De Cesare, G.

Oehy, Ch.

Schleiss, Anton

Infoscience - École polytechnique fédérale de Lausanne

 Experiments on turbidity currents influenced by solid and permeable obstacles and water jet screens Giovanni De Cesare1*, Christoph D. Oehy2 and Anton J. Schleiss11 Laboratory of Hydraulic Constructions (LCH), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 18, CH - 1015 Lausanne, Switzerland (*Corresponding author, e-mail: giovanni.decesare@epfl.ch). 2 formerly at the EPFL-LCH, now at Swiss Reinsurance Company, Mythenquai 50/60, P.O. Box, CH - 8022 Zurich, Switzerland The sustainable use of reservoirs for irrigation, flood protection, water supply, and hydropower may be endangered due to unavoidable reservoir sedimentation. In many reservoirs turbidity currents are the main process for the transport and deposit of sediments. Besides other measures, turbidity currents can be influenced by means of solid obstacles, permeable geotextile screens or the injection of water jets. Physical experiments and numerical simulations of turbidity currents flowing over an obstacle, against a textile screen or through a water jet screen were carried out. In each experiment vertical velocity profiles in the body of the turbidity current were measured with an ultrasonic velocity profiler (UVP). The velocity measurements were made at three locations upstream of the various obstructions and one location downstream. The investigations showed that turbidity currents could be influenced effectively by properly designed constructive measures. Based on the results of the physical experiments and numerical simulations, some design recommendations for solid and permeable obstacles as well as for a jet screen are proposed. Keywords: Reservoir, sedimentation, turbidity current, water jet, physical modeling, numerical simulations, velocity profiles, sediment deposits 1 INTRODUCTION The aim behind the efforts to create reservoirs is storing water, however solid material is carried along by the water and is, as a rule deposited there. Long-lasting operation of reservoirs in terms of sustainable use of available water resources involves the need for sedimentation control and release. Wise development of hydropower resources regarding sedimentation has frequently not been implemented in the past and the sustainable use of reservoirs is not always guaranteed in the long term. In narrow reservoirs with quite steep bottom slopes, turbidity currents are frequently the main process for the transport and deposit of sediments [1] (Fan and Morris 1992). These turbidity currents with high sediment concentrations mainly occur during floods and follow the thalweg to the deepest zones of the reservoir near the dam. Depending on the slope of the thalweg, density currents reach velocities in the range of 0.5–0.8 m/s, and exceptionally up to 2 m/s during floods [2] (Fan 1986). Sediments, which have already settled down, can therefore be eroded again and transported toward the dam. The resulting introduction of additional suspended sediments into a turbidity current increases its density and consequently its velocity [3] (Parker et al. 1986). On the other hand, turbidity currents slow down on low slopes or after a hydraulic jump, which causes the sediments to settle and the current to die out [4] (Altinakar et al. 1990). If turbidity currents can be entirely stopped in a reservoir, or influenced in such a way that the sediments are not deposited in critical locations like in front of intakes and bottom outlets, the sustainability of the reservoir operation may be increased considerably. Such technical measures to control reservoir sedimentation due to turbidity currents have in principal the purpose to stop, dilute, or divert the flow influencing the location of major sediment deposits. This can be done by a solid or permeable obstacle [5] (Oehy and Schleiss 2007) or a jet screen placed inside the reservoir (Figure 1).  Figure 1: Inves ty currents. 