Jelisaveta Muskatirovic1
and Zvonimir Predic2
1“Jaroslav Cerni” Institute for the Development of Water Resources
Jaroslava Cernog 80, 11223 Beli Potok, Yugoslavia
2A.P. Company, Belgrade, Yugoslavia
tel: +381 11 3906469; fax: +381 11 3906481; E-mail: muski@eunet.yu
Abstract: Information, on behaviour of discharging structures and other control released facilities under extreme hydrodynamic loads, are very important in seeking optimal solutions which will assure their safe and reliable operation. That is a reason for presentation of the extensive investigations of the bottom outlet, located on Yugoslav part of the Hydro Plant “Iron Gate I.” The main purpose of the field tests was to obtain the necessary data for judgment of possibility to use the bottom outlets also as reliable flood discharging structures.
The paper considers the hydrodynamic characteristics of flow along bottom outlet, as well as behaviour of the control gate for different operating regimes and reservoir water levels higher than designed one. On the basis of test results: dynamic pressures acting on the control gate, bottom outlet and stilling basin walls; vibrations of gate and concrete structure; lowering and lifting force etc., it was possible to conclude that under some definite conditions, the bottom outlets can be used for passing floods up to the maximum water level in reservoir.
Keywords: discharging structures, bottom outlet, hydrodynamic loads, field tests
In the scope of wide spectrum of activities for improvement of dam safety, the measures for increasing the capacity of discharging structures and for assuring the safe operation of related hydromechanical equipment at all designed regimes, should be foreseen. This statement is stressed by the fact that many dams require some modifications, reconstructions or revision of discharging structures, aiming to increase discharge capacity, due to changed hydrologic and hydraulic conditions or to ensure optimal use of available water resources.
Although many innovative designs are being developed during recent years, for each specific project some additional tests are necessary. The objective of these investigations is to verify the discharge capacity and behaviour of other release facilities for extreme discharges. For this purpose the physical and mathematical models, as well as field tests, can be used. Bearing in mind that some phenomena, e.g. cavitation, vibration, large local hydrodynamic loading etc., may become a threat to reliable operation, and in some cases, to dam stability, the field tests are of a special importance. On the basis of measuring results, it is possible to prove the design assumptions or to determine some discrepancies. Of course, field tests for large flows are most useful. Unfortunately, such investigations are very rare. Generally, field tests have been performed in cases of appearance of some undesired phenomena or accidents. The main goal of these investigations is to find out the causes and measures for their elimination.
As extreme flow can be simulated in the prototype for some configuration of discharging structures, the field tests results are very important not only for tested structure, but also for similar ones. That is a reason for analysis of the results of field tests carried out on the middle bottom outlet, located on Yugoslav part of “Iron Gate I” Hydro Plant.
“Iron Gate I” Dam, beside 14 gated spillways, located at middle part of the dam, maximum capacity of 15,400 m3/s, has 6 bottom outlets (3 on Yugoslav and 3 on Romanian side of the Danube). Three bottom outlets on each side are located between the powerhouse and navigation lock. Maximum discharge of all outlets according design is 5,056 m3/s for reservoir water level 63.00 m a.s.l. As it can see, the bottom outlets are of large capacity. According to the design, the outlets can operate at the above mentioned water level or lower, although maximum water level in the reservoir is 69.50 m a.s.l.
The motive for field tests on one of the bottom outlet of “Iron Gate I” was the judgment of the possibility of transforming the bottom outlets in reliable flood discharging structure for all range of reservoir levels. Among the measures for optimal use of available hydropotential of the reservoir, the solution with additional power units was considered. This solution requires closure of one spillway bay on each side of dam and usage of the bottom outlets as discharging structures. Although the “Iron Gate I” Hydro Plant is in operation since 1972., the bottom outlets were in operation only during second stage of dam construction. From that period, there are unclear memories that vibrations appeared. After that, bottom outlets were only regularly maintained and checked, especially after hydromechanical equipment.
In order to prove operation of bottom outlets, not only under the designed reservoir water level, but also at higher levels and for different gate openings, the extensive field tests were anticipated. Unfortunately, it was not possible to realize the field tests for hydraulic conditions for which the model tests at scale of 1:40 were carried. But it does not mean that model data were not used for the analysis of field tests results.
