Obaidullah safieDepartment ofCivil Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-kuNagoya, 466-8555,Japan Akihiro tominagaDepartment ofCivil Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku Nagoya, 466-8555,Japan Impermeable and permeable spur dikes are used toprotect banks of an open channel.

Impermeable spur dike suffers from structuralinstability due to local scour; whereas, high permeable spur dike cannot protectthe bank sufficiently. In this study, pile-group dikes are investigated inorder to reduce the velocity behind structure for the purpose of bankprotection and improving aquatic habitat with expected reduced local scouraround structure. An experimental study on the flow characteristics around pile-groupdike structures is conducted using particle image velocimetry (PIV) method. Pile-groups were arranged with in-line andstaggered arrays. Quantitative analysis on flow field around impermeable spurdike and various permeability pile-group dikes are considered.

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The resultsindicate that staggered arrangement pile-group dike demonstrated efficient significancethan impermeable and in-line arrangement dikes.  1          IntroductionFor protecting the river bank from erosion, several common measures such asuse of spur dikes (or groins), revetments, riprap structures, concrete armor,gabion mattress, etc., are presented as a result of years of practice and research1. Spur dikes (or groins) are hydraulic structures constructed projectingfrom a bank of the channel into the current.

They redirect the flow in adesired direction, reduce the flow velocity along the riverbanks, and createrecirculation zones downstream of the structure providing a favorable aquatichabitat 2. Spur dikes are classified into impermeable andpermeable types according to the flow penetration through it. Various spur diketypes produce various flow characteristics and patterns around structure and inthe mainstream.

Selection of a specific type of spur dike depends on thepurpose of its construction. If a dike is mainly applied for navigationpurposes, the mainstream velocity to secure appropriate depth is considered.However, if a dike is aimed at protecting a bank or improving the stream environment,flow velocity reduction behind the structure and the recirculation zonesaffected by flow separation become important. Therefore, their analysis isnecessary to select the appropriate type of spur dike for a specific purpose.Kang et al. 2 studied flowpatterns and characteristics of impermeable, permeable pile of single row, andtriangular shape groins.

It is stated for impermeable groin, an increasedvelocity at the groin tip and in the mainstream; and a broad spectrum ofrecirculation zones were observed which have high effect on the local scour atthe groin tip and the bed changes of the main channel. While, for permeablepile groins, decreased velocity at the groin tip and strength of the vortex isreported which has the advantages of excellent stability and relatively easy maintenance2.In addition, pile-groups have been alsostudied to protect other structures along the bank of a river, e.g. the bridge abutments,irrigation intakes, or an impermeable spur dike 1.

Installationof a pile group at the upstream adjacent of the spur dike can reduce the localscour and volume of scouring 3. Additionally, due to the penetration of flow througha pile dike, it is also useful to prevent changes such as a sudden increase inthe levels at river bends 2. Nevertheless, flow characteristics around pile-groupdikes are not treated sufficiently.

It needs for further investigations. Herein, quantitative analysis on flow characteristics around pile-groupdike is considered. A series of experiments were conducted using particle imagevelocimetry (PIV) method.

Pile-groups are arranged in two patterns of in-lineand staggered arrays with different permeability as indicated in Figure1. The main purposes were to analyze the velocity fields around structure, inthe main stream, and behind the structure near protected bank to evaluate theperformance of different pile-group dikes for bank protection and creation ofdiverse ecological habitat.2          Experimental proceduresThe experiments were conducted in a 7.5m long,0.

3m wide, and 0.4m high rectangular flume. The slope of the flume S was set to0.001. The pile-groups were made of acrylic cylinders of 0.5cm diameter and heighthd of 10cm. Theexperimental conditions and the schematic view of the flume with structure areshown in Table 1 and Figure 1 respectively.

