SEDIMENT MANAGEMENT IN HYDROPOWER PLANTS AN OVERVIEW

Transcription

1 International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun SEDIMENT MANAGEMENT IN HYDROPOWER PLANTS AN OVERVIEW P.K. Pande, Former Professor of Civil Engineering, University of Roorkee (IIT, Roorkee), Roorkee, India INTRODUCTION Hydroelectric power plants can be either resrvoir based or run of river (RoR). While storage based projects require construction of a dam and store water in the reservoir created upstream, RoR schemes rely on the diversion of water from the river to the power house via a power channel. The diversion structure is generally a barrage or a low dam without sufficient storage. Both types of schemes, however present problems of sediment management, though differing in nature. Thus while storage projects have the problem of sediment deposition in the reservoir, RoR schemes have to ensure appropriate measures for sediment extraction. This paper reviews in brief the problems of both the types and the mitigation measures. RESERVOIR BASED SCHEMES Reservoir Sedimentation Reservoir based schemes present sediment problems of various types. These problems affect both the upstream and downstream of the reservoirs. On the upstream side deposition of sediment results in delta deposition causing aggradation, reduction of reservoir capacity and consequent loss of energy besides affecting navigation as well as ecological changes. The downstream impacts include change in stream morphology, altered nutrient dynamics and temperature changes. The relatively clear water flowing out of the dam causes degradation and armouring on the downstream which at times can make it unsuitable as ecological habitat or spawning grounds. Loss of Capacity One of the major problems is the deposition of sediment in the reservoir and consequent loss of storage capacity. It is estimated that approximately 1% of the storage volume of the world s reservoirs are lost annually due to sediment deposition (Mahmood, 1987 and Yoon, 1992). The estimated rates of reservoir sedimentation are summarised in Table1 (adapted from Shen 2010). 85

2 ICHPSD-2015 Table 1: Reduction in Storage Volume of Reservoirs Location Percent sedimentation rate (annual) India 0.5% China 2.3% U.S.A. 0.22% Turkey 1.2% Morocco 0.7% Tunisia 2.3% World 1% A preliminary estimate of the progressive reduction of the storage capacity of a reservoir can be made if the annual sediment yield from the catchment and the trap efficiency of the reservoir are known. There are many equations developed for estimation of the sediment yield from a catchment based on regression analysis. These include the Universal Soil Loss Equation (USLE) developed by the USDA Agriculture Research Station and the Modified Universal Soil Loss Equation (MUSLE). The USLE groups the numerous interrelated physical and management parameters that influence the erosion rate under six major factors, of which site specific values can be expressed numerically. The USLE is represented as: A = R*K*L*S*C*P where, A = Average annual soil loss in tons per ha per year; R = rainfall erosivity factor; K = soil erodibility factor; LS = slope length and slope steepness factor; C = cover management factor; and P = support practice factor. For Indian catchments a model developed by Garde and Kothiyari (1986) based on data from 50 catchments, that includes the physiographic factors such as slope, drainage density, soils, land use and climatic factor i.e. rainfall could be used. The model is given by Y = 0.2 F e 1.7 S D d 0.10 (P max /P) 0.9 P a m where, Y is annual sediment yield in cm, D d is the drainage density, S is the land slope, P a is annual rainfall in cm, P max is average maximum monthly rainfall in cm and F e is erosion factor defined as F e = (1/ A i ) (0.8A A +0.6A G + 0.3A F + 0.1A W ) where, A A, A G, A F and A W are the arable, grass and scrub, protected forest and waste areas respectively in km 2 and A i is an arbitrary coefficient. 86

3 International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun The trap efficiency can also be determined from many relationships available for this purpose. One of the simplest relationships is that developed by Brune (1953) who related the trap efficiency (T e ) to the ratio of the storage capacity (V) and the annual water inflow volume (I). The results are presented in the figure below: Fig 1: Relationship between T e and V/I A combination of the sediment yield and trap efficiency equations can then be used to get the reduction in storage capacity. Deposition Pattern The coarsest material entering a reservoir deposits early and forms what is generally known as the delta deposits. The finer material travels further downstream into the reservoir and deposits closer to the dam. This is shown in Figure 2. The basic characteristics of reservoir deltas are (Fan and Morris 1992) There is an abrupt change between the slope of topset and foreset deposits. Sediment particles on the topset bed are coarser than on the foreset bed, and there is an abrupt change in particle diameter between topset and foreset deposits. The elevation of the transition zone from the topset to the foreset bed depends on the reservoir operating rule and pool elevation. 87

