Physical models application of flow analysis in regulated reservoir dams

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1 River Basin Management III 15 Physical models application of flow analysis in regulated reservoir dams M. R. M. Tabatabai 1, S. Faghihirad 2 & M. Kolahdoozan 3 1 Water Engineering Department, Power and Water Institute of Technology, Iran 2 Water Research Institute, Iran 3 Amir Kabir University of Technology (Tehran Polytechnique), Iran Abstract Nowadays lack of water resources in semi arid countries and also optimum operation of water systems play major roles in water management decisionsmakings. River regulated reservoirs are constructed downstream of reservoir dams with relatively low elevation appropriately to supply water to river riparian. They are mainly used for agricultural and industrial consumptions. However, the major difference between these and reservoir dams appear in dam elevation as well as generated turbulent flow in the regulated reservoir by flow diversion for various regulatory conditions. These characteristics have caused complex flow pattern generation, which may be difficult to forecast. Physical hydraulic models are actually constructed to obtain a better understanding of flow behaviour and its impact on hydraulic structures. In this paper, physical modelling of regulated reservoir dams associated with water intakes are studied in details to establish a relationship between water discharge and the proportion by which it is flown through the intakes. Velocity measurements were also made to establish velocity field upstream of diversion dam as well as intakes. Optimum water elevation in the regulated reservoir was also determined while intakes were operating at maximum capacity. Keywords: reservoirs, hydraulic models, physical models, flow pattern, water intake, diversion dam. 1 Introduction A hydraulic model cannot reproduce an equal ratio of all the prototype forces and satisfy perfect similarity. Also practical limitations of available space, funds

2 16 River Basin Management III and bed materials make it necessary in many studies to deviate from the desired scale ratios. Therefore, it depends on the engineer to use his knowledge, experience and even intuition to determine what laws of similarity must be satisfied and what special modeling techniques must be applied to accurately reproduce prototype occurrences. The engineer must first determine the type of problem to be studied and the type of data to be obtained from the model. This information will dictate the type of model to be used, the space to be reproduced and the range of scales suitable for the study. There are generally two types of river models, which are fixed-bed and moveable-bed. The fixed-bed model is constructed of concrete, pea gravel, or some other material that cannot be moved by the force of the fluid in the model. The moveable-bed model usually has fixed banklines and overbank areas with a movable-bed of crushed-coal, sand and some other materials that can be moved by the forces developed in the model stream [1]. In addition, models may be constructed with the same horizontal and vertical scales (undistorted) or with different horizontal and vertical scales (distorted). Even though it is desirable in most cases to have an undistorted model, in some cases distortion is necessary. It necessitates as the prototype area is so large that if the vertical scale was made the same as the horizontal scale, the changes in water surface elevation would be of such small magnitude that accuracy would be lost [2]. Where hydraulic performance of river channels as well as the crossstructures to be predicted, it is appropriate to apply models with fixed-bed. These are used mainly when problems of water levels and flow patterns are only investigated [3]. 2 Background to the project Hamidieh regulated reservoir is located 11 km away from Hamidieh Town. It has been constructed on Karkheh River downstream of Karkheh reservoir dam. There are two water intakes located on both sides of it, namely Ghods and Vosaileh. Currently, Vosaileh water intake channel is 10.8 km long with maximum discharge of 60 m 3 /s while Ghods operates with 25 km channel length under maximum carrying capacity of 13 m 3 /s [4]. Due to development in irrigation and drainage networks of Azadegan and Chamran plains, present conditions of the water intakes are not able to meet the water demand sufficiently. Hence, it is inevitable to increase the rate of flow intakes by redesigning new dimensions and geometric characteristics for the structures. Hamidieh reservoir dam is 192 m long, 4.5 m high with 19 spillway bays opening and 10 floodgates. Azadegan replaces Ghods water intake with an inlet width of 56 m, 8 bays opening and 4 under sluice gates. It is meant to increase the carrying capacity of 13 m 3 /s to 75 m 3 /s. Vosialeh water intake is also replaced by Chamran with an inlet width of 86.6 m, 16 bays opening and 13 trash racks opening to increase the carrying capacity of 60 m 3 /s to 90 m 3 /s, fig. 1. An undistorted 1/20-scale model of Hamidieh regulated reservoir and its relevant