The bottom outlets are rectangular cross section (width 5.00m and height 8.80m), length of 84.20m, Figure 1. Each outlet is equipped with the control and emergency gate at the entrance and with emergency gate at downstream end. The control gate is vertical leaf gate, divided into three parts, with upstream sealing. The gates are designed for maximum head, which corresponds to maximum reservoir water level of 69.50 m a.s.l., and for lowering in the flow by own weight and ballast. It was accepted to operate in close or fully open position. Downstream of three bottom outlets is appertaining stilling basin, total length 100 m along the axis, with three chute blocks, middle and end sill. The bottom width of stilling basin at the entrance is 20 m. The end sill is at angle to stilling basin axis. The final solution for stilling basin was obtained by means of physical model tests.
A detailed inspection and verification of the operation reliability of all control and emergency gates preceded the field tests. Revision of the outlets involved checking out of the inner surfaces and bed load deposition upstream of the control gate and downstream.
Along with above mentioned activities, the Programme of field tests was prepared. The model results were used for selection of a measuring points in the stilling basin. Due to very unfavourable flow conditions in the stilling basin, registered at the model, when only one bottom outlet was in operation it was decided to limit discharge. The Programme of field tests included the following measurements: upstream and downstream water level; water levels in the gate slots; pressures in outlet and stilling basin; pressure fluctuations in selected points of the control gate; vibration of the control gate and concrete structures; lifting force (using strain gages and balance on crane) and gate operating regime. Observations of the phenomena in the zone of the gate, as well as flow regime in the stilling basin, especially along dividing walls (towards tail race and navigation lock) were also foreseen. The location of measuring points were shown in Figure 1. The detail positions of pressure and vibration transducers, installed on the gate lip, are presented in Figure 2.
Fig. 1 Layout and longitudinal section of the bottom outlet with positions of measuring sections and measuring devices
Fig. 2 Position of pressure transducers (S) and vibration devices (V) at gate lip
According to the Programme, it was accepted to perform investigations for maximum reservoir water level. Due to hydrological regime of the Danube River, reservoir level was 66.80, i.e. 3.8 m lower then maximum one in the period of field tests. Investigations were carried out for total head of 24.40 m.
All tests can be grouped in four series. The first series involves the trial tests. The tests were carried out during lifting and lowering the control gate to 5, 50, 80 and 100 cm openings. Due to safety reason, in these tests the emergency gate was opened 2 m. During the second test series, the gate was lifted to openings 2, 4 and 6 m. At each of these opening, the gate was stopped and after that lowered. The third test series was most important one due to a fact that measurement results were obtained for 7m gate opening, i.e. for maximum designed discharge through one outlet. This discharge provoked in the stilling basin unfavourable flow conditions, which caused tearing of the transducers and cables, besides all undertaken measures for their protection. The fourth test series was carried out for the different gate operatng regimes.
Bearing in mind a characteristics of phenomena under consideration, simultaneous measurements at all measuring points were carried out. Such concept ensured the analysis of obtained data as a function of gate opening. For data acquisition, two parallel systems were used. The physical values were proceessed and partially analyzed during measurements. Simultaneously, all analogue signals were registered on the tape recorder. Detail processing of all data in time and frequency domains was completed later on.
Numerous and different test results enabled the analysis of gate operation reliability and the analysis of flow conditions along bottom outlet and partially in stilling basin, during bottom outlet operation. Obtained measuring results and observations show that the control gate can open and close at different gate regimes without any problem. The tests performed for the higher gate openings indicated that the limiting factor in gate operation is appearance of unfavourable flow conditions in the stilling basin. The maximum discharge, obtained at 7 m gate opening, provoked asymmetrical flow conditions with intensive reverse flow, excessive turbulence, waves, splashing and partial overtopping of the left wall as seen in Figure 3. Observed flow conditions, as well as measuring results, showed that maximum discharge has to be limited. It was stressed by the fact that on the basis of model tests, it was concluded that operation of one bottom outlet, with maximum discharge, should be avoided, due possible cavitation and big local erosion in the river channel. It was reason that the field tests were not performed for full gate opening of 8.8 m.
