CHARACTERISTICS OF NEAR-BOUNDARY FLOW THROUGH TWO RIPARIAN VEGETATION TYPES A FLUME … ·...

Transactions of the ASABE

Vol. 55(6): 2101-2109 © 2012 American Society of Agricultural and Biological Engineers ISSN 2151-0032 2101

CHARACTERISTICS OF NEAR-BOUNDARY FLOW THROUGH TWO RIPARIAN VEGETATION TYPES: A FLUME STUDY

L. C. Hopkinson

ABSTRACT. Stream restoration designs frequently involve establishing vegetation on the bank face and floodplain, result-ing in mixed patterns of riparian vegetation. This study examined the effects of varied patterns of simulated vegetation on time-averaged velocity and turbulence statistics. A flume experiment was conducted with eight submerged vegetation treatments composed of three characteristics: flexible grass height (0.7 and 3.9 cm), percent grass cover (100% and 50%), and the presence of herbaceous tubelings (with and without). Turbulence statistics (e.g., turbulence intensities, turbulent kinetic energy, and Reynolds stresses) were examined using near-bed velocity measurements. The 50% grass cover treat-ments introduced additional boundaries located at the vegetation edges that were not present in the 100% cover treat-ments. These additional edges resulted in complex flow patterns and velocity gradients, as well as heterogeneous distribu-tion of turbulence statistics. Elevated Reynolds stresses located at the vegetation patch edges indicated potential for localized particle entrainment, consistent with previous observations of erosion. The presence of dowels also contributed to flow complexity while not significantly elevating overall turbulence. Results indicate that combining dense vegetation with varying lengths and sparsely positioned herbaceous tubelings will promote flow complexity.

Keywords. Flume, Stream restoration, Submerged vegetation, Turbulence.

stablishing riparian vegetation along streambanks and floodplains is a common technique used to control streambank retreat (Bernhardt et al., 2005; Bernhardt and Palmer, 2007). Bank vegetation in-

creases geotechnical stability through root reinforcement (Simon and Collison, 2002; Wynn and Mostaghimi, 2006; Simon et al., 2006), and interactions between streambank vegetation and near-bank flow result in altered near-bank ve-locity and shear stress distributions (Thorne and Furbish, 1995; McBride et al., 2007; Hopkinson and Wynn, 2009). Generally, multiple planting methods (e.g., live staking, bare-root plantings, and potted plants) combined with seeding and other soil bioengineering systems are suggested for bank sta-bilization (USDA-NRCS, 1996; Doll et al., 2003). This mixed planting method results in a distribution of vegetation with varying densities, time of establishment, length, and flexibility. In addition to the desired stability management, this heterogeneous planting scheme may generate flow com-plexity (e.g., velocity gradients, eddies, etc.) that are desira-ble habitat features for aquatic organisms (Fausch and White, 1981; Hayes and Jowett, 1994; Crowder and Diplas, 2002).

Flow through vegetation has been studied extensively for idealized vegetation in controlled experiments. Flow re-sistance and turbulence structure have been quantified for flow through an array of mimicked vegetation (Ikeda and

Kanazawa 1996; Nepf, 1999; Garcia et al., 2004; Ghisal-berti and Nepf, 2002, 2006; Tanino and Nepf, 2008; Liu et al., 2008). These studies primarily utilized a single vegeta-tion type (e.g., rigid cylinders, plastic strips) located along the flume bed. This work was extended by defining the stepped velocity profile of flow through a combination of short and long rigid dowels (Liu et al., 2010) and through artificial vegetation with frontal densities that varied with depth (Lightbody and Nepf, 2006).