The dike was installedperpendicular to the flume axis 3.0m downstream from the channel entrance. Threetypes of dikes were applied; the impermeable (permeability P=0%), the in-line,and the staggered arrays permeable pile-groups (P of 46.7, 60, and 73.3%). Permeabilityrate P is defined by Equation 1.                                                                                                                          (1) Length Ld and the width Wdof dikes were 0.

075m in all cases. Diameter of pile was d=0.5cm and nis the number of piles per row (Figure 1). Permeability rate was controlled bychanging the number of piles, and then the spacing between piles was calculatedaccording to it. For each case, the same number of piles per row and column (n=m)hence the same center to center spacing of the piles in the x and y directions(Sx=Sy) was kept. The discharge Qwas 0.003m3/s, then the mean velocity U0 was 0.

251m/sand the water depth h was set to 0.04m. Velocity vectors were measured by PIV method in horizontal planes. Forvisualization of the flow, nylon resin particles with 80 micron in diameter and1.02 in specific weight were used. A green laser light sheet was projected onhorizontal (x-y) planes. For each case, the height of the laser projection inhorizontal planes was from 5 to 35mm with 5mm interval. The visual images weretaken by a high speed video camera with 200 frames in a second and they were recordedas AVI files with 1024 x 1024 pixels.

The velocity vectors were measured with across-correlation method by using the commercial PIV software (FlowExpert byKatokoken). Time averaged velocity vectors were obtained by processing 3200successive images in 16 seconds. Table 1. Experimental conditions Discharge Q (m3/s) 0.003   Permeability P 73.

3% 60% 46.7% 0% Water Depth h (m) 0.04 Number of piles (n × m) (per row x per column) 4 × 4 6 × 6 8 × 8 imper-meable Mean velocity U0 (m/s) 0.251 Pile spacing Sx = Sy (mm) 23.3 14 10 Channel Slope S 0.001 Pile dimeter d(mm) 5 Channel width B (m) 0.3 Dike length Ld (mm) 75 Froude number Fr 0.4 Dike width Wd (mm) 75                  (a)                                                                               (b)              (c)                                            (d)                                            (e)   Figure 1.

Flume and dikes layout: (a) Plan view of dike placement on the flume bed, (b) side veiw of the flume, (c) in-line array, (d) staggered array, and (e) impermeable dikes. All dimention are in meters (m). 3          Results and discussionThe experimental results were analyzed for the velocity in the mainstream,the recirculating vortices, and the local velocity in the vicinity of protectedbank downstream of structure.

Velocity measurements were conducted at a rangeof -1Ld to 4Ld in the x directionand for the entire width of the channel (0 to B) in the ydirection. Figure 2(a)-(g) show the contours of longitudinal velocity U/U0in the plane of z=2.0cm. Longitudinal velocity U is normalized bythe mean velocity U0 and the y axis by the width ofthe channel B. Figure 2(h) summarizes the maximum longitudinal velocity valuesUmax in the mainstream that was generated due to installation ofthe dike.

The values of the longitudinal velocity in the vicinity of theprotected bank Ub downstream of the structure are presentedby Figure 2(i). The values of Umax and Ubare calculated as regional averages. Umax was obtained byaveraging the values in an area of (2Ld ? x ? 3Ld)and (0.6B ? y ? 0.

8B), while the values of Ub wasobtained by averaging the longitudinal velocity in the section of y=2cm for (1Ld ? x ? 4Ld)for each case. (a) P=73.3%                                         (b) P=60%                                             (c) P=46.

7%     (d) P=73.3% staggered                        (e) P=60% staggered                          (f) P=46.7% Staggered                               (g) P=0, Impermeable                                      (h) Umax/U0                                               (i) Ub/U0     Figure 2. Longitudinal velocity: (a) – (g) Contours of longitudinal velocity for different permeability rate P, (h) maximum longitudinal velocity in the mainstream, and (i) longitudinal velocity in the vicinity of protected bank. All values are for z = 20mm.    3.