Fig. 3 View of flow conditions in the stilling basin for maximum designed discharge
As an example of obtained measuring results, Figure 4 shows registered pressure variations on the gate lip (S1), in outlet at revision shaft (S6) and on left and right wall of stilling basin, (S8) and (S12) during gate raising to 7 m opening and its stoppage. The variation of the gate lifting force and differential pressure in the slot of the gate for same operating regime are presented in Figure 5.
On the basis of obtained results, it is possible to conclude that small negative pressure (1kPa) and small pressure fluctuations (double amplitude is 1kPa) were registered on the gate lip when the air entrainment through the slot was interrupted ( probably due to formation of water plug above gate, at the gate opening of 6.4 m). The appearance of negative pressures and pressure fluctuations coincides with the maximum value of lifting force. Maximum pressure fluctuations during the gate lifting were registered for gate opening of 2 m ( 2kPa). In the presence of the emergency gate, pressure fluctuations at the control gate lip were up to 10kPA. During the gate lowering, the negative pressures were observed for the gate opening higher than 6.4 m. The pressure fluctuations were significantly higher (8kPa).
Fig.4 Variation of pressures in selected measuring points for presented gate operating regime
The variation of pressures along the outlet and in stilling basin are presented through the pressure elevations. At the gate opening of 7 m, the water level at the measuring point in the outlet (revision shaft) was 8 m lower than the downstream water level. The maximum pressure fluctuations during lifting the gate were up to 10 kPa. Unfavourable flow conditions in the stilling basin are reflected also in significant pressure fluctuations, registered at transducers, located in first part of the basin. Due to deviation of main flow towards the left wall, maximum pressure fluctuations were 7kPa on the left wall, while in same profile on the right wall were 3kPa. The mean water levels on both sides were practically the same. It is interesting to mention that maximum pressure fluctuations registered at model, with same profile, were 7kPa on the left wall, during operation of all three bottom outlets. The water levels along right wall, in the second part of stilling basin were 3 m higher than in first part, with maximum fluctuations 2kPa.
Fig. 5 Changes of lifting force in gears and changes of differential pressure in gate slot
The analysis of pressure fluctuations in frequency domain, for the gate opening of 7 m gave dominant frequencies in all five piezometers at the gate lip 0.5Hz and 3Hz. The dominant pressure frequencies in the outlet are 0.1 to 0.2Hz and 12Hz and in stilling basin 0.1 to 0.2Hz and 2 to 3Hz. Registered higher frequencies have lower energy. As it can be seen, macro vortices are dominant. The vibration measuring results did not indicate presence of some considerable vibrations, neither the gate, nor outlet slab and stilling basin dividing walls. According to measurements of lifting force during the gate opening and closing, the maximum and minimum forces were determined. It was found that that maximum weight capacity of the crane was not reached , as well as the gate can be lowered under full flow .
The field tests of the middle bottom outlet on Yugoslav part of “Iron Gate I” Hydro Plant gave the possibility to analyze the behaviour of the control gate and flow conditions along outlet and stilling basin for water level higher than designed one. On the basis of measuring results and observations, it is possible to conclude:
l The bottom outlets can be transformed in reliable and safe discharging structures;
l The flow in the zone of the control gate, as well as response of gate, are such that gate can contol releasing the flow from reservoir for designed and maximum discharge;
l The flow conditions in stilling basin, for maximum design discharge through the middle outlet, are unacceptable due to asymmetric flow directed towards left dividing wall, intensive reversed flow, excessive turbulence, significant waves, splashing and spray.
l Releasing the water through one bottom outlet should be limited due to very unfavourable flow conditions in the stilling basin;
l Utilization of the bottom outlets as flood discharging structures requires some additional tests for determining optimum schedule of gates opening.
Acknowledgments
Such and similar field tests can not be performed without adequate help of the staff charged for maintenance and operation of hydraulic engineering structures. In the specific case study, the engineering and other working staff of “Iron Gate I” provided exceptional help and support in preparation and realization of the presented tests.