Recent research has begun to address the impacts of streamside vegetation and vegetation patches (i.e., regions of vegetation with a finite area) on the flow field. The turbu-lence structure resulting from idealized vegetation located on the floodplain (Thornton et al., 2000; McBride et al., 2007; Yang et al., 2007), as streamside vegetation (White and Nepf, 2007; Afzalimehr and Dey, 2009; Zong and Nepf, 2010), and on sloping streambanks (Hopkinson and Wynn, 2009) has been quantified. High turbulent shear stresses are moved away from the streambank in the presence of dense, stream-bank vegetation (Hopkinson and Wynn, 2009). Recent re-search also characterized flow structure near vegetation/main channel boundaries of vegetation patches (Zong and Nepf, 2010; Siniscalchi et al., 2012). There is a need to expand this knowledge, considering flow transitions at vegetation patch boundaries (Nepf, 2012) and the flow effects resulting from heterogeneous vegetation patterns (Nezu and Sanjou, 2008).

The objective of this research was to examine the effects of varied patterns of simulated vegetation on time-averaged velocity and turbulence statistics. Measurements of veloci-ty were taken at a high spatial density within vegetation treatments combining flexible and rigid vegetation mimics that created two vertical layers. Artificial vegetation spac-ing was determined by a recent restoration plan.

Submitted for review in December 2011 as manuscript number

SW 9561; approved for publication by the Soil & Water Division ofASABE in October 2012.

The author is Leslie C. Hopkinson, Assistant Professor, Departmentof Civil and Environmental Engineering, West Virginia University, P.O.Box 6103, Morgantown, WV 26506; phone: 304-293-9932; e-mail:[email protected].

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2102 TRANSACTIONS OF THE ASABE

METHODS FLUME SETUP AND VEGETATION TREATMENTS

A flume experiment was conducted in a 1.21 m wide × 6 m long flume (0% bed slope) at the St. Anthony Falls La-boratory (Minneapolis, Minn.). Flume vegetation models were constructed based on a planting pattern followed by an actual stream restoration site (Stroubles Creek, Blacks-burg, Va.). The stream restoration design involved reshap-ing the banks and establishing bank and riparian vegetation (Thompson et al., 2012). In the Stroubles Creek restoration design, the lower bank of the restoration site was seeded with a wetland grass mixture, and herbaceous tubelings were planted in rows with a 0.15 m spacing and 0.76 m off-set (Theresa Thompson, BSE Virginia Tech, personal com-munication, 27 August 2010).

From these characteristics, eight vegetation treatments were constructed involving three vegetation factors: flexi-ble grass length (0.7 and 3.9 cm), percent cover (100% and 50%), and herbaceous plantings (with and without) (fig. 1 and table 1). The grass was simulated using synthetic grass mats (RealGrass) with blade lengths of 0.7 cm (127 stems cm-2) and 3.9 cm (22 stems cm-2); the two grass lengths represented the wetland grass at two stages of development. Because vegetation often does not develop in a uniform pattern, the flexible vegetation density was reduced by 50% by randomly removing square sections (30.5 cm × 30.5 cm) of the grass. This decreased area with vegetation was used to mimic patchy vegetation (figs. 1c and 1d). Wooden dow-els were used to represent the herbaceous tubelings and were meant to represent the plants immediately after plant-ing (diameter = 0.6 cm; length = 15.24 cm). The dowels

were positioned vertically in rows with a 0.15 m spacing and a 0.76 m offset, following the Stroubles Creek planting arrangement (Theresa Thompson, BSE Virginia Tech, per-sonal communication, 27 August 2010; figs. 1b and 1d).

MICROADV VELOCITY MEASUREMENTS The eight treatments were examined under two dis-

charges (0.02 and 0.03 m3 s-1; tailgate height = 13.3 cm; water depth = 17 to 18 cm). Experimental discharges were guided by previous studies (e.g., McBride et al., 2007; Afzalimehr and Dey, 2009) and by flow rates and velocities expected in the riparian area of Stroubles Creek. Discharge was measured with a sharp-crest weir located at the end of

Figure 1. (a-d) Plan views of flume open channel indicating the location of flexible vegetation (shaded areas) and staggered dowels (•) for one flexible vegetation stem length. The main flow direction is the positive x-direction, and velocity measurement locations (marked as ×) are identi-fied in the expanded views of the measurement regions. (e-l) Treatments for both flexible vegetation stem lengths: (e) S-100%, (f) S-100%-d, (g) S-50%, (h) S-50%-d, (i) L-100%, (j) L-100%-d, (k) L-50%, and (l) L-50%-d, where S = short grass (0.7 cm), L = long grass (3.9 cm), and d = dowel. For images (e) through (l), the main flow direction is into the camera.

