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Using a bed sill as a scour countermeasure downstream of an apron
Ain Shams Engineering Journal xxx (2017) xxx–xxx
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Using a bed sill as a scour countermeasure downstream of an apron H. Hamidifar a,⇑, M. Nasrabadi b, M.H. Omid b a b
Water Engineering Department, Shiraz University, Shiraz, Iran Department of Irrigation and Reclamation Engineering, University of Tehran, Karaj, Iran
a r t i c l e
i n f o
Article history: Received 19 March 2016 Revised 2 August 2016 Accepted 21 August 2016 Available online xxxx Keywords: Scour Apron Bed sill Protective structures Maximum scour depth
a b s t r a c t In order to protect hydraulic structures against scouring, engineers have designed and used many different types of countermeasures. In this paper, local scour downstream of a rigid apron using a bed sill is studied. The main objective of the present study was to investigate the efficiency of scour reduction by means of a single bed sill located downstream of the apron as countermeasure, and to evaluate its effectiveness at various distances from the end of the apron. It was found that the maximum scour downstream of the apron reduces up to 95%. Furthermore, variations of the characteristic lengths of the scour hole were investigated. Also, it was observed that completely buried sills may not be useful. Finally, a regression based equation is proposed to predict the shape of the scour hole with the sill. Ó 2016 Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Scour occurs naturally as a part of a river’s morphological changes and as a result of built structures, and threatens the safety of the structure. Aprons downstream of sluices and energydissipating devices can also be threatened by the erosion of sediments in their vicinity. This is also a problem for hydraulic structures in irrigation networks, where the aprons and their bed protection downstream of hydraulic structures don’t have enough length for a submerged jump, allowing scour holes to eventually form behind the aprons [1,2]. The decay of maximum velocity for a submerged jump is slower than for a free jump, and the proportion of jump length to submergence factor is linear. Thus, a long apron for a submerged jump is often uneconomic [3]. Prediction of local scour holes that develop downstream of hydraulic structures plays an important role in their design [4,5]. Excessive local scour can progressively undermine the foundation of the structure leading to failure. Because complete protection against scour is too expensive, generally, the main characteristics lengths of the scour hole such as the maximum scour depth have to be predicted to minimize the risk of failure [6]. Rajaratnam and Berry and Rajaratnam studied the scour by circular wall jets [7,8]. Mason, Canepa and Hager showed that the scour process depends on different parameters, i.e. discharge, densimetric jet
Peer review under responsibility of Ain Shams University. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Hamidifar), [email protected] (M. Nasrabadi), [email protected] (M.H. Omid).
Froude number, jet impact angle, jet air content, upstream flow, tailwater depth, and granulometric characteristics of the sediment [9,10]. Pagliara et al. studied the effect of ridge removal on the geometry of the scour hole [11]. Abdelhaleem used a set of semicircular baffle blocks to minimize the scour downstream of a weir [12]. He tested several configuration of the blocks and found that all suggested baffle block arrangements reduce the maximum scour depth. However, the configuration with Lb/Lf = 0.4 and Hb/ Do = 1.33, where Lb, Lf, Hb and Do are distance between baffles line and the toe of the weir, floor length, baffle’s height and outer baffle’s diameter, respectively, gives the maximum reduction in the scour length which ranged from 77.06% to 93.66%. Mesbahi et al. applied gene-expression programming to predict local scour depth at downstream of stilling basins and determined the Froude number as the most important parameter on the prediction of maximum scour depth [13]. To mitigate local scouring and its risk to structure safety, protection facilities are often applied to control the scouring at the downstream face of the structure. Petts and Calow proposed that the basic objective of any restoration project is to enhance morphological, hydraulic, and sedimentological variabilities consistent with the natural constraints of the streams [14]. For this, impacts of the project on stream stability and hydraulics have to be carefully examined. When a protection structure is introduced, the whole scour mechanism changes depending on the type and the location of the structure [15]. The common method of preventing scour below vertical gates is to place riprap stones or build a concrete apron. These countermeasures can be expensive and, in the latter case,
http://dx.doi.org/10.1016/j.asej.2016.08.016 2090-4479/Ó 2016 Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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failure may occur as a result of uplift pressures or of undermining caused by erosion proceeding upstream under the apron. Rajaratnam and Aderibigbe conducted an experimental study on a method for reducing scour below vertical gates [16]. In their method, a screen is placed on the erodible bed immediately downstream of the gate. They concluded that placing a screen on the sand bed reduces significantly the depth of scour. Sarkar and Dey proposed a number of equations to design a launching apron as protective countermeasures for scour downstream of an apron [17]. Dey and Sarkar conducted a comprehensive study on scour downstream of aprons and presented some criteria for determining the characteristic lengths of the scour hole [18]. Ali et al. conducted an experimental study on the effect of different spaced corrugated aprons on the downstream local scour due to submerged jump [19]. Their results showed that the proposed method minimize the scour depth up to 63.4% in comparing with classical jump. Also, Farhoudi and Khalili-Shayan and Khalili-Shayan and Farhoudi studied local scour downstream of adverse stilling basins. Their results showed that the volume of scour hole downstream of an adverse stilling basin is larger than that of horizontal aprons. Also, they found that the maximum depth of scour hole decreases as the length and slope of stilling basin increases [20,21]. Recently, Dodaro et al. and Dodaro et al. developed a numerical model through modifying the Einstein sediment transport formula to simulate scour evolution downstream of an apron under steady and unsteady flow conditions [22,23]. The construction of sequences of sills is a widely applied countermeasure to control excessive erosion, forcing the longitudinal profile of the stream to follow a stair-like pattern. The ultimate result of bed sills is the formation of sequences of steps followed by reaches at a milder slope than the original channel gradient [24]. Studies verified that channel-traversing structures like bedsills can increase the roughness of the streambed, dissipate excess energy, decrease the mean flow velocity, and reduce streambed shear force and thus reduces sediment movement especially under a mild channel gradient and steep slopes equipped with 2–4 and 1–2 times the average channel width interval, respectively [25]. Although some researchers have studied the interaction of flow and sediment near the sills, the potential ability of a single bedsill in reducing the scour adjacent the hydraulic structures has not received much attention yet. Lin et al. conducted model experiments on various types of serial ground-sills to determine the appropriate spacing to best protect the downstream bed of check-dams [25]. They concluded that Ground-sills can effectively protect the streambed from scouring under a suitable equipped condition. Pagliara and Palermo proposed a method of reducing the plunge pool scour introducing vertical protection structures with different permeability [15]. They concluded that if suitably located, the proposed structures can reduce the maximum scour depth up to 30%. Grimaldi et al. proposed the use of a bed sill downstream of bridge piers [26]. They found that the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. Pagliara et al. studied the 3D behavior of plunge pool scour in presence of two protections at selected positions, as an extension of the previous studies on 2D and 3D plunge pool scour [27]. They found that in presence of a protection the phenomenon becomes even more complex as the jet is deflected. Gaudio et al. and Tafarojnoruz et al. enhanced the efficiency of a bed-sill as bridge pier scour countermeasure through combining it with other countermeasures [28,29]. Although bed-sills have the ability of controlling the riverbed souring problem, they also have a negative impact on the downstream channel stability. Ashida et al. compared a riverbed before and after the implementation of bed-sills and found that the streambed upstream of the bed-sills received the expected out-
come in scouring control, while the average streambed souring depth increased downstream of the protected sections [30]. Nasrabadi et al. concluded that by increasing distance between the sill and apron (for a given sill height), scour depth gradually decreases [31]. The main objective of the present study was verifying the possibility of using the bed-sill as a countermeasure downstream of an apron. Hence, a series of laboratory experiments was conducted and the geometry of the scour holes was measured. The effect of parameters, including the height of the sill above the original bed and its distance from the apron, was examined.
