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Chapter 2 An Experimental Study of Flow through Rigid Vegetation1
Abstract
Better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation. Instead of spatial averaging, this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels. Velocity data was collected with a one dimensional laser Doppler velocimeter (LDV) under emergent and submerged flow conditions. The effects of dowel arrangement, density, and roughness were also examined. The results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it, however the boundaries are marked by inflection points. The strongest vortices and turbulence intensities can be found there, especially in the region immediately downstream of a dowel. These results support the idea that the flow in the region near the bed and at the top of the dowel array is very unstable leading to the formation of coherent structures and are areas of significant mass and momentum exchange.
Introduction
Vegetation has traditionally been viewed as a nuisance and obstruction to channel flow by increasing flow resistance and water depth. However, in recent years, vegetation has become a major component of erosion control and stream restoration (e.g. Simon et al., 2004). Vegetation is known to increase bank stability, reduce erosion and turbidity, provide habitat for aquatic and terrestrial wildlife, attenuate floods, present aesthetic properties, and filter pollutants.
Better understanding and possible quantitative assessment of the many benefits provided by vegetation in the stream requires improved knowledge of the detailed flow structure. Flow through and above agricultural and forest canopies has been extensively studied (e.g. Raupach and Thom, 1981), but until recent decades, research of water flow through and above vegetation has been sparse. Furthermore, the majority of these studies have focused on the effects of vegetation on the bulk flow properties. For example, many researchers have attempted to quantify the effects of vegetation on the flow depth into a resistance parameter. Early attempts at describing vegetation roughness use the Manning roughness coefficient, n (Petryk and Bosmajian, 1975), and the Darcy-Weisbach friction factor, f (Chen, 1976). Such methods provide inaccurate estimates. The flow depth in wetlands can change significantly and as a result the corresponding Manning n and Darcy-Weisbach friction factor values vary considerably (James et al., 2004). To improve resistance relationships, researchers have been simulating vegetation with artificial roughness, both flexible and rigid elements, in laboratory flume experiments (e.g. Li and Shen, 1973; Tsujimoto et al., 1992, Nepf, 1999; Stone and Shen, 2002; Garcia et al., 2004; Ikeda and Kanazawa, 1996; Carollo et al. 2002; James et al., 2004). Most of these research efforts focus on determining drag coefficients and empirical formulas for resistance under various vegetation configurations. While it is important to develop empirical solutions to vegetative resistance, it is also important to understand the detailed characteristics of the flow through vegetation. Some research efforts attempt to describe the flow characteristics using velocity and turbulence intensity profiles from a single location in the flow (e.g. Tsujimoto et al., 1992). Other studies use spatial averaging of velocity measurements obtained from several locations to create a single profile (e.g. Nepf, 1999; Garcia et al., 2004). Such results are indicative of bulk flow behavior.
The objective of the present work is to describe the detailed characteristics of flow through rigid vegetation. This is accomplished by collecting measurements along verticals at locations selected to serve as a template to provide an adequate representation of the flow conditions and their variability anywhere within the vegetation array. The main focus is to examine how the mean longitudinal and vertical velocities, as well as their turbulence intensities, are affected by simulated vegetation arranged in emergent and submerged conditions. In addition, the effect of dowel density, configuration, and channel bed and stem roughness are examined. Bulk velocities and Manning n are calculated to determine how the vegetation affects the overall flow resistance of the channel.
Experimental Approach
The experiments were conducted at the Baker Environmental Hydraulics laboratory at Virginia Tech in a water-recirculating, tilting flume with vegetation simulated by acrylic dowels. The flume was 4.3 m long by 0.3 m wide and kept at a constant slope of 0.003. The acrylic dowels were 76 mm tall and 6.35 mm in diameter. They were attached to a 13 mm thick sheet of smooth Plexiglas bolted to the bottom of the flume. The flow became fully developed within twelve flow depths from the start of the dowel section. Beyond that point the flow was uniform. To ensure flow uniformity all the way to the channel outlet, stop logs were placed at the end of the flume. The simulated vegetation area was 3.0 m long by 0.3 m wide and placed 1.3 m from the entrance of the flume. Instantaneous velocity measurements were taken via a Dantec one-dimensional laser Doppler velocimeter (LDV), mounted onto a vertical traverse, 2.25 m downstream from the start of the vegetated section to ensure that the flow was, on average, fully developed. The dowels were arranged either in a staggered [Fig. 1(a)] or linear pattern [Fig 1(c)]. The spacing of the dowels was determined by a non-dimensional parameter, s/d, where s is the distance between the center of two rows of dowels and d is the diameter of the dowels.
