Improving the Pressurized Flushing Efficiency in Reservoirs: an Experimental Study Mohamad Reza Madadi, Majid Rahimpour & Kourosh Qaderi
Water Resources Management An International Journal - Published for the European Water Resources Association (EWRA) ISSN 0920-4741 Water Resour Manage DOI 10.1007/s11269-017-1770-y
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Author's personal copy Water Resour Manage DOI 10.1007/s11269-017-1770-y
Improving the Pressurized Flushing Efficiency in Reservoirs: an Experimental Study Mohamad Reza Madadi 1 & Majid Rahimpour 1 & Kourosh Qaderi 1
Received: 25 July 2016 / Accepted: 26 June 2017 # Springer Science+Business Media B.V. 2017
Abstract Sediment flushing in many reservoirs of the world is accomplished with low efficiency. In this study, a new configuration was proposed for reservoir bottom outlet to increase the pressurized flushing efficiency. In the new configuration, a projecting semicircular structure was connected to the upstream edge of bottom outlet. It was observed that by employing the projecting bottom outlet, the sediment removal efficiency increased significantly compared to the flushing via typical bottom outlet. In the case of new-configuration bottom outlet with Lsc/Doutlet = 5.26 and Dsc/Doutlet = 1.32, the dimensionless length, width and depth of flushing cone increased 280%, 45% and 14%, respectively, compared to the reference test. The proposed structure can ensure the sustainable use of reservoirs. Keywords Reservoir desilting . Flushing efficiency . Bottom outlet . Semi-circular structure
1 Introduction Sedimentation is one of the most serious problems for the sustainable use of reservoirs. Several techniques have been employed to alleviate the sedimentation problem. One of the best methods is sediment flushing, through which, the previously deposited sediments in the reservoir are hydraulically removed by opening the bottom outlets of dam. Flushing can be classified into two types of free flow flushing (reservoir complete drawdown) and pressurized flushing (reservoir partial
* Mohamad Reza Madadi
[email protected] Majid Rahimpour
[email protected] Kourosh Qaderi
[email protected]
1
Department of Water Engineering, Shahid Bahonar University of Kerman, P.O.Box: 76169-133, Kerman, Iran
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drawdown). Previous studies reported that the pressurized flushing has very low efficiency in removal of deposited sediments from the reservoirs (Mahmood 1987; Sloff 1991; Fan and Morris 1992; Kantoush 2008). In this type of flushing, a great amount of water is released from the bottom outlets but a little effect occurs in restoring the reservoir active storage (Jansson and Erlingsson 2000). White and Bettes (1984) demonstrated that the developing rate of flushing cone is negligible during the pressurized flushing. Hotchkiss (1989) and Emamgholizadeh et al. (2006) reported that without sufficient drawdown, recovery in reservoir storage capacity is negligible. Janssen and Shen (1997) reported that by flushing operation, little erosion and poor channel widening occurs within the reservoir. Jansson and Erlingsson (2000) found that by pressurized flushing, the scouring will happen in a very limited area in the vicinity of bottom outlet. Scheuerlein et al. (2004) claimed that the flushing operation can only be effective when it is carried out with a substantial water level drawdown. Based on their experimental data and the data of ten reservoirs in China, Lai and Shen (1996) investigated the mechanism of free flow flushing and presented equations to predict the characteristics of flushing cone. Talebbeydokhti and Naghshineh (2004) performed an experimental work on a physical model of reservoir and concluded that the amount of flushed sediment during pressurized flushing depends on the outflowing discharge, water level gradient and width of flushing channel. Powell and Khan (2012, 2015) conducted laboratory experiments to investigate the flow characteristics and the sediment transport upstream of circular outlets and reported the formation of vortices near the outlet. Fathi-Moghadam et al. (2010) and Emamgholizadeh and Fathi-Moghadam (2014) provided relations to predict the size of flushing cone and concluded that the size of flushing cone has a reverse relation with the water depth and the sediment size and a direct relation with the outflowing discharge. Meshkati et al. (2009) investigated the effect of shape and dimensions of bottom outlet on the size of flushing cone. Ahadpour Dodaran et al. (2012) experimentally studied the effect of frequency and location of a vibrating plate located at the reservoir on the size of flushing cone. They concluded that the vibrating plate has positive effect on the dimensions of flushing cone. Althaus (2011) investigated the effect of jet induced flow on sediment evacuation from the reservoirs. Madadi et al. (2016) proposed a confined piles group structure to increase the sediment removal through a large circular orifice. As can be seen, most of the previous researches were devoted to the recognition of flushing mechanism and the factors that influence on the flushing operation. There are very few studies