2. Experimental setup and procedure Experiments were carried out at the central laboratory of water researches at the University of Tehran, Iran. A recirculating rectangular open channel flume 9 m long, 0.5 m wide and 0.6 m deep was used. The flume side walls are made of transparent Plexiglas in order to allow the direct observation of the scouring phenomena. A horizontal wall jet issuing from the sluice gate installed near the upstream end of the flume of 2 cm opening size was operated to permit only local scour and there was no net transport of the sand beyond the edge of the bed. A solid Perspex platform approximately 1 m long was constructed to simulate a rigid apron next the sluice gate. As the main aim of the present study was to study out the effect of the bed sill on the characteristic lengths of the scour hole, the length of the apron has not been changed in different tests. A 0.2 m-deep and 1.65 m-long sediment recess box was located at the end of the apron. It was filled with the bed material and flushed level with the apron. The median bed grain diameter, p D50, was 1.85 mm and the geometric standard deviation rg = (D84/D16) = 1.1 was less than the threshold of rg = 1.35 proposed by Breusers and Raudkivi for the definition of non-uniform gradings [32]. The relative submerged density of the sediments was D = 1.63. The downstream end of the flume was equipped with a trap to prevent any accidental transport of the bed material into the flow system. The scour profiles and flow depth were measured using a point gauge with an accuracy of ±0.1 mm. The point gauge was mounted on a traverse arrangement which can move longitudinally along the flume and transversely across the flume cross-section. The scour profiles were measured at three longitudinal sections, i.e., along the jet centerline, close to the front Plexiglas wall, and close to the far wall after dewatering the flume. Bed sills used in the tests were 8 mm thick transparent Plexiglas plates, fully fitted across the flume width. The sills were positioned at different heights respect to the initial bed level, with different distances L downstream to the end of the apron (Fig. 1). Discharges were measured with a calibrated sharp crested rectangular weir located into the reservoir upstream of the sluice gate, with accuracy of 0.1 l/s. Flow discharge was checked regularly to ensure the consistency of the flow. At the end of the flume, a tailgate was used in order to regulate the design tailwater depth, that is fully submerged horizontal jet. Whereas the main objective of the present study was verifying the possibility of using the bedsill as a countermeasure downstream of an apron, the discharge (Q) and tailwater depth (Tw) were remained constant in the experiments as 17.72 l/s and 0.20 m, respectively. Tw values were measured several meters downstream of the scour hole to minimize the error in reading the flow depth due to the oscillating water surface downstream of the hydraulic jump. Preliminary test runs found that there was a very considerable decrease in scour when a bed sill was installed, and that about twelve hours would be required for the scoured bed to reach an
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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Fig. 2 shows the effect of sill height for the tests where the sill is introduced at the location of maximum scour depth for the control test. The vertical axis is the percent scour reduction at equilibrium (rds) of the depth behind and in front of the bed sill, ds1 and ds2 respectively:
rdsi ð%Þ ¼
Fig. 1. Characteristics lengths of the scour hole, (a) no sill condition and (b) after introducing the bed sill.
asymptotic condition where the maximum scour depth at the centerline of the flume did not change significantly. A reference test was carried out without the protection structure to find the maximum scour depth (ds0) and the longitudinal length of the scour hole (LS0) in basic conditions. Fig. 1 shows the schematic view of the main geometrical parameters of the scour hole. Altogether, 41 experiments were conducted under deeply submerged jet conditions; i.e., the tailwater depth was greater than 10 times the size of the jet opening according to the criteria of Lim and Yu [33]. The experiments covered a wide range of bedsill height and distance (Table 1 lists the range of the test conditions). A grid of 40 control positions including eight longitudinal and five vertical positions was used for the geometry of the sill. The longitudinal distances and the height of the sills for different tests were changed with intervals of 2 cm and 1 cm, respectively.
3. Results and discussion Totally 41 tests have been carried out using bed sill located at different longitudinal and vertical positions. All scour holes obtained were two-dimensional, with minor occasional wall effects at the mound region. Based on experimental results, channel bed profiles under different combinations of variables, i.e. sill height and position, indicated that the depth of the local scouring area downstream from the horizontal apron decreased with increasing resistance of the water flow over the bed sill. Results of the preliminary tests showed that the shape of the scour hole was completely different when a bed sill was introduced as a countermeasure. At a certain distance from the end of the apron, a deep hole forms behind the sill. As the jet plunging over the crest diffuses its energy in turbulent rollers inside the pools below, another scour hole forms in front of the sill. In other words, bed sills encourage the formation of two distinct scour holes with different characteristics: one upstream and another downstream of the sill. The latter is similar in shape to the one formed without any countermeasures (the control test).
ds0 dsi 100 ds0
ð1Þ
where the subscript i takes the value of 1 or 2. Negative values on vertical axis in Fig. 2 indicate that the maximum scour depth after using the sill is greater than that without the sill. Negative values for the sill height show the depth to which the sill is buried under the zero bed level. It is observed from Fig. 2 that ds1 (the maximum scour depth behind the sill) is practically not depending on h. Also, it is always greater than ds0. However, as the height of the sill above the zero bed level increases, the percent reduction of the maximum scour depth decreases in front of the sill (ds2). Also, in the cases which the sill was buried, except the zero-height sill cases, the maximum scour depth in front of the sill (ds2) was the same as the control test, but the maximum scour depth behind the sill increased in both buried and unburied sills. This caused some controversy about the study’s preliminary hypothesis about the usefulness of the bed sill. However, there were some sill heights where the percent reduction of ds2 was considerable. As an example, although the maximum scour depth between the apron and the sill increased about 53% with a 2 cm high sill, the maximum scour depth in front of the sill was reduced up to 95%. Results also suggested that buried sills may not be useful, because they showed no reduction for either ds1 or ds2. Therefore, the study continued to consider only the cases where the sill was unburied (i.e., h/b = 0.0, 0.5 and 1.0) where b is the jet opening size. However, even when not taking into account the area behind the bed sill, the question remained of which distance between the end of the apron and the sill would be the most efficient? Fig. 3 shows the variations of the dimensionless scour depth behind the sill (ds1/ds0) against the dimensionless distance between the sill and the end of the apron (L/Ls0) for different sill heights. The trend of the variations of ds1 for all three heights of the sill tested in this study is the same. It can be noted that the greater and smaller dependence on L is respectively achieved in the cases of h/b = 1.0 and h/b = 0. However, the difference is greater than 20% in some cases. It is observed that ds1 has a maximum value for each sill height. It is notable that the maximum of ds1 occurs at the distance of the maximum scour depth for the control test (L/LS0 1). So, if the sill is introduced at the location of the maximum scour depth for the reference situation (i.e. no sill condition), the maximum scour depth is respected. This may be explained by the action of the overflow jet over the end of the apron. Visual observations indicated that the flow field in the vicinity of the bed resembled a plunging jet with a recirculating region in the area of the channel bed. Balachandar and Kells confirmed this by observing flow patterns using dye injection [34]. As shown in Fig. 4, in the control conditions, the flow entering the scour hole may be divided in two when it reaches the bottom of the canal. When a sill is introduced at the dividing point of the backward and forward flow on the bottom of the canal, the sill may force the forward flow to move deeper in the canal, deepening the bed. As the height of the sill above the bed increases, the flow resistance increases. This decreases the potential for the flow to scour downstream, and a smaller scour depth in front of the sill is expected. Fig. 5 shows the variations of the dimensionless maximum scour depth in front of the sill (ds2/ds0) against the dimensionless parameter L/LS0 for three heights of the sill above the original bed. It is obvious from this figure that the height of the sill has a significant effect on the scour depth after the sill. For the case of h/b = 0.0, where less resistance against the flow occurs, the max-
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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Table 1 Summary of the experimental conditions and results.