Three sets of experiments were performed. The first set consisted of six experiments with low to medium density dowel arrangements and measurement locations at various points behind a dowel and in the free stream region. The velocity measurement locations of each experiment, shown in Fig. 1(a – c), were selected to observe the variation of the flow as it moved through the dowel array. Velocity profiles were obtained at four locations in line with the dowels at equal distance intervals starting immediately (2d) behind a dowel (Fig. 1). For some experiments, two more locations were chosen in the free stream region, between lines of dowels. These experiments focused only on longitudinal velocity. Velocity readings were taken at 14 -18 measurement points along the vertical direction at each location for the emergent experiments and 20 – 23 points for completely submerged experiments starting approximately 0.5 mm above the channel bed. Approximately 5,000 instantaneous velocity readings were taken at each measurement point over a period of 20 to 30 seconds. The estimated uncertainty in the mean streamwise velocity was 1% while the uncertainty in the RMS velocities was 3%. The measurement points were more closely spaced near the bed and further apart near the top of the flow. Experiments 1.1 – 1.3 were emergent. Experiment 1.1 had a staggered dowel arrangement. Experiments 1.2 and 1.3 were linear with the lowest (s/d = 16) and highest (s/d = 8) dowel density, respectively. The dowels in experiments 1.4 – 1.6 were completely submerged and set up in the same order as the emergent experiments. The flow rates for the emergent and submerged experiments were 0.0057 and 0.0114 m3/s, respectively. The experimental conditions for each experiment in this set are summarized in Table 1.
The second set, which consisted of six experiments, focused on a high density (s/d = 5), staggered dowel arrangement under emergent conditions. Velocity measurements were taken at six locations, four in line with the dowels starting 1d downstream of a dowel, and two in the free stream region, shown in Fig. 1(d). However, due to the density of the dowel arrangement, the measurement locations differ from the first set of experiments (Fig. 1). The third set of six experiments was exactly the same as the second one, but with the dowels completely submerged. Velocity readings were taken at 19 -20 measurement points along the vertical direction at each location for the emergent experiments and 31 points for completely submerged experiments starting 0.5 mm above the channel bed. A larger number of measurement points were taken near the bed. The longitudinal velocity component was measured in Exp 2.1 – 2.4 and 3.1 – 3.4, and the vertical velocity component in Exp 2.5, 2.6, 3.5, and 3.6. Vertical velocity measurements were taken by rotating the scope of the LDV 90°. The effects of bed and dowel roughness were also tested. Experiments with bed roughness were simulated by 35 grit sand belt sander strips with a median grain size diameter (d50) of 0.7 mm glued to the entire bed using waterproof adhesive. Dowel roughness was simulated with 100 and 40 grit sandpaper with median grain size diameters of 0.2 mm and 0.45 mm in an effort to replicate fine and coarse roughness of tree bark or similar types of stems in other vegetation. The flow rates used for the emergent and submerged experiments were 0.0044 m3/s and 0.0114 m3/s, respectively. The experiment conditions for the emergent and submerged flow runs are summarized in Table 1.
CHAPTER 1 INTRODUCTION
CHAPTER 2 AN EXPERIMENTAL STUDY OF FLOW THROUGH RIGID VEGETATION
Abstract
1 Introduction
2 Experimental Approach.
3 Experimental Results and Discussion
4 Conclusion
Acknowledgement
References
CHAPTER 3 A DOUBLE LAYER APPROACH TO FLOW THROUGH RIGID VEGETATION HYDRODYNAMICS
Abstract.
1 Introduction
2 Experimental Conditions
3 Results.
4 Discussion
5 Conclusion
Acknowledgement
References.
CHAPTER 4 CONCLUSIONS
NOTATIONS
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