. & MacVicar, B.J. (2021). Dynamics of shallow wakes on gravel-bed floodplains: dataset from field experiments. Earth System Science Data, 13(4), 1519-1529. https://doi.org/10.5194/essd-13-1519-2021
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Photo by Nathan Dumlao on Unsplash