a
Run No.
Q (lit/s)
L (cm)
ha (cm)
Ls1 (cm)
ds1 (cm)
Ls2 (cm)
ds2 (cm)
Lds (cm)
hds (cm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72
without sill 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16
without sill 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2
9 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 9 12 14 16 2 3.5 6 8 10 12 14 16 1.5 4 6 8 10 12 14 16
2.9 3.2 3.52 4.02 4.17 3.97 3.3 3.11 2.91 3.7 4.21 4.34 4.4 4.48 4.14 3.1 2.57 3.33 4.08 4.73 4.94 3.93 3.59 2.8 2.29 2.91 3.62 4 4.1 4.21 3.96 3.27 2.96 2.87 3.27 3.67 4.05 4.08 4.16 3.09 2.91
9 16 18 19 20 21 22 24 27 18 18.5 19.5 21 22 23.5 25.5 26 21 23 23.5 24 25 26 27 29 9 14 6 12 17 19 24 23 9 14 16 15 14 16 18 20
2.9 2.28 2.3 2.47 2.26 2.12 1.79 1.7 1.65 1.47 1.53 1.3 1.11 0.89 0.75 0.66 0.54 0.13 0.17 0.3 0.2 0.11 0.05 0.07 0.06 3.37 3.2 2.92 2.94 2.81 2.9 1.94 1.82 3.55 3.3 2.9 2.9 2.85 2.72 2.42 2.57
78 63 61 61 60 64 58 61 63 57 55 55 54 56 60 55 55 52 50 49 53 52 52 53 54 67 62 60 64 62 57 58 59 58 66 60 66 57 62 60 59
7.09 6.35 6.44 6.43 6.57 5.86 5.6 5.59 5.83 4.12 4.67 4.66 4.7 4.41 4.94 4.55 4.2 2.21 2.52 3.05 3.14 3 3.23 3.57 3.44 7.93 6.62 7.02 7 6.77 5.84 5.39 5.51 6.31 7.23 6.66 7.12 6.6 6.92 6.71 6.89
Negative values indicate that the sill was buried.
Fig. 2. Effect of the sill height on the maximum scour depth.
imum scour depth for all values of L/LS0 is much greater than for those of the other two cases (h/b = 0.5 and h/b = 1.0). In spite of some scatter of the data points, it is worth noting that for a given sill height, as the sill recedes (that is, L/LS0 increases), ds2/ds0 decreases. However, it is observed from Figs. 3 and 5 that in the
Fig. 3. Variations of the scour depth behind the sill against the distance to the sill.
range of the tests of the present study the best sill configuration is h/b = 1.0 and L/LS0 = 1.8. Another characteristic of the scour hole is the height of the sand wave (dune) formed downstream of the scour hole. As in this study the tailwater depth was set so that no general scour would occur,
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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a)
b)
Sand bed
Sand bed
Fig. 4. Flow pattern in the scour hole, (a) without bed sill and (b) with bed sill.
Fig. 5. Variations of the maximum scour depth against the distance to the sill.
any sediment grain that was eroded from the alluvial bed was then gathered on the downstream bed, as the flow was unable to keep it in motion. The presence of the sand wave could affect the flow characteristics and needed to be considered. Fig. 6 shows the variations of the dimensionless dune height (hds/hd0) against the dimensionless parameter L/LS0 for three heights of the bed sill. It is seen from this figure that the dune height is affected by the sill height. As discussed earlier, for smaller sills the scour depth is greater. Hence, the amount of sediment that exits from the scour hole will be greater, and a higher dune may be expected for small sill heights. Also, for a given sill height, the dune height does not vary noticeably as the sill recedes from the end of the apron. How-
Fig. 6. Variations of the dimensionless dune height against the dimensionless distance to the sill.
ever, for all three heights of the sill tested in this study, the parameter hds/hd0 was less than 1. This means that introducing a bed sill of any height significantly reduces the height of the dune at the range of distances tested. Fig. 7 shows the variations of the dimensionless location of the occurrence of the maximum scour depth behind the sill (Ls1/LS0) against the dimensionless parameter L/LS0 for three different sill heights. Increasing the distance of the sill from the end of the apron caused the location of ds1 to recede. Also, the height of the sill did not affect the parameter Ls1/LS0 for the three sill heights tested in this study. It should be mentioned that the linear trend of the variations of the Ls1/LS0 is due to the fact that for all tests the Ls1 was located adjacent to the sill. Fig. 8 shows the variations of the dimensionless location of the maximum scour depth in front of the sill (Ls2/LS0) against the dimensionless parameter L/LS0. As the same as the Ls1/LS0, by increasing the distance between the sill and the end of the apron, the location of the occurrence of the maximum scour depth in front of the sill recedes. Also, the trend of variations for three sill heights are the same, but in some cases there are a considerable difference specially when the sill is closer to the apron. However, the rate of variation (the slope of the trendline) of the Ls2/LS0 is much smaller than that of the Ls1/LS0. Fig. 9 shows the variations of the dimensionless location of the maximum dune height (Lsd/LS0) against the non-dimensional parameter L/LS0. It is clear from this figure that for a given height of the sill, Lsd/LS0 does not vary significantly and remains constant as L/LS0 increases. Fig. 10 shows the scour profiles along the centerline of the channel. In this study many attempts were made to collapse all scour profiles onto each other by making dimensionless the eroded depth and the distance from the end of the apron. So, the use of different scaling variables was attempted. But as observed in this
Fig. 7. Variations of Ls1/LS0 against the L/LS0.
