AN EXPERIMENTAL STUDY OF SUSPENDED SEDIMENT TRANSPORT IN A VEGETATED CHANNEL

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1 Full Paper th ISE 206, Melbourne, Australia AN EXPERIMENTAL STUDY OF SUSPENDED SEDIMENT TRANSPORT IN A VEGETATED CHANNEL YING SHI Department of Hydraulic Engineering, Tsinghua University Beijing, 00084, China CHUNBO JIANG Department of Hydraulic Engineering, Tsinghua University Beijing, 00084, China HEIDI NEPF Department of Civil and Environmental Engineering, Massachusetts Institute of Technology Cambridge, Massachusetts, USA The vegetation patch serves as an ecosystem engineer in natural systems mainly through its impact on the sediment transport. Previous studies show that both sedimentation and erosion can occur surrounding the vegetation patch. This study aims at exploring the space heterogeneity in deposition created by vegetation patch. By changing flow velocity and sediment particles, different sets of flow and deposition patterns have been observed. Though varied in velocities, all flow conditions show consistent flow characteristics with low velocity in the wake and high in the adjacent. The deposition in these two regions has three patterns in total: high deposition in both the wake and the adjacent zones, low deposition in both the wake and the adjacent zones and high deposition in the wake while low deposition in the adjacent regions. INTRODUCTION Aquatic plants serve as the engineers in natural river systems. By taking roots into the earth, they can provide extra drag thus stabilize the bed [Bouma et al., 2007; Rominger et al., 200]. By taking up heavy metals and surplus nutrients, they can improve water quality [Schulz et al., 2003; Cotton et al., 2006]. Through the exchange with water, they can alter the local flow field and the fate of particle[jones et al., 202; Meire et al., 204]. Generally, vegetation patch is understood to reduce flow and enhance deposition [Cotton et al., 2006; Tanaka and Yagisawa, 200]. For example, Chen et al. [202] observed enhanced deposition in the steady wake region behind a porous obstruction where both velocity and turbulent levels are low. Yet, diminished deposition has also been observed. For example, flow deflection at the edge of a patch creates enhanced flow thus diminished deposition [Bouma et al., 2007; Rominger et al., 200]. Ortiz et al. [203] contribute low or high deposition to the levels of turbulence. They think enhanced deposition only occur when both turbulence and velocity are low while low deposition occurs when turbulent levels are high regardless of local velocity. Thus the vegetation patch can cast an influence of space heterogeneity in deposition. Producing different sets of flow conditions and particles, this laboratory work aims at exploring how vegetation patch influence the behavior of suspended particles and how the space heterogeneity in deposition affected by the patch. 2 EXPERIMENT SETUP The experiments were conducted in a 6 m long,.2 m wide recirculating flume with a test section 3 m long, and.2 m wide. A circular patch of model vegetation was constructed in the center of the flume using rigid wooden circular cylinders. The patch diameter (D) was 20 cm and the cylinder diameter (d) was 6.4 mm. The patch density, described by the frontal area per volume, a nd, was 25 m -, where n is the number of stems per bed area. For an emergent patch, the magnitude of flow diversion and the strength of the lateral shear layer were set by the non-dimensional flow blockage (ad). In this case, ad 5. corresponding to high flow blockage [Rominger and Nepf, 20; Chen et al., 202]. The solid volume fraction within the patch was ad / 4 0.3, which is representative of density found in real aquatic vegetation [Nepf, 202].