Table 1. Description of experimental flume runs.

Run Discharge

(m3 s-1)

Flexible Vegetation

Stem Length (cm)

Percent Flexible Cover (%)

Herbaceous Plantings

1 0.03 0.7 100 None 2 0.03 0.7 100 Dowels 3 0.03 0.7 50 None 4 0.03 0.7 50 Dowels 5 0.03 3.9 100 None 6 0.03 3.9 100 Dowels 7 0.03 3.9 50 None 8 0.03 3.9 50 Dowels 9 0.02 0.7 100 None 10 0.02 0.7 100 Dowels 11 0.02 0.7 50 None 12 0.02 0.7 50 Dowels 13 0.02 3.9 100 None 14 0.02 3.9 100 Dowels 15 0.02 3.9 50 None 16 0.02 3.9 50 Dowels

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the open channel and was verified with MicroADV velocity measurements. The discharge of 0.02 m3 s-1 was the mini-mum discharge that could be measured with the sharp-crest weir. Flow was fully turbulent for all experiments (Reyn-olds numbers of the order of 18,700), and vegetation was fully submerged. Due to local fluctuations in the water sur-face, precise water surface slopes could not be measured over the short longitudinal distance of the open channel.

Three-dimensional velocity was measured with a down-looking Sontek 16 MHz MicroADV (San Diego, Cal.) at 25 Hz for 120 s, meeting the minimum record length for calculating turbulence statistics (Buffin-Bélanger and Roy, 2005). Velocity was measured under steady flow in the measurement region (30.5 × 38.1 cm) located approximate-ly 20 flow depths from the flume entrance. Velocity meas-urements were taken in a grid pattern at one vertical dis-tance per location (n = 98 for treatments with dowels; n = 99 for treatments without dowels; figs. 1a to 1d). Velocity was measured at a distance of 4 cm from the flume bed. This distance was as close to the flume bed as allowed without significant interference of the velocity probe for all treatments.

The MicroADV velocity data were filtered to reduce er-roneous data that may bias mean velocity values and turbu-lence characteristics. Instantaneous velocity values with an average signal correlation value less than 70% or an aver-age signal to noise ratio less than 15 were removed (Wahl, 2000; McBride et al., 2007; Hopkinson and Wynn, 2009). All recorded time series were included in the data analysis because more than 75% of the instantaneous values re-mained after filtering (McBride et al., 2007; Hopkinson and Wynn, 2009).

DATA ANALYSIS Each velocity time series was analyzed for turbulence

intensities, turbulent kinetic energy, and Reynolds stresses. The root mean square (RMS) of the fluctuating velocities was calculated for each velocity component, representing the turbulence intensities:

2RMS u u'= (1)

2RMS v v'= (2)

2RMS w w'= (3)

where u is the downstream velocity component (m s-1), v is the lateral velocity component (m s-1), and w is the vertical velocity component (m s-1). This analysis required separat-ing the instantaneous velocity fluctuations (u′, v′, and w′) from the time-averaged velocity ( u , v , and w ) for each ve-locity component.

The overall average turbulent kinetic energy (TKE) was calculated:

( )2 2 2TKE 0 5. u' v' w'= ρ + + (4)

where ρ is the fluid density (kg m-3).