regarding the methods of increasing the sediment removal efficiency during the pressurized flushing operation, and to the author’s knowledge, no applicable measure has yet been provided to this purpose. Accordingly, the major novelty of this work is that, in this study, a new configuration was proposed for dam bottom outlet to increase the pressurized flushing efficiency. In the new configuration, a projecting semi-circular structure (PSC structure) is connected to the dam body from one hand, and is mounted on beams which penetrate to a depth of reservoir bed by piles, from the other hand, as shown in Fig. 1. There are several structural and hydraulic advantages regarding the PSC structure. The risk of failure or structural instability due to the external forces is negligible in this structure. The semicircular shape of PSC structure prevents the concentration of stresses (induced by the water hydro-static force) on the crown of structure. In this way, the vertical forces are transferred to the reservoir bed through beams and piles. The other advantage of PSC structure is that, the fine suspended sediments in the reservoir will not accumulate on the structure because of arcuate shape of the structure which causes slipping of sediments from the crown of structure. Furthermore, PSC structure constricts the outflowing water from the bottom outlet and reinforces the erosional power of the flow for evacuating the deposited sediments. In addition, it changes the axis of the vertical vortices (that forms behind the bottom outlet) to the horizontal axis. In this way
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Fig. 1 New configuration for reservoir bottom outlet
the strength and the extent of the vortices increases. Therefore, a significant increment occurs in removal of the deposited sediments from the reservoir by using the PSC structure. So, the projecting semi-circular structure can satisfy both hydraulic and structural requirements that are needed for design, construction and operation of a hydraulic structure. Accordingly, in this study, PSC structure was proposed and successfully tested with the aim of strengthening and extending the vortices that are established behind the bottom outlet during the flushing operation. The temporal variations and the geometrical characteristics of flushing cone for different sizes of semi-circular structure and different hydraulic conditions was
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investigated. The flushing efficiency was calculated and a relation was offered for predicting the volume of flushing cone.
1.1 Experimental Set-Up The experiments were conducted at the Hydraulic Laboratory of Shahid Bahonar University of Kerman, Iran. A model of reservoir was constructed with the shape of a prismatic tank having 2.5 m length, 1.3 m wide and 1.5 m height. One quarter of the reservoir’s side wall and the middle part of the front wall were made of glass, providing transparency for visual observations, whereas the other walls were made out of steel plates. The reservoir bottom outlet was a 9.5 cm diameter circular orifice. In the laboratory, the water was supplied from an under-floor sump using a centrifugal pump and thereafter was fed to the reservoir model through 76 mm diameter pipe with an adjustable valve for controlling the discharge. Provisions (weir wall and perforated bricks) were embedded within the first 50 cm of the reservoir, across the entire cross-section to assist in turbulence damping and overall uniformity of approaching flow. The outflow from reservoir was finally returned to the sump and recirculated through this system. The flow rate was measured with a calibrated 90° V-notch weir located at the downstream wall of the second stilling basin. It was also measured using volumetric method to enhance the accuracy of discharge measurement. More details of the experimental setup is available in Madadi et al. (2016). Series of experiments carried out for outflow discharge rate of 14.5, 12.5 and 10 L/s, for constant flow head of 0.6 m at the reservoir. The effect of variations of flow head were previously investigated by Emamgholizadeh et al. (2006) and Meshkati et al. (2009). The flushing cone topography were measured longitudinally and transversally using a manual pointer gauge with ±1 mm of reading accuracy. In addition, the videos were recorded during the experiments by Canon Powershot SX 160 IS camera and used to visualize the scour processes and temporal variations of flushing cone. The sediment used in the experiments consisted of non-cohesive sand particles with a median diameter of D50 = 0.36 mm, relative density of 2.65 and geometric standard deviation of 2.24. To improve the pressurized flushing efficiency, a semi-circular projected structure was connected to the dam bottom outlet. The experiments were performed for four different diameters (0.11 m, 0.125 m, 0.15 m and 0.16 m) and four lengths (0.15 m, 0.25 m, 0.4 m and 0.5 m). The presence of semi-circular structure causes the flow to be constricted in the structure. The constricted flow imposes stronger forces (stresses) to the surface of deposited sediments and erodes more materials from the reservoir.