2 Figure. Experiment set up. (a) top view; (b) front view. Modeled vegetation patch was shown as grey circle. Placement of glass slides were shown as squares. x=0 was at the upstream edge of the patch, y=0 was at the centerline of the patch and flume. Figure not to scale. Given that we focused on emergent vegetation, the flow condition can be simplified as two-dimensional, with flow diversion and dominant shear layers in the horizontal plane [Chen et al., 202; Follett and Nepf, 202]. The coordinate system was defined as x in the streamwise direction, with x 0 at the leading edge of the patch, y in the lateral direction, with y 0 at the center of the patch and flume, and z in the vertical direction with z 0 at the bed of flume. Velocity measurements were taken from 5 D upstream of the patch to 30 D downstream of the patch using a Nortek Vectrino (acoustic Doppler velocimeter, ADV). At each position, the velocity was measured at mid-depth for 240 s with a sample rate of 25 Hz. In previous studies in the same flume, measurements at mid-depth were shown to be within 5% of the depth-averaged velocity [White and Nepf, 2007; Chen et al., 202; Ortiz et al., 203]. Three components of instantaneous velocity ( u, v, w ) were collected at each point, with u the velocity in the streamwise direction, v the lateral velocity and w the vertical velocity. Each velocity record was decomposed into time-averaged components ( u, v, w ) and fluctuating components ' ' ' ( u, v, w ). The average velocity upstream of the patch was defined as the reference velocity U. Three 0 different flow conditions were considered with the reference velocities U cm/s, U cm/s, 02 U cm/s.the water depth at the upstream end for these three cases was cm, cm 03 and cm, respectively. Net deposition was measured using glass slides (2.5 cm 2.5 cm) placed on the bed of the flume. The clean and dry glass slides were weighed before placement in the flume. The placement was arranged based on the flow characteristics with one slide ahead of the patch, one in the center of the patch, four adjacent to the patch, three rows of slides behind the patch and one column through the centerline of the flume (seen in Figure ). The desired flow conditions were set by adjusting the pump rate and the height of the weir gate. 600 g of sediment was first mixed with water in a small container, and the mixture was added to the tail box of the flume. Once mixed, the flume was run for 4 hours, after which the pump was slowly stopped and the weir was lifted by 4 cm off the bed to slowly drain the water. The slides were left to air dry for at least 2 days. Afterward, the slides were placed in an oven at 50 C for 4h to remove the moisture and then reweighed. The net deposition at each point was calculated by the difference in weight before and after the experiment, divided by the area of the glass slide. 3 RESULTS 3. Flow characteristics

3 Figure 2. Velocity distribution. (a) Steamwise velocity profile along the centerline of flume and patch for different flow conditions. Patch is located between the solid vertical line at x 0 and the dashed vertical line at x D; (b) Lateral profile of streamwise velocity at x.25d and x 4D along for all three flow conditions. High flow resistance within the patch caused upstream flow to divert laterally away from the patch, towards the adjacent area, resulting in higher streamwise velocity ( U2 U0 in the adjacent area and lower streamwise velocity ( U U0)in the area directly behind the patch, which has been called the steady wake region [Zong and Nepf, 20; Chen et al., 202; Follett and Nepf, 202]. Since the experiment was designed to compare the net deposition within the regions of diminished velocity in the wake and the region of enhanced velocity adjacent to the patch, we used the measured velocity field to define a wake zone and an adjacent zone, which could be used consistently across all cases. The streamwise velocity ( U )within the steady wake region, remains constant over a length scale of L. Since U is a function of solid volume fraction ( ) only [Zong and Nepf, 20], all three cases should have the same length scale of L which was consistent with our velocity measurements (Figure 2a). The measured velocity profiles indicated that L 3D, extending from x D to x 4D. L was used to define the length of the wake zone. The width of the wake zone was defined as the region within which the velocity was laterally uniform at U. As shown in the lateral velocity profile (Figure 2b), the low velocity ( U ) extended to y 0.5D, which was the edge of the patch, then quickly increased to a higher velocity ( U 2 ) at y D So the wake zone was defined as the region D L ( x: D to 4D, y: 0.5D to 0.5D ). The adjacent zone was defined as the region of elevated velocity to the side of the patch, which began at the leading edge of the patch and extended to the end of the wake, i.e., x 0 to x 4D. The velocity was elevated above U 2 over the lateral distance y 0.5D, and to simplify the comparison, we chose the adjacent region to have a width similar to the wake, i.e. width D. This choice was supported by a recent simulation, which showed that the region of elevated velocity adjacent to the patch extended lateral over a distance comparable to the wake width ( Lima et al., 205).Thus the width of the adjacent region was defined from y 0.5D to y 0.5D.

4 3.2 Deposition patterns Figure 3. Three representative deposition patterns across all cases with varied upcoming streamwise velocity, particle size and density. (a) 2.5 g/cm 3, 24 m, U 4.9 cm/s; (b).02 g/c m 3, 45 m, U2 9.6 cm/s; (c) 2.5 g/c m 3, 0 m, U3 9 cm/s. Basically, there are three deposition patterns across all running cases as shown in Figure 3. The first pattern shown in Figure 3a is the deposition of particle (2.5 g/cm 3, 24 m ) at the lowest velocity (5 cm/s). In this case, we have the highest deposition (20-38 g/m 2 ) and it s constantly high all over the channel. This suggests that resuspension is inactive and high deposition occurs everywhere in the channel. The opposite pattern, namely, pattern 2 displayed in Figure 3b has little deposition (0-8 g/m 2 ) across the whole channel. This corresponds to the case of the lightest particle (.02 g/c m 3, 45 m ) at 9.6 cm/s which suggests that resuspension is active everywhere and makes it uniform low deposition throughout the channel. Another pattern, pattern 3 presented in Figure 3c, is the case of a medium particle (2.5 g/c m 3, 0 m ) at 9 cm/s. In this pattern, a wider variety change in deposition occurs, i.e. the deposition in the adjacent zones is as low as that observed in pattern 2 while deposition in the wake zones can reach as high as that in pattern. This implies that resuspension is active in the adjacent areas but inactive in the wake areas. Previous works also observe the similar phenomenon as in pattern