3 River Basin Management III 17 structures were constructed under the title of new development project to investigate the operation of the entire system to improve the understanding of flow behaviour in the vicinity of the structures. Azadegan intake Flow direction Right floodgate Left floodgate Chamran intake Figure 1: Plan view of Hamidieh diversion dam and associated structures. 3 Project objectives Application of empirical relations in many hydraulic phenomena may lead to some degrees of errors. This could be on account of the complex nature of them or the simplistic approach by which they are treated [5]. In river engineering projects, it is necessary to understand the interaction between water and sediment discharge resulted from complex morphological processes in three dimensions [6]. Physical models of rivers are offered as essential tools to enable us to obtain more accurate and reliable data, particularly, where these changes are made by cross-structures. In addition, the present condition of the project and also the expectations of new developments have even made the physical model study more significant. These key issues are listed as Water supply to two main irrigation and drainage networks with an area of about 2x10 6 hectares, Irregular water intake from the river which is approximately up to 90% of the river flow, Inappropriate location of Hamidieh regulated reservoir dam, Lack of sufficient discharge through intakes in normal water level conditions in the reservoir, Improvement of intakes locations.

4 18 River Basin Management III 4 Physical model To validate model results in comparison with prototype, the set of conditions associated with each must be physically similar. When studying free surface flow models, it is necessary to consider both gravity and friction forces to achieve dynamic similarity, therefore, the Froude and Reynolds numbers should ideally have the same value in both the prototype and models [7]. For the study of 1/20 undistorted scale model of Hamidieh regulated reservoir dam, the achievement of full Froude and Reynolds criterion require the ratio of the kinematic viscosity of the model fluid to that of the prototype to be 1/20. No such-low viscosity of natural liquid exists. Therefore, scaling of the model results must as usual be carried out in accordance with a selected primary criterion with consideration given to the other criterion as a second priority [8]. For free surface flow, the Reynolds number is very high in the prototype, the flow fully turbulent, and the frictional effect is controlled essentially by the relative roughness of the surface and is fairly insensitive to Reynolds number changes. Therefore, the model conditions are derived from the Froude number similarity criterion as gravity provides the driving force in river mechanics. Accordingly, satisfaction of the Froude law will make the model operate at a much smaller Reynolds number than exists in the prototype and care must be taken to ensure that the model operates within the same flow regime as the prototype; as energy losses associated with laminar and turbulent flow cannot be easily scaled [9]. In order to model turbulent flow in natural streams more accurately, effect of bed roughness (k) on water depth must be considered. This can be achieved by coupling up Manning-Strickler with Darcy-Weisbach to obtain [3]. 1 λ 1 6 k = 2. 9 (1) R where λ = Darcy factor, and R = hydraulic radius. In wide channels R may be replaced by water depth y. A comparison of this empirical formula for the roughness coefficient with the general function based on the fundamental experiment by Nikuradse yields the range of relative roughness (k/y) by simple experimental form with good approximation 2x10-3 < (k/y) < 2x10-1 [10]. The majority of natural open channels falls within these limits, with the exception of the very large streams and of rivers in coastal regions. Flows in natural channels are usually within the hydraulically rough range so that the roughness coefficient only depends upon the relative roughness hence the magnitude of the Reynolds number is of no importance [2].

