INTRODUCTION TO HYDRODYNAMIC DESIGN OF SPILLWAY FOR HYDRAULIC JUMP TYPE STILLING BASIN AS AN ENERGY DISSIPATOR

Transcription

1 INTRODUCTION TO HYDRODYNAMIC DESIGN OF SPILLWAY FOR HYDRAULIC JUMP TYPE STILLING BASIN AS AN ENERGY DISSIPATOR Kaoustubh Tiwari 1 Utkarsh Nigam 2, BalKrishna 3 Dr. R.P Dubey 4 1 Engineer (Ports &Harbour), WAPCOS Limited, Pune, kaoustubhtiwari@gmail.com 2 Asst. Prof., SRPEC, Unjha, Gujarat, India. ;-utkarsh.nigam99@gmail.com Dy. Chief Engineer (Ports &Harbour), WAPCOS Limited, Pune, Senior General Manager (Ports &Harbour), WAPCOS Limited, New Delhi Abstract: This paper mainly deals with the energy dissipation of spillways through hydraulic jump type stilling basins and a complete overview of hydraulic uplift and other hydrodynamic forces has been provided and comparison with other energy dissipation is also studied. Also discussion includes that for finalizing the structural design of stilling basin floor, uplift forces likely to be experienced by the individual floor monoliths are required to be assessed. Dissipation of the huge energy generated at the base of a spillway at downstream is essential. Hence, bringing the flow into the downstream river to the normal (almost pre-dam) condition in as short of a distance as possible. This is necessary, not only to protect the riverbed and banks from erosion, but also to ensure that the dam itself and adjoining structures like powerhouse, canal, etc. are not undetermined by the high velocity turbulent flow. Although a variety of devices are used for energy dissipation at the base of spillways, the dissipation of energy is through internal friction and turbulence or impact and diffusion of the high velocity flow in the mass of water. Various types of energy dissipators are used to dissipate kinetic turbulence of water into potential reach at downstream. Uplift and piping failures also have a main concern. Keywords: Energy Dissipatiors, Hydraulic Jump and its types, Spillways, Types of Energy Dissipators. 1. INTRODUCTION A spillway is a hydraulic structure designed to prevent overtopping of a dam at a place and to spill and release water as and when required. A reservoir will overflow if its capacity is less than the difference between the volumes of inflow and outflow. The spillway has five basic components which forms an integral part of it. These are (a). an entrance channel, (b). A control structure, (c). A discharge carrier, (d).an energy disspator and (e).an outlet channel. The main concern here is to depict and describes the advantages of fouth component i.e. energy dissipators for spillway and its design concern. Energy disspators converts potential energy into kinetic energy and then into turbulence and finally into heat. At the base of spillway, the dissipation of energy is through internal friction and turbulence and diffusin of high velocity into mass of fluid as given in Khatsuriya R.M.(2005). Principal types of energy dissipators are have studied, compared and the design aspect and characteristics of Stilling jump type energy. Spoljaric, A. et. al. (1982) studied the Unsteady dynamic force due to pressure fluctuations on the bottom of an energy dissipator.toso, J. W.and Bowers, C. E.(1988) researched on 1

2 Extreme pressures in hydraulic jump stilling basins.farhoudi and Narayanan (1991) studied experimentally the drag force induced by hydraulic jump on baffle blocks of stilling basin downstream of sluice gate. Firotto and Rinaldo (1992b) studied studied the features of hydraulic jump downstream of sluice gate, where Froude number ranges between 5 to 9.5. the function of induced dynamic force in stilling basins was experimentally studied by Bellin and Firotto (1995). The present work would be devoted to investigate and study the hydrodynamic design aspects of Stilling Jump type energy dissipators and the methods for calculating uplift force by analytical or experimental means is also studied along with comparision of various energy dissipators. Also the characteristics and properties of various forces action on a stilling jump type energy dissipators are studied. 2. SPILLWAYS AND TYPES OF SPILLWAYS A spillway has various functions and also there are different types of spillways which can be classified according to numerous criteria s. 2.1 Functions of A Spillway:Seven functions that can be assigned to spillway as discussed by Takasu et al. (1988). 1) Maintaining normal river water functions (compensation water supply) 2) Discharging water for utilization 3) Maintaining initial water level in the flood-control operation 4) Controlling floods 5) Controlling additional floods 6) Releasing surplus water (securing dam and reservoir safety) 7) Lowering water levels (depleting water levels in an emergency) 2.2 Classification of Spillways Spillways have been classified according to various criteria as shown below. 1) According to the most prominent feature These following are of this types: Ogee spillway, Chute spillway, Side channel spillway, Shaft spillway, Siphon spillway, Straight drop or overfall spillway, Tunnel spillway/culvert spillway, Labyrinth spillwayandstepped spillway. 2) According to Function Service spillway, Auxiliary spillwayandfuse plug or emergency spillway 3) According to Control Structure Gated spillway, Ungated spillway and Orifice of sluice spillway. 2