Reynolds stresses τxz and τxy, representing vertical and lateral momentum exchange, respectively, were also calcu-lated:

xz u' w'τ = −ρ (5)

xy u' v'τ = −ρ (6)

STATISTICAL ANALYSIS Statistical tests were used to determine differences

among the near-bed time-averaged velocities and calculated turbulence statistics. First, the distributions of time-averaged velocities and turbulence statistics were tested for normality using the Shapiro-Wilk test. The normal quartile plots were also examined. The data did not follow a Gauss-ian distribution; therefore, the Wilcoxon test was used to test for differences in medians. Differences among factors (i.e., discharge, flexible stem length, percent flexible cover, and herbaceous plantings) were tested for each time-averaged velocity component ( u , v , and w ), TKE, Reyn-olds stress (τxz and τxy), and turbulence intensity (RMSu, RMSv, and RMSw). To eliminate the error contributed by position, data were blocked by measurement location (figs. 1a to 1d). Blocks without data (i.e., dowel location; figs. 1b and 1d) were removed from the analysis (Devore, 2000). A significance level of α = 0.05 was assumed for all tests. Statistical analysis was conducted using JMP soft-ware (v.9.0.0, SAS Institute, Inc., Cary, N.C.).

RESULTS AND DISCUSSION For both discharges, u was typically two orders of

magnitude greater than the time-averaged lateral ( v ) and vertical ( w ) velocity components for each individual run. The RMS values represent the strength of the velocity fluc-tuations and were on the same order of magnitude for all three components (100 cm s-1). On average, RMSu was an order of magnitude less than u , while RMSv and RMSw were an order of magnitude greater than v and w , respec-tively (table 2). These results indicate that turbulence fluc-tuations larger than the average velocity were frequent for the lateral and vertical components of velocity.

To further compare the RMS values, relative turbu-lence intensity values were calculated by dividing the RMS values by the magnitude of the three-dimensional velocity vector component M for each velocity measure-

ment ( 2 2 2 M u v w= + + ). The relative turbulence in-tensity values varied by location and treatment, but simi-larities for each component were observed. Excluding measurement locations directly behind a dowel, RMSu/M was greater than both RMSv/M and RMSw/M. Including all runs, relative turbulence intensity ranges were 0.11 to 0.54, 0.11 to 0.46, and 0.07 to 0.22 for RMSu/M, RMSv/M, and RMSw/M, respectively. As such, the downstream ve-locity component was the dominant contribution to over-all turbulence; however, lateral and vertical components also substantially contributed. RMSu, RMSv, and RMSw contributed 46% to 54%, 31% to 36%, and 12% to 16% to


the overall TKE magnitude, respectively. For all runs, median τxz and τxy values ranged from 0.04

to 0.23 Pa and from -0.03 to 0.01 Pa, respectively. The me-dian τxy values suggest little variability among treatments, but the interquartile range (IQR) within the treatments had a value of up to 0.15 Pa (run 7, table 2). This result indi-cates increased variability within the treatment with 50% flexible cover, as is discussed in the following sections. Lastly, τxz was on the same order of magnitude as τxy for the short grass treatments (10-2 Pa), and τxz was an order of magnitude greater than τxy for the long grass treatments (ta-ble 2), suggesting greater vertical momentum exchange in the long grass treatments.

The following sections discuss the impacts of discharge, flexible stem length, vegetation arrangement, and vertical

layers on time-averaged velocities, turbulence intensities, turbulent kinetic energy, and Reynolds stresses.

EFFECTS OF DISCHARGE The effect of discharge was significant for time-

averaged velocity components u and w (p < 0.0001) and turbulence characteristics RMSu, RMSv, RMSw, TKE, and τxz (p < 0.0001) (table 3). Time-averaged velocities for the high discharge runs (0.03 m3 s-1) were 25% to 27% greater than those measured at 0.02 m3 s-1. Turbulence intensities, TKE, and τxz measured at the high discharge were 29% to 39%, 56%, and 69% greater than those measured at the low discharge, respectively. While there were differences in magnitude, spatial trends of velocities and turbulence for each vegetation treatment were the same for both levels of

Table 2. Median, range (in italics), and interquartile range (in parentheses) of velocity and turbulence data for each flume run: u is the time-averaged downstream velocity; v is the time-averaged lateral velocity; w is the time-averaged vertical velocity; RMSu, RMSv, and RMSw are

turbulence intensities; TKE is turbulent kinetic energy; and τxz and τxy are Reynolds stresses. Treatments for runs are defined in table 1.