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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An analysis of the profile of the eroded bed at asymptotic state in the plane of symmetry indicates that the profiles for the hole (not for the mound region) are approximately similar if made dimensionless using suitable parameters. For this reason, and because of the effects of the height and distance of the sill, the depth and longitudinal extension of the hole were made dimensionless as (ds ds2)/(ds2 h) and (x L)/(ds2 h) respectively, as shown in Fig. 10. In this figure h is height of the sill above the original bed, L is the distance of the sill from the end of the apron, ds is the scour depth at the distance x from the end of the apron and ds2 is the maximum scour depth in front of the sill. Thus, the effects of sill height and its distance from the end of the apron are absorbed by the proper choice of scaling variables. Finally, regression analysis collapsed all data points reasonably well onto a single line. This study proposes Eq. (2) for predicting the shape of the scour hole:
y ¼ 0:538 þ 0:409 cosð0:347x þ 0:956Þ Fig. 8. Variations of Ls2/LS0 against L/LS0.
ð2Þ
in which x and y are the predefined non-dimensional parameters used for longitudinal extension and depth of the scour hole respectively. The proposed equation has a standard error of S = 0.18 and correlation coefficient of r = 0.82. Also, the results of the experimental measured scour depths are compared in Fig. 11 with the computed scour depths using Eq. (2). It is seen that the computed scour depths are in good agreement with observed values and most computed values fall in the ±30% lines. 4. Conclusion
Fig. 9. Variations of Lsd/LS0 against L/LS0.
This study examined scour caused by a wall jet downstream of a rigid apron interacting with non-cohesive sand beds to further understand the effects of introducing a bed sill as a countermeasure. An extensive range sill location for five sill heights was used under constant hydraulic conditions such as flow discharge and tailwater depth. Variations of the characteristic lengths of the scour hole were investigated. Results showed that the geometry of the scour hole was completely different when a bed sill was introduced as a countermeasure. It was found that a bed sill placed downstream of a rigid apron, not too far from it, reduces local scouring. Two distinct scour holes develop behind and in front of the sill. For all tests performed in this study the maximum scour depth behind the sill (between the sill and the end of the apron) was greater than when there was no sill. Nevertheless, for all tests, an outstanding sill reduced the maximum scour depth in front of the sill up to 95%. It was observed that completely buried sills
Fig. 10. Similarity of non-dimensional scour hole profiles for different experiments.
study and as reported by Deshpande et al., scour patterns were somewhat three-dimensional in the sand wave region. Hence, a great scatter was observed for this region [35]. Fig. 11. Comparison of the observed and computed values of the scour depth.
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may not be useful, because no reduction occurs for scour depth either behind or in front of the sill. Results also showed that scour profiles differed for sills at different locations and heights. Finally, this method appears to have a great deal of potential if proper aprons can be designed, and it should be explored further. As the apron length was constant in all the tests, the proposed equation for prediction of the scour hole profile is only valid in the range of the parameters used in the present study.
References [1] Hamidifar H, Omid MH, Nasrabadi M. Bed scour downstream of sluice gates. J Water Soil 2010;24(4):728–36. [2] Hamidifar H, Omid MH, Nasrabadi M. Scour downstream of a rough rigid apron. World Appl Sci J 2011;14(8):1169–78. [3] Wu S, Rajaratnam N. Free jumps, submerged jumps and wall jets. J Hydraul Res 1995;33(2):197–212. [4] Moussa YAM. Modeling of local scour depth downstream hydraulic structures in trapezoidal channel using GEP and ANNs. Ain Shams Eng J 2013;4 (4):717–22. [5] Ghani AA, Mohammadpour R. Temporal variation of clear-water scour at compound Abutments. Ain Shams Eng J 2015. doi: http://dx.doi.org/10.1016/j. asej.2015.07.005. [6] Khwairakpam P, Mazumdar DA. Local Scour around hydraulic structures. Int J Recent Trends Eng 2009;1(6):59–61. [7] Rajaratnam N, Berry B. Erosion by circular turbulent wall jets. J Hydraul Res 1977;15(3):277–89. [8] Rajaratnam N. Erosion by circular wall jets in cross flow. J Hydraul Divis ASCE 1980;106(HY11):1867–83. [9] Mason PJ. Review of plunge pool rock scour downstream of Srisailam dam. In: Schleiss A, Bolleart E, EPFL-LCH, editors. Rock Scour Due to Falling HighVelocity Jets. A.A. Balkema-Swets & Zeitlinger, The Netherlands; 2002. p. 25– 31. [10] Canepa S, Hager WH. Effect of air jet content on plunge pool scour. J Hydraul Eng 2003;128(5):358–65. [11] Pagliara S, Hager WH, Minor HE. Hydraulics of plane plunge pool scour. J Hydraul Eng 2006;132(5):450–61. [12] Abdelhaleem FSF. Effect of semi-circular baffle blocks on local scour downstream clear-overfall weirs. Ain Shams Eng J 2013;4(4):675–84. [13] Mesbahi M, Talebbeydokhti N, Hosseini SA, Afzali SH. Gene expression programming to predict local scour depth at downstream of stilling basins. Scientia Iranica. Trans Civil Eng 2015. [14] Petts G, Calow P. River restoration. Blackwell Science Ltd.; 1996. [15] Pagliara S, Palermo M. Plane plunge pool scour with protection structures. J Hydro-environ Res 2008;2:182–91. [16] Rajaratnam N, Aderibigbe O. A method for reducing scour below vertical gates. In: Proceedings of the Institution of Civil Engineers-Water, Maritime and Energy, 101, June 1993. p. 73–83. [17] Sarkar A, Dey S. Scour downstream of aprons caused by sluices. Proc ICE Water Manage 2005;158(2):55–64. [18] Dey S, Sarkar A. Scour downstream of an apron due to submerged horizontal jets. J Hydraul Eng 2006;132(3):246–57. [19] Ali HM, El Gendy MM, Mirdan AMH, Ali AAM, Abdelhaleem FSF. Minimizing downstream scour due to submerged hydraulic jump using corrugated aprons. Ain Shams Eng J 2014;5(4):1059–69. [20] Farhoudi J, Khalili-Shayan H. Investigation on local scour downstream of adverse stilling basins. Ain Shams Eng J 2014;5(2):361–75. [21] Khalili-Shayan HK, Farhoudi J. Local scour profiles downstream of adverse stilling basins. Scientia Iranica Trans A Civ Eng 2015;22(1):1. [22] Dodaro G, Tafarojnoruz A, Stefanucci F, Adduce C, Calomino F, Gaudio R, Sciortino G. An experimental and numerical study on the spatial and temporal evolution of a scour hole downstream of a rigid bed. In: Proc the international conference on fluvial hydraulics, River Flow, Lausanne, Switzerland; 2014. p. 1415–22. [23] Dodaro G, Tafarojnoruz A, Sciortino G, Adduce C, Calomino F, Gaudio R. Modified Einstein sediment transport method to simulate the local Scour evolution downstream of a rigid bed. J Hydraul Eng ASCE 2016;142:1–11. [24] Tregnaghi M, Marion A, Coleman S. Scouring at bed sills as a response to flash floods. J Hydraul Eng ASCE 2009;135(6):466–75. [25] Lin B, Yeh C, Lien H. The experimental study for the allocation of ground-sills downstream of check dams. Int J Sediment Res 2008;23:28–43.