3 Fig. 1 Classification of spillways (A-1 to A-5 & C-1 to C-5) (shown in VischeretalSanfrancisco, 1988) 3. ENERGY DISSIPATORS Dissipation of the kinetic energy generated at the base of a spillway is essential for bringing the flow into the downstream river to the normal (almost pre-dam) condition in as short of a distance as possible. This is necessary, not only to protect the riverbed and banks from erosion, but also to ensure that the dam itself and adjoining structures like powerhouse, canal, etc. are not underminedby the high velocity turbulent flow.although a variety of devices are used for energy dissipation at the base of spillways, the dissipation of energy is through internal friction and turbulence or impact and diffusion of the high velocity flow in the mass of water. 3.1 Classification of EnergyDissipators Energy dissipators for the spillways can be classified in several ways as mentioned below. Fog 1 shows types of energy dissipaters (D-1 to D-4). 1) Based on Hydraulic Action Turbulence and internal friction as in hydraulic jump stilling basins, roller buckets, and impact and pool diffusion as with ski jump buckets and plunge pools. 2) Based on the Mode of Dissipation Horizontal as in the hydraulic jump, vertical as with ski jump buckets/free jets, and oblique as with spatial and cross flows. The vertical dissipation may be in the downward direction as with free jets and plunge pools and in upward direction as with roller buckets. 3. Based on Geometry or Form of the Main Flow Situations involving sudden expansion, contraction, counter acting flows, impact, etc. 3

4 4) Based On The Geometry Or Form Of The Structure Stilling basin employs hydraulic jump with or without appurtenances like chute blocks, baffle piers, etc. Buckets (ski jump or flip buckets) include special shapes like serrated, dentated buckets, and roller buckets that are either solid roller bucket or slotted buckets. 3.2 Prinicipal Types of Energy Dissipators The energy dissipators for spillways can be grouped under the following five categories: a) Hydraulic jump stilling basins b) Free jets and trajectory buckets c) Roller buckets d) Dissipation by spatial hydraulic jump e) Impact type energy dissipaters Hydraulic jump stilling basins include horizontal and sloping aprons and basins equipped with energy dissipating appurtenances such as chute blocks, baffle piers, and dentated end sills. This is the most common type of energy dissipator for the spillways and outlets and effects up to 60% dissipation of the energy entering the basin, depending on the Froude number of the flow. For heads exceeding about 100 m, hydraulic jump stilling basins are not recommended because of the problems associated with turbulence like intermittent cavitation, vibration, uplift, and hydrodynamic loading. Free jets and trajectory buckets are not dissipators of energy in real sense. The bucket deflects the high velocity jet into the air and is made to strike the riverbed at a considerable distance from the structure. Any scour that may occur in the impingement zone remains away from the structure and hence does not endanger the stability of the structure. Nappe splitters and dispersers contribute to the dissipation of energy by spreading and aerating the jet. Nevertheless, at some projects, problems of spray and retrogression of the scour hole towards the structure threatened the stability. Coupled with the plunge pools, part of energy of the deflected jet can be dissipated by pool diffusion. Roller buckets can be conceptualized as hydraulic jump on a curved floor, as its performance is closely related to the Froude number of the incoming flow. 4. HYDRAULIC JUMP TYPE OF ENERGY DISSIPATOR These are fundamentally be divided into two types.(1). Horizontal apron type and (2).Sloping apron type. 4