Run u

(cm s-1) v

(cm s-1) w

(cm s-1) RMSu

(cm s-1) RMSv

(cm s-1) RMSw

(cm s-1) TKE

(N m-2) τxz

(Pa) τxy

(Pa) 1 14.2 -0.53 0.29 2.37 1.93 1.21 0.54 0.09 -0.03 12.5 to 16.6 1.09 to 0.21 -0.06 to 0.77 1.87 to 2.74 1.72 to 2.28 1.01 to 1.47 0.38 to 0.71 0.05 to 0.15 -0.13 to 0.11 (2.2) (0.38) (0.36) (0.24) (0.14) (0.11) (0.08) (0.03) (0.09)

2 14.1 -0.51 0.34 2.19 1.84 1.13 0.48 0.06 -0.01 10.9 to 15.7 -1.27 to 1.31 -0.08 to 1.01 1.89 to 3.19 1.61 to 4.16 0.98 to 1.75 0.38 to 1.53 0.03 to 0.11 -0.60 to 0.15 (0.93) (0.44) (0.26) (0.23) (0.17) (0.08) (0.08) (0.02) (0.10)



5 13.1 -0.27 0.29 3.09 2.44 1.51 0.90 0.19 0.00 10.4 to 14.9 -0.97 to 0.22 -0.05 to 0.64 2.54 to 3.83 2.15 to 2.86 1.29 to 1.77 0.69 to 1.30 0.09 to 0.35 -0.12 to 0.13 (0.97) (0.34) (0.26) (0.37) (0.20) (0.13) (0.17) (0.06) (0.09)



8 13.2 -0.29 0.04 2.83 2.32 1.61 0.81 0.19 0.00 4.7 to 14.7 -0.92 to 0.86 -0.98 to 2.51 1.99 to 3.64 1.45 to 4.03 0.64 to 1.99 0.35 to 1.42 -0.02 to 0.38 -0.29 to 0.16 (1.29) (0.58) (0.73) (0.79) (0.39) (0.23) (0.35) (0.12) (0.13)

9 10.8 -0.42 0.25 1.57 1.32 0.79 0.25 0.04 -0.01 9.4 to 11.8 -0.71 to -0.08 -0.03 to 0.66 1.25 to 1.98 1.11 to 1.53 0.66 to 0.92 0.17 to 0.34 0.02 to 0.06 -0.07 to 0.04 (1.07) (0.20) (0.24) (0.15) (0.12) (0.07) (0.04) (0.01) (0.03)







16 10.7 -0.42 0.08 2.20 1.92 1.30 0.53 0.11 -0.01 -0.30 to 11.9 -1.27 to 0.65 -0.71 to 1.97 1.49 to 3.04 1.03 to 3.48 0.34 to 1.64 0.17 to 1.02 -0.03 to 0.25 -0.17 to 0.11 (1.41) (0.39) (0.58) (0.70) (0.46) (0.30) (0.28) (0.09) (0.06)

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discharge. These findings are consistent with McBride et al. (2007), who reported that the pattern and distribution of ve-locities and turbulence did not change with discharge level in a flume study with forested and non-forested riparian vegetation.

EFFECTS OF STEM LENGTH The effect of stem length was significant for all velocity

components ( u , v , and w ; p < 0.0001) and turbulence characteristics (RMSu, RMSv, RMSw, TKE, τxz, and τxy; p < 0.0001) (table 3), likely due to the increase in overall drag. The long grass cross-sectional blockage factor (i.e., B = Av/A, where Av is the area of plants in the cross-section, and A is the area of the cross-section) was 5.5 times the short grass cross-sectional blockage factor for both the 100% and 50% flexible cover vegetation treatments. Increasing drag along the streambank will reduce flow within the bank veg-etation and direct flow into the main channel. As observed by Rominger et al. (2010), this increased drag and associat-ed flow acceleration at the vegetation edge can result in lo-cal scour along the vegetation boundary.