1      Velocity in the mainstreamVelocity contours of Figure 2 indicate that the increase of velocity inmainstream is inversely proportional to the permeability rate. In addition, consideringthe same P rate, staggered pile-group dike enhanced the mainstream morethan in-line type. While for high permeability rate due to the large openingbetween piles, the behavior of both tend to become identical. On the other hand, impermeable dike strongly enhancedthe velocity in the mainstream. The maximum velocity in the mainstream wasabout 1.9U0 for impermeable dike while it was lower for pile-groupdikes of any arrangement. 3.

2     Recirculating vortices and velocity in thevicinity of protected bank behind the dike For the impermeable dike, a large vortex wasgenerated downstream of the structure. Two additional smaller vortices rotatingin opposite direction occurred in front of and behind the dike. For pile-groupdikes this large recirculating vortex was not generated, rather small vorticesbehind the piles appeared.

The change of velocity due to the presence of pile-groupdike occurred in both the regions, in the mainstream and behind the dike. Thechange in these two regions is inversely related to each other. Figure 2(i)indicates that staggered arrangement has significant influence on velocityreduction near the bank while the mainstream does not affected strongly. Furthermore, for any pair of pile-group dike with different arrangement,but having the same permeability rate, regardless of the magnitudes ofvelocities, the contour shapes express similar pattern in the mainstream butdifferent behind the structure (Figure 2). Behind the structure, for in-linearrangement pile-group dikes the velocity becomes higher near the bank and thendecreases toward the mainstream up to the width of dike, while for staggeredpile-group; the velocity is minimized near the bank and increases regularly tothe mainstream.

However, the impermeable dike has a returned flow near thebank. 4          conclusionThree types of dikes were investigatedexperimentally, namely; the impermeable, in-line, and staggered pile-groupdikes. Flow characteristics around the above mentioned dikes were studied.

Theresults stated that, impermeable dike enhanced strongly the mainstream. Anincreased velocity around the structure and a large recirculation zone wereobserved. These have a high effect on the local scour around dike and the bedchanges of the main channel. In contrast, it may provide favorable aquatichabitat in the recirculation zone.On the other hand, for the pile-group dikes, the velocity behind thestructure was decreased while the mainstream was not enhanced strongly. The flowpenetrated thorough pile-group dikes and did not deviate suddenly to themainstream. In addition, the penetrated flow discharged from the pile-groupdike with reduced velocity.

This function of pile-group dikes can reduce theflow velocity for downstream bank protection purpose while reducing the localscour around structure. Furthermore, by varying the openings between the piles,the flow gradient along the bank can be adjusted. In addition, slow velocityzone behind the dike can encourage vegetation growth, providing further stabilizationof the banks as well as improved habitat for aquatic species. Application ofthese dikes can be most appropriate for narrow sections where installation ofan impermeable dike results in extreme acceleration in the mainstream. Pile-groupdikes are expected not to enhance the main stream strongly and to reduce localscour around structure while protecting the bank. Among the two arrangements of pile dikes, the staggered arrays demonstratedbetter significance regarding flow pattern. It reduced the velocity near thebank and then it is increased gradually to the mainstream, while for thein-line arrays it was in opposite. This phenomenon needs for furtherinvestigations to clarify its mechanism.

 REFERENCES 1    Sayed HashmatSADAT and Akihiro TOMINAGA, “Optimal distance betweenpile-group and spur-dike to reduce local scour”, Journal of Japan Society of Civil Engineers, Ser. B1 (HydraulicEngineering), Vol. 71, No.

4, (2015), pp I_187-I_192.2    JOONGU KANG, HONGKOO YEO, SUNGJUNG KIM and UN JI, “Permeability effects of single groin on flow characteristics”, Journal of Hydraulic Research, Vol. 49, No. 6, (2011), pp 728-735.

3    Sayed HashmatSADAT and Akihiro TOMINAGA, “Influence of pile groupdensity on minimizing local scour of a double spur dike group”, Journal of Japan Society of Civil Engineers, Ser. B1(Hydraulic Engineering), Vol. 70, No. 4, (2014), pp I_85-I_90.

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