5 Fig. 2: Horizonalapron Stilling Basin with end sill Fig. 3: Sloping apron Stilling Basin with end sill 4.1 Classification of Hydraulic Jump Hydraulic jumps can be classified according to the geometrical form, pre-jump Froude number of the flow relating it to the energy dissipation efficiency, or as a free, forced, or submerged jump. In the first category, the jump is designated as classical jump, A-type, B- type, C-type, or D-type. A classical hydraulic jump is the transition from supercritical to subcritical flow in a horizontal prismatic channel. An A-jump is the hydraulic jump formed at the junction of a sloping channel with the horizontal floor as shown in Figure 4. If the jump forms at a location on the slope but ends on the horizontal floor, it is termed B-jump. The C- jump occurs in sloping channels with a horizontal channel portion when the end of the jump is located at the junction. In a D-jump, the entire jump is formed on the sloping portion. Fig. 4: Type of Hydraulic Jump Hydraulic jumps have also been classified according to the pre-jump Froude number (F1). For values of F1 up to about 1.7, a slight ruffle on the water surface is the only apparent 5

6 feature for such a jump, often termed as undular jump. For the higher range of F1, the classification is 1) 1.7 to 2.5 (pre-jump): low energy loss. 2) 2.5 to 4.5 (transition or oscillatory jump): energy loss 25 to 50. 3) 4.5 to 9.0 (steady or good jump): energy loss 50 to 70. 4) Greater than 9 (effective but rough jump): energy loss morethan 70. Fig. 5: Hydraulic Jump according to Froude number 5. HYDRODYNAMIC DESIGN OF STILLING BASIN For finalizing the structural design of stilling basin floor, uplift forces likely to be experienced by the individual floor monoliths are required to be assessed. The assessment of hydrodynamic uplift force on the apron of the stilling basin may be carried out on a hydraulic model by measurement of hydrodynamic forces acting on stilling basin using transducers. 5.1 THEORY AND MECHANISM OF HYDRODYNAMIC UPLIFT The uplift force beneath the apron of the hydraulic jump could be caused due to one or combination of the following: 1) Hydrodynamic uplift caused by the seepage gradient below the stilling basin. 2) Propagation of undamped fluctuating pressures below the lining i.e. at the concrete rock interface, due to cracks or unsealed joints between the panels causing uplift whenever instantaneous difference between the pressures on the upper and lower surface exceeds weight of the concrete including anchorage forces and is including anchorage forces and is acting upwards. The procedure in regard to determination of the hydrostatic uplift due to seepage gradient has been standardized and available in the relevant Indian Standard IS: 11527(1985). The procedure allows for 50 % reduction of the uplift force if adequate drainage arrangement below the apron has been provided. Following the failure of stilling basin aprons of some dams, the concept of hydrodynamic uplift has gained considerable attention.. 6

7 During last decades studies have been done on hydrodynamic uplift forces. There are two methods of assessing hydrodynamic uplift viz. based on the measurement of fluctuating pressures with their spatial correlation and direct measurement of force. Contributions by Bribiesca and Mariles(1979), Spoljaric and Hajdin (1982), Hajdin and stevanovic (1982), Lopardo and Henning (1985), Toso and Bowers (1988) and fiorotto and Rinaldo (1992) involved pressure measurements. In all these studies, propagation of fluctuating pressures below the panel was not considered. Studies by Peiquing et al (1996) have considered this aspect. The other method involves direct measurement of uplift force employing force transducer. Farhoudi and Narayanan (1991) were the first to conduct such a study. In their studies however, propagation of fluctuating pressures below the panel were not considered. Studies conducted by Bellin and Fiorotto (1995) have considered such a propagation and presented a method of calculating uplift force. Various approaches as indicated above can be applied to calculate uplift force and thickness of apron slab etc. in the case of any stilling basin for spillways. 5.2 MEASUREMENT OF HYDRODYNAMIC FORCES ACTING ON STILLING BASIN The most serious problem with the hydraulic jump dissipator is more of structural strength rather than hydraulic efficiency. Many examples of stilling basins suffering serious damages arising from uplift, vibration, cavitation, abrasion, and hydrodynamic loading are there. The uplift of the apron slab could be caused due to one or a combination of the following: 1) Hydrostatic uplift caused by the seepage gradient below the stilling basin. 2) Intermittent pressure depressions due to turbulence, especially in the initial reach of the jump. Such pressures may cause suction effect on the upper face of the slab, trying to lift it from its position. 3) Difference between the fluctuating pressures on the upper and lower faces of the slab monolith. Such a difference can result due to the transmission of pressure peaks from the upper to the lower face of the slab, through exposed construction joints, cracks, etc. on the slab. The uplift pressures tending to lift the slab are caused by the intermittent conversion of kinetic energy into pressure energy, transmitted through any opening, joint, or crack that may be in the apron floor. 7