A shear layer likely developed above the submerged, flexible vegetation. For the long grass treatments, it is ex-pected that the shear layer was moved away from the flume bed as compared to the short grass treatments. Therefore, the median downstream velocity for the long grass (11.7 cm s-1) was less than that measured for the short grass (12.6 cm s-1) at the measurement location. Similarly, the median values for all calculated turbulence statistics were greater than those calculated for the short grass treatments. Turbulence intensities, TKE, and τxz for the long grass treatments were 30% to 16%, 70%, and 150% greater than that measured for the short grass treatments, respectively (table 3).

EFFECTS OF VEGETATION ARRANGEMENT (PERCENT FLEXIBLE COVER)

The effect of percent flexible cover was significant for w and v (p ≤ 0.012), as illustrated in figure 2. The addi-tional vegetation/main channel boundaries introduced by

the vegetation patches created complex flow patterns that provide potential habitat benefits. For the treatments with 50% of flexible grass cover, velocity vectors are oriented in the positive z-direction at the dense vegetation patch en-trance (x = 34.2 cm) and in the negative z-direction be-tween patches (x = 7.6 to 26.6 cm, y = 0 to 15.2 cm; figs. 2c, 2d, 2g, and 2h). For treatments with 100% dense vegetation, 1% to 8% of the measurement locations had ve-locity vectors pointed in the negative z-direction; however, 19% to 23% and 44% to 46% of the velocity vectors were pointed in the negative z-direction for the 50% short grass and 50% long grass treatments, respectively. This flow het-erogeneity was also illustrated by the dispersion of the data.

Table 3. Median values reported by factor (discharge, flexible stem length, percent flexible cover, and herbaceous plantings); p-values for each pair are reported in parentheses.

Discharge

Flexible Stem Length

Percent Flexible Cover

Herbaceous Plantings

0.03 m3 s-1 0.02 m3 s-1 3.9 cm 0.7 cm 100% 50% Plantings None

u (cm s-1) 13.7 10.8 11.7 12.6 12.3 12.0 10.1 10.3 (<0.0001) (<0.0001) (0.9387) (0.5377)

v (cm s-1) -0.39 -0.39 -0.31 -0.45 -0.40 -0.38 -0.39 -0.37 (0.7775) (<0.0001) (0.0012) (0.7765)

w (cm s-1) 0.30 0.24 0.20 0.32 0.32 0.21 0.28 0.25 (<0.0001) (<0.0001) (<0.0001) (0.0126)

RMSu (cm s-1) 2.56 1.99 2.68 2.00 2.33 2.29 2.23 2.41

(<0.0001) (<0.0001) (0.1413) (<0.0001)

RMSv (cm s-1) 2.13 1.65 2.19 1.68 1.90 1.90 1.84 1.97

(<0.0001) (<0.0001) (0.3404) (0.0004)

RMSw (cm s-1) 1.39 1.07 1.44 1.06 1.19 1.28 1.19 1.27

(<0.0001) (<0.0001) (<0.0001) (0.0067)

TKE (N m-2) 0.65 0.41 0.70 0.41 0.77 0.68 0.49 0.58

(<0.0001) (<0.0001) (0.8452) (0.0001)

τxz (Pa) 0.131 0.078 0.158 0.064 0.097 0.105 0.092 0.113

(<0.0001) (<0.0001) (0.0005) (<0.0001)

τxy (Pa) -0.007 -0.009 0.002 -0.015 -0.010 -0.008 -0.010 -0.007

(0.8133) (<0.0001) (0.5040) (0.3397)

Figure 2. Three-dimensional velocity vector plots showing mixing in patchy vegetation and outward exchange with the fully covered treatments. The main flow direction is in the positive x-direction. Plots only show the 0.03 m3 s-1 results; trends were similar for the low-discharge experiments (S = short grass, L = long grass, and d = dowel; exaggerated vertical scale).


The IQR for the vertical velocity component was at least 20% larger in patchy treatments (50% flexible cover) as compared to treatments with 100% flexible cover (table 2).

