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The siltation is a rather minor problem for many dams but may reduce by decades the possible long life of 50 % of them and may be a key problem within few years or few decades for over 10% of large or small dams.
The cost efficiency of various solutions for siltation mitigation has varied greatly and it is not easy to optimize their choice because the local data are never the same and the likely siltation rate itself may hardly be known precisely before some years of dam operation.
The key problems are:
– The loss of storage, especially for irrigation or drinkable water
– The damages to turbines of hydropower plants
– The impacts to the river, especially downstream of the dam.
5.1. Present conditions
The total reservoirs storage is 7000 Km3 for 50 000 large dams including very roughly:
– 5000 km3 for 10 000 hydropower dams
– 500 km3 for 30 000 irrigation or water storage dams.
– 1500 km3 for dams devoted to several targets including hydropower.
Siltation impacts and relevant solutions vary greatly with the reservoir target and volume and with the river data.
The annual rivers inflow is 40 000 km3. The annual sediment storage may presently reach 40 Billions m3, 0,6% of the reservoirs storage; the total present sediment storage in reservoirs is close to 1 500 km3 but a large part is in the designed dead storage of hydropower dams.
The overall cost of world reservoirs siltation has been evaluated between 15 and 20 Billion U.S. $ per year, i.e. 30% of the overall annual expenses of 60 Billion for dams or 10% of the 200 Billion $ annual dam benefits. But these ratios are much higher for 10 or 20 % of dams which are most prone to siltation; the siltation problem should impact their overall design including the choice of the dam site, the reservoir volume and all spilling facilities. Care of siltation may be a large part of their investment but it may be possible to postpone by decades a part of this specific investment (by pass tunnels, dredging facilities, dam upgrading) and to adjust its extent and timing to the precise actual rate of reservoir siltation. Huge expenses for siltation mitigation are much more acceptable when the basic dam and plants investments are already paid for.
Some efficient solutions such as sluicing or flushing are well known from a long experience worldwide: some more recent solutions (such as sediment bypass tunnels, specific dredging facilities or managed sediment storage) are promising especially for very large schemes and are analyzed below.
5.2. Variability in sediments:
– 90% of the world catchment areas generate less than 500 tons of sediment per year and per km2 but 10% generate 5000 tons/km2 as average and some supply well over 10 000 tons. The annual sediment load reaching many small reservoirs is thousands m3 but many large hydropower reservoirs are reached by million or dozen Million m3/year and the load of 10 rivers is over 150 Million m3/year.
– The concentration varies greatly along the year; 50% of the annual load may be discharged in few days and 90% in one month. Most siltation may be at the beginning of the flood season. The total annual load may also vary greatly.
Measurement of sediment load is thus a difficult problem but it is a key target as well at design time as during operation; many usual methods and procedures are unefficient and were the reason of unadapted designs.
5.3. Main impacts of reservoirs sedimentation
5.3.1. Storage loss
It is usualy the main impact for dams devoted to water storage as their benefit is quite proportional to the storage. This impact is lower for dams devoted to hydropower: their benefit may possibly be reduced by under 20% when the reservoir is 80% filled (including a large part in the designed dead storage).
5.3.2. Turbines abrasion:
Sediment coarser than 0,1 mm may greatly accelerate the erosion of turbines parts; even smaller grain sizes may cause damages if containing quartz. It may be the main siltation problem for high head hydropower. Also sediment concentration and total head are important factors.
5.3.3. Downstream impacts
River reaches downstream of dams suffer large environmental impacts due to flow changes, reduction of sediment load, altered nutrient dynamics, temperature changes, and the presence of the migration barrier imposed by the structure and the upstream impoundment.
Clear water released from the reservoir will cause down stream erosion and possibly bank failures.
Sediment trapping by dams can even affect coastal morphology. It sometimes becomes a major factor contributing to the rapid shoreline recession and subsidence (Nile, Mississipi…). One way of reducing this impact may be to build run-of-river hydroelectric projects which would allow passage of 100% of the fines and an important portion of the bed load.
