Les problèmes de la sédimentation dans les prises d'eau des centrales hydro-électriques

Transcription

1 Sediment problems at intakes for hydropower plants H. D, Sharma and H. R. Sharma Abstract. Unique solutions evolved to solve sediment problems at the intake structures of some major Himalayan run-of-river schemes have been described. Considerations for fixing the layout of a power intake to minimize sediment entry have been discussed. Different solutions of sediment problems at power intakes discussed here are expected to help the designer in finding a suitable solution to his problem depending upon the project constraints and the topographical features of the site. Les problèmes de la sédimentation dans les prises d'eau des centrales hydro-électriques Résumé. On décrit les solutions mises au point pour résoudre les problèmes de sédimentation aux prises d'eau de quelques grands projets "au fil de l'eau" de la région de l'himalaya. On discute des éléments à prendre en considération pour déterminer l'emplacement d'un ouvrage de prise conçu pour réduire l'envasement dans toute la mesure du possible. On peut espérer que les diverses solutions proposées ici pour résoudre les problèmes de sédimentation dans les prises des centrales hydro-électriques aideront le concepteur à trouver une solution convenable à son problème en fonction des contraintes du projet et des caractéristiques topographiques du site. INTRODUCTION One of the essential requirements for the design of an intake for a hydropower plant is that the water drawn in should be free of sediment as far as possible. The presence of sediment, especially sharp-edged sand particles, may cause wear of the turbine runner vanes and other steel parts besides causing damage to the tunnel lining. Abrasion effects become more pronounced with increase in head. Severe damage to abrasion has been observed at a number of hydropower plants in different parts of the world. The turbines of the Florida Alta plant in Chile, operating under a head of 95 m, were worn out entirely after 2000 h of operation. In a number of recently constructed power plants in Norway operating under a head of 300^500 m severe abrasion damage has been observed (Sharma, 1973). Hydropower plants can be broadly classified into two categories, (1) run-of-river schemes, and (2) storage reservoir schemes. In the case of run-of-river, a relatively small dam or a barrage is built across a mountain stream to divert the river water into the intake which in turn feeds the power house through a water conductor system. The latter is usually an open channel for low head power plants and a pressure conduit for high head ones. Due to the large quantities of sediment likely to be transported by the rivers, as in the case of Himalayan rivers in India, the diversion dam may get silted up to the crest within a few years of construction. The new river bed would stabilize at almost the original slope of the river. Under these circumstances a large quantity of sediment may enter the intake and may cause damage to the tunnel lining, the turbine blades and auxiliaries. A carefully designed layout, based on the principles of sediment transport, can reduce the sediment entry into the power intake considerably. An intake located on the outer curve of a river would draw a comparatively small quantity of sediment. In some cases, it may be desirable to induce favourable flow conditions near the intake 330