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[26] Grimaldi C, Gaudio R, Calomino F, Cardoso AH. Control of Scour at Bridge Piers by a Downstream Bed Sill. J Hydraul Eng 2009;135(1):13–21. [27] Pagliara S, Roy D, Palermo M. 3D plunge pool scour with protection countermeasures. J Hydro-environ Res 2010;4:225–33. [28] Gaudio R, Tafarojnoruz A, Calomino F. Combined flow-altering countermeasures against bridge pier scour. J Hydraul Res IAHR 2012;50 (1):35–43. [29] Tafarojnoruz A, Gaudio R, Calomino F. Bridge pier scour mitigation under steady and unsteady flow conditions. Acta Geophys 2012;60(4):1076–97. [30] Ashida K, Takahash T, Michiue M. River sediment disaster and its’ treatment. Shen Ba Publish 1983:255–321. [31] Nasrabadi M, Omid MH, Hamidifar H.Local scouring at bed sill downstream of turbulent jets. In: The 36th IAHR world congress, The Hague, The Netherlands, 28 June–3 July, 2015. [32] Breusers HNC, Raudkivi AJ, Scouring. Hydraulic structures design manual. IAHR, A.A. Balkema: Rotterdam; 1991. [33] Lim S, Yu G. Scouring downstream of sluice gate. In: First international conference on scour of foundation, Texas Transportation Institute, Nov. 17–20, Texas A & M University, College Station, Texas, USA. [34] Balachandar R, Kells JA. Local channel scour in uniformly graded sediments: the time-scale problem. Canad J Civil Eng 1997;24(5):799–807. [35] Deshpande NP, Balachandar R, Mazurek KA. Effects of submergence and test startup conditions on local scour by plane turbulent wall jets. J Hydraul Res 2007;45(3):370–87.
Hossein Hamidifar, Ph.D., Assistant Professor Water Engineering Deptartment, Shiraz University, Shiraz, Iran. The author graduated from Shiraz University and received his B.Sc. in Water Engineering in 2006 and then graduated from University of Tehran and received his M. Sc and Ph.D. degrees in Hydraulic Structures 2008 and 2012, respectively. He has many published papers in referred national and international journals and international conferences. His main field of research interests are Scour, Energy dissipation, Stilling Basin, Sediment and Pollutant Transport, and Hydraulic Structures.
Mohsen Nasrabadi, Ph.D. student in Hydraulic Structures, Department of Irrigation and Reclamation, Engineering, University of Tehran, Tehran, Iran. The author received his M.Sc. Degree from University of Tehran in 2012. His research interests include Open Channel Hydraulics, Irrigation Structures, Local Scour at the downstream of Sydraulic Structures and Sediment Transport.
Mohammad Hossein Omid, Ph.D., Distinguished Professor of hydraulic structures and river engineering, Department of Irrigation and Reclamation Engineering, Faculty of Agricultural Engineering and Technology, UTCAN, University of Tehran, Iran. He received his Ph.D. in civil engineering from Department of Civil and Structural Engineering, UMIST Manchester, UK. He has many published papers in referred national and international journals and international conferences. His main field of research interests focused on scouring downstream of stilling basins and hydraulic jump as well as river engineering. Beside his academic activities, he was serving the designers of hydraulic structures in the country.
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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Using a bed sill as a scour countermeasure downstream of an apron H. Hamidifar a,⇑, M. Nasrabadi b, M.H. Omid b a b
Water Engineering Department, Shiraz University, Shiraz, Iran Department of Irrigation and Reclamation Engineering, University of Tehran, Karaj, Iran
a r t i c l e
i n f o
Article history: Received 19 March 2016 Revised 2 August 2016 Accepted 21 August 2016 Available online xxxx Keywords: Scour Apron Bed sill Protective structures Maximum scour depth
a b s t r a c t In order to protect hydraulic structures against scouring, engineers have designed and used many different types of countermeasures. In this paper, local scour downstream of a rigid apron using a bed sill is studied. The main objective of the present study was to investigate the efficiency of scour reduction by means of a single bed sill located downstream of the apron as countermeasure, and to evaluate its effectiveness at various distances from the end of the apron. It was found that the maximum scour downstream of the apron reduces up to 95%. Furthermore, variations of the characteristic lengths of the scour hole were investigated. Also, it was observed that completely buried sills may not be useful. Finally, a regression based equation is proposed to predict the shape of the scour hole with the sill. Ó 2016 Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Scour occurs naturally as a part of a river’s morphological changes and as a result of built structures, and threatens the safety of the structure. Aprons downstream of sluices and energydissipating devices can also be threatened by the erosion of sediments in their vicinity. This is also a problem for hydraulic structures in irrigation networks, where the aprons and their bed protection downstream of hydraulic structures don’t have enough length for a submerged jump, allowing scour holes to eventually form behind the aprons [1,2]. The decay of maximum velocity for a submerged jump is slower than for a free jump, and the proportion of jump length to submergence factor is linear. Thus, a long apron for a submerged jump is often uneconomic [3]. Prediction of local scour holes that develop downstream of hydraulic structures plays an important role in their design [4,5]. Excessive local scour can progressively undermine the foundation of the structure leading to failure. Because complete protection against scour is too expensive, generally, the main characteristics lengths of the scour hole such as the maximum scour depth have to be predicted to minimize the risk of failure [6]. Rajaratnam and Berry and Rajaratnam studied the scour by circular wall jets [7,8]. Mason, Canepa and Hager showed that the scour process depends on different parameters, i.e. discharge, densimetric jet
Peer review under responsibility of Ain Shams University. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Hamidifar), [email protected] (M. Nasrabadi), [email protected] (M.H. Omid).