5.3.4. Concepts Of Reservoir Life
With reasonable levels of maintenance, the structural life of dams is virtually unlimited, and most reservoirs are designed and operated on the concept of a finite life which will ultimately be terminated by sediment accumulation rather than structural obsolescence.
Design life is the planning period used for designing the reservoir project. Planning and economic studies are typically based on a period not exceeding 50 years, whereas engineering studies often incorporate a 100-year sediment storage pool in the design.
The target of a very long reservoir life should be a key point of a right design and management of siltation problems.
5.4. Reservoir sedimentation management
5.4.1. Sediment trapping upstream of the dam
There are only two strategies to reduce the sediment yield entering a reservoir: either prevent erosion or trap eroded sediment before it reaches the reservoir.
The rehabilitation of some watersheds can dramatically reduce the rate at which sediment, nutrients, and other contaminants are delivered to a reservoir.
– A number of reservoirs in Russia are undergoing sedimentation problems. An efficient proposed option was the construction of upstream reservoirs that will act only as sediment retention structures. (Aliev et Al 2009,ICOLD, Q.89)
5.4.2. Sediment Routing
Sediment routing includes any method to manipulate reservoir hydraulics, geometry, or both, to pass sediment through or around reservoir or intake areas while minimizing objectionable deposition.
Routing is the most environmentally benign sediment management strategy.
Sediment routing techniques include sediment passing through and sediment by pass.
5.4.2.1. Sediment pass through
It may be by seasonal drawdown, by drawdown adapted to floods or by turbidity currents. This requires initial implementation of the necessary bottom gates to be designed with great care.
– A reservoir operated under seasonal drawdown is either partially or completely emptied during the beginning of flood season. Seasonal drawdown is conducted during a predetermined period each year, as opposed to flood routing, which requires that the reservoir level be drawn down for individual flood events when they occur. At some sites routing can be implemented at very low cost
A major disadvantage of sediment routing is that a significant amount of water must be released during floods to transport sediments. Sediment routing is most applicable at hydrologically small reservoirs where the water discharged by large sediment-transporting floods exceeds the reservoir capacity, making water available for sediment release without infringing on beneficial uses.
From among the case studies, Sanmenxia, Heisonglin, Cachi, Gebidem, and Sefid-Rud all employ reservoir emptying for sediment management. Sanmenxia is an example of sediment routing because the reservoir is empty during the flood seasons, with the primary emphasis placed on passing sediment through the impoundment without deposition. This was successfully achieved before 1960 with the Old Aswan Dam storing 6 billion m3 of Nile water.
– Density of the sediment laden water at the time of the flood may be high and the sediment laden water may sneak and drift along the bottom of the reservoir. Turbidity currents may then be vented through low-level outlets, reducing the sediment accumulation within the impoundment without drawing down the pool level. Jolanda M.I Jenzer Althaus et al. provide the concept of turbidity currents based on case studies of several Alpine reservoirs in Switzerland. Analysis of this intervention showed that the reservoir could last 20 to 50 years longer. (2009, ICOLD, Q.89)
5.4.2.2. Sediment Bypass For Reservoirs
When topographic conditions are favourable (for this reason also the site selection is important), a large-capacity channel or tunnel can be constructed to bypass sediment-laden flow around the reservoir or part of it. It may be built initially and possibly used for flood control during construction. Most may also be built according to precise needs after years or decades of operation. Such tunnel may be associated with a low dam in the upstream part of the reservoir. It may bypass some hundred m3/s or even thousand m3/s of water. This solution may have much future for large schemes.
– Mitsuzumi, Kato and Omoto provide the results of the monitoring activities that had been conducted on the sediment bypass system at the Asahi Dam in Japan. The 2350 m long bypass system has a maximum discharge capacity of 140m3/s. It can flush out both bed load and/or turbid water. The by pass system managed to control 90% of the sediment. (ICOLD, Q.89)
5.4.3. Sediment flushing
Hydraulic flushing involves reservoir drawdown by opening a low-level outlet to temporarily establish riverine flow along the impounded reach, eroding a channel through the deposits and flushing the eroded sediment through the outlet.