4 ICHPSD-2015 Fig. 2: Typical Deposition Pattern in a Reservoir The delta profile varies with time as is clear from Fig.3, which shows the profiles for the Bhakra dam for the years 1971, 1980 and Fig. 3: Timewise Delta Profile for Bhakra Dam (source: Morris 1998) Mitigation Measures The strategies to deal with reservoir sedimentation essentially fall under three categories as discussed below: 1. Reduction of Sediment Yield Reduction in sediment yield can be achieved either by erosion control or by trapping eroded sediment before it reaches the reservoir. This may include catchment treatment by vegetative measures, gulley control and structural and other measures such as sediment traps and check dams etc. 2. Sediment Routing The sediment load in a stream varies both temporally and also within the cross section, the sediment routing methods seek to manipulate sediment laden flows through or around the reservoir. These methods include sediment pass through by partial reservoir drawdown or venting density currents or sediment bye pass. An 88

5 International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun example of the partial Drawdown strategy for sediment routing is the Three Gorges Project in China (Lin et.al. 1989). Fig. 4: Sediment Routing Strategies (after Morris 1998) 3. Reservoir Flushing Flushing involves opening a dam s bottom outlets and allowing the accumulated sediment to be re-suspended and flushed out. The flushing can be done without allowing the pool level in the reservoir to drop down significantly called partial drawdown flushing or full drawdown flushing in which the reservoir pool level is allowed to be completely drawn down. While full drawdown flushing is more effective, it has certain other problems associated with it besides consuming a lot of water. The figure below shows both the flushing options. Fig. 5: Partial (left) and Full (right) Drawdown Flushing (Wang and Hu 2009) The environmental impacts of flushing may include fish gill clogging, changes in riverine habitats, increased temperature, reduced dissolved oxygen etc. and need to be considered while planning flushing. (Baran et.al.2011) Some case studies for sediment routing and flushing have been described in detail by Morris et.al RUN OF RIVER SCHEMES In run of river schemes, water is diverted to a channel for conveying it to the power house. The diversion structure is normally a barrage or a low dam without much storage and the sediment accumulated upstream is easily flushed down. Further, a considerable amount of sediment both suspended and bed load finds its way into the diversion channel. Two issues need consideration as far as sediment management in such schemes is concerned viz. (i) the design of the diversion channel so as to be able carry the sediment entering the same without deposition and (ii) extracting the sediment above a particular size before the water is led to the turbines. 89

6 ICHPSD-2015 It is generally accepted that sediment coarser than 0.2mm may harm the turbine blades and therefore needs to be extracted from power channels. Further, sediment extraction may also be required if the sediment load entering the power channel is more than its carrying capacity. Two devices are commonly used for sediment extraction in situations where a significant amount of suspended sediment needs to be extracted. These are vortex chamber sediment extractors and settling basins. A brief description of these follows. Vortex Chamber Extractors This type of extractor consists of a cylindrical chamber with an orifice at the centre of its bottom. Water is led tangentially into the chamber at high velocity giving rise to a Rankine Combined Vortex type of flow with a forced vortex at the periphery and a free vortex forming near the orifice. The sediment particles tend to settle at the bottom and move towards the orifice and can be flushed out through the orifice into a channel or pipe. This type of extractor has been analysed in detail by many investigators including Cecen and Bayazit (1975), Mashauri (1986) and Athar et.al. (2002). The use of this device is however limited to small schemes only as for efficient extraction, the chamber diameter required is about five times the inflow channel width. Fig. 6: Vortex Chamber Extractor 90

7 International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun Settling Basins The settling basins work on the principle of reducing the velocity of flow within the basin and ensuring that sediments above a certain size settle on the floor of the basin. The reduction in velocity is brought about by increasing the width and depth, while the length is provided to ensure desired efficiency of removal of sediment of the smallest size to be removed. A schematic of a settling basin is shown in Fig.7. The sediment thus settled on the floor may be removed by continuous flushing or removed periodically by flushing or mechanical means. Design Considerations Fig. 7: Schematic of a Settling Basin The design of a settling basin involves determination of a combination of width, depth and length of the basin for desired removal efficiency of sediment above a given size. There are many empirical and semi empirical procedures available for determining the efficiency of removal (Dobbins 1944, Camp 1946, Sumer 1977). An analysis for determining the accuracy of the different relationships based on the data available was made by Dongre (2002), who proposed the following relation for the efficiency a settling basin without flushing, which could predict the efficiency with a maximum error within 25%: ɳ = {1-exp (-0.3(A b /A a ))}{1-exp (-0.1(L/D))}{1-exp (-0.42(ω/u * ))} 91