Froude number, jet impact angle, jet air content, upstream flow, tailwater depth, and granulometric characteristics of the sediment [9,10]. Pagliara et al. studied the effect of ridge removal on the geometry of the scour hole [11]. Abdelhaleem used a set of semicircular baffle blocks to minimize the scour downstream of a weir [12]. He tested several configuration of the blocks and found that all suggested baffle block arrangements reduce the maximum scour depth. However, the configuration with Lb/Lf = 0.4 and Hb/ Do = 1.33, where Lb, Lf, Hb and Do are distance between baffles line and the toe of the weir, floor length, baffle’s height and outer baffle’s diameter, respectively, gives the maximum reduction in the scour length which ranged from 77.06% to 93.66%. Mesbahi et al. applied gene-expression programming to predict local scour depth at downstream of stilling basins and determined the Froude number as the most important parameter on the prediction of maximum scour depth [13]. To mitigate local scouring and its risk to structure safety, protection facilities are often applied to control the scouring at the downstream face of the structure. Petts and Calow proposed that the basic objective of any restoration project is to enhance morphological, hydraulic, and sedimentological variabilities consistent with the natural constraints of the streams [14]. For this, impacts of the project on stream stability and hydraulics have to be carefully examined. When a protection structure is introduced, the whole scour mechanism changes depending on the type and the location of the structure [15]. The common method of preventing scour below vertical gates is to place riprap stones or build a concrete apron. These countermeasures can be expensive and, in the latter case,
http://dx.doi.org/10.1016/j.asej.2016.08.016 2090-4479/Ó 2016 Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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failure may occur as a result of uplift pressures or of undermining caused by erosion proceeding upstream under the apron. Rajaratnam and Aderibigbe conducted an experimental study on a method for reducing scour below vertical gates [16]. In their method, a screen is placed on the erodible bed immediately downstream of the gate. They concluded that placing a screen on the sand bed reduces significantly the depth of scour. Sarkar and Dey proposed a number of equations to design a launching apron as protective countermeasures for scour downstream of an apron [17]. Dey and Sarkar conducted a comprehensive study on scour downstream of aprons and presented some criteria for determining the characteristic lengths of the scour hole [18]. Ali et al. conducted an experimental study on the effect of different spaced corrugated aprons on the downstream local scour due to submerged jump [19]. Their results showed that the proposed method minimize the scour depth up to 63.4% in comparing with classical jump. Also, Farhoudi and Khalili-Shayan and Khalili-Shayan and Farhoudi studied local scour downstream of adverse stilling basins. Their results showed that the volume of scour hole downstream of an adverse stilling basin is larger than that of horizontal aprons. Also, they found that the maximum depth of scour hole decreases as the length and slope of stilling basin increases [20,21]. Recently, Dodaro et al. and Dodaro et al. developed a numerical model through modifying the Einstein sediment transport formula to simulate scour evolution downstream of an apron under steady and unsteady flow conditions [22,23]. The construction of sequences of sills is a widely applied countermeasure to control excessive erosion, forcing the longitudinal profile of the stream to follow a stair-like pattern. The ultimate result of bed sills is the formation of sequences of steps followed by reaches at a milder slope than the original channel gradient [24]. Studies verified that channel-traversing structures like bedsills can increase the roughness of the streambed, dissipate excess energy, decrease the mean flow velocity, and reduce streambed shear force and thus reduces sediment movement especially under a mild channel gradient and steep slopes equipped with 2–4 and 1–2 times the average channel width interval, respectively [25]. Although some researchers have studied the interaction of flow and sediment near the sills, the potential ability of a single bedsill in reducing the scour adjacent the hydraulic structures has not received much attention yet. Lin et al. conducted model experiments on various types of serial ground-sills to determine the appropriate spacing to best protect the downstream bed of check-dams [25]. They concluded that Ground-sills can effectively protect the streambed from scouring under a suitable equipped condition. Pagliara and Palermo proposed a method of reducing the plunge pool scour introducing vertical protection structures with different permeability [15]. They concluded that if suitably located, the proposed structures can reduce the maximum scour depth up to 30%. Grimaldi et al. proposed the use of a bed sill downstream of bridge piers [26]. They found that the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. Pagliara et al. studied the 3D behavior of plunge pool scour in presence of two protections at selected positions, as an extension of the previous studies on 2D and 3D plunge pool scour [27]. They found that in presence of a protection the phenomenon becomes even more complex as the jet is deflected. Gaudio et al. and Tafarojnoruz et al. enhanced the efficiency of a bed-sill as bridge pier scour countermeasure through combining it with other countermeasures [28,29]. Although bed-sills have the ability of controlling the riverbed souring problem, they also have a negative impact on the downstream channel stability. Ashida et al. compared a riverbed before and after the implementation of bed-sills and found that the streambed upstream of the bed-sills received the expected out-
come in scouring control, while the average streambed souring depth increased downstream of the protected sections [30]. Nasrabadi et al. concluded that by increasing distance between the sill and apron (for a given sill height), scour depth gradually decreases [31]. The main objective of the present study was verifying the possibility of using the bed-sill as a countermeasure downstream of an apron. Hence, a series of laboratory experiments was conducted and the geometry of the scour holes was measured. The effect of parameters, including the height of the sill above the original bed and its distance from the apron, was examined.
2. Experimental setup and procedure Experiments were carried out at the central laboratory of water researches at the University of Tehran, Iran. A recirculating rectangular open channel flume 9 m long, 0.5 m wide and 0.6 m deep was used. The flume side walls are made of transparent Plexiglas in order to allow the direct observation of the scouring phenomena. A horizontal wall jet issuing from the sluice gate installed near the upstream end of the flume of 2 cm opening size was operated to permit only local scour and there was no net transport of the sand beyond the edge of the bed. A solid Perspex platform approximately 1 m long was constructed to simulate a rigid apron next the sluice gate. As the main aim of the present study was to study out the effect of the bed sill on the characteristic lengths of the scour hole, the length of the apron has not been changed in different tests. A 0.2 m-deep and 1.65 m-long sediment recess box was located at the end of the apron. It was filled with the bed material and flushed level with the apron. The median bed grain diameter, p D50, was 1.85 mm and the geometric standard deviation rg = (D84/D16) = 1.1 was less than the threshold of rg = 1.35 proposed by Breusers and Raudkivi for the definition of non-uniform gradings [32]. The relative submerged density of the sediments was D = 1.63. The downstream end of the flume was equipped with a trap to prevent any accidental transport of the bed material into the flow system. The scour profiles and flow depth were measured using a point gauge with an accuracy of ±0.1 mm. The point gauge was mounted on a traverse arrangement which can move longitudinally along the flume and transversely across the flume cross-section. The scour profiles were measured at three longitudinal sections, i.e., along the jet centerline, close to the front Plexiglas wall, and close to the far wall after dewatering the flume. Bed sills used in the tests were 8 mm thick transparent Plexiglas plates, fully fitted across the flume width. The sills were positioned at different heights respect to the initial bed level, with different distances L downstream to the end of the apron (Fig. 1). Discharges were measured with a calibrated sharp crested rectangular weir located into the reservoir upstream of the sluice gate, with accuracy of 0.1 l/s. Flow discharge was checked regularly to ensure the consistency of the flow. At the end of the flume, a tailgate was used in order to regulate the design tailwater depth, that is fully submerged horizontal jet. Whereas the main objective of the present study was verifying the possibility of using the bedsill as a countermeasure downstream of an apron, the discharge (Q) and tailwater depth (Tw) were remained constant in the experiments as 17.72 l/s and 0.20 m, respectively. Tw values were measured several meters downstream of the scour hole to minimize the error in reading the flow depth due to the oscillating water surface downstream of the hydraulic jump. Preliminary test runs found that there was a very considerable decrease in scour when a bed sill was installed, and that about twelve hours would be required for the scoured bed to reach an
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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Fig. 2 shows the effect of sill height for the tests where the sill is introduced at the location of maximum scour depth for the control test. The vertical axis is the percent scour reduction at equilibrium (rds) of the depth behind and in front of the bed sill, ds1 and ds2 respectively:
rdsi ð%Þ ¼
Fig. 1. Characteristics lengths of the scour hole, (a) no sill condition and (b) after introducing the bed sill.
asymptotic condition where the maximum scour depth at the centerline of the flume did not change significantly. A reference test was carried out without the protection structure to find the maximum scour depth (ds0) and the longitudinal length of the scour hole (LS0) in basic conditions. Fig. 1 shows the schematic view of the main geometrical parameters of the scour hole. Altogether, 41 experiments were conducted under deeply submerged jet conditions; i.e., the tailwater depth was greater than 10 times the size of the jet opening according to the criteria of Lim and Yu [33]. The experiments covered a wide range of bedsill height and distance (Table 1 lists the range of the test conditions). A grid of 40 control positions including eight longitudinal and five vertical positions was used for the geometry of the sill. The longitudinal distances and the height of the sills for different tests were changed with intervals of 2 cm and 1 cm, respectively.