5 3 that enhanced deposition occurs in the wake downstream of the patch and diminished deposition at the lateral edge of the patch [e.g., Cotton et al., 2006; Bouma et al., 2007; Follett and Nepf, 202]. Previous studies show that vegetation favors in growing in the wake where lots of organic matters deposited, our results supplemented the idea that whether there is positive feedback between vegetation patch and deposition patterns only falls into a limited range of velocities for a given particle. For example, if the velocity is too high, even if there are enough particles available, no pattern is going to be seen because resuspensition is active everywhere and there is little deposition at all. While if the velocity is too low, there is no big difference in deposition between the adjacent zones and the wake zones, thus plants will not strongly prefer to grow in the wake zones either. Only when the velocity falls into the range that the adjacent shear velocity is above the critical shear velocity and the wake shear velocity is below the critical shear velocity will the wake zones created by the vegetation patch be an ideally perfect growing choice for plantation. 4 CONCLUSIONS Flow around a circular vegetation patch creates a stable low flow velocity region which we call the wake zones and high flow velocity region which we call the adjacent zones. The vegetation patch has a positive feedback on the deposition patterns only happens when a limited range of flowing velocity and particle sizes. When velocity is too low, everywhere in the channel has a shear velocity below the particle s critical shear velocity, and resuspension is inactive everywhere thus high deposition is everywhere. While velocity is too high, the opposite trend is happening because resuspension is active across the whole channel and low deposition left in the channel. The supremely positive feedback appears when the critical shear velocity of the particle falls between the shear velocity of the wake and adjacent zones that the flow condition creating the wake perfect zones for vegetation to grow up. 5 ACKNOWLEDGMENTS The first author was supported by funds from the National Natural Science Foundation of China ( ) and the International Cooperation Project of the National Natural Science Foundation of China ( ). Funding for the experiments was provided by NSF EAR REFERENCES [] Bouma T. J., van Duren L. A., Temmerman S., Claverie T., Blanco-Garcia A., Ysebaert T. and Herman P. M. J., Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments, Cont. Shelf Res., Vol. 27, (2007), pp [2] Chen Z., Ortiz A. C., Zong L. and Nepf H. M., The wake structure behind a porous obstruction and its implications for deposition near a finite patch of emergent vegetation, Water Resour. Res., Vol. 48, (202), pp W0957. [3] Cotton J. A., Wharton G., Bass J. A. B., Heppell C. M. and Wotton R. S., The effects of seasonal changes to in-stream vegetation cover on patterns of flow and accumulation of sediment, Geomorphology, Vol. 77, (2006), pp [4] De Lima P. H. S., Janzen J. G. and Nepf H. M., Flow patterns around two neighboring patches of emergent vegetation and possible implications for deposition and vegetation growth, Environ. Fluid Mech., Vol. 5, No., (205), pp -8. [5] Jones J. I., Collins A. L., Naden P. S. and Sear D. A., The relationship between fine sediment and macrophytes in rivers, River Res. Appl., Vol. 28, No. 7, (202), pp [6] Meire D. W. S. A., Kondziolka J. M. and Nepf H. M., Interaction between neighboring vegetation patches: Impact on flow and deposition, Water Resour. Res., Vol. 50, No. 5, (204), pp [7] Ortiz A. C., Ashton A. and Nepf H. M., Mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition, J. Geophys. Res. Earth Surf. Vol. 8, No. 4, (203), pp [8] Rominger J. T., Lightbody A. F. and Nepf H. M., Effects of added vegetation on sand bar stability and stream hydrodynamics, J. Hydraul. Eng., Vol. 36, No. 2, (200), pp [9] Schulz M., Kozerski H. P., Pluntke T. and Rinke K., The influence of macrophytes on sedimentation and nutrient retention in the lower River Spree (Germany), Water Res., Vol. 37, (2003), pp

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