Laboratory investigation of hydraulic performance of silt screens

9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China

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2010, 22(5), supplement :312-317 DOI: 10.1016/S1001-6058(09)60212-2

Laboratory investigation of hydraulic performance of silt screens

Thu Trang Vu *1 , Soon Keat Tan 2 1School of Civil and Environmental Engineering, Nanyang Technological University (NTU)

2DHI-NTU Centre, Nanyang Environment and Water Research Institute, NTU Singapore 639798

* E-mail: [email protected]

ABSTRACT: Silt screen is a type of flexible barrier that has been widely deployed for the purpose of sediment containment in an aquatic environment. Yet, its working mechanism is still largely unknown. In this study, the authors presented the findings of an investigation of the hydraulic performance of silt screen using Particles Imaging Velocimetry (PIV) technique. The experiments were conducted in a 30 cm-wide flume, with a silt screen spanned across the whole width, normal to the flow. Three average flow velocities, U0 of 0.5 m/s, 0.1 m/s, and 0.2 m/s and different values of the penetration ratio (the ration between screen’s penetration depth and flow depth) were simulated in the experiments. PIV images of the flow approaching the screen and that at the lee side of the silt screen were captured in the vertical mid-plane of the flume. The images were then processed, and analyzed to determine the velocity distribution, turbulence characteristics and streamlines patterns of the flow in the vicinity of the silt screen. Preliminary results showed that underflow through the gap between silt screen’s lower end and the flume’s bottom could be large. The experimental results also suggested a distinct change of flow patterns with the formation of recirculation areas at both sides of the screen. This paper attempts to elucidate the hydraulic performance of silt-screen in response to the magnitude of the environmental flow and different scenarios of screen configurations. KEY WORDS: silt screen; Particles Imaging Velocimetry; recirculation; flow diversion; sediment containment, silt screen deformation. 1 INTRODUCTION The spread of sediment plumes during dredging is usually mitigated with the deployment of silt screen, a flexible woven structure of polymeric fibers. At the dredging sites, the screen is often deployed vertically and laid out in three typical configurations as depicted in Fig. 1. According to silt screen manufacturers and literature on silt screens, the performance of a silt screen in controlling sediment movement is primarily

determined by the interactions between silt screens and the flow – hence the term “hydraulics performance” of silt screens.

Fig. 1 Typical layouts of silt screen deployment at dredging sites: (1) closed configuration, (2) maze configuration, and (3) open configuration Structurally, a silt screen is made up of multi-filament fiber threads stretching in both warp and weft directions. The presence of such screen-like structure was reported to affect flow/hydraulic parameters such as pressure drop, velocity distribution and turbulence structure [1-5]. The screen properties such as rigidity, inclination and porosity are important factors in determining the influence of the screens on flow characteristics. On the other hand, the configuration of a silt screen in water and the silt screen’s low porosity suggest that eddies and/or recirculation will be formed in the immediate vicinity – such as those being


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observed when an oil boom is deployed in the sea [6-8]. To date, there is no rigorous investigation on the characteristics of the flow in the vicinity of a silt screen. Although literature briefly mentioned the effects of silt screens on velocity distribution and turbulence[9-12], the reported information are qualitative. The objective of this paper is to present the flow characteristics obtained from the investigation on the flow in the vicinity of a silt screen and hence establish a method of estimating the hydraulic performance of the screen. 2 EXPERIMENTS The effects of silt screens on flow were investigated experimentally in an open channel flume (5.0 m (L) x 0.6 m (H) x 0.3 m (W)). The experiments were conducted with a silt screen which spanned the whole channel’s width. The screen sample was kept submerged using a 1 kg weight attached at the lower edge of the screen. The properties of the selected silt screen, as obtained from the manufacturer, are included in Table 1. Table 1. Properties of the selected silt screen

Material Polyester

Weight >600 g/m2

Thickness >1.2 mm

Mean wide width tensile strength-wet condition >150 kN/m

Mean tensile tension at maximum load – wet condition

<18%

Shrinkage in seawater <2%

Seawater permeability <7.5x10-3 cm/s

UV Resistance (obtained after 60 days of exposure to tropical sunlight)