Unlike sediment routing, which attempts to prevent deposition during flood events, flushing uses drawdown and emptying to scour and release sediment after it has been deposited.
Usually flushing is less efficient if the sediment is coarse or is consolidated clay materials. The width of a flushing channel within sediments is often in the range of 100 m and may reach few hundred m; flushing is thus much more efficient in narrow valleys.
– Guo and Cao (China) provide the experience of Sanmenxia Reservoir showing that the operation mode has an important effect on reservoir sedimentation. The reconstruction was done to increase the flood discharge capacity of the dam so that as much as possible of all flood sediment laden water is flushed out, which makes the reservoir in an equilibrium state. Guo and Cao futher provide that having cascade dams upstream greatly reduces the sedimentation in the downstream reservoirs and in the long run prolongs the life span of the downstream reservoirs. (2009,ICOLD, Q.89)
– Sumi et al. (Japan) present their finding on the effect of sediment flushing and environmental mitigation measures in the Kurobe River for two major dams namely Unazuki and Dashidaira. The sediment flushing and sluicing operations for two dams are conducted in a coordinated manner. It was concluded that drawing sediment while operating at low water level significantly improves the flushing efficiency. Evacuation channels have been used successfully in the evacuation of fish and other creatures whenever turbidity levels are high due to flushing.
5.4.4. Managing the silt storage
– It is usually difficult to drawdown completely the large hydropower reservoirs but it is usually possible to drawdown the reservoir by 20 % or 30 % at the flood time in order to store the sediment in the dead storage and to pass through most sediment when the dead storage is full. Corresponding gates may be 20 to 50 m under the dam crest.
– Turbines abrasion may be a key problem for high head hydropower and many desilting underground chambers along head race tunnels designed for avoiding it proofed costly and poorly efficient. It may be possible to design the reservoir itself as a desilting structure upstream of the entrance of the head race to the power house. This solution which is presented in the ICOLD Bulletin 144 appears very promising. The stored sediment may be flushed from time to time or dredged permanently.
5.4.5. Removing stored sediment:
Sediment deposits may be mechanically removed from reservoirs by dry excavation or hydraulic dredging and hydro-suction.
The annual worldwide stored sediment is close to 40 Billion m3; possibly half is in the designed dead storage. Harmfull sediment is thus in the range of 20 Billion m3 for a total relevant damage close to 20 Billion $, i.e as average 1$/m3 but much higher or lower according to dams. This means that for many dams, removing materials at a cost of one or few $/m3 may be cost effective.
5.4.5.1. Dry Excavation
All methods of mechanical excavation are costly because of the large volumes of material involved and, frequently, the difficulty of obtaining suitable sites for placement of the excavated material within an economic distance of the impoundment. However, once sediments are deposited in a reservoir, excavation may be the best management option available.
– Sumi et al. discuss a concept of asset management but specifically on its applicability to reservoir sediment management at Kizu River in Japan, using a number of case studies. It was concluded that dry excavation while the reservoir is empty may be the most economically feasible countermeasure even if reduced power production and water production is taken into consideration. (2009, ICOLD, Q.89)
5.4.5.2. Hydraulic dredging
The solutions may be very different for small irrigation dams and for large hydropower reservoirs. They have been presented in ICOLD Bulletin 144 (chapter 3.4.8.)
– For rather small irrigation reservoirs siphon dredges may be used and do not need pumps. The slurry is forced through the pipeline by the differential head between the water surface in the reservoir and the discharge point located at the lowest possible place downstream of the dam. It seems adapted to some dozen thousands m3 of silt per year.