2 Sediment problems for hydropower plants 331 artificially, with the help of suitable training works and by providing an appropriate length of divide wall, if the diversion structure is a barrage. In the latter case, provision of a sediment excluder may also prove successful in controlling the sediment entry into the intake (Sharma and Asthana, 1975). The suitability of a particular type of sediment exclusion arrangement for a particular case would depend upon the topographical features of the project site. The principle involved in all these arrangements is the same, namely, to settle the sediment in settling tanks or basins by reducing the velocity of flow through them and to flush the settling sediment simultaneously under gravity flow or to occasionally clean the settling basin by mechanical means. The different arrangements have been discussed in detail elsewhere (Sharma, 1973). The present paper discusses four examples of major Himalayan run-of-river schemes in which unique solutions were evolved to solve the sediment problems at the intake structures. THE INTAKE FOR THE CHIBRO UNDERGROUND POWER HOUSE Yamuna Hydel Scheme Stage II, Part-I comprises a 53-m high concrete dam at Ichari across the River Tons to divert a discharge of 53 m 3 /s into a 7-m diameter and 6.3-km long pressure tunnel to feed the Chibro underground power house working under a gross head of 124 m and having an installed capacity of 240 MW (4 X 60 MW). The river carries a large quantity of sediment and sediment load studies have indicated that the reservoir is likely to be silted up to the crest of the dam within a few years, resulting in excessive sediment entry into the intake. Initially, a tower intake located centrally at a distance of 29 m from the axis of the dam was proposed to draw the required discharge for power generation. In order to withdraw comparatively sediment free water, the lip of the intake was kept 2.25 m higher than the spillway crest. To settle out the sediment finding its way into the intake and to flush it out before it entered the power tunnel, a settling chamber along with a flushing conduit was proposed. The model studies, however, indicated that under the worst test conditions there was an afflux of about 4 m and considerable airentrainment took place, even with the provision of anti-vortex devices. Moreover, the intake structure practically masked two spillway bays. The length of the settling chamber was also found to be inadequate to settle the desired particle sizes. To increase the length of the settling chamber and to reduce the flow velocity therein, the intake was shifted to the right abutment and, instead of a tower intake, a side intake having two settling chambers under the river bed was provided. Each settling chamber consisted of three hoppers for collecting the sediment. To flush out' the sediment simultaneously, flushing conduits were provided at the bottom of each hopper, the first two passing through the piers and the third one through the left abutment (Fig. 1). The size and transitions of the hoppers, and the size and layout of the flushing conduits were decided on the basis of hydraulic model studies to achieve the maximum flushing efficiency. The velocity in the individual flushing conduits was kept to about 4 5 m/s. The flushing discharge worked out to be 40 per cent of the design discharge during monsoons. To minimize turbulence in the hoppers and thereby to induce quick settlement of sediment, an artificial shear plane was created between the comparatively high velocity water in the settling chamber and the stagnant water in the hoppers by providing a grid similar to that in the Jaybird rock trap (Mattimore et al, 1964). Several constraints governed the design of the exclusion arrangement. A coffer dam had already been constructed to suit the tower intake originally proposed, and excavation of the dam foundations had reached an advanced stage. The width of the settling chamber could not therefore be increased. A detailed description of the intake is given by Garg et al. (1971).

3 332 H. D. Sharma and H. R. Sharma FIGURE 1. Iehari intake. Side intake at Ichari: (a) section through Ichari intake, (b) layout plan of Studies conducted on a geometrically similar model built to a scale of 1:25 indicated that the proposed arrangement was expected to exclude about per cent of the sediment which would otherwise enter the head race tunnel. The scheme has since come into operation and the performance of the intake structure has been found satisfactory.

4 Sediment problems for hydropower plants 333 FLUSHING TUNNEL *" I 2-2m dia j FIGURE 2. Plan of the intake works at Maneri. THE INTAKE WORKS AT MANERI Maneri-Bhali Hydel Scheme envisages utilization of the power potential of the River Bhagirathi, a tributary of the River Ganga, between Maneri and Uttarkashi. A 41-m high concrete diversion dam will divert the water of the River Bhagirathi into a 4.75-m diameter and 8.66-km long pressure tunnel to feed the surface power house at Uttarkashi under a gross head of 175 m and utilizing 70 m 3 /s to generate 93 MW (3X31 MW) of electric power. The River Bhagirathi has an acute bend at the proposed dam site and carries a heavy sediment load. The intake has been located on the outer curve of the river on a terrace on the left bank at a distance of about 70 m from the dam axis. The level of the intake crest has been kept 2.75 m higher than the crest level of the dam so as to withdraw comparatively sediment free water. The intake consists of three bays of 9 m each separated by 1.5 m thick piers, and one bay of 9 m width for the winter channel. The latter will draw 40 m 3 /s during winter when the river water is sediment free and will if necessary enable the sediment traps and the flushing conduits to be cleaned manually or mechanically without closing the power house. The location and the orientation of the intake, and the sediment exclusion arrangement were finalized with the help of hydraulic model studies, (Sharma and Singh, 1976). The sediment exclusion arrangement consists of two rows of four hoppers each 15 X 15.7 m. To prevent any deposition of material on the slopes of the hoppers, the latter were kept at an inclination of 40. Flushing conduits of 0.6 m X 1.5 m take off from each of the eight hoppers. Two such conduits join into one conduit of 1.2 X 1.5 m and then ultimately merge into a flushing channel of 2.2 m diameter. To keep the flushing conduits clear of sediment, a minimum velocity of 4 m/s was ensured through each conduit. The overall flushing discharge worked out to be 41 per cent of the design discharge for power generation. The proposed intake arrangement is shown in Figs. 2 and 3. The sediment exclusion efficiency is estimated at 42 per cent and it is expected that sediment coarser than 0.3 mm will be excluded. THE INTAKE FOR THE GIRI HYDEL SCHEME The Giri Hydel Scheme envisages the construction of a diversion barrage at Jateon in