5 River Basin Management III 19 According to [11], the limit between the hydraulically rough flow regions and the transition is given by Re λk = 2000 (2) 4y According to equation (1), the requirement of hydraulically rough flow condition is satisfied for 3 k Re (3) y 5 Model construction The construction of Hamidieh regulated reservoir physical model and associated structures were based on three principles: type of model, model scale and preparation [12]. With reference to similarity theories, a fixed-bed undistorted physical model seemed appropriate to enable us to observe hydraulic phenomena in the model. However, it must be borne in mind that the flow turbulence should exist even at low discharges. In general, model scale is selected by trial and error in such a way that the constructed model is to be fitted in the laboratory spaces, then the possibility to generate hydraulic model parameters such as water discharge, bed roughness and flow hydraulic characteristics to be assessed. An appropriate scale is selected to meet all these criteria otherwise the above procedures have to be repeated. A 1/20-scale model of Hamidieh regulated reservoir dam was constructed, as this meets all the above requirements regarding flow hydraulic conditions. The relationship between the hydraulic parameters of the model and the prototype are demonstrated, table 1. Table 1: Ratios of calculated hydraulic parameters [8]. quantity symbols ratio scale ratio length L r L r 1:20 height h r L r 1:20 area A r (L r ) 2 1:400 slope S r 1 1 velocity V r (L r ) 1/2 1:4.472 discharge Q r (L r ) 5/2 1: Manning s roughness n r (L r ) 1/6 1:1.648 On the basis of provided 1/500-scale maps, a plan view of Hamidieh regulated reservoir physical model was designed and constructed [12]. This includes relevant structures such as stilling and sedimentation basins as well as Azadegan and Chamran water intakes. A laboratory setup was then established

6 20 River Basin Management III on the basis of the designed dimensions of the physical model and associated structures (i.e. inlet and outlet channels, water storage tank, stilling and sedimentation basins) at Water Research Centre [12]. Water discharge was adjusted by pumping station through an inlet channel to the system and carried out of it by an outlet channel leading to the storage tank. Discharge measurements were made by weirs installed in different locations, desired water level was also maintained by a tail gate while the stream pattern was observed by tracers and velocities were measured by means of current-metre. 6 Results Water discharge values were simulated for three different conditions regulatory, high flow and flooding [12]. In general, Manning s roughness in the model is less than that of the prototype; however, this can be compensated by applying a number of factors to be increased. The roughness coefficient in the prototype was estimated in the range of to [13], this was simulated to be to in the model, which could easily be obtained by a plane concrete surface. Water elevation was considered to be at normal condition m in the regulated reservoir while it was controlled in the intake channels for different months of the year as well as the extreme conditions [12]. Intake structures are applied where water abstraction from rivers or reservoirs is required. Therefore, design of their inlet structures need being handled with care and attention. In fact, the design has to be made in such a way to meet all the requirements such as structural stability and hydraulic characteristics (i.e. flow rate of intake and intake water elevation during low flow periods). In general, the inlet velocity varies in the range m/s [10], while any transition or bending at the entrance should occur gradually to minimize the energy losses in the inlet channel. A uniform velocity distribution is also recommended at the entrance, as non-uniform velocity distribution and unstable water surface upstream of the intake channel as well as vortex generation in the vicinity of that could reduce the rate of flow into the system [10]. In the model, hydraulic operation of the intake structures is assessed by velocity measurements as well as flow pattern behaviour in the vicinity of the structures. Flow distribution could also be determined by velocity profile variations in plan, which is obtained by point velocity measurements using a current metre. Flow behaviour is also investigated in the vicinity of the intakes at 20 and 40 metres upstream by the study of velocity variations profile. This shows a uniform flow velocity distribution upstream of Azadegan intake which stablises more by a reduction in distance to confirm proper design and operation of the system, fig. 2, whereas a non-uniform velocity distribution is observed upstream of Chamran intake to confirm inability of the transition channel in the reduction of this nonuniformity, fig. 3.

7 River Basin Management III 21 Velocity (m/s) m from intake 40 m from intake Distance from mid-channel (m) Figure 2: Velocity variations profile in the vicinity of Azadegan intake at m water elevation in the reservoir Velocity (m/s) m from intake 40 m from intake Distance from mid-channel (m) Figure 3: Velocity variations profile in the vicinity of Chamran intake at m water elevation in the reservoir. Water discharge measurements downstream of the intakes states that Chamran cannot meet its maximum carrying capacity of 90 m 3 /s while 75 m 3 /s is easily achieved by Azadegan at m water elevation in the reservoir, table 2.

8 22 River Basin Management III Table 2: Results of hydraulic experiments, Discharge (m 3 /s), Bay opening (m 2 ), Elevation (m) and R. F. is right floodgates and L. F. is left floodgates. Operation System Reservoir Az.intake Ch.intake Under sluice Az.intake R. F. L. F. Remark Discharge * Bay opening Elevation Discharge * Bay opening Elevation Discharge * Bay opening Elevation Discharge * Bay opening ** Elevation Discharge * Bay opening ** Elevation *** Discharge * Bay opening Elevation Discharge * Bay opening Elevation Discharge * Bay opening Elevation Discharge **** Bay opening Elevation *without trash racks, **head loss at Chamran intake gates, *** maximum carrying capacity for the intakes at the elevation 20.20m, **** with trash racks and considering bay opening surface trash racks is 80% total surface.