– Many large hydropower reservoirs are designed with a huge dead storage witch may be filled in some years or decades: flushing may be inefficient or unacceptable but the cost of dredging millions m3/year may be acceptable if the dredging equipment is designed according to local facilities including fine materials, calm water, electric energy, and paid for along decades of operations. This is studied in ICOLD Bulletin 144. For a scheme of 1 Billion $, investing 20 to 50 Millions $ of dredging equipment after 10 or 20 years of operation may be cost effective.
– Dredged sediments may be sent downstream to the river. They may also be sent to a disposal area and the water flown back to the reservoir (Mechra reservoir in Morocco: ICOLD Q.89).
5.4.6. Phased dam construction:
It may be difficult to avoid the reservoir siltation but many earth fill irrigation dams may be designed for an easy level increase after 20 or 100 years of operation. Raising by 5 m a 30 m high dam may quite double the storage when necessary.
5.5. Asset management for reservoir sediment management
Sumi et al. (Japan) provide a study on the applicability of the asset management for reservoir sediment management. (2009, ICOLD, Q.89)
5.5.1. Effects and costs of countermeasures
In order to sustain function of dams for more than a century, we must clarify challenges to apply asset management to dams, which are made up of a variety of elements such as dam body, spillways mechanical and electric equipments with different service lifetimes and functions to lower their life cycle costs (Lcc) and to smooth their annual maintenance costs.
In the near future, the sedimentation countermeasure cost may exceed for many dams the cost of electrical and machinery equipment categories. Therefore, it is important to apply asset management not only to the electrical and the machinery equipment categories, but also to sedimentation countermeasures.
The effects and cost of each sedimentation countermeasure are presented on Table 3 as an example.
Table 3 Sedimentation Countermeasures Cost
assumed by the sedimentation measure case in Japan
5.5.2. Study of optimization of sediment removal method
– For this example, it has been shown that the dry excavation with the reservoir emptying may be economically feasible even if it needs compensation both for reduced power production and water use loss rather than other countermeasures requiring initial facility investment such as sediment bypass, flushing sediment etc.
– The above result suggests the possibility of “the optimization of sediment removal method” if associating the management of several dams.
5.6. Conclusion
Siltation may be a key problem of many reservoirs
The design of such dams including the site choice and layout should be linked with it.
It is necessary to investigate following items for sediment treatment in reservoir:
– To measure efficiently the sediment load
– To evaluate the cost of sediment in long term at the time of planning dam structure
– To select most suitable sediment treatment method with consideration of topography and flows of river, effectiveness, economical, environmental and various conditions with overall judgment.
– To apply, not only one, but several measures in rivers with great volume of sediment, such as sediment routing, bypassing, dredging, or measures to origin of occurrence upstream of dam.
– To apply measures for sediment with coordination of several reservoirs of one area or basin.
– In the near future, strategic dam asset management including preventive countermeasures for reservoir sedimentation will be an important challenge.
The sediment in dams is critical. Leaving sediment management as it is may lead to not only reduction but full loss of reservoir functions. With proper treatment of sediment, it is possible to maintain its function economically along centuries.
Since the progress of sedimentation is slow in general, part of solutions may possibly be delayed but the long term possible solutions should be analysed initially and partly implemented from the beginning: they may thus impact the full design.
REFERENCES
[1] G.R.BASSON: MANAGEMENT OF SILTATION IN EXISTING AND NEW RESERVOIRS, GENERAL REPORT Q89 2009
[2] Gregory L.Morris and Jiahua Fan: Reservoir Sedimentation Handbook : Design and Management of Dams , Reservoirs , and Watersheds for Sustainable Use, 1998
[3] Tetsuya SUMI, Kiyoshi KOBAYASHI, Kenichiro YAMAGUCHI, Yasufumi TAKATA: STUDY ON THE APPLICABILITY OF THE ASSET MANAGEMENT FOR RESERVOIR SEDIMENT MANAGEMENT, Japan q89-r.4, 2009
[4] Qingchao GUO, Wenhong CAO: RESERVOIR SEDIMENTATION AND ITS CONTROL､IWHR China Q89-R.4, 2009
[5] ICOLD Bulletin 144 : Costs Savings in Dams (2010)
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