2 MODELLI RRENTS Turbid n studied in 9] (Altinakar  . al e  tigated methods to stop turbidiNG OF TURBIDITY CUity currents have extensively beethe laboratory by many investigators [6-et al. 1996, De Cesare and Schleiss 1999, Baas etal. 2005, Hosseini et al. 2006). [10] De Cesare et al(2001) presented three-dimensional numericsimulations of turbidity currents in the Luzzon                                                                                                                                                                                                                           Top                                                    6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering                     41Reservoir in Switzerland. The model used in this study is based on the CFX-4 flow solver, where user routines were added to take into account the settling of the suspended sediments and the erosion-deposition at the bottom. The effects of solid obstacles or permeable screens on turbidity currents have been investigated in the case study of Lake Grimsel [11] (Oehy and Schleiss 2001) and Lake Lugano [12] (De Cesare et al. 2006). [13] Bühler et al. (2006) give an excellent overview on the phenomena of flows on inclines passing through water jets directed upstream. 3 EXPERIMENTS 3.1 Experimental set-up The experiments were carried out in an 8.55 m long, 0.27 m wide and 0.90 m deep multipurpose flume (Figure 2 left). The flume can be tilted in a slope range n the upper part of the flume a re installed. A sluice gate e turbidity current in the between 0 and 5%. Istilling box and head tank weallowed the release of thdownstream part simulating a 7.1 m long straight reservoir. An adjacent mixing tank with a capacity of 1.5 m3 was used to prepare the dense fluid mixture.    Figure 2: Photograph of the experimental flume (left) and the UVP transducer as well as the bottom and reference electrodes (right). The turbidity current was created by a rapid opening of a sliding gate. Do nstream of this gate a y distribution. A compartment at the downstream end  m from its inlet. It had a   as applied r cited model tests for the g only slightly as can be seen in the photographic sequence in Figure 3.    Figure 3: Sequ wing over a Gaussian obst ach front flow velocity Uf m/s, height of the cuh = 0.106 m, grid spacing 0.10 m. w  tranquilizer composed of small rectangular tubes reduced the scale of the initial turbulence of thereleased mixture and generated a uniform velocitof the flume trapped the turbidity current for withdrawal. For the experiments a cohesionless, fine polymer powder with a density of ρs = 1'135 kg/m3 and a particle diameter of d50 = 90 μm was chosen. More details on the experimental set-up can be found in [14] (Oehy 2002). 3.2 Modeling of obstacles (solid and permeable) and of an inclined jet screen The solid obstacle used in this study was a ridge of 24 cm height, extending across the full width of the flume at a distance of x = 5Gaussian shape. This particular form is usedbecause it does not have any edges creating flowsingularities, and can simulate an embankment dam. Furthermore, [15] Prinos (1999) investigated two-dimensional density currents over semicircular and triangular obstacles and found that there is no significant effect of the obstacle geometry. Furthermore, five turbidity current runs through a permeable screen made of two different types of Tricopor® geotextiles of 0.5 m height, also located at x = 5 m from the inlet, were carried out. The respective porosities were 36% and 41%. In order to investigate the turbidity current flow across an inclined multiport-diffuser, water jets emerged from a rectangular box 60 cm long, 27.2 cm wide and 7 mm thick placed inside the flume on the channel bottom. The jet screen was located x = 5.15 m from the inlet gate. 3.3 Measuring devices In each experiment four vertical velocity profiles in the quasi-steady body of the turbidity current were measured with an ultrasonic velocity profiler (UVP, Figure 2 right). This method wsuccessfully in the earliemonitoring of laboratory turbidity currents, as well as in a laboratory reservoir sedimentation study by [16] (Kantoush et al. 2008). A device to measure the local evolution of sediment layer thickness during the experiments was developed based on the fact that the electrical resistance of a layer of particles depends on its thickness [17] (De Rooij et al. 1999). The thickness of the sediment deposits can thus be determined by measuring its resistance. The resistance was measured between a 6 mm stainless-steel rod, 6.5 m long, mounted 0.5 m above the flume bottom and 62 electrodes on the bottom (Figure 2 right). 4 RESULTS 4.1 Flow over a solid obstacle When the turbidity current reached the obstacle, it climbed up, deceleratin  t = 0 s t = 10 st = 30 st = 20 sence of a turbidity current floacle at time intervals of 10 s. Appro = 0.039 rrent 6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering                                                                                                                        42The turbid stacle, whereas a g head. A less importa on and kept flowing n the flume. The normal  region was reestablishe ream of the obstacle. Du cle, the flow rate changed and an internal raveled upstream. Bores are moving hydraulic jumps and also occur in open-channel flows. 