1.2 Test Procedure The experiments included the tests with typical bottom outlet (reference tests) and the tests with new-configuration bottom outlet. Before each run was started, sediments were evenly leveled with the outlet invert with a thickness of 40 cm. In order to avoid the undesirable erosion of the sediment bed, the reservoir was initially filled with water from a hosepipe. Once the water depth reached the desired level over the sediment bed, the motor was started and the experiment was run by adjusting the discharge to the desired rate. At the same time, the bottom outlet valve was manually opened so that the inflow to the reservoir became equal to the outflow. Experiments were run until the flushing cone reached to the equilibrium condition. For this study, the flushing cone was assessed as having reached equilibrium when the side walls of flushing cone did not change more than 1 mm within a 1-h period. This was
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determined by analyzing the video recording of flushing cone during the flushing cone development process. At the end of each experiment, in order to preserve the shape of the resulting flushing cone, both the reservoir inlet valve and the bottom outlet valve was gradually closed and the remained water in the reservoir was allowed to drain off gradually through a 10 mm valve which was placed at the floor of the reservoir. After the water was drained, the topography of the flushing cone was measured using a manual point gage and mesh grid system. The measured parameters of flushing cone were: (1) the maximum scour depth (dc)defined as the vertical distance from the initial (flat) bed elevation to the deepest point in the stable flushing cone. (2) The maximum length of flushing cone (Lc), defined as the longitudinal distance from the bottom outlet to the upstream limit of the flushing cone, (3) the maximum width of flushing cone (Wc), which represents the extent of lateral expansion of the flushing cone; and (4) the volume of flushing cone(∀cone)which indicates the amount of evacuated sediments through flushing operation.
1.3 Dimensional Analysis In the pressurized flushing, the flow pattern close to the bottom outlet is three dimensional and a high number of parameters involved in the phenomena (Scheuerlein et al. 2004). The volume of the flushing cone(∀cone)depends on outflow discharge (Qoutlet), flow depth above the outlet(H), mass density of the sediment(ρs), mass density of the fluid(ρ), dynamic viscosity of the fluid(μ), gravitational acceleration(g), median size of the sediment particles(D50), diameter of outlet(Doutlet), diameter of semi-circular structure(Dsc) and length of semicircular structure (Lsc). The set of characteristic parameters appropriate for the determination of ∀cone can be represented by the following functional form: f ð∀cone ; Qoutlet ; H; ρs ; ρw ; μ; g; D50 ; Doutlet ; Dsc ; Lsc Þ ¼ 0
ð1Þ
Where, f denotes a Bfunction of^. Using Buckingham π−theorem, the significant nondimensional parameters controlling the volume of flushing cone in functional form can be obtained as below: 0 1 H D50 Dsc Lsc B ∀cone Qoutlet C ffi; ; ; ; ; Gs; ReA ¼ 0 f @ 3 ; qffiffiffiffiffiffiffiffiffiffiffiffiffi D D D D Dcone 5 outlet outlet outlet outlet gDoutlet
ð2Þ
Where, Re ¼ ρuoutletμDoutlet is the outlet Reynolds number, and Gs ¼ ρρs is the specific gravity of w sediment. The effect of Re was negligible due to the fully turbulent flow from bottom outlet. Also, Gs was dropped because both water and sand densities had constant value through the experiH ments. Since the value of Doutlet, H and D50 was constant in all tests of this study, the value of Doutlet 50 and DDoutlet remained constant, as well. Therefore, Eq. (2) can be simplified as Eq. (3): 1 0
∀cone Dsc Lsc C B Qoutlet ffi; ¼ f 1 @ qffiffiffiffiffiffiffiffiffiffiffiffiffi ; A 3 Doutlet Doutlet Doutlet gD5
ð3Þ
outlet
According to Eq. (3), the bottom outlet discharge, the diameter and the length of the semicircular structure are three effective parameters on the volume of flushing cone. In the present study the effect of Qoutlet, Dsc and Lscon the geometric characteristics of flushing cone was
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investigated. The coordinate system was set with the origin at the invert of the bottom outlet. The x, y and z positive location was measured upstream of the bottom outlet, laterally, and vertically across the outlet, respectively.
