Effects of Submerged Vegetation Density on Turbulent Flow Characteristics in an Open Channel

Abstract

The vegetation density λ affects turbulent flow type in the submerged vegetated river. This laboratory study investigates different types of vegetated turbulent flow, especially the flow at 0.04 < λ < 0.1 and λ = 1.44 by setting the experimental λ within a large range. Vertical distributions of turbulent statistics (velocity, shear stress and skewness coefficients), turbulence kinetic generation rate and turbulence spectra in different λ conditions have been presented and compared. Results indicate that for flow at 0.04 < λ < 0.1, the profiles of turbulent statistics manifest characteristics that are similar to those of both the bed-shear flow and the free-shear flow, and the turbulence spectral curves are characterized with some slight humps within the low-frequency range. For λ = 1.44, the turbulent statistics above the vegetation top demonstrate the characteristics of boundary-shear flow. The spectral curves fluctuate intensely within the low-frequency range, and the spectra of low-frequency eddies above vegetation top are significantly larger than the values below. The change of turbulent flow type induced by an increase of λ would increase the maximum value of turbulence kinetic generation rate G S and change the point where G S is vertically maximum upwards to the vegetation top.Vegetation density λ can be defined as the frontal vegetation area per unit bed area. Nepf [5] proposed that the bed-shear flow and free-shear flow occur at 0 < λ < 0.04 and 0.1 < λ < λ NB , respectively. Here, λ NB is the critical density which indicates the change from the free-shear flow to the secondary boundary-shear flow. The value of λ NB has not been determined yet, but according to the comparison and summary of many investigations, λ NB might lie in the range between 0.512 and 1.55 [6].Previous studies have investigated the bed-shear flow (0 < λ< 0.04) and the free-shear flow (0.1 < λ< λ NB ), especially the latter one in the vegetated channel. Flow at 0.04 < λ< 0.1 and λ> λ NB (the secondary boundary-shear flow may appear) have been rarely analyzed. In the present investigation, flow characteristics have been measured and analyzed as λvaries within a large range. Based on vertical distributions of turbulent statistics (velocity, shear stress, skewness coefficients), turbulence kinetic generation rate and turbulence spectra, different flow types have been verified. Furthermore, flow characteristics and velocity distribution laws at 0.04 < λ< 0.1 and λ> λ NB are discussed specifically. Experimental Setup and MeasurementLaboratory experiments were conducted in a 12-m-long, 60-cm-wide (B = 60 cm), 60-cm-deep recirculating flume (Figure 1). An ultrasonic flow meter (Krohne, Germany) with an accuracy of 0.001 L/s was used to monitor flow discharge Q, which was maintained constant by a variable frequency pump system. The flow from the water tank passed through a flow-straightener upstream of the vegetation section to smooth the inflow. A tailgate was mounted at the outlet of the flume to control the flow depth H in the experiment...

Spectral shortcut in turbulence energy transfer in open channel flow over submerged vegetation

“…As to the turbulence in the mixing layer, the F(f) attains its maximum at some specific low frequency f, which is regarded as the KH vortex frequency fKH (Ghisalberti & Nepf, 2002). The fKH value dose not vary spatially (Zhao et al, 2019). We thus speculate that the fKH was about 0.15 Hz in Case B-3-30 (Figs.…”

“…The arrangement of gauging points was similar to that in Huai et al (2009) and Tang et al (2014), and the rationality of overall flow measurement has been validate by Zhao et al (2019). The sampling duration was set to be 180 seconds with a frequency of 200 Hz.…”

Techniques and Applications in Water Science and Engineering

“…However, the world is faced with serious water-related problems, which pose a major challenge to water resources engineers and scientists. In hydraulic systems, transient flow often occurs in pipelines and may interact with pipe structures [1,2]; aquatic vegetation widely occurs in surface water, affecting the flow field [3], and then further interfering with the water environment and aquatic biodiversity; and, due to climate change and rapid urbanization, extreme floods are becoming more frequent and intense, threatening human lives and causing significant damage to property [4]. An understanding of the water borne transport of emerging contaminants, such as plastic debris and antibiotics, is relevant to the remediation of rivers, lakes, and oceans [5,6].…”

“…Addressing different water resources problems through hydraulic modelling [1][2][3][4], hydroenvironmental modelling [5,6], and hydro-ecological modelling [7,8], the papers in this Special Issue report on hydraulic mechanisms [1][2][3], modelling techniques [4][5][6], and natural effects [7,8]. Analytical and numerical tools are used to improve simulation calibration and validation [1][2][3][4][5][6] as well as future ecological modeling performance [7,8]. In this study, a computational fluid dynamics (CFD) method based on Fluent software was used to investigate cavitation flow in a pipeline.…”

Mechanism of Riparian Vegetation Growth and Sediment Transport Interaction in Floodplain: A Dynamic Riparian Vegetation Model (DRIPVEM) Approach

“…Riparian vegetation plays a significant role in the ecological balance of a riparian area [1]. Riparian area is a transitional semiterrestrial zone extending from the edges of water bodies to the edges of upland communities and regularly interacts with river water, flow alterations, sediment, and nutrients [2][3][4]. Flood disturbance is one of the major causes of vegetation mortality in the riparian zone either by inundation or bank erosion [5][6][7].…”