Due to the hydraulic resistance introduced by the vege-tation, flow was directed over the vegetation patch (x = 32.4 cm; figs. 2c, 2d, 2g, and 2h). However, downstream of the measurement region (x > 40 cm), it is expected that ver-tical mass and momentum exchange occurred as flow moved over the patch. Assuming there is sufficient re-sistance to generate an inflection point in the velocity pro-file, a shear layer will form due to Kelvin-Helmholtz insta-bility (Ghisalberti and Nepf, 2002; Poggi et al., 2004), and vortices will dominate vertical momentum exchange (Ghi-salberti and Nepf, 2005).

Two turbulence parameters, RMSw and τxz, were deter-mined to be statistically significant for the effect of percent flexible cover (p ≤ 0.0005; table 3). Figures 3a to 3h illus-trates the distribution of τxz in the measurement region for the high-discharge runs; trends for the low-discharge meas-urements were similar in distribution and approximately 40% less in magnitude. Especially apparent for the long grass, treatments with 50% flexible vegetation cover (both with and without dowels) resulted in a localized area of re-

duced τxz (≈0.1 Pa; x > 20 cm, y > 15 cm; figs. 3g and 3h) where the vegetation was omitted for the full longitudinal length of the measurement region. Maximum τxz values were located between long grass vegetation patches (y < 15 cm, x < 30 cm) and were at least double those measured in the area where the vegetation was omitted for the full longitudinal length of the measurement region. The maxi-mum τxz measured between the vegetation patches was 3.4 times the median τxz value.

Additionally, treatments with 50% long grass (runs 7, 8, 15, and 16) resulted in the greatest median τxz and IQR at both discharges; the IQR values were more than double those of all other treatments at each discharge (table 2). This result indicates greater variability in the Reynolds stresses for the 50% long grass treatments due to the addi-tional boundaries introduced to the patches of vegetation (i.e., vegetation/main channel edges). However, the τxz val-ues were of relatively low magnitude, ranging from -0.03 to 0.26 Pa and from -0.02 to 0.45 Pa for the low and high dis-charge runs, respectively.

Fluvial entrainment, typically quantified by the excess shear stress model, results when the applied shear stress ex-

Figure 3. (a-h) Distributions of the Reynolds stress tensor τxz (Pa) in the measurement region, and (i-p) distributions of the Reynolds stress ten-sor τxy (Pa) in the measurement region (S = short grass, L = long grass, and d = dowel). Only the high-discharge runs (0.03 m3 s-1) of each treat-ment are shown, as the trends were similar. The diamond-shaped areas with no data were due to dowel interference with the ADV probe.

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ceeds the critical shear stress of the soil (Partheniades, 1965). The applied shear stress is often represented by near-boundary Reynolds stresses (Nepf, 2012); therefore, the elevated τxz values in this study indicate potential local-ized erosion on the edges of the vegetation patches. This result supports previous observations of erosion along the edge of vegetation patches in tidal flats (Temmerman et al., 2007) and along the edge of a vegetated, sandy point bar (Rominger et al., 2010). This localized erosion may be ben-eficial by creating complex bank geometry where biota can hide from predators and locate low-velocity refugia. Bank erosion also leads to riparian vegetation succession, essen-tial for riparian diversity (Florsheim et al., 2008).

While there was no significant difference in TKE when defined by percent cover, the spatial distribution indicated trends. For the high-discharge runs, contour plots indicate a location of increased TKE along the edges of vegetation patches, which was most apparent for the long grass treat-ments (TKE > 1 N m-2; figs. 4g and 4h). Maximum TKE measured between long grass vegetation patches (y < 15 cm, x < 30 cm) was 1.6 times the median TKE value for treat-ments with dowels and 1.5 times the median TKE value for treatments without dowels. This flow heterogeneity was also supported by the IQR values. For these treatments (runs 7

and 8), the IQR was more than double the IQR for all other treatments at the high discharge. The IQR for the 50% long grass treatments were 53% greater than for all other treat-ments at the low discharge (table 2).