3. Results and discussion Totally 41 tests have been carried out using bed sill located at different longitudinal and vertical positions. All scour holes obtained were two-dimensional, with minor occasional wall effects at the mound region. Based on experimental results, channel bed profiles under different combinations of variables, i.e. sill height and position, indicated that the depth of the local scouring area downstream from the horizontal apron decreased with increasing resistance of the water flow over the bed sill. Results of the preliminary tests showed that the shape of the scour hole was completely different when a bed sill was introduced as a countermeasure. At a certain distance from the end of the apron, a deep hole forms behind the sill. As the jet plunging over the crest diffuses its energy in turbulent rollers inside the pools below, another scour hole forms in front of the sill. In other words, bed sills encourage the formation of two distinct scour holes with different characteristics: one upstream and another downstream of the sill. The latter is similar in shape to the one formed without any countermeasures (the control test).
ds0 dsi 100 ds0
ð1Þ
where the subscript i takes the value of 1 or 2. Negative values on vertical axis in Fig. 2 indicate that the maximum scour depth after using the sill is greater than that without the sill. Negative values for the sill height show the depth to which the sill is buried under the zero bed level. It is observed from Fig. 2 that ds1 (the maximum scour depth behind the sill) is practically not depending on h. Also, it is always greater than ds0. However, as the height of the sill above the zero bed level increases, the percent reduction of the maximum scour depth decreases in front of the sill (ds2). Also, in the cases which the sill was buried, except the zero-height sill cases, the maximum scour depth in front of the sill (ds2) was the same as the control test, but the maximum scour depth behind the sill increased in both buried and unburied sills. This caused some controversy about the study’s preliminary hypothesis about the usefulness of the bed sill. However, there were some sill heights where the percent reduction of ds2 was considerable. As an example, although the maximum scour depth between the apron and the sill increased about 53% with a 2 cm high sill, the maximum scour depth in front of the sill was reduced up to 95%. Results also suggested that buried sills may not be useful, because they showed no reduction for either ds1 or ds2. Therefore, the study continued to consider only the cases where the sill was unburied (i.e., h/b = 0.0, 0.5 and 1.0) where b is the jet opening size. However, even when not taking into account the area behind the bed sill, the question remained of which distance between the end of the apron and the sill would be the most efficient? Fig. 3 shows the variations of the dimensionless scour depth behind the sill (ds1/ds0) against the dimensionless distance between the sill and the end of the apron (L/Ls0) for different sill heights. The trend of the variations of ds1 for all three heights of the sill tested in this study is the same. It can be noted that the greater and smaller dependence on L is respectively achieved in the cases of h/b = 1.0 and h/b = 0. However, the difference is greater than 20% in some cases. It is observed that ds1 has a maximum value for each sill height. It is notable that the maximum of ds1 occurs at the distance of the maximum scour depth for the control test (L/LS0 1). So, if the sill is introduced at the location of the maximum scour depth for the reference situation (i.e. no sill condition), the maximum scour depth is respected. This may be explained by the action of the overflow jet over the end of the apron. Visual observations indicated that the flow field in the vicinity of the bed resembled a plunging jet with a recirculating region in the area of the channel bed. Balachandar and Kells confirmed this by observing flow patterns using dye injection [34]. As shown in Fig. 4, in the control conditions, the flow entering the scour hole may be divided in two when it reaches the bottom of the canal. When a sill is introduced at the dividing point of the backward and forward flow on the bottom of the canal, the sill may force the forward flow to move deeper in the canal, deepening the bed. As the height of the sill above the bed increases, the flow resistance increases. This decreases the potential for the flow to scour downstream, and a smaller scour depth in front of the sill is expected. Fig. 5 shows the variations of the dimensionless maximum scour depth in front of the sill (ds2/ds0) against the dimensionless parameter L/LS0 for three heights of the sill above the original bed. It is obvious from this figure that the height of the sill has a significant effect on the scour depth after the sill. For the case of h/b = 0.0, where less resistance against the flow occurs, the max-
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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Table 1 Summary of the experimental conditions and results.
a
Run No.
Q (lit/s)
L (cm)
ha (cm)
Ls1 (cm)
ds1 (cm)
Ls2 (cm)
ds2 (cm)
Lds (cm)
hds (cm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72 17.72
without sill 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16
without sill 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2
9 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 9 12 14 16 2 3.5 6 8 10 12 14 16 1.5 4 6 8 10 12 14 16
2.9 3.2 3.52 4.02 4.17 3.97 3.3 3.11 2.91 3.7 4.21 4.34 4.4 4.48 4.14 3.1 2.57 3.33 4.08 4.73 4.94 3.93 3.59 2.8 2.29 2.91 3.62 4 4.1 4.21 3.96 3.27 2.96 2.87 3.27 3.67 4.05 4.08 4.16 3.09 2.91
9 16 18 19 20 21 22 24 27 18 18.5 19.5 21 22 23.5 25.5 26 21 23 23.5 24 25 26 27 29 9 14 6 12 17 19 24 23 9 14 16 15 14 16 18 20
2.9 2.28 2.3 2.47 2.26 2.12 1.79 1.7 1.65 1.47 1.53 1.3 1.11 0.89 0.75 0.66 0.54 0.13 0.17 0.3 0.2 0.11 0.05 0.07 0.06 3.37 3.2 2.92 2.94 2.81 2.9 1.94 1.82 3.55 3.3 2.9 2.9 2.85 2.72 2.42 2.57
78 63 61 61 60 64 58 61 63 57 55 55 54 56 60 55 55 52 50 49 53 52 52 53 54 67 62 60 64 62 57 58 59 58 66 60 66 57 62 60 59
7.09 6.35 6.44 6.43 6.57 5.86 5.6 5.59 5.83 4.12 4.67 4.66 4.7 4.41 4.94 4.55 4.2 2.21 2.52 3.05 3.14 3 3.23 3.57 3.44 7.93 6.62 7.02 7 6.77 5.84 5.39 5.51 6.31 7.23 6.66 7.12 6.6 6.92 6.71 6.89
Negative values indicate that the sill was buried.
Fig. 2. Effect of the sill height on the maximum scour depth.
imum scour depth for all values of L/LS0 is much greater than for those of the other two cases (h/b = 0.5 and h/b = 1.0). In spite of some scatter of the data points, it is worth noting that for a given sill height, as the sill recedes (that is, L/LS0 increases), ds2/ds0 decreases. However, it is observed from Figs. 3 and 5 that in the
Fig. 3. Variations of the scour depth behind the sill against the distance to the sill.
range of the tests of the present study the best sill configuration is h/b = 1.0 and L/LS0 = 1.8. Another characteristic of the scour hole is the height of the sand wave (dune) formed downstream of the scour hole. As in this study the tailwater depth was set so that no general scour would occur,
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H. Hamidifar et al. / Ain Shams Engineering Journal xxx (2017) xxx–xxx
a)
b)
Sand bed
Sand bed
Fig. 4. Flow pattern in the scour hole, (a) without bed sill and (b) with bed sill.