2 Results and Discussion As previously said, the laboratory tests were divided into two series of experiments with typical bottom outlets (reference tests) and the experiments with new configuration bottom outlet (tests with PSC structure). Followings are the main results of the study. First, the general observations are reported. Next, the effect of employing the newconfiguration bottom outlet on the performance of flushing operation and dimensions of flushing cone are discussed.
2.1 Flushing with Typical Bottom Outlet The primary mechanism of sediment transport during the first seconds of this set of experiments was the excess flow shear stresses, generated by the accelerated outflowing water through the bottom outlet. The outflowing water scoured the sediments and created a flushing cone upstream of the bottom outlet. The flushing cone extended symmetrically toward the upstream with a half-cone shape. During this stage, a large amount of hyperconcentrated flow was released from the reservoir. After about 3 min, due to increasing the depth of flushing cone, the velocities and shear stresses in the flushing cone decreased. Next, two counter-rotating vortices begun to form below the invert of the bottom outlet (Fig. 2). Nagahara et al. (2003) claimed that the velocity and pressure distribution of these submerged vortices is such a way that, at the center of the vortex the velocity is maximum and the pressure is minimum, so that, by increasing the distance from the center, the velocity decreases and the pressure increases. These vortices caused
Fig. 2 Typical flow patterns upstream of typical outlet, reference test
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lifting the sediments from the flushing cone into the outflowing water and afterward became the dominant mechanism for removing the sediments from the flushing cone. Then, the vortices were mixed together and produced a stronger drifting vortex. Sediment particles begun to roll down the sides of the flushing cone, then was fed into the central drifting vortex and sucked vertically out of the cone and through the bottom outlet. After about 240 min, the equilibrium condition was reached, after which, the outflow from reservoir was almost clear. At the end of each experiment, a series of ridges and troughs were observed within the flushing cone (Figs. 3 and 4). Powell (2007) stated that the strength of the flushing vortices can be estimated by analyzing the shape and orientation of these ridges and troughs.
2.2 Flushing with New-Configuration Bottom Outlet By employing the new-configuration bottom outlet, it was observed that two further scouring processes were generated in addition to the vortices action, as shown in Figs. 3
Fig. 3 Typical flow patterns upstream of the new configuration outlet
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Fig. 4 Topography of flushing cone after removing the semi-circular structure
and 4. (i) The constricted axial flow passing through the projecting semi-circular structure pushed bed material along the entire length of the structure, first by progressive erosion and then by retrogressive erosion. (ii) The lateral flow along the structure caused piping erosion, through which, a large amount of bed materials was scoured from the semi-circular structure environs. In each test of this set of experiments, there was strong interactions between the semicircular structure, the size of the flushing cone and the strength of vortices. The flow had to pass through the semi-circular structure before reaching the bottom outlet. This led to establishing a constricted flow with higher velocity and tractive forces which swept away the bed material and developed the flushing cone at a faster rate compared to the reference test. During the first seconds of this set of experiments, the rate of flushing scour was very rapid rather than the reference test but it decreased with time. Scouring was continued until the equilibrium condition was reached, i.e., when the flow shear stresses and turbulent agitation inside the semi-circular structure were no longer able to transport bed material from the flushing cone. At the end of each experiment, it was observed that the material that was been eroded from the semi-circular structure perimeter, formed a single ridge (bar) along the central axis of the structure with a height of approximately one-eighths the pipe diameter (Fig. 3). The ridge was covered by a layer of relatively coarser particles. In addition, it was observed that, almost in all tests of this study, the maximum scour depth occurred at the center of the flushing cone due to the effect of central vortex (Fig. 4).