8 ICHPSD-2015 where ɳ is the efficiency, A b and A a the cross sectional areas of the basin and approach channel respectively, L is the basin length, D the basin depth ω the fall velocity of the given size of particle and u * is the shear velocity. The effect of flushing on the efficiency was examined by Rangaraju et.al. (1999), who proposed ɳ f /ɳ = Q f {ω/u * } where ɳ f is the efficiency with flushing and Q f is the flushing discharge expressed as a percentage of the discharge entering the basin. It is worth mentioning that continuous flushing uses about 15 to 20% of the channel discharge and thus the channel upto the settling basin has to be designed to carry more than the discharge required at the turbine. Certain innovative technologies for sediment sluicing in settling basins have also been proposed. These include the Serpent Sediment Sluicing System (S4) and the Slotted Pipe Sediment Sluicers (SPSSs) which have been used in hydropower plants in Nepal (Shreshta) successfully. The sediment flushed from settling basins finds its way into the river. Since the river discharge is likely to be low, part of it may get deposited at the outfall and have an effect albeit temporary on the river morphology. The impact is temporary because the sediment thus deposited will be washed away during floods. EROSION OF TURBINE COMPONENTS The erosion of turbine components depends not only of the size of sediment entering the turbine but also on the shape and composition of the sediment particles. Sediment which has a large proportion particles having a hardness of more than 5 in the Mohs scale are considered to be more harmful. Thus quartz, which has a hardness of 7, can cause considerable erosion of turbine components. According to Shrestha, the data from Nepal shows that the sediment in many Himalayan rivers has quartz content ranging from about 40% to 70%. The shape of the sediment particles also has an effect on the erosion. While rounded particles will cause less erosion, the erosion will be more if the particles have sharp edges as is likely to be the case in the higher reaches of a river. Other factors which are likely to affect the erosion of turbine components are the chemical composition of water, the material hardness and elasticity of the turbine components and the angle of impingement of the sediment particles. 92

9 International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun CONCLUSIONS Hydroelectric projects both dam based and RoR give rise to problems of sediment management, though of different types. A review of the problems in both and the mitigation measures has been carried out. Issues to be considered in reservoir flushing as well as in settling basin design have been discussed in brief. REFERENCES 1. Athar, M., Kothyari, U.C. and Garde, R J. (2002). Sediment Removal Efficiency of Vortex Chamber Type Sediment Extractor, Journal of Hydraulic Engineering, ASCE, 128(12), Atkinson E. (1996). The Feasibility of Flushing Sediment From Reservoirs. HR Wallinford Group Limited, Oxon, UK. 21 pp. 3. Baran Eric and Nasielski Joshua.(2011), Reservoir Sediment Flushing and Fish Resources, World Fish Centre. 4. Brune, G.M. (1953). Trap Efficiency of Reservoirs, Trans. AGU. 14(3). 5. Camp, T. R. (1946). Sedimentation and the Design of Settling Tanks. Trans. ASCE, Cecen, K. and Bayazit, M. (1975). Some Laboratory Studies of Sediment Controlling Structures, 9th Congress of ICID, Moscow, pp Dobbins, W. E. (1944). Effect of Turbulence on Sedimentation. Trans. ASCE, 109, Dongre, N. B. (2002). Settling Basin Design, M. Tech. Thesis, Dept. of Civil Engineering, Indian Institute of Technology, Roorkee, India. 9. Fan, J., and Morris, G. L., 1992,"Reservoir Sedimentation. I: Delta and Density Current Deposits," J. Hydraulic Engineering ASCE, 118 (3): Garde, R.J. and Kothyari, U.C. (1986). Erosion in Indian Catchments, 3rd Int. Symposium on River Sedimentation, Jackson (Miss), U.S.A. 11. Lin, B., Dou, G., Xie, J., Dai, D., Chen, J., Tang, R., Zhang, R., "On Some Key Sedimentation Problems of Three Gorges Project (TGP)," Intl. J. Sediment Research, 4 (1): Mahmood, K., (1987), Reservoir Sedimentation Impact, Extent and Mitigation, World Bank Technical Paper No Mashauri, D.A. (1986). Modelling of Vortex Settling Basin for Primary Clarification Of Water, Ph.D. Thesis, Tamperi University of Technology, Finland, 217 pp. 14. Morris, Gregory L. and Fan, Jiahua. (1998). Reservoir Sedimentation Handbook, McGraw-Hill Book Co., New York. 15. Ranga Raju, K.G., Kothyari, U.C., Srivastav, S. and Saxena, M. (1999). Sediment Removal Efficiency of Settling Basins, Journal of Irrigation and Drainage Engineering, ASCE, 125(5), pp Shen, H.W. (1999) Flushing Sediment through Reservoirs, Journal of Hydraulic Research, 37: 6, Online publication

10 ICHPSD Shrestha, K., Sediment Problems in ROR Hydropower Project. Upload.Wkimedia.org. 18. Sumer, M. S. (1977). Settlement of Solid Particles in Open Channel Flow. J. Hydr. Div., ASCE, 103(11), Walling, D.E. (1994). Erosion and Sediment in a Changing Environment. Proc. of the International Symposium, East-West, North-South Encounter on the State-of-the art in River Engineering Methods and Design Philosophies, St. Petersburg, Russia. 20. Wang Z., Hu C. (2009). Strategies for Managing Reservoir Sedimentation. International Journal of Sediment Research. 24; Yoon, Y., N.,(1992), "The State and the Perspective of the Direct Sediment Removal Methods from Reservoirs." International Journal of Sediment Research, Vol.7, No.2. 94