Fig. 5. Variations of the maximum scour depth against the distance to the sill.
any sediment grain that was eroded from the alluvial bed was then gathered on the downstream bed, as the flow was unable to keep it in motion. The presence of the sand wave could affect the flow characteristics and needed to be considered. Fig. 6 shows the variations of the dimensionless dune height (hds/hd0) against the dimensionless parameter L/LS0 for three heights of the bed sill. It is seen from this figure that the dune height is affected by the sill height. As discussed earlier, for smaller sills the scour depth is greater. Hence, the amount of sediment that exits from the scour hole will be greater, and a higher dune may be expected for small sill heights. Also, for a given sill height, the dune height does not vary noticeably as the sill recedes from the end of the apron. How-
Fig. 6. Variations of the dimensionless dune height against the dimensionless distance to the sill.
ever, for all three heights of the sill tested in this study, the parameter hds/hd0 was less than 1. This means that introducing a bed sill of any height significantly reduces the height of the dune at the range of distances tested. Fig. 7 shows the variations of the dimensionless location of the occurrence of the maximum scour depth behind the sill (Ls1/LS0) against the dimensionless parameter L/LS0 for three different sill heights. Increasing the distance of the sill from the end of the apron caused the location of ds1 to recede. Also, the height of the sill did not affect the parameter Ls1/LS0 for the three sill heights tested in this study. It should be mentioned that the linear trend of the variations of the Ls1/LS0 is due to the fact that for all tests the Ls1 was located adjacent to the sill. Fig. 8 shows the variations of the dimensionless location of the maximum scour depth in front of the sill (Ls2/LS0) against the dimensionless parameter L/LS0. As the same as the Ls1/LS0, by increasing the distance between the sill and the end of the apron, the location of the occurrence of the maximum scour depth in front of the sill recedes. Also, the trend of variations for three sill heights are the same, but in some cases there are a considerable difference specially when the sill is closer to the apron. However, the rate of variation (the slope of the trendline) of the Ls2/LS0 is much smaller than that of the Ls1/LS0. Fig. 9 shows the variations of the dimensionless location of the maximum dune height (Lsd/LS0) against the non-dimensional parameter L/LS0. It is clear from this figure that for a given height of the sill, Lsd/LS0 does not vary significantly and remains constant as L/LS0 increases. Fig. 10 shows the scour profiles along the centerline of the channel. In this study many attempts were made to collapse all scour profiles onto each other by making dimensionless the eroded depth and the distance from the end of the apron. So, the use of different scaling variables was attempted. But as observed in this
Fig. 7. Variations of Ls1/LS0 against the L/LS0.
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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H. Hamidifar et al. / Ain Shams Engineering Journal xxx (2017) xxx–xxx
An analysis of the profile of the eroded bed at asymptotic state in the plane of symmetry indicates that the profiles for the hole (not for the mound region) are approximately similar if made dimensionless using suitable parameters. For this reason, and because of the effects of the height and distance of the sill, the depth and longitudinal extension of the hole were made dimensionless as (ds ds2)/(ds2 h) and (x L)/(ds2 h) respectively, as shown in Fig. 10. In this figure h is height of the sill above the original bed, L is the distance of the sill from the end of the apron, ds is the scour depth at the distance x from the end of the apron and ds2 is the maximum scour depth in front of the sill. Thus, the effects of sill height and its distance from the end of the apron are absorbed by the proper choice of scaling variables. Finally, regression analysis collapsed all data points reasonably well onto a single line. This study proposes Eq. (2) for predicting the shape of the scour hole:
y ¼ 0:538 þ 0:409 cosð0:347x þ 0:956Þ Fig. 8. Variations of Ls2/LS0 against L/LS0.
ð2Þ
in which x and y are the predefined non-dimensional parameters used for longitudinal extension and depth of the scour hole respectively. The proposed equation has a standard error of S = 0.18 and correlation coefficient of r = 0.82. Also, the results of the experimental measured scour depths are compared in Fig. 11 with the computed scour depths using Eq. (2). It is seen that the computed scour depths are in good agreement with observed values and most computed values fall in the ±30% lines. 4. Conclusion
Fig. 9. Variations of Lsd/LS0 against L/LS0.
This study examined scour caused by a wall jet downstream of a rigid apron interacting with non-cohesive sand beds to further understand the effects of introducing a bed sill as a countermeasure. An extensive range sill location for five sill heights was used under constant hydraulic conditions such as flow discharge and tailwater depth. Variations of the characteristic lengths of the scour hole were investigated. Results showed that the geometry of the scour hole was completely different when a bed sill was introduced as a countermeasure. It was found that a bed sill placed downstream of a rigid apron, not too far from it, reduces local scouring. Two distinct scour holes develop behind and in front of the sill. For all tests performed in this study the maximum scour depth behind the sill (between the sill and the end of the apron) was greater than when there was no sill. Nevertheless, for all tests, an outstanding sill reduced the maximum scour depth in front of the sill up to 95%. It was observed that completely buried sills
Fig. 10. Similarity of non-dimensional scour hole profiles for different experiments.
study and as reported by Deshpande et al., scour patterns were somewhat three-dimensional in the sand wave region. Hence, a great scatter was observed for this region [35]. Fig. 11. Comparison of the observed and computed values of the scour depth.
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016
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may not be useful, because no reduction occurs for scour depth either behind or in front of the sill. Results also showed that scour profiles differed for sills at different locations and heights. Finally, this method appears to have a great deal of potential if proper aprons can be designed, and it should be explored further. As the apron length was constant in all the tests, the proposed equation for prediction of the scour hole profile is only valid in the range of the parameters used in the present study.
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Hossein Hamidifar, Ph.D., Assistant Professor Water Engineering Deptartment, Shiraz University, Shiraz, Iran. The author graduated from Shiraz University and received his B.Sc. in Water Engineering in 2006 and then graduated from University of Tehran and received his M. Sc and Ph.D. degrees in Hydraulic Structures 2008 and 2012, respectively. He has many published papers in referred national and international journals and international conferences. His main field of research interests are Scour, Energy dissipation, Stilling Basin, Sediment and Pollutant Transport, and Hydraulic Structures.
Mohsen Nasrabadi, Ph.D. student in Hydraulic Structures, Department of Irrigation and Reclamation, Engineering, University of Tehran, Tehran, Iran. The author received his M.Sc. Degree from University of Tehran in 2012. His research interests include Open Channel Hydraulics, Irrigation Structures, Local Scour at the downstream of Sydraulic Structures and Sediment Transport.
Mohammad Hossein Omid, Ph.D., Distinguished Professor of hydraulic structures and river engineering, Department of Irrigation and Reclamation Engineering, Faculty of Agricultural Engineering and Technology, UTCAN, University of Tehran, Iran. He received his Ph.D. in civil engineering from Department of Civil and Structural Engineering, UMIST Manchester, UK. He has many published papers in referred national and international journals and international conferences. His main field of research interests focused on scouring downstream of stilling basins and hydraulic jump as well as river engineering. Beside his academic activities, he was serving the designers of hydraulic structures in the country.
Please cite this article in press as: Hamidifar H et al. Using a bed sill as a scour countermeasure downstream of an apron. Ain Shams Eng J (2017), http://dx. doi.org/10.1016/j.asej.2016.08.016