2.2.1 Effect of Length of Semi-Circular Structure The observations indicated that the length of projecting semi-circular structure has a significant effect on the dimensions of flushing cone. As shown in Fig. 5, by increasing the dimensionless length of the projecting structure (Lsc/Doutlet) from 1.58 to 5.26, the maximum relative length (Lc/Doutlet) and the maximum relative width (Wc/Doutlet) of
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Fig. 5 Variations of flushing cone dimensions vs. relative length of PSC structure
flushing cone increased up to 248% and 28%, respectively, compared to the test with typical outlet. Eqs. (4) to (6) represent the relationship between the flushing cone dimensions and the relative length of the semi-circular structure. Ls Lsc ¼ 0:728 ð4Þ þ 1:37 R2 ¼ 0:975 Doutlet Doutlet Ws Lsc ¼ 0:0428 þ 3:32 R2 ¼ 0:997 Doutlet Doutlet ds Doutlet
Lsc ¼ −0:01789 Doutlet
ð5Þ
þ 0:85 R2 ¼ 0:882
Fig. 6 Variations of flushing cone dimensions vs. relative diameter of PSC structure
ð6Þ
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2.2.2 Effect of Diameter of Semi-Circular Structure Four different diameters were used for semi-circular structure to investigate the effect of structure diameter on the geometric characteristics of flushing cone. Fig. 6 shows the effect of relative diameter (Dsc/Doutlet) on the dimensions of flushing cone. It can be seen that, all the Lc/Doutlet, Wc/Doutlet and dc/Doutlet parameters have their maximum values in Dsc/Doutlet = 1.32, indicates the optimum dimensionless diameter of PSC structure for the experimental conditions of this study. In the case of new-configuration bottom outlet with Lsc/Doutlet = 5.26 and Dsc/Doutlet = 1.32, the dimensionless length, width and depth of flushing cone increased 280%, 45% and 14%, respectively, compared to the reference test. Eqs. (7) to (9) illustrate the nonlinear correlation between PSC structure diameter and dimensions of flushing cone. Ls Doutlet
¼ −2:6695
Dsc Doutlet
2
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Dsc ¼ −0:1908 Doutlet
2
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ð7Þ
ð8Þ
þ 0:85 R2 ¼ 0:915
ð9Þ
2.3 Slope Angles of Flushing Cone The experiments showed that, the side slope of flushing cones was approximately equal to the submerged angle of repose of used sediments, 26o. This is consistent with the observations of Emamgholizadeh et al. (2006) and Fathi-Moghadam et al. (2010). Fang and Cao (1996) reported that, for granular sediments, the slope of flushing cone may be estimated by the submerged angle of repose. For the outlet with new configuration, it was observed that the slope angle of cone was slightly diminished compared to the slope angle of cone at the reference test.
2.4 Flushing Cone Cross-Sections for Semi-Circular Structure The flushing cone cross-sectional data may be used for the calibration, assessment and development of numerical models. Fig. 7 indicates the effect of PSC structure on the longitudinal development of flushing cone, for three outflow discharges of 14.5, 12.5 and 10 L/s. Fig. 8 shows the transverse cross-sections of flushing cone for three different distances of y/Doutlet = 0.53, y/Doutlet = 1 and y/Doutlet = 1.53 from the outlet, for both cases of with- and without semi-circular structure, for different outflow discharges. From the figure, the dimensions of flushing cone for the reference test decreased by increasing the distance from bottom outlet. In other word, the flushing cone cross-sectional area at distance y/Doutlet = 0.53 was 9.22 times the cross-sectional area at y/Doutlet = 1.53. While for the test with new-configuration outlet with Lsc/Doutlet = 5.26 and Dsc/ Doutlet = 1.58, the ratio of cross-sectional areas at the same distances was close to unity (1.14), indicates that the dimensions of flushing cone was nearly constant along the entire length of the
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semi-circular structure. For the closest cross-section to bottom outlet, the dimensions of flushing cone for both of the reference test and the test with new-configuration outlet were almost the same, while by increasing the distance from the outlet, the effect of PSC structure on dimensions of flushing cone became incontrovertible. In other word, for y/Doutlet = 0.53, the ratio of cross sectional area in new-configuration bottom outlet to that of the reference test was about 1.25, while this ratio increased to 10.2 for y/Doutlet = 1.53.