5 334 H. D. Sharma and H. R. Sharma I/600 -OPERATING PLATFORM FOR TRASH CLEANING MACHINE EL Q 3-05$H. j2-3 --GROOVE FOR GATE 0-7x0-5 ANTIVORTEX GRID l2b4-c7» 7 '"*» A K- I CONTRACTION JO'NT"» ANTICIPATED FIRM ROCK PROFILE FIGURE 3. Longitudinal section and cross section of the intake works at Maneri. Himachal Pradesh across the River Giri, a tributary of the River Yamuna. The barrage, designed for a flood of 5180 m 3 /s, consists of six barrage bays each of m and four undersluice bays each of 8.07 m. An open head regulator (intake) located on the right flank and consisting of six bays will divert the river water through a 7.12-km long and 3.65-m diameter pressure tunnel to the power house at Majri in the Bata Valley to utilize a drop of 180 m and a maximum discharge of 47 m 3 /s to generate 62 MW (2X31 MW) of electric power. Initially, the sediment exclusion arrangement comprised a settling chamber provided with a low level exclusion tunnel. The latter was proposed to be operated occasionally to flush out the sediment deposited in the settling chamber and discharge this into the river downstream of the barrage through the construction adit (Fig. 4). At the barrage site, the River Giri is boulder strewn and carries a large quantity of sediment, the river bed slope being 1 in 230. In spite of the fact that the intake crest has been kept 3.5 m higher than the general river bed level, a considerable quantity of sediment is likely to enter the intake in suspension. Computations indicate that particles up to 4 mm in size will remain in suspension. The possibility of larger sized particles jumping over the intake crest cannot be ruled out, especially in the case of deposition upstream of the barrage. Approximate computations based on Camp's Charts indicated that about 80 per cent of 2 mm particles were likely to enter the power tunnel. Model studies conducted on a 1:10 geometrically similar part-width model indicated that only 28 per cent of the sediment injected into the intake settled in the settling chamber and the remaining 72 per cent entered the power tunnel. Since the works described above had already been constructed at the site, an attempt was made to minimize the sediment entry into the intake by inducing proper curvature of flow by providing an appropriate length of divide wall. Further, for removal of the sediment entering the power tunnel a sand trap-cum-sediment ejector was also provided in the head race tunnel. Model studies (Sharma and Sharma, 1976) conducted on a 1:25 geometrically similar comprehensive model indicated that increasing the length of the upstream divide wall from to m improved the flow conditions at the intake considerably.

6 Sediment problems for hydropower plants 335 BARRAGE AXIS SURFACE LINES OF FLOW BED LINES OF FLOW... FIGURE 4. INITIALLY PROPOSED SAND- TRAP a.ô/'a' est* HEAD RACE ~^ TUNNEL Intake foi the Giri Hydel Scheme. CONSTRUCTION TO RIVER ADIT NOTE-ALL DIMENSIONS ARE IN METRES FIGURE 5. Giri intake: layout of sand trap. Practically all the bed lines were diverted towards the barrage bays creating a convex curvature towards the intake (Fig. 4). The quantity of sediment entering the intake was reduced by about 60 per cent. The proposed sand trap was initially located in the head race tunnel immediately after the settling chamber (Fig. 4). However, it was not considered economically