70%

The objectives of the experiments were to identify the flow patterns in the vicinity of the silt screen, and to investigate the effects of flow velocity and the silt screen’s configuration on the resulting flow. The configuration or layout of the silt screen was

defined using two parameters: (1) screen penetration ratio and (2) screen deflection angle. The screen penetration ratio (r) is calculated as the ratio of the original length of the silt screen (l0) to water depth (H0). As the silt screen might be deflected by the flow, the resulting effective screen penetration ratio (re) is thus defined as the ratio of the projected screen’s length along the vertical direction (le) to the water depth H0. The other parameter, screen deflection angle, refers to the angle that is formed between the screen and the vertical plane. The experiments were conducted using three average (ambient) velocity (0.05 m/s; 0.1 m/s and 0.2 m/s). During the experiments, the penetration depth of the silt screen was varied. For each ambient flow velocity, the experiments were repeated with various screen penetration ratio r of 0.50, 0.75 and 1. In addition, two anchor conditions were included to study the interactions between flow and the screen’s deflection. The anchor conditions tested were: (1) when the lower end of the silt screen sample was held to the flume bottom – forming a deflection angle of 0˚, and (2) when the lower end of the silt screen sample was free to move. The latter usually resulted in a deflected silt screen. The flow characteristics were investigated using the technique of Particles Imaging Velocimetry (PIV). The experimental setup is presented schematically in Fig. 2. Images of the flow field at mid-plane of the flume were captured using a 12-bit charged-couple device (CCD) camera with a resolution of 1600x1200 pixels and a frame rate of 15 Hz. The viewing area was set at 300 mm × 250 mm.

Fig. 2 Laboratory setup to investigate the hydraulics of silt

screens In this study, the flow field along the longitudinal direction, from -40 cm (upstream of the silt screen’s location) to 40 cm downstream was captured using the PIV system which was mounted on a traversing unit.


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Due to the design of the flume, the flow in the region of 4.5 cm from the bottom of the flume could not be captured by the camera. For each scenario, the time-average flow field was obtained by the ensemble averaging of 420 images captured. Based on the time-average flow field, the flow patterns in the vicinity of the silt screen were established. 3 RESULTS The data obtained from the experiments were analyzed and presented in the x-z coordinates system described in Fig. 3 below.

Fig. 3 Coordinate system employed in analysis For each experimental data set, time-averaged velocity profiles were extracted at five locations in the test section. These five locations cover locations upstream of the silt screen (at x/H0=-1 and x/H0=-0.5); the point at x/H0=0, which is located right in front of the silt screen in the stream-wise direction; and two locations downstream of the silt screen (at x/H0=0.5 and x/H0=1, respectively). The typical velocity profiles obtained are presented in Fig. 4. As can be seen in Fig. 4, the flow is from right to left and the presence of the silt screen significantly changes the velocity profile of the flow in its vicinity. The effect is intensified close to and upstream of the silt screen, as can be observed in Figs. 4 (a), (b), and (c). In particular, as shown in Fig. 4 (a) and (b), the flow at the surface layer is retarded and the zone of retardation extends to deeper water. Right in front of the silt screen, the velocity profile depicts two distinct regions: (1) a region of retarded flow at the upper layer of the water column, and (2) a more rapid flow at the lower layer of the water column. The feature of the rapid flow at the lower water layer remains at location downstream of the silt screen, as observed in Fig. 4 (d) and (e). Further downstream of the screen, the diverted flow gradually transforms into a jet-like flow as seen in Fig. 4 (e).

Fig. 4 Typical velocity profiles at different locations along the test section: (a) at x/H0=-1, (b) at x/H0=-0.5, (c) at x/H0=0, (d) at x/H0=0.5, and (e) at x/H0=1

3.1 Characteristics of the flow in the upper water

layer The time-averaged streamlines distribution suggested the presence of recirculation zones in the upper layer at both sides of the silt screen. Upstream of the silt screen, depending on screen configuration and average flow velocity, a recirculation zone could be observed at the upper water layer. For example, Fig. 5 suggested the dependence of the existence of the


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recirculation zone on screen deflection angle. A recirculation zone was observed when the screen was anchored to the floor (the screen deflection angle was 0˚). However, no recirculation zone was observed in the time-averaged streamlines distribution of an unanchored screen (which deflected at an angle of 19.5˚). Such effect of deflection angle on the presence of recirculation zones is probably due to the role of screen deflection angle which in turns influences the longitudinal pressure distribution. It is plausible that the deflected screen provides a favorable pressure gradient for an incoming “streamline” to deflect or guide the flow towards the lower layer instead of forming a re-circulated flow. Consequently, a recirculation zone was not observed in the time-averaged flow patterns in the case when the screen was deflected.