4.2 Flow through a permeable scrn, low en ht of 2 - 3 times the 4.3 Flow thr jet screen Figure 5 shows a photographic progression of the turbidity current flowing through the jet screen.  Figure 5: Tu h the 45° inclined jet screen at ti w velocity f  passes through while the major ed upwards. were in the quasi-steady part of the turbidity current body. The computed velocity profiles are compared with the experimental results as shown in Figure 6 for the passage over a solid obstacle and through a permeable screen.ity current head passed over the obap developed behind the nt current followed the head regi over the top of the obstacle dowobstabore t  shape of the frontald at some distance downste to the presence of the eenWhen the turbidity current reached the vertical screeit was nearly blocked due to the increased fresistance (Figure 4). The turbidity current thclimbed up the screen to a heigheight of the oncoming turbidity current, and decelerated as it rose. The turbidity current then started to seep through at the bottom of the screen driven by the pressure gradient. As the interface upstream rises, more fluid passes through the screen, forming a small and slow outgoing turbidity current. Due to the flow resistance of the screen, part of the flow is reflected and moves upstream as an internal bore similar to the experiments with a solid obstacle.    Figure 4: Sequence of a turbidity current flowing through a permeable screen (41% porosity) at time intervals of 10 s. Approach front flow velocity Uf = 0.051 m/s, height of the current h = 0.112 m. ough a water rbidity current flowing througme intervals of 10 s. Approach floUf = 0.042 m/s, height of the current h = 0.106 m. The sequence starts just before the turbidity current reaches the inclined jet screen. A small amount othe turbidity currentpart of it remains upstream and is push4.4 Velocity profiles of the turbidity currents Computed profiles of streamwise velocity compared with the values determined from measurements by UVP using time-averaged profiles  a) b)  Figure 6: Vertical velocity profiles upstream and down-stream of the obstacle nd b) and of the permeablscreen c) and d). Computed velocity, u (thin solid line); measured velocity (plus signs); and computed concentration, cs (dashed line). In addition, the computed concentration distribution using CFX-4 is shown on the same graphs. It can be seen that the numerical results agree fairly well with the measured distribution of the streamwise velocity. The height at which the concentration vanishes coincides approximately with the height of zerovelocity. Due to the effect of the obstacle nd screen, the downstream velocity as well as  concentration is strongly reduced, whereas the height is increased. 4.5 Influence on sediment deposition The measured evolution of the sediment deposits along the flume is shown in Figure 7 and has been compared to the results from the numerical model. Both approaches showed that sedimentation downstream of the diffuser is significantly reduceda) a e   a thet = 10 st = 0 s t = 30 st = 20 s t = 0 s t = 10 st = 20 s c) d)                                                                                                                                                                                                                           Top                                                    6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering                     43while the upstream deposition is increased duethe flow and sediment retention.  to  Figure 7: Measured evolution of sediment deposit of the turbidity current partially blocked by the 45° upward inclined water jet screen at time intervals of 10 s. 5 CONCLUSIONS The long term use of reservoirs for irrigation, flood protection, water supply and hydropower may be endangered due to unavoidable reservoir sedimentation. The challenge for designers and dam operators is to achieve sustainable storage volumes by means of wise reservoir sedimentation management. Very often turbidity currents are the governing process for the transport and deposition of suspended sediments in reservoirs. Physical experiments and numerical simulations of turbidity currents flowing over a solid obstacle and through a geotextile screen as well as through an inclined water jet screen were carried out.  