7 336 H. D. Sharma and H. R. Sharma FIGURE 6. Intake for the Rishikesh-Hardwar Hydel Scheme. feasible to expand the already constructed head race tunnel which was supported by closely spaced steel ribs. Moreover, the expansion of the existing head race tunnel would have interfered with the construction schedule and would have delayed the completion of the project. Therefore, the proposed sand trap was provided in the additional U-bend of the head race tunnel (Fig. 5). Model studies conducted on a 1:10 geometrically similar part-width model indicated that the sediment exclusion efficiency of the new arrangement would be of the order of 53 per cent, assuming the settling chamber to be silted up. Considering the settling chamber to be effective, the exclusion lefficiency is expected to be 85 per cent. THE INTAKE FOR THE RISHIKESH-HARDWAR HYDEL SCHEME The Rishikesh-Hardwar Hydel Scheme envisages the construction of a 312-m wide barrage across the River Ganga at Rishikesh to divert the river water into a 14-km long power channel to feed the surface powerhouse at Hardwar working under a head of 30 m and utilizing a maximum discharge of 680 m 3 /s to generate 108 MW of electric power. The head regulator (intake) for the power channel consists of five spans each of 11 m. The crest of the head regulator has been kept 2.3 m higher than the crest of the undersluice bays in order to draw relatively sediment free water. The River Ganga is boulder strewn at the barrage site and during floods carries a large quantity of sediment varying from boulders to fine sand in size. A trash rack of 75 mm opening is proposed for the intake. Accordingly, sediment smaller than 75 mm is likely to enter the intake and travel down the power channel and may damage the runner blades. Further, if the quantity of sediment entering the intake exceeds the transporting capacity of the power channel, the excess sediment will deposit In the channel and will reduce its discharging capacity, thereby affecting the power generation adversely. To minimize sediment entry into the power channel, the head regulator was aligned at an angle of 108 with the barrage axis and the length of the upstream divide wall was kept at 64 m, as indicated by model studies. For controlling the entry of larger sized sediment, a shingle excluder was provided just upstream of the head regulator. In addition, a sediment ejector is also to be constructed 200 m downstream of the head regulator to exclude sediment from the power channel (Fig. 6).

8 Sediment problems for hydropower plants 337 The hydraulic designs of the sediment excluder and ejector were evolved with the help of model studies conducted on geometrically similar scale models built to scales of 1:60 and 1:20 respectively (UP1RI, 1975c, 1975a). The sediment was simulated in the model by keeping the value of the terms (W/V. d/d) and V/W the same in both model and prototype, where W = fall velocity of the particle, V- flow velocity, d = particle size, and ) = depth of flow. The efficiency of the sediment excluder was observed to be 52 per cent in the model. Particles coarser than 18 mm are expected to be prevented from entering the power channel. The efficiency of the sediment ejector was found to be 66.6 per cent in the model. The ejector was expected to exclude all particles coarser than 1 mm and up to 75 per cent of particles of 0.5 mm size. Sediment excluders and ejectors have been provided on a number of irrigation and power channels in India both in boulder as well as alluvial floored rivers. The performance of these sediment excluding devices has been found to be very satisfactory (UPIRI, 1975b). CONCLUSIONS Four interesting examples have been discussed of the sediment exclusion arrangements at intakes for run-of-river schemes in northern India. It has been shown that efficient solutions can be evolved to suit the site conditions and the project constraints with the help of model studies. In the absence of suitable topographical features, favourable flow curvature can be induced at the intake by providing an appropriate length of divide wall, if the diversion work is a barrage. REFERENCES Garg, S.P. et al. (1971) Sediment exclusion at Yamuna intake. Water Power, London, June. Mattimore, J.J., Tinney, E.R. and Wolcott, W.W. (1964) Rock trap experience in unlined tunnels. /. Power Div., Proc. Amer. Soc. Civ. Engrs, October. Sharma, H.D. and Asthana, B.N. (1975) Problems of sediment control on canals in India. Paper presented at 9th Congress of the International Commission on Irrigation and Drainage, Moscow, May. Sharma, H.D. and Sharma, H.R. (1976) Sediment exclusion at the intake for the Giri Hydel Scheme. In Proceedings of the 45th Annual Research Session ofcbi & P, Hyderabad, June, vol. II. Sharma, H.D. and Singh, S.V. (1976) Hydraulic model studies for the Maneri Bhali Hydel Scheme, stage I. Indian J. Power and River Valley Development, March. Sharma, H.R. (1973) Sediment problems of hydropower plants. In Proceedings of the 15th Congress oflahr, Ankara, September. UPIRI (1975a) Hydraulic Design of Sediment Ejector for Rishikesh-Hardwar Hydel Channel - A Model Study: T.M. no. 46 R.R. (H,-4), Roorkee, October. UPIRI (1975b) Sediment Excluders and Ejectors: Design Monograph: Roorkee, March. UPIRI (197'5c) Model Studies for Sediment Excluder at Virbhadra Barrage: T.M. no. 47 R.R. (H 2 -l), Roorkee.