River Basin Management III 23 It is shown in table 2 that water elevation at intakes was measured in irrigation channel Following this, several solutions have been put forward to enable Chamran to

One of the solutions which were actually adopted in the experimental model was to install permeable groins on the opposite bank to deflect flow direction towards the intake, as it could activate

9 River Basin Management III 23 It is shown in table 2 that water elevation at intakes was measured in irrigation channel Following this, several solutions have been put forward to enable Chamran to reach its maximum carrying capacity. One of the solutions which were actually adopted in the experimental model was to install permeable groins on the opposite bank to deflect flow direction towards the intake, as it could activate stagnant zone to some extent, however, Chamran carrying capacity was increased only by 5%, fig. 4. Alternatively, where a permeable wall obstructed the entire reservoir width, a local increase in the reservoir water elevation was observed which was resulted in 90 m 3 /s water discharge at Chamran, fig. 5. Nevertheless, has this experiment been idealistic and is not very practical for implementation, as this would confirm that maximum carrying capacity at Chamran can only be obtainable by a raise in water elevation to m in the reservoir. Figure 4: Location of permeable groins on the opposite bank. Figure 5: Obstruction of reservoir width by a permeable wall.

10 24 River Basin Management III 7 Conclusions A series of experiments on Hamidieh regulated reservoir physical model could reveal the governing phenomena in the system to some extent. It is necessary to remember that governing conditions and simplifications of the experiments play major role in the analysis of the results. The results obtained from hydraulic experiments confirm that: - Chamran water intake could reach its maximum carrying capacity of 90 m 3 /s provided that the water elevation is increased in the reservoir. - An increase of 30% in compensation releases downstream of the Hamidieh reservoir is necessary, as this can effectively improve the operation of Azadegan sluice gate. It could also provide a constant release from Chamran sluice gate, which is to be applied in normal regulatory conditions. - Decreasing the number of opening bays of the sluice gate as well as redesigning a single contraction rather a double one can reduce energy losses in Chamran intake system. Azadegan water intake can operate at maximum designed discharge of 75 m 3 /s and even a little more than that due to its appropriate location and geometric plan. Therefore, it assures a reliable flow intake during the operation. It is also advisable to keep the present geometric plan as it may be necessary to increase carrying capacity of the intake due to the problems raised by the location of Chamran water intake. References [1] Ivicsisc, L., Hydraulic Models, VITUKI, Budapest, [2] French, R. H., Open Channel Hydraulics, McGraw-Hill, New York, [3] Chadwick, A. J. and Morfett, J. C., Hydraulics in Civil Engineering, Allen & Unwin, London, [4] Water and Electricity Organisation of Khuzestan Province (WEOKP), Karkheh Irrigation Network Operation Company, Bulletin No. 31, pp , [5] De Vries, M., Use of models for river problems, Int. Hydro. Prog. Studies and Reports on Hydrology, 51, Paris, UNESCO Pub., [6] De Vries, M. and Van Der Zwaard, J. J., Movable-bed river-models, ASCE Symp. On Modelling, Pub. No. 156, San Francisco, 3-5 September, pp. 1-17, [7] Henderson, F. M., Open Channel Flow, Macmillan, New York, [8] Sharp, J. J., Hydraulic Modelling, Butterworths, London, [9] Yalin, M. S., Theory of Hydraulic Models, Macmillan, London, [10] Kobus, H., Hydraulic Modelling, German Assoc. for Water Resources Improvement, Bulletin No. 7, [11] Moody, L. F., Friction factors for pipe flows, Trans. Am. Soc. Mech. Engrn. 66, pp , [12] Water Research Centre, Physical Model of Hamidieh Regulated Reservoir, Report No , 148pp, [13] Chow, V. T., Open Channel Hydraulics, McGraw-Hill, New York, 1973.

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