Based on the tests and simulations it can be concluded that solid obstacles with reasonable heights (at least twice the high of the approachin s ficant ed also urations, turbidity currents can ams (SwissCoD) and Met-Flow for their support of this study. eiss, A. (1999). Turbidity sical model flume using e in a current, J. Geophys. Res.-Oceans. 0.1029/2004JC002668.. ity currents using an 59-68. ng., 127(1), 6-16. Switzerland, Proc., s due to flood-induced flow on inclines, Proc., Int. Conference on  (CD-ROM), Measurement and diment layer thickness under g turbidity current) allow for efficient blocking ofturbidity currents. The laboratory tests as well anumerical case studies revealed this signieffect of obstacles. The investigations showthat, in certain configbe considerably slowed down by a geotextile or an inclined water jet screen and therefore most of the sediments can be retained upstream. The technical measures presented may also be of interest in combination with other traditional methods, such as flushing or turbidity current venting, but also with new concepts of sediment management in reservoirs. ACKNOWLEDGEMENT The authors gratefully thank the PSEL foundation of the Swiss Union of Electricity Producers, grant N° 175, the Swiss Committee on DREFERENCES [1] Fan, J., and Morris, G. L. (1992). Reservoir sedimentation I: Delta and density current deposits, J. Hydraul. Eng., 118(3), 354–369. [2] Fan, J. (1986). Turbid density currents in reservoirs, Water Int., 11(3), 107-116. [3] Parker, G., Fukushima, Y., and Pantin, H. M. (1986). “Self-accelerating turbidity currents.” J. Fluid Mech., 171, 145–181. [4] Altinakar, M. S., Graf, W. H., and Hopfinger, E. J. (1990). Weakly depositing turbidity current on a small slope, J. Hydraul. Res., 28(1), 55–80. [5] Oehy, Ch., and Schleiss, A. (2007). Control of turbidity currents in reservoirs by solid and permeable obstacles, J. Hydraul. Eng., 133(6), 637-648. [6] Altinakar, M., Graf, W., and Hopfinger, E. (1996). Flow structure of turbidity currents, J. Hydraul. Res., 34(5), 713–718. [7] De Cesare, G., and Schlcurrent monitoring in a phyultrasonic Doppler method, Proc., 2nd ISUD, Villigen PSI, Switzerland, 61-64. [8] Baas, J. H., McCaffrey, W. D., Haughton, P. D. W., and Choux, C. (2005). Coupling between suspended sediment distribution and turbulence structurlaboratory turbidity 110 (C11015), doi:1[9] Hosseini S. A., Shamsai, A., and Ataie-Ashtiani, B. (2006). Synchronous measurements of the velocity and concentration in low density turbidAcoustic Doppler Velocimeter, Flow Measurement and Instrumentation, 17 (2006), [10] De Cesare, G., Schleiss, A., and Hermann, F. (2001). Impact of turbidity currents on reservoir sedimentation, J. Hydraul. E[11] Oehy, Ch., and Schleiss, A. (2001). Numerical modeling of a turbidity current passing over an obstacle - Practical application in Lake Grimsel, Int. Symp. on Env. Hydr. (CD-ROM), Tempe, Ariz. [12] De Cesare, G., Boillat, J.-L., and Schleiss A. J. (2006). Circulation in stratified laketurbidity currents, J. Environ. Eng., 132(11), 1508-1517. [13] Bühler, J., Oehy, Ch., and Schleiss, A. (2006). Jets opposing Fluvial Hydraulics, River Flow 2006, Lisbonne, 1617-1622. [14] Oehy, Ch. (2002). Effects of obstacles and jets on reservoir sedimentation due to turbidity currents, Communication No. 15 of the Laboratory of Hydraulic Constructions (LCH), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. [15] Prinos, P. (1999). Two-dimensional density currents over obstacles, Proc., 28th IAHR CongressGraz, Austria. [16] Kantoush, S. A., De Cesare, G., Boillat, J.-L., and Schleiss, A. J. (2008). Flow field investigation in a rectangular shallow reservoir using UVP, LSPIV and numerical modeling, Flow Instrumentation, Elsevier, 19 (2008), pp. 139–144.[17] De Rooij, F., Dalziel, S. B., and Linden, P. F. (1999). Electrical measurement of sesuspension flows, Exp. Fluids, 26, 470–474. 6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering                                                                                                                        44

Experiments on turbidity currents influenced by solid and permeable obstacles and water jet screens

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Experiments on turbidity currents influenced by solid and permeable obstacles and water jet screens

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