EFFECTS OF VERTICAL LAYERS The effect of two vegetation layers (with or without

dowels) was significant for all turbulence intensities: RMSu, RMSv, and RMSw (p ≤ 0.0067). For treatments with dowels, median turbulence intensities were 2.23, 1.84, and 1.19 cm s-1 for RMSu, RMSv, and RMSw, respectively. Without multiple layers, median turbulence intensities val-ues were 2.41, 1.97, and 1.27 cm s-1 for RMSu, RMSv, and RMSw, respectively. With two layers of vegetation (i.e., long grass with dowel treatments), the dowels likely reduced the velocity of the flow above the grass. This de-creasing momentum exchange resulted in the decrease in RMSu for the treatments with dowels compared to treat-ments without dowels.

The effect of two vegetation layers was also significant for TKE (p < 0.0001). Median TKE values decreased with the presence of dowels (table 3); however, localized areas of elevated TKE (TKE > 1 N m-2) were present within a downstream distance of 7.5 cm from the dowels (figs. 4b, 4d, 4f, and 4h). While the dense, flexible vegetation likely produced turbulence in the form of a shear layer over the vegetation, the turbulence produced by the dowels was due to vortex shedding from individual dowels. This dif-ference in turbulence generation is illustrated in figure 4. Elevated TKE values were located downstream of the dowels because the TKE calculation incorporates all ve-locity components (eq. 4). The vortex shedding due to the dowels occurred in the lateral direction, making the veloc-ity component (v) an important component to define tur-bulence at this location. The effect of multiple layers was not statistically significant for τxy; however, spatial pat-terns of τxy similar to TKE were observed (figs. 3j, 3l, 3n, and 3p), and the τxy values with the greatest absolute magnitude (-0.29 to -0.60 Pa) were located directly down-stream of the dowels.

While a local area of elevated mean turbulence was pre-sent in the dowel treatments, the addition of the dowels did not result in an overall increase in TKE or Reynolds stress-es (table 3). These results suggest that the presence of sparse vegetation promoted lateral momentum exchange while not significantly contributing overall instability.

CONCLUSIONS Mixed planting methods are often suggested compo-

nents of stream restoration designs and result in varied pat-terns of riparian vegetation. This research was intended to improve our understanding of flow complexity resulting from these planting methods. Vegetation patches composed of dense, flexible vegetation created boundaries where the edge of the vegetation met the main channel. These addi-tional edges resulted in complex flow patterns and velocity gradients, as well as heterogeneous distribution of turbu-lence statistics. Elevated near-bed Reynolds stresses locat-

Figure 4. Distribution of turbulent kinetic energy (TKE, N m-2) in the measurement region of each treatment for the high-discharge flumeruns; trends were similar in the low-discharge treatments (S = short grass, L = long grass, and d = dowel). The diamond-shaped areas withno data were due to dowel interference with the ADV probe.


ed at the vegetation patch edges, mainly for the long grass treatments, indicate a potential for localized particle en-trainment, consistent with previous observations of erosion along vegetation edges. The presence of dowels also con-tributed to flow complexity while not significantly elevat-ing overall turbulence. Elevated TKE and Reynolds stresses only occurred in close proximity to the dowels. Therefore, sparsely planted herbaceous tubules will contribute to lat-eral mass and momentum exchange. Dense grass patches at varying stem heights in combination with sparsely planted herbaceous tubelings will likely provide the benefits of flow complexity while minimizing the vegetation/main channel edges that are susceptible to particle entrainment.

ACKNOWLEDGEMENTS Support for this work was provided by the STC program

of the National Science Foundation via the National Center for Earth-Surface Dynamics under the agreement number EAR-0120914 and by the National Science Foundation's ADVANCE IT Program under Award HRD-1007978. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foun-dation. The author would like to thank Dr. Theresa Thomp-son of the Department of Biological Systems Engineering at Virginia Tech for information about the Stroubles Creek restoration.

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CHARACTERISTICS OF NEAR-BOUNDARY FLOW THROUGH TWO RIPARIAN VEGETATION TYPES A FLUME … ·...

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