8 This mechanism poses a threat especially at high Froude numbers and is accentuated by incoming turbulence by which the energy is dissipated in the hydraulic jump. When the pressure becomes negative at a point on the apron, there may be a short local instability if there is a steady uplift pressure at the concrete-rock contact or at any other interface within the thickness of the slab. When this uplift is greater than the submerged weight of the concrete plus the water load, the floor slab is lifted up. Damage to many stilling basins indicated that the probability of occurrence of this unfavorable combination is far from being negligible Analytical There are two methods of assessing hydrodynamic uplift, one based on measurement of fluctuating pressures with their spatial correlation and another based on direct measurement of fluctuating force.fig. 6 shows hydraulic jump formation with notations. Fig. 6: Typical formation of hydraulic jump showing notations TABLE I :MEASUREMENT OF HYDRODYNAMIC UPLIFT MEASUREMENT OF HYDRODYNAMIC UPLIFT Measurement of pressure Measurement of uplift fluctuations force 1. Bribiesta et al (1979) 1. Farhaudi et al (1991) 2. Spaljaric et al (1982) 2. Bellin et al (1995) 3. Hajdin et al (1982) 4. Lopardo et al (1985) 5. Toso et al (1988) 8

9 6. Fiorotto (1992) 1. Hajdin et al (1982):- Uplift force F is given by F = 1 2 ρv 1 2 AKC P L B Where, ρ= relative density, V 1 = velocity at entry point, A= area of the slab panel, K= a factor defining the probability of occurrence of force; generally K= 3.09 corresponding to 99.8 % probability of occurrence. C P = pressure fluctuation coefficient, Φ L = coefficient of correlation along length L, Φ B = coefficient of correlation along length B, The equivalent thickness of monolith t s is then, Where, A= area of the slab, t s = thickness of monolith slab, γ c = specific weight of concrete, γ w =specific weight of water, F = A t s (γ c γ w ) 2. Bribiesca et al (1979): Obtain an expression for the time average of the square of the total vertical force acting on the slab S P 2 as S p 2 = f p 2 S p 2 A 2 With f p = 2 L+ B e L + L 1 [e βb + βb 1 ] Where, S p 2 = variance of the total pressure acting on the upper face of the slab. f p = coefficient of distribution of pressure, The thickness is given by, Where, t s = γ w γ c γ w f p S p 2l n (τf m ) S p = standard deviation of the depth of flow at the centre of graviry of the area A, in m, τ= useful life of concrete lining of the slab in seconds, f m = main frequency of purpose fluctuations, Hz. 9

10 1. Toso et al. (1988): State that for practical purposes, the pressure fluctuations tend to approach a definite limit, of the order of 80 to 90 % of the head. By selecting an appropriate value of Cp from table given by him, the maximum deviation from the mean pressure is worked out as 2 V 1 P = C P 2g This deviation pressure P is assumed to act on the centre of an area 8y 1 * 13y 1 moving out of the centre of the area, the pressure would drop off to the mean pressure. The uplift force is given by Where, P = mean pressure, L c = length of concrete slab, B c = breadth of concrete slab, γ w = specific weight of water. F = P 1 2 L cb c γ w 1. Farhaudi et al (1991): performed direct measurements of uplift force using a force transducers in a model set up. Results have been presented in terms of RMS coefficient C f is defined as, C f = (F ) ρ wv 1 2 Peak instantaneous value of force are 3.5 times the RMS value 1 F max. = 3. 5C ρ f 2 wv 2 1 A And the thickness of the slab, t s = F max. A(γ s γ w ) 2. Bellin et al (1995): Conducted laboratory studies simulating this phenomena with a direct force measurement system. The maximum uplift force F max. just exceeding the submerged weight of the slab was measured and related to the dimensionless pressure coefficients C P + and C P and uplift coefficient Ω s considering standard deviation of fluctuating force and pressure. The relationship is, 10