Fig. 5 Streamlines distribution from the time-average velocity

field obtained from experiments with environmental velocity of 0.2 m/s, screen penetration ratio of 0.75, when the screen was tied vertically (a) and deflected (b)

The instantaneous snapshots of the flow patterns clearly showed the periodic characteristics. Fig. 6 shows that a complete cycle of the development of the recirculation zone may consist of four phases:

(1) Phase 1: The evolution and growth of a primary recirculation zone, which arises from an eddy formed as the flow encounter an obstacle;

(2) Phase 2: The decay of the first eddy formation and cascade into secondary eddies in the recirculation zone

(3) Phase 3: Development of the eddies into secondary recirculation zone, and

(4) Phase 4: Collapse of the secondary eddies and the regeneration of the primary eddy formation, marking the beginning of another cycle.

To assess the strength of the upstream recirculation zone, a parameter named recirculation ratio is proposed. In this study, the recirculation ratio was calculated as the ratio of the largest backward velocity observed in the recirculation zone to the incident velocity. As can be seen in Fig. 7, the recirculation ratio is inversely proportional to the magnitude of the incident velocity. A lower value of recirculation ratio

corresponds to the flow with a higher incident velocity. This phenomenon suggested the weakening of the recirculation zone with increasing environmental flow.

Fig. 6 Instantaneous snapshots of streamlines distribution at

time (a) t=16.93 s, (b) t=18.2 s, (c) at t=19.13 s, (d) at t=21.87 s

Fig. 7 Variation of recirculation ratios at different incident

velocity The authors noted a large recirculation zone in the water area in the lee of the silt screen, so called downstream recirculation zone, in both the time-averaged streamlines distribution and the velocity profiles obtained. As can be seen in Fig. 8, the instantaneous flow patterns obtained from PIV measurement suggest that the downstream flow


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patterns consist of two recirculation zones of opposite directions: anti-clockwise at the upper layer and clockwise at the lower layer.

Fig. 8 The flow images of the downstream recirculation zones

(a) at t=17.33 s, (b) at t=17.53 s, (c) at t=18 s, and (d) at t=18.33 s.

Both recirculation zones are dynamic and their locations and intensity vary with time, as can be deduced from the flow patterns shown in Fig. 8. 3.2 Characteristics of the flow in the lower layer of

the water column The velocity profiles presented in Fig. 4 also depicted a region of strong flow at the lower layer of the water column in the vicinity of the silt screen. The flow velocity in this region may be 3-7 times as large as that of the incident velocity. The largest normalized velocity exhibited a quasi-linear relationship to the effective screen penetration ratio, as shown in Fig. 9. The higher the screen penetration ratio is, the larger the area that is “blocked” by the screen. As a result, the flow is diverted and channeled through the smaller “unblocked” area, leading to a higher velocity observed in this region.

Fig. 9: Largest normalized gap velocity vs. effective screen

peneration ratio 4 CONCLUSION The results obtained from the experiments showed that the deployment of a silt screen significantly affects the flow distribution. In the vicinity of the silt screen, the velocity profile formed two distinct regions: (1) an upper layer of retarded flow, and (2) a jet-like flow at the lower layer of the water column. The flow patterns and the velocity distribution in the two layers were found to be dependent on the incident velocity and the configuration of the screen. The authors also noted the significance of the two regions in determining the movement of sediment in the vicinity of the screen. The recirculation zones at the upper water layer suggested possible enhancement of sediment trapping in the zone. However, the magnitude of the jet-like flow in the lower layer raised concerns over the possibility of sediment entrainment, escape and dispersion of sediment in this region. Further study is necessary to develop the techniques and methodology to quantify the role of the above mentioned flow patterns in the containment of sediment using silt screen. ACKNOWLEDGEMENTS This study was conducted with the technical assistance from Dr. Wang Xikun (Maritime Research Centre, Nanyang Technological University). Financial support of NRF (EWT) scholarship and DHI-NTU Centre (Project No. 34) are duly acknowledged. REFERENCES [1] Bailey B J, Montero J I, Parra J P, et al. Airflow Resistance

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Laboratory investigation of hydraulic performance of silt screens