2.5 Temporal Development of Flushing Cone One of the main objectives of this study was to determine the time varying development of flushing cone. To this purpose, the time development of flushing cone was investigated during the scouring process using a high speed photography system. Fig. 9 indicates the development pattern of flushing cone with time. As can be seen, in the case of typical outlet, the scour was occurred in front of the bottom outlet immediately after opening the flushing valve and thereafter extended symmetrically toward the upstream with a half-cone shape. The scour rate was very high at the early stage of the scouring process, then decreased with time. For the test with new-configuration bottom outlet, the scouring process initiated at the inlet edges of the PSC structure and then propagated rather rapidly along the entire length of the structure. In this case, the rate of temporal development of flushing cone increased significantly. Finally, an oval groove was established around the structure. At about 240 min after the commencement of the test, the variations rate of flushing cone was decreased and the final shape of flushing cone was formed.
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2.6 Flushing Efficiency Flushing efficiency (E) is an index to describe the effectiveness of hydraulic flushing. Following equation was suggested by Qian (1982) for calculating the flushing efficiency. It implies the ratio of volume of flushed sediment (∀C) to the volume of water used (∀w) during the flushing operation. ∀c ∀w
E¼
ð10Þ
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Fig. 8 Longitudinal cross sections of flushing cone along the Y-axis for typical outlet and PSC outlet (LSC/ Doutlet = 5.26 and DSC/Doutlet = 1.58)
Author's personal copy Improving the pressurized flushing efficiency in reservoirs
Fig. 9 Flushing cone cross-sections in different distances from the outlet
By employing bottom outlet with new configuration, the flow field became constricted inside the semi-circular structure and a flow jet formed with stronger erosional forces and shear stresses. Therefore, the flushing efficiency increased. To calculate the volume of flushing cone, after each run, the x,y,z points of flushing cone were imported to SURFER-10 software. It was found that, the volume of flushing cone at the test with semi-circular structure was about 4.5 times more than the reference test, indicating an impressive effect of new-configuration bottom outlet on improving the sediment removal efficiency. Based on the experimental results of this study, a nonlinear correlation of the PSC structure dimensionless length and diameter for estimation of flushing cone volume was obtained as Eq. (5). 10:53 0 ∀cone Q DSC 0:58 LSC 0:91 C B ð11Þ ¼ 0:31 @ qffiffiffiffiffiffiffiffiffiffiffiffiffiffi A R2 ¼ 0:89 Doutlet Doutlet D3outlet gD5outlet
Fig. 10 Observed vs. calculated dimensionless volume of flushing cone
Author's personal copy Madadi M.R. et al.
Fig. 10 represents the observed values of dimensionless volume of flushing cone versus the calculated values by Eq. (11). As shown, there is a good agreement between the observed and calculated values.
3 Conclusion This research was performed with two purposes: 1) to study the flow characteristics and sediment scour during pressurized flushing operation, when the sediments are accumulated up to the invert of orifice, and 2) to improve the flushing efficiency by employing a semi-circular structure in the reservoir. The effect of geometrical properties of proposed structure on the dimensions of flushing cone was investigated. The time development of flushing cone was followed with and without the structure. The results showed that, by increasing the dimensionless length of the PSC structure from 1.58 to 5.26, the maximum relative length and the maximum relative width of flushing cone increased 248% and 28%, respectively. In addition, the diameter of semi-circular structure had a significant effect on geometrical characteristics of flushing cone. Using PSC structure, the flushing efficiency increased significantly compared to the flushing with typical outlet. The results of this study can be used for dam designers and managers to optimize sediment management in reservoirs. The design of proposed bottom outlet should be incorporated as early as possible in the initial design of a dam, however, it is possible to carry out it for existing small dams. The following symbols were used in this paper: ρs mass density of the sediment (kgm−3), ρ mass density of the fluid (kgm−3), σg geometric standard deviation of sediment (−), ∀cone volume of flushing cone (m−3), ∀w volume of water (m−3), μ dynamic viscosity of the fluid (Pa.s), D50 median size of the sediment particles (m), Doutlet diameter of bottom outlet (m), Dsc diameter of PSC structure (m), dc maximum depth of flushing cone (m), E flushing efficiency (−), g acceleration due to gravity (ms−2), Gs specific gravity of sediment (−), H flow head above the outlet (m), Lc maximum length of flushing cone (m), Lsc length of PSC structure (m), Re bottom outlet Reynolds number (−), uoutlet mean outflow velocity from bottom outlet (ms−1), Wc maximum width of flushing cone (m).
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