11 Where, 2 F max. = Ω s (C + P + C v 1 P )γ W 2g LB Dimensionless pressure coefficients are C P + and C P and uplift coefficient is Ω s Hydraulic Model Studies Hydrodynamic model studies would be a suitable tool for measurement of hydrodynamic uplift on the stilling basin for finalizing the structural design of stilling basin floor. Since the hydrodynamic uplift is caused due to the simultaneous action of fluctuating pressures on the both upper and lower surfaces of the concrete lining (due to transmission of fluctuating forces through unsealed joints, cracks, etc.), it was preferred to measure the uplift force directly by a force transducer.the measurement system should include a force transducer coupled to a typical panel of stilling basin slab, whose signal output was fed to a PC based data acquisition system. The data received from transducer system will be analysed using statistical methods. The data indicates the percentage of time a panel experience uplift force on the stilling basin as per the position of the panel. This analysis of uplift forces would be useful in deciding the design uplift force for various panels considering the frequency of floods, the duration of flood and the strength of anchors in the prototype. 1) Instrumental Setup And Measurement Ststem Hydraulic model studies involve running of the physical model for various discharge conditions, measurement of hydrodynamic uplift forces using force transducers and statistical analysis of the data obtained. The force transducers are used to obtain the hydrodynamic pressures acting on the stilling basin slab for different loading conditions. A typical force transducer is shown in photo 3 and location of embedded force transducers for a typical model studies in shown in figure 7 Fig. 7: Plan and Elevation of model embedded with Force Transducers The measurement system comprises a force transducer coupled to a typical panel of concrete slab reduced to model scale, which is isolated from rest of the structure in such a way that 2 mm wide gaps around its four sides and at the bottom facilitated simulation of 11

12 seepage of water through unsealed joints and consequently transmission of forces below the slab resulting in fluctuating forces. The measurement system consists of a force transducer with known capacity (say 1-2kN) with an excitation voltage of 15 volts whose signal output was fed to a PC based Data Acquisition System. Figure 8 shows details of the connection of stilling basin floor slab panel to force transducer. Figure 9 shows details and specifications of a typical force transducer used for hydraulic model studies. Fig. 8: Details of Transducer mounting on hydraulic model Fig. 9: Force Transducer details and specifications A series of tests are required to be conducted to estimate the natural frequency of force transducer system and to determine if the natural frequency of the dampening of the system would influence measurement of forces, through resonance effects. Location of the transducer along the length of the stilling basin is important and critical, since the peak of the pressure fluctuations occur at a location which is governed by various parameters such as Froude s number, entrance condition, length of the jump, as also whether the jump is submerged or unsubmerged. 2) Conditions of Experiments 1. The studies are to be carried out for several dischargesfor MWL/ FRL, maintaining normal tail water levels as per the Gauge Discharge (G-Q) curve. 12

13 2. The measurements are to be carried out for specific acquisition time, say sampling time of one millisecond to 10 milliseconds. The acquisition time should in fact correspond to the time of outflow hydrograph corresponding to various floods. Studies conducted by Bellin et al (1995) with acquisition time varying from 5 minutes to 20 hrs. indicated that an experiment that an experiment duration of 30 minutes was satisfactory for obtaining a good estimation of the uplift co-efficient in their studies. 3. An elaborate system of drainage is required below the stilling floor with a network of half round pipes connected to drainage galleries and pump sump. In hydraulic model, simulation of draining out of the seepage water accumulated under the slab can be done qualitatively, in as much as that the peripheral space between the yoke of the transducer and the rest of the housing could be opened and sealed as required, as shown in figure 8. 3) Statistical Analysis of the Data The stilling basin floor would experience the dynamic pulsations which could cause uplift and downthrust as shown in typical time history records acquired from the measurement shown in fig 10. However, due to inertia, concrete in the thick slab of the stilling basin with anchors at the base would not respond to the instantaneous peak of the uplift pressures as fast as they occur. This time lag is suggested of a sustained near average value of uplift forces which would be more appropriate for the structural design of the stilling basin floor rather than transient peak values of much higher magnitude. So, results be analysed to obtain: 1) Time average value of uplift force, considering only uplift part of the time history record (without considering the downthrust). 2) Probability of time duration of uplift forces of various magnitudes. Data to be analysed for the entire run time (say of 30 minutes) of experiments for different discharges for various panels to obtain peak values of uplift and downthrust, mean and RMS values of uplift period of the time history records. 13

14 Fig. 10: Time History Record and variation in forces at different discharges The analysis should be in terms of cumulative probability (percentage of time) corresponding to forces of different magnitudes. This gives the percentage of time a panel experienceuplift force. 6. CONCLUSIONS & RECOMMENDATIONS The present work deals with the hydrodynamic design aspects of Stilling Jump type energy dissipators along with comparison of various energy dissipators. Also the characteristics and properties of various forces acting on a stilling jump type energy dissipatoris studied.various methods of calculating the Uplift force/drag either analytically and experimentally are mentioned in paper. How the experiments are carried out and how the force transducers are used to measure and calibrate the forces is also discussed. In India Stilling Jump type energy dissipators with only one end sill is sufficient to dissipate the energy in Himalayan and plain region because the velocity of rivers in those areas are very high. Other energy dissipators such as Trajectory bucket, roller buckets with baffle blocks should be used to increase velocity in a low-velocity river flowing in any region. Here after this study we can recommend that various energy disspators may be used as requirement and experimental study and further research may be done for estimating the uplift and hydrodynamic forces on energy dissipators. Also Hydraulic jump type energy dissipator is not recommended for head above 100 meter. 14

15 ACKNOWLEDGMENT The authors are thankfully acknowledge to Mr. J.N.Patel, ChairmainVidyabharti Trust, Mr. K.N.Patel, Hon. Secretary, Vidyabharti Trust, Dr. H.R.Patel, Director, Dr.J.A.Shah, Principal, S.N.P.I.T.&R.C.,Umrakh, Bardoli, Gujarat,India for their motivational & infrastructural supports to carry out this research. REFERENCES 1. Bellin, A.; Fiorotto, V. Direct dynamic force measurement on slabs in spillwaystilling basins. ASCE, Jnl. of Hyd. Div, Oct. 1995, 121(No.10). 2. Bribiesca, J. L. S.; Mariles, O. A. F. Experimental analysis of Macroturbulence effects on the lining of stilling basins, Q50, R613th ICOLD, Farhaudi, J.; Narayanan, R. Force on slab beneath hydraulic jump. ASCE, Jnl. of Hyd. Engg, 1991, 117(1). 4. Fiorotto, V.; Rinaldo, A. Fluctuating uplift and lining design in spillway stilling basins. ASCE, Jnl. of Hyd. Engg, 1992-a, 118(4). 5. Hajdin, Georgije Contribution to the evaluation of fluctuation pressure on fluid currents limit areas- based on the pressures recorded at several points of the area, VIII Conference of Yugoslav Hydraulics Association. Portoroz, Khatsuriya.R.M. Spillways and Energy Dissipators. Marcel Dekker Publishers, Lopardo, R. A.; Henning, R. E. Experimental advances on pressure fluctuation beneath hydraulic jump Proc. 21st IAHR Congress. Melbourne, Novak. P, Moffat A.I.B, Nalluri. C., Narayanan. R. Hydraulic structures. Taylor & Francis, New York, Spoljaric, A.; Maskimovic, C.; Hajdin, G. Unsteady dynamic force due to pressure fluctuations on the bottom of an energy dissipator An example, Proc. Intnl. Conf. on Hyd. Modelling of Civ. Engg. Structures, BHRA, Takasu, S.; Yamaguchi, J. Principle for selecting type of spillway for flood control dams in Japan, Q-63, R- 19, ibid, Toso, J. W.; Bowers, C. E. Extreme pressures in hydraulic jump stilling basins. ASCE, Jnl. of Hyd. Engg, 1988, 114(8). 12. Vischer, D.; Rutschmann, P. Spillway facilities Typology and General Safety Questions, Q-63, R-23, Proc. 16th